InfoMagic Standards 1994 January/InfoMagic Standards - January 1994.iso/ccitt/1988/troff/8_2_04.tro

Text File | 1991-12-22 | 833KB | 33,264 lines

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.nr LL 40.5P
.nr ll 40.5P
.nr HM 3P
.nr FM 6P
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.nr PD 9p
.po 4P

.rs
\v'|.5i'
.LP
\fBMONTAGE : FIN DE L'ANNEXE A DE LA RECOMMANDATION X.21
EN\(hyT\*\|ETE DE CETTE PAGE\fR
.sp 2P
.LP
\v'28P'
\fBRecommendation\ X.22\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fBMULTIPLEX\ DTE/DCE\ INTERFACE\fR \fB\ FOR\ USER\ CLASSES\ 3\(hy6\fR
.EF '%    Fascicle\ VIII.2\ \(em\ Rec.\ X.22''
.OF '''Fascicle\ VIII.2\ \(em\ Rec.\ X.22    %'
.ce 0
.sp 1P
.ce 1000
\fIGeneva, 1980, amended at Melbourne, 1988)\fR
.sp 9p
.RT
.ce 0
.sp 1P
.sp 2P
.LP
    The\ CCITT,
.sp 1P
.RT
.sp 1P
.LP
\fIconsidering\fR
.sp 9p
.RT
.PP
(a)
that Recommendations X.1 and X.2 define the
services and facilities to be provided by a public data network;
.LP
.PP
(b)
that Recommendation\ X.21 defines the interface between a Data Terminal
Equipment (DTE) and Data Circuit\(hyterminating Equipment (DCE) for synchronous
operation on public data networks;
.PP
(c)
that it is desirable for characteristics of the
interface carrying a multiplexed bit stream between a DTE and a multiplex
DCE of a public data network to be standardized;
.sp 1P
.LP
\fIunanimously declares\fR
.sp 9p
.RT
.PP
that the interface between the DTE and the DCE in a public data
network using a multiplexed channel configuration employing synchronous
transmission should be as defined in this Recommendation.
.bp
.sp 2P
.LP
\fB1\fR     \fBScope\fR
.sp 1P
.RT
.PP
1.1
This Recommendation defines the interface between a DTE and a multiplex
DCE, operating at 48\|000\ bit/s and multiplexing a number of
Recommendation\ X.21 subscriber channels employing
synchronous
transmission.
.sp 9p
.RT
.LP
.PP
1.2
The number of Recommendation X.21 subscriber channels is
limited by the number of subscriber channels allowed in the network multiplex
structure (see\ \(sc\ 4).
.PP
1.3
The provision of all services supported by Recommendation\ X.21
is possible.
.sp 2P
.LP
\fB2\fR     \fBDTE/DCE physical interface elements\fR (see Table 1/X.22)
.sp 1P
.RT
.sp 1P
.LP
2.1
    \fIElectrical characteristics\fR
.sp 9p
.RT
.PP
The electrical characteristics of the interchange circuits at both the
DCE side and the DTE side of the interface will comply with
Recommendation\ X.27 with implementation of the cable termination in the
load.
.RT
.sp 1P
.LP
2.2
    \fIMechanical characteristics\fR
.sp 9p
.RT
.PP
Refer to ISO 4903 (15\(hypole DTE/DCE interface connector and contact number
assignments) for mechanical arrangements.
.RT
.sp 1P
.LP
2.3
    \fIFunctional characteristics of the\fR
\fIinterchange circuits\fR
.sp 9p
.RT
.PP
Definitions of the interchange circuits G, T, R, C, I, S and F are given
in Recommendation\ X.24 and in\ \(sc\ 4 below.
.RT
.LP
.sp 1
.ce
.line
.ce
\fBTable [1/X.22] [T1.22], p.\fR
.LP
2.4
\fICall control and failure detection procedures\fR
.LP
\fICall control\fR
\|and \fIfailure detection\fR
 \|procedures shall operate as specified in Recommendation\ X.21 on each
subscriber channel independent of other subscriber channels.
.LP
2.4.1
\fIQuiescent states\fR
.LP
The quiescent states shall be in accordance with
Recommendation\ X.21, \(sc\ 2.5.
.LP
2.4.2
\fIFailure detection\fR
.LP
See Recommendation\ X.27, \(sc\ 9 for association of the
receiver circuit failure detection types.
.LP
2.4.2.1
\fIFault conditions on interchange circuits\fR
.LP
The DTE should interpret a fault condition on circuit\ R as r\ =\ 0 on
all channels using failure detection type\ 2, a fault condition on circuit\
I as i\ =\ OFF on all channels using failure detection type\ 1, and a fault
condition on both circuits\ R and\ I as r\ =\ 0, i\ =\ OFF \fI(DCE not
ready)\fR
on all channels.
.LP
Alternatively a fault condition on one of these circuits,
R or I, may be interpreted by the DTE as r\ =\ 0, i\ =\ OFF
\fI(DCE not ready)\fR
, using failure detection type\ 3.
.LP
The DCE will interpret a fault condition on circuit\ T as t\ =\ 0
on all channels using failure detection type\ 2, a fault
condition on circuit\ C as c\ =\ OFF on all channels using failure
detection type\ 1, and a fault condition on both circuits\ T
and\ C as t\ =\ 0, c\ =\ OFF on all channels \fI(DTE uncontrolled not\fR
\fIready)\fR
.LP
Alternatively, a fault condition on one of these circuits,
T or C, may be interpreted by the DCE as t\ =\ 0, c\ =\ OFF
\fI(DTE uncontrolled not ready)\fR
, using failure detection
type\ 3.
.LP
2.4.2.2
\fIDCE fault condition\fR
.LP
Indication of the DCE failure condition shall be in accordance
with Recommendation\ X.21, \(sc\ 2.6.2.
.LP
A DCE failure condition may effect all subscriber channels
at the DTE/DCE interface.
.LP
2.4.2.3
\fISignal element timing provision\fR
.LP
The provision of signal element timing shall be in accordance
with Recommendation\ X.21, \(sc\ 2.6.3.
.LP
2.4.3
\fIElements of the call control phase\fR
.LP
The elements of the call control phase, for each channel, shall be in accordance
with Recommendation\ X.21, \(sc\ 4 with the exception that byte
timing is not used.
.LP
2.4.4
\fIData transfer phase\fR
.LP
The data transfer phase, for each channel, shall be in accordance with
Recommendation\ X.21, \(sc\ 5.
.LP
2.4.5
\fIClearing phase\fR
.LP
The clearing phase, for each channel, shall be in accordance with Recommendation\
X.21, \(sc\ 6.
.LP
\fB3\fR
\fBAlignment of call control characters\fR
\fBand\fR
\fBerror\fR
\fBchecking\fR
.LP
\fB
3.1
\fICharacter alignment\fR
.LP
For the interchange of information between the DTE and the DCE for call
control purposes, it is necessary to establish correct alignment of
characters. Each sequence of call control characters to and from the DCE
shall be preceded by two or more contiguous\ 1/6 (\*QSYN\*U) characters.
.LP
3.1.1
Certain Administrations will require the DTE to align call
control characters transmitted from the DTE to either SYN characters delivered
to the DTE or to the signals on the \fIframe start identification\fR
interchange
circuit\ (F).
.LP
3.1.2
Certain Administrations will permit call control characters to be transmitted
from the DTE independently of the SYN characters delivered to
the DTE.
.LP
3.2
\fIError checking\fR
.LP
Odd parity according to Recommendation\ X.4 applies for the
interchange of IA5 characters for call control purposes.
.LP
\fB4\fR
\fBMultiplex structure\fR
.LP
Depending on the multiplex structure used by the network, the
structure of the multiplexed bit stream will be one of two different
types.
.LP
4.1
\fIMultiplex structure in networks providing 6\ bit\(hybytes\fR
.LP
The DCE shall deliver to and receive from the DTE a 6\(hybit byte
interleaved multiplexed bit stream containing a number of subscriber channels.
The allocation of the subscriber channels should be:
.LP
\ 5\ channels\ (phases)\ of\ 9600\ bit/s\ or
10\ channels\ \ \ \ \ \ \ \ of\ 4800\ bit/s\ or
20\ channels\ \ \ \ \ \ \ \ of\ 2400\ bit/s\ or
80\ channels\ \ \ \ \ \ \ \ of\ \ 600\ bit/s\ or
an appropriate mix of channel data signalling rates having an aggregate bit
rate of 48\ kbit/s.
.LP
The multiplex structure is divided into five phases of 9600\ bit/s, where
each phase shall be homogeneous with regard to the subscriber data
signalling rates.
.LP
4.1.1
\fIInterchange circuits and\fR
\fIinterface signalling\fR
\fIscheme\fR
.LP
The interchange circuits between the DTE and the DCE are shown in Figure\
1/X.22 and a timing diagram for the signals is given in
Figure\ 2/X.22.
.LP
The signalling over the interchange circuits is as follows.
.LP
The transmit (T) and receive (R) circuits will convey in one
time slot six consecutive user data bits for one subscriber
channel (see Figure\ 2/X.22).
.LP
The control (C) and indication (I) circuits will convey the
appropriate signal levels in accordance with Recommendation\ X.21 for the
data channel which in the same time slot have bits conveyed over the respective
data circuits.
.LP
Change of condition on circuit C shall take place at the OFF to ON
transition of circuit\ S at the beginning of the first bit in the 6\(hybit
byte.
The condition on circuit\ C shall be steady for the whole 6\(hybit byte.
.LP
Change of condition on circuit I will take place at the OFF to ON
transition of circuit\ S at the beginning of the first bit in the 6\(hybit byte
and the condition will be steady for the whole 6\(hybit byte.
.LP
The signal element timing (S) will operate for continuous isochronous transmission
at 48\ kbit/s.
.LP
The \fIframe start identification\fR
\| circuit (F) will indicate the frame start with an OFF condition appearing
in the last bit of each frame. For
length will be 480\ bits. For networks using Recommendation\ X.50 division\ 3
multiplexing in which the user rate of 600\ bit/s is not included, the frame
length will be 120\ bits.
.LP
4.2
\fIMultiplex structure in networks providing 8\(hybit bytes\fR
.LP
The DCE shall deliver to and receive from the DTE an 8\(hybit byte
interleaved multiplexed bit stream containing a number of subscriber channels.
The allocation of the subscriber channels should be:
.LP
\ 5\ channels\ (phases)\ of\ 9600\ bit/s\ or
10\ channels\
(phases)

20\ channels\
(phases)
\ of\ 2400\ bit/s\ or
80\ channels\
(phases)
\ of\ \ 600\ bit/s\ or
an appropriate mix of channel data signalling rates having an aggregate bit
rate of 48\ kbit/s.
.LP
The multiplex bit stream is divided into five phases of
9600\ bit/s, where each phase shall be homogeneous with regard to the subscriber
data signalling rates.
.LP
4.2.1
\fIInterchange circuits and interface signalling scheme\fR
.LP
The interchange circuits between the DTE and DCE are shown in
Figure\ 1/X.22 and a timing diagram for the signals is given in Figure\
3/X.22. The signalling over the interchange circuits is as follows.
.LP
The transmit (T) and receive (R) circuits will convey in one time slot
eight consecutive user data bits for one subscriber channel (see
Figure\ 3/X.22).
.LP
The control (C) and indication (I) circuits will convey the
appropriate signal levels in accordance with Recommendation\ X.21 for the
data channel which in the same time slot have bits conveyed over the respective
data circuits.
.LP
Change of condition on circuit C shall take place at the OFF to ON
transition of circuit\ S at the beginning of the first bit in the 8\(hybit
byte.
The condition on circuit\ C shall be steady for the whole 8\(hybit byte.
.LP
Change of condition on circuit I will take place at the OFF to ON
transition of circuit\ S at the beginning of the first bit in the 8\(hybit byte
and the condition will be steady for the whole 8\(hybit byte.
.LP
The signal element timing (S) will operate for continuous isochronous transmission
at 48\ kbit/s.
.LP
The
\fIframe start identification\fR
\| circuit
(F) will indicate
the
frame start with an OFF condition appearing in the position of the last
bit of each 640\(hybit frame. As an optional facility each frame start
could be followed by a code which will indicate the actual channel allocation.





Establishment of test loops for DTE tests and network maintenance is for

.LP
.ce
.ce
\fBFigure 1/X.22, p.\fR
.ce
.line
.ce
\fBFigure 2/X.22, p.\fR
.ce
.line
.ce
\fBFigure 3/X.22, p.\fR
.ce
\fBRecommendation\ X.24\fR
.ce
.line
.ce
\fBLIST\ OF\fR
.ce
\
.ce
\fBDEFINITIONS\ FOR\ INTERCHANGE\ CIRCUITS\ BETWEEN\ DATA\fR
.ce

.ce
\fBTERMINAL\ EQUIPMENT\ (DTE)\ AND\ DATA\ CIRCUIT\(hyTERMINATING\fR
.ce
\fBEQUIPMENT\ (DCE)\fR
.ce
\fB\ ON\ PUBLIC\ DATA\ NETWORKS\fR
.ce
\fI(Geneva, 1976; amended at Geneva, 1980, Malaga\(hyTorremolinos, 1984,\fR
.ce
\fIand Melbourne, 1988)\fR
.ce
The\ CCITT,
.ce
.LP

\fIconsidering that\fR

.LP

(a)

the interface between DTE and DCE on public data neworks requires, in addition
to the electrical and functional characteristics of the interchange circuits,
the definition of procedural characteristics for call

control functions and selection of the facilities according to

Recommendation\ X.2;

.LP



(b)

the functions of the circuits defined in

Recommendation\ V.24 are based on the requirements of data transmission
over the general telephone network and are not appropriate for use at DTE/DCE
interfaces in public data networks;

.LP

\fIunanimously declares\fR

.LP

a Recommendation to include the list of definitions of

interchange circuits for use in public data networks is required.

.LP



\fB1\fR

\fBScope\fR

.LP

1.1

This Recommendation applies to the functions of the interchange circuits
provided at the interface between DTE and DCE of data networks

for the transfer of binary data, call control signals and timing signals.

.LP



For any type of practical equipment, a selection will be made from the
range of interchange circuits defined in this Recommendation, as

appropriate. The actual interchange circuits to be used in a particular
DCE for a user class of service according to Recommendation\ X.1 and defined
user

facilities according to Recommendation\ X.2, are those indicated in the

relevant Recommendation for the procedural characteristics of the interface,

e.g.,\ Recommendation\ X.20 or\ X.21.

.LP





To enable a standard DTE to be developed, the use and termination by the
DTE of certain circuits even when implemented in the DCE are not mandatory.
This is covered by the individual interface Recommendations.

.LP

The interchange circuits defined for the transfer of binary data are also
used for the exchange of call control signals.

.LP

The electrical characteristics of the interchange circuits are

detailed in the appropriate Recommendation for electrical characteristics of

interchange circuits. The application of those characteristics for a particular
DCE is specified in the Recommendation for the procedural characteristics

of the interface.

.LP

1.2

The range of interchange circuits defined in this

Recommendation is applicable to the range of services which could be offered
on a public data network, e.g.,\ circuit switching services (synchronous
and

start/stop), telex service, packet switching services, message registration
and retransmission service and facsimile service.

.LP

\fB2\fR

\fBLine of demarcation\fR

.LP





The interface between DTE and DCE is located at a connector which is the
interchange point between these two classes of equipment shown in

Figure\ 1/X.24.

.LP

.LP

\fBFigure 1/X.24, p.\fR

2.1

The connector will not necessarily be physically attached to

the DCE and may be mounted in a fixed position near the DTE. The female part

of the connector belongs to the DCE.

.LP

2.2

An interconnecting cable will normally be provided together

with the DTE. The cable length is limited by electrical parameters specified

in the appropriate Recommendations for the electrical characteristics of the

interchange circuits.

.LP

\fB3\fR

\fBDefinition of\fR



\fBinterchange circuits\fR

.LP

A list of the data network series interchange circuits is presented in
tabular form in Table\ 1/X.24.

.LP

.LP

\fBTable 1/X.24 [T1.24], p.\fR

3.1

\fICircuit\ G\ \(em\ Signal ground or common return\fR

.LP

This conductor establishes the signal common reference potential

for unbalanced double\(hycurrent interchange circuits with electrical

characteristics according to Recommendation\ V.28. In the case of

interchange circuits according to Recommendations\ V.10 and\ V.11, it

interconnects the zero volt reference points of a generator and a receiver
to reduce environmental signal interference, if required.

.LP

Within the DCE, this conductor shall be brought to one point,

protective ground or earth, by means of a metallic strap within the equipment.
This metallic strap can be connected or removed at installation, as may
be

required, to minimize the introduction of noise into electronic circuitry
or to meet applicable regulations.

.LP

\fINote\fR

\ \(em\ Where a shielded interconnecting cable is used at the

interface, the shield may be connected either to circuit\ G, or to protective

ground in accordance with national regulations. Protective ground may be

further connected to external grounds as required by applicable electrical

safety regulations.

.LP



For unbalanced interchange circuits with electrical characteristics in
accordance with Recommendation\ V.10, two common\(hyreturn conductors are
required, one for each direction of signalling, each conductor being connected
to ground only on the generator side of the interface. Where used, these
shall be

designated circuits\ Ga and\ Gb, and they are defined as follows:

.LP



\fICircuit\ Ga\ \(em\ DTE common return\fR

.LP

This conductor is connected to the DTE circuit common and is used

as the reference potential for the unbalanced\ X.26 type interchange circuit

receivers within the DCE.

.LP

\fICircuit\ Gb\ \(em\ DCE common return\fR

.LP

This conductor is connected to the DCE circuit common and is used

as the reference potential for the unbalanced\ X.26 type interchange circuit

receivers within the DTE.

.LP

3.2

\fICircuit\ T\ \(em\ Transmit\fR

.LP

\fIDirection:\fR

To DCE

.LP

The binary signals originated by the DTE to be transmitted during the data
transfer phase via the data circuit to one or more remote DTEs are

transferred on this circuit to the DCE.

.LP





This circuit also transfers the call control signals originated by the
DTE, to be transmitted to the DCE in the call establishment and other call

control phases as specified by the relevant Recommendations for the procedural
characteristic of the interface.

.LP

The DCE monitors this circuit for detection of electrical circuit

fault conditions, according to the specifications of the electrical

characteristics of the interface. A circuit fault is to be interpreted
by the DCE as defined in the Recommendation for the procedural characteristics
of

the interface.

.LP

3.3

\fICircuit\ R\ \(em\ Receive\fR

.LP

\fIDirection:\fR

\ From DCE

.LP

The binary signals sent by the DCE as received during the data

transfer phase from a remote DTE, are transferred on this circuit to the DTE.

.LP



This circuit also transfers the call control signals sent by the DCE as
received during the call establishment and other call control phases as

specified by the relevant Recommendations for the procedural characteristics
of the interface.

.LP



The DTE monitors this circuit for detection of electrical circuit

fault conditions, according to the specifications of the electrical

characteristics of the interface. A circuit fault is to be interpreted
by the DTE as defined in the Recommendation for the procedural characteristics
of

the interface.

.LP

3.4

\fICircuit\ C\ \(em\ Control\fR

.LP

\fIDirection:\fR

\ To DCE

.LP

Signals on this circuit control the DCE for a particular signalling process.

.LP

Representation of a control signal requires additional coding of

circuit\ T\(hy\fITransmit\fR

\|as specified in the relevant Recommendation for the

procedural characteristics of the interface. During the data phase, this

circuit shall remain ON. During the call control phases, the condition
of this circuit shall be as specified in the relevant Recommendation for
the procedural characteristics of the interface.

.LP

\fINote\fR

\ \(em\ After appropriate selection of special user facilities (not yet
defined), it might be required to change the ON condition after entering

the data phase in accordance with the regulations for the use of these

facilities. This subject is for further study.

.LP

The DCE monitors this circuit for detection of electrical circuit

fault conditions, according to the specifications of the electrical

characteristics of the interface. A circuit fault is to be interpreted
by the DCE as defined in the Recommendation for the procedural characteristics
of

the interface.

.LP

3.5

\fICircuit\ I\ \(em\ Indication\fR

.LP

\fIDirection:\fR

\ From DCE

.LP

Signals on this circuit indicate to the DTE the state of the call control
process.

.LP

Representation of a control signal requires additional coding of

circuit\ R\(hy\fIReceive\fR

, as specified in the relevant Recommendation for the

procedural characteristics of the interface. The ON condition of this circuit
signifies that signals on circuit\ R contain information from the distant
DTE. The OFF condition signifies a control signalling condition which is
defined by the bit sequence on circuit\ R as specified by the procedural
characteristics

of the interface.

.LP



The DTE monitors this circuit for detection of electrical circuit

fault conditions, according to the specifications of the electrical

characteristics of the interface. A circuit fault is to be interpreted
by the DTE as defined in the Recommendation for the procedural characteristics
of

the interface.

.LP

\fINote\fR

\ \(em\ For use with special user facilities (not yet defined) it

might be required to use the OFF condition after entering the data transfer

phase in accordance with the regulations for the use of these facilities.

This subject is for further study.

.LP

3.6

\fICircuit\ S\ \(em\ \fR

\fISignal element timing\fR

.LP

\fIDirection:\fR

\ From DCE

.LP

Signals on this circuit provide the DTE with signal element timing information.
The condition of this circuit shall be ON and OFF for nominally

equal periods of time. However, for burst isochronous operations, longer

periods of OFF condition may be permitted equal to an integer odd number of

the nominal period of the ON condition as specified by the relevant procedural
characteristics of the interface.

.LP





The DTE shall present a binary signal on circuit\ T\(hy\fITransmit\fR

\|and a   condition on circuit\ C\(hy\fIControl\fR

, in which the transitions nominally occur at the time of the transitions
from OFF to ON condition of this circuit.

.LP



The DCE presents a binary signal on circuit\ R\(hy\fIReceive\fR

\|and a

condition on circuit\ I\(hy\fIIndication\fR

 \|in which the transitions nominally occur at the time of the transitions
from OFF to ON condition of this circuit.

.LP



The transition from ON to OFF condition shall nominally indicate the centre
of each signal element on circuit\ R.

.LP

The DCE shall transfer signal element timing information on this

circuit across the interface at all times that the timing source is capable
of generating this information.

.LP

3.7

\fICircuit\ B\ \(em\ \fR

\fIByte timing\fR

(see Note 2)

.LP

\fIDirection:\fR

\ From DCE

.LP

Signals on this circuit provide the DTE with 8\(hybit byte timing

information. The condition of this circuit shall be OFF for nominally the

period of the ON condition of circuit\ S\(hy\fISignal element timing\fR

 which indicates the last bit of an 8\(hybit byte and shall be ON at all
other times within the

period of the 8\(hybit byte.

.LP





During the call control phases, the call control characters and steady
state conditions used for all information transfers between the DCE and
the

DTE, in either direction, shall be correctly aligned to the signals of

circuit\ B.

.LP

The DTE shall present the beginning of the first bit of each call

control character on circuit\ T\(hy\fITransmit\fR

nominally at the time of the

OFF to ON transition of circuit\ S which follows the OFF to ON transition of

circuit\ B.

.LP

A change of condition of circuit\ C\(hy\fIControl\fR

\|may occur at any

OFF to ON transition of circuit\ S, but it will be sampled in the DCE at the

time of the OFF to ON transition of circuit\ B, i.e., for evaluation of the

following call control character on circuit\ T.

.LP

The centre of the last bit of each call control character will be

presented by the DCE on circuit\ R\(hy\fIReceive\fR

nominally at the time of the

OFF to ON transition of circuit\ B.

.LP

A change of condition of circuit\ I\(hy\fIIndication\fR

 \|will occur nominally at the OFF to ON transition of circuit\ S which
follows the OFF to ON

transition of circuit\ B.

.LP





The DCE shall transfer byte timing information on this circuit across the
interface at all times that the timing source is capable of generating
this information.

.LP

\fINote\ 1\fR

\ \(em\ During the data transfer phase, DTEs communicating by

means of an 8\(hybit code may utilize the byte timing information for mutual

character alignment.

.LP

It is a prerequisite for the provision of this feature that

character alignment is preserved after the call has entered the data transfer
phase and that the alignment obtained at one interface is synchronized
to the alignment at the other interface. (This is only possible on some
connections.)

.LP

Furthermore, where this feature is available, a change of condition

on circuit\ C as defined above may result in an equivalent change in the

relative alignment on circuit\ I at the distant interface.

.LP

\fINote\ 2\fR

\ \(em\ In some Recommendations for the procedural

characteristics of the interface (e.g.,\ X.21), the use and termination
of this circuit by the DTE is not mandatory even when implemented in the
DCE.

.LP



3.8

\fICircuit\ F\ \(em\ Frame start identification\fR

.LP

\fIDirection:\fR

\ From DCE

.LP

Signals on this circuit continuously provide the DTE with a

multiplex frame start indication when connected to a multiplexed DTE/DCE

interface.

.LP

The condition on this circuit shall be OFF for the nominal period of one
bit, indicating the last bit of the multiplex frame. At other times the

circuit shall remain\ ON.

.LP

The first data bit on subscriber channel 1 shall be transmitted or

received beginning nominally at the OFF to ON transition of circuit\ F.

.LP



3.9

\fICircuit\ X\ \(em\ DTE transmit signal element timing\fR

(see Note)

.LP



\fIDirection:\fR

\ To DCE

.LP

Signals on this circuit provide signal element timing information for the
transmit direction in cases where circuit\ S only provides signal

element timing for the receive direction. The condition of this circuit
shall be ON and OFF for nominally equal periods of time. However, for burst

isochronous operations, longer periods of OFF condition may be permitted

equal to an integer odd number of the nominal period of the ON condition as

specified by the relevant procedural characteristics of the interface.

.LP



The DTE shall present a binary signal on the circuit T\(hy\fITransmit\fR

\|and a condition on circuit\ C\(hy\fIControl\fR

, in which the transitions nominally occur at the time of the transitions
from OFF to ON condition of this circuit.

.LP



The transition from ON to OFF condition shall nominally indicate the centre
of each signal element on circuit\ T.

.LP

\fINote\fR

\ \(em\ The use and termination of this circuit by the DCE is a

national matter.

.LP



\fBRecommendation X.25\fR

.LP

\fBINTERFACE\ BETWEEN\ DATA\ TERMINAL\ EQUIPMENT\ (DTE)\ AND\ DATA\fR



\fBCIRCUIT\(hyTERMINATING\ EQUIPMENT\ (DCE)\ FOR\ TERMINALS\ OPERATING\fR

\fBIN\ THE\ PACKET\ MODE\ AND\ CONNECTED\ TO\ PUBLIC\fR

\fBDATA\ NETWORKS\ BY\ DEDICATED\ CIRCUIT\fR

\fI(Geneva, 1976; amended at Geneva, 1980\fR

,

\fIMalaga\(hyTorremolinos, 1984 and Melbourne, 1988)\fR



The establishment in various countries of public data networks providing
packet switched data transmission services creates a need to produce standards
to facilitate international interworking.

.LP

The\ CCITT,

.LP

\fIconsidering\fR

.LP



(a)

that Recommendation X.1 includes specific user classes of service for data
terminal equipments operating in the packet mode,

Recommendation\ X.2 defines user facilities, Recommendation\ X.10 defines

categories of access, Recommendations\ X.21 and X.21\|\fIbis\fR

define DTE/DCE

physical layer interface characteristics, Recommendation\ X.92 defines the

hypothetical reference connections for packet switched data transmission

service and Recommendation\ X.96 defines \fIcall progress\fR

signals;

.LP

(b)

that data terminal equipments operating in the packet

mode will send and receive network control information in the form of packets;

.LP



(c)

that certain data terminal equipments operating in the packet mode will
use a

packet interleaved synchronous data circuit

;

.LP

(d)

the desirability of being able to use a single data

circuit to a Data Switching Exchange\ (DSE) for all user facilities;

.LP



(e)

that Recommendation X.2 specifies which of the various data transmission
services and optional user facilities described in the

present Recommendation are \*Qessential\*U and have thus to be made available

internationally, and which are not;

.LP



(f

)

the need for defining an international

Recommendation for the exchange between DTE and DCE of control information
for the use of packet switched data transmission services;

.LP

(g)

that this definition is made in Recommendation X.32 with regard to the
access through a public switched telephone network, an integrated services
digital network (ISDN), or a circuit switched public data network;

.LP



(h)

that Recommendation X.31 defines the support of

packet\(hymode

terminal equipment by an integrated services digital network (ISDN);

.LP

(i)

that, when this Recommendation is used to support the

Network Service defined in Recommendation\ X.213, the physical, data link and

packet

layers correspond to the Physical, Data link and Network Layers respectively,
as defined in Recommendation\ X.200;

.LP

(j)

that this Recommendation includes all the features

necessary to support the services included in Recommendation\ X.213 as
well as other features; that Recommendation\ X.223 defines the use of X.25
packet layer protocol to provide the OSI connection mode Network service;

.LP

(k)

that the necessary elements for an interface

Recommendation should be defined independently as:

.LP



\fIPhysical layer\fR

\ \(em\ the mechanical, electrical, functional and

procedural characteristics to activate, maintain and deactivate

the physical link between the DTE and the DCE;

.LP

\fIData link layer\fR

\ \(em\ the link access procedure for data interchange

across the link between the DTE and the DCE;

.LP

\fIPacket layer\fR

\ \(em\ the packet format and control procedures for the

exchange of packets containing control information and user

data between the DTE and the DCE;

.LP

\fIunanimously declares\fR

.LP



that for public data networks accessed via dedicated circuits by data terminal
equipments operating in the packet mode:

.LP

(1)

the mechanical, electrical, functional and procedural

characteristics to activate, maintain and deactivate the physical link
between the DTE and the DCE should be as specified in \(sc\ 1 below, \fIDTE/DCE
interface\fR

\fIcharacteristics\fR

;

.LP

(2)

the link access procedure for data interchange across

the link between the DTE and the DCE should be as specified in \(sc\ 2 below,

\fILink access procedure across the DTE/DCE interface\fR

;

.LP

(3)

the packet layer procedures for the exchange of control information and
user data at the DTE/DCE interface should be as specified in

\(sc\ 3 below, \fIDescription of the packet layer DTE/DCE interface\fR

;

.LP

(4)

the procedures for virtual call and permanent virtual

circuit services should be as specified in \(sc\ 4 below, \fIProcedures
for virtual\fR

\fIcircuit services\fR

;

.LP



(5)

the format for packets exchanged between the DTE and the DCE should be
as specified in \(sc\ 5 below, \fIPacket formats\fR

;

.LP

(6)

the procedures for optional user facilities should be as specified in \(sc\
6 below, \fIProcedures for optional user facilities\fR

;

.LP

(7)

the formats for optional user facilities should be as

specified in \(sc\ 7 below, \fIFormats for facility fields and registration
fields\fR

.

.LP



\fINote\fR

\ \(em\ This Recommendation fully specifies the behaviour of the DCE. In
addition, a minimum set of requirements is specified for the DTE.

Additional guidance for the design of DTEs is available in ISO standards

ISO\ 7776 (data link layer) and ISO\ 8208 (packet layer). It is not required
by this Recommendation that these ISO standards be used. If using these
ISO

standards, note must be taken that their scope is expanded beyond that
of just interfacing with packet switched public data networks.

.LP

It should also be noted that this Recommendation uses the term DTE to refer
to the equipment to which the DCE interfaces. In ISO\ 8208, distinction
is made between a DTE and a packet switched private data network, which
are both considered as DTEs in this Recommendation.

.LP

CONTENTS

1

\fIDTE/DCE interface characteristics (physical layer)\fR

.LP

1.1

X.21 interface

.LP

1.2

X.21\|\fIbis\fR

interface

.LP

1.3

V\(hyseries interface

.LP

1.4

X.31 interface

.LP

2

\fILink access procedures across the DTE/DCE interface\fR

.LP

2.1

Scope and field of application

.LP

2.2

Frame structure

.LP

2.3

LAPB elements of procedures

.LP

2.4

Description of the LAPB procedure

.LP

2.5

Multilink procedure (MLP)

.LP

2.6

LAP elements of procedure

.LP

2.7

Description of the LAP procedure

.LP

3

\fIDescription of the packet layer DTE/DCE interface\fR

.LP

3.1

Logical channels

.LP

3.2

Basic structure of packets

.LP

3.3

Procedure for restart

.LP

3.4

Error handling

.LP

4

\fIProcedures for virtual circuit services\fR

.LP

4.1

Procedures for virtual call service

.LP

4.2

Procedures for permanent virtual circuit service

.LP

4.3

Procedures for data and interrupt transfer

.LP

4.4

Procedures for flow control

.LP

4.5

Effects of clear, reset and restart procedures

on the transfer of packets

.LP

4.6

Effects of the physical and the data link layer on the

packet layer

.LP

5

\fIPacket formats\fR

.LP

5.1

General

.LP



5.2

Call set\(hyup and clearing packets

.LP

5.3

Data and interrupt packets

.LP

5.4

Flow control and reset packets

.LP

5.5

Restart packets

.LP

5.6

Diagnostic packet

.LP

5.7

Packets required for optional user facilities

.LP



6

\fIProcedures for optional user facilities (packet layer)\fR

.LP



6.1

On\(hyline facility registration

.LP

6.2

Extended packet sequence numbering

.LP

6.3

D bit modification

.LP

6.4

Packet retransmission

.LP

6.5

Incoming calls barred

.LP

6.6

Outgoing calls barred

.LP

6.7

One\(hyway logical channel outgoing

.LP

6.8

One\(hyway logical channel incoming

.LP

6.9

Non\(hystandard default packet sizes

.LP

6.10

Non\(hystandard default window sizes

.LP



6.11

Default throughput classes assignment

.LP

6.12

Flow control parameter negotiation

.LP

6.13

Throughput class negotiation

.LP

6.14

Closed user group related facilities

.LP

6.15

Bilateral closed user group related facilities

.LP

6.16

Fast select

.LP

6.17

Fast select acceptance

.LP

6.18

Reverse charging

.LP

6.19

Reverse charging acceptance

.LP

6.20

Local charging prevention

.LP

6.21

Network user identification (NUI) related facilities

.LP



6.22

Charging information

.LP

6.23

RPOA related facilities

.LP

6.24

Hunt group

.LP

6.25

Call redirection and call deflection related facilities

.LP



6.26

Called line address modified notification

.LP

6.27

Transit delay selection and indication

.LP

6.28

TOA/NEI address subscription

.LP

7

\fIFormats for facility fields and registration fields\fR

.LP

7.1

General

.LP

7.2

Coding of facility field in call set\(hyup

and clearing packets

.LP

7.3

Coding of the registration field of

registration packets

.LP

\fIAnnex\ A\fR

\(em

Range of logical channels used for virtual calls

and permanent virtual circuits

.LP

\fIAnnex\ B\fR

\(em

Packet layer DTE/DCE interface state diagrams

.LP



\fIAnnex\ C\fR

\(em

Actions taken by the DCE on receipt of packets

in a given state of the packet layer DTE/DCE interface as

perceived by the DCE

.LP

\fIAnnex\ D\fR

\(em

Packet layer DCE time\(hyouts and DTE time\(hylimits

.LP



\fIAnnex\ E\fR

\(em

Coding of X.25 network generated

diagnostic fields in clear, reset and restart indication,

registration confirmation, and diagnostic packets

.LP

\fIAnnex\ F\fR

\(em

Applicability of the on\(hyline registration facility

to other facilities

.LP

\fIAnnex\ G\fR

\(em

CCITT\(hyspecified DTE facilities to support the

OSI Network service

.LP

\fIAnnex\ H\fR

\(em

Subscription\(hytime optional user facilities that may be

associated with a network user identifier in conjunction with the NUI

override facility

.LP

\fIAppendix\ I\fR

\(em

Examples of data link layer transmitted bit patterns by   the DCE and the DTE

.LP

\fIAppendix\ II\fR

\(em

An explanation of how the values for N1 in \(sc 2.4.8.5 are derived

.LP

\fIAppendix\ III\fR

\(em

Examples of multilink resetting procedures

.LP



\fIAppendix\ IV\fR

\(em

Information on addresses in call set\(hyup and clearing

packets

.LP

\fB1\fR

\fBDTE/DCE interface characteristics\fR

\fB(physical layer)\fR

.LP



Administrations may offer one or more of the interfaces specified below.
The exact use of the relevant points in these Recommendations is

detailed below.

.LP

1.1

\fIX.21 interface\fR

.LP

1.1.1

\fIDTE/DCE physical interface elements\fR

.LP

The DTE/DCE physical interface elements shall be according to

\(sc\(sc\ 2.1 through\ 2.5 of Recommendation\ X.21.

.LP

1.1.2

\fIProcedures for entering operational phases\fR

.LP

The procedures for entering operational phases shall be as

described in \(sc\ 5.2 of Recommendation\ X.21. The data exchanged on circuits\
T

and\ R when the interface is in states\ 13S, 13R and\ 13 of Figure A\(hy3/X.21
will be as described in subsequent sections of this Recommendation.

.LP

The \fInot ready\fR

\| states given in \(sc\ 2.5 of Recommendation X.21 are

considered to be \fInon\(hyoperational\fR

states and may be considered by the higher   layers to be \fIout of order\fR

states (see \(sc\ 4.6 below).

.LP

1.1.3

\fIFailure detection and test loops\fR

.LP

The failure detection principles shall be according to \(sc\ 2.6 of

Recommendation\ X.21. In addition, i\ =\ OFF may be signalled due to momentary

transmission failures. Higher layers may delay for several seconds before

considering the interface to be out of order.

.LP

The definitions of test loops and the principles of maintenance

testing using the test loops are provided in Recommendation\ X.150.

.LP

A description of the test loops and the procedures for their use is

given in \(sc\ 7 of Recommendation\ X.21.

.LP



Automatic activation by a DTE of a test loop\ 2 in the DCE at the

remote terminal is not possible. However, some Administrations may permit
the DTE to control the equivalent of a test loop\ 2, at the local DSE,
to verify the operation of the leased line or subscriber line and/or all
or part of the DCE or line terminating equipment. Control of the loop,
if provided, may be either manual or automatic, as described in Recommendations\
X.150 and\ X.21

respectively.

.LP

1.1.4

\fISignal element timing\fR

.LP

Signal element timing shall be in accordance with \(sc\ 2.6.3 of

Recommendation\ X.21.

.LP

1.2

\fIX.21\|bis interface\fR

.LP

1.2.1

\fIDTE/DCE physical interface elements\fR

.LP

The DTE/DCE physical interface elements shall be according to \(sc\ 1.2
of Recommendation\ X.21\|\fIbis\fR

.

.LP

1.2.2

\fIOperational phases\fR

.LP

When circuit 107 is in the ON condition, and circuits\ 105, 106, 108 and
109, if provided, are in the ON condition, data exchange on circuits\ 103

and\ 104 will be as described in subsequent sections of this

Recommendation.

.LP



When circuit 107 is in the OFF condition, or any of circuits 105, 106,
108 or\ 109, if provided, are in the OFF condition, this is considered
to be in a \fInon\(hyoperational\fR

 state, and may be considered by the higher layers to be in an \fIout of
order\fR

state (see \(sc\ 4.6 below).

.LP

1.2.3

\fIFailure detection and test loops\fR

.LP

The failure detection principles, the description of test loops and the
procedures for their use shall be according to \(sc\(sc\ 3.1 through\ 3.3
of

Recommendation\ X.21\|\fIbis\fR

. In addition, circuits\ 106 and\ 109 may enter the OFF condition due to
momentary transmission failures. Higher layers may delay for several seconds
before considering the interface to be out of order.

.LP



Automatic activation by a DTE of test loop 2 in the DCE at the remote terminal
is not possible. However, some Administrations may permit the DTE to control
the equivalent of a test loop\ 2, at the local DSE, to verify the

operation of the leased line or subscriber line and/or all or part of the
DCE or line terminating equipment. Control of the loop, if provided, may
be either manual or automatic, as described in Recommendations\ X.150 and\
X.21\|\fIbis\fR



respectively.

.LP

1.2.4

\fISignal element timing\fR

.LP

Signal element timing shall be in accordance with \(sc\ 3.4 of

Recommendation\ X.21\|\fIbis\fR

.

.LP

1.3

\fIV\(hySeries interface\fR

.LP

General operation with V\(hySeries modems is as described in \(sc\ 1.2

above. However, for specific details, particularly related to failure detection
principles, loop testing, and the use of circuits\ 107, 109, 113 and\ 114,
refer to the appropriate V\(hySeries Recommendations.

.LP

The delay between 105\(hyON and 106\(hyON (when these

circuits are present) will be more than 10\ ms and less than 1\ s. In addition,
circuits\ 106 or\ 109 may enter the OFF condition due to momentary transmission
failures or modem retraining. Higher layers may delay for several seconds

before considering the interface to be out of order.

.LP

1.4

\fIX.31 interface\fR

.LP

1.4.1

\fIDTE/DCE physical interface\fR

.LP

The DTE/DCE physical interface shall coincide with the R reference point
between the DTE and the Terminal Adaptor (TA). The purpose of the TA is

to allow the operation of a DTE over an ISDN. The functionalities of such
a TA when accessing a packet switched data transmission service through
a

semi\(hypermanent ISDN connection (i.e.,\ a non switched B\(hychannel)
are described in \(sc\ 7 of Recommendation\ X.31.

.LP

\fINote\ 1\fR

\ \(em\ This type of access is considered a dedicated access to a public
switched data transmission service. Non dedicated access to a public

switched data transmission service is defined in Recommendations\ X.32

and\ X.31.

.LP

\fINote\ 2\fR

\ \(em\ The DTE and the TA functionalities may be implemented in

the same piece of equipment in the case of a packet mode terminal TE1

conforming to the I\(hyseries Recommendations. In this case, this

Recommendation covers layer\ 2 and layer\ 3 operation on the semi\(hypermanent

B\(hychannel.

.LP

1.4.2

\fIOperational phases\fR

.LP

The operational phases are as described in \(sc 7 of

Recommendation\ X.31.

.LP

1.4.3

\fIMaintenance\fR

.LP



The maintenance shall be made as described in \(sc 7.6 of

Recommendation\ X.31.

.LP

1.4.4

\fISynchronization\fR

.LP

The synchronization shall be made as described in \(sc 7 of

Recommendation\ X.31.

.LP

\fB2\fR

\fBLink access procedures across the DTE/DCE interface\fR

.LP



2.1

\fIScope and field of applications\fR

.LP

2.1.1

The

Link Access Procedures

(LAPB and LAP) are

described as the

Data Link Layer Element

and are used for data

interchange

between a DCE and a DTE over a single physical circuit (LAPB and LAP), or

optionally over multiple physical circuits (LAPB), operating in user classes
of service\ 8 to\ 11 as indicated in Recommendation\ X.1. The optional,

subscription\(hytime selectable, multiple physical circuit operation with LAPB

(known as multilink operation) is required if the effects of circuit failures
are not to disrupt the Packet Layer operation.

.LP

The single link procedures (SLPs) described in \(sc\(sc\ 2.2, 2.3

and\ 2.4 (LAPB) and in \(sc\(sc\ 2.2, 2.6 and 2.7 (LAP) are used for data
interchange over a single physical circuit, conforming to the description
given in \(sc\ 1,

between a DTE and a DCE. When the optional multilink operation is employed
with LAPB, a single link procedure (SLP) is used independently on each
physical

circuit, and the multilink procedure (MLP) described in \(sc\ 2.5 is used for

data interchange over these multiple parallel LAPB data links. In addition,

when only a single physical circuit is employed with LAPB, agreements may be

made with the Administration to use this optional

multilink procedure

over the one LAPB data link.

.LP

2.1.2

The

single link procedures (SLPs)

use the principles

and terminology of the High\(hylevel Data Link Control (HDLC) procedures

specified by the International Organization for Standardization (ISO). The

multilink pro

cedure\ (MLP) is based on the principles and terminology of the Multilink
Control Procedures specified by ISO.

.LP

2.1.3

Each transmission facility is duplex.

.LP

2.1.4

DCE compatibility of operation with the ISO balanced classes

of procedure (Class BA with options\ 2, 8 and Class\ BA with options\ 2,
8, 10) is achieved using the LAPB procedure described in \(sc\(sc\ 2.3
and\ 2.4. Of these

classes, Class\ BA with options\ 2, 8 (LAPB modulo\ 8) is the basic service,
and is available in all networks. Class\ BA with options\ 2, 8, 10 (LAPB
modulo\ 128) is recognized as an optional, subscription\(hytime selectable,
extended sequence numbering service that may be available in those networks
wishing to serve DTE applications having a need for modulo\ 128 sequence
numbering.

.LP



DTE manufacturers and implementors must be aware that the

procedure hereunder described as LAPB modulo\ 8 will be the only one available
in all networks.

.LP

Likewise, a DTE may continue to use the LAP procedure described in

\(sc\(sc\ 2.2, 2.6 and\ 2.7 (in those networks supporting such a procedure),
but for

new

DTE implementations, LAPB should be preferred. The LAP procedures are defined
for modulo\ 8 basic service only.

.LP

\fINote\fR

\ \(em\ Other possible applications for further study are, for

example:

.LP

\(em

two\(hyway alternate, asynchronous response mode;

.LP

\(em

two\(hyway simultaneous, normal response mode;

.LP

\(em

two\(hyway alternate, normal response mode.

.LP



2.1.5

For those networks that choose to support both the basic and

extended LAPB sequence numbering services, the choice of either basic mode

(modulo\ 8) or extended mode (modulo\ 128) may be made at subscription
time. The choice of the mode employed for each data link procedure is independent
of all others and of the choice of mode for the corresponding Packet Layer
procedures. All choices are matters for agreement for a period of time
with the

Administration.

.LP

2.1.6

In the case of those networks that support both the LAPB procedure and
the LAP procedure, the DCE will maintain an internal mode variable\ B,
which it will set as follows:

.LP

\(em

to 1, upon acceptance of an SABM/SABME (modulo 8/modulo\ 128)

command from the DTE, or upon issuance of an SABM/SABME command

by the DCE;

.LP

\(em

to 0, upon acceptance of an SARM command from the DTE.

.LP



Whenever B is 1, the DCE will use the LAPB procedure described in \(sc\(sc\
2.2, 2.3 and 2.4 below, and is said to be in the LAPB (balanced) mode.

.LP



Whenever B is 0, the DCE will use the LAP procedure described in

\(sc\(sc\ 2.2, 2.6 and 2.7 below, and is said to be in the LAP mode.

.LP

Changes to the mode variable B by the DTE should occur only when the data
link has been disconnected as described in \(sc\(sc\ 2.4.4.3 or\ 2.7.3.3
below.

.LP



Should a DCE malfunction occur that negates the current setting of

internal mode variable\ B, the DCE will, upon restoration of operation,
not send either a SARM or SABM/SABME command. The DCE may send a DISC command
or a

DM\ response to notify the DTE that the DCE is in the disconnected phase.
This will result in the DTE attempting to reinitialize the data link with
what the DTE

considers to be the proper mode\(hysetting command (SARM or SABM/SABME).
The DCE will then be able to set the internal mode variable\ B to its proper

value.

.LP

2.2

\fIFrame structure\fR

.LP

2.2.1

All transmissions on an SLP are in frames conforming to one of the formats
of Table\ 1/X.25 for basic (modulo\ 8) operation, or alternatively one
of the formats of Table\ 2/X.25 for extended (modulo\ 128) operation. The
flag

preceding the address field is defined as the opening flag. The flag following
the FCS field is defined as the closing flag.

.LP

.LP

\fBTable 1/X.25 [T1.25], p.\fR

.LP

\fBTable 2/X.25 [T2.25], p.\fR

2.2.2

\fIFlag sequence\fR

.LP

All frames shall start and end with the flag sequence consisting of one
0\ bit followed by six contiguous 1\ bits and one 0\ bit. The DTE and DCE

shall only send complete eight\(hybit flag sequences when sending multiple flag

sequences (see \(sc\ 2.2.11). A single flag may be used as both the

closing

flag

for one frame and the

opening flag

for the next frame.

.LP

2.2.3

\fIAddress field\fR

.LP

The address field shall consist of one octet. The address field

identifies the intended receiver of a command frame and the transmitter of a

response frame. The coding of the address field is described in \(sc\ 2.4.2
(LAPB) and in \(sc\ 2.7.1 (LAP) below.

.LP

2.2.4

\fIControl field\fR

.LP

For modulo\ 8 (basic) operation, the control field shall consist of one
octet. For modulo\ 128 (extended) operation, the control field shall consist
of two octets for frame formats that contain sequence numbers, and one
octet

for frame formats that do not contain sequence numbers. The content of this

field is described in \(sc\ 2.3.2\ (LAPB) and in \(sc\ 2.6.2 (LAP) below.

.LP

2.2.5

\fIInformation field\fR

.LP

The information field of a frame, when present, follows the control field
(see \(sc\ 2.2.4 above) and precedes the

frame check sequence field

(see \(sc\ 2.2.7 below).

.LP

See \(sc\(sc 2.3.4.9, 2.5.2, 2.6.4.8 and 5 for the various codings and

groupings of bits in the information field as used in this Recommendation.

.LP



See \(sc\(sc 2.3.4.9, 2.4.8.5, 2.6.4.8 and 2.7.7.5 below with regard to
the maximum information field length.

.LP

2.2.6

\fITransparency\fR

.LP

The DCE or DTE, when transmitting, shall examine the frame content between
the two flag sequences including the address, control, information and
FCS fields and shall insert a 0 bit after all sequences of 5 contiguous
1\ bits (including the last 5\ bits of the FCS) to ensure that a flag sequence
is not

simulated. The DCE or DTE, when receiving, shall examine the frame content
and shall discard any 0\ bit which directly follows\ 5 contiguous 1\ bits.

.LP

2.2.7

\fIFrame check sequence (FCS)\fR

\fI field\fR

.LP

The notation used to describe the FCS is based on the property of cyclic
codes that a code vector such as 1000000100001 can be represented by a
polynomial \fIP\fR

(\fIx\fR

)\ =\ \fIx\fR

\u1\d\u2\d\ +\ \fIx\fR

\u5\d\ +\ 1. The elements of an \fIn\fR

\(hyelement

code word are thus the coefficients of a polynomial of order \fIn\fR

\ \(em\ 1. In this application, these coefficients can have the value\
0 or\ 1 and the polynomial

operations are performed modulo\ 2. The polynomial representing the content
of a frame is generated using the first bit received after the frame opening
flag as the coefficient of the highest order term.

.LP

The FCS field shall be a 16\(hybit sequence. It shall be the ones

complement of the sum (modulo\ 2) of:

.LP

1)

the remainder of

\fIx\fR

\uD\dlFk\fR

(\fIx\fR

\u1\d\u5\d\uD\dlF036+\ \fIx\fR

\u1\d\u4\d\ +\ \fIx\fR

\u1\d\u3\d\ +

\fIx\fR

\u1\d\u2\d\ +\ \fIx\fR

\u1\d\u1\d\ +\ \fIx\fR

\u1\d\u0\d\ +\ \fIx\fR

\u9\d\ +\ \fIx\fR

\u8\d\ +

\fIx\fR

\u7\d\ +\ \fIx\fR

\u6\d\ +\ \fIx\fR

\u5\d\ +\ \fIx\fR

\u4\d\uD\dlF036+\ \fIx\fR

\u3\d\ +

+\ \fIx\fR

\u2\d\ +\ \fIx\fR

\ +\ 1) divided (modulo\ 2)

by the

generator polynomial \fIx\fR

\u1\d\u6\d\ +\ \fIx\fR

\u1\d\u2\d\ +\ \fIx\fR

\u5\d\ +\ 1,

where \fIk\fR

is the number of bits in the frame existing between,

but not including, the final bit of the opening flag and the

first bit of the FCS, excluding bits inserted for transparency,

and

.LP

2)

the remainder of the division (modulo 2) by the generator

polynomial \fIx\fR

\u1\d\u6\d\ +\ \fIx\fR

\u1\d\u2\d\ +\ \fIx\fR

\u5\d\ +\ 1 of the

product of \fIx\fR

\u1\d\u6\d by the content of the frame, existing

between but not including, the final bit of the opening flag and

the first bit of the FCS, excluding bits inserted for

transparency.

.LP



As a typical implementation, at the transmitter, the initial

content of the register of the device computing the remainder of the division
is preset to all 1s and is then modified by division by the generator

polynomial (as described above) on the address, control and information
fields; the ones complement of the resulting remainder is transmitted as
the 16\(hybit

FCS.

.LP

At the receiver, the initial content of the register of the device

computing the remainder is preset to all 1s. The final remainder, after

multiplication by \fIx\fR

\u1\d\u6\d and then division (modulo\ 2) by the generator

polynomial \fIx\fR

\u1\d\u6\d\ +\ \fIx\fR

\u1\d\u2\d\ +\ \fIx\fR

\u5\d\ +\ 1 of the serial incoming protected bits and the FCS, will be
0001110100001111 (\fIx\fR

\u1\d\u5\d through

\fIx\fR

\u0\d, respectively) in the absence of transmission errors.

.LP

\fINote\fR

\ \(em\ Examples of transmitted bit patterns by the DCE and the DTE illustrating
application of the transparency mechanism and the frame check

sequence to the SABM command and the UA response are given in Appendix\ I.

.LP

2.2.8

\fIOrder of bit transmission\fR

.LP

Addresses, commands, responses and sequence numbers shall be

transmitted with the low\(hyorder bit first (for example, the first bit of the

sequence number that is transmitted shall have the weight 2\u0\d). The
order of transmitting bits within the information field is not specified
under \(sc\ 2 of

this Recommendation. The FCS shall be transmitted to the line commencing
with the coefficient of the highest term, which is found in bit position\
16 of the FCS\ field (see Tables\ 1/X.25 and 2/X.25).

.LP

\fINote\fR

\ \(em\ In Tables 1/X.25 to 13/X.25, bit 1 is defined as the

low\(hyorder bit.

.LP

2.2.9

\fIInvalid frames\fR

.LP

The definition of an invalid frame is described in \(sc\ 2.3.5.3 (LAPB)
and in \(sc\ 2.6.5.3 (LAP) below.

.LP

2.2.10

\fIFrame abortion\fR

.LP

Aborting a frame is performed by transmitting at least seven

contiguous 1\ bits (with no inserted 0\ bits).

.LP

2.2.11

\fIInterframe time fill\fR

.LP

Interframe time fill is accomplished by transmitting contiguous

flags between frames, i.e.\ multiple eight\(hybit flag sequences (see

\(sc\ 2.2.2).

.LP

2.2.12

\fILink channel states\fR

.LP

A link channel as defined here is the means for transmission for

one direction.

.LP

2.2.12.1

\fIActive channel state\fR

.LP

The DCE incoming or outgoing channel is defined to be in an active condition
when it is receiving or transmitting, respectively, a frame, an

abortion sequence or interframe time fill.

.LP

2.2.12.2

\fIIdle channel state\fR

.LP

The DCE incoming or outgoing channel is defined to be in an idle

condition when it is receiving or transmitting, respectively, a continuous\
1s state for a period of at least 15\ bit times.

.LP

See \(sc\ 2.3.5.5 for a description of DCE action when an idle condition
exists on its incoming channel for an excessive period of time.

.LP



2.3

\fILAPB elements of procedures\fR

.LP

2.3.1

The LAPB elements of procedures are defined in terms of actions

that occur on receipt of frames at the DCE or\ DTE.

.LP

The elements of procedures specified below contain the selection

of commands and responses relevant to the LAPB data link and system

configurations

described in \(sc\ 2.1 above. Together, \(sc\(sc\ 2.2 and 2.3 form the general

requirements for the proper management of a LAPB access data link.

.LP



2.3.2

\fILAPB control field formats and parameters\fR

.LP



2.3.2.1

\fIControl field formats\fR

.LP

The control field contains a command or a response, and sequence

numbers where applicable.

.LP

Three types of control field formats are used to perform numbered

information transfer (I\ format), numbered supervisory functions (S\ format)
and unnumbered control functions (U\ format).

.LP

The control field formats for basic (modulo\ 8) operation are depicted
in Table\ 3/X.25.

.LP



The control field formats for extended (modulo 128) operation are

depicted in Table\ 4/X.25.

.LP

.LP

\fBTable 3/X.25 [T3.25], p.\fR

.LP

\fBTable 4/X.25 [T4/X.25], p.\fR

2.3.2.1.1

\fIInformation transfer format\fR

\fI \(em I\fR

.LP

The I format is used to perform an information transfer. The

functions of N(S), N(R) and P are independent; i.e.,\ each I\ frame has
an N(S), an N(R) which may or may not acknowledge additional I\ frames
received by the

DCE or DTE, and a P\ bit that may be set to\ 0 or\ 1.

.LP

2.3.2.1.2

\fISupervisory format\fR

\fI \(em S\fR

.LP

The S format is used to perform data link supervisory control

functions such as acknowledge I\ frames, request retransmission of I\ frames,
and to request a temporary suspension of transmission of I\ frames. The
functions of N(R) and P/F are independent; i.e.,\ each supervisory frame
has an N(R) which

may or may not acknowledge additional I\ frames received by the DCE or\
DTE, and a P/F\ bit that may be set to\ 0 or\ 1.

.LP

2.3.2.1.3

\fIUnnumbered format\fR

\fI \(em U\fR

.LP

The U format is used to provide additional data link control

functions. This format contains no sequence numbers, but does include a
P/F\ bit that may be set to\ 0 or\ 1. The unnumbered frames have the same
control field

length (one octet) in both basic (modulo\ 8) operation and extended (modulo\
128) operation.

.LP

2.3.2.2

\fIControl field parameters\fR

.LP

The various parameters associated with the control field formats

are described below.

.LP

2.3.2.2.1

\fIModulus\fR

.LP

Each I frame is sequentially numbered and may have the value\ 0

through modulus minus\ 1 (where \*Qmodulus\*U is the modulus of the sequence

numbers). The modulus equals either\ 8 or\ 128 and the sequence numbers cycle

through the entire range.

.LP

2.3.2.2.2

\fISend state variable\fR

\fI V(S)\fR

.LP

The send state variable V(S) denotes the sequence number of the

next in\(hysequence I\ frame to be transmitted. V(S) can take on the values\ 0

through modulus minus\ 1. The value of V(S) is incremented by\ 1 with each

successive I\ frame transmission, but cannot exceed the N(R) of the last

received\ I or supervisory frame by more than the maximum number of outstanding
I\ frames\ (\fIk\fR

). The value of\ k is defined in \(sc\ 2.4.8.6 below.

.LP



2.3.2.2.3

\fISend sequence number\fR

\fI N(S)\fR

.LP

Only I frames contain N(S), the send sequence number of transmitted I\
frames. At the time that an in\(hysequence I\ frame is designated for

transmission, the value of N(S) is set equal to the value of the send state

variable\ V(S).

.LP

2.3.2.2.4

\fIReceive state variable\fR

\fIV(R)\fR

.LP

The receive state variable V(R) denotes the sequence number of the next
in\(hysequence I\ frame expected to be received. V(R) can take on the values
0 through modulus minus\ 1. The value of V(R) is incremented by\ 1 by the
receipt of an error\(hyfree, in\(hysequence I\ frame whose send sequence
number N(S) equals

the receive state variable V(R).

.LP

2.3.2.2.5

\fIReceive sequence number\fR

\fI N(R)\fR

.LP

All I frames and supervisory frames contain N(R), the expected send sequence
number of the next received I\ frame. At the time that a frame of the above
types is designated for transmission, the value of N(R) is set equal to
the current value of the receive state variable V(R). N(R) indicates that
the DCE or DTE transmitting the N(R) has received correctly all I\ frames
numbered up to and including N(R)\ \(em\ 1.

.LP

2.3.2.2.6

\fIPoll/Final bit\fR

\fI P/F\fR

.LP

All frames contain P/F, the Poll/Final bit. In command frames, the P/F
bit is referred to as the P bit. In response frames, it is referred to
as the F\ bit.

.LP

2.3.3

\fIFunctions of the Poll/Final bit\fR

.LP

The Poll bit set to 1 is used by the DCE or DTE to solicit (poll) a response
from the DTE or DCE, respectively. The Final bit set to\ 1 is used by the
DCE or DTE to indicate the response frame transmitted by the DTE or DCE,

respectively, as a result of the soliciting (poll) command.

.LP

The use of the P/F bit is described in \(sc\ 2.4.3 below.

.LP

2.3.4

\fICommands and responses\fR

.LP

For basic (modulo 8) operation, the commands and responses

represented in Table\ 5/X.25 will be supported by the DCE and the\ DTE.

.LP



For extended (modulo 128) operation, the commands and responses

represented in Table\ 6/X.25 will be supported by the DCE and the DTE.

.LP



For purposes of the LAPB procedures, the supervisory function bit encoding
\*Q11\*U and those encodings of the modifier function bits in

Tables\ 3/X.25 and 4/X.25 not identified in Tables\ 5/X.25 or 6/X.25 are

identified as \*Qundefined or not implemented\*U command and response control

fields.

.LP

The commands and responses in Tables 5/X.25 and 6/X.25 are defined as  follows:

.LP

2.3.4.1

\fIInformation (I)\fR

\fI command\fR

.LP

The function of the information (I) command is to transfer across a data
link a sequentially numbered frame containing an information field.

.LP

.LP

\fBTable 5/X.25 [T5.25], p.\fR

.LP

\fBTable 6/X.25 [T6.25], p.\fR

2.3.4.2

\fIReceive ready (RR)\fR

\fI command and response\fR

.LP

The receive ready (RR) supervisory frame is used by the DCE

or DTE to:

.LP



1)

indicate it is ready to receive an I frame; and

.LP

2)

acknowledge previously received I frames numbered up to and

including N(R)\ \(em\ 1.

.LP

An RR frame may be used to indicate the clearance of a busy

condition that was reported by the earlier transmission of an RNR frame
by that same station (DCE or\ DTE). In addition to indicating the DCE or
DTE status, the RR command with the P\ bit set to\ 1 may be used by the
DCE or DTE to ask for the status of the DTE or DCE, respectively.

.LP

2.3.4.3

\fIReceive not ready (RNR) command and response\fR

.LP

The receive not ready (RNR) supervisory frame is used by the DCE or DTE
to indicate a busy condition; i.e.\ temporary inability to accept

additional incoming I\ frames. I\ frames numbered up to and including N(R)\
\(em\ 1

are acknowledged. I\ frame N(R) and any subsequent I\ frames received, if any,

are not acknowledged; the acceptance status of these I\ frames will be
indicated in subsequent exchanges.

.LP

In addition to indicating the DCE or DTE status, the RNR command with the
P bit set to\ 1 may be used by an DCE or DTE to ask for the status of the

DTE or DCE, respectively.

.LP

2.3.4.4

\fIReject (REJ) command and response\fR

.LP

The reject (REJ) supervisory frame is used by the DCE or DTE to request
transmission of I\ frames starting with the frame numbered N(R).

I\ frames numbered N(R)\ \(em\ 1 and below are acknowledged. Additional
I\ frames

pending initial transmission may be transmitted following the retransmitted

I\ frame(s).

.LP



Only one REJ exception condition for a given direction of information transfer
may be established at any time. The REJ exception condition is cleared
(reset) upon the receipt of an I\ frame with an N(S) equal to the N(R)
of the

REJ\ frame.

.LP

An REJ frame may be used to indicate the clearance of a busy condition
that was reported by the earlier transmission of an RNR frame by that same

station (DCE or\ DTE). In addition to indicating the DCE or DTE status, the

REJ\ command with the P bit set to\ 1 may be used by the DCE or\ DTE to ask for

the status of the DTE or\ DCE, respectively.

.LP

\fR

2.3.4.5

\fISet asynchronous balanced mode (SABM)\fR

\fIcommand/\fR

\fISet asynchronous balanced mode extended (SABME)\fR



\fIcommand (subscription time option)\fR

.LP



The SABM unnumbered command is used to place the addressed DCE or DTE in
an asynchronous balanced mode (ABM) information transfer phase where all
command/response control fields will be one\ octet in length.

.LP

The SABME unnumbered command is used to place the addressed DCE or DTE
in an asynchronous balanced mode\ (ABM) information transfer phase where

numbered command/response control fields will be two octets in length, and

unnumbered command/response control fields will be one octet in length.

.LP



No information field is permitted with the SABM or SABME command. The transmission
of a SABM/SABME command indicates the clearance of a busy

condition that was reported by the earlier trans

mission\ of an RNR\ frame by   that

same station (DCE or\ DTE). The DCE or DTE confirms acceptance of SABM/SABME

[modulo\ 8 (basic) operation/modulo\ 128 (extended) operation] command by the

transmission, at the first opportunity, of a UA\ response. Upon acceptance of

this command, the DCE or DTE send state variable\ V(S) and receive state

variable V(R) are set to\ 0.

.LP

Previously transmitted I\ frames that are unacknowledged when this

command is actioned remain unac

knowledged.\ It is the responsibility of a

higher layer (e.g.\ Packet Layer or MLP) to recover from the possible loss of

the contents (e.g.\ packets) of such I\ frames.

.LP

\fINote\fR

\ \(em\ The mode of operation of a data link [basic (modulo 8) or

extended (modulo\ 128)] is determined at subscription time and is only
changed by going through a new subscription process.

.LP

2.3.4.6

\fIDisconnect (DISC)\fR

\fI command\fR

.LP

The DISC unnumbered command is used to terminate the mode

previously set. It is used to inform the DCE or DTE receiving the DISC
command that the DTE or DCE sending the DISC command is suspending operation.
No

information field is permitted with the DISC command. Prior to actioning the

DISC command, the DCE or DTE receiving the DISC command confirms the acceptance
of the DISC command by the transmission of a UA response. The DTE or DCE

sending the DISC command enters the disconnected phase when it receives the

acknowledging UA response.

.LP

Previously transmitted I frames that are unacknowledged when this

command is actioned remain unacknowledged. It is the responsibility of
a higher layer (e.g.\ Packet Layer or MLP) to recover from the possible
loss of the

contents (e.g.,\ packets) of such I frames.

.LP

2.3.4.7

\fIUnnumbered acknowledgement (UA)\fR

\fI response\fR

.LP

The UA unnumbered response is used by the DCE or DTE to acknowledge the
receipt and acceptance of the mode\(hysetting commands. Received mode\(hysetting
commands are not actioned until the UA response is transmitted. The

transmission of a UA response indicates the clearance of a busy condition
that was reported by the earlier transmission of an RNR frame by that same
station (DCE or DTE). No information field is permitted with the UA response.

.LP



2.3.4.8

\fIDisconnected mode (DM)\fR

\fI response\fR

.LP

The DM unnumbered response is used to report a status where the DCE or
DTE is logically disconnected from the data link, and is in the disconnected
phase. The DM response may be sent to indicate that the DCE or DTE has
entered the disconnected phase without benefit of having received a DISC
command, or, if sent in response to the reception of a mode setting command,
is sent to

inform the DTE or DCE that the DCE or DTE, respectively, is still in the

disconnected phase and cannot execute the set mode command. No information

field is permitted with the DM response.

.LP

A DCE or DTE in a disconnected phase will monitor received commands

and will react to an SABM/SABME command as outlined in \(sc\ 2.4.4 below,
and will respond with a DM response with the F bit set to\ 1 to any other
command

received with the P\ bit set to\ 1.

.LP

2.3.4.9

\fIFrame reject (FRMR)\fR

\fI response\fR

.LP

The FRMR unnumbered response is used by the DCE or DTE to report an error
condition not recoverable by retransmission of the identical frame;

i.e.\ at least one of the following conditions, which results from the
receipt of a valid frame:

.LP

1)

the receipt of a command or response control field that is

undefined or not implemented;

.LP

2)

the receipt of an I frame with an information field which

exceeds the maximum established length;

.LP

3)

the receipt of an invalid N(R); or

.LP

4)

the receipt of a frame with an information field which is

not permitted or the receipt of a supervisory or unnumbered

frame with incorrect length.

.LP

An undefined or not implemented control field is any of the

control field encodings that are not identified in Tables\ 5/X.25 or\ 6/X.25.

.LP



A valid N(R) must be within the range from the lowest send sequence

number N(S) of the still unacknowledged frame(s) to the current DCE send
state variable inclusive (or to the current internal variable \fIx\fR

if the DCE is in

the timer recovery condition as described in \(sc\ 2.4.5.9).

.LP



An information field which immediately follows the control field, and consists
of\ 3 or 5\ octets [modulo\ 8 (basic) operation or modulo\ 128 (extended)
operation, respectively], is returned with this response and provides the

reason for the FRMR response. These formats are given in Tables\ 7/X.25

and\ 8/X.25.

.LP

2.3.5

\fIException condition reporting and recovery\fR

.LP

The error recovery procedures which are available to effect

recovery following the detection/occurrence of an

exception condition

 at the Data Link Layer are described below. Exception conditions described
are those situations which may occur as the result of transmission errors,
DCE or DTE malfunction, or operational situations.

.LP

2.3.5.1

\fIBusy condition\fR

.LP

The busy condition results when the DCE or DTE is temporarily

unable to continue to receive I frames due to internal constraints,

e.g.\ receive buffering limitations. In this case an RNR frame is transmitted

from the busy DCE or DTE. I\ frames pending transmission may be transmitted
from the busy DCE or DTE prior to or following the RNR\ frame.

.LP

An indication that the busy condition has cleared is communicated by the
transmission of a UA (only in response to a SABM/SABME command), RR, REJ
or SABM/SABME (modulo\ 8/modulo\ 128) frame.

.LP

.LP

\fBTable 7/X.25 [T7.25], p.\fR

.LP

\fBTable 8/X.25 [T8.25], p.\fR

2.3.5.2

\fIN(S) sequence error condition\fR

.LP

The information field of all I frames received whose N(S) does not equal
the receive state variable V(R) will be discarded.

.LP

An N(S) sequence error exception condition occurs in the receiver when
an I\ frame received contains an N(S) which is not equal to the receive
state

variable V(R) at the receiver. The receiver does not acknowledge (increment
its receive state variable) the I\ frame causing the sequence error, or
any I\ frame which may follow, until an I\ frame with the correct N(S)
is received.

.LP



A DCE or DTE which receives one or more valid I frames having sequence
errors or subsequent supervisory frames (RR, RNR and REJ) shall accept
the

control information contained in the N(R) field and the P or F bit to perform
data link control functions; e.g.\ to receive acknowledgement of previously

transmitted I\ frames and to cause the DCE or DTE to respond (P bit set to 1).

.LP



The means specified in \(sc\(sc\ 2.3.5.2.1 and 2.3.5.2.2 shall be available
for initiating the retransmission of lost or errored I\ frames following
the

occurrence of an N(S) sequence error condition.

.LP

2.3.5.2.1

\fIREJ recovery\fR

.LP

The REJ frame is used by a receiving DCE or DTE to initiate a

recovery (retransmission) following the detection of an N(S) sequence error.

.LP



With respect to each direction of transmission on the data link, only one
\*Qsent REJ\*U exception condition from a DCE or DTE, to a DTE or DCE,
is

established at a time. A \*Qsent REJ\*U exception condition is cleared when the

requested I\ frame is received.

.LP



A DCE or DTE receiving a REJ frame initiates sequential

(re\(hy)transmission of I\ frames starting with the I\ frame indicated
by the N(R) contained in the REJ frame. The retransmitted frames may contain
an N(R) and a P bit that are updated from, and therefore different from,
the ones contained in the originally transmitted I\ frames.

.LP

2.3.5.2.2

\fITime\(hyout recovery\fR

.LP

If a DCE or DTE, due to a transmission error, does not receive (or receives
and discards) a single I\ frame or the last I\ frame(s) in a sequence of
I\ frames, it will not detect an N(S) sequence error condition and, therefore,
will not transmit a REJ frame. The DTE or DCE which transmitted the

unacknowledged I\ frame(s) shall, following the completion of a system
specified time\(hyout period (see \(sc\(sc\ 2.4.5.1 and 2.4.5.9 below),
take appropriate recovery action to determine at which I\ frame retransmission
must begin. The

retransmitted frame(s) may contain an N(R) and a P bit that is updated from,

and therefore different from, the ones contained in the originally transmitted
frame(s).

.LP

2.3.5.3

\fIInvalid frame\fR

\fI condition\fR

.LP

Any frame which is invalid will be discarded, and no action is

taken as the result of that frame. An invalid frame is defined as one

which:

.LP

a)

is not properly bounded by two flags;

.LP

b)

in basic (modulo 8) operation, contains fewer than 32 bits

between flags; in extended (modulo\ 128) operation, contains

fewer than 40\ bits between flags of frames that contain sequence

numbers or 32\ bits between flags of frames that do not contain

sequence numbers;

.LP

c)

contains a Frame Check Sequence (FCS) error; or

.LP

d)

contains an address other than A or B (for single link

operation) or other than C or D (for multilink operation).

.LP



For those networks that are octet aligned, a detection of

non\(hyoctet alignment may be made at the Data Link Layer by adding a frame

validity check that requires the number of bits between the opening flag and

the closing flag, excluding bits inserted for transparency, to be an integral
number of octets in length, or the frame is considered invalid.

.LP

2.3.5.4

\fIFrame rejection\fR

\fI condition\fR

.LP

A frame rejection condition is established upon the receipt of an error\(hyfree
frame with one of the conditions listed in \(sc\ 2.3.4.9 above.

.LP



At the DCE or DTE, this frame rejection exception condition is

reported by an FRMR response for appropriate DTE or DCE action, respectively.
Once a DCE has established such an exception condition, no additional I\
frames are accepted until the condition is reset by the DTE, except for
examination of the P bit. The FRMR response may be repeated at each opportunity,
as specified in \(sc\ 2.4.7.3, until recovery is effected by the DTE, or
until the DCE initiates its own recovery in case the DTE does not respond.

.LP

2.3.5.5

\fIExcessive idle channel state condition on incoming\fR



\fIchannel\fR

.LP

Upon detection of an idle channel state condition (see \(sc\ 2.2.12.2 above)
on the incoming channel, the DCE shall wait for a period\ T3 (see

\(sc\ 2.4.8.3 below) without taking any specific action, waiting for detection
of a return to the active channel state (i.e.,\ detection of at least one
flag

sequence). After the period\ T3, the DCE shall notify the higher layer
(e.g.\ the Packet Layer or the MLP) of the excessive idle channel state
condition, but

shall not take any action that would preclude the DTE from establishing the

data link by normal data link set\(hyup procedures.

.LP

\fINote\fR

\ \(em\ Other actions to be taken by the DCE at the Data Link Layer upon
expiration of period\ T3 is a subject for further study.

.LP



2.4

\fIDescription of the \fR

\fILAPB procedure\fR

.LP

2.4.1

\fILAPB basic and extended modes of operation\fR

.LP

In accordance with the system choice made by the DTE at

subscription time, the DCE will either support modulo\ 8 (basic) operation or

will support modulo\ 128 (extended) operation. Changing from basic operation
to extended operation, or vice versa, in the DCE requires resubscription
by the

DTE for the desired service, and is not supported dynamically.

.LP

Table 5/X.25 indicates the command and response control field formats used
with the basic (modulo\ 8) service. The mode\(hysetting command employed
to

initialize (set up) or reset the basic mode is the SABM command. Table
6/X.25 indicates the command and response control field formats used with
the extended (modulo\ 128) service. The mode\(hysetting command employed
to initialize (set up) or reset the extended mode is the SABME command.

.LP

2.4.2

\fILAPB procedure for addressing\fR

.LP

The address field identifies a frame as either a command or a

response. A command frame contains the address of the DCE or DTE to which
the command is being sent. A response frame contains the address of the
DCE or DTE sending the frame.

.LP

In order to allow differentiation between single link operation and

the optional multilink operation for diagnostic and/or maintenance reasons,

different address pair encodings are assigned to data links operating with

multilink procedure compared to data links operating with the single link

procedure.

.LP

Frames containing commands transferred from the DCE to the DTE will

contain the address\ A for the single link operation and address\ C for the

multilink operation.

.LP

Frames containing responses transferred from the DCE to the DTE will contain
the address\ B for the single link operation and address\ D for the

multilink operation.

.LP



Frames containing commands transferred from the DTE to the DCE shall contain
the address\ B for the single link operation and address\ D for the

multilink operation.

.LP

Frames containing responses transferred from the DTE to the DCE shall contain
the address\ A for the single link operation and address\ C for the

multilink operation.

.LP

These addresses are coded as follows:

.LP

Address

1\ 2\ 3\ 4\ 5\ 6\ 7\ 8

.LP

Single link operation

\ \ A

1\ 1\ 0\ 0\ 0\ 0\ 0\ 0

.LP

\ \ B

1\ 0\ 0\ 0\ 0\ 0\ 0\ 0

.LP

Multilink operation

\ \ C

1\ 1\ 1\ 1\ 0\ 0\ 0\ 0

.LP

\ \ D

1\ 1\ 1\ 0\ 0\ 0\ 0\ 0

.LP

\fINote\fR

\ \(em\ The DCE will discard all frames received with an address other
than\ A or\ B (single link operation), or\ C or\ D (multilink operation).

.LP

2.4.3

\fILAPB procedure for the use of the P/F bit\fR

.LP

The DCE or DTE receiving an SABM/SABME, DISC, supervisory command or I\
frame with the P\ bit set to\ 1 will set the F\ bit to\ 1 in the next response
frame it transmits.

.LP

The response frame returned by the DCE to an SABM/SABME or DISC

command with the P\ bit set to\ 1 will be a UA or DM response with the
F\ bit set to\ 1. The response frame returned by the DCE to an I\ frame
with the P\ bit set to\ 1, received during the information transfer phase,
will be an RR, REJ, RNR or FRMR response with the F\ bit set to\ 1. The
response frame returned by the

DCE to a supervisory command with the P\ bit set to\ 1, received during the

information transfer phase, will be an RR, REJ, RNR or FRMR response with
the F\ bit set to\ 1. The response frame returned by the DCE to an I\ frame
or

supervisory frame with the P\ bit set to\ 1, received during the disconnected

phase, will be a DM response with the F\ bit set to\ 1.

.LP

The P bit may be used by the DCE in conjunction with the timer

recovery condition (see \(sc\ 2.4.5.9 below).

.LP

\fINote\fR

\ \(em\ Other use of the P bit by the DCE is a subject for further

study.

.LP

2.4.4

\fILAPB procedure for data link set\(hyup and disconnection\fR

.LP



2.4.4.1

\fIData link set\(hyup\fR

.LP

The DCE will indicate that it is able to set up the data link by

transmitting contiguous flags (active channel state).

.LP

Either the DTE or the DCE may initiate data link set\(hyup. Prior to

initiation of data link set\(hyup, either the DCE or the DTE may initiate data

link

disconnection (see \(sc\ 2.4.4.3) for the purpose of insuring that the
DCE and the DTE are in the same phase. The DCE may also transmit an unsolicited
DM response to request the DTE to initiate data link set\(hyup.

.LP

The DTE shall initiate data link set\(hyup by transmitting an SABM/SABME
command to the DCE. If, upon receipt of the SABM/SABME command correctly,
the DCE determines that it can enter the information transfer phase, it
will return a UA response to the DTE, will reset its send and receive state
variables V(S) and V(R) to zero, and will consider that the data link is
set up. If, upon

receipt

of the SABM/SABME command correctly, the DCE determines that it cannot enter

the information transfer phase, it will return a DM response to the DTE as a

denial to the data link set\(hyup initialization and will consider that the

data link is

\fInot\fR

 set up. In order to avoid misinterpretation of the DM response received,
it is suggested that the DTE always sends its SABM/SABME command with the
P\ bit set to\ 1. Otherwise, it is not possible to differentiate a DM response
intended as a denial to data link set\(hyup from a DM response that is
issued in a separate unsolicited sense as a request for a mode\(hysetting
command (as described

in\ \(sc\ 2.4.4.4.2).

.LP

The DCE will initiate data link set\(hyup by transmitting an SABM/SABME
command to the DTE and starting its Timer\ T1 in order to determine when
too

much time has elapsed waiting for a reply (see \(sc\ 2.4.8.1 below). Upon
reception of a UA response from the DTE, the DCE will reset its send and
receive state

variables\ V(S) and\ V(R) to zero, will stop its Timer\ T1, and will consider
that the data link is set up. Upon reception of a DM response from the
DTE as a

denial

to the data link set\(hyup initialization, the DCE will stop its Timer\
T1 and will consider that the data link is \fInot\fR

set up.

.LP



The DCE, having sent the SABM/SABME command, will ignore and discard any
frames except an SABM/SABME or DISC command, or a UA or DM response

received from the DTE. The receipt of an SABM/SABME or DISC command from the

DTE will result in a collision situation that is resolved per \(sc\ 2.4.4.5
below. Frames other than the UA and DM responses sent in response to a
received

SABM/SABME or DISC command will be sent only after the data link is set
up and if no outstanding SABM/SABME command exists.

.LP

After the DCE sends the SABM/SABME command, if a UA or DM response is not
received correctly, Timer\ T1 will run out in the DCE. The DCE will then

resend the SABM/SABME command and will restart Timer\ T1. After transmission
of the SABM/SABME command N2 times by the DCE, appropriate higher layer
recovery action will be initiated. The value of N2 is defined in \(sc\
2.4.8.4 below.

.LP

2.4.4.2

\fIInformation transfer phase\fR

.LP

After having transmitted the UA response to the SABM/SABME command or having
received the UA response to a transmitted SABM/SABME command, the DCE will
accept and transmit I and supervisory frames according to the procedures
described in \(sc\ 2.4.5 below.

.LP



When receiving the SABM/SABME command while in the information

transfer phase, the DCE will conform to the data link resetting procedure

described in \(sc\ 2.4.7 below.

.LP

2.4.4.3

\fIData link disconnection\fR

.LP

The DTE shall initiate a disconnect of the data link by

transmitting a DISC command to the DCE. On correctly receiving a DISC command
in the information transfer phase, the DCE will send a UA response and
enter

the disconnected phase. On correctly receiving a DISC command in the

disconnected phase, the DCE will send a DM response and remain in the

disconnected phase. In order to avoid misinterpretation of the DM response

received, it is suggested that the DTE always sends its DISC command with
the P\ bit set to\ 1. Otherwise, it is not possible to differentiate a
DM response

intended as an indication that the DCE is already in the disconnected phase

from a DM response that is issued in a separate unsolicited sense as a
request for a mode\(hysetting command (as described in \(sc\ 2.4.4.4.2).

.LP



The DCE will initiate a disconnect of the data link by transmitting a DISC
command to the DTE and starting its Timer\ T1 (see \(sc\ 2.4.8.1 below).
Upon reception of an UA response from the DTE, the DCE will stop its Timer\
T1 and

will enter the disconnected phase. Upon reception of a DM response from
the DTE as an indication that the DTE was already in the disconnected phase,
the DCE

will stop its Timer\ T1 and will enter the disconnected phase.

.LP

The DCE, having sent the DISC command, will ignore and discard any

frames except an SABM/SABME or DISC command, or a UA or DM response received

from the DTE. The receipt of an SABM/SABME or DISC command from the DTE will

result in a collision situation that is resolved per \(sc\ 2.4.4.5 below.

.LP



After the DCE sends the DISC command, if a UA or DM response is not

received correctly, Timer\ T1 will run out in the DCE. The DCE will then
resend the DISC command and will restart Timer\ T1. After transmission
of the DISC

command N2 times by the DCE, appropriate higher layer recovery action will
be initiated. The value of N2 is defined in \(sc\ 2.4.8.4 below.

.LP

2.4.4.4

\fIDisconnected phase\fR

.LP

2.4.4.4.1

After having received a DISC command from the DTE and

returned a UA response to the DTE, or having received the UA response to a

transmitted DISC command, the DCE will enter the disconnected phase.

.LP

In the disconnected phase, the DCE may initiate data link set\(hyup. In
the disconnected phase, the DCE will react to the receipt of an SABM/SABME
command as described in \(sc\ 2.4.4.1 above and will transmit a DM response
in

answer to a received DISC command. When receiving any other command (defined,
or undefined or not implemented) with the P\ bit set to\ 1, the DCE will
transmit a DM response with the F\ bit set to\ 1. Other frames received
in the

disconnected phase will be ignored by the DCE.

.LP

2.4.4.4.2

When the DCE enters the disconnected phase after

detecting error conditions as listed in \(sc\ 2.4.6 below, or after an internal

malfunction, it may indicate this by sending a DM response rather than
a DISC command. In these cases, the DCE will transmit a DM response and
start its

Timer\ T1 (see \(sc\ 2.4.8.1 below).

.LP

If Timer T1 runs out before the reception of an SABM/SABME or DISC command
from the DTE, the DCE will retransmit the DM response and restart

Timer\ T1. After transmission of the DM response N2 times, the DCE will
remain in the disconnected phase and appropriate recovery actions will
be initiated. The value of N2 is defined in \(sc\ 2.4.8.4 below.

.LP

Alternatively, after an internal malfunction, the DCE may either

initiate a data link resetting procedure (see \(sc\ 2.4.7 below) or disconnect
the data link (see \(sc\ 2.4.4.3 above) prior to initiating a data link
set\(hyup

procedure (see \(sc\ 2.4.4.1 above).

.LP

2.4.4.5

\fICollision of unnumbered commands\fR

.LP

Collision situations shall be resolved in the following way:

.LP

2.4.4.5.1

If the sent and received unnumbered commands are the

same, the DCE and the DTE shall each send the UA response at the earliest

possible opportunity. The DCE shall enter the indicated phase either,

.LP



1)

after receiving the UA response,

.LP

2)

after sending the UA response, or

.LP

3)

after timing out waiting for the UA response having sent a

UA response.

.LP

In the case of 2) above, the DCE will accept a subsequent UA

response to the mode\(hysetting command it issued without causing an exception

condition if received within the time\(hyout interval.

.LP



2.4.4.5.2

If the sent and received unnumbered commands are

different, the DCE and the DTE shall each enter the disconnected phase and

issue a DM response at the earliest possible opportunity.

.LP

2.4.4.6

\fICollision of DM response with SABM/SABME or DISC\fR



\fIcommand\fR

.LP

When a DM response is issued by the DCE or DTE as an unsolicited

response to request the DTE or DCE, respectively, to issue a mode\(hysetting

command as described in \(sc\ 2.4.4.4, a collision between an SABM/SABME
or DISC

command and the unsolicited DM response may occur. In order to avoid

misinterpretation of the DM response received, the DTE always sends its

SABM/SABME or DISC command with the P\ bit set to\ 1.

.LP

2.4.4.7

\fICollision of DM responses\fR

.LP

A contention situation may occur when both the DCE and the DTE

issue a DM response to request a mode\(hysetting command. In this case, the DTE

will issue an SABM/SABME command to resolve the contention situation.

.LP



2.4.5

\fILAPB procedures for information transfer\fR

.LP

The procedures which apply to the transmission of I\ frames in each direction
during the information transfer phase are described below.

.LP

In the following, \*Qnumber one higher\*U is in reference to a

continuously repeated sequence series, i.e., 7 is 1\ higher than 6 and 0 is

1\ higher than 7 for modulo\ 8 series, and 127 is 1\ higher than 126 and 0 is

1\ higher than 127 for modulo\ 128 series.

.LP

2.4.5.1

\fISending I frames\fR

.LP

When the DCE has an I frame to transmit (i.e. an I frame not

already transmitted, or having to be retransmitted as described in \(sc\
2.4.5.6

below), it will transmit it with an N(S) equal to its current send state

variable V(S), and an N(R) equal to its current receive state variable
V(R). At the end of the transmission of the I\ frame, the DCE will increment
its send

state variable V(S) by\ 1.

.LP

If Timer T1 is not running at the time of transmission of an I frame, it
will be started.

.LP

If the send state variable V(S) is equal to the last value of N(R)

received plus \fIk\fR

(where \fIk\fR

is the maximum number of outstanding I\ frames \(em

see \(sc\ 2.4.8.6 below), the DCE will not transmit any new I\ frames, but may

retransmit an I\ frame as described in \(sc\(sc\ 2.4.5.6 or 2.4.5.9 below.

.LP

When the DCE is in the busy condition, it may still transmit I frames,
provided that the DTE is not busy. When the DCE is in the frame rejection

condition, it will stop transmitting I\ frames.

.LP

2.4.5.2

\fIReceiving an I frame\fR

.LP

2.4.5.2.1

When the DCE is not in a busy condition and receives a valid I\ frame whose
send sequence number N(S) is equal to the DCE receive state

variable V(R), the DCE will accept the information field of this frame,

increment by one its receive state variable V(R), and act as follows:

.LP



a)

If the DCE is still not in a busy condition:

.LP

i)

If an I frame is available for transmission by the

DCE, it may act as in \(sc\ 2.4.5.1 above and acknowledge

the received I frame by setting N(R) in the control field

of the next transmitted I\ frame to the value of the DCE

receive state variable V(R). Alternatively, the DCE may

acknowledge the received I\ frame by transmitting an RR

frame with the N(R) equal to the value of the DCE receive

state variable V(R).

.LP

ii)

If no I frame is available for transmission by the

DCE, it will transmit an RR frame with N(R) equal to the

value of the DCE receive state variable V(R).

.LP



b)

If the DCE is now in a busy condition, it will transmit an

RNR frame with N(R) equal to the value of the DCE receive

state variable V(R) (see \(sc\ 2.4.5.8).

.LP



2.4.5.2.2

When the DCE is in a busy condition, it may ignore the

information field contained in any received I\ frame.

.LP

2.4.5.3

\fIReception of invalid frames\fR

.LP

When the DCE receives an invalid frame (see \(sc\ 2.3.5.3), this frame
will be discarded.

.LP

2.4.5.4

\fIReception of out\(hyof\(hysequence I frames\fR

.LP

When the DCE receives a valid I frame whose send sequence number

N(S) is incorrect, i.e.,\ not equal to the current DCE receive state variable

V(R), it will discard the information field of the I\ frame and transmit
an REJ frame with the N(R) set to one higher than the N(S) of the last
correctly

received I\ frame. The REJ frame will be a command frame with the P\ bit
set to\ 1 if an acknowledged transfer of the retransmission request is
required;

otherwise the REJ frame may be either a command or a response frame. The DCE

will then discard the information field of all I\ frames received until the

expected I\ frame is correctly received. When receiving the expected I\ frame,

the DCE will then acknowledge the I\ frame as described in \(sc\ 2.4.5.2
above. The DCE will use the N(R) and P\ bit information in the discarded
I\ frames as

described in \(sc\ 2.3.5.2 above.

.LP

2.4.5.5

\fIReceiving acknowledgement\fR

.LP

When correctly receiving an I frame or a supervisory frame (RR, RNR or
REJ), even in the busy condition, the DCE will consider the N(R) contained
in this frame as an acknowledgement for all I\ frames it has transmitted
with an N(S) up to and including the received N(R)\(em1. The DCE will stop
Timer\ T1 when it correctly receives an I\ frame or a supervisory frame
with the N(R) higher

than the last received N(R) (actually acknowledging some I\ frames), or
an REJ frame with an N(R) equal to the last received N(R).

.LP



If Timer T1 has been stopped by the receipt on an I, RR or RNR frame, and
if there are outstanding I\ frames still unacknowledged, the DCE will

restart Timer\ T1. If Timer\ T1 then runs out, the DCE will follow the
recovery procedure (\(sc\ 2.4.5.9 below) with respect to the unacknowledged
I\ frames. If

Timer\ T1 has been stopped by the receipt of an REJ frame, the DCE will
follow the retransmission procedures in \(sc\ 2.4.5.6 below.

.LP

2.4.5.6

\fIReceiving an REJ frame\fR

.LP

When receiving an REJ frame, the DCE will set its send state

variable V(S) to the N(R) received in the REJ control field. It will transmit
the corresponding I\ frame as soon as it is available or retransmit it
in

accordance with the procedures described in \(sc\ 2.4.5.1 above. (Re)transmission
will conform to the following procedure:

.LP

i)

if the DCE is transmitting a supervisory command or response

when it receives the REJ frame, it will complete that

transmission before commencing transmission of the requested

I\ frame;

.LP



ii)

if the DCE is transmitting an unnumbered command or

response when it receives the REJ frame, it will ignore the

request for retransmission;

.LP

iii)

if the DCE is transmitting an I frame when the REJ frame

is received, it may abort the I\ frame and commence transmission

of the requested I\ frame immediately after abortion;

.LP

iv)

if the DCE is not transmitting any frame when the REJ frame

is received, it will commence transmission of the requested

I\ frame immediately.

.LP

In all cases, if other unacknowledged I frames had already been

transmitted following the one indicated in the REJ frame, then those I
frames will be retransmitted by the DCE following the retransmission of
the requested I\ frame. Other I\ frames not yet transmitted may be transmitted
following the

retransmitted I\ frames.

.LP

If the REJ frame was received from the DTE as a command with the P bit
set to\ 1, the DCE will transmit an RR, RNR or REJ response with the F
bit set to\ 1 before transmitting or retransmitting the corresponding I\
frame.

.LP

2.4.5.7

\fIReceiving an RNR frame\fR

.LP

After receiving an RNR frame whose N(R) acknowledges all frames

previously transmitted, the DCE will stop Timer\ T1 and may then transmit

an I\ frame, with the P\ bit set to\ 0, whose send sequence number is

equal to the N(R) indicated in the RNR frame, restarting Timer\ T1 as it
does. After receiving an RNR frame whose N(R) indicates a previously transmitted

frame, the DCE will not transmit or retransmit any I\ frame, Timer\ T1 being

already running. In either case, if the Timer\ T1 runs out before receipt
of a busy clearance indication, the DCE will follow the procedure described
in

\(sc\ 2.4.5.9\ below. In any case, the DCE will not transmit any other
I\ frames

before receiving an RR or REJ frame, or before the completion of a link

resetting procedure.

.LP

Alternatively, after receiving an RNR frame, the DCE may wait for a

period of time (e.g.,\ the length of the Timer\ T1) and then transmit a

supervisory command frame (RR, RNR or REJ) with the P\ bit set to\ 1, and
start Timer\ T1, in order to determine if there is any change in the receive
status of the DTE. The DTE shall respond to the P\ bit set to\ 1 with a
supervisory

response frame (RR, RNR or REJ) with the F\ bit set to\ 1 indicating either

continuance of the busy condition (RNR) or clearance of the busy condition
(RR or REJ). Upon receipt of the DTE response, Timer\ T1 is stopped.

.LP



1)

If the response is the RR or REJ response, the busy

condition is cleared and the DCE may transmit I\ frames beginning

with the I\ frame identified by the N(R) in the received response

frame.

.LP

2)

If the response is the RNR response, the busy condition

still exists, and the DCE will after a period of time (e.g.\ the

length of Timer\ T1) repeat the enquiry of the DTE receive

status.

.LP

If Timer T1 runs out before a status response is received, the

enquiry process above is repeated. If N2 attempts to get a status response
fail (i.e.\ Timer\ T1 runs out N2\ times), the DCE will initiate a data
link resetting procedure as described in \(sc\ 2.4.7.2\ below or will transmit
a DM response to ask the DTE to initiate a data link set\(hyup procedure
as described in \(sc\ 2.4.4.1 and enter the disconnected phase. The value
of N2 is defined in \(sc\ 2.4.8.4\ below.

.LP



If, at any time during the enquiry process, an unsolicited RR or REJ frame
is received from the DTE, it will be considered to be an indication of

clearance of the busy condition. Should the unsolicited RR or REJ frame be a

command frame with the P bit set to\ 1, the appropriate response frame
with the F\ bit set to 1 must be transmitted before the DCE may resume
transmission of

I\ frames. If Timer\ T1 is running, the DCE will wait for the non\(hybusy
response with the F bit set to\ 1 or will wait for Timer\ T1 to run out
and then either

may reinitiate the enquiry process in order to realize a successful P/F bit

exchange or may resume transmission of I frames beginning with the I\ frame

identified by the N(R) in the received RR or REJ frame.

.LP

2.4.5.8

\fIDCE busy condition\fR

.LP

When the DCE enters a busy condition, it will transmit an RNR frame at
the earliest opportunity. The RNR frame will be a command frame with the
P bit set to\ 1 if an acknowledged transfer of the busy condition indication
is

required; otherwise the RNR frame may be either a command or a response
frame. While in the busy condition, the DCE will accept and process supervisory

frames, will accept and process the contents of the N(R) fields of I\ frames,

and will return an RNR response with the F bit set to\ 1 if it receives a

supervisory command or I command frame with the P bit set to\ 1. To clear the

busy condition, the DCE will transmit either an REJ frame or an RR frame, with

N(R) set to the current receive state variable V(R), depending on whether or

not it discarded information fields of correctly received I\ frames. The REJ

frame or the RR frame will be a command frame with the P bit set to\ 1 if an

acknowledged transfer of the busy\(hyto\(hynon\(hybusy transition is required,
otherwise the REJ frame or the RR frame may be either a command or a response
frame.

.LP



2.4.5.9

\fIWaiting acknowledgement\fR

.LP

The DCE maintains an internal transmission attempt variable which is set
to\ 0 when the DCE sends a UA response, when the DCE receives a UA

response or an RNR command or response, or when the DCE correctly receives
an I\ frame or supervisory frame with the N(R) higher than the last received
N(R) (actually acknowledging some outstanding I\ frames).

.LP

If Timer T1 runs out waiting for the acknowledgement from the DTE for an
I\ frame transmitted, the DCE will enter the timer recovery condition,
add

one to its transmission attempt variable and set an internal variable \fIx\fR

to

the current value of its send state variable V(S). The DCE will then restart

Timer T1, set its send state variable V(S) to the last value of N(R) received
from the DTE and retransmit the corresponding I\ frame with the P bit set
to\ 1, or transmit an appropriate supervisory command frame (RR, RNR or
REJ) with the P\ bit set to\ 1.

.LP



The timer recovery condition is cleared when the DCE receives a valid supervisory
frame with the F\ bit\ set to\ 1.

.LP

If, while in the timer recovery condition, the DCE correctly receives a
supervisory frame with the F bit set to\ 1 and with the N(R) within the
range from its current send state variable V(S) to \fIx\fR

included, it will clear the

timer recovery condition (including stopping Timer\ T1) and set its send
state variable V(S) to the value of the received N(R), and may then resume
with

I\ frame transmission or retransmission, as appropriate.

.LP

If, while in the timer recovery condition, the DCE correctly receives an
I or supervisory frame with the P/F bit set to\ 0 and with a valid N(R)
(see \(sc\ 2.3.4.9), it will not clear the timer recovery condition. The
value of the

received N(R) may be used to update the send state variable V(S). However,
the DCE may decide to keep the last transmitted I\ frame in store (even
if it is

acknowledged) in order to be able to retransmit it with the P bit set to\
1 when Timer\ T1 runs out at a later time.

.LP



If the received supervisory frame with the P/F bit set to\ 0 is an REJ
frame with a valid N(R), the DCE may either immediately initiate

(re)transmission from the value of the send state variable V(S), or it may

ignore the request for retransmission and wait until the supervisory frame
with the F bit set to\ 1 is received before initiating (re)transmission
of frames

from the value identified in the N(R) field of the supervisory frame with
the F\ bit set to\ 1. In the case of immediate retransmission, in order
to prevent

duplicate retransmissions following the clearance of the timer recovery

condition, the DCE shall inhibit retransmission of a specific I\ frame [same

N(R) in the same numbering cycle] if the DCE has retransmitted that I\
frame as the result of a received REJ frame with the P/F bit set to\ 0.

.LP

If, while in the timer recovery condition, the DCE receives a REJ

command with the P bit set to\ 1, the DCE will respond immediately with an

appropriate supervisory response with the F bit set to\ 1. The DCE may
then use the value of the N(R) in the REJ command to update the send state
variable

V(S), and may either immediately begin (re)transmission from the value N(R)

indicated in the REJ frame or ignore the request for retransmission and wait

until the supervisory frame with the F bit set to\ 1 is received before

initiating (re)transmission of I\ frames from the value identified in the
N(R) field of the supervisory frame with the F\ bit set to\ 1. In the case
of

immediate retransmission, in order to prevent duplicate retransmissions

following the clearance of the timer recovery condition, the DCE shall
inhibit retransmission of a specific I\ frame [same N(R) in the same numbering
cycle]

if the DCE has retransmitted that I\ frame as the result of the received REJ

command with the P\ bit set to\ 1.

.LP

If Timer T1 runs out in the timer recovery condition, and no I or

supervisory frame with the P/F bit set to 0 and with a valid N(R) has been

received, or no REJ command with the P bit set to\ 1 and with a valid N(R)
has been received, the DCE will add one to its transmission attempt variable,

restart Timer\ T1, and either retransmit the I frame sent with the P bit set

to\ 1 or transmit an appropriate supervisory command with the P bit set to\ 1.

.LP



If the transmission attempt variable is equal to N2, the DCE will

initiate a data link resetting procedure as described in \(sc\ 2.4.7.2
below, or

will transmit a DM response to ask the DTE to initiate a data link set\(hyup

procedure as described in \(sc\ 2.4.4.1 above and enter the disconnected
phase. N2 is a system parameter (see \(sc\ 2.4.8.4 below).

.LP

\fINote\fR

\ \(em\ Although the DCE may implement the internal variable \fIx\fR

,

other mechanisms do exist that achieve the identical function.

.LP

2.4.6

\fILAPB conditions for \fR

\fIdata link resetting or data link\fR



\fIre\(hyinitialization\fR

\fI(data link set\(hyup)\fR

.LP

2.4.6.1

When the DCE receives, during the information transfer phase, a

frame which is not invalid (see \(sc\ 2.3.5.3) with one of the conditions
listed in \(sc\ 2.3.4.9 above, the DCE will request the DTE to initiate
a data link resetting procedure by transmitting an FRMR response to the
DTE as described in

\(sc\ 2.4.7.3.

.LP

2.4.6.2

When the DCE receives, during the information transfer phase, an FRMR response
from the DTE, the DCE will either initiate the data link

resetting procedures itself as described in \(sc\ 2.4.7.2 or return a DM
response to ask the DTE to initiate the data link set\(hyup (initialization)
procedure as described in \(sc\ 2.4.4.1. After transmitting a DM response,
the DCE will enter

the disconnected phase as described in \(sc\ 2.4.4.4.2.

.LP

2.4.6.3

When the DCE receives, during the information transfer phase, a UA response,
or an unsolicited response with the F bit set to\ 1, the DCE may

either initiate the data link resetting procedures itself as described in

\(sc\ 2.4.7.2, or return a DM response to ask the DTE to initiate the data link

set\(hyup (initialization) procedure as described in \(sc\ 2.4.4.1. After
transmitting a DM response, the DCE will enter the disconnected phase as
described in

\(sc\ 2.4.4.4.2.

.LP

2.4.6.4

When the DCE receives, during the information transfer phase, a DM response
from the DTE, the DCE will either initiate the data link set\(hyup

(initialization) procedures itself as described in \(sc\ 2.4.4.1, or return
a DM

response to ask the DTE to initiate the data link set\(hyup (initialization)

procedures as described in \(sc\ 2.4.4.1. After transmitting a DM response,
the DCE will enter the disconnected phase as described in \(sc\ 2.4.4.4.2.

.LP



2.4.7

\fILAPB procedure for data link resetting\fR

.LP

2.4.7.1

The data link resetting procedure is used to initialize both

directions of information transfer according to the procedure described
below. The data link resetting procedure only applies during the information
transfer phase.

.LP

2.4.7.2

Either the DTE or the DCE may initiate the data link resetting

procedure. The data link resetting procedure indicates a clearance of a DCE

and/or DTE busy condition, if present.

.LP

The DTE shall initiate a data link resetting by transmitting an

SABM/SABME command to the DCE. If, upon correct receipt of the SABM/SABME

command, the DCE determines that it can continue in the information transfer

phase, it will return a UA response to the DTE, will reset its send and
receive state variables V(S) and V(R) to zero, and will remain in the information

transfer phase. If, upon correct receipt of the SABM/SABME command, the DCE

determines that it cannot remain in the information transfer phase, it will

return a DM response as a denial to the resetting request and will enter the

disconnected phase.

.LP

The DCE will initiate a data link resetting by transmitting an

SABM/SABME command to the DTE and starting its Timer\ T1 (see \(sc\ 2.4.8.1
below). Upon reception of a UA response from the DTE, the DCE will reset
its send and receive state variables V(S) and V(R) to zero, will stop its
Timer\ T1, and will remain in the information transfer phase. Upon reception
of a DM response from the DTE as a denial to the data link resetting request,
the DCE will stop its Timer\ T1 and will enter the disconnected phase.

.LP

The DCE, having sent an SABM/SABME command, will ignore and discard

any frames received from the DTE except an SABM/SABME or DISC command,
or a UA or DM response. The receipt of an SABM/SABME or DISC command from
the DTE will result in a collision situation that is resolved per \(sc\
2.4.4.5 above. Frames

other than the UA or DM response sent in response to a received SABM/SABME
or DISC command will be sent only after the data link is reset and if no

outstanding SABM/SABME command exists.

.LP



After the DCE sends the SABM/SABME command, if a UA or DM response is not
received correctly, Timer\ T1 will run out in the DCE. The DCE will then

resend the SABM/SABME command and will restart Timer\ T1. After N2 attempts
to reset the data link, the DCE will initiate appropriate higher layer
recovery

action and will enter the disconnected phase. The value of N2 is defined in

\(sc\ 2.4.8.4 below.

.LP

2.4.7.3

The DCE may ask the DTE to reset the data link by transmitting an FRMR
response (see \(sc\ 2.4.6.1 above). After transmitting an FRMR response,

the DCE will enter the frame rejection condition.

.LP

The frame rejection condition is cleared when the DCE receives an SABM/SABME
command, a DISC command, a FRMR response, or a DM response; or if

the DCE transmits an SABM/SABME command, a DISC command, or a DM response.

Other commands received while in the frame rejection condition will cause
the DCE to retransmit the FRMR response with the same information field
as

originally transmitted.

.LP

The DCE may start Timer\ T1 on transmission of the FRMR response. If

Timer\ T1 runs out before the frame rejection condition is cleared, the
DCE may retransmit the FRMR response, and restart T1. After N2 attempts
(time outs) to get the DTE to reset the data link, the DCE may reset the
data link itself as described in \(sc\ 2.4.7.2 above. The value of N2 is
defined in \(sc\ 2.4.8.4 below.

.LP



In the frame rejection condition, I frames and supervisory frames will
not be transmitted by the DCE. Also, received I frames and supervisory
frames will be discarded by the DCE except for the observance of a P bit
set to\ 1.

When an additional FRMR response must be transmitted by the DCE as a result
of the receipt of a P\ bit set to 1 while Timer\ T1 is running, Timer\
T1 will

continue to run. Upon reception of an FRMR response (even during a frame

rejection condition), the DCE will initiate a resetting procedure by

transmitting an SABM/SABME command as described in \(sc\ 2.4.7.2 above, or will

transmit a DM response to ask the DTE to initiate the data link set\(hyup

procedure as described in \(sc\ 2.4.4.1 and enter the disconnected phase.

.LP

2.4.8

\fIList of \fR

\fILAPB system parameters\fR

.LP

The DCE and DTE system parameters are as follows:

.LP



2.4.8.1

\fITimer\fR

\fIT1\fR

.LP

The value of the DTE Timer T1 system parameter may be different

than the value of the DCE Timer T1 system parameter. These values shall
be made known to both the DTE and the DCE, and agreed to for a period of
time by both the DTE and the DCE.

.LP

The period of Timer T1, at the end of which retransmission of a frame may
be initiated (see \(sc\ 2.4.4 and \(sc\ 2.4.5 above for the DCE), shall
take into account whether T1 is started at the beginning or the end of
the transmission of a frame.

.LP

The proper operation of the procedure requires that the transmitter's (DCE
or DTE) Timer\ T1 be greater than the maximum time between transmission
of a frame (SABM/SABME, DISC, I\ or supervisory command, or DM or FRMR
response)

and the reception of the corresponding frame returned as an answer to that

frame (UA, DM or acknowledging frame). Therefore, the receiver (DCE or DTE)

should not delay the response or acknowledging frame returned to one of the

above frames by more than a value\ T2, where T2 is a system parameter (see

\(sc\ 2.4.8.2).

.LP



The DCE will not delay the response or acknowledging frame returned to
one of the above DTE frames by more than a period\ T2.

.LP

2.4.8.2

\fIParameter T2\fR

.LP

The value of the DTE parameter T2 may be different than the value of the
DCE parameter T2. These values shall be made known to both the DTE and
the DCE, and agreed to for a period of time by both the DTE and the DCE.

.LP



The period of parameter T2 shall indicate the amount of time

available at the DCE or DTE before the acknowledging frame must be initiated
in order to ensure its receipt by the DTE or DCE, respectively, prior to
Timer\ T1 running out at the DTE or DCE (parameter\ T2\ <\ Timer\ T1).

.LP

\fINote\fR

\ \(em\ The period of parameter T2 shall take into account the

following timing factors: the transmission time of the acknowledging frame,
the propagation time over the access data link, the stated processing times
at the DCE and the DTE, and the time to complete the transmission of the
frame(s) in the DCE or DTE transmit queue that are neither displaceable
or modifiable in an orderly manner.

.LP



Given a value for Timer T1 for the DTE or DCE, the value of parameter T2
at the DCE or DTE, respectively, must be no larger than T1 minus\ 2 times
the propagation time over the access data link, minus the frame processing
time at the DCE, minus the frame processing time at the DTE, and minus
the transmission time of the acknowledging frame by the DCE or DTE, respectively.

.LP



2.4.8.3

\fITimer T3\fR

.LP

The DCE shall support a Timer T3 system parameter, the value of

which shall be made known to the DTE.

.LP

The period of Timer T3, at the end of which an indication of an

observed excessively long idle channel state condition is passed to the
Packet Layer, shall be sufficiently greater than the period of the DCE
Timer T1

(i.e.\ T3\ >\ T1) so that the expiration of T3 provides the desired level of

assurance that the data link channel is in a non\(hyactive, non\(hyoperational
state, and is in need of data link set\(hyup before normal data link operation
can

resume.

.LP

2.4.8.4

\fIMaximum number of attempts\fR

\fIto complete a\fR



\fItransmission N2\fR

.LP



The value of the DTE N2 system parameter may be different than the value
of the DCE N2 system parameter. These values shall be made known to both
the DTE and the DCE, and agreed to for a period of time by both the DTE
and the DCE.

.LP

The value of N2 shall indicate the maximum number of attempts made by the
DCE or DTE to complete the successful transmission of a frame to the DTE
or DCE, respectively.

.LP

2.4.8.5

\fIMaximum number of bits in an I frame N1\fR

.LP

The value of the DTE N1 system parameter may be different than the value
of the DCE N1 system parameter. These values shall be made known to both
the DTE and the DCE.

.LP

The values of N1 shall indicate the maximum number of bits in an

I\ frame (excluding flags and 0\ bits inserted for transparency) that the
DCE or DTE is willing to accept from the DTE or DCE, respectively.

.LP

In order to allow for universal operation, a DTE should support a

value of DTE N1 which is not less than 1080\ bits (135\ octets). DTEs should
be aware that the network may transmit longer packets (see \(sc\ 5.2),
that may result in a data link layer problem.

.LP



All networks shall offer to a DTE which requires it, a value of DCE N1
which is greater than or equal to 2072\ bits (259\ octets) plus the length
of the address, control and FCS fields at the DTE/DCE interface, and greater
than or equal to the maximum length of the data packets which may cross
the DTE/DCE

interface plus the length of the address, control and FCS fields at the
DTE/DCE interface.

.LP

Appendix II provides a description of how the values stated above are  derived.

.LP

2.4.8.6

\fIMaximum number of \fR

\fIoutstanding I frames k\fR

.LP

The value of the DTE k system parameter shall be the same as the

value of the DCE k system parameter. This value shall be agreed to for a

period of time by both the DTE and the DCE.

.LP

The value of k shall indicate the maximum number of sequentially

numbered I\ frames that the DTE or DCE may have outstanding

(i.e.\ unacknowledged) at any given time. The value of k shall never exceed

seven for modulo\ 8 operation, or one hundred and twenty\(hyseven for modulo\
128

operation. All networks (DCEs) shall support a value of seven. Other values
of k (less than and greater than seven) may also be supported by networks

(DCEs).

.LP

.LP

\fBMONTAGE : \(sc 2.5 de cette Recommandation sur le reste de cette page\fR

.LP



.LP
.ad r
\fBTable [1/X.22] [T1.22], p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
.sp 1
2.4
    \fICall control and failure detection procedures\fR
.sp 9p
.RT
.PP
\fICall control\fR \|and \fIfailure detection\fR \|procedures shall operate
as specified in Recommendation\ X.21 on each subscriber channel independent
of other subscriber channels.
.RT
.sp 1P
.LP
2.4.1
    \fIQuiescent states\fR
.sp 9p
.RT
.PP
The quiescent states shall be in accordance with
Recommendation\ X.21, \(sc\ 2.5.
.bp
.RT
.sp 1P
.LP
2.4.2
    \fIFailure detection\fR
.sp 9p
.RT
.PP
See Recommendation\ X.27, \(sc\ 9 for association of the
receiver circuit failure detection types.
.RT
.sp 1P
.LP
2.4.2.1
    \fIFault conditions on interchange circuits\fR
.sp 9p
.RT
.PP
The DTE should interpret a fault condition on circuit\ R as r\ =\ 0 on
all channels using failure detection type\ 2, a fault condition on circuit\
I as i\ =\ OFF on all channels using failure detection type\ 1, and a fault
condition on both circuits\ R and\ I as r\ =\ 0, i\ =\ OFF \fI(DCE not
ready)\fR on all channels.
.PP
Alternatively a fault condition on one of these circuits,
R or I, may be interpreted by the DTE as r\ =\ 0, i\ =\ OFF
\fI(DCE not ready)\fR , using failure detection type\ 3.
.PP
The DCE will interpret a fault condition on circuit\ T as t\ =\ 0
on all channels using failure detection type\ 2, a fault
condition on circuit\ C as c\ =\ OFF on all channels using failure
detection type\ 1, and a fault condition on both circuits\ T
and\ C as t\ =\ 0, c\ =\ OFF on all channels \fI(DTE uncontrolled not\fR
\fIready)\fR .
.PP
Alternatively, a fault condition on one of these circuits,
T or C, may be interpreted by the DCE as t\ =\ 0, c\ =\ OFF
\fI(DTE uncontrolled not ready)\fR , using failure detection
type\ 3.
.RT
.sp 1P
.LP
2.4.2.2
    \fIDCE fault condition\fR
.sp 9p
.RT
.PP
Indication of the DCE failure condition shall be in accordance
with Recommendation\ X.21, \(sc\ 2.6.2.
.PP
A DCE failure condition may effect all subscriber channels
at the DTE/DCE interface.
.RT
.sp 1P
.LP
2.4.2.3
    \fISignal element timing provision\fR
.sp 9p
.RT
.PP
The provision of signal element timing shall be in accordance
with Recommendation\ X.21, \(sc\ 2.6.3.
.RT
.sp 1P
.LP
2.4.3
    \fIElements of the call control phase\fR
.sp 9p
.RT
.PP
The elements of the call control phase, for each channel, shall be in accordance
with Recommendation\ X.21, \(sc\ 4 with the exception that byte
timing is not used.
.RT
.sp 1P
.LP
2.4.4
    \fIData transfer phase\fR
.sp 9p
.RT
.PP
The data transfer phase, for each channel, shall be in accordance with
Recommendation\ X.21, \(sc\ 5.
.RT
.sp 1P
.LP
2.4.5
    \fIClearing phase\fR
.sp 9p
.RT
.PP
The clearing phase, for each channel, shall be in accordance with Recommendation\
X.21, \(sc\ 6.
.RT
.sp 2P
.LP
\fB3\fR     \fBAlignment of call control characters\fR \fBand\fR
\fBerror\fR
\fBchecking\fR
.sp 1P
.RT
.sp 1P
.LP
\fB
3.1
    \fICharacter alignment\fR
.sp 9p
.RT
.LP
.PP
For the interchange of information between the DTE and the DCE for call
control purposes, it is necessary to establish correct alignment of
characters. Each sequence of call control characters to and from the DCE
shall be preceded by two or more contiguous\ 1/6 (\*QSYN\*U) characters.
.RT
.PP
3.1.1
Certain Administrations will require the DTE to align call
control characters transmitted from the DTE to either SYN characters delivered
to the DTE or to the signals on the \fIframe start identification\fR interchange
circuit\ (F).
.sp 9p
.RT
.PP
3.1.2
Certain Administrations will permit call control characters to be transmitted
from the DTE independently of the SYN characters delivered to
the DTE.
.bp
.sp 9p
.RT
.sp 1P
.LP
3.2
    \fIError checking\fR
.sp 9p
.RT
.PP
Odd parity according to Recommendation\ X.4 applies for the
interchange of IA5 characters for call control purposes.
.RT
.sp 2P
.LP
\fB4\fR     \fBMultiplex structure\fR
.sp 1P
.RT
.PP
Depending on the multiplex structure used by the network, the
structure of the multiplexed bit stream will be one of two different
types.
.RT
.sp 1P
.LP
4.1
    \fIMultiplex structure in networks providing 6\ bit\(hybytes\fR
.sp 9p
.RT
.PP
The DCE shall deliver to and receive from the DTE a 6\(hybit byte
interleaved multiplexed bit stream containing a number of subscriber channels.
The allocation of the subscriber channels should be:
.RT
.ce 1000
\ 5\ channels\ (phases)\ of\ 9600\ bit/s\ or
.ce 0
.ce 1000
10\ channels\ \ \ \ \ \ \ \ of\ 4800\ bit/s\ or
.ce 0
.ce 1000
20\ channels\ \ \ \ \ \ \ \ of\ 2400\ bit/s\ or
.ce 0
.sp 1P
.ce 1000
80\ channels\ \ \ \ \ \ \ \ of\ \ 600\ bit/s\ or
.ce 0
.sp 1P
.LP
an appropriate mix of channel data signalling rates having an aggregate bit
rate of 48\ kbit/s.
.PP
The multiplex structure is divided into five phases of 9600\ bit/s, where
each phase shall be homogeneous with regard to the subscriber data
signalling rates.
.sp 1P
.LP
4.1.1
    \fIInterchange circuits and\fR
\fIinterface signalling\fR \fIscheme\fR
.sp 9p
.RT
.PP
The interchange circuits between the DTE and the DCE are shown in Figure\
1/X.22 and a timing diagram for the signals is given in
Figure\ 2/X.22.
.PP
The signalling over the interchange circuits is as follows.
.PP
The transmit (T) and receive (R) circuits will convey in one
time slot six consecutive user data bits for one subscriber
channel (see Figure\ 2/X.22).
.PP
The control (C) and indication (I) circuits will convey the
appropriate signal levels in accordance with Recommendation\ X.21 for the
data channel which in the same time slot have bits conveyed over the respective
data circuits.
.PP
Change of condition on circuit C shall take place at the OFF to ON
transition of circuit\ S at the beginning of the first bit in the 6\(hybit
byte.
The condition on circuit\ C shall be steady for the whole 6\(hybit byte.
.PP
Change of condition on circuit I will take place at the OFF to ON
transition of circuit\ S at the beginning of the first bit in the 6\(hybit byte
and the condition will be steady for the whole 6\(hybit byte.
.PP
The signal element timing (S) will operate for continuous isochronous transmission
at 48\ kbit/s.
.PP
The \fIframe start identification\fR \| circuit (F) will indicate the frame
start with an OFF condition appearing in the last bit of each frame. For
networks using Recommendation\ X.50 division\ 2 multiplexing, the frame
length will be 480\ bits. For networks using Recommendation\ X.50 division\ 3
multiplexing in which the user rate of 600\ bit/s is not included, the frame
length will be 120\ bits.
.RT
.sp 1P
.LP
4.2
    \fIMultiplex structure in networks providing 8\(hybit bytes\fR
.sp 9p
.RT
.PP
The DCE shall deliver to and receive from the DTE an 8\(hybit byte
interleaved multiplexed bit stream containing a number of subscriber channels.
The allocation of the subscriber channels should be:
.RT
.ce 1000
\ 5\ channels\ (phases)\ of\ 9600\ bit/s\ or
.ce 0
.ce 1000
10\ channels\
(phases)
\ of\ 4800\ bit/s\ or
.ce 0
.ce 1000
20\ channels\
(phases)
\ of\ 2400\ bit/s\ or
.ce 0
.sp 1P
.ce 1000
80\ channels\
(phases)
\ of\ \ 600\ bit/s\ or
.ce 0
.sp 1P
.LP
an appropriate mix of channel data signalling rates having an aggregate bit
rate of 48\ kbit/s.
.PP
The multiplex bit stream is divided into five phases of
9600\ bit/s, where each phase shall be homogeneous with regard to the subscriber
data signalling rates.
.bp
.sp 1P
.LP
4.2.1
    \fIInterchange circuits and interface signalling scheme\fR
.sp 9p
.RT
.PP
The interchange circuits between the DTE and DCE are shown in
Figure\ 1/X.22 and a timing diagram for the signals is given in Figure\
3/X.22. The signalling over the interchange circuits is as follows.
.PP
The transmit (T) and receive (R) circuits will convey in one time slot
eight consecutive user data bits for one subscriber channel (see
Figure\ 3/X.22).
.PP
The control (C) and indication (I) circuits will convey the
appropriate signal levels in accordance with Recommendation\ X.21 for the
data channel which in the same time slot have bits conveyed over the respective
data circuits.
.PP
Change of condition on circuit C shall take place at the OFF to ON
transition of circuit\ S at the beginning of the first bit in the 8\(hybit
byte.
The condition on circuit\ C shall be steady for the whole 8\(hybit byte.
.PP
Change of condition on circuit I will take place at the OFF to ON
transition of circuit\ S at the beginning of the first bit in the 8\(hybit byte
and the condition will be steady for the whole 8\(hybit byte.
.PP
The signal element timing (S) will operate for continuous isochronous transmission
at 48\ kbit/s.
.PP
The
\fIframe start identification\fR \| circuit
(F) will indicate
the
frame start with an OFF condition appearing in the position of the last
bit of each 640\(hybit frame. As an optional facility each frame start
could be followed by a code which will indicate the actual channel allocation.
This facility is for further study.
.RT
.sp 2P
.LP
\fB5\fR     \fBTest loops\fR
.sp 1P
.RT
.PP
Establishment of test loops for DTE tests and network maintenance is for
further study.
.RT
.LP
.rs
.sp 11P
.ad r
\fBFigure 1/X.22, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.rs
.sp 17P
.ad r
\fBFigure 2/X.22, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 22P
.ad r
\fBFigure 3/X.22, p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 2P
.LP
\fBRecommendation\ X.24\fR
.RT
.sp 2P
.ce 1000
\fBLIST\ OF\fR \
\fBDEFINITIONS\ FOR\ INTERCHANGE\ CIRCUITS\ BETWEEN\ DATA\fR
.EF '%    Fascicle\ VIII.2\ \(em\ Rec.\ X.24''
.OF '''Fascicle\ VIII.2\ \(em\ Rec.\ X.24    %'
.ce 0
.ce 1000
\fBTERMINAL\ EQUIPMENT\ (DTE)\ AND\ DATA\ CIRCUIT\(hyTERMINATING\fR
.ce 0
.sp 1P
.ce 1000
\fBEQUIPMENT\ (DCE)\fR \fB\ ON\ PUBLIC\ DATA\ NETWORKS\fR
.ce 0
.sp 1P
.ce 1000
\fI(Geneva, 1976; amended at Geneva, 1980, Malaga\(hyTorremolinos, 1984,\fR
.sp 9p
.RT
.ce 0
.sp 1P
.ce 1000
\fIand Melbourne, 1988)\fR
.ce 0
.sp 1P
.sp 2P
.LP
    The\ CCITT,
.sp 1P
.RT
.sp 1P
.LP
\fIconsidering that\fR
.sp 9p
.RT
.PP
(a)
the interface between DTE and DCE on public data neworks requires, in addition
to the electrical and functional characteristics of the interchange circuits,
the definition of procedural characteristics for call
control functions and selection of the facilities according to
Recommendation\ X.2;
.PP
(b)
the functions of the circuits defined in
Recommendation\ V.24 are based on the requirements of data transmission
over the general telephone network and are not appropriate for use at DTE/DCE
interfaces in public data networks;
.sp 1P
.LP
\fIunanimously declares\fR
.sp 9p
.RT
.PP
a Recommendation to include the list of definitions of
interchange circuits for use in public data networks is required.
.bp
.sp 2P
.LP
\fB1\fR     \fBScope\fR
.sp 1P
.RT
.PP
1.1
This Recommendation applies to the functions of the interchange circuits
provided at the interface between DTE and DCE of data networks
for the transfer of binary data, call control signals and timing signals.
.sp 9p
.RT
.PP
For any type of practical equipment, a selection will be made from the
range of interchange circuits defined in this Recommendation, as
appropriate. The actual interchange circuits to be used in a particular
DCE for a user class of service according to Recommendation\ X.1 and defined
user
facilities according to Recommendation\ X.2, are those indicated in the
relevant Recommendation for the procedural characteristics of the interface,
e.g.,\ Recommendation\ X.20 or\ X.21.
.PP
To enable a standard DTE to be developed, the use and termination by the
DTE of certain circuits even when implemented in the DCE are not mandatory.
This is covered by the individual interface Recommendations.
.PP
The interchange circuits defined for the transfer of binary data are also
used for the exchange of call control signals.
.PP
The electrical characteristics of the interchange circuits are
detailed in the appropriate Recommendation for electrical characteristics of
interchange circuits. The application of those characteristics for a particular
DCE is specified in the Recommendation for the procedural characteristics
of the interface.
.RT
.PP
1.2
The range of interchange circuits defined in this
Recommendation is applicable to the range of services which could be offered
on a public data network, e.g.,\ circuit switching services (synchronous
and
start/stop), telex service, packet switching services, message registration
and retransmission service and facsimile service.
.sp 9p
.RT
.sp 2P
.LP
\fB2\fR     \fBLine of demarcation\fR
.sp 1P
.RT
.LP
.PP
The interface between DTE and DCE is located at a connector which is the
interchange point between these two classes of equipment shown in
Figure\ 1/X.24.
.RT
.LP
.rs
.sp 15P
.ad r
\fBFigure 1/X.24, p.\fR
.sp 1P
.RT
.ad b
.RT
.PP
2.1
The connector will not necessarily be physically attached to
the DCE and may be mounted in a fixed position near the DTE. The female part
of the connector belongs to the DCE.
.sp 9p
.RT
.PP
2.2
An interconnecting cable will normally be provided together
with the DTE. The cable length is limited by electrical parameters specified
in the appropriate Recommendations for the electrical characteristics of the
interchange circuits.
.bp
.sp 9p
.RT
.sp 2P
.LP
\fB3\fR     \fBDefinition of\fR
\fBinterchange circuits\fR
.sp 1P
.RT
.PP
A list of the data network series interchange circuits is presented in
tabular form in Table\ 1/X.24.
.RT
.ce
.line
.ce
\fBTable 1/X.24 [T1.24], p.\fR
.ce
3.1
.ce
\fICircuit\ G\ \(em\ Signal ground or common return\fR

.LP

This conductor establishes the signal common reference potential

for unbalanced double\(hycurrent interchange circuits with electrical

characteristics according to Recommendation\ V.28. In the case of

interchange circuits according to Recommendations\ V.10 and\ V.11, it

interconnects the zero volt reference points of a generator and a receiver
to reduce environmental signal interference, if required.

.LP

Within the DCE, this conductor shall be brought to one point,

protective ground or earth, by means of a metallic strap within the equipment.
This metallic strap can be connected or removed at installation, as may
be

required, to minimize the introduction of noise into electronic circuitry
or to meet applicable regulations.

.LP

\fINote\fR

\ \(em\ Where a shielded interconnecting cable is used at the

interface, the shield may be connected either to circuit\ G, or to protective

ground in accordance with national regulations. Protective ground may be

further connected to external grounds as required by applicable electrical

safety regulations.

.LP



For unbalanced interchange circuits with electrical characteristics in
accordance with Recommendation\ V.10, two common\(hyreturn conductors are
required, one for each direction of signalling, each conductor being connected
to ground only on the generator side of the interface. Where used, these
shall be

designated circuits\ Ga and\ Gb, and they are defined as follows:

.LP



\fICircuit\ Ga\ \(em\ DTE common return\fR

.LP

This conductor is connected to the DTE circuit common and is used

as the reference potential for the unbalanced\ X.26 type interchange circuit

receivers within the DCE.

.LP

\fICircuit\ Gb\ \(em\ DCE common return\fR

.LP

This conductor is connected to the DCE circuit common and is used

as the reference potential for the unbalanced\ X.26 type interchange circuit

receivers within the DTE.

.LP

3.2

\fICircuit\ T\ \(em\ Transmit\fR

.LP

\fIDirection:\fR

To DCE

.LP

The binary signals originated by the DTE to be transmitted during the data
transfer phase via the data circuit to one or more remote DTEs are

transferred on this circuit to the DCE.

.LP





This circuit also transfers the call control signals originated by the
DTE, to be transmitted to the DCE in the call establishment and other call

control phases as specified by the relevant Recommendations for the procedural
characteristic of the interface.

.LP

The DCE monitors this circuit for detection of electrical circuit

fault conditions, according to the specifications of the electrical

characteristics of the interface. A circuit fault is to be interpreted
by the DCE as defined in the Recommendation for the procedural characteristics
of

the interface.

.LP

3.3

\fICircuit\ R\ \(em\ Receive\fR

.LP

\fIDirection:\fR

\ From DCE

.LP

The binary signals sent by the DCE as received during the data

transfer phase from a remote DTE, are transferred on this circuit to the DTE.

.LP



This circuit also transfers the call control signals sent by the DCE as
received during the call establishment and other call control phases as

specified by the relevant Recommendations for the procedural characteristics
of the interface.

.LP



The DTE monitors this circuit for detection of electrical circuit

fault conditions, according to the specifications of the electrical

characteristics of the interface. A circuit fault is to be interpreted
by the DTE as defined in the Recommendation for the procedural characteristics
of

the interface.

.LP

3.4

\fICircuit\ C\ \(em\ Control\fR

.LP

\fIDirection:\fR

\ To DCE

.LP

Signals on this circuit control the DCE for a particular signalling process.

.LP

Representation of a control signal requires additional coding of

circuit\ T\(hy\fITransmit\fR

\|as specified in the relevant Recommendation for the

procedural characteristics of the interface. During the data phase, this

circuit shall remain ON. During the call control phases, the condition
of this circuit shall be as specified in the relevant Recommendation for
the procedural characteristics of the interface.

.LP

\fINote\fR

\ \(em\ After appropriate selection of special user facilities (not yet
defined), it might be required to change the ON condition after entering

the data phase in accordance with the regulations for the use of these

facilities. This subject is for further study.

.LP

The DCE monitors this circuit for detection of electrical circuit

fault conditions, according to the specifications of the electrical

characteristics of the interface. A circuit fault is to be interpreted
by the DCE as defined in the Recommendation for the procedural characteristics
of

the interface.

.LP

3.5

\fICircuit\ I\ \(em\ Indication\fR

.LP

\fIDirection:\fR

\ From DCE

.LP

Signals on this circuit indicate to the DTE the state of the call control
process.

.LP

Representation of a control signal requires additional coding of

circuit\ R\(hy\fIReceive\fR

, as specified in the relevant Recommendation for the

procedural characteristics of the interface. The ON condition of this circuit
signifies that signals on circuit\ R contain information from the distant
DTE. The OFF condition signifies a control signalling condition which is
defined by the bit sequence on circuit\ R as specified by the procedural
characteristics

of the interface.

.LP



The DTE monitors this circuit for detection of electrical circuit

fault conditions, according to the specifications of the electrical

characteristics of the interface. A circuit fault is to be interpreted
by the DTE as defined in the Recommendation for the procedural characteristics
of

the interface.

.LP

\fINote\fR

\ \(em\ For use with special user facilities (not yet defined) it

might be required to use the OFF condition after entering the data transfer

phase in accordance with the regulations for the use of these facilities.

This subject is for further study.

.LP

3.6

\fICircuit\ S\ \(em\ \fR

\fISignal element timing\fR

.LP

\fIDirection:\fR

\ From DCE

.LP

Signals on this circuit provide the DTE with signal element timing information.
The condition of this circuit shall be ON and OFF for nominally

equal periods of time. However, for burst isochronous operations, longer

periods of OFF condition may be permitted equal to an integer odd number of

the nominal period of the ON condition as specified by the relevant procedural
characteristics of the interface.

.LP





The DTE shall present a binary signal on circuit\ T\(hy\fITransmit\fR

\|and a   condition on circuit\ C\(hy\fIControl\fR

, in which the transitions nominally occur at the time of the transitions
from OFF to ON condition of this circuit.

.LP



The DCE presents a binary signal on circuit\ R\(hy\fIReceive\fR

\|and a

condition on circuit\ I\(hy\fIIndication\fR

 \|in which the transitions nominally occur at the time of the transitions
from OFF to ON condition of this circuit.

.LP



The transition from ON to OFF condition shall nominally indicate the centre
of each signal element on circuit\ R.

.LP

The DCE shall transfer signal element timing information on this

circuit across the interface at all times that the timing source is capable
of generating this information.

.LP

3.7

\fICircuit\ B\ \(em\ \fR

\fIByte timing\fR

(see Note 2)

.LP

\fIDirection:\fR

\ From DCE

.LP

Signals on this circuit provide the DTE with 8\(hybit byte timing

information. The condition of this circuit shall be OFF for nominally the

period of the ON condition of circuit\ S\(hy\fISignal element timing\fR

 which indicates the last bit of an 8\(hybit byte and shall be ON at all
other times within the

period of the 8\(hybit byte.

.LP





During the call control phases, the call control characters and steady
state conditions used for all information transfers between the DCE and
the

DTE, in either direction, shall be correctly aligned to the signals of

circuit\ B.

.LP

The DTE shall present the beginning of the first bit of each call

control character on circuit\ T\(hy\fITransmit\fR

nominally at the time of the

OFF to ON transition of circuit\ S which follows the OFF to ON transition of

circuit\ B.

.LP

A change of condition of circuit\ C\(hy\fIControl\fR

\|may occur at any

OFF to ON transition of circuit\ S, but it will be sampled in the DCE at the

time of the OFF to ON transition of circuit\ B, i.e., for evaluation of the

following call control character on circuit\ T.

.LP

The centre of the last bit of each call control character will be

presented by the DCE on circuit\ R\(hy\fIReceive\fR

nominally at the time of the

OFF to ON transition of circuit\ B.

.LP

A change of condition of circuit\ I\(hy\fIIndication\fR

 \|will occur nominally at the OFF to ON transition of circuit\ S which
follows the OFF to ON

transition of circuit\ B.

.LP





The DCE shall transfer byte timing information on this circuit across the
interface at all times that the timing source is capable of generating
this information.

.LP

\fINote\ 1\fR

\ \(em\ During the data transfer phase, DTEs communicating by

means of an 8\(hybit code may utilize the byte timing information for mutual

character alignment.

.LP

It is a prerequisite for the provision of this feature that

character alignment is preserved after the call has entered the data transfer
phase and that the alignment obtained at one interface is synchronized
to the alignment at the other interface. (This is only possible on some
connections.)

.LP

Furthermore, where this feature is available, a change of condition

on circuit\ C as defined above may result in an equivalent change in the

relative alignment on circuit\ I at the distant interface.

.LP

\fINote\ 2\fR

\ \(em\ In some Recommendations for the procedural

characteristics of the interface (e.g.,\ X.21), the use and termination
of this circuit by the DTE is not mandatory even when implemented in the
DCE.

.LP



3.8

\fICircuit\ F\ \(em\ Frame start identification\fR

.LP

\fIDirection:\fR

\ From DCE

.LP

Signals on this circuit continuously provide the DTE with a

multiplex frame start indication when connected to a multiplexed DTE/DCE

interface.

.LP

The condition on this circuit shall be OFF for the nominal period of one
bit, indicating the last bit of the multiplex frame. At other times the

circuit shall remain\ ON.

.LP

The first data bit on subscriber channel 1 shall be transmitted or

received beginning nominally at the OFF to ON transition of circuit\ F.

.LP



3.9

\fICircuit\ X\ \(em\ DTE transmit signal element timing\fR

(see Note)

.LP



\fIDirection:\fR

\ To DCE

.LP

Signals on this circuit provide signal element timing information for the
transmit direction in cases where circuit\ S only provides signal

element timing for the receive direction. The condition of this circuit
shall be ON and OFF for nominally equal periods of time. However, for burst

isochronous operations, longer periods of OFF condition may be permitted

equal to an integer odd number of the nominal period of the ON condition as

specified by the relevant procedural characteristics of the interface.

.LP



The DTE shall present a binary signal on the circuit T\(hy\fITransmit\fR

\|and a condition on circuit\ C\(hy\fIControl\fR

, in which the transitions nominally occur at the time of the transitions
from OFF to ON condition of this circuit.

.LP



The transition from ON to OFF condition shall nominally indicate the centre
of each signal element on circuit\ T.

.LP

\fINote\fR

\ \(em\ The use and termination of this circuit by the DCE is a

national matter.

.LP



\fBRecommendation X.25\fR

.line

\fBINTERFACE\ BETWEEN\ DATA\ TERMINAL\ EQUIPMENT\ (DTE)\ AND\ DATA\fR



\fBCIRCUIT\(hyTERMINATING\ EQUIPMENT\ (DCE)\ FOR\ TERMINALS\ OPERATING\fR

\fBIN\ THE\ PACKET\ MODE\ AND\ CONNECTED\ TO\ PUBLIC\fR

\fBDATA\ NETWORKS\ BY\ DEDICATED\ CIRCUIT\fR

\fI(Geneva, 1976; amended at Geneva, 1980\fR

,

\fIMalaga\(hyTorremolinos, 1984 and Melbourne, 1988)\fR



The establishment in various countries of public data networks providing
packet switched data transmission services creates a need to produce standards
to facilitate international interworking.

.LP

The\ CCITT,

.LP

\fIconsidering\fR

.LP



(a)

that Recommendation X.1 includes specific user classes of service for data
terminal equipments operating in the packet mode,

Recommendation\ X.2 defines user facilities, Recommendation\ X.10 defines

categories of access, Recommendations\ X.21 and X.21\|\fIbis\fR

define DTE/DCE

physical layer interface characteristics, Recommendation\ X.92 defines the

hypothetical reference connections for packet switched data transmission

service and Recommendation\ X.96 defines \fIcall progress\fR

signals;

.LP

(b)

that data terminal equipments operating in the packet

mode will send and receive network control information in the form of packets;

.LP



(c)

that certain data terminal equipments operating in the packet mode will
use a

packet interleaved synchronous data circuit

;

.LP

(d)

the desirability of being able to use a single data

circuit to a Data Switching Exchange\ (DSE) for all user facilities;

.LP



(e)

that Recommendation X.2 specifies which of the various data transmission
services and optional user facilities described in the

present Recommendation are \*Qessential\*U and have thus to be made available

internationally, and which are not;

.LP



(f

)

the need for defining an international

Recommendation for the exchange between DTE and DCE of control information
for the use of packet switched data transmission services;

.LP

(g)

that this definition is made in Recommendation X.32 with regard to the
access through a public switched telephone network, an integrated services
digital network (ISDN), or a circuit switched public data network;

.LP



(h)

that Recommendation X.31 defines the support of

packet\(hymode

terminal equipment by an integrated services digital network (ISDN);

.LP

(i)

that, when this Recommendation is used to support the

Network Service defined in Recommendation\ X.213, the physical, data link and

packet

layers correspond to the Physical, Data link and Network Layers respectively,
as defined in Recommendation\ X.200;

.LP

(j)

that this Recommendation includes all the features

necessary to support the services included in Recommendation\ X.213 as
well as other features; that Recommendation\ X.223 defines the use of X.25
packet layer protocol to provide the OSI connection mode Network service;

.LP

(k)

that the necessary elements for an interface

Recommendation should be defined independently as:

.LP



\fIPhysical layer\fR

\ \(em\ the mechanical, electrical, functional and

procedural characteristics to activate, maintain and deactivate

the physical link between the DTE and the DCE;

.LP

\fIData link layer\fR

\ \(em\ the link access procedure for data interchange

across the link between the DTE and the DCE;

.LP

\fIPacket layer\fR

\ \(em\ the packet format and control procedures for the

exchange of packets containing control information and user

data between the DTE and the DCE;

.LP

\fIunanimously declares\fR

.LP



that for public data networks accessed via dedicated circuits by data terminal
equipments operating in the packet mode:

.LP

(1)

the mechanical, electrical, functional and procedural

characteristics to activate, maintain and deactivate the physical link
between the DTE and the DCE should be as specified in \(sc\ 1 below, \fIDTE/DCE
interface\fR

\fIcharacteristics\fR

;

.LP

(2)

the link access procedure for data interchange across

the link between the DTE and the DCE should be as specified in \(sc\ 2 below,

\fILink access procedure across the DTE/DCE interface\fR

;

.LP

(3)

the packet layer procedures for the exchange of control information and
user data at the DTE/DCE interface should be as specified in

\(sc\ 3 below, \fIDescription of the packet layer DTE/DCE interface\fR

;

.LP

(4)

the procedures for virtual call and permanent virtual

circuit services should be as specified in \(sc\ 4 below, \fIProcedures
for virtual\fR

\fIcircuit services\fR

;

.LP



(5)

the format for packets exchanged between the DTE and the DCE should be
as specified in \(sc\ 5 below, \fIPacket formats\fR

;

.LP

(6)

the procedures for optional user facilities should be as specified in \(sc\
6 below, \fIProcedures for optional user facilities\fR

;

.LP

(7)

the formats for optional user facilities should be as

specified in \(sc\ 7 below, \fIFormats for facility fields and registration
fields\fR

.

.LP



\fINote\fR

\ \(em\ This Recommendation fully specifies the behaviour of the DCE. In
addition, a minimum set of requirements is specified for the DTE.

Additional guidance for the design of DTEs is available in ISO standards

ISO\ 7776 (data link layer) and ISO\ 8208 (packet layer). It is not required
by this Recommendation that these ISO standards be used. If using these
ISO

standards, note must be taken that their scope is expanded beyond that
of just interfacing with packet switched public data networks.

.LP

It should also be noted that this Recommendation uses the term DTE to refer
to the equipment to which the DCE interfaces. In ISO\ 8208, distinction
is made between a DTE and a packet switched private data network, which
are both considered as DTEs in this Recommendation.

.LP

CONTENTS

1

\fIDTE/DCE interface characteristics (physical layer)\fR

.LP

1.1

X.21 interface

.LP

1.2

X.21\|\fIbis\fR

interface

.LP

1.3

V\(hyseries interface

.LP

1.4

X.31 interface

.LP

2

\fILink access procedures across the DTE/DCE interface\fR

.LP

2.1

Scope and field of application

.LP

2.2

Frame structure

.LP

2.3

LAPB elements of procedures

.LP

2.4

Description of the LAPB procedure

.LP

2.5

Multilink procedure (MLP)

.LP

2.6

LAP elements of procedure

.LP

2.7

Description of the LAP procedure

.LP

3

\fIDescription of the packet layer DTE/DCE interface\fR

.LP

3.1

Logical channels

.LP

3.2

Basic structure of packets

.LP

3.3

Procedure for restart

.LP

3.4

Error handling

.LP

4

\fIProcedures for virtual circuit services\fR

.LP

4.1

Procedures for virtual call service

.LP

4.2

Procedures for permanent virtual circuit service

.LP

4.3

Procedures for data and interrupt transfer

.LP

4.4

Procedures for flow control

.LP

4.5

Effects of clear, reset and restart procedures

on the transfer of packets

.LP

4.6

Effects of the physical and the data link layer on the

packet layer

.LP

5

\fIPacket formats\fR

.LP

5.1

General

.LP



5.2

Call set\(hyup and clearing packets

.LP

5.3

Data and interrupt packets

.LP

5.4

Flow control and reset packets

.LP

5.5

Restart packets

.LP

5.6

Diagnostic packet

.LP

5.7

Packets required for optional user facilities

.LP



6

\fIProcedures for optional user facilities (packet layer)\fR

.LP



6.1

On\(hyline facility registration

.LP

6.2

Extended packet sequence numbering

.LP

6.3

D bit modification

.LP

6.4

Packet retransmission

.LP

6.5

Incoming calls barred

.LP

6.6

Outgoing calls barred

.LP

6.7

One\(hyway logical channel outgoing

.LP

6.8

One\(hyway logical channel incoming

.LP

6.9

Non\(hystandard default packet sizes

.LP

6.10

Non\(hystandard default window sizes

.LP



6.11

Default throughput classes assignment

.LP

6.12

Flow control parameter negotiation

.LP

6.13

Throughput class negotiation

.LP

6.14

Closed user group related facilities

.LP

6.15

Bilateral closed user group related facilities

.LP

6.16

Fast select

.LP

6.17

Fast select acceptance

.LP

6.18

Reverse charging

.LP

6.19

Reverse charging acceptance

.LP

6.20

Local charging prevention

.LP

6.21

Network user identification (NUI) related facilities

.LP



6.22

Charging information

.LP

6.23

RPOA related facilities

.LP

6.24

Hunt group

.LP

6.25

Call redirection and call deflection related facilities

.LP



6.26

Called line address modified notification

.LP

6.27

Transit delay selection and indication

.LP

6.28

TOA/NEI address subscription

.LP

7

\fIFormats for facility fields and registration fields\fR

.LP

7.1

General

.LP

7.2

Coding of facility field in call set\(hyup

and clearing packets

.LP

7.3

Coding of the registration field of

registration packets

.LP

\fIAnnex\ A\fR

\(em

Range of logical channels used for virtual calls

and permanent virtual circuits

.LP

\fIAnnex\ B\fR

\(em

Packet layer DTE/DCE interface state diagrams

.LP



\fIAnnex\ C\fR

\(em

Actions taken by the DCE on receipt of packets

in a given state of the packet layer DTE/DCE interface as

perceived by the DCE

.LP

\fIAnnex\ D\fR

\(em

Packet layer DCE time\(hyouts and DTE time\(hylimits

.LP



\fIAnnex\ E\fR

\(em

Coding of X.25 network generated

diagnostic fields in clear, reset and restart indication,

registration confirmation, and diagnostic packets

.LP

\fIAnnex\ F\fR

\(em

Applicability of the on\(hyline registration facility

to other facilities

.LP

\fIAnnex\ G\fR

\(em

CCITT\(hyspecified DTE facilities to support the

OSI Network service

.LP

\fIAnnex\ H\fR

\(em

Subscription\(hytime optional user facilities that may be

associated with a network user identifier in conjunction with the NUI

override facility

.LP

\fIAppendix\ I\fR

\(em

Examples of data link layer transmitted bit patterns by   the DCE and the DTE

.LP

\fIAppendix\ II\fR

\(em

An explanation of how the values for N1 in \(sc 2.4.8.5 are derived

.LP

\fIAppendix\ III\fR

\(em

Examples of multilink resetting procedures

.LP



\fIAppendix\ IV\fR

\(em

Information on addresses in call set\(hyup and clearing

packets

.LP

\fB1\fR

\fBDTE/DCE interface characteristics\fR

\fB(physical layer)\fR

.LP



Administrations may offer one or more of the interfaces specified below.
The exact use of the relevant points in these Recommendations is

detailed below.

.LP

1.1

\fIX.21 interface\fR

.LP

1.1.1

\fIDTE/DCE physical interface elements\fR

.LP

The DTE/DCE physical interface elements shall be according to

\(sc\(sc\ 2.1 through\ 2.5 of Recommendation\ X.21.

.LP

1.1.2

\fIProcedures for entering operational phases\fR

.LP

The procedures for entering operational phases shall be as

described in \(sc\ 5.2 of Recommendation\ X.21. The data exchanged on circuits\
T

and\ R when the interface is in states\ 13S, 13R and\ 13 of Figure A\(hy3/X.21
will be as described in subsequent sections of this Recommendation.

.LP

The \fInot ready\fR

\| states given in \(sc\ 2.5 of Recommendation X.21 are

considered to be \fInon\(hyoperational\fR

states and may be considered by the higher   layers to be \fIout of order\fR

states (see \(sc\ 4.6 below).

.LP

1.1.3

\fIFailure detection and test loops\fR

.LP

The failure detection principles shall be according to \(sc\ 2.6 of

Recommendation\ X.21. In addition, i\ =\ OFF may be signalled due to momentary

transmission failures. Higher layers may delay for several seconds before

considering the interface to be out of order.

.LP

The definitions of test loops and the principles of maintenance

testing using the test loops are provided in Recommendation\ X.150.

.LP

A description of the test loops and the procedures for their use is

given in \(sc\ 7 of Recommendation\ X.21.

.LP



Automatic activation by a DTE of a test loop\ 2 in the DCE at the

remote terminal is not possible. However, some Administrations may permit
the DTE to control the equivalent of a test loop\ 2, at the local DSE,
to verify the operation of the leased line or subscriber line and/or all
or part of the DCE or line terminating equipment. Control of the loop,
if provided, may be either manual or automatic, as described in Recommendations\
X.150 and\ X.21

respectively.

.LP

1.1.4

\fISignal element timing\fR

.LP

Signal element timing shall be in accordance with \(sc\ 2.6.3 of

Recommendation\ X.21.

.LP

1.2

\fIX.21\|bis interface\fR

.LP

1.2.1

\fIDTE/DCE physical interface elements\fR

.LP

The DTE/DCE physical interface elements shall be according to \(sc\ 1.2
of Recommendation\ X.21\|\fIbis\fR

.

.LP

1.2.2

\fIOperational phases\fR

.LP

When circuit 107 is in the ON condition, and circuits\ 105, 106, 108 and
109, if provided, are in the ON condition, data exchange on circuits\ 103

and\ 104 will be as described in subsequent sections of this

Recommendation.

.LP



When circuit 107 is in the OFF condition, or any of circuits 105, 106,
108 or\ 109, if provided, are in the OFF condition, this is considered
to be in a \fInon\(hyoperational\fR

 state, and may be considered by the higher layers to be in an \fIout of
order\fR

state (see \(sc\ 4.6 below).

.LP

1.2.3

\fIFailure detection and test loops\fR

.LP

The failure detection principles, the description of test loops and the
procedures for their use shall be according to \(sc\(sc\ 3.1 through\ 3.3
of

Recommendation\ X.21\|\fIbis\fR

. In addition, circuits\ 106 and\ 109 may enter the OFF condition due to
momentary transmission failures. Higher layers may delay for several seconds
before considering the interface to be out of order.

.LP



Automatic activation by a DTE of test loop 2 in the DCE at the remote terminal
is not possible. However, some Administrations may permit the DTE to control
the equivalent of a test loop\ 2, at the local DSE, to verify the

operation of the leased line or subscriber line and/or all or part of the
DCE or line terminating equipment. Control of the loop, if provided, may
be either manual or automatic, as described in Recommendations\ X.150 and\
X.21\|\fIbis\fR



respectively.

.LP

1.2.4

\fISignal element timing\fR

.LP

Signal element timing shall be in accordance with \(sc\ 3.4 of

Recommendation\ X.21\|\fIbis\fR

.

.LP

1.3

\fIV\(hySeries interface\fR

.LP

General operation with V\(hySeries modems is as described in \(sc\ 1.2

above. However, for specific details, particularly related to failure detection
principles, loop testing, and the use of circuits\ 107, 109, 113 and\ 114,
refer to the appropriate V\(hySeries Recommendations.

.LP

The delay between 105\(hyON and 106\(hyON (when these

circuits are present) will be more than 10\ ms and less than 1\ s. In addition,
circuits\ 106 or\ 109 may enter the OFF condition due to momentary transmission
failures or modem retraining. Higher layers may delay for several seconds

before considering the interface to be out of order.

.LP

1.4

\fIX.31 interface\fR

.LP

1.4.1

\fIDTE/DCE physical interface\fR

.LP

The DTE/DCE physical interface shall coincide with the R reference point
between the DTE and the Terminal Adaptor (TA). The purpose of the TA is

to allow the operation of a DTE over an ISDN. The functionalities of such
a TA when accessing a packet switched data transmission service through
a

semi\(hypermanent ISDN connection (i.e.,\ a non switched B\(hychannel)
are described in \(sc\ 7 of Recommendation\ X.31.

.LP

\fINote\ 1\fR

\ \(em\ This type of access is considered a dedicated access to a public
switched data transmission service. Non dedicated access to a public

switched data transmission service is defined in Recommendations\ X.32

and\ X.31.

.LP

\fINote\ 2\fR

\ \(em\ The DTE and the TA functionalities may be implemented in

the same piece of equipment in the case of a packet mode terminal TE1

conforming to the I\(hyseries Recommendations. In this case, this

Recommendation covers layer\ 2 and layer\ 3 operation on the semi\(hypermanent

B\(hychannel.

.LP

1.4.2

\fIOperational phases\fR

.LP

The operational phases are as described in \(sc 7 of

Recommendation\ X.31.

.LP

1.4.3

\fIMaintenance\fR

.LP



The maintenance shall be made as described in \(sc 7.6 of

Recommendation\ X.31.

.LP

1.4.4

\fISynchronization\fR

.LP

The synchronization shall be made as described in \(sc 7 of

Recommendation\ X.31.

.LP

\fB2\fR

\fBLink access procedures across the DTE/DCE interface\fR

.LP



2.1

\fIScope and field of applications\fR

.LP

2.1.1

The

Link Access Procedures

(LAPB and LAP) are

described as the

Data Link Layer Element

and are used for data

interchange

between a DCE and a DTE over a single physical circuit (LAPB and LAP), or

optionally over multiple physical circuits (LAPB), operating in user classes
of service\ 8 to\ 11 as indicated in Recommendation\ X.1. The optional,

subscription\(hytime selectable, multiple physical circuit operation with LAPB

(known as multilink operation) is required if the effects of circuit failures
are not to disrupt the Packet Layer operation.

.LP

The single link procedures (SLPs) described in \(sc\(sc\ 2.2, 2.3

and\ 2.4 (LAPB) and in \(sc\(sc\ 2.2, 2.6 and 2.7 (LAP) are used for data
interchange over a single physical circuit, conforming to the description
given in \(sc\ 1,

between a DTE and a DCE. When the optional multilink operation is employed
with LAPB, a single link procedure (SLP) is used independently on each
physical

circuit, and the multilink procedure (MLP) described in \(sc\ 2.5 is used for

data interchange over these multiple parallel LAPB data links. In addition,

when only a single physical circuit is employed with LAPB, agreements may be

made with the Administration to use this optional

multilink procedure

over the one LAPB data link.

.LP

2.1.2

The

single link procedures (SLPs)

use the principles

and terminology of the High\(hylevel Data Link Control (HDLC) procedures

specified by the International Organization for Standardization (ISO). The

multilink pro

cedure\ (MLP) is based on the principles and terminology of the Multilink
Control Procedures specified by ISO.

.LP

2.1.3

Each transmission facility is duplex.

.LP

2.1.4

DCE compatibility of operation with the ISO balanced classes

of procedure (Class BA with options\ 2, 8 and Class\ BA with options\ 2,
8, 10) is achieved using the LAPB procedure described in \(sc\(sc\ 2.3
and\ 2.4. Of these

classes, Class\ BA with options\ 2, 8 (LAPB modulo\ 8) is the basic service,
and is available in all networks. Class\ BA with options\ 2, 8, 10 (LAPB
modulo\ 128) is recognized as an optional, subscription\(hytime selectable,
extended sequence numbering service that may be available in those networks
wishing to serve DTE applications having a need for modulo\ 128 sequence
numbering.

.LP



DTE manufacturers and implementors must be aware that the

procedure hereunder described as LAPB modulo\ 8 will be the only one available
in all networks.

.LP

Likewise, a DTE may continue to use the LAP procedure described in

\(sc\(sc\ 2.2, 2.6 and\ 2.7 (in those networks supporting such a procedure),
but for

new

DTE implementations, LAPB should be preferred. The LAP procedures are defined
for modulo\ 8 basic service only.

.LP

\fINote\fR

\ \(em\ Other possible applications for further study are, for

example:

.LP

\(em

two\(hyway alternate, asynchronous response mode;

.LP

\(em

two\(hyway simultaneous, normal response mode;

.LP

\(em

two\(hyway alternate, normal response mode.

.LP



2.1.5

For those networks that choose to support both the basic and

extended LAPB sequence numbering services, the choice of either basic mode

(modulo\ 8) or extended mode (modulo\ 128) may be made at subscription
time. The choice of the mode employed for each data link procedure is independent
of all others and of the choice of mode for the corresponding Packet Layer
procedures. All choices are matters for agreement for a period of time
with the

Administration.

.LP

2.1.6

In the case of those networks that support both the LAPB procedure and
the LAP procedure, the DCE will maintain an internal mode variable\ B,
which it will set as follows:

.LP

\(em

to 1, upon acceptance of an SABM/SABME (modulo 8/modulo\ 128)

command from the DTE, or upon issuance of an SABM/SABME command

by the DCE;

.LP

\(em

to 0, upon acceptance of an SARM command from the DTE.

.LP



Whenever B is 1, the DCE will use the LAPB procedure described in \(sc\(sc\
2.2, 2.3 and 2.4 below, and is said to be in the LAPB (balanced) mode.

.LP



Whenever B is 0, the DCE will use the LAP procedure described in

\(sc\(sc\ 2.2, 2.6 and 2.7 below, and is said to be in the LAP mode.

.LP

Changes to the mode variable B by the DTE should occur only when the data
link has been disconnected as described in \(sc\(sc\ 2.4.4.3 or\ 2.7.3.3
below.

.LP



Should a DCE malfunction occur that negates the current setting of

internal mode variable\ B, the DCE will, upon restoration of operation,
not send either a SARM or SABM/SABME command. The DCE may send a DISC command
or a

DM\ response to notify the DTE that the DCE is in the disconnected phase.
This will result in the DTE attempting to reinitialize the data link with
what the DTE

considers to be the proper mode\(hysetting command (SARM or SABM/SABME).
The DCE will then be able to set the internal mode variable\ B to its proper

value.

.LP

2.2

\fIFrame structure\fR

.LP

2.2.1

All transmissions on an SLP are in frames conforming to one of the formats
of Table\ 1/X.25 for basic (modulo\ 8) operation, or alternatively one
of the formats of Table\ 2/X.25 for extended (modulo\ 128) operation. The
flag

preceding the address field is defined as the opening flag. The flag following
the FCS field is defined as the closing flag.

.LP

.line

\fBTable 1/X.25 [T1.25], p.\fR

.line

\fBTable 2/X.25 [T2.25], p.\fR

2.2.2

\fIFlag sequence\fR

.LP

All frames shall start and end with the flag sequence consisting of one
0\ bit followed by six contiguous 1\ bits and one 0\ bit. The DTE and DCE

shall only send complete eight\(hybit flag sequences when sending multiple flag

sequences (see \(sc\ 2.2.11). A single flag may be used as both the

closing

flag

for one frame and the

opening flag

for the next frame.

.LP

2.2.3

\fIAddress field\fR

.LP

The address field shall consist of one octet. The address field

identifies the intended receiver of a command frame and the transmitter of a

response frame. The coding of the address field is described in \(sc\ 2.4.2
(LAPB) and in \(sc\ 2.7.1 (LAP) below.

.LP

2.2.4

\fIControl field\fR

.LP

For modulo\ 8 (basic) operation, the control field shall consist of one
octet. For modulo\ 128 (extended) operation, the control field shall consist
of two octets for frame formats that contain sequence numbers, and one
octet

for frame formats that do not contain sequence numbers. The content of this

field is described in \(sc\ 2.3.2\ (LAPB) and in \(sc\ 2.6.2 (LAP) below.

.LP

2.2.5

\fIInformation field\fR

.LP

The information field of a frame, when present, follows the control field
(see \(sc\ 2.2.4 above) and precedes the

frame check sequence field

(see \(sc\ 2.2.7 below).

.LP

See \(sc\(sc 2.3.4.9, 2.5.2, 2.6.4.8 and 5 for the various codings and

groupings of bits in the information field as used in this Recommendation.

.LP



See \(sc\(sc 2.3.4.9, 2.4.8.5, 2.6.4.8 and 2.7.7.5 below with regard to
the maximum information field length.

.LP

2.2.6

\fITransparency\fR

.LP

The DCE or DTE, when transmitting, shall examine the frame content between
the two flag sequences including the address, control, information and
FCS fields and shall insert a 0 bit after all sequences of 5 contiguous
1\ bits (including the last 5\ bits of the FCS) to ensure that a flag sequence
is not

simulated. The DCE or DTE, when receiving, shall examine the frame content
and shall discard any 0\ bit which directly follows\ 5 contiguous 1\ bits.

.LP

2.2.7

\fIFrame check sequence (FCS)\fR

\fI field\fR

.LP

The notation used to describe the FCS is based on the property of cyclic
codes that a code vector such as 1000000100001 can be represented by a
polynomial \fIP\fR

(\fIx\fR

)\ =\ \fIx\fR

\u1\d\u2\d\ +\ \fIx\fR

\u5\d\ +\ 1. The elements of an \fIn\fR

\(hyelement

code word are thus the coefficients of a polynomial of order \fIn\fR

\ \(em\ 1. In this application, these coefficients can have the value\
0 or\ 1 and the polynomial

operations are performed modulo\ 2. The polynomial representing the content
of a frame is generated using the first bit received after the frame opening
flag as the coefficient of the highest order term.

.LP

The FCS field shall be a 16\(hybit sequence. It shall be the ones

complement of the sum (modulo\ 2) of:

.LP

1)

the remainder of

\fIx\fR

\uD\dlFk\fR

(\fIx\fR

\u1\d\u5\d\uD\dlF036+\ \fIx\fR

\u1\d\u4\d\ +\ \fIx\fR

\u1\d\u3\d\ +

\fIx\fR

\u1\d\u2\d\ +\ \fIx\fR

\u1\d\u1\d\ +\ \fIx\fR

\u1\d\u0\d\ +\ \fIx\fR

\u9\d\ +\ \fIx\fR

\u8\d\ +

\fIx\fR

\u7\d\ +\ \fIx\fR

\u6\d\ +\ \fIx\fR

\u5\d\ +\ \fIx\fR

\u4\d\uD\dlF036+\ \fIx\fR

\u3\d\ +

+\ \fIx\fR

\u2\d\ +\ \fIx\fR

\ +\ 1) divided (modulo\ 2)

by the

generator polynomial \fIx\fR

\u1\d\u6\d\ +\ \fIx\fR

\u1\d\u2\d\ +\ \fIx\fR

\u5\d\ +\ 1,

where \fIk\fR

is the number of bits in the frame existing between,

but not including, the final bit of the opening flag and the

first bit of the FCS, excluding bits inserted for transparency,

and

.LP

2)

the remainder of the division (modulo 2) by the generator

polynomial \fIx\fR

\u1\d\u6\d\ +\ \fIx\fR

\u1\d\u2\d\ +\ \fIx\fR

\u5\d\ +\ 1 of the

product of \fIx\fR

\u1\d\u6\d by the content of the frame, existing

between but not including, the final bit of the opening flag and

the first bit of the FCS, excluding bits inserted for

transparency.

.LP



As a typical implementation, at the transmitter, the initial

content of the register of the device computing the remainder of the division
is preset to all 1s and is then modified by division by the generator

polynomial (as described above) on the address, control and information
fields; the ones complement of the resulting remainder is transmitted as
the 16\(hybit

FCS.

.LP

At the receiver, the initial content of the register of the device

computing the remainder is preset to all 1s. The final remainder, after

multiplication by \fIx\fR

\u1\d\u6\d and then division (modulo\ 2) by the generator

polynomial \fIx\fR

\u1\d\u6\d\ +\ \fIx\fR

\u1\d\u2\d\ +\ \fIx\fR

\u5\d\ +\ 1 of the serial incoming protected bits and the FCS, will be
0001110100001111 (\fIx\fR

\u1\d\u5\d through

\fIx\fR

\u0\d, respectively) in the absence of transmission errors.

.LP

\fINote\fR

\ \(em\ Examples of transmitted bit patterns by the DCE and the DTE illustrating
application of the transparency mechanism and the frame check

sequence to the SABM command and the UA response are given in Appendix\ I.

.LP

2.2.8

\fIOrder of bit transmission\fR

.LP

Addresses, commands, responses and sequence numbers shall be

transmitted with the low\(hyorder bit first (for example, the first bit of the

sequence number that is transmitted shall have the weight 2\u0\d). The
order of transmitting bits within the information field is not specified
under \(sc\ 2 of

this Recommendation. The FCS shall be transmitted to the line commencing
with the coefficient of the highest term, which is found in bit position\
16 of the FCS\ field (see Tables\ 1/X.25 and 2/X.25).

.LP

\fINote\fR

\ \(em\ In Tables 1/X.25 to 13/X.25, bit 1 is defined as the

low\(hyorder bit.

.LP

2.2.9

\fIInvalid frames\fR

.LP

The definition of an invalid frame is described in \(sc\ 2.3.5.3 (LAPB)
and in \(sc\ 2.6.5.3 (LAP) below.

.LP

2.2.10

\fIFrame abortion\fR

.LP

Aborting a frame is performed by transmitting at least seven

contiguous 1\ bits (with no inserted 0\ bits).

.LP

2.2.11

\fIInterframe time fill\fR

.LP

Interframe time fill is accomplished by transmitting contiguous

flags between frames, i.e.\ multiple eight\(hybit flag sequences (see

\(sc\ 2.2.2).

.LP

2.2.12

\fILink channel states\fR

.LP

A link channel as defined here is the means for transmission for

one direction.

.LP

2.2.12.1

\fIActive channel state\fR

.LP

The DCE incoming or outgoing channel is defined to be in an active condition
when it is receiving or transmitting, respectively, a frame, an

abortion sequence or interframe time fill.

.LP

2.2.12.2

\fIIdle channel state\fR

.LP

The DCE incoming or outgoing channel is defined to be in an idle

condition when it is receiving or transmitting, respectively, a continuous\
1s state for a period of at least 15\ bit times.

.LP

See \(sc\ 2.3.5.5 for a description of DCE action when an idle condition
exists on its incoming channel for an excessive period of time.

.LP



2.3

\fILAPB elements of procedures\fR

.LP

2.3.1

The LAPB elements of procedures are defined in terms of actions

that occur on receipt of frames at the DCE or\ DTE.

.LP

The elements of procedures specified below contain the selection

of commands and responses relevant to the LAPB data link and system

configurations

described in \(sc\ 2.1 above. Together, \(sc\(sc\ 2.2 and 2.3 form the general

requirements for the proper management of a LAPB access data link.

.LP



2.3.2

\fILAPB control field formats and parameters\fR

.LP



2.3.2.1

\fIControl field formats\fR

.LP

The control field contains a command or a response, and sequence

numbers where applicable.

.LP

Three types of control field formats are used to perform numbered

information transfer (I\ format), numbered supervisory functions (S\ format)
and unnumbered control functions (U\ format).

.LP

The control field formats for basic (modulo\ 8) operation are depicted
in Table\ 3/X.25.

.LP



The control field formats for extended (modulo 128) operation are

depicted in Table\ 4/X.25.

.LP

.line

\fBTable 3/X.25 [T3.25], p.\fR

.line

\fBTable 4/X.25 [T4/X.25], p.\fR

2.3.2.1.1

\fIInformation transfer format\fR

\fI \(em I\fR

.LP

The I format is used to perform an information transfer. The

functions of N(S), N(R) and P are independent; i.e.,\ each I\ frame has
an N(S), an N(R) which may or may not acknowledge additional I\ frames
received by the

DCE or DTE, and a P\ bit that may be set to\ 0 or\ 1.

.LP

2.3.2.1.2

\fISupervisory format\fR

\fI \(em S\fR

.LP

The S format is used to perform data link supervisory control

functions such as acknowledge I\ frames, request retransmission of I\ frames,
and to request a temporary suspension of transmission of I\ frames. The
functions of N(R) and P/F are independent; i.e.,\ each supervisory frame
has an N(R) which

may or may not acknowledge additional I\ frames received by the DCE or\
DTE, and a P/F\ bit that may be set to\ 0 or\ 1.

.LP

2.3.2.1.3

\fIUnnumbered format\fR

\fI \(em U\fR

.LP

The U format is used to provide additional data link control

functions. This format contains no sequence numbers, but does include a
P/F\ bit that may be set to\ 0 or\ 1. The unnumbered frames have the same
control field

length (one octet) in both basic (modulo\ 8) operation and extended (modulo\
128) operation.

.LP

2.3.2.2

\fIControl field parameters\fR

.LP

The various parameters associated with the control field formats

are described below.

.LP

2.3.2.2.1

\fIModulus\fR

.LP

Each I frame is sequentially numbered and may have the value\ 0

through modulus minus\ 1 (where \*Qmodulus\*U is the modulus of the sequence

numbers). The modulus equals either\ 8 or\ 128 and the sequence numbers cycle

through the entire range.

.LP

2.3.2.2.2

\fISend state variable\fR

\fI V(S)\fR

.LP

The send state variable V(S) denotes the sequence number of the

next in\(hysequence I\ frame to be transmitted. V(S) can take on the values\ 0

through modulus minus\ 1. The value of V(S) is incremented by\ 1 with each

successive I\ frame transmission, but cannot exceed the N(R) of the last

received\ I or supervisory frame by more than the maximum number of outstanding
I\ frames\ (\fIk\fR

). The value of\ k is defined in \(sc\ 2.4.8.6 below.

.LP



2.3.2.2.3

\fISend sequence number\fR

\fI N(S)\fR

.LP

Only I frames contain N(S), the send sequence number of transmitted I\
frames. At the time that an in\(hysequence I\ frame is designated for

transmission, the value of N(S) is set equal to the value of the send state

variable\ V(S).

.LP

2.3.2.2.4

\fIReceive state variable\fR

\fIV(R)\fR

.LP

The receive state variable V(R) denotes the sequence number of the next
in\(hysequence I\ frame expected to be received. V(R) can take on the values
0 through modulus minus\ 1. The value of V(R) is incremented by\ 1 by the
receipt of an error\(hyfree, in\(hysequence I\ frame whose send sequence
number N(S) equals

the receive state variable V(R).

.LP

2.3.2.2.5

\fIReceive sequence number\fR

\fI N(R)\fR

.LP

All I frames and supervisory frames contain N(R), the expected send sequence
number of the next received I\ frame. At the time that a frame of the above
types is designated for transmission, the value of N(R) is set equal to
the current value of the receive state variable V(R). N(R) indicates that
the DCE or DTE transmitting the N(R) has received correctly all I\ frames
numbered up to and including N(R)\ \(em\ 1.

.LP

2.3.2.2.6

\fIPoll/Final bit\fR

\fI P/F\fR

.LP

All frames contain P/F, the Poll/Final bit. In command frames, the P/F
bit is referred to as the P bit. In response frames, it is referred to
as the F\ bit.

.LP

2.3.3

\fIFunctions of the Poll/Final bit\fR

.LP

The Poll bit set to 1 is used by the DCE or DTE to solicit (poll) a response
from the DTE or DCE, respectively. The Final bit set to\ 1 is used by the
DCE or DTE to indicate the response frame transmitted by the DTE or DCE,

respectively, as a result of the soliciting (poll) command.

.LP

The use of the P/F bit is described in \(sc\ 2.4.3 below.

.LP

2.3.4

\fICommands and responses\fR

.LP

For basic (modulo 8) operation, the commands and responses

represented in Table\ 5/X.25 will be supported by the DCE and the\ DTE.

.LP



For extended (modulo 128) operation, the commands and responses

represented in Table\ 6/X.25 will be supported by the DCE and the DTE.

.LP



For purposes of the LAPB procedures, the supervisory function bit encoding
\*Q11\*U and those encodings of the modifier function bits in

Tables\ 3/X.25 and 4/X.25 not identified in Tables\ 5/X.25 or 6/X.25 are

identified as \*Qundefined or not implemented\*U command and response control

fields.

.LP

The commands and responses in Tables 5/X.25 and 6/X.25 are defined as  follows:

.LP

2.3.4.1

\fIInformation (I)\fR

\fI command\fR

.LP

The function of the information (I) command is to transfer across a data
link a sequentially numbered frame containing an information field.

.LP

.line

\fBTable 5/X.25 [T5.25], p.\fR

.line

\fBTable 6/X.25 [T6.25], p.\fR

2.3.4.2

\fIReceive ready (RR)\fR

\fI command and response\fR

.LP

The receive ready (RR) supervisory frame is used by the DCE

or DTE to:

.LP



1)

indicate it is ready to receive an I frame; and

.LP

2)

acknowledge previously received I frames numbered up to and

including N(R)\ \(em\ 1.

.LP

An RR frame may be used to indicate the clearance of a busy

condition that was reported by the earlier transmission of an RNR frame
by that same station (DCE or\ DTE). In addition to indicating the DCE or
DTE status, the RR command with the P\ bit set to\ 1 may be used by the
DCE or DTE to ask for the status of the DTE or DCE, respectively.

.LP

2.3.4.3

\fIReceive not ready (RNR) command and response\fR

.LP

The receive not ready (RNR) supervisory frame is used by the DCE or DTE
to indicate a busy condition; i.e.\ temporary inability to accept

additional incoming I\ frames. I\ frames numbered up to and including N(R)\
\(em\ 1

are acknowledged. I\ frame N(R) and any subsequent I\ frames received, if any,

are not acknowledged; the acceptance status of these I\ frames will be
indicated in subsequent exchanges.

.LP

In addition to indicating the DCE or DTE status, the RNR command with the
P bit set to\ 1 may be used by an DCE or DTE to ask for the status of the

DTE or DCE, respectively.

.LP

2.3.4.4

\fIReject (REJ) command and response\fR

.LP

The reject (REJ) supervisory frame is used by the DCE or DTE to request
transmission of I\ frames starting with the frame numbered N(R).

I\ frames numbered N(R)\ \(em\ 1 and below are acknowledged. Additional
I\ frames

pending initial transmission may be transmitted following the retransmitted

I\ frame(s).

.LP



Only one REJ exception condition for a given direction of information transfer
may be established at any time. The REJ exception condition is cleared
(reset) upon the receipt of an I\ frame with an N(S) equal to the N(R)
of the

REJ\ frame.

.LP

An REJ frame may be used to indicate the clearance of a busy condition
that was reported by the earlier transmission of an RNR frame by that same

station (DCE or\ DTE). In addition to indicating the DCE or DTE status, the

REJ\ command with the P bit set to\ 1 may be used by the DCE or\ DTE to ask for

the status of the DTE or\ DCE, respectively.

.LP

\fR

2.3.4.5

\fISet asynchronous balanced mode (SABM)\fR

\fIcommand/\fR

\fISet asynchronous balanced mode extended (SABME)\fR



\fIcommand (subscription time option)\fR

.LP



The SABM unnumbered command is used to place the addressed DCE or DTE in
an asynchronous balanced mode (ABM) information transfer phase where all
command/response control fields will be one\ octet in length.

.LP

The SABME unnumbered command is used to place the addressed DCE or DTE
in an asynchronous balanced mode\ (ABM) information transfer phase where

numbered command/response control fields will be two octets in length, and

unnumbered command/response control fields will be one octet in length.

.LP



No information field is permitted with the SABM or SABME command. The transmission
of a SABM/SABME command indicates the clearance of a busy

condition that was reported by the earlier trans

mission\ of an RNR\ frame by   that

same station (DCE or\ DTE). The DCE or DTE confirms acceptance of SABM/SABME

[modulo\ 8 (basic) operation/modulo\ 128 (extended) operation] command by the

transmission, at the first opportunity, of a UA\ response. Upon acceptance of

this command, the DCE or DTE send state variable\ V(S) and receive state

variable V(R) are set to\ 0.

.LP

Previously transmitted I\ frames that are unacknowledged when this

command is actioned remain unac

knowledged.\ It is the responsibility of a

higher layer (e.g.\ Packet Layer or MLP) to recover from the possible loss of

the contents (e.g.\ packets) of such I\ frames.

.LP

\fINote\fR

\ \(em\ The mode of operation of a data link [basic (modulo 8) or

extended (modulo\ 128)] is determined at subscription time and is only
changed by going through a new subscription process.

.LP

2.3.4.6

\fIDisconnect (DISC)\fR

\fI command\fR

.LP

The DISC unnumbered command is used to terminate the mode

previously set. It is used to inform the DCE or DTE receiving the DISC
command that the DTE or DCE sending the DISC command is suspending operation.
No

information field is permitted with the DISC command. Prior to actioning the

DISC command, the DCE or DTE receiving the DISC command confirms the acceptance
of the DISC command by the transmission of a UA response. The DTE or DCE

sending the DISC command enters the disconnected phase when it receives the

acknowledging UA response.

.LP

Previously transmitted I frames that are unacknowledged when this

command is actioned remain unacknowledged. It is the responsibility of
a higher layer (e.g.\ Packet Layer or MLP) to recover from the possible
loss of the

contents (e.g.,\ packets) of such I frames.

.LP

2.3.4.7

\fIUnnumbered acknowledgement (UA)\fR

\fI response\fR

.LP

The UA unnumbered response is used by the DCE or DTE to acknowledge the
receipt and acceptance of the mode\(hysetting commands. Received mode\(hysetting
commands are not actioned until the UA response is transmitted. The

transmission of a UA response indicates the clearance of a busy condition
that was reported by the earlier transmission of an RNR frame by that same
station (DCE or DTE). No information field is permitted with the UA response.

.LP



2.3.4.8

\fIDisconnected mode (DM)\fR

\fI response\fR

.LP

The DM unnumbered response is used to report a status where the DCE or
DTE is logically disconnected from the data link, and is in the disconnected
phase. The DM response may be sent to indicate that the DCE or DTE has
entered the disconnected phase without benefit of having received a DISC
command, or, if sent in response to the reception of a mode setting command,
is sent to

inform the DTE or DCE that the DCE or DTE, respectively, is still in the

disconnected phase and cannot execute the set mode command. No information

field is permitted with the DM response.

.LP

A DCE or DTE in a disconnected phase will monitor received commands

and will react to an SABM/SABME command as outlined in \(sc\ 2.4.4 below,
and will respond with a DM response with the F bit set to\ 1 to any other
command

received with the P\ bit set to\ 1.

.LP

2.3.4.9

\fIFrame reject (FRMR)\fR

\fI response\fR

.LP

The FRMR unnumbered response is used by the DCE or DTE to report an error
condition not recoverable by retransmission of the identical frame;

i.e.\ at least one of the following conditions, which results from the
receipt of a valid frame:

.LP

1)

the receipt of a command or response control field that is

undefined or not implemented;

.LP

2)

the receipt of an I frame with an information field which

exceeds the maximum established length;

.LP

3)

the receipt of an invalid N(R); or

.LP

4)

the receipt of a frame with an information field which is

not permitted or the receipt of a supervisory or unnumbered

frame with incorrect length.

.LP

An undefined or not implemented control field is any of the

control field encodings that are not identified in Tables\ 5/X.25 or\ 6/X.25.

.LP



A valid N(R) must be within the range from the lowest send sequence

number N(S) of the still unacknowledged frame(s) to the current DCE send
state variable inclusive (or to the current internal variable \fIx\fR

if the DCE is in

the timer recovery condition as described in \(sc\ 2.4.5.9).

.LP



An information field which immediately follows the control field, and consists
of\ 3 or 5\ octets [modulo\ 8 (basic) operation or modulo\ 128 (extended)
operation, respectively], is returned with this response and provides the

reason for the FRMR response. These formats are given in Tables\ 7/X.25

and\ 8/X.25.

.LP

2.3.5

\fIException condition reporting and recovery\fR

.LP

The error recovery procedures which are available to effect

recovery following the detection/occurrence of an

exception condition

 at the Data Link Layer are described below. Exception conditions described
are those situations which may occur as the result of transmission errors,
DCE or DTE malfunction, or operational situations.

.LP

2.3.5.1

\fIBusy condition\fR

.LP

The busy condition results when the DCE or DTE is temporarily

unable to continue to receive I frames due to internal constraints,

e.g.\ receive buffering limitations. In this case an RNR frame is transmitted

from the busy DCE or DTE. I\ frames pending transmission may be transmitted
from the busy DCE or DTE prior to or following the RNR\ frame.

.LP

An indication that the busy condition has cleared is communicated by the
transmission of a UA (only in response to a SABM/SABME command), RR, REJ
or SABM/SABME (modulo\ 8/modulo\ 128) frame.

.LP

.line

\fBTable 7/X.25 [T7.25], p.\fR

.line

\fBTable 8/X.25 [T8.25], p.\fR

2.3.5.2

\fIN(S) sequence error condition\fR

.LP

The information field of all I frames received whose N(S) does not equal
the receive state variable V(R) will be discarded.

.LP

An N(S) sequence error exception condition occurs in the receiver when
an I\ frame received contains an N(S) which is not equal to the receive
state

variable V(R) at the receiver. The receiver does not acknowledge (increment
its receive state variable) the I\ frame causing the sequence error, or
any I\ frame which may follow, until an I\ frame with the correct N(S)
is received.

.LP



A DCE or DTE which receives one or more valid I frames having sequence
errors or subsequent supervisory frames (RR, RNR and REJ) shall accept
the

control information contained in the N(R) field and the P or F bit to perform
data link control functions; e.g.\ to receive acknowledgement of previously

transmitted I\ frames and to cause the DCE or DTE to respond (P bit set to 1).

.LP



The means specified in \(sc\(sc\ 2.3.5.2.1 and 2.3.5.2.2 shall be available
for initiating the retransmission of lost or errored I\ frames following
the

occurrence of an N(S) sequence error condition.

.LP

2.3.5.2.1

\fIREJ recovery\fR

.LP

The REJ frame is used by a receiving DCE or DTE to initiate a

recovery (retransmission) following the detection of an N(S) sequence error.

.LP



With respect to each direction of transmission on the data link, only one
\*Qsent REJ\*U exception condition from a DCE or DTE, to a DTE or DCE,
is

established at a time. A \*Qsent REJ\*U exception condition is cleared when the

requested I\ frame is received.

.LP



A DCE or DTE receiving a REJ frame initiates sequential

(re\(hy)transmission of I\ frames starting with the I\ frame indicated
by the N(R) contained in the REJ frame. The retransmitted frames may contain
an N(R) and a P bit that are updated from, and therefore different from,
the ones contained in the originally transmitted I\ frames.

.LP

2.3.5.2.2

\fITime\(hyout recovery\fR

.LP

If a DCE or DTE, due to a transmission error, does not receive (or receives
and discards) a single I\ frame or the last I\ frame(s) in a sequence of
I\ frames, it will not detect an N(S) sequence error condition and, therefore,
will not transmit a REJ frame. The DTE or DCE which transmitted the

unacknowledged I\ frame(s) shall, following the completion of a system
specified time\(hyout period (see \(sc\(sc\ 2.4.5.1 and 2.4.5.9 below),
take appropriate recovery action to determine at which I\ frame retransmission
must begin. The

retransmitted frame(s) may contain an N(R) and a P bit that is updated from,

and therefore different from, the ones contained in the originally transmitted
frame(s).

.LP

2.3.5.3

\fIInvalid frame\fR

\fI condition\fR

.LP

Any frame which is invalid will be discarded, and no action is

taken as the result of that frame. An invalid frame is defined as one

which:

.LP

a)

is not properly bounded by two flags;

.LP

b)

in basic (modulo 8) operation, contains fewer than 32 bits

between flags; in extended (modulo\ 128) operation, contains

fewer than 40\ bits between flags of frames that contain sequence

numbers or 32\ bits between flags of frames that do not contain

sequence numbers;

.LP

c)

contains a Frame Check Sequence (FCS) error; or

.LP

d)

contains an address other than A or B (for single link

operation) or other than C or D (for multilink operation).

.LP



For those networks that are octet aligned, a detection of

non\(hyoctet alignment may be made at the Data Link Layer by adding a frame

validity check that requires the number of bits between the opening flag and

the closing flag, excluding bits inserted for transparency, to be an integral
number of octets in length, or the frame is considered invalid.

.LP

2.3.5.4

\fIFrame rejection\fR

\fI condition\fR

.LP

A frame rejection condition is established upon the receipt of an error\(hyfree
frame with one of the conditions listed in \(sc\ 2.3.4.9 above.

.LP



At the DCE or DTE, this frame rejection exception condition is

reported by an FRMR response for appropriate DTE or DCE action, respectively.
Once a DCE has established such an exception condition, no additional I\
frames are accepted until the condition is reset by the DTE, except for
examination of the P bit. The FRMR response may be repeated at each opportunity,
as specified in \(sc\ 2.4.7.3, until recovery is effected by the DTE, or
until the DCE initiates its own recovery in case the DTE does not respond.

.LP

2.3.5.5

\fIExcessive idle channel state condition on incoming\fR



\fIchannel\fR

.LP

Upon detection of an idle channel state condition (see \(sc\ 2.2.12.2 above)
on the incoming channel, the DCE shall wait for a period\ T3 (see

\(sc\ 2.4.8.3 below) without taking any specific action, waiting for detection
of a return to the active channel state (i.e.,\ detection of at least one
flag

sequence). After the period\ T3, the DCE shall notify the higher layer
(e.g.\ the Packet Layer or the MLP) of the excessive idle channel state
condition, but

shall not take any action that would preclude the DTE from establishing the

data link by normal data link set\(hyup procedures.

.LP

\fINote\fR

\ \(em\ Other actions to be taken by the DCE at the Data Link Layer upon
expiration of period\ T3 is a subject for further study.

.LP



2.4

\fIDescription of the \fR

\fILAPB procedure\fR

.LP

2.4.1

\fILAPB basic and extended modes of operation\fR

.LP

In accordance with the system choice made by the DTE at

subscription time, the DCE will either support modulo\ 8 (basic) operation or

will support modulo\ 128 (extended) operation. Changing from basic operation
to extended operation, or vice versa, in the DCE requires resubscription
by the

DTE for the desired service, and is not supported dynamically.

.LP

Table 5/X.25 indicates the command and response control field formats used
with the basic (modulo\ 8) service. The mode\(hysetting command employed
to

initialize (set up) or reset the basic mode is the SABM command. Table
6/X.25 indicates the command and response control field formats used with
the extended (modulo\ 128) service. The mode\(hysetting command employed
to initialize (set up) or reset the extended mode is the SABME command.

.LP

2.4.2

\fILAPB procedure for addressing\fR

.LP

The address field identifies a frame as either a command or a

response. A command frame contains the address of the DCE or DTE to which
the command is being sent. A response frame contains the address of the
DCE or DTE sending the frame.

.LP

In order to allow differentiation between single link operation and

the optional multilink operation for diagnostic and/or maintenance reasons,

different address pair encodings are assigned to data links operating with

multilink procedure compared to data links operating with the single link

procedure.

.LP

Frames containing commands transferred from the DCE to the DTE will

contain the address\ A for the single link operation and address\ C for the

multilink operation.

.LP

Frames containing responses transferred from the DCE to the DTE will contain
the address\ B for the single link operation and address\ D for the

multilink operation.

.LP



Frames containing commands transferred from the DTE to the DCE shall contain
the address\ B for the single link operation and address\ D for the

multilink operation.

.LP

Frames containing responses transferred from the DTE to the DCE shall contain
the address\ A for the single link operation and address\ C for the

multilink operation.

.LP

These addresses are coded as follows:

.LP

Address

1\ 2\ 3\ 4\ 5\ 6\ 7\ 8

.LP

Single link operation

\ \ A

1\ 1\ 0\ 0\ 0\ 0\ 0\ 0

.line

\ \ B

1\ 0\ 0\ 0\ 0\ 0\ 0\ 0

.LP

Multilink operation

\ \ C

1\ 1\ 1\ 1\ 0\ 0\ 0\ 0

.line

\ \ D

1\ 1\ 1\ 0\ 0\ 0\ 0\ 0

.LP

\fINote\fR

\ \(em\ The DCE will discard all frames received with an address other
than\ A or\ B (single link operation), or\ C or\ D (multilink operation).

.LP

2.4.3

\fILAPB procedure for the use of the P/F bit\fR

.LP

The DCE or DTE receiving an SABM/SABME, DISC, supervisory command or I\
frame with the P\ bit set to\ 1 will set the F\ bit to\ 1 in the next response
frame it transmits.

.LP

The response frame returned by the DCE to an SABM/SABME or DISC

command with the P\ bit set to\ 1 will be a UA or DM response with the
F\ bit set to\ 1. The response frame returned by the DCE to an I\ frame
with the P\ bit set to\ 1, received during the information transfer phase,
will be an RR, REJ, RNR or FRMR response with the F\ bit set to\ 1. The
response frame returned by the

DCE to a supervisory command with the P\ bit set to\ 1, received during the

information transfer phase, will be an RR, REJ, RNR or FRMR response with
the F\ bit set to\ 1. The response frame returned by the DCE to an I\ frame
or

supervisory frame with the P\ bit set to\ 1, received during the disconnected

phase, will be a DM response with the F\ bit set to\ 1.

.LP

The P bit may be used by the DCE in conjunction with the timer

recovery condition (see \(sc\ 2.4.5.9 below).

.LP

\fINote\fR

\ \(em\ Other use of the P bit by the DCE is a subject for further

study.

.LP

2.4.4

\fILAPB procedure for data link set\(hyup and disconnection\fR

.LP



2.4.4.1

\fIData link set\(hyup\fR

.LP

The DCE will indicate that it is able to set up the data link by

transmitting contiguous flags (active channel state).

.LP

Either the DTE or the DCE may initiate data link set\(hyup. Prior to

initiation of data link set\(hyup, either the DCE or the DTE may initiate data

link

disconnection (see \(sc\ 2.4.4.3) for the purpose of insuring that the
DCE and the DTE are in the same phase. The DCE may also transmit an unsolicited
DM response to request the DTE to initiate data link set\(hyup.

.LP

The DTE shall initiate data link set\(hyup by transmitting an SABM/SABME
command to the DCE. If, upon receipt of the SABM/SABME command correctly,
the DCE determines that it can enter the information transfer phase, it
will return a UA response to the DTE, will reset its send and receive state
variables V(S) and V(R) to zero, and will consider that the data link is
set up. If, upon

receipt

of the SABM/SABME command correctly, the DCE determines that it cannot enter

the information transfer phase, it will return a DM response to the DTE as a

denial to the data link set\(hyup initialization and will consider that the

data link is

\fInot\fR

 set up. In order to avoid misinterpretation of the DM response received,
it is suggested that the DTE always sends its SABM/SABME command with the
P\ bit set to\ 1. Otherwise, it is not possible to differentiate a DM response
intended as a denial to data link set\(hyup from a DM response that is
issued in a separate unsolicited sense as a request for a mode\(hysetting
command (as described

in\ \(sc\ 2.4.4.4.2).

.LP

The DCE will initiate data link set\(hyup by transmitting an SABM/SABME
command to the DTE and starting its Timer\ T1 in order to determine when
too

much time has elapsed waiting for a reply (see \(sc\ 2.4.8.1 below). Upon
reception of a UA response from the DTE, the DCE will reset its send and
receive state

variables\ V(S) and\ V(R) to zero, will stop its Timer\ T1, and will consider
that the data link is set up. Upon reception of a DM response from the
DTE as a

denial

to the data link set\(hyup initialization, the DCE will stop its Timer\
T1 and will consider that the data link is \fInot\fR

set up.

.LP



The DCE, having sent the SABM/SABME command, will ignore and discard any
frames except an SABM/SABME or DISC command, or a UA or DM response

received from the DTE. The receipt of an SABM/SABME or DISC command from the

DTE will result in a collision situation that is resolved per \(sc\ 2.4.4.5
below. Frames other than the UA and DM responses sent in response to a
received

SABM/SABME or DISC command will be sent only after the data link is set
up and if no outstanding SABM/SABME command exists.

.LP

After the DCE sends the SABM/SABME command, if a UA or DM response is not
received correctly, Timer\ T1 will run out in the DCE. The DCE will then

resend the SABM/SABME command and will restart Timer\ T1. After transmission
of the SABM/SABME command N2 times by the DCE, appropriate higher layer
recovery action will be initiated. The value of N2 is defined in \(sc\
2.4.8.4 below.

.LP

2.4.4.2

\fIInformation transfer phase\fR

.LP

After having transmitted the UA response to the SABM/SABME command or having
received the UA response to a transmitted SABM/SABME command, the DCE will
accept and transmit I and supervisory frames according to the procedures
described in \(sc\ 2.4.5 below.

.LP



When receiving the SABM/SABME command while in the information

transfer phase, the DCE will conform to the data link resetting procedure

described in \(sc\ 2.4.7 below.

.LP

2.4.4.3

\fIData link disconnection\fR

.LP

The DTE shall initiate a disconnect of the data link by

transmitting a DISC command to the DCE. On correctly receiving a DISC command
in the information transfer phase, the DCE will send a UA response and
enter

the disconnected phase. On correctly receiving a DISC command in the

disconnected phase, the DCE will send a DM response and remain in the

disconnected phase. In order to avoid misinterpretation of the DM response

received, it is suggested that the DTE always sends its DISC command with
the P\ bit set to\ 1. Otherwise, it is not possible to differentiate a
DM response

intended as an indication that the DCE is already in the disconnected phase

from a DM response that is issued in a separate unsolicited sense as a
request for a mode\(hysetting command (as described in \(sc\ 2.4.4.4.2).

.LP



The DCE will initiate a disconnect of the data link by transmitting a DISC
command to the DTE and starting its Timer\ T1 (see \(sc\ 2.4.8.1 below).
Upon reception of an UA response from the DTE, the DCE will stop its Timer\
T1 and

will enter the disconnected phase. Upon reception of a DM response from
the DTE as an indication that the DTE was already in the disconnected phase,
the DCE

will stop its Timer\ T1 and will enter the disconnected phase.

.LP

The DCE, having sent the DISC command, will ignore and discard any

frames except an SABM/SABME or DISC command, or a UA or DM response received

from the DTE. The receipt of an SABM/SABME or DISC command from the DTE will

result in a collision situation that is resolved per \(sc\ 2.4.4.5 below.

.LP



After the DCE sends the DISC command, if a UA or DM response is not

received correctly, Timer\ T1 will run out in the DCE. The DCE will then
resend the DISC command and will restart Timer\ T1. After transmission
of the DISC

command N2 times by the DCE, appropriate higher layer recovery action will
be initiated. The value of N2 is defined in \(sc\ 2.4.8.4 below.

.LP

2.4.4.4

\fIDisconnected phase\fR

.LP

2.4.4.4.1

After having received a DISC command from the DTE and

returned a UA response to the DTE, or having received the UA response to a

transmitted DISC command, the DCE will enter the disconnected phase.

.LP

In the disconnected phase, the DCE may initiate data link set\(hyup. In
the disconnected phase, the DCE will react to the receipt of an SABM/SABME
command as described in \(sc\ 2.4.4.1 above and will transmit a DM response
in

answer to a received DISC command. When receiving any other command (defined,
or undefined or not implemented) with the P\ bit set to\ 1, the DCE will
transmit a DM response with the F\ bit set to\ 1. Other frames received
in the

disconnected phase will be ignored by the DCE.

.LP

2.4.4.4.2

When the DCE enters the disconnected phase after

detecting error conditions as listed in \(sc\ 2.4.6 below, or after an internal

malfunction, it may indicate this by sending a DM response rather than
a DISC command. In these cases, the DCE will transmit a DM response and
start its

Timer\ T1 (see \(sc\ 2.4.8.1 below).

.LP

If Timer T1 runs out before the reception of an SABM/SABME or DISC command
from the DTE, the DCE will retransmit the DM response and restart

Timer\ T1. After transmission of the DM response N2 times, the DCE will
remain in the disconnected phase and appropriate recovery actions will
be initiated. The value of N2 is defined in \(sc\ 2.4.8.4 below.

.LP

Alternatively, after an internal malfunction, the DCE may either

initiate a data link resetting procedure (see \(sc\ 2.4.7 below) or disconnect
the data link (see \(sc\ 2.4.4.3 above) prior to initiating a data link
set\(hyup

procedure (see \(sc\ 2.4.4.1 above).

.LP

2.4.4.5

\fICollision of unnumbered commands\fR

.LP

Collision situations shall be resolved in the following way:

.LP

2.4.4.5.1

If the sent and received unnumbered commands are the

same, the DCE and the DTE shall each send the UA response at the earliest

possible opportunity. The DCE shall enter the indicated phase either,

.LP



1)

after receiving the UA response,

.LP

2)

after sending the UA response, or

.LP

3)

after timing out waiting for the UA response having sent a

UA response.

.LP

In the case of 2) above, the DCE will accept a subsequent UA

response to the mode\(hysetting command it issued without causing an exception

condition if received within the time\(hyout interval.

.LP



2.4.4.5.2

If the sent and received unnumbered commands are

different, the DCE and the DTE shall each enter the disconnected phase and

issue a DM response at the earliest possible opportunity.

.LP

2.4.4.6

\fICollision of DM response with SABM/SABME or DISC\fR



\fIcommand\fR

.LP

When a DM response is issued by the DCE or DTE as an unsolicited

response to request the DTE or DCE, respectively, to issue a mode\(hysetting

command as described in \(sc\ 2.4.4.4, a collision between an SABM/SABME
or DISC

command and the unsolicited DM response may occur. In order to avoid

misinterpretation of the DM response received, the DTE always sends its

SABM/SABME or DISC command with the P\ bit set to\ 1.

.LP

2.4.4.7

\fICollision of DM responses\fR

.LP

A contention situation may occur when both the DCE and the DTE

issue a DM response to request a mode\(hysetting command. In this case, the DTE

will issue an SABM/SABME command to resolve the contention situation.

.LP



2.4.5

\fILAPB procedures for information transfer\fR

.LP

The procedures which apply to the transmission of I\ frames in each direction
during the information transfer phase are described below.

.LP

In the following, \*Qnumber one higher\*U is in reference to a

continuously repeated sequence series, i.e., 7 is 1\ higher than 6 and 0 is

1\ higher than 7 for modulo\ 8 series, and 127 is 1\ higher than 126 and 0 is

1\ higher than 127 for modulo\ 128 series.

.LP

2.4.5.1

\fISending I frames\fR

.LP

When the DCE has an I frame to transmit (i.e. an I frame not

already transmitted, or having to be retransmitted as described in \(sc\
2.4.5.6

below), it will transmit it with an N(S) equal to its current send state

variable V(S), and an N(R) equal to its current receive state variable
V(R). At the end of the transmission of the I\ frame, the DCE will increment
its send

state variable V(S) by\ 1.

.LP

If Timer T1 is not running at the time of transmission of an I frame, it
will be started.

.LP

If the send state variable V(S) is equal to the last value of N(R)

received plus \fIk\fR

(where \fIk\fR

is the maximum number of outstanding I\ frames \(em

see \(sc\ 2.4.8.6 below), the DCE will not transmit any new I\ frames, but may

retransmit an I\ frame as described in \(sc\(sc\ 2.4.5.6 or 2.4.5.9 below.

.LP

When the DCE is in the busy condition, it may still transmit I frames,
provided that the DTE is not busy. When the DCE is in the frame rejection

condition, it will stop transmitting I\ frames.

.LP

2.4.5.2

\fIReceiving an I frame\fR

.LP

2.4.5.2.1

When the DCE is not in a busy condition and receives a valid I\ frame whose
send sequence number N(S) is equal to the DCE receive state

variable V(R), the DCE will accept the information field of this frame,

increment by one its receive state variable V(R), and act as follows:

.LP



a)

If the DCE is still not in a busy condition:

.LP

i)

If an I frame is available for transmission by the

DCE, it may act as in \(sc\ 2.4.5.1 above and acknowledge

the received I frame by setting N(R) in the control field

of the next transmitted I\ frame to the value of the DCE

receive state variable V(R). Alternatively, the DCE may

acknowledge the received I\ frame by transmitting an RR

frame with the N(R) equal to the value of the DCE receive

state variable V(R).

.LP

ii)

If no I frame is available for transmission by the

DCE, it will transmit an RR frame with N(R) equal to the

value of the DCE receive state variable V(R).

.LP



b)

If the DCE is now in a busy condition, it will transmit an

RNR frame with N(R) equal to the value of the DCE receive

state variable V(R) (see \(sc\ 2.4.5.8).

.LP



2.4.5.2.2

When the DCE is in a busy condition, it may ignore the

information field contained in any received I\ frame.

.LP

2.4.5.3

\fIReception of invalid frames\fR

.LP

When the DCE receives an invalid frame (see \(sc\ 2.3.5.3), this frame
will be discarded.

.LP

2.4.5.4

\fIReception of out\(hyof\(hysequence I frames\fR

.LP

When the DCE receives a valid I frame whose send sequence number

N(S) is incorrect, i.e.,\ not equal to the current DCE receive state variable

V(R), it will discard the information field of the I\ frame and transmit
an REJ frame with the N(R) set to one higher than the N(S) of the last
correctly

received I\ frame. The REJ frame will be a command frame with the P\ bit
set to\ 1 if an acknowledged transfer of the retransmission request is
required;

otherwise the REJ frame may be either a command or a response frame. The DCE

will then discard the information field of all I\ frames received until the

expected I\ frame is correctly received. When receiving the expected I\ frame,

the DCE will then acknowledge the I\ frame as described in \(sc\ 2.4.5.2
above. The DCE will use the N(R) and P\ bit information in the discarded
I\ frames as

described in \(sc\ 2.3.5.2 above.

.LP

2.4.5.5

\fIReceiving acknowledgement\fR

.LP

When correctly receiving an I frame or a supervisory frame (RR, RNR or
REJ), even in the busy condition, the DCE will consider the N(R) contained
in this frame as an acknowledgement for all I\ frames it has transmitted
with an N(S) up to and including the received N(R)\(em1. The DCE will stop
Timer\ T1 when it correctly receives an I\ frame or a supervisory frame
with the N(R) higher

than the last received N(R) (actually acknowledging some I\ frames), or
an REJ frame with an N(R) equal to the last received N(R).

.LP



If Timer T1 has been stopped by the receipt on an I, RR or RNR frame, and
if there are outstanding I\ frames still unacknowledged, the DCE will

restart Timer\ T1. If Timer\ T1 then runs out, the DCE will follow the
recovery procedure (\(sc\ 2.4.5.9 below) with respect to the unacknowledged
I\ frames. If

Timer\ T1 has been stopped by the receipt of an REJ frame, the DCE will
follow the retransmission procedures in \(sc\ 2.4.5.6 below.

.LP

2.4.5.6

\fIReceiving an REJ frame\fR

.LP

When receiving an REJ frame, the DCE will set its send state

variable V(S) to the N(R) received in the REJ control field. It will transmit
the corresponding I\ frame as soon as it is available or retransmit it
in

accordance with the procedures described in \(sc\ 2.4.5.1 above. (Re)transmission
will conform to the following procedure:

.LP

i)

if the DCE is transmitting a supervisory command or response

when it receives the REJ frame, it will complete that

transmission before commencing transmission of the requested

I\ frame;

.LP



ii)

if the DCE is transmitting an unnumbered command or

response when it receives the REJ frame, it will ignore the

request for retransmission;

.LP

iii)

if the DCE is transmitting an I frame when the REJ frame

is received, it may abort the I\ frame and commence transmission

of the requested I\ frame immediately after abortion;

.LP

iv)

if the DCE is not transmitting any frame when the REJ frame

is received, it will commence transmission of the requested

I\ frame immediately.

.LP

In all cases, if other unacknowledged I frames had already been

transmitted following the one indicated in the REJ frame, then those I
frames will be retransmitted by the DCE following the retransmission of
the requested I\ frame. Other I\ frames not yet transmitted may be transmitted
following the

retransmitted I\ frames.

.LP

If the REJ frame was received from the DTE as a command with the P bit
set to\ 1, the DCE will transmit an RR, RNR or REJ response with the F
bit set to\ 1 before transmitting or retransmitting the corresponding I\
frame.

.LP

2.4.5.7

\fIReceiving an RNR frame\fR

.LP

After receiving an RNR frame whose N(R) acknowledges all frames

previously transmitted, the DCE will stop Timer\ T1 and may then transmit

an I\ frame, with the P\ bit set to\ 0, whose send sequence number is

equal to the N(R) indicated in the RNR frame, restarting Timer\ T1 as it
does. After receiving an RNR frame whose N(R) indicates a previously transmitted

frame, the DCE will not transmit or retransmit any I\ frame, Timer\ T1 being

already running. In either case, if the Timer\ T1 runs out before receipt
of a busy clearance indication, the DCE will follow the procedure described
in

\(sc\ 2.4.5.9\ below. In any case, the DCE will not transmit any other
I\ frames

before receiving an RR or REJ frame, or before the completion of a link

resetting procedure.

.LP

Alternatively, after receiving an RNR frame, the DCE may wait for a

period of time (e.g.,\ the length of the Timer\ T1) and then transmit a

supervisory command frame (RR, RNR or REJ) with the P\ bit set to\ 1, and
start Timer\ T1, in order to determine if there is any change in the receive
status of the DTE. The DTE shall respond to the P\ bit set to\ 1 with a
supervisory

response frame (RR, RNR or REJ) with the F\ bit set to\ 1 indicating either

continuance of the busy condition (RNR) or clearance of the busy condition
(RR or REJ). Upon receipt of the DTE response, Timer\ T1 is stopped.

.LP



1)

If the response is the RR or REJ response, the busy

condition is cleared and the DCE may transmit I\ frames beginning

with the I\ frame identified by the N(R) in the received response

frame.

.LP

2)

If the response is the RNR response, the busy condition

still exists, and the DCE will after a period of time (e.g.\ the

length of Timer\ T1) repeat the enquiry of the DTE receive

status.

.LP

If Timer T1 runs out before a status response is received, the

enquiry process above is repeated. If N2 attempts to get a status response
fail (i.e.\ Timer\ T1 runs out N2\ times), the DCE will initiate a data
link resetting procedure as described in \(sc\ 2.4.7.2\ below or will transmit
a DM response to ask the DTE to initiate a data link set\(hyup procedure
as described in \(sc\ 2.4.4.1 and enter the disconnected phase. The value
of N2 is defined in \(sc\ 2.4.8.4\ below.

.LP



If, at any time during the enquiry process, an unsolicited RR or REJ frame
is received from the DTE, it will be considered to be an indication of

clearance of the busy condition. Should the unsolicited RR or REJ frame be a

command frame with the P bit set to\ 1, the appropriate response frame
with the F\ bit set to 1 must be transmitted before the DCE may resume
transmission of

I\ frames. If Timer\ T1 is running, the DCE will wait for the non\(hybusy
response with the F bit set to\ 1 or will wait for Timer\ T1 to run out
and then either

may reinitiate the enquiry process in order to realize a successful P/F bit

exchange or may resume transmission of I frames beginning with the I\ frame

identified by the N(R) in the received RR or REJ frame.

.LP

2.4.5.8

\fIDCE busy condition\fR

.LP

When the DCE enters a busy condition, it will transmit an RNR frame at
the earliest opportunity. The RNR frame will be a command frame with the
P bit set to\ 1 if an acknowledged transfer of the busy condition indication
is

required; otherwise the RNR frame may be either a command or a response
frame. While in the busy condition, the DCE will accept and process supervisory

frames, will accept and process the contents of the N(R) fields of I\ frames,

and will return an RNR response with the F bit set to\ 1 if it receives a

supervisory command or I command frame with the P bit set to\ 1. To clear the

busy condition, the DCE will transmit either an REJ frame or an RR frame, with

N(R) set to the current receive state variable V(R), depending on whether or

not it discarded information fields of correctly received I\ frames. The REJ

frame or the RR frame will be a command frame with the P bit set to\ 1 if an

acknowledged transfer of the busy\(hyto\(hynon\(hybusy transition is required,
otherwise the REJ frame or the RR frame may be either a command or a response
frame.

.LP



2.4.5.9

\fIWaiting acknowledgement\fR

.LP

The DCE maintains an internal transmission attempt variable which is set
to\ 0 when the DCE sends a UA response, when the DCE receives a UA

response or an RNR command or response, or when the DCE correctly receives
an I\ frame or supervisory frame with the N(R) higher than the last received
N(R) (actually acknowledging some outstanding I\ frames).

.LP

If Timer T1 runs out waiting for the acknowledgement from the DTE for an
I\ frame transmitted, the DCE will enter the timer recovery condition,
add

one to its transmission attempt variable and set an internal variable \fIx\fR

to

the current value of its send state variable V(S). The DCE will then restart

Timer T1, set its send state variable V(S) to the last value of N(R) received
from the DTE and retransmit the corresponding I\ frame with the P bit set
to\ 1, or transmit an appropriate supervisory command frame (RR, RNR or
REJ) with the P\ bit set to\ 1.

.LP



The timer recovery condition is cleared when the DCE receives a valid supervisory
frame with the F\ bit\ set to\ 1.

.LP

If, while in the timer recovery condition, the DCE correctly receives a
supervisory frame with the F bit set to\ 1 and with the N(R) within the
range from its current send state variable V(S) to \fIx\fR

included, it will clear the

timer recovery condition (including stopping Timer\ T1) and set its send
state variable V(S) to the value of the received N(R), and may then resume
with

I\ frame transmission or retransmission, as appropriate.

.LP

If, while in the timer recovery condition, the DCE correctly receives an
I or supervisory frame with the P/F bit set to\ 0 and with a valid N(R)
(see \(sc\ 2.3.4.9), it will not clear the timer recovery condition. The
value of the

received N(R) may be used to update the send state variable V(S). However,
the DCE may decide to keep the last transmitted I\ frame in store (even
if it is

acknowledged) in order to be able to retransmit it with the P bit set to\
1 when Timer\ T1 runs out at a later time.

.LP



If the received supervisory frame with the P/F bit set to\ 0 is an REJ
frame with a valid N(R), the DCE may either immediately initiate

(re)transmission from the value of the send state variable V(S), or it may

ignore the request for retransmission and wait until the supervisory frame
with the F bit set to\ 1 is received before initiating (re)transmission
of frames

from the value identified in the N(R) field of the supervisory frame with
the F\ bit set to\ 1. In the case of immediate retransmission, in order
to prevent

duplicate retransmissions following the clearance of the timer recovery

condition, the DCE shall inhibit retransmission of a specific I\ frame [same

N(R) in the same numbering cycle] if the DCE has retransmitted that I\
frame as the result of a received REJ frame with the P/F bit set to\ 0.

.LP

If, while in the timer recovery condition, the DCE receives a REJ

command with the P bit set to\ 1, the DCE will respond immediately with an

appropriate supervisory response with the F bit set to\ 1. The DCE may
then use the value of the N(R) in the REJ command to update the send state
variable

V(S), and may either immediately begin (re)transmission from the value N(R)

indicated in the REJ frame or ignore the request for retransmission and wait

until the supervisory frame with the F bit set to\ 1 is received before

initiating (re)transmission of I\ frames from the value identified in the
N(R) field of the supervisory frame with the F\ bit set to\ 1. In the case
of

immediate retransmission, in order to prevent duplicate retransmissions

following the clearance of the timer recovery condition, the DCE shall
inhibit retransmission of a specific I\ frame [same N(R) in the same numbering
cycle]

if the DCE has retransmitted that I\ frame as the result of the received REJ

command with the P\ bit set to\ 1.

.LP

If Timer T1 runs out in the timer recovery condition, and no I or

supervisory frame with the P/F bit set to 0 and with a valid N(R) has been

received, or no REJ command with the P bit set to\ 1 and with a valid N(R)
has been received, the DCE will add one to its transmission attempt variable,

restart Timer\ T1, and either retransmit the I frame sent with the P bit set

to\ 1 or transmit an appropriate supervisory command with the P bit set to\ 1.

.LP



If the transmission attempt variable is equal to N2, the DCE will

initiate a data link resetting procedure as described in \(sc\ 2.4.7.2
below, or

will transmit a DM response to ask the DTE to initiate a data link set\(hyup

procedure as described in \(sc\ 2.4.4.1 above and enter the disconnected
phase. N2 is a system parameter (see \(sc\ 2.4.8.4 below).

.LP

\fINote\fR

\ \(em\ Although the DCE may implement the internal variable \fIx\fR

,

other mechanisms do exist that achieve the identical function.

.LP

2.4.6

\fILAPB conditions for \fR

\fIdata link resetting or data link\fR



\fIre\(hyinitialization\fR

\fI(data link set\(hyup)\fR

.LP

2.4.6.1

When the DCE receives, during the information transfer phase, a

frame which is not invalid (see \(sc\ 2.3.5.3) with one of the conditions
listed in \(sc\ 2.3.4.9 above, the DCE will request the DTE to initiate
a data link resetting procedure by transmitting an FRMR response to the
DTE as described in

\(sc\ 2.4.7.3.

.LP

2.4.6.2

When the DCE receives, during the information transfer phase, an FRMR response
from the DTE, the DCE will either initiate the data link

resetting procedures itself as described in \(sc\ 2.4.7.2 or return a DM
response to ask the DTE to initiate the data link set\(hyup (initialization)
procedure as described in \(sc\ 2.4.4.1. After transmitting a DM response,
the DCE will enter

the disconnected phase as described in \(sc\ 2.4.4.4.2.

.LP

2.4.6.3

When the DCE receives, during the information transfer phase, a UA response,
or an unsolicited response with the F bit set to\ 1, the DCE may

either initiate the data link resetting procedures itself as described in

\(sc\ 2.4.7.2, or return a DM response to ask the DTE to initiate the data link

set\(hyup (initialization) procedure as described in \(sc\ 2.4.4.1. After
transmitting a DM response, the DCE will enter the disconnected phase as
described in

\(sc\ 2.4.4.4.2.

.LP

2.4.6.4

When the DCE receives, during the information transfer phase, a DM response
from the DTE, the DCE will either initiate the data link set\(hyup

(initialization) procedures itself as described in \(sc\ 2.4.4.1, or return
a DM

response to ask the DTE to initiate the data link set\(hyup (initialization)

procedures as described in \(sc\ 2.4.4.1. After transmitting a DM response,
the DCE will enter the disconnected phase as described in \(sc\ 2.4.4.4.2.

.LP



2.4.7

\fILAPB procedure for data link resetting\fR

.LP

2.4.7.1

The data link resetting procedure is used to initialize both

directions of information transfer according to the procedure described
below. The data link resetting procedure only applies during the information
transfer phase.

.LP

2.4.7.2

Either the DTE or the DCE may initiate the data link resetting

procedure. The data link resetting procedure indicates a clearance of a DCE

and/or DTE busy condition, if present.

.LP

The DTE shall initiate a data link resetting by transmitting an

SABM/SABME command to the DCE. If, upon correct receipt of the SABM/SABME

command, the DCE determines that it can continue in the information transfer

phase, it will return a UA response to the DTE, will reset its send and
receive state variables V(S) and V(R) to zero, and will remain in the information

transfer phase. If, upon correct receipt of the SABM/SABME command, the DCE

determines that it cannot remain in the information transfer phase, it will

return a DM response as a denial to the resetting request and will enter the

disconnected phase.

.LP

The DCE will initiate a data link resetting by transmitting an

SABM/SABME command to the DTE and starting its Timer\ T1 (see \(sc\ 2.4.8.1
below). Upon reception of a UA response from the DTE, the DCE will reset
its send and receive state variables V(S) and V(R) to zero, will stop its
Timer\ T1, and will remain in the information transfer phase. Upon reception
of a DM response from the DTE as a denial to the data link resetting request,
the DCE will stop its Timer\ T1 and will enter the disconnected phase.

.LP

The DCE, having sent an SABM/SABME command, will ignore and discard

any frames received from the DTE except an SABM/SABME or DISC command,
or a UA or DM response. The receipt of an SABM/SABME or DISC command from
the DTE will result in a collision situation that is resolved per \(sc\
2.4.4.5 above. Frames

other than the UA or DM response sent in response to a received SABM/SABME
or DISC command will be sent only after the data link is reset and if no

outstanding SABM/SABME command exists.

.LP



After the DCE sends the SABM/SABME command, if a UA or DM response is not
received correctly, Timer\ T1 will run out in the DCE. The DCE will then

resend the SABM/SABME command and will restart Timer\ T1. After N2 attempts
to reset the data link, the DCE will initiate appropriate higher layer
recovery

action and will enter the disconnected phase. The value of N2 is defined in

\(sc\ 2.4.8.4 below.

.LP

2.4.7.3

The DCE may ask the DTE to reset the data link by transmitting an FRMR
response (see \(sc\ 2.4.6.1 above). After transmitting an FRMR response,

the DCE will enter the frame rejection condition.

.LP

The frame rejection condition is cleared when the DCE receives an SABM/SABME
command, a DISC command, a FRMR response, or a DM response; or if

the DCE transmits an SABM/SABME command, a DISC command, or a DM response.

Other commands received while in the frame rejection condition will cause
the DCE to retransmit the FRMR response with the same information field
as

originally transmitted.

.LP

The DCE may start Timer\ T1 on transmission of the FRMR response. If

Timer\ T1 runs out before the frame rejection condition is cleared, the
DCE may retransmit the FRMR response, and restart T1. After N2 attempts
(time outs) to get the DTE to reset the data link, the DCE may reset the
data link itself as described in \(sc\ 2.4.7.2 above. The value of N2 is
defined in \(sc\ 2.4.8.4 below.

.LP



In the frame rejection condition, I frames and supervisory frames will
not be transmitted by the DCE. Also, received I frames and supervisory
frames will be discarded by the DCE except for the observance of a P bit
set to\ 1.

When an additional FRMR response must be transmitted by the DCE as a result
of the receipt of a P\ bit set to 1 while Timer\ T1 is running, Timer\
T1 will

continue to run. Upon reception of an FRMR response (even during a frame

rejection condition), the DCE will initiate a resetting procedure by

transmitting an SABM/SABME command as described in \(sc\ 2.4.7.2 above, or will

transmit a DM response to ask the DTE to initiate the data link set\(hyup

procedure as described in \(sc\ 2.4.4.1 and enter the disconnected phase.

.LP

2.4.8

\fIList of \fR

\fILAPB system parameters\fR

.LP

The DCE and DTE system parameters are as follows:

.LP



2.4.8.1

\fITimer\fR

\fIT1\fR

.LP

The value of the DTE Timer T1 system parameter may be different

than the value of the DCE Timer T1 system parameter. These values shall
be made known to both the DTE and the DCE, and agreed to for a period of
time by both the DTE and the DCE.

.LP

The period of Timer T1, at the end of which retransmission of a frame may
be initiated (see \(sc\ 2.4.4 and \(sc\ 2.4.5 above for the DCE), shall
take into account whether T1 is started at the beginning or the end of
the transmission of a frame.

.LP

The proper operation of the procedure requires that the transmitter's (DCE
or DTE) Timer\ T1 be greater than the maximum time between transmission
of a frame (SABM/SABME, DISC, I\ or supervisory command, or DM or FRMR
response)

and the reception of the corresponding frame returned as an answer to that

frame (UA, DM or acknowledging frame). Therefore, the receiver (DCE or DTE)

should not delay the response or acknowledging frame returned to one of the

above frames by more than a value\ T2, where T2 is a system parameter (see

\(sc\ 2.4.8.2).

.LP



The DCE will not delay the response or acknowledging frame returned to
one of the above DTE frames by more than a period\ T2.

.LP

2.4.8.2

\fIParameter T2\fR

.LP

The value of the DTE parameter T2 may be different than the value of the
DCE parameter T2. These values shall be made known to both the DTE and
the DCE, and agreed to for a period of time by both the DTE and the DCE.

.LP



The period of parameter T2 shall indicate the amount of time

available at the DCE or DTE before the acknowledging frame must be initiated
in order to ensure its receipt by the DTE or DCE, respectively, prior to
Timer\ T1 running out at the DTE or DCE (parameter\ T2\ <\ Timer\ T1).

.LP

\fINote\fR

\ \(em\ The period of parameter T2 shall take into account the

following timing factors: the transmission time of the acknowledging frame,
the propagation time over the access data link, the stated processing times
at the DCE and the DTE, and the time to complete the transmission of the
frame(s) in the DCE or DTE transmit queue that are neither displaceable
or modifiable in an orderly manner.

.LP



Given a value for Timer T1 for the DTE or DCE, the value of parameter T2
at the DCE or DTE, respectively, must be no larger than T1 minus\ 2 times
the propagation time over the access data link, minus the frame processing
time at the DCE, minus the frame processing time at the DTE, and minus
the transmission time of the acknowledging frame by the DCE or DTE, respectively.

.LP



2.4.8.3

\fITimer T3\fR

.LP

The DCE shall support a Timer T3 system parameter, the value of

which shall be made known to the DTE.

.LP

The period of Timer T3, at the end of which an indication of an

observed excessively long idle channel state condition is passed to the
Packet Layer, shall be sufficiently greater than the period of the DCE
Timer T1

(i.e.\ T3\ >\ T1) so that the expiration of T3 provides the desired level of

assurance that the data link channel is in a non\(hyactive, non\(hyoperational
state, and is in need of data link set\(hyup before normal data link operation
can

resume.

.LP

2.4.8.4

\fIMaximum number of attempts\fR

\fIto complete a\fR



\fItransmission N2\fR

.LP



The value of the DTE N2 system parameter may be different than the value
of the DCE N2 system parameter. These values shall be made known to both
the DTE and the DCE, and agreed to for a period of time by both the DTE
and the DCE.

.LP

The value of N2 shall indicate the maximum number of attempts made by the
DCE or DTE to complete the successful transmission of a frame to the DTE
or DCE, respectively.

.LP

2.4.8.5

\fIMaximum number of bits in an I frame N1\fR

.LP

The value of the DTE N1 system parameter may be different than the value
of the DCE N1 system parameter. These values shall be made known to both
the DTE and the DCE.

.LP

The values of N1 shall indicate the maximum number of bits in an

I\ frame (excluding flags and 0\ bits inserted for transparency) that the
DCE or DTE is willing to accept from the DTE or DCE, respectively.

.LP

In order to allow for universal operation, a DTE should support a

value of DTE N1 which is not less than 1080\ bits (135\ octets). DTEs should
be aware that the network may transmit longer packets (see \(sc\ 5.2),
that may result in a data link layer problem.

.LP



All networks shall offer to a DTE which requires it, a value of DCE N1
which is greater than or equal to 2072\ bits (259\ octets) plus the length
of the address, control and FCS fields at the DTE/DCE interface, and greater
than or equal to the maximum length of the data packets which may cross
the DTE/DCE

interface plus the length of the address, control and FCS fields at the
DTE/DCE interface.

.LP

Appendix II provides a description of how the values stated above are  derived.

.LP

2.4.8.6

\fIMaximum number of \fR

\fIoutstanding I frames k\fR

.LP

The value of the DTE k system parameter shall be the same as the

value of the DCE k system parameter. This value shall be agreed to for a

period of time by both the DTE and the DCE.

.LP

The value of k shall indicate the maximum number of sequentially

numbered I\ frames that the DTE or DCE may have outstanding

(i.e.\ unacknowledged) at any given time. The value of k shall never exceed

seven for modulo\ 8 operation, or one hundred and twenty\(hyseven for modulo\
128

operation. All networks (DCEs) shall support a value of seven. Other values
of k (less than and greater than seven) may also be supported by networks

(DCEs).

.LP

.line

\fBMONTAGE : \(sc 2.5 de cette Recommandation sur le reste de cette page\fR

.parag
.ce

.ce
.line
.ad r
\fBTable 1/X.24 [T1.24], p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
3.1
    \fICircuit\ G\ \(em\ Signal ground or common return\fR
.sp 9p
.RT
.PP
This conductor establishes the signal common reference potential
for unbalanced double\(hycurrent interchange circuits with electrical
characteristics according to Recommendation\ V.28. In the case of
interchange circuits according to Recommendations\ V.10 and\ V.11, it
interconnects the zero volt reference points of a generator and a receiver
to reduce environmental signal interference, if required.
.PP
Within the DCE, this conductor shall be brought to one point,
protective ground or earth, by means of a metallic strap within the equipment.
This metallic strap can be connected or removed at installation, as may
be
required, to minimize the introduction of noise into electronic circuitry
or to meet applicable regulations.
.PP
\fINote\fR \ \(em\ Where a shielded interconnecting cable is used at the
interface, the shield may be connected either to circuit\ G, or to protective
ground in accordance with national regulations. Protective ground may be
further connected to external grounds as required by applicable electrical
safety regulations.
.PP
For unbalanced interchange circuits with electrical characteristics in
accordance with Recommendation\ V.10, two common\(hyreturn conductors are
required, one for each direction of signalling, each conductor being connected
to ground only on the generator side of the interface. Where used, these
shall be
designated circuits\ Ga and\ Gb, and they are defined as follows:
.RT
.LP
    \fICircuit\ Ga\ \(em\ DTE common return\fR
.LP
    This conductor is connected to the DTE circuit common and is used
as the reference potential for the unbalanced\ X.26 type interchange circuit
receivers within the DCE.
.LP
    \fICircuit\ Gb\ \(em\ DCE common return\fR
.LP
    This conductor is connected to the DCE circuit common and is used
as the reference potential for the unbalanced\ X.26 type interchange circuit
receivers within the DTE.
.bp
.sp 1P
.LP
3.2
    \fICircuit\ T\ \(em\ Transmit\fR
.sp 9p
.RT
.LP
    \fIDirection:\fR To DCE
.PP
The binary signals originated by the DTE to be transmitted during the data
transfer phase via the data circuit to one or more remote DTEs are
transferred on this circuit to the DCE.
.PP
This circuit also transfers the call control signals originated by the
DTE, to be transmitted to the DCE in the call establishment and other call
control phases as specified by the relevant Recommendations for the procedural
characteristic of the interface.
.PP
The DCE monitors this circuit for detection of electrical circuit
fault conditions, according to the specifications of the electrical
characteristics of the interface. A circuit fault is to be interpreted
by the DCE as defined in the Recommendation for the procedural characteristics
of
the interface.
.RT
.sp 1P
.LP
3.3
    \fICircuit\ R\ \(em\ Receive\fR
.sp 9p
.RT
.LP
    \fIDirection:\fR \ From DCE
.PP
The binary signals sent by the DCE as received during the data
transfer phase from a remote DTE, are transferred on this circuit to the DTE.
.PP
This circuit also transfers the call control signals sent by the DCE as
received during the call establishment and other call control phases as
specified by the relevant Recommendations for the procedural characteristics
of the interface.
.PP
The DTE monitors this circuit for detection of electrical circuit
fault conditions, according to the specifications of the electrical
characteristics of the interface. A circuit fault is to be interpreted
by the DTE as defined in the Recommendation for the procedural characteristics
of
the interface.
.RT
.sp 1P
.LP
3.4
    \fICircuit\ C\ \(em\ Control\fR
.sp 9p
.RT
.LP
    \fIDirection:\fR \ To DCE
.PP
Signals on this circuit control the DCE for a particular signalling process.
.PP
Representation of a control signal requires additional coding of
circuit\ T\(hy\fITransmit\fR \|as specified in the relevant Recommendation
for the
procedural characteristics of the interface. During the data phase, this
circuit shall remain ON. During the call control phases, the condition
of this circuit shall be as specified in the relevant Recommendation for
the procedural characteristics of the interface.
.PP
\fINote\fR \ \(em\ After appropriate selection of special user facilities
(not yet defined), it might be required to change the ON condition after
entering
.PP
the data phase in accordance with the regulations for the use of these
facilities. This subject is for further study.
.PP
The DCE monitors this circuit for detection of electrical circuit
fault conditions, according to the specifications of the electrical
characteristics of the interface. A circuit fault is to be interpreted
by the DCE as defined in the Recommendation for the procedural characteristics
of
the interface.
.RT
.sp 1P
.LP
3.5
    \fICircuit\ I\ \(em\ Indication\fR
.sp 9p
.RT
.LP
    \fIDirection:\fR \ From DCE
.PP
Signals on this circuit indicate to the DTE the state of the call control
process.
.PP
Representation of a control signal requires additional coding of
circuit\ R\(hy\fIReceive\fR , as specified in the relevant Recommendation
for the
procedural characteristics of the interface. The ON condition of this circuit
signifies that signals on circuit\ R contain information from the distant
DTE. The OFF condition signifies a control signalling condition which is
defined by the bit sequence on circuit\ R as specified by the procedural
characteristics
of the interface.
.PP
The DTE monitors this circuit for detection of electrical circuit
fault conditions, according to the specifications of the electrical
characteristics of the interface. A circuit fault is to be interpreted
by the DTE as defined in the Recommendation for the procedural characteristics
of
the interface.
.PP
\fINote\fR \ \(em\ For use with special user facilities (not yet defined) it
might be required to use the OFF condition after entering the data transfer
phase in accordance with the regulations for the use of these facilities.
This subject is for further study.
.bp
.RT
.sp 1P
.LP
3.6
    \fICircuit\ S\ \(em\ \fR \fISignal element timing\fR
.sp 9p
.RT
.LP
    \fIDirection:\fR \ From DCE
.PP
Signals on this circuit provide the DTE with signal element timing information.
The condition of this circuit shall be ON and OFF for nominally
equal periods of time. However, for burst isochronous operations, longer
periods of OFF condition may be permitted equal to an integer odd number of
the nominal period of the ON condition as specified by the relevant procedural
characteristics of the interface.
.PP
The DTE shall present a binary signal on circuit\ T\(hy\fITransmit\fR \|and
a condition on circuit\ C\(hy\fIControl\fR , in which the transitions nominally
occur at the time of the transitions from OFF to ON condition of this circuit.
.PP
The DCE presents a binary signal on circuit\ R\(hy\fIReceive\fR \|and a
condition on circuit\ I\(hy\fIIndication\fR \|in which the transitions
nominally occur at the time of the transitions from OFF to ON condition
of this circuit.
.PP
The transition from ON to OFF condition shall nominally indicate the centre
of each signal element on circuit\ R.
.PP
The DCE shall transfer signal element timing information on this
circuit across the interface at all times that the timing source is capable
of generating this information.
.RT
.sp 1P
.LP
3.7
    \fICircuit\ B\ \(em\ \fR \fIByte timing\fR (see Note 2)
.sp 9p
.RT
.LP
    \fIDirection:\fR \ From DCE
.PP
Signals on this circuit provide the DTE with 8\(hybit byte timing
information. The condition of this circuit shall be OFF for nominally the
period of the ON condition of circuit\ S\(hy\fISignal element timing\fR
which indicates the last bit of an 8\(hybit byte and shall be ON at all
other times within the
period of the 8\(hybit byte.
.PP
During the call control phases, the call control characters and steady
state conditions used for all information transfers between the DCE and
the
DTE, in either direction, shall be correctly aligned to the signals of
circuit\ B.
.PP
The DTE shall present the beginning of the first bit of each call
control character on circuit\ T\(hy\fITransmit\fR nominally at the time of the
OFF to ON transition of circuit\ S which follows the OFF to ON transition of
circuit\ B.
.PP
A change of condition of circuit\ C\(hy\fIControl\fR \|may occur at any
OFF to ON transition of circuit\ S, but it will be sampled in the DCE at the
time of the OFF to ON transition of circuit\ B, i.e., for evaluation of the
following call control character on circuit\ T.
.PP
The centre of the last bit of each call control character will be
presented by the DCE on circuit\ R\(hy\fIReceive\fR nominally at the time
of the
OFF to ON transition of circuit\ B.
.PP
A change of condition of circuit\ I\(hy\fIIndication\fR \|will occur nominally
at the OFF to ON transition of circuit\ S which follows the OFF to ON
transition of circuit\ B.
.PP
The DCE shall transfer byte timing information on this circuit across the
interface at all times that the timing source is capable of generating
this information.
.PP
\fINote\ 1\fR \ \(em\ During the data transfer phase, DTEs communicating by
means of an 8\(hybit code may utilize the byte timing information for mutual
character alignment.
.RT
.LP
It is a prerequisite for the provision of this feature that
character alignment is preserved after the call has entered the data transfer
phase and that the alignment obtained at one interface is synchronized
to the alignment at the other interface. (This is only possible on some
connections.)
.LP
Furthermore, where this feature is available, a change of condition
on circuit\ C as defined above may result in an equivalent change in the
relative alignment on circuit\ I at the distant interface.
.PP
\fINote\ 2\fR \ \(em\ In some Recommendations for the procedural
characteristics of the interface (e.g.,\ X.21), the use and termination
of this circuit by the DTE is not mandatory even when implemented in the
DCE.
.bp
.LP
.sp 1P
.LP
3.8
    \fICircuit\ F\ \(em\ Frame start identification\fR
.sp 9p
.RT
.LP
    \fIDirection:\fR \ From DCE
.PP
Signals on this circuit continuously provide the DTE with a
multiplex frame start indication when connected to a multiplexed DTE/DCE
interface.
.PP
The condition on this circuit shall be OFF for the nominal period of one
bit, indicating the last bit of the multiplex frame. At other times the
circuit shall remain\ ON.
.PP
The first data bit on subscriber channel 1 shall be transmitted or
received beginning nominally at the OFF to ON transition of circuit\ F.
.RT
.sp 1P
.LP
3.9
    \fICircuit\ X\ \(em\ DTE transmit signal element timing\fR (see Note)
.sp 9p
.RT
.LP
    \fIDirection:\fR \ To DCE
.PP
Signals on this circuit provide signal element timing information for the
transmit direction in cases where circuit\ S only provides signal
element timing for the receive direction. The condition of this circuit
shall be ON and OFF for nominally equal periods of time. However, for burst
isochronous operations, longer periods of OFF condition may be permitted
.PP
equal to an integer odd number of the nominal period of the ON condition as
specified by the relevant procedural characteristics of the interface.
.PP
The DTE shall present a binary signal on the circuit T\(hy\fITransmit\fR
\|and a condition on circuit\ C\(hy\fIControl\fR , in which the transitions
nominally occur at the time of the transitions from OFF to ON condition
of this circuit.
.PP
The transition from ON to OFF condition shall nominally indicate the centre
of each signal element on circuit\ T.
.PP
\fINote\fR \ \(em\ The use and termination of this circuit by the DCE is a
national matter.
\v'6p'
.RT
.LP
.sp 2P
.LP
\fBRecommendation X.25\fR
.RT
.sp 2P
.ce 1000
\fBINTERFACE\ BETWEEN\ DATA\ TERMINAL\ EQUIPMENT\ (DTE)\ AND\ DATA\fR
.EF '%    Fascicle\ VIII.2\ \(em\ Rec.\ X.25''
.OF '''Fascicle\ VIII.2\ \(em\ Rec.\ X.25    %'
.ce 0
.ce 1000
\fBCIRCUIT\(hyTERMINATING\ EQUIPMENT\ (DCE)\ FOR\ TERMINALS\ OPERATING\fR
.ce 0
.ce 1000
\fBIN\ THE\ PACKET\ MODE\ AND\ CONNECTED\ TO\ PUBLIC\fR
.ce 0
.sp 1P
.ce 1000
\fBDATA\ NETWORKS\ BY\ DEDICATED\ CIRCUIT\fR
.ce 0
.sp 1P
.ce 1000
\fI(Geneva, 1976; amended at Geneva, 1980\fR ,
.sp 9p
.RT
.ce 0
.sp 1P
.ce 1000
\fIMalaga\(hyTorremolinos, 1984 and Melbourne, 1988)\fR
.ce 0
.sp 1P
.PP
The establishment in various countries of public data networks providing
packet switched data transmission services creates a need to produce standards
to facilitate international interworking.
.sp 1P
.RT
.sp 2P
.LP
    The\ CCITT,
.sp 1P
.RT
.sp 1P
.LP
\fIconsidering\fR
.sp 9p
.RT
.PP
(a)
that Recommendation X.1 includes specific user classes of service for data
terminal equipments operating in the packet mode,
Recommendation\ X.2 defines user facilities, Recommendation\ X.10 defines
categories of access, Recommendations\ X.21 and X.21\|\fIbis\fR define DTE/DCE
physical layer interface characteristics, Recommendation\ X.92 defines the
hypothetical reference connections for packet switched data transmission
service and Recommendation\ X.96 defines \fIcall progress\fR signals;
.PP
(b)
that data terminal equipments operating in the packet
mode will send and receive network control information in the form of packets;
.PP
(c)
that certain data terminal equipments operating in the packet mode will
use a
packet interleaved synchronous data circuit
;
.bp
.PP
(d)
the desirability of being able to use a single data
circuit to a Data Switching Exchange\ (DSE) for all user facilities;
.PP
(e)
that Recommendation X.2 specifies which of the various data transmission
services and optional user facilities described in the
present Recommendation are \*Qessential\*U and have thus to be made available
internationally, and which are not;
.PP
(f
)
the need for defining an international
Recommendation for the exchange between DTE and DCE of control information
for the use of packet switched data transmission services;
.PP
(g)
that this definition is made in Recommendation X.32 with regard to the
access through a public switched telephone network, an integrated services
digital network (ISDN), or a circuit switched public data network;
.PP
(h)
that Recommendation X.31 defines the support of
packet\(hymode
terminal equipment by an integrated services digital network (ISDN);
.PP
(i)
that, when this Recommendation is used to support the
Network Service defined in Recommendation\ X.213, the physical, data link and
packet
layers correspond to the Physical, Data link and Network Layers respectively,
as defined in Recommendation\ X.200;
.PP
(j)
that this Recommendation includes all the features
necessary to support the services included in Recommendation\ X.213 as
well as other features; that Recommendation\ X.223 defines the use of X.25
packet layer protocol to provide the OSI connection mode Network service;
.PP
(k)
that the necessary elements for an interface
Recommendation should be defined independently as:
.LP
    \fIPhysical layer\fR \ \(em\ the mechanical, electrical, functional and
procedural characteristics to activate, maintain and deactivate
the physical link between the DTE and the DCE;
.LP
    \fIData link layer\fR \ \(em\ the link access procedure for data interchange
across the link between the DTE and the DCE;
.LP
    \fIPacket layer\fR \ \(em\ the packet format and control procedures for the
exchange of packets containing control information and user
data between the DTE and the DCE;
.sp 1P
.LP
\fIunanimously declares\fR
.sp 9p
.RT
.PP
that for public data networks accessed via dedicated circuits by data terminal
equipments operating in the packet mode:
.PP
(1)
the mechanical, electrical, functional and procedural
characteristics to activate, maintain and deactivate the physical link
between the DTE and the DCE should be as specified in \(sc\ 1 below, \fIDTE/DCE
interface\fR \fIcharacteristics\fR ;
.PP
(2)
the link access procedure for data interchange across
the link between the DTE and the DCE should be as specified in \(sc\ 2 below,
\fILink access procedure across the DTE/DCE interface\fR ;
.PP
(3)
the packet layer procedures for the exchange of control information and
user data at the DTE/DCE interface should be as specified in
\(sc\ 3 below, \fIDescription of the packet layer DTE/DCE interface\fR ;
.PP
(4)
the procedures for virtual call and permanent virtual
circuit services should be as specified in \(sc\ 4 below, \fIProcedures
for virtual\fR \fIcircuit services\fR ;
.PP
(5)
the format for packets exchanged between the DTE and the DCE should be
as specified in \(sc\ 5 below, \fIPacket formats\fR ;
.PP
(6)
the procedures for optional user facilities should be as specified in \(sc\
6 below, \fIProcedures for optional user facilities\fR ;
.PP
(7)
the formats for optional user facilities should be as
specified in \(sc\ 7 below, \fIFormats for facility fields and registration
fields\fR .
.PP
\fINote\fR \ \(em\ This Recommendation fully specifies the behaviour of
the DCE. In addition, a minimum set of requirements is specified for the
DTE.
Additional guidance for the design of DTEs is available in ISO standards
ISO\ 7776 (data link layer) and ISO\ 8208 (packet layer). It is not required
by this Recommendation that these ISO standards be used. If using these
ISO
standards, note must be taken that their scope is expanded beyond that
of just interfacing with packet switched public data networks.
.PP
It should also be noted that this Recommendation uses the term DTE to refer
to the equipment to which the DCE interfaces. In ISO\ 8208, distinction
is made between a DTE and a packet switched private data network, which
are both considered as DTEs in this Recommendation.
.bp
.RT
.sp 1P
.ce 1000
CONTENTS
.ce 0
.sp 1P
.sp 2P
.LP
1
    \fIDTE/DCE interface characteristics (physical layer)\fR \v'3p'
.sp 1P
.RT
.LP
    1.1
    X.21 interface
.LP
    1.2
    X.21\|\fIbis\fR interface
.LP
    1.3
    V\(hyseries interface
.LP
    1.4
    X.31 interface
.sp 1P
.LP
2
    \fILink access procedures across the DTE/DCE interface\fR \v'3p'
.sp 9p
.RT
.LP
    2.1
    Scope and field of application
.LP
    2.2
    Frame structure
.LP
    2.3
    LAPB elements of procedures
.LP
    2.4
    Description of the LAPB procedure
.LP
    2.5
    Multilink procedure (MLP)
.LP
    2.6
    LAP elements of procedure
.LP
    2.7
    Description of the LAP procedure
.sp 1P
.LP
3
    \fIDescription of the packet layer DTE/DCE interface\fR \v'3p'
.sp 9p
.RT
.LP
    3.1
    Logical channels
.LP
    3.2
    Basic structure of packets
.LP
    3.3
    Procedure for restart
.LP
    3.4
    Error handling
.sp 1P
.LP
4
    \fIProcedures for virtual circuit services\fR \v'3p'
.sp 9p
.RT
.LP
    4.1
    Procedures for virtual call service
.LP
    4.2
    Procedures for permanent virtual circuit service
.LP
    4.3
    Procedures for data and interrupt transfer
.LP
    4.4
    Procedures for flow control
.LP
    4.5
    Effects of clear, reset and restart procedures
on the transfer of packets
.LP
    4.6
    Effects of the physical and the data link layer on the
packet layer
.sp 1P
.LP
5
    \fIPacket formats\fR \v'3p'
.sp 9p
.RT
.LP
    5.1
    General
.LP
    5.2
    Call set\(hyup and clearing packets
.LP
    5.3
    Data and interrupt packets
.LP
    5.4
    Flow control and reset packets
.LP
    5.5
    Restart packets
.LP
    5.6
    Diagnostic packet
.LP
    5.7
    Packets required for optional user facilities
.sp 1P
.LP
6
    \fIProcedures for optional user facilities (packet layer)\fR \v'3p'
.sp 9p
.RT
.LP
    6.1
    On\(hyline facility registration
.LP
    6.2
    Extended packet sequence numbering
.LP
    6.3
    D bit modification
.LP
    6.4
    Packet retransmission
.LP
    6.5
    Incoming calls barred
.LP
    6.6
    Outgoing calls barred
.bp
.LP
    6.7
    One\(hyway logical channel outgoing
.LP
    6.8
    One\(hyway logical channel incoming
.LP
    6.9
    Non\(hystandard default packet sizes
.LP
    6.10
    Non\(hystandard default window sizes
.LP
    6.11
    Default throughput classes assignment
.LP
    6.12
    Flow control parameter negotiation
.LP
    6.13
    Throughput class negotiation
.LP
    6.14
    Closed user group related facilities
.LP
    6.15
    Bilateral closed user group related facilities
.LP
    6.16
    Fast select
.LP
    6.17
    Fast select acceptance
.LP
    6.18
    Reverse charging
.LP
    6.19
    Reverse charging acceptance
.LP
    6.20
    Local charging prevention
.LP
    6.21
    Network user identification (NUI) related facilities
.LP
    6.22
    Charging information
.LP
    6.23
    RPOA related facilities
.LP
    6.24
    Hunt group
.LP
    6.25
    Call redirection and call deflection related facilities
.LP
    6.26
    Called line address modified notification
.LP
    6.27
    Transit delay selection and indication
.LP
    6.28
    TOA/NEI address subscription
.sp 1P
.LP
7
    \fIFormats for facility fields and registration fields\fR \v'3p'
.sp 9p
.RT
.LP
    7.1
    General
.LP
    7.2
    Coding of facility field in call set\(hyup
and clearing packets
.LP
    7.3
    Coding of the registration field of
registration packets
.sp 1P
.LP
\fIAnnex\ A\fR     \(em
    Range of logical channels used for virtual calls
and permanent virtual circuits
.sp 9p
.RT
.LP
\fIAnnex\ B\fR     \(em
    Packet layer DTE/DCE interface state diagrams
.LP
\fIAnnex\ C\fR     \(em
    Actions taken by the DCE on receipt of packets
in a given state of the packet layer DTE/DCE interface as
perceived by the DCE
.LP
\fIAnnex\ D\fR     \(em
    Packet layer DCE time\(hyouts and DTE time\(hylimits
.LP
\fIAnnex\ E\fR     \(em
    Coding of X.25 network generated
diagnostic fields in clear, reset and restart indication,
registration confirmation, and diagnostic packets
.LP
\fIAnnex\ F\fR     \(em
    Applicability of the on\(hyline registration facility
to other facilities
.LP
\fIAnnex\ G\fR     \(em
    CCITT\(hyspecified DTE facilities to support the
OSI Network service
.LP
\fIAnnex\ H\fR     \(em
    Subscription\(hytime optional user facilities that may be
associated with a network user identifier in conjunction with the NUI
override facility
.LP
\fIAppendix\ I\fR     \(em
    Examples of data link layer transmitted bit patterns by   the DCE and the DTE
.LP
\fIAppendix\ II\fR     \(em
    An explanation of how the values for N1 in \(sc 2.4.8.5 are derived
.LP
\fIAppendix\ III\fR     \(em
    Examples of multilink resetting procedures
.LP
\fIAppendix\ IV\fR     \(em
    Information on addresses in call set\(hyup and clearing
packets
.bp
.sp 2P
.LP
\fB1\fR     \fBDTE/DCE interface characteristics\fR \fB(physical layer)\fR
.sp 1P
.RT
.PP
Administrations may offer one or more of the interfaces specified below.
The exact use of the relevant points in these Recommendations is
detailed below.
.RT
.sp 2P
.LP
1.1
    \fIX.21 interface\fR
.sp 1P
.RT
.sp 1P
.LP
1.1.1
    \fIDTE/DCE physical interface elements\fR
.sp 9p
.RT
.PP
The DTE/DCE physical interface elements shall be according to
\(sc\(sc\ 2.1 through\ 2.5 of Recommendation\ X.21.
.RT
.sp 1P
.LP
1.1.2
    \fIProcedures for entering operational phases\fR
.sp 9p
.RT
.PP
The procedures for entering operational phases shall be as
described in \(sc\ 5.2 of Recommendation\ X.21. The data exchanged on circuits\
T
and\ R when the interface is in states\ 13S, 13R and\ 13 of Figure A\(hy3/X.21
will be as described in subsequent sections of this Recommendation.
.PP
The \fInot ready\fR \| states given in \(sc\ 2.5 of Recommendation X.21 are
considered to be \fInon\(hyoperational\fR states and may be considered
by the higher layers to be \fIout of order\fR states (see \(sc\ 4.6 below).
.RT
.sp 1P
.LP
1.1.3
    \fIFailure detection and test loops\fR
.sp 9p
.RT
.PP
The failure detection principles shall be according to \(sc\ 2.6 of
Recommendation\ X.21. In addition, i\ =\ OFF may be signalled due to momentary
transmission failures. Higher layers may delay for several seconds before
considering the interface to be out of order.
.PP
The definitions of test loops and the principles of maintenance
testing using the test loops are provided in Recommendation\ X.150.
.PP
A description of the test loops and the procedures for their use is
given in \(sc\ 7 of Recommendation\ X.21.
.PP
Automatic activation by a DTE of a test loop\ 2 in the DCE at the
remote terminal is not possible. However, some Administrations may permit
the DTE to control the equivalent of a test loop\ 2, at the local DSE,
to verify the operation of the leased line or subscriber line and/or all
or part of the DCE or line terminating equipment. Control of the loop,
if provided, may be either manual or automatic, as described in Recommendations\
X.150 and\ X.21
respectively.
.RT
.sp 1P
.LP
1.1.4
    \fISignal element timing\fR
.sp 9p
.RT
.PP
Signal element timing shall be in accordance with \(sc\ 2.6.3 of
Recommendation\ X.21.
.RT
.sp 2P
.LP
1.2
    \fIX.21\|bis interface\fR
.sp 1P
.RT
.sp 1P
.LP
1.2.1
    \fIDTE/DCE physical interface elements\fR
.sp 9p
.RT
.PP
The DTE/DCE physical interface elements shall be according to \(sc\ 1.2
of Recommendation\ X.21\|\fIbis\fR .
.RT
.sp 1P
.LP
1.2.2
    \fIOperational phases\fR
.sp 9p
.RT
.PP
When circuit 107 is in the ON condition, and circuits\ 105, 106, 108 and
109, if provided, are in the ON condition, data exchange on circuits\ 103
and\ 104 will be as described in subsequent sections of this
Recommendation.
.PP
When circuit 107 is in the OFF condition, or any of circuits 105, 106,
108 or\ 109, if provided, are in the OFF condition, this is considered
to be in a \fInon\(hyoperational\fR state, and may be considered by the
higher layers to be in an \fIout of order\fR state (see \(sc\ 4.6 below).
.RT
.sp 1P
.LP
1.2.3
    \fIFailure detection and test loops\fR
.sp 9p
.RT
.PP
The failure detection principles, the description of test loops and the
procedures for their use shall be according to \(sc\(sc\ 3.1 through\ 3.3
of
Recommendation\ X.21\|\fIbis\fR . In addition, circuits\ 106 and\ 109 may
enter the OFF condition due to momentary transmission failures. Higher
layers may delay for several seconds before considering the interface to
be out of order.
.bp
.PP
Automatic activation by a DTE of test loop 2 in the DCE at the remote terminal
is not possible. However, some Administrations may permit the DTE to control
the equivalent of a test loop\ 2, at the local DSE, to verify the
operation of the leased line or subscriber line and/or all or part of the
DCE or line terminating equipment. Control of the loop, if provided, may
be either manual or automatic, as described in Recommendations\ X.150 and\
X.21\|\fIbis\fR
respectively.
.RT
.sp 1P
.LP
1.2.4
    \fISignal element timing\fR
.sp 9p
.RT
.PP
Signal element timing shall be in accordance with \(sc\ 3.4 of
Recommendation\ X.21\|\fIbis\fR .
.RT
.sp 1P
.LP
1.3
    \fIV\(hySeries interface\fR
.sp 9p
.RT
.PP
General operation with V\(hySeries modems is as described in \(sc\ 1.2
above. However, for specific details, particularly related to failure detection
principles, loop testing, and the use of circuits\ 107, 109, 113 and\ 114,
refer to the appropriate V\(hySeries Recommendations.
.PP
The delay between 105\(hyON and 106\(hyON (when these
circuits are present) will be more than 10\ ms and less than 1\ s. In addition,
circuits\ 106 or\ 109 may enter the OFF condition due to momentary transmission
failures or modem retraining. Higher layers may delay for several seconds
before considering the interface to be out of order.
.RT
.sp 2P
.LP
1.4
    \fIX.31 interface\fR \v'3p'
.sp 1P
.RT
.sp 1P
.LP
1.4.1
    \fIDTE/DCE physical interface\fR
.sp 9p
.RT
.PP
The DTE/DCE physical interface shall coincide with the R reference point
between the DTE and the Terminal Adaptor (TA). The purpose of the TA is
.PP
to allow the operation of a DTE over an ISDN. The functionalities of such
a TA when accessing a packet switched data transmission service through
a
semi\(hypermanent ISDN connection (i.e.,\ a non switched B\(hychannel)
are described in \(sc\ 7 of Recommendation\ X.31.
.PP
\fINote\ 1\fR \ \(em\ This type of access is considered a dedicated access
to a public switched data transmission service. Non dedicated access to
a public
switched data transmission service is defined in Recommendations\ X.32
and\ X.31.
.PP
\fINote\ 2\fR \ \(em\ The DTE and the TA functionalities may be implemented in
the same piece of equipment in the case of a packet mode terminal TE1
conforming to the I\(hyseries Recommendations. In this case, this
Recommendation covers layer\ 2 and layer\ 3 operation on the semi\(hypermanent
B\(hychannel.
.RT
.sp 1P
.LP
1.4.2
    \fIOperational phases\fR
.sp 9p
.RT
.PP
The operational phases are as described in \(sc 7 of
Recommendation\ X.31.
.RT
.sp 1P
.LP
1.4.3
    \fIMaintenance\fR
.sp 9p
.RT
.PP
The maintenance shall be made as described in \(sc 7.6 of
Recommendation\ X.31.
.RT
.sp 1P
.LP
1.4.4
    \fISynchronization\fR
.sp 9p
.RT
.PP
The synchronization shall be made as described in \(sc 7 of
Recommendation\ X.31.
.RT
.sp 2P
.LP
\fB2\fR     \fBLink access procedures across the DTE/DCE interface\fR
.sp 1P
.RT
.sp 2P
.LP
2.1
    \fIScope and field of applications\fR \v'3p'
.sp 1P
.RT
.PP
2.1.1
The
Link Access Procedures
(LAPB and LAP) are
described as the
Data Link Layer Element
and are used for data
interchange
between a DCE and a DTE over a single physical circuit (LAPB and LAP), or
optionally over multiple physical circuits (LAPB), operating in user classes
of service\ 8 to\ 11 as indicated in Recommendation\ X.1. The optional,
subscription\(hytime selectable, multiple physical circuit operation with LAPB
(known as multilink operation) is required if the effects of circuit failures
are not to disrupt the Packet Layer operation.
.bp
.sp 9p
.RT
.PP
The single link procedures (SLPs) described in \(sc\(sc\ 2.2, 2.3
and\ 2.4 (LAPB) and in \(sc\(sc\ 2.2, 2.6 and 2.7 (LAP) are used for data
interchange over a single physical circuit, conforming to the description
given in \(sc\ 1,
between a DTE and a DCE. When the optional multilink operation is employed
with LAPB, a single link procedure (SLP) is used independently on each
physical
circuit, and the multilink procedure (MLP) described in \(sc\ 2.5 is used for
.LP
data interchange over these multiple parallel LAPB data links. In addition,
when only a single physical circuit is employed with LAPB, agreements may be
made with the Administration to use this optional
multilink procedure
over the one LAPB data link.
.PP
2.1.2
The
single link procedures (SLPs)
use the principles
and terminology of the High\(hylevel Data Link Control (HDLC) procedures
specified by the International Organization for Standardization (ISO). The
multilink pro
cedure\ (MLP) is based on the principles and terminology of the Multilink
Control Procedures specified by ISO.
.PP
2.1.3
Each transmission facility is duplex.
.PP
2.1.4
DCE compatibility of operation with the ISO balanced classes
of procedure (Class BA with options\ 2, 8 and Class\ BA with options\ 2,
8, 10) is achieved using the LAPB procedure described in \(sc\(sc\ 2.3
and\ 2.4. Of these
classes, Class\ BA with options\ 2, 8 (LAPB modulo\ 8) is the basic service,
and is available in all networks. Class\ BA with options\ 2, 8, 10 (LAPB
modulo\ 128) is recognized as an optional, subscription\(hytime selectable,
extended sequence numbering service that may be available in those networks
wishing to serve DTE applications having a need for modulo\ 128 sequence
numbering.
.PP
DTE manufacturers and implementors must be aware that the
procedure hereunder described as LAPB modulo\ 8 will be the only one available
in all networks.
.PP
Likewise, a DTE may continue to use the LAP procedure described in
\(sc\(sc\ 2.2, 2.6 and\ 2.7 (in those networks supporting such a procedure),
but for
new
DTE implementations, LAPB should be preferred. The LAP procedures are defined
for modulo\ 8 basic service only.
.PP
\fINote\fR \ \(em\ Other possible applications for further study are, for
example:
.RT
.LP
    \(em
    two\(hyway alternate, asynchronous response mode;
.LP
    \(em
    two\(hyway simultaneous, normal response mode;
.LP
    \(em
    two\(hyway alternate, normal response mode.
.PP
2.1.5
For those networks that choose to support both the basic and
extended LAPB sequence numbering services, the choice of either basic mode
(modulo\ 8) or extended mode (modulo\ 128) may be made at subscription
time. The choice of the mode employed for each data link procedure is independent
of all others and of the choice of mode for the corresponding Packet Layer
procedures. All choices are matters for agreement for a period of time
with the
Administration.
.PP
2.1.6
In the case of those networks that support both the LAPB procedure and
the LAP procedure, the DCE will maintain an internal mode variable\ B,
which it will set as follows:
.LP
    \(em
    to 1, upon acceptance of an SABM/SABME (modulo 8/modulo\ 128)
command from the DTE, or upon issuance of an SABM/SABME command
by the DCE;
.LP
    \(em
    to 0, upon acceptance of an SARM command from the DTE.
.PP
Whenever B is 1, the DCE will use the LAPB procedure described in \(sc\(sc\
2.2, 2.3 and 2.4 below, and is said to be in the LAPB (balanced) mode.
.PP
Whenever B is 0, the DCE will use the LAP procedure described in
\(sc\(sc\ 2.2, 2.6 and 2.7 below, and is said to be in the LAP mode.
.PP
Changes to the mode variable B by the DTE should occur only when the data
link has been disconnected as described in \(sc\(sc\ 2.4.4.3 or\ 2.7.3.3
below.
.PP
Should a DCE malfunction occur that negates the current setting of
internal mode variable\ B, the DCE will, upon restoration of operation,
not send either a SARM or SABM/SABME command. The DCE may send a DISC command
or a
.PP
DM\ response to notify the DTE that the DCE is in the disconnected phase.
This will result in the DTE attempting to reinitialize the data link with
what the DTE
considers to be the proper mode\(hysetting command (SARM or SABM/SABME).
The DCE will then be able to set the internal mode variable\ B to its proper
value.
.RT
.sp 1P
.LP
2.2
    \fIFrame structure\fR \v'3p'
.sp 9p
.RT
.PP
2.2.1
All transmissions on an SLP are in frames conforming to one of the formats
of Table\ 1/X.25 for basic (modulo\ 8) operation, or alternatively one
of the formats of Table\ 2/X.25 for extended (modulo\ 128) operation. The
flag
preceding the address field is defined as the opening flag. The flag following
the FCS field is defined as the closing flag.
.bp
.ce
.line
.ce
\fBTable 1/X.25 [T1.25], p.\fR
.ce
.line
.ce
\fBTable 2/X.25 [T2.25], p.\fR
.ce
2.2.2
.ce
\fIFlag sequence\fR
.ce
.parag
.ce
All frames shall start and end with the flag sequence consisting of one
0\ bit followed by six contiguous 1\ bits and one 0\ bit. The DTE and DCE
.ce
shall only send complete eight\(hybit flag sequences when sending multiple flag
.ce
sequences (see \(sc\ 2.2.11). A single flag may be used as both the
.ce
closing
.ce
flag
.ce
for one frame and the
.ce
opening flag
.ce
for the next frame.
.ce
.parag
.ce
2.2.3
.ce
\fIAddress field\fR
.ce
.parag
.ce
The address field shall consist of one octet. The address field
.ce
identifies the intended receiver of a command frame and the transmitter of a
.ce
response frame. The coding of the address field is described in \(sc\ 2.4.2
(LAPB) and in \(sc\ 2.7.1 (LAP) below.
.ce
.parag
.ce
2.2.4
.ce
\fIControl field\fR
.ce
.parag
.ce
For modulo\ 8 (basic) operation, the control field shall consist of one
octet. For modulo\ 128 (extended) operation, the control field shall consist
of two octets for frame formats that contain sequence numbers, and one
octet
.ce
for frame formats that do not contain sequence numbers. The content of this
.ce
field is described in \(sc\ 2.3.2\ (LAPB) and in \(sc\ 2.6.2 (LAP) below.
.ce
.parag
.ce
2.2.5
.ce
\fIInformation field\fR
.ce
.parag
.ce
The information field of a frame, when present, follows the control field
(see \(sc\ 2.2.4 above) and precedes the
.ce
frame check sequence field
.ce
(see \(sc\ 2.2.7 below).
.ce
.parag
.ce
See \(sc\(sc 2.3.4.9, 2.5.2, 2.6.4.8 and 5 for the various codings and
.ce
groupings of bits in the information field as used in this Recommendation.
.ce
.parag
.ce

.ce
See \(sc\(sc 2.3.4.9, 2.4.8.5, 2.6.4.8 and 2.7.7.5 below with regard to
the maximum information field length.
.ce
.parag
.ce
2.2.6
.ce
\fITransparency\fR
.ce
.parag
.ce
The DCE or DTE, when transmitting, shall examine the frame content between
the two flag sequences including the address, control, information and
FCS fields and shall insert a 0 bit after all sequences of 5 contiguous
1\ bits (including the last 5\ bits of the FCS) to ensure that a flag sequence
is not
.ce
simulated. The DCE or DTE, when receiving, shall examine the frame content
and shall discard any 0\ bit which directly follows\ 5 contiguous 1\ bits.
.ce
.parag
.ce
2.2.7
.ce
\fIFrame check sequence (FCS)\fR
.ce
\fI field\fR
.ce
.parag
.ce
The notation used to describe the FCS is based on the property of cyclic
codes that a code vector such as 1000000100001 can be represented by a
polynomial \fIP\fR
.ce
(\fIx\fR
.ce
)\ =\ \fIx\fR
.ce
\u1\d\u2\d\ +\ \fIx\fR
.ce
\u5\d\ +\ 1. The elements of an \fIn\fR
.ce
\(hyelement
.ce
code word are thus the coefficients of a polynomial of order \fIn\fR
.ce
\ \(em\ 1. In this application, these coefficients can have the value\
0 or\ 1 and the polynomial
.ce
operations are performed modulo\ 2. The polynomial representing the content
of a frame is generated using the first bit received after the frame opening
flag as the coefficient of the highest order term.
.ce
.parag
.ce
The FCS field shall be a 16\(hybit sequence. It shall be the ones
.ce
complement of the sum (modulo\ 2) of:
.ce
.parag
.ce
1)
.ce
the remainder of
.ce
\fIx\fR
.ce
\uD\dlFk\fR
.ce
(\fIx\fR
.ce
\u1\d\u5\d\uD\dlF036+\ \fIx\fR
.ce
\u1\d\u4\d\ +\ \fIx\fR
.ce
\u1\d\u3\d\ +
.ce
\fIx\fR
.ce
\u1\d\u2\d\ +\ \fIx\fR
.ce
\u1\d\u1\d\ +\ \fIx\fR
.ce
\u1\d\u0\d\ +\ \fIx\fR
.ce
\u9\d\ +\ \fIx\fR
.ce
\u8\d\ +
.ce
\fIx\fR
.ce
\u7\d\ +\ \fIx\fR
.ce
\u6\d\ +\ \fIx\fR
.ce
\u5\d\ +\ \fIx\fR
.ce
\u4\d\uD\dlF036+\ \fIx\fR
.ce
\u3\d\ +
.ce
+\ \fIx\fR
.ce
\u2\d\ +\ \fIx\fR
.ce
\ +\ 1) divided (modulo\ 2)
.ce
by the
.ce
generator polynomial \fIx\fR
.ce
\u1\d\u6\d\ +\ \fIx\fR
.ce
\u1\d\u2\d\ +\ \fIx\fR
.ce
\u5\d\ +\ 1,
.ce
where \fIk\fR
.ce
is the number of bits in the frame existing between,
.ce
but not including, the final bit of the opening flag and the
.ce
first bit of the FCS, excluding bits inserted for transparency,
.ce
and
.ce
.parag
.ce
2)
.ce
the remainder of the division (modulo 2) by the generator
.ce
polynomial \fIx\fR
.ce
\u1\d\u6\d\ +\ \fIx\fR
.ce
\u1\d\u2\d\ +\ \fIx\fR
.ce
\u5\d\ +\ 1 of the
.ce
product of \fIx\fR
.ce
\u1\d\u6\d by the content of the frame, existing
.ce
between but not including, the final bit of the opening flag and
.ce
the first bit of the FCS, excluding bits inserted for
.ce
transparency.
.ce
.parag
.ce

.ce
As a typical implementation, at the transmitter, the initial
.ce
content of the register of the device computing the remainder of the division
is preset to all 1s and is then modified by division by the generator
.ce
polynomial (as described above) on the address, control and information
fields; the ones complement of the resulting remainder is transmitted as
the 16\(hybit
.ce
FCS.
.ce
.parag
.ce
At the receiver, the initial content of the register of the device
.ce
computing the remainder is preset to all 1s. The final remainder, after
.ce
multiplication by \fIx\fR
.ce
\u1\d\u6\d and then division (modulo\ 2) by the generator
.ce
polynomial \fIx\fR
.ce
\u1\d\u6\d\ +\ \fIx\fR
.ce
\u1\d\u2\d\ +\ \fIx\fR
.ce
\u5\d\ +\ 1 of the serial incoming protected bits and the FCS, will be
0001110100001111 (\fIx\fR
.ce
\u1\d\u5\d through
.ce
\fIx\fR
.ce
\u0\d, respectively) in the absence of transmission errors.
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ Examples of transmitted bit patterns by the DCE and the DTE illustrating
application of the transparency mechanism and the frame check
.ce
sequence to the SABM command and the UA response are given in Appendix\ I.
.ce
.parag
.ce
2.2.8
.ce
\fIOrder of bit transmission\fR
.ce
.parag
.ce
Addresses, commands, responses and sequence numbers shall be
.ce
transmitted with the low\(hyorder bit first (for example, the first bit of the
.ce
sequence number that is transmitted shall have the weight 2\u0\d). The
order of transmitting bits within the information field is not specified
under \(sc\ 2 of
.ce
this Recommendation. The FCS shall be transmitted to the line commencing
with the coefficient of the highest term, which is found in bit position\
16 of the FCS\ field (see Tables\ 1/X.25 and 2/X.25).
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ In Tables 1/X.25 to 13/X.25, bit 1 is defined as the
.ce
low\(hyorder bit.
.ce
.parag
.ce
2.2.9
.ce
\fIInvalid frames\fR
.ce
.parag
.ce
The definition of an invalid frame is described in \(sc\ 2.3.5.3 (LAPB)
and in \(sc\ 2.6.5.3 (LAP) below.
.ce
.parag
.ce
2.2.10
.ce
\fIFrame abortion\fR
.ce
.parag
.ce
Aborting a frame is performed by transmitting at least seven
.ce
contiguous 1\ bits (with no inserted 0\ bits).
.ce
.parag
.ce
2.2.11
.ce
\fIInterframe time fill\fR
.ce
.parag
.ce
Interframe time fill is accomplished by transmitting contiguous
.ce
flags between frames, i.e.\ multiple eight\(hybit flag sequences (see
.ce
\(sc\ 2.2.2).
.ce
.parag
.ce
2.2.12
.ce
\fILink channel states\fR
.ce
.parag
.ce
A link channel as defined here is the means for transmission for
.ce
one direction.
.ce
.parag
.ce
2.2.12.1
.ce
\fIActive channel state\fR
.ce
.parag
.ce
The DCE incoming or outgoing channel is defined to be in an active condition
when it is receiving or transmitting, respectively, a frame, an
.ce
abortion sequence or interframe time fill.
.ce
.parag
.ce
2.2.12.2
.ce
\fIIdle channel state\fR
.ce
.parag
.ce
The DCE incoming or outgoing channel is defined to be in an idle
.ce
condition when it is receiving or transmitting, respectively, a continuous\
1s state for a period of at least 15\ bit times.
.ce
.parag
.ce
See \(sc\ 2.3.5.5 for a description of DCE action when an idle condition
exists on its incoming channel for an excessive period of time.
.ce
.parag
.ce

.ce
2.3
.ce
\fILAPB elements of procedures\fR
.ce
.parag
.ce
2.3.1
.ce
The LAPB elements of procedures are defined in terms of actions
.ce
that occur on receipt of frames at the DCE or\ DTE.
.ce
.parag
.ce
The elements of procedures specified below contain the selection
.ce
of commands and responses relevant to the LAPB data link and system
.ce
configurations
.ce
described in \(sc\ 2.1 above. Together, \(sc\(sc\ 2.2 and 2.3 form the general
.ce
requirements for the proper management of a LAPB access data link.
.ce
.parag
.ce

.ce
2.3.2
.ce
\fILAPB control field formats and parameters\fR
.ce
.parag
.ce

.ce
2.3.2.1
.ce
\fIControl field formats\fR
.ce
.parag
.ce
The control field contains a command or a response, and sequence
.ce
numbers where applicable.
.ce
.parag
.ce
Three types of control field formats are used to perform numbered
.ce
information transfer (I\ format), numbered supervisory functions (S\ format)
and unnumbered control functions (U\ format).
.ce
.parag
.ce
The control field formats for basic (modulo\ 8) operation are depicted
in Table\ 3/X.25.
.ce
.parag
.ce

.ce
The control field formats for extended (modulo 128) operation are
.ce
depicted in Table\ 4/X.25.
.ce
.parag
.ce
.line
.ce
\fBTable 3/X.25 [T3.25], p.\fR
.ce
.line
.ce
\fBTable 4/X.25 [T4/X.25], p.\fR
.ce
2.3.2.1.1
.ce
\fIInformation transfer format\fR
.ce
\fI \(em I\fR
.ce
.parag
.ce
The I format is used to perform an information transfer. The
.ce
functions of N(S), N(R) and P are independent; i.e.,\ each I\ frame has
an N(S), an N(R) which may or may not acknowledge additional I\ frames
received by the
.ce
DCE or DTE, and a P\ bit that may be set to\ 0 or\ 1.
.ce
.parag
.ce
2.3.2.1.2
.ce
\fISupervisory format\fR
.ce
\fI \(em S\fR
.ce
.parag
.ce
The S format is used to perform data link supervisory control
.ce
functions such as acknowledge I\ frames, request retransmission of I\ frames,
and to request a temporary suspension of transmission of I\ frames. The
functions of N(R) and P/F are independent; i.e.,\ each supervisory frame
has an N(R) which
.ce
may or may not acknowledge additional I\ frames received by the DCE or\
DTE, and a P/F\ bit that may be set to\ 0 or\ 1.
.ce
.parag
.ce
2.3.2.1.3
.ce
\fIUnnumbered format\fR
.ce
\fI \(em U\fR
.ce
.parag
.ce
The U format is used to provide additional data link control
.ce
functions. This format contains no sequence numbers, but does include a
P/F\ bit that may be set to\ 0 or\ 1. The unnumbered frames have the same
control field
.ce
length (one octet) in both basic (modulo\ 8) operation and extended (modulo\
128) operation.
.ce
.parag
.ce
2.3.2.2
.ce
\fIControl field parameters\fR
.ce
.parag
.ce
The various parameters associated with the control field formats
.ce
are described below.
.ce
.parag
.ce
2.3.2.2.1
.ce
\fIModulus\fR
.ce
.parag
.ce
Each I frame is sequentially numbered and may have the value\ 0
.ce
through modulus minus\ 1 (where \*Qmodulus\*U is the modulus of the sequence
.ce
numbers). The modulus equals either\ 8 or\ 128 and the sequence numbers cycle
.ce
through the entire range.
.ce
.parag
.ce
2.3.2.2.2
.ce
\fISend state variable\fR
.ce
\fI V(S)\fR
.ce
.parag
.ce
The send state variable V(S) denotes the sequence number of the
.ce
next in\(hysequence I\ frame to be transmitted. V(S) can take on the values\ 0
.ce
through modulus minus\ 1. The value of V(S) is incremented by\ 1 with each
.ce
successive I\ frame transmission, but cannot exceed the N(R) of the last
.ce
received\ I or supervisory frame by more than the maximum number of outstanding
I\ frames\ (\fIk\fR
.ce
). The value of\ k is defined in \(sc\ 2.4.8.6 below.
.ce
.parag
.ce

.ce
2.3.2.2.3
.ce
\fISend sequence number\fR
.ce
\fI N(S)\fR
.ce
.parag
.ce
Only I frames contain N(S), the send sequence number of transmitted I\
frames. At the time that an in\(hysequence I\ frame is designated for
.ce
transmission, the value of N(S) is set equal to the value of the send state
.ce
variable\ V(S).
.ce
.parag
.ce
2.3.2.2.4
.ce
\fIReceive state variable\fR
.ce
\fIV(R)\fR
.ce
.parag
.ce
The receive state variable V(R) denotes the sequence number of the next
in\(hysequence I\ frame expected to be received. V(R) can take on the values
0 through modulus minus\ 1. The value of V(R) is incremented by\ 1 by the
receipt of an error\(hyfree, in\(hysequence I\ frame whose send sequence
number N(S) equals
.ce
the receive state variable V(R).
.ce
.parag
.ce
2.3.2.2.5
.ce
\fIReceive sequence number\fR
.ce
\fI N(R)\fR
.ce
.parag
.ce
All I frames and supervisory frames contain N(R), the expected send sequence
number of the next received I\ frame. At the time that a frame of the above
types is designated for transmission, the value of N(R) is set equal to
the current value of the receive state variable V(R). N(R) indicates that
the DCE or DTE transmitting the N(R) has received correctly all I\ frames
numbered up to and including N(R)\ \(em\ 1.
.ce
.parag
.ce
2.3.2.2.6
.ce
\fIPoll/Final bit\fR
.ce
\fI P/F\fR
.ce
.parag
.ce
All frames contain P/F, the Poll/Final bit. In command frames, the P/F
bit is referred to as the P bit. In response frames, it is referred to
as the F\ bit.
.ce
.parag
.ce
2.3.3
.ce
\fIFunctions of the Poll/Final bit\fR
.ce
.parag
.ce
The Poll bit set to 1 is used by the DCE or DTE to solicit (poll) a response
from the DTE or DCE, respectively. The Final bit set to\ 1 is used by the
DCE or DTE to indicate the response frame transmitted by the DTE or DCE,
.ce
respectively, as a result of the soliciting (poll) command.
.ce
.parag
.ce
The use of the P/F bit is described in \(sc\ 2.4.3 below.
.ce
.parag
.ce
2.3.4
.ce
\fICommands and responses\fR
.ce
.parag
.ce
For basic (modulo 8) operation, the commands and responses
.ce
represented in Table\ 5/X.25 will be supported by the DCE and the\ DTE.
.ce
.parag
.ce

.ce
For extended (modulo 128) operation, the commands and responses
.ce
represented in Table\ 6/X.25 will be supported by the DCE and the DTE.
.ce
.parag
.ce

.ce
For purposes of the LAPB procedures, the supervisory function bit encoding
\*Q11\*U and those encodings of the modifier function bits in
.ce
Tables\ 3/X.25 and 4/X.25 not identified in Tables\ 5/X.25 or 6/X.25 are
.ce
identified as \*Qundefined or not implemented\*U command and response control
.ce
fields.
.ce
.parag
.ce
The commands and responses in Tables 5/X.25 and 6/X.25 are defined as  follows:
.ce
.parag
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2.3.4.1
.ce
\fIInformation (I)\fR
.ce
\fI command\fR
.ce
.parag
.ce
The function of the information (I) command is to transfer across a data
link a sequentially numbered frame containing an information field.
.ce
.parag
.ce
.line
.ce
\fBTable 5/X.25 [T5.25], p.\fR
.ce
.line
.ce
\fBTable 6/X.25 [T6.25], p.\fR
.ce
2.3.4.2
.ce
\fIReceive ready (RR)\fR
.ce
\fI command and response\fR
.ce
.parag
.ce
The receive ready (RR) supervisory frame is used by the DCE
.ce
or DTE to:
.ce
.parag
.ce

.ce
1)
.ce
indicate it is ready to receive an I frame; and
.ce
.parag
.ce
2)
.ce
acknowledge previously received I frames numbered up to and
.ce
including N(R)\ \(em\ 1.
.ce
.parag
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An RR frame may be used to indicate the clearance of a busy
.ce
condition that was reported by the earlier transmission of an RNR frame
by that same station (DCE or\ DTE). In addition to indicating the DCE or
DTE status, the RR command with the P\ bit set to\ 1 may be used by the
DCE or DTE to ask for the status of the DTE or DCE, respectively.
.ce
.parag
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2.3.4.3
.ce
\fIReceive not ready (RNR) command and response\fR
.ce
.parag
.ce
The receive not ready (RNR) supervisory frame is used by the DCE or DTE
to indicate a busy condition; i.e.\ temporary inability to accept
.ce
additional incoming I\ frames. I\ frames numbered up to and including N(R)\
\(em\ 1
.ce
are acknowledged. I\ frame N(R) and any subsequent I\ frames received, if any,
.ce
are not acknowledged; the acceptance status of these I\ frames will be
indicated in subsequent exchanges.
.ce
.parag
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In addition to indicating the DCE or DTE status, the RNR command with the
P bit set to\ 1 may be used by an DCE or DTE to ask for the status of the
.ce
DTE or DCE, respectively.
.ce
.parag
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2.3.4.4
.ce
\fIReject (REJ) command and response\fR
.ce
.parag
.ce
The reject (REJ) supervisory frame is used by the DCE or DTE to request
transmission of I\ frames starting with the frame numbered N(R).
.ce
I\ frames numbered N(R)\ \(em\ 1 and below are acknowledged. Additional
I\ frames
.ce
pending initial transmission may be transmitted following the retransmitted
.ce
I\ frame(s).
.ce
.parag
.ce

.ce
Only one REJ exception condition for a given direction of information transfer
may be established at any time. The REJ exception condition is cleared
(reset) upon the receipt of an I\ frame with an N(S) equal to the N(R)
of the
.ce
REJ\ frame.
.ce
.parag
.ce
An REJ frame may be used to indicate the clearance of a busy condition
that was reported by the earlier transmission of an RNR frame by that same
.ce
station (DCE or\ DTE). In addition to indicating the DCE or DTE status, the
.ce
REJ\ command with the P bit set to\ 1 may be used by the DCE or\ DTE to ask for
.ce
the status of the DTE or\ DCE, respectively.
.ce
.parag
.ce
\fR
.ce
2.3.4.5
.ce
\fISet asynchronous balanced mode (SABM)\fR
.ce
\fIcommand/\fR
.ce
\fISet asynchronous balanced mode extended (SABME)\fR
.ce

.ce
\fIcommand (subscription time option)\fR
.ce
.parag
.ce

.ce
The SABM unnumbered command is used to place the addressed DCE or DTE in
an asynchronous balanced mode (ABM) information transfer phase where all
command/response control fields will be one\ octet in length.
.ce
.parag
.ce
The SABME unnumbered command is used to place the addressed DCE or DTE
in an asynchronous balanced mode\ (ABM) information transfer phase where
.ce
numbered command/response control fields will be two octets in length, and
.ce
unnumbered command/response control fields will be one octet in length.
.ce
.parag
.ce

.ce
No information field is permitted with the SABM or SABME command. The transmission
of a SABM/SABME command indicates the clearance of a busy
.ce
condition that was reported by the earlier trans
.ce
mission\ of an RNR\ frame by   that
.ce
same station (DCE or\ DTE). The DCE or DTE confirms acceptance of SABM/SABME
.ce
[modulo\ 8 (basic) operation/modulo\ 128 (extended) operation] command by the
.ce
transmission, at the first opportunity, of a UA\ response. Upon acceptance of
.ce
this command, the DCE or DTE send state variable\ V(S) and receive state
.ce
variable V(R) are set to\ 0.
.ce
.parag
.ce
Previously transmitted I\ frames that are unacknowledged when this
.ce
command is actioned remain unac
.ce
knowledged.\ It is the responsibility of a
.ce
higher layer (e.g.\ Packet Layer or MLP) to recover from the possible loss of
.ce
the contents (e.g.\ packets) of such I\ frames.
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ The mode of operation of a data link [basic (modulo 8) or
.ce
extended (modulo\ 128)] is determined at subscription time and is only
changed by going through a new subscription process.
.ce
.parag
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2.3.4.6
.ce
\fIDisconnect (DISC)\fR
.ce
\fI command\fR
.ce
.parag
.ce
The DISC unnumbered command is used to terminate the mode
.ce
previously set. It is used to inform the DCE or DTE receiving the DISC
command that the DTE or DCE sending the DISC command is suspending operation.
No
.ce
information field is permitted with the DISC command. Prior to actioning the
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DISC command, the DCE or DTE receiving the DISC command confirms the acceptance
of the DISC command by the transmission of a UA response. The DTE or DCE
.ce
sending the DISC command enters the disconnected phase when it receives the
.ce
acknowledging UA response.
.ce
.parag
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Previously transmitted I frames that are unacknowledged when this
.ce
command is actioned remain unacknowledged. It is the responsibility of
a higher layer (e.g.\ Packet Layer or MLP) to recover from the possible
loss of the
.ce
contents (e.g.,\ packets) of such I frames.
.ce
.parag
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2.3.4.7
.ce
\fIUnnumbered acknowledgement (UA)\fR
.ce
\fI response\fR
.ce
.parag
.ce
The UA unnumbered response is used by the DCE or DTE to acknowledge the
receipt and acceptance of the mode\(hysetting commands. Received mode\(hysetting
commands are not actioned until the UA response is transmitted. The
.ce
transmission of a UA response indicates the clearance of a busy condition
that was reported by the earlier transmission of an RNR frame by that same
station (DCE or DTE). No information field is permitted with the UA response.
.ce
.parag
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.ce
2.3.4.8
.ce
\fIDisconnected mode (DM)\fR
.ce
\fI response\fR
.ce
.parag
.ce
The DM unnumbered response is used to report a status where the DCE or
DTE is logically disconnected from the data link, and is in the disconnected
phase. The DM response may be sent to indicate that the DCE or DTE has
entered the disconnected phase without benefit of having received a DISC
command, or, if sent in response to the reception of a mode setting command,
is sent to
.ce
inform the DTE or DCE that the DCE or DTE, respectively, is still in the
.ce
disconnected phase and cannot execute the set mode command. No information
.ce
field is permitted with the DM response.
.ce
.parag
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A DCE or DTE in a disconnected phase will monitor received commands
.ce
and will react to an SABM/SABME command as outlined in \(sc\ 2.4.4 below,
and will respond with a DM response with the F bit set to\ 1 to any other
command
.ce
received with the P\ bit set to\ 1.
.ce
.parag
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2.3.4.9
.ce
\fIFrame reject (FRMR)\fR
.ce
\fI response\fR
.ce
.parag
.ce
The FRMR unnumbered response is used by the DCE or DTE to report an error
condition not recoverable by retransmission of the identical frame;
.ce
i.e.\ at least one of the following conditions, which results from the
receipt of a valid frame:
.ce
.parag
.ce
1)
.ce
the receipt of a command or response control field that is
.ce
undefined or not implemented;
.ce
.parag
.ce
2)
.ce
the receipt of an I frame with an information field which
.ce
exceeds the maximum established length;
.ce
.parag
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3)
.ce
the receipt of an invalid N(R); or
.ce
.parag
.ce
4)
.ce
the receipt of a frame with an information field which is
.ce
not permitted or the receipt of a supervisory or unnumbered
.ce
frame with incorrect length.
.ce
.parag
.ce
An undefined or not implemented control field is any of the
.ce
control field encodings that are not identified in Tables\ 5/X.25 or\ 6/X.25.
.ce
.parag
.ce

.ce
A valid N(R) must be within the range from the lowest send sequence
.ce
number N(S) of the still unacknowledged frame(s) to the current DCE send
state variable inclusive (or to the current internal variable \fIx\fR
.ce
if the DCE is in
.ce
the timer recovery condition as described in \(sc\ 2.4.5.9).
.ce
.parag
.ce

.ce
An information field which immediately follows the control field, and consists
of\ 3 or 5\ octets [modulo\ 8 (basic) operation or modulo\ 128 (extended)
operation, respectively], is returned with this response and provides the
.ce
reason for the FRMR response. These formats are given in Tables\ 7/X.25
.ce
and\ 8/X.25.
.ce
.parag
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2.3.5
.ce
\fIException condition reporting and recovery\fR
.ce
.parag
.ce
The error recovery procedures which are available to effect
.ce
recovery following the detection/occurrence of an
.ce
exception condition
.ce
 at the Data Link Layer are described below. Exception conditions described
are those situations which may occur as the result of transmission errors,
DCE or DTE malfunction, or operational situations.
.ce
.parag
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2.3.5.1
.ce
\fIBusy condition\fR
.ce
.parag
.ce
The busy condition results when the DCE or DTE is temporarily
.ce
unable to continue to receive I frames due to internal constraints,
.ce
e.g.\ receive buffering limitations. In this case an RNR frame is transmitted
.ce
from the busy DCE or DTE. I\ frames pending transmission may be transmitted
from the busy DCE or DTE prior to or following the RNR\ frame.
.ce
.parag
.ce
An indication that the busy condition has cleared is communicated by the
transmission of a UA (only in response to a SABM/SABME command), RR, REJ
or SABM/SABME (modulo\ 8/modulo\ 128) frame.
.ce
.parag
.ce
.line
.ce
\fBTable 7/X.25 [T7.25], p.\fR
.ce
.line
.ce
\fBTable 8/X.25 [T8.25], p.\fR
.ce
2.3.5.2
.ce
\fIN(S) sequence error condition\fR
.ce
.parag
.ce
The information field of all I frames received whose N(S) does not equal
the receive state variable V(R) will be discarded.
.ce
.parag
.ce
An N(S) sequence error exception condition occurs in the receiver when
an I\ frame received contains an N(S) which is not equal to the receive
state
.ce
variable V(R) at the receiver. The receiver does not acknowledge (increment
its receive state variable) the I\ frame causing the sequence error, or
any I\ frame which may follow, until an I\ frame with the correct N(S)
is received.
.ce
.parag
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.ce
A DCE or DTE which receives one or more valid I frames having sequence
errors or subsequent supervisory frames (RR, RNR and REJ) shall accept
the
.ce
control information contained in the N(R) field and the P or F bit to perform
data link control functions; e.g.\ to receive acknowledgement of previously
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transmitted I\ frames and to cause the DCE or DTE to respond (P bit set to 1).
.ce
.parag
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.ce
The means specified in \(sc\(sc\ 2.3.5.2.1 and 2.3.5.2.2 shall be available
for initiating the retransmission of lost or errored I\ frames following
the
.ce
occurrence of an N(S) sequence error condition.
.ce
.parag
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2.3.5.2.1
.ce
\fIREJ recovery\fR
.ce
.parag
.ce
The REJ frame is used by a receiving DCE or DTE to initiate a
.ce
recovery (retransmission) following the detection of an N(S) sequence error.
.ce
.parag
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.ce
With respect to each direction of transmission on the data link, only one
\*Qsent REJ\*U exception condition from a DCE or DTE, to a DTE or DCE,
is
.ce
established at a time. A \*Qsent REJ\*U exception condition is cleared when the
.ce
requested I\ frame is received.
.ce
.parag
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.ce
A DCE or DTE receiving a REJ frame initiates sequential
.ce
(re\(hy)transmission of I\ frames starting with the I\ frame indicated
by the N(R) contained in the REJ frame. The retransmitted frames may contain
an N(R) and a P bit that are updated from, and therefore different from,
the ones contained in the originally transmitted I\ frames.
.ce
.parag
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2.3.5.2.2
.ce
\fITime\(hyout recovery\fR
.ce
.parag
.ce
If a DCE or DTE, due to a transmission error, does not receive (or receives
and discards) a single I\ frame or the last I\ frame(s) in a sequence of
I\ frames, it will not detect an N(S) sequence error condition and, therefore,
will not transmit a REJ frame. The DTE or DCE which transmitted the
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unacknowledged I\ frame(s) shall, following the completion of a system
specified time\(hyout period (see \(sc\(sc\ 2.4.5.1 and 2.4.5.9 below),
take appropriate recovery action to determine at which I\ frame retransmission
must begin. The
.ce
retransmitted frame(s) may contain an N(R) and a P bit that is updated from,
.ce
and therefore different from, the ones contained in the originally transmitted
frame(s).
.ce
.parag
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2.3.5.3
.ce
\fIInvalid frame\fR
.ce
\fI condition\fR
.ce
.parag
.ce
Any frame which is invalid will be discarded, and no action is
.ce
taken as the result of that frame. An invalid frame is defined as one
.ce
which:
.ce
.parag
.ce
a)
.ce
is not properly bounded by two flags;
.ce
.parag
.ce
b)
.ce
in basic (modulo 8) operation, contains fewer than 32 bits
.ce
between flags; in extended (modulo\ 128) operation, contains
.ce
fewer than 40\ bits between flags of frames that contain sequence
.ce
numbers or 32\ bits between flags of frames that do not contain
.ce
sequence numbers;
.ce
.parag
.ce
c)
.ce
contains a Frame Check Sequence (FCS) error; or
.ce
.parag
.ce
d)
.ce
contains an address other than A or B (for single link
.ce
operation) or other than C or D (for multilink operation).
.ce
.parag
.ce

.ce
For those networks that are octet aligned, a detection of
.ce
non\(hyoctet alignment may be made at the Data Link Layer by adding a frame
.ce
validity check that requires the number of bits between the opening flag and
.ce
the closing flag, excluding bits inserted for transparency, to be an integral
number of octets in length, or the frame is considered invalid.
.ce
.parag
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2.3.5.4
.ce
\fIFrame rejection\fR
.ce
\fI condition\fR
.ce
.parag
.ce
A frame rejection condition is established upon the receipt of an error\(hyfree
frame with one of the conditions listed in \(sc\ 2.3.4.9 above.
.ce
.parag
.ce

.ce
At the DCE or DTE, this frame rejection exception condition is
.ce
reported by an FRMR response for appropriate DTE or DCE action, respectively.
Once a DCE has established such an exception condition, no additional I\
frames are accepted until the condition is reset by the DTE, except for
examination of the P bit. The FRMR response may be repeated at each opportunity,
as specified in \(sc\ 2.4.7.3, until recovery is effected by the DTE, or
until the DCE initiates its own recovery in case the DTE does not respond.
.ce
.parag
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2.3.5.5
.ce
\fIExcessive idle channel state condition on incoming\fR
.ce

.ce
\fIchannel\fR
.ce
.parag
.ce
Upon detection of an idle channel state condition (see \(sc\ 2.2.12.2 above)
on the incoming channel, the DCE shall wait for a period\ T3 (see
.ce
\(sc\ 2.4.8.3 below) without taking any specific action, waiting for detection
of a return to the active channel state (i.e.,\ detection of at least one
flag
.ce
sequence). After the period\ T3, the DCE shall notify the higher layer
(e.g.\ the Packet Layer or the MLP) of the excessive idle channel state
condition, but
.ce
shall not take any action that would preclude the DTE from establishing the
.ce
data link by normal data link set\(hyup procedures.
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ Other actions to be taken by the DCE at the Data Link Layer upon
expiration of period\ T3 is a subject for further study.
.ce
.parag
.ce

.ce
2.4
.ce
\fIDescription of the \fR
.ce
\fILAPB procedure\fR
.ce
.parag
.ce
2.4.1
.ce
\fILAPB basic and extended modes of operation\fR
.ce
.parag
.ce
In accordance with the system choice made by the DTE at
.ce
subscription time, the DCE will either support modulo\ 8 (basic) operation or
.ce
will support modulo\ 128 (extended) operation. Changing from basic operation
to extended operation, or vice versa, in the DCE requires resubscription
by the
.ce
DTE for the desired service, and is not supported dynamically.
.ce
.parag
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Table 5/X.25 indicates the command and response control field formats used
with the basic (modulo\ 8) service. The mode\(hysetting command employed
to
.ce
initialize (set up) or reset the basic mode is the SABM command. Table
6/X.25 indicates the command and response control field formats used with
the extended (modulo\ 128) service. The mode\(hysetting command employed
to initialize (set up) or reset the extended mode is the SABME command.
.ce
.parag
.ce
2.4.2
.ce
\fILAPB procedure for addressing\fR
.ce
.parag
.ce
The address field identifies a frame as either a command or a
.ce
response. A command frame contains the address of the DCE or DTE to which
the command is being sent. A response frame contains the address of the
DCE or DTE sending the frame.
.ce
.parag
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In order to allow differentiation between single link operation and
.ce
the optional multilink operation for diagnostic and/or maintenance reasons,
.ce
different address pair encodings are assigned to data links operating with
.ce
multilink procedure compared to data links operating with the single link
.ce
procedure.
.ce
.parag
.ce
Frames containing commands transferred from the DCE to the DTE will
.ce
contain the address\ A for the single link operation and address\ C for the
.ce
multilink operation.
.ce
.parag
.ce
Frames containing responses transferred from the DCE to the DTE will contain
the address\ B for the single link operation and address\ D for the
.ce
multilink operation.
.ce
.parag
.ce

.ce
Frames containing commands transferred from the DTE to the DCE shall contain
the address\ B for the single link operation and address\ D for the
.ce
multilink operation.
.ce
.parag
.ce
Frames containing responses transferred from the DTE to the DCE shall contain
the address\ A for the single link operation and address\ C for the
.ce
multilink operation.
.ce
.parag
.ce
These addresses are coded as follows:
.ce
.parag
.ce
Address
.ce
1\ 2\ 3\ 4\ 5\ 6\ 7\ 8
.ce
.parag
.ce
Single link operation
.ce
\ \ A
.ce
1\ 1\ 0\ 0\ 0\ 0\ 0\ 0
.ce
.line
.ce
\ \ B
.ce
1\ 0\ 0\ 0\ 0\ 0\ 0\ 0
.ce
.parag
.ce
Multilink operation
.ce
\ \ C
.ce
1\ 1\ 1\ 1\ 0\ 0\ 0\ 0
.ce
.line
.ce
\ \ D
.ce
1\ 1\ 1\ 0\ 0\ 0\ 0\ 0
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ The DCE will discard all frames received with an address other
than\ A or\ B (single link operation), or\ C or\ D (multilink operation).
.ce
.parag
.ce
2.4.3
.ce
\fILAPB procedure for the use of the P/F bit\fR
.ce
.parag
.ce
The DCE or DTE receiving an SABM/SABME, DISC, supervisory command or I\
frame with the P\ bit set to\ 1 will set the F\ bit to\ 1 in the next response
frame it transmits.
.ce
.parag
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The response frame returned by the DCE to an SABM/SABME or DISC
.ce
command with the P\ bit set to\ 1 will be a UA or DM response with the
F\ bit set to\ 1. The response frame returned by the DCE to an I\ frame
with the P\ bit set to\ 1, received during the information transfer phase,
will be an RR, REJ, RNR or FRMR response with the F\ bit set to\ 1. The
response frame returned by the
.ce
DCE to a supervisory command with the P\ bit set to\ 1, received during the
.ce
information transfer phase, will be an RR, REJ, RNR or FRMR response with
the F\ bit set to\ 1. The response frame returned by the DCE to an I\ frame
or
.ce
supervisory frame with the P\ bit set to\ 1, received during the disconnected
.ce
phase, will be a DM response with the F\ bit set to\ 1.
.ce
.parag
.ce
The P bit may be used by the DCE in conjunction with the timer
.ce
recovery condition (see \(sc\ 2.4.5.9 below).
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ Other use of the P bit by the DCE is a subject for further
.ce
study.
.ce
.parag
.ce
2.4.4
.ce
\fILAPB procedure for data link set\(hyup and disconnection\fR
.ce
.parag
.ce

.ce
2.4.4.1
.ce
\fIData link set\(hyup\fR
.ce
.parag
.ce
The DCE will indicate that it is able to set up the data link by
.ce
transmitting contiguous flags (active channel state).
.ce
.parag
.ce
Either the DTE or the DCE may initiate data link set\(hyup. Prior to
.ce
initiation of data link set\(hyup, either the DCE or the DTE may initiate data
.ce
link
.ce
disconnection (see \(sc\ 2.4.4.3) for the purpose of insuring that the
DCE and the DTE are in the same phase. The DCE may also transmit an unsolicited
DM response to request the DTE to initiate data link set\(hyup.
.ce
.parag
.ce
The DTE shall initiate data link set\(hyup by transmitting an SABM/SABME
command to the DCE. If, upon receipt of the SABM/SABME command correctly,
the DCE determines that it can enter the information transfer phase, it
will return a UA response to the DTE, will reset its send and receive state
variables V(S) and V(R) to zero, and will consider that the data link is
set up. If, upon
.ce
receipt
.ce
of the SABM/SABME command correctly, the DCE determines that it cannot enter
.ce
the information transfer phase, it will return a DM response to the DTE as a
.ce
denial to the data link set\(hyup initialization and will consider that the
.ce
data link is
.ce
\fInot\fR
.ce
 set up. In order to avoid misinterpretation of the DM response received,
it is suggested that the DTE always sends its SABM/SABME command with the
P\ bit set to\ 1. Otherwise, it is not possible to differentiate a DM response
intended as a denial to data link set\(hyup from a DM response that is
issued in a separate unsolicited sense as a request for a mode\(hysetting
command (as described
.ce
in\ \(sc\ 2.4.4.4.2).
.ce
.parag
.ce
The DCE will initiate data link set\(hyup by transmitting an SABM/SABME
command to the DTE and starting its Timer\ T1 in order to determine when
too
.ce
much time has elapsed waiting for a reply (see \(sc\ 2.4.8.1 below). Upon
reception of a UA response from the DTE, the DCE will reset its send and
receive state
.ce
variables\ V(S) and\ V(R) to zero, will stop its Timer\ T1, and will consider
that the data link is set up. Upon reception of a DM response from the
DTE as a
.ce
denial
.ce
to the data link set\(hyup initialization, the DCE will stop its Timer\
T1 and will consider that the data link is \fInot\fR
.ce
set up.
.ce
.parag
.ce

.ce
The DCE, having sent the SABM/SABME command, will ignore and discard any
frames except an SABM/SABME or DISC command, or a UA or DM response
.ce
received from the DTE. The receipt of an SABM/SABME or DISC command from the
.ce
DTE will result in a collision situation that is resolved per \(sc\ 2.4.4.5
below. Frames other than the UA and DM responses sent in response to a
received
.ce
SABM/SABME or DISC command will be sent only after the data link is set
up and if no outstanding SABM/SABME command exists.
.ce
.parag
.ce
After the DCE sends the SABM/SABME command, if a UA or DM response is not
received correctly, Timer\ T1 will run out in the DCE. The DCE will then
.ce
resend the SABM/SABME command and will restart Timer\ T1. After transmission
of the SABM/SABME command N2 times by the DCE, appropriate higher layer
recovery action will be initiated. The value of N2 is defined in \(sc\
2.4.8.4 below.
.ce
.parag
.ce
2.4.4.2
.ce
\fIInformation transfer phase\fR
.ce
.parag
.ce
After having transmitted the UA response to the SABM/SABME command or having
received the UA response to a transmitted SABM/SABME command, the DCE will
accept and transmit I and supervisory frames according to the procedures
described in \(sc\ 2.4.5 below.
.ce
.parag
.ce

.ce
When receiving the SABM/SABME command while in the information
.ce
transfer phase, the DCE will conform to the data link resetting procedure
.ce
described in \(sc\ 2.4.7 below.
.ce
.parag
.ce
2.4.4.3
.ce
\fIData link disconnection\fR
.ce
.parag
.ce
The DTE shall initiate a disconnect of the data link by
.ce
transmitting a DISC command to the DCE. On correctly receiving a DISC command
in the information transfer phase, the DCE will send a UA response and
enter
.ce
the disconnected phase. On correctly receiving a DISC command in the
.ce
disconnected phase, the DCE will send a DM response and remain in the
.ce
disconnected phase. In order to avoid misinterpretation of the DM response
.ce
received, it is suggested that the DTE always sends its DISC command with
the P\ bit set to\ 1. Otherwise, it is not possible to differentiate a
DM response
.ce
intended as an indication that the DCE is already in the disconnected phase
.ce
from a DM response that is issued in a separate unsolicited sense as a
request for a mode\(hysetting command (as described in \(sc\ 2.4.4.4.2).
.ce
.parag
.ce

.ce
The DCE will initiate a disconnect of the data link by transmitting a DISC
command to the DTE and starting its Timer\ T1 (see \(sc\ 2.4.8.1 below).
Upon reception of an UA response from the DTE, the DCE will stop its Timer\
T1 and
.ce
will enter the disconnected phase. Upon reception of a DM response from
the DTE as an indication that the DTE was already in the disconnected phase,
the DCE
.ce
will stop its Timer\ T1 and will enter the disconnected phase.
.ce
.parag
.ce
The DCE, having sent the DISC command, will ignore and discard any
.ce
frames except an SABM/SABME or DISC command, or a UA or DM response received
.ce
from the DTE. The receipt of an SABM/SABME or DISC command from the DTE will
.ce
result in a collision situation that is resolved per \(sc\ 2.4.4.5 below.
.ce
.parag
.ce

.ce
After the DCE sends the DISC command, if a UA or DM response is not
.ce
received correctly, Timer\ T1 will run out in the DCE. The DCE will then
resend the DISC command and will restart Timer\ T1. After transmission
of the DISC
.ce
command N2 times by the DCE, appropriate higher layer recovery action will
be initiated. The value of N2 is defined in \(sc\ 2.4.8.4 below.
.ce
.parag
.ce
2.4.4.4
.ce
\fIDisconnected phase\fR
.ce
.parag
.ce
2.4.4.4.1
.ce
After having received a DISC command from the DTE and
.ce
returned a UA response to the DTE, or having received the UA response to a
.ce
transmitted DISC command, the DCE will enter the disconnected phase.
.ce
.parag
.ce
In the disconnected phase, the DCE may initiate data link set\(hyup. In
the disconnected phase, the DCE will react to the receipt of an SABM/SABME
command as described in \(sc\ 2.4.4.1 above and will transmit a DM response
in
.ce
answer to a received DISC command. When receiving any other command (defined,
or undefined or not implemented) with the P\ bit set to\ 1, the DCE will
transmit a DM response with the F\ bit set to\ 1. Other frames received
in the
.ce
disconnected phase will be ignored by the DCE.
.ce
.parag
.ce
2.4.4.4.2
.ce
When the DCE enters the disconnected phase after
.ce
detecting error conditions as listed in \(sc\ 2.4.6 below, or after an internal
.ce
malfunction, it may indicate this by sending a DM response rather than
a DISC command. In these cases, the DCE will transmit a DM response and
start its
.ce
Timer\ T1 (see \(sc\ 2.4.8.1 below).
.ce
.parag
.ce
If Timer T1 runs out before the reception of an SABM/SABME or DISC command
from the DTE, the DCE will retransmit the DM response and restart
.ce
Timer\ T1. After transmission of the DM response N2 times, the DCE will
remain in the disconnected phase and appropriate recovery actions will
be initiated. The value of N2 is defined in \(sc\ 2.4.8.4 below.
.ce
.parag
.ce
Alternatively, after an internal malfunction, the DCE may either
.ce
initiate a data link resetting procedure (see \(sc\ 2.4.7 below) or disconnect
the data link (see \(sc\ 2.4.4.3 above) prior to initiating a data link
set\(hyup
.ce
procedure (see \(sc\ 2.4.4.1 above).
.ce
.parag
.ce
2.4.4.5
.ce
\fICollision of unnumbered commands\fR
.ce
.parag
.ce
Collision situations shall be resolved in the following way:
.ce
.parag
.ce
2.4.4.5.1
.ce
If the sent and received unnumbered commands are the
.ce
same, the DCE and the DTE shall each send the UA response at the earliest
.ce
possible opportunity. The DCE shall enter the indicated phase either,
.ce
.parag
.ce

.ce
1)
.ce
after receiving the UA response,
.ce
.parag
.ce
2)
.ce
after sending the UA response, or
.ce
.parag
.ce
3)
.ce
after timing out waiting for the UA response having sent a
.ce
UA response.
.ce
.parag
.ce
In the case of 2) above, the DCE will accept a subsequent UA
.ce
response to the mode\(hysetting command it issued without causing an exception
.ce
condition if received within the time\(hyout interval.
.ce
.parag
.ce

.ce
2.4.4.5.2
.ce
If the sent and received unnumbered commands are
.ce
different, the DCE and the DTE shall each enter the disconnected phase and
.ce
issue a DM response at the earliest possible opportunity.
.ce
.parag
.ce
2.4.4.6
.ce
\fICollision of DM response with SABM/SABME or DISC\fR
.ce

.ce
\fIcommand\fR
.ce
.parag
.ce
When a DM response is issued by the DCE or DTE as an unsolicited
.ce
response to request the DTE or DCE, respectively, to issue a mode\(hysetting
.ce
command as described in \(sc\ 2.4.4.4, a collision between an SABM/SABME
or DISC
.ce
command and the unsolicited DM response may occur. In order to avoid
.ce
misinterpretation of the DM response received, the DTE always sends its
.ce
SABM/SABME or DISC command with the P\ bit set to\ 1.
.ce
.parag
.ce
2.4.4.7
.ce
\fICollision of DM responses\fR
.ce
.parag
.ce
A contention situation may occur when both the DCE and the DTE
.ce
issue a DM response to request a mode\(hysetting command. In this case, the DTE
.ce
will issue an SABM/SABME command to resolve the contention situation.
.ce
.parag
.ce

.ce
2.4.5
.ce
\fILAPB procedures for information transfer\fR
.ce
.parag
.ce
The procedures which apply to the transmission of I\ frames in each direction
during the information transfer phase are described below.
.ce
.parag
.ce
In the following, \*Qnumber one higher\*U is in reference to a
.ce
continuously repeated sequence series, i.e., 7 is 1\ higher than 6 and 0 is
.ce
1\ higher than 7 for modulo\ 8 series, and 127 is 1\ higher than 126 and 0 is
.ce
1\ higher than 127 for modulo\ 128 series.
.ce
.parag
.ce
2.4.5.1
.ce
\fISending I frames\fR
.ce
.parag
.ce
When the DCE has an I frame to transmit (i.e. an I frame not
.ce
already transmitted, or having to be retransmitted as described in \(sc\
2.4.5.6
.ce
below), it will transmit it with an N(S) equal to its current send state
.ce
variable V(S), and an N(R) equal to its current receive state variable
V(R). At the end of the transmission of the I\ frame, the DCE will increment
its send
.ce
state variable V(S) by\ 1.
.ce
.parag
.ce
If Timer T1 is not running at the time of transmission of an I frame, it
will be started.
.ce
.parag
.ce
If the send state variable V(S) is equal to the last value of N(R)
.ce
received plus \fIk\fR
.ce
(where \fIk\fR
.ce
is the maximum number of outstanding I\ frames \(em
.ce
see \(sc\ 2.4.8.6 below), the DCE will not transmit any new I\ frames, but may
.ce
retransmit an I\ frame as described in \(sc\(sc\ 2.4.5.6 or 2.4.5.9 below.
.ce
.parag
.ce
When the DCE is in the busy condition, it may still transmit I frames,
provided that the DTE is not busy. When the DCE is in the frame rejection
.ce
condition, it will stop transmitting I\ frames.
.ce
.parag
.ce
2.4.5.2
.ce
\fIReceiving an I frame\fR
.ce
.parag
.ce
2.4.5.2.1
.ce
When the DCE is not in a busy condition and receives a valid I\ frame whose
send sequence number N(S) is equal to the DCE receive state
.ce
variable V(R), the DCE will accept the information field of this frame,
.ce
increment by one its receive state variable V(R), and act as follows:
.ce
.parag
.ce

.ce
a)
.ce
If the DCE is still not in a busy condition:
.ce
.parag
.ce
i)
.ce
If an I frame is available for transmission by the
.ce
DCE, it may act as in \(sc\ 2.4.5.1 above and acknowledge
.ce
the received I frame by setting N(R) in the control field
.ce
of the next transmitted I\ frame to the value of the DCE
.ce
receive state variable V(R). Alternatively, the DCE may
.ce
acknowledge the received I\ frame by transmitting an RR
.ce
frame with the N(R) equal to the value of the DCE receive
.ce
state variable V(R).
.ce
.parag
.ce
ii)
.ce
If no I frame is available for transmission by the
.ce
DCE, it will transmit an RR frame with N(R) equal to the
.ce
value of the DCE receive state variable V(R).
.ce
.parag
.ce

.ce
b)
.ce
If the DCE is now in a busy condition, it will transmit an
.ce
RNR frame with N(R) equal to the value of the DCE receive
.ce
state variable V(R) (see \(sc\ 2.4.5.8).
.ce
.parag
.ce

.ce
2.4.5.2.2
.ce
When the DCE is in a busy condition, it may ignore the
.ce
information field contained in any received I\ frame.
.ce
.parag
.ce
2.4.5.3
.ce
\fIReception of invalid frames\fR
.ce
.parag
.ce
When the DCE receives an invalid frame (see \(sc\ 2.3.5.3), this frame
will be discarded.
.ce
.parag
.ce
2.4.5.4
.ce
\fIReception of out\(hyof\(hysequence I frames\fR
.ce
.parag
.ce
When the DCE receives a valid I frame whose send sequence number
.ce
N(S) is incorrect, i.e.,\ not equal to the current DCE receive state variable
.ce
V(R), it will discard the information field of the I\ frame and transmit
an REJ frame with the N(R) set to one higher than the N(S) of the last
correctly
.ce
received I\ frame. The REJ frame will be a command frame with the P\ bit
set to\ 1 if an acknowledged transfer of the retransmission request is
required;
.ce
otherwise the REJ frame may be either a command or a response frame. The DCE
.ce
will then discard the information field of all I\ frames received until the
.ce
expected I\ frame is correctly received. When receiving the expected I\ frame,
.ce
the DCE will then acknowledge the I\ frame as described in \(sc\ 2.4.5.2
above. The DCE will use the N(R) and P\ bit information in the discarded
I\ frames as
.ce
described in \(sc\ 2.3.5.2 above.
.ce
.parag
.ce
2.4.5.5
.ce
\fIReceiving acknowledgement\fR
.ce
.parag
.ce
When correctly receiving an I frame or a supervisory frame (RR, RNR or
REJ), even in the busy condition, the DCE will consider the N(R) contained
in this frame as an acknowledgement for all I\ frames it has transmitted
with an N(S) up to and including the received N(R)\(em1. The DCE will stop
Timer\ T1 when it correctly receives an I\ frame or a supervisory frame
with the N(R) higher
.ce
than the last received N(R) (actually acknowledging some I\ frames), or
an REJ frame with an N(R) equal to the last received N(R).
.ce
.parag
.ce

.ce
If Timer T1 has been stopped by the receipt on an I, RR or RNR frame, and
if there are outstanding I\ frames still unacknowledged, the DCE will
.ce
restart Timer\ T1. If Timer\ T1 then runs out, the DCE will follow the
recovery procedure (\(sc\ 2.4.5.9 below) with respect to the unacknowledged
I\ frames. If
.ce
Timer\ T1 has been stopped by the receipt of an REJ frame, the DCE will
follow the retransmission procedures in \(sc\ 2.4.5.6 below.
.ce
.parag
.ce
2.4.5.6
.ce
\fIReceiving an REJ frame\fR
.ce
.parag
.ce
When receiving an REJ frame, the DCE will set its send state
.ce
variable V(S) to the N(R) received in the REJ control field. It will transmit
the corresponding I\ frame as soon as it is available or retransmit it
in
.ce
accordance with the procedures described in \(sc\ 2.4.5.1 above. (Re)transmission
will conform to the following procedure:
.ce
.parag
.ce
i)
.ce
if the DCE is transmitting a supervisory command or response
.ce
when it receives the REJ frame, it will complete that
.ce
transmission before commencing transmission of the requested
.ce
I\ frame;
.ce
.parag
.ce

.ce
ii)
.ce
if the DCE is transmitting an unnumbered command or
.ce
response when it receives the REJ frame, it will ignore the
.ce
request for retransmission;
.ce
.parag
.ce
iii)
.ce
if the DCE is transmitting an I frame when the REJ frame
.ce
is received, it may abort the I\ frame and commence transmission
.ce
of the requested I\ frame immediately after abortion;
.ce
.parag
.ce
iv)
.ce
if the DCE is not transmitting any frame when the REJ frame
.ce
is received, it will commence transmission of the requested
.ce
I\ frame immediately.
.ce
.parag
.ce
In all cases, if other unacknowledged I frames had already been
.ce
transmitted following the one indicated in the REJ frame, then those I
frames will be retransmitted by the DCE following the retransmission of
the requested I\ frame. Other I\ frames not yet transmitted may be transmitted
following the
.ce
retransmitted I\ frames.
.ce
.parag
.ce
If the REJ frame was received from the DTE as a command with the P bit
set to\ 1, the DCE will transmit an RR, RNR or REJ response with the F
bit set to\ 1 before transmitting or retransmitting the corresponding I\
frame.
.ce
.parag
.ce
2.4.5.7
.ce
\fIReceiving an RNR frame\fR
.ce
.parag
.ce
After receiving an RNR frame whose N(R) acknowledges all frames
.ce
previously transmitted, the DCE will stop Timer\ T1 and may then transmit
.ce
an I\ frame, with the P\ bit set to\ 0, whose send sequence number is
.ce
equal to the N(R) indicated in the RNR frame, restarting Timer\ T1 as it
does. After receiving an RNR frame whose N(R) indicates a previously transmitted
.ce
frame, the DCE will not transmit or retransmit any I\ frame, Timer\ T1 being
.ce
already running. In either case, if the Timer\ T1 runs out before receipt
of a busy clearance indication, the DCE will follow the procedure described
in
.ce
\(sc\ 2.4.5.9\ below. In any case, the DCE will not transmit any other
I\ frames
.ce
before receiving an RR or REJ frame, or before the completion of a link
.ce
resetting procedure.
.ce
.parag
.ce
Alternatively, after receiving an RNR frame, the DCE may wait for a
.ce
period of time (e.g.,\ the length of the Timer\ T1) and then transmit a
.ce
supervisory command frame (RR, RNR or REJ) with the P\ bit set to\ 1, and
start Timer\ T1, in order to determine if there is any change in the receive
status of the DTE. The DTE shall respond to the P\ bit set to\ 1 with a
supervisory
.ce
response frame (RR, RNR or REJ) with the F\ bit set to\ 1 indicating either
.ce
continuance of the busy condition (RNR) or clearance of the busy condition
(RR or REJ). Upon receipt of the DTE response, Timer\ T1 is stopped.
.ce
.parag
.ce

.ce
1)
.ce
If the response is the RR or REJ response, the busy
.ce
condition is cleared and the DCE may transmit I\ frames beginning
.ce
with the I\ frame identified by the N(R) in the received response
.ce
frame.
.ce
.parag
.ce
2)
.ce
If the response is the RNR response, the busy condition
.ce
still exists, and the DCE will after a period of time (e.g.\ the
.ce
length of Timer\ T1) repeat the enquiry of the DTE receive
.ce
status.
.ce
.parag
.ce
If Timer T1 runs out before a status response is received, the
.ce
enquiry process above is repeated. If N2 attempts to get a status response
fail (i.e.\ Timer\ T1 runs out N2\ times), the DCE will initiate a data
link resetting procedure as described in \(sc\ 2.4.7.2\ below or will transmit
a DM response to ask the DTE to initiate a data link set\(hyup procedure
as described in \(sc\ 2.4.4.1 and enter the disconnected phase. The value
of N2 is defined in \(sc\ 2.4.8.4\ below.
.ce
.parag
.ce

.ce
If, at any time during the enquiry process, an unsolicited RR or REJ frame
is received from the DTE, it will be considered to be an indication of
.ce
clearance of the busy condition. Should the unsolicited RR or REJ frame be a
.ce
command frame with the P bit set to\ 1, the appropriate response frame
with the F\ bit set to 1 must be transmitted before the DCE may resume
transmission of
.ce
I\ frames. If Timer\ T1 is running, the DCE will wait for the non\(hybusy
response with the F bit set to\ 1 or will wait for Timer\ T1 to run out
and then either
.ce
may reinitiate the enquiry process in order to realize a successful P/F bit
.ce
exchange or may resume transmission of I frames beginning with the I\ frame
.ce
identified by the N(R) in the received RR or REJ frame.
.ce
.parag
.ce
2.4.5.8
.ce
\fIDCE busy condition\fR
.ce
.parag
.ce
When the DCE enters a busy condition, it will transmit an RNR frame at
the earliest opportunity. The RNR frame will be a command frame with the
P bit set to\ 1 if an acknowledged transfer of the busy condition indication
is
.ce
required; otherwise the RNR frame may be either a command or a response
frame. While in the busy condition, the DCE will accept and process supervisory
.ce
frames, will accept and process the contents of the N(R) fields of I\ frames,
.ce
and will return an RNR response with the F bit set to\ 1 if it receives a
.ce
supervisory command or I command frame with the P bit set to\ 1. To clear the
.ce
busy condition, the DCE will transmit either an REJ frame or an RR frame, with
.ce
N(R) set to the current receive state variable V(R), depending on whether or
.ce
not it discarded information fields of correctly received I\ frames. The REJ
.ce
frame or the RR frame will be a command frame with the P bit set to\ 1 if an
.ce
acknowledged transfer of the busy\(hyto\(hynon\(hybusy transition is required,
otherwise the REJ frame or the RR frame may be either a command or a response
frame.
.ce
.parag
.ce

.ce
2.4.5.9
.ce
\fIWaiting acknowledgement\fR
.ce
.parag
.ce
The DCE maintains an internal transmission attempt variable which is set
to\ 0 when the DCE sends a UA response, when the DCE receives a UA
.ce
response or an RNR command or response, or when the DCE correctly receives
an I\ frame or supervisory frame with the N(R) higher than the last received
N(R) (actually acknowledging some outstanding I\ frames).
.ce
.parag
.ce
If Timer T1 runs out waiting for the acknowledgement from the DTE for an
I\ frame transmitted, the DCE will enter the timer recovery condition,
add
.ce
one to its transmission attempt variable and set an internal variable \fIx\fR
.ce
to
.ce
the current value of its send state variable V(S). The DCE will then restart
.ce
Timer T1, set its send state variable V(S) to the last value of N(R) received
from the DTE and retransmit the corresponding I\ frame with the P bit set
to\ 1, or transmit an appropriate supervisory command frame (RR, RNR or
REJ) with the P\ bit set to\ 1.
.ce
.parag
.ce

.ce
The timer recovery condition is cleared when the DCE receives a valid supervisory
frame with the F\ bit\ set to\ 1.
.ce
.parag
.ce
If, while in the timer recovery condition, the DCE correctly receives a
supervisory frame with the F bit set to\ 1 and with the N(R) within the
range from its current send state variable V(S) to \fIx\fR
.ce
included, it will clear the
.ce
timer recovery condition (including stopping Timer\ T1) and set its send
state variable V(S) to the value of the received N(R), and may then resume
with
.ce
I\ frame transmission or retransmission, as appropriate.
.ce
.parag
.ce
If, while in the timer recovery condition, the DCE correctly receives an
I or supervisory frame with the P/F bit set to\ 0 and with a valid N(R)
(see \(sc\ 2.3.4.9), it will not clear the timer recovery condition. The
value of the
.ce
received N(R) may be used to update the send state variable V(S). However,
the DCE may decide to keep the last transmitted I\ frame in store (even
if it is
.ce
acknowledged) in order to be able to retransmit it with the P bit set to\
1 when Timer\ T1 runs out at a later time.
.ce
.parag
.ce

.ce
If the received supervisory frame with the P/F bit set to\ 0 is an REJ
frame with a valid N(R), the DCE may either immediately initiate
.ce
(re)transmission from the value of the send state variable V(S), or it may
.ce
ignore the request for retransmission and wait until the supervisory frame
with the F bit set to\ 1 is received before initiating (re)transmission
of frames
.ce
from the value identified in the N(R) field of the supervisory frame with
the F\ bit set to\ 1. In the case of immediate retransmission, in order
to prevent
.ce
duplicate retransmissions following the clearance of the timer recovery
.ce
condition, the DCE shall inhibit retransmission of a specific I\ frame [same
.ce
N(R) in the same numbering cycle] if the DCE has retransmitted that I\
frame as the result of a received REJ frame with the P/F bit set to\ 0.
.ce
.parag
.ce
If, while in the timer recovery condition, the DCE receives a REJ
.ce
command with the P bit set to\ 1, the DCE will respond immediately with an
.ce
appropriate supervisory response with the F bit set to\ 1. The DCE may
then use the value of the N(R) in the REJ command to update the send state
variable
.ce
V(S), and may either immediately begin (re)transmission from the value N(R)
.ce
indicated in the REJ frame or ignore the request for retransmission and wait
.ce
until the supervisory frame with the F bit set to\ 1 is received before
.ce
initiating (re)transmission of I\ frames from the value identified in the
N(R) field of the supervisory frame with the F\ bit set to\ 1. In the case
of
.ce
immediate retransmission, in order to prevent duplicate retransmissions
.ce
following the clearance of the timer recovery condition, the DCE shall
inhibit retransmission of a specific I\ frame [same N(R) in the same numbering
cycle]
.ce
if the DCE has retransmitted that I\ frame as the result of the received REJ
.ce
command with the P\ bit set to\ 1.
.ce
.parag
.ce
If Timer T1 runs out in the timer recovery condition, and no I or
.ce
supervisory frame with the P/F bit set to 0 and with a valid N(R) has been
.ce
received, or no REJ command with the P bit set to\ 1 and with a valid N(R)
has been received, the DCE will add one to its transmission attempt variable,
.ce
restart Timer\ T1, and either retransmit the I frame sent with the P bit set
.ce
to\ 1 or transmit an appropriate supervisory command with the P bit set to\ 1.
.ce
.parag
.ce

.ce
If the transmission attempt variable is equal to N2, the DCE will
.ce
initiate a data link resetting procedure as described in \(sc\ 2.4.7.2
below, or
.ce
will transmit a DM response to ask the DTE to initiate a data link set\(hyup
.ce
procedure as described in \(sc\ 2.4.4.1 above and enter the disconnected
phase. N2 is a system parameter (see \(sc\ 2.4.8.4 below).
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ Although the DCE may implement the internal variable \fIx\fR
.ce
,
.ce
other mechanisms do exist that achieve the identical function.
.ce
.parag
.ce
2.4.6
.ce
\fILAPB conditions for \fR
.ce
\fIdata link resetting or data link\fR
.ce

.ce
\fIre\(hyinitialization\fR
.ce
\fI(data link set\(hyup)\fR
.ce
.parag
.ce
2.4.6.1
.ce
When the DCE receives, during the information transfer phase, a
.ce
frame which is not invalid (see \(sc\ 2.3.5.3) with one of the conditions
listed in \(sc\ 2.3.4.9 above, the DCE will request the DTE to initiate
a data link resetting procedure by transmitting an FRMR response to the
DTE as described in
.ce
\(sc\ 2.4.7.3.
.ce
.parag
.ce
2.4.6.2
.ce
When the DCE receives, during the information transfer phase, an FRMR response
from the DTE, the DCE will either initiate the data link
.ce
resetting procedures itself as described in \(sc\ 2.4.7.2 or return a DM
response to ask the DTE to initiate the data link set\(hyup (initialization)
procedure as described in \(sc\ 2.4.4.1. After transmitting a DM response,
the DCE will enter
.ce
the disconnected phase as described in \(sc\ 2.4.4.4.2.
.ce
.parag
.ce
2.4.6.3
.ce
When the DCE receives, during the information transfer phase, a UA response,
or an unsolicited response with the F bit set to\ 1, the DCE may
.ce
either initiate the data link resetting procedures itself as described in
.ce
\(sc\ 2.4.7.2, or return a DM response to ask the DTE to initiate the data link
.ce
set\(hyup (initialization) procedure as described in \(sc\ 2.4.4.1. After
transmitting a DM response, the DCE will enter the disconnected phase as
described in
.ce
\(sc\ 2.4.4.4.2.
.ce
.parag
.ce
2.4.6.4
.ce
When the DCE receives, during the information transfer phase, a DM response
from the DTE, the DCE will either initiate the data link set\(hyup
.ce
(initialization) procedures itself as described in \(sc\ 2.4.4.1, or return
a DM
.ce
response to ask the DTE to initiate the data link set\(hyup (initialization)
.ce
procedures as described in \(sc\ 2.4.4.1. After transmitting a DM response,
the DCE will enter the disconnected phase as described in \(sc\ 2.4.4.4.2.
.ce
.parag
.ce

.ce
2.4.7
.ce
\fILAPB procedure for data link resetting\fR
.ce
.parag
.ce
2.4.7.1
.ce
The data link resetting procedure is used to initialize both
.ce
directions of information transfer according to the procedure described
below. The data link resetting procedure only applies during the information
transfer phase.
.ce
.parag
.ce
2.4.7.2
.ce
Either the DTE or the DCE may initiate the data link resetting
.ce
procedure. The data link resetting procedure indicates a clearance of a DCE
.ce
and/or DTE busy condition, if present.
.ce
.parag
.ce
The DTE shall initiate a data link resetting by transmitting an
.ce
SABM/SABME command to the DCE. If, upon correct receipt of the SABM/SABME
.ce
command, the DCE determines that it can continue in the information transfer
.ce
phase, it will return a UA response to the DTE, will reset its send and
receive state variables V(S) and V(R) to zero, and will remain in the information
.ce
transfer phase. If, upon correct receipt of the SABM/SABME command, the DCE
.ce
determines that it cannot remain in the information transfer phase, it will
.ce
return a DM response as a denial to the resetting request and will enter the
.ce
disconnected phase.
.ce
.parag
.ce
The DCE will initiate a data link resetting by transmitting an
.ce
SABM/SABME command to the DTE and starting its Timer\ T1 (see \(sc\ 2.4.8.1
below). Upon reception of a UA response from the DTE, the DCE will reset
its send and receive state variables V(S) and V(R) to zero, will stop its
Timer\ T1, and will remain in the information transfer phase. Upon reception
of a DM response from the DTE as a denial to the data link resetting request,
the DCE will stop its Timer\ T1 and will enter the disconnected phase.
.ce
.parag
.ce
The DCE, having sent an SABM/SABME command, will ignore and discard
.ce
any frames received from the DTE except an SABM/SABME or DISC command,
or a UA or DM response. The receipt of an SABM/SABME or DISC command from
the DTE will result in a collision situation that is resolved per \(sc\
2.4.4.5 above. Frames
.ce
other than the UA or DM response sent in response to a received SABM/SABME
or DISC command will be sent only after the data link is reset and if no
.ce
outstanding SABM/SABME command exists.
.ce
.parag
.ce

.ce
After the DCE sends the SABM/SABME command, if a UA or DM response is not
received correctly, Timer\ T1 will run out in the DCE. The DCE will then
.ce
resend the SABM/SABME command and will restart Timer\ T1. After N2 attempts
to reset the data link, the DCE will initiate appropriate higher layer
recovery
.ce
action and will enter the disconnected phase. The value of N2 is defined in
.ce
\(sc\ 2.4.8.4 below.
.ce
.parag
.ce
2.4.7.3
.ce
The DCE may ask the DTE to reset the data link by transmitting an FRMR
response (see \(sc\ 2.4.6.1 above). After transmitting an FRMR response,
.ce
the DCE will enter the frame rejection condition.
.ce
.parag
.ce
The frame rejection condition is cleared when the DCE receives an SABM/SABME
command, a DISC command, a FRMR response, or a DM response; or if
.ce
the DCE transmits an SABM/SABME command, a DISC command, or a DM response.
.ce
Other commands received while in the frame rejection condition will cause
the DCE to retransmit the FRMR response with the same information field
as
.ce
originally transmitted.
.ce
.parag
.ce
The DCE may start Timer\ T1 on transmission of the FRMR response. If
.ce
Timer\ T1 runs out before the frame rejection condition is cleared, the
DCE may retransmit the FRMR response, and restart T1. After N2 attempts
(time outs) to get the DTE to reset the data link, the DCE may reset the
data link itself as described in \(sc\ 2.4.7.2 above. The value of N2 is
defined in \(sc\ 2.4.8.4 below.
.ce
.parag
.ce

.ce
In the frame rejection condition, I frames and supervisory frames will
not be transmitted by the DCE. Also, received I frames and supervisory
frames will be discarded by the DCE except for the observance of a P bit
set to\ 1.
.ce
When an additional FRMR response must be transmitted by the DCE as a result
of the receipt of a P\ bit set to 1 while Timer\ T1 is running, Timer\
T1 will
.ce
continue to run. Upon reception of an FRMR response (even during a frame
.ce
rejection condition), the DCE will initiate a resetting procedure by
.ce
transmitting an SABM/SABME command as described in \(sc\ 2.4.7.2 above, or will
.ce
transmit a DM response to ask the DTE to initiate the data link set\(hyup
.ce
procedure as described in \(sc\ 2.4.4.1 and enter the disconnected phase.
.ce
.parag
.ce
2.4.8
.ce
\fIList of \fR
.ce
\fILAPB system parameters\fR
.ce
.parag
.ce
The DCE and DTE system parameters are as follows:
.ce
.parag
.ce

.ce
2.4.8.1
.ce
\fITimer\fR
.ce
\fIT1\fR
.ce
.parag
.ce
The value of the DTE Timer T1 system parameter may be different
.ce
than the value of the DCE Timer T1 system parameter. These values shall
be made known to both the DTE and the DCE, and agreed to for a period of
time by both the DTE and the DCE.
.ce
.parag
.ce
The period of Timer T1, at the end of which retransmission of a frame may
be initiated (see \(sc\ 2.4.4 and \(sc\ 2.4.5 above for the DCE), shall
take into account whether T1 is started at the beginning or the end of
the transmission of a frame.
.ce
.parag
.ce
The proper operation of the procedure requires that the transmitter's (DCE
or DTE) Timer\ T1 be greater than the maximum time between transmission
of a frame (SABM/SABME, DISC, I\ or supervisory command, or DM or FRMR
response)
.ce
and the reception of the corresponding frame returned as an answer to that
.ce
frame (UA, DM or acknowledging frame). Therefore, the receiver (DCE or DTE)
.ce
should not delay the response or acknowledging frame returned to one of the
.ce
above frames by more than a value\ T2, where T2 is a system parameter (see
.ce
\(sc\ 2.4.8.2).
.ce
.parag
.ce

.ce
The DCE will not delay the response or acknowledging frame returned to
one of the above DTE frames by more than a period\ T2.
.ce
.parag
.ce
2.4.8.2
.ce
\fIParameter T2\fR
.ce
.parag
.ce
The value of the DTE parameter T2 may be different than the value of the
DCE parameter T2. These values shall be made known to both the DTE and
the DCE, and agreed to for a period of time by both the DTE and the DCE.
.ce
.parag
.ce

.ce
The period of parameter T2 shall indicate the amount of time
.ce
available at the DCE or DTE before the acknowledging frame must be initiated
in order to ensure its receipt by the DTE or DCE, respectively, prior to
Timer\ T1 running out at the DTE or DCE (parameter\ T2\ <\ Timer\ T1).
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ The period of parameter T2 shall take into account the
.ce
following timing factors: the transmission time of the acknowledging frame,
the propagation time over the access data link, the stated processing times
at the DCE and the DTE, and the time to complete the transmission of the
frame(s) in the DCE or DTE transmit queue that are neither displaceable
or modifiable in an orderly manner.
.ce
.parag
.ce

.ce
Given a value for Timer T1 for the DTE or DCE, the value of parameter T2
at the DCE or DTE, respectively, must be no larger than T1 minus\ 2 times
the propagation time over the access data link, minus the frame processing
time at the DCE, minus the frame processing time at the DTE, and minus
the transmission time of the acknowledging frame by the DCE or DTE, respectively.
.ce
.parag
.ce

.ce
2.4.8.3
.ce
\fITimer T3\fR
.ce
.parag
.ce
The DCE shall support a Timer T3 system parameter, the value of
.ce
which shall be made known to the DTE.
.ce
.parag
.ce
The period of Timer T3, at the end of which an indication of an
.ce
observed excessively long idle channel state condition is passed to the
Packet Layer, shall be sufficiently greater than the period of the DCE
Timer T1
.ce
(i.e.\ T3\ >\ T1) so that the expiration of T3 provides the desired level of
.ce
assurance that the data link channel is in a non\(hyactive, non\(hyoperational
state, and is in need of data link set\(hyup before normal data link operation
can
.ce
resume.
.ce
.parag
.ce
2.4.8.4
.ce
\fIMaximum number of attempts\fR
.ce
\fIto complete a\fR
.ce

.ce
\fItransmission N2\fR
.ce
.parag
.ce

.ce
The value of the DTE N2 system parameter may be different than the value
of the DCE N2 system parameter. These values shall be made known to both
the DTE and the DCE, and agreed to for a period of time by both the DTE
and the DCE.
.ce
.parag
.ce
The value of N2 shall indicate the maximum number of attempts made by the
DCE or DTE to complete the successful transmission of a frame to the DTE
or DCE, respectively.
.ce
.parag
.ce
2.4.8.5
.ce
\fIMaximum number of bits in an I frame N1\fR
.ce
.parag
.ce
The value of the DTE N1 system parameter may be different than the value
of the DCE N1 system parameter. These values shall be made known to both
the DTE and the DCE.
.ce
.parag
.ce
The values of N1 shall indicate the maximum number of bits in an
.ce
I\ frame (excluding flags and 0\ bits inserted for transparency) that the
DCE or DTE is willing to accept from the DTE or DCE, respectively.
.ce
.parag
.ce
In order to allow for universal operation, a DTE should support a
.ce
value of DTE N1 which is not less than 1080\ bits (135\ octets). DTEs should
be aware that the network may transmit longer packets (see \(sc\ 5.2),
that may result in a data link layer problem.
.ce
.parag
.ce

.ce
All networks shall offer to a DTE which requires it, a value of DCE N1
which is greater than or equal to 2072\ bits (259\ octets) plus the length
of the address, control and FCS fields at the DTE/DCE interface, and greater
than or equal to the maximum length of the data packets which may cross
the DTE/DCE
.ce
interface plus the length of the address, control and FCS fields at the
DTE/DCE interface.
.ce
.parag
.ce
Appendix II provides a description of how the values stated above are  derived.
.ce
.parag
.ce
2.4.8.6
.ce
\fIMaximum number of \fR
.ce
\fIoutstanding I frames k\fR
.ce
.parag
.ce
The value of the DTE k system parameter shall be the same as the
.ce
value of the DCE k system parameter. This value shall be agreed to for a
.ce
period of time by both the DTE and the DCE.
.ce
.parag
.ce
The value of k shall indicate the maximum number of sequentially
.ce
numbered I\ frames that the DTE or DCE may have outstanding
.ce
(i.e.\ unacknowledged) at any given time. The value of k shall never exceed
.ce
seven for modulo\ 8 operation, or one hundred and twenty\(hyseven for modulo\
128
.ce
operation. All networks (DCEs) shall support a value of seven. Other values
of k (less than and greater than seven) may also be supported by networks
.ce
(DCEs).
.ce
.parag
.ce
.line
.ce
\fBMONTAGE : \(sc 2.5 de cette Recommandation sur le reste de cette page\fR
.ce
.parag
.ce

.ce
.line
.ad r
\fBTable 1/X.25 [T1.25], p.\fR
.sp 1P
.RT
.ad b
.RT
.ce
.line
.ce
\fBTable 2/X.25 [T2.25], p.\fR
.ce
2.2.2
.ce
\fIFlag sequence\fR
.ce
.parag
.ce
All frames shall start and end with the flag sequence consisting of one
0\ bit followed by six contiguous 1\ bits and one 0\ bit. The DTE and DCE
.ce
shall only send complete eight\(hybit flag sequences when sending multiple flag
.ce
sequences (see \(sc\ 2.2.11). A single flag may be used as both the
.ce
closing
.ce
flag
.ce
for one frame and the
.ce
opening flag
.ce
for the next frame.
.ce
.parag
.ce
2.2.3
.ce
\fIAddress field\fR
.ce
.parag
.ce
The address field shall consist of one octet. The address field
.ce
identifies the intended receiver of a command frame and the transmitter of a
.ce
response frame. The coding of the address field is described in \(sc\ 2.4.2
(LAPB) and in \(sc\ 2.7.1 (LAP) below.
.ce
.parag
.ce
2.2.4
.ce
\fIControl field\fR
.ce
.parag
.ce
For modulo\ 8 (basic) operation, the control field shall consist of one
octet. For modulo\ 128 (extended) operation, the control field shall consist
of two octets for frame formats that contain sequence numbers, and one
octet
.ce
for frame formats that do not contain sequence numbers. The content of this
.ce
field is described in \(sc\ 2.3.2\ (LAPB) and in \(sc\ 2.6.2 (LAP) below.
.ce
.parag
.ce
2.2.5
.ce
\fIInformation field\fR
.ce
.parag
.ce
The information field of a frame, when present, follows the control field
(see \(sc\ 2.2.4 above) and precedes the
.ce
frame check sequence field
.ce
(see \(sc\ 2.2.7 below).
.ce
.parag
.ce
See \(sc\(sc 2.3.4.9, 2.5.2, 2.6.4.8 and 5 for the various codings and
.ce
groupings of bits in the information field as used in this Recommendation.
.ce
.parag
.ce

.ce
See \(sc\(sc 2.3.4.9, 2.4.8.5, 2.6.4.8 and 2.7.7.5 below with regard to
the maximum information field length.
.ce
.parag
.ce
2.2.6
.ce
\fITransparency\fR
.ce
.parag
.ce
The DCE or DTE, when transmitting, shall examine the frame content between
the two flag sequences including the address, control, information and
FCS fields and shall insert a 0 bit after all sequences of 5 contiguous
1\ bits (including the last 5\ bits of the FCS) to ensure that a flag sequence
is not
.ce
simulated. The DCE or DTE, when receiving, shall examine the frame content
and shall discard any 0\ bit which directly follows\ 5 contiguous 1\ bits.
.ce
.parag
.ce
2.2.7
.ce
\fIFrame check sequence (FCS)\fR
.ce
\fI field\fR
.ce
.parag
.ce
The notation used to describe the FCS is based on the property of cyclic
codes that a code vector such as 1000000100001 can be represented by a
polynomial \fIP\fR
.ce
(\fIx\fR
.ce
)\ =\ \fIx\fR
.ce
\u1\d\u2\d\ +\ \fIx\fR
.ce
\u5\d\ +\ 1. The elements of an \fIn\fR
.ce
\(hyelement
.ce
code word are thus the coefficients of a polynomial of order \fIn\fR
.ce
\ \(em\ 1. In this application, these coefficients can have the value\
0 or\ 1 and the polynomial
.ce
operations are performed modulo\ 2. The polynomial representing the content
of a frame is generated using the first bit received after the frame opening
flag as the coefficient of the highest order term.
.ce
.parag
.ce
The FCS field shall be a 16\(hybit sequence. It shall be the ones
.ce
complement of the sum (modulo\ 2) of:
.ce
.parag
.ce
1)
.ce
the remainder of
.ce
\fIx\fR
.ce
\uD\dlFk\fR
.ce
(\fIx\fR
.ce
\u1\d\u5\d\uD\dlF036+\ \fIx\fR
.ce
\u1\d\u4\d\ +\ \fIx\fR
.ce
\u1\d\u3\d\ +
.ce
\fIx\fR
.ce
\u1\d\u2\d\ +\ \fIx\fR
.ce
\u1\d\u1\d\ +\ \fIx\fR
.ce
\u1\d\u0\d\ +\ \fIx\fR
.ce
\u9\d\ +\ \fIx\fR
.ce
\u8\d\ +
.ce
\fIx\fR
.ce
\u7\d\ +\ \fIx\fR
.ce
\u6\d\ +\ \fIx\fR
.ce
\u5\d\ +\ \fIx\fR
.ce
\u4\d\uD\dlF036+\ \fIx\fR
.ce
\u3\d\ +
.ce
+\ \fIx\fR
.ce
\u2\d\ +\ \fIx\fR
.ce
\ +\ 1) divided (modulo\ 2)
.ce
by the
.ce
generator polynomial \fIx\fR
.ce
\u1\d\u6\d\ +\ \fIx\fR
.ce
\u1\d\u2\d\ +\ \fIx\fR
.ce
\u5\d\ +\ 1,
.ce
where \fIk\fR
.ce
is the number of bits in the frame existing between,
.ce
but not including, the final bit of the opening flag and the
.ce
first bit of the FCS, excluding bits inserted for transparency,
.ce
and
.ce
.parag
.ce
2)
.ce
the remainder of the division (modulo 2) by the generator
.ce
polynomial \fIx\fR
.ce
\u1\d\u6\d\ +\ \fIx\fR
.ce
\u1\d\u2\d\ +\ \fIx\fR
.ce
\u5\d\ +\ 1 of the
.ce
product of \fIx\fR
.ce
\u1\d\u6\d by the content of the frame, existing
.ce
between but not including, the final bit of the opening flag and
.ce
the first bit of the FCS, excluding bits inserted for
.ce
transparency.
.ce
.parag
.ce

.ce
As a typical implementation, at the transmitter, the initial
.ce
content of the register of the device computing the remainder of the division
is preset to all 1s and is then modified by division by the generator
.ce
polynomial (as described above) on the address, control and information
fields; the ones complement of the resulting remainder is transmitted as
the 16\(hybit
.ce
FCS.
.ce
.parag
.ce
At the receiver, the initial content of the register of the device
.ce
computing the remainder is preset to all 1s. The final remainder, after
.ce
multiplication by \fIx\fR
.ce
\u1\d\u6\d and then division (modulo\ 2) by the generator
.ce
polynomial \fIx\fR
.ce
\u1\d\u6\d\ +\ \fIx\fR
.ce
\u1\d\u2\d\ +\ \fIx\fR
.ce
\u5\d\ +\ 1 of the serial incoming protected bits and the FCS, will be
0001110100001111 (\fIx\fR
.ce
\u1\d\u5\d through
.ce
\fIx\fR
.ce
\u0\d, respectively) in the absence of transmission errors.
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ Examples of transmitted bit patterns by the DCE and the DTE illustrating
application of the transparency mechanism and the frame check
.ce
sequence to the SABM command and the UA response are given in Appendix\ I.
.ce
.parag
.ce
2.2.8
.ce
\fIOrder of bit transmission\fR
.ce
.parag
.ce
Addresses, commands, responses and sequence numbers shall be
.ce
transmitted with the low\(hyorder bit first (for example, the first bit of the
.ce
sequence number that is transmitted shall have the weight 2\u0\d). The
order of transmitting bits within the information field is not specified
under \(sc\ 2 of
.ce
this Recommendation. The FCS shall be transmitted to the line commencing
with the coefficient of the highest term, which is found in bit position\
16 of the FCS\ field (see Tables\ 1/X.25 and 2/X.25).
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ In Tables 1/X.25 to 13/X.25, bit 1 is defined as the
.ce
low\(hyorder bit.
.ce
.parag
.ce
2.2.9
.ce
\fIInvalid frames\fR
.ce
.parag
.ce
The definition of an invalid frame is described in \(sc\ 2.3.5.3 (LAPB)
and in \(sc\ 2.6.5.3 (LAP) below.
.ce
.parag
.ce
2.2.10
.ce
\fIFrame abortion\fR
.ce
.parag
.ce
Aborting a frame is performed by transmitting at least seven
.ce
contiguous 1\ bits (with no inserted 0\ bits).
.ce
.parag
.ce
2.2.11
.ce
\fIInterframe time fill\fR
.ce
.parag
.ce
Interframe time fill is accomplished by transmitting contiguous
.ce
flags between frames, i.e.\ multiple eight\(hybit flag sequences (see
.ce
\(sc\ 2.2.2).
.ce
.parag
.ce
2.2.12
.ce
\fILink channel states\fR
.ce
.parag
.ce
A link channel as defined here is the means for transmission for
.ce
one direction.
.ce
.parag
.ce
2.2.12.1
.ce
\fIActive channel state\fR
.ce
.parag
.ce
The DCE incoming or outgoing channel is defined to be in an active condition
when it is receiving or transmitting, respectively, a frame, an
.ce
abortion sequence or interframe time fill.
.ce
.parag
.ce
2.2.12.2
.ce
\fIIdle channel state\fR
.ce
.parag
.ce
The DCE incoming or outgoing channel is defined to be in an idle
.ce
condition when it is receiving or transmitting, respectively, a continuous\
1s state for a period of at least 15\ bit times.
.ce
.parag
.ce
See \(sc\ 2.3.5.5 for a description of DCE action when an idle condition
exists on its incoming channel for an excessive period of time.
.ce
.parag
.ce

.ce
2.3
.ce
\fILAPB elements of procedures\fR
.ce
.parag
.ce
2.3.1
.ce
The LAPB elements of procedures are defined in terms of actions
.ce
that occur on receipt of frames at the DCE or\ DTE.
.ce
.parag
.ce
The elements of procedures specified below contain the selection
.ce
of commands and responses relevant to the LAPB data link and system
.ce
configurations
.ce
described in \(sc\ 2.1 above. Together, \(sc\(sc\ 2.2 and 2.3 form the general
.ce
requirements for the proper management of a LAPB access data link.
.ce
.parag
.ce

.ce
2.3.2
.ce
\fILAPB control field formats and parameters\fR
.ce
.parag
.ce

.ce
2.3.2.1
.ce
\fIControl field formats\fR
.ce
.parag
.ce
The control field contains a command or a response, and sequence
.ce
numbers where applicable.
.ce
.parag
.ce
Three types of control field formats are used to perform numbered
.ce
information transfer (I\ format), numbered supervisory functions (S\ format)
and unnumbered control functions (U\ format).
.ce
.parag
.ce
The control field formats for basic (modulo\ 8) operation are depicted
in Table\ 3/X.25.
.ce
.parag
.ce

.ce
The control field formats for extended (modulo 128) operation are
.ce
depicted in Table\ 4/X.25.
.ce
.parag
.ce
.line
.ce
\fBTable 3/X.25 [T3.25], p.\fR
.ce
.line
.ce
\fBTable 4/X.25 [T4/X.25], p.\fR
.ce
2.3.2.1.1
.ce
\fIInformation transfer format\fR
.ce
\fI \(em I\fR
.ce
.parag
.ce
The I format is used to perform an information transfer. The
.ce
functions of N(S), N(R) and P are independent; i.e.,\ each I\ frame has
an N(S), an N(R) which may or may not acknowledge additional I\ frames
received by the
.ce
DCE or DTE, and a P\ bit that may be set to\ 0 or\ 1.
.ce
.parag
.ce
2.3.2.1.2
.ce
\fISupervisory format\fR
.ce
\fI \(em S\fR
.ce
.parag
.ce
The S format is used to perform data link supervisory control
.ce
functions such as acknowledge I\ frames, request retransmission of I\ frames,
and to request a temporary suspension of transmission of I\ frames. The
functions of N(R) and P/F are independent; i.e.,\ each supervisory frame
has an N(R) which
.ce
may or may not acknowledge additional I\ frames received by the DCE or\
DTE, and a P/F\ bit that may be set to\ 0 or\ 1.
.ce
.parag
.ce
2.3.2.1.3
.ce
\fIUnnumbered format\fR
.ce
\fI \(em U\fR
.ce
.parag
.ce
The U format is used to provide additional data link control
.ce
functions. This format contains no sequence numbers, but does include a
P/F\ bit that may be set to\ 0 or\ 1. The unnumbered frames have the same
control field
.ce
length (one octet) in both basic (modulo\ 8) operation and extended (modulo\
128) operation.
.ce
.parag
.ce
2.3.2.2
.ce
\fIControl field parameters\fR
.ce
.parag
.ce
The various parameters associated with the control field formats
.ce
are described below.
.ce
.parag
.ce
2.3.2.2.1
.ce
\fIModulus\fR
.ce
.parag
.ce
Each I frame is sequentially numbered and may have the value\ 0
.ce
through modulus minus\ 1 (where \*Qmodulus\*U is the modulus of the sequence
.ce
numbers). The modulus equals either\ 8 or\ 128 and the sequence numbers cycle
.ce
through the entire range.
.ce
.parag
.ce
2.3.2.2.2
.ce
\fISend state variable\fR
.ce
\fI V(S)\fR
.ce
.parag
.ce
The send state variable V(S) denotes the sequence number of the
.ce
next in\(hysequence I\ frame to be transmitted. V(S) can take on the values\ 0
.ce
through modulus minus\ 1. The value of V(S) is incremented by\ 1 with each
.ce
successive I\ frame transmission, but cannot exceed the N(R) of the last
.ce
received\ I or supervisory frame by more than the maximum number of outstanding
I\ frames\ (\fIk\fR
.ce
). The value of\ k is defined in \(sc\ 2.4.8.6 below.
.ce
.parag
.ce

.ce
2.3.2.2.3
.ce
\fISend sequence number\fR
.ce
\fI N(S)\fR
.ce
.parag
.ce
Only I frames contain N(S), the send sequence number of transmitted I\
frames. At the time that an in\(hysequence I\ frame is designated for
.ce
transmission, the value of N(S) is set equal to the value of the send state
.ce
variable\ V(S).
.ce
.parag
.ce
2.3.2.2.4
.ce
\fIReceive state variable\fR
.ce
\fIV(R)\fR
.ce
.parag
.ce
The receive state variable V(R) denotes the sequence number of the next
in\(hysequence I\ frame expected to be received. V(R) can take on the values
0 through modulus minus\ 1. The value of V(R) is incremented by\ 1 by the
receipt of an error\(hyfree, in\(hysequence I\ frame whose send sequence
number N(S) equals
.ce
the receive state variable V(R).
.ce
.parag
.ce
2.3.2.2.5
.ce
\fIReceive sequence number\fR
.ce
\fI N(R)\fR
.ce
.parag
.ce
All I frames and supervisory frames contain N(R), the expected send sequence
number of the next received I\ frame. At the time that a frame of the above
types is designated for transmission, the value of N(R) is set equal to
the current value of the receive state variable V(R). N(R) indicates that
the DCE or DTE transmitting the N(R) has received correctly all I\ frames
numbered up to and including N(R)\ \(em\ 1.
.ce
.parag
.ce
2.3.2.2.6
.ce
\fIPoll/Final bit\fR
.ce
\fI P/F\fR
.ce
.parag
.ce
All frames contain P/F, the Poll/Final bit. In command frames, the P/F
bit is referred to as the P bit. In response frames, it is referred to
as the F\ bit.
.ce
.parag
.ce
2.3.3
.ce
\fIFunctions of the Poll/Final bit\fR
.ce
.parag
.ce
The Poll bit set to 1 is used by the DCE or DTE to solicit (poll) a response
from the DTE or DCE, respectively. The Final bit set to\ 1 is used by the
DCE or DTE to indicate the response frame transmitted by the DTE or DCE,
.ce
respectively, as a result of the soliciting (poll) command.
.ce
.parag
.ce
The use of the P/F bit is described in \(sc\ 2.4.3 below.
.ce
.parag
.ce
2.3.4
.ce
\fICommands and responses\fR
.ce
.parag
.ce
For basic (modulo 8) operation, the commands and responses
.ce
represented in Table\ 5/X.25 will be supported by the DCE and the\ DTE.
.ce
.parag
.ce

.ce
For extended (modulo 128) operation, the commands and responses
.ce
represented in Table\ 6/X.25 will be supported by the DCE and the DTE.
.ce
.parag
.ce

.ce
For purposes of the LAPB procedures, the supervisory function bit encoding
\*Q11\*U and those encodings of the modifier function bits in
.ce
Tables\ 3/X.25 and 4/X.25 not identified in Tables\ 5/X.25 or 6/X.25 are
.ce
identified as \*Qundefined or not implemented\*U command and response control
.ce
fields.
.ce
.parag
.ce
The commands and responses in Tables 5/X.25 and 6/X.25 are defined as  follows:
.ce
.parag
.ce
2.3.4.1
.ce
\fIInformation (I)\fR
.ce
\fI command\fR
.ce
.parag
.ce
The function of the information (I) command is to transfer across a data
link a sequentially numbered frame containing an information field.
.ce
.parag
.ce
.line
.ce
\fBTable 5/X.25 [T5.25], p.\fR
.ce
.line
.ce
\fBTable 6/X.25 [T6.25], p.\fR
.ce
2.3.4.2
.ce
\fIReceive ready (RR)\fR
.ce
\fI command and response\fR
.ce
.parag
.ce
The receive ready (RR) supervisory frame is used by the DCE
.ce
or DTE to:
.ce
.parag
.ce

.ce
1)
.ce
indicate it is ready to receive an I frame; and
.ce
.parag
.ce
2)
.ce
acknowledge previously received I frames numbered up to and
.ce
including N(R)\ \(em\ 1.
.ce
.parag
.ce
An RR frame may be used to indicate the clearance of a busy
.ce
condition that was reported by the earlier transmission of an RNR frame
by that same station (DCE or\ DTE). In addition to indicating the DCE or
DTE status, the RR command with the P\ bit set to\ 1 may be used by the
DCE or DTE to ask for the status of the DTE or DCE, respectively.
.ce
.parag
.ce
2.3.4.3
.ce
\fIReceive not ready (RNR) command and response\fR
.ce
.parag
.ce
The receive not ready (RNR) supervisory frame is used by the DCE or DTE
to indicate a busy condition; i.e.\ temporary inability to accept
.ce
additional incoming I\ frames. I\ frames numbered up to and including N(R)\
\(em\ 1
.ce
are acknowledged. I\ frame N(R) and any subsequent I\ frames received, if any,
.ce
are not acknowledged; the acceptance status of these I\ frames will be
indicated in subsequent exchanges.
.ce
.parag
.ce
In addition to indicating the DCE or DTE status, the RNR command with the
P bit set to\ 1 may be used by an DCE or DTE to ask for the status of the
.ce
DTE or DCE, respectively.
.ce
.parag
.ce
2.3.4.4
.ce
\fIReject (REJ) command and response\fR
.ce
.parag
.ce
The reject (REJ) supervisory frame is used by the DCE or DTE to request
transmission of I\ frames starting with the frame numbered N(R).
.ce
I\ frames numbered N(R)\ \(em\ 1 and below are acknowledged. Additional
I\ frames
.ce
pending initial transmission may be transmitted following the retransmitted
.ce
I\ frame(s).
.ce
.parag
.ce

.ce
Only one REJ exception condition for a given direction of information transfer
may be established at any time. The REJ exception condition is cleared
(reset) upon the receipt of an I\ frame with an N(S) equal to the N(R)
of the
.ce
REJ\ frame.
.ce
.parag
.ce
An REJ frame may be used to indicate the clearance of a busy condition
that was reported by the earlier transmission of an RNR frame by that same
.ce
station (DCE or\ DTE). In addition to indicating the DCE or DTE status, the
.ce
REJ\ command with the P bit set to\ 1 may be used by the DCE or\ DTE to ask for
.ce
the status of the DTE or\ DCE, respectively.
.ce
.parag
.ce
\fR
.ce
2.3.4.5
.ce
\fISet asynchronous balanced mode (SABM)\fR
.ce
\fIcommand/\fR
.ce
\fISet asynchronous balanced mode extended (SABME)\fR
.ce

.ce
\fIcommand (subscription time option)\fR
.ce
.parag
.ce

.ce
The SABM unnumbered command is used to place the addressed DCE or DTE in
an asynchronous balanced mode (ABM) information transfer phase where all
command/response control fields will be one\ octet in length.
.ce
.parag
.ce
The SABME unnumbered command is used to place the addressed DCE or DTE
in an asynchronous balanced mode\ (ABM) information transfer phase where
.ce
numbered command/response control fields will be two octets in length, and
.ce
unnumbered command/response control fields will be one octet in length.
.ce
.parag
.ce

.ce
No information field is permitted with the SABM or SABME command. The transmission
of a SABM/SABME command indicates the clearance of a busy
.ce
condition that was reported by the earlier trans
.ce
mission\ of an RNR\ frame by   that
.ce
same station (DCE or\ DTE). The DCE or DTE confirms acceptance of SABM/SABME
.ce
[modulo\ 8 (basic) operation/modulo\ 128 (extended) operation] command by the
.ce
transmission, at the first opportunity, of a UA\ response. Upon acceptance of
.ce
this command, the DCE or DTE send state variable\ V(S) and receive state
.ce
variable V(R) are set to\ 0.
.ce
.parag
.ce
Previously transmitted I\ frames that are unacknowledged when this
.ce
command is actioned remain unac
.ce
knowledged.\ It is the responsibility of a
.ce
higher layer (e.g.\ Packet Layer or MLP) to recover from the possible loss of
.ce
the contents (e.g.\ packets) of such I\ frames.
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ The mode of operation of a data link [basic (modulo 8) or
.ce
extended (modulo\ 128)] is determined at subscription time and is only
changed by going through a new subscription process.
.ce
.parag
.ce
2.3.4.6
.ce
\fIDisconnect (DISC)\fR
.ce
\fI command\fR
.ce
.parag
.ce
The DISC unnumbered command is used to terminate the mode
.ce
previously set. It is used to inform the DCE or DTE receiving the DISC
command that the DTE or DCE sending the DISC command is suspending operation.
No
.ce
information field is permitted with the DISC command. Prior to actioning the
.ce
DISC command, the DCE or DTE receiving the DISC command confirms the acceptance
of the DISC command by the transmission of a UA response. The DTE or DCE
.ce
sending the DISC command enters the disconnected phase when it receives the
.ce
acknowledging UA response.
.ce
.parag
.ce
Previously transmitted I frames that are unacknowledged when this
.ce
command is actioned remain unacknowledged. It is the responsibility of
a higher layer (e.g.\ Packet Layer or MLP) to recover from the possible
loss of the
.ce
contents (e.g.,\ packets) of such I frames.
.ce
.parag
.ce
2.3.4.7
.ce
\fIUnnumbered acknowledgement (UA)\fR
.ce
\fI response\fR
.ce
.parag
.ce
The UA unnumbered response is used by the DCE or DTE to acknowledge the
receipt and acceptance of the mode\(hysetting commands. Received mode\(hysetting
commands are not actioned until the UA response is transmitted. The
.ce
transmission of a UA response indicates the clearance of a busy condition
that was reported by the earlier transmission of an RNR frame by that same
station (DCE or DTE). No information field is permitted with the UA response.
.ce
.parag
.ce

.ce
2.3.4.8
.ce
\fIDisconnected mode (DM)\fR
.ce
\fI response\fR
.ce
.parag
.ce
The DM unnumbered response is used to report a status where the DCE or
DTE is logically disconnected from the data link, and is in the disconnected
phase. The DM response may be sent to indicate that the DCE or DTE has
entered the disconnected phase without benefit of having received a DISC
command, or, if sent in response to the reception of a mode setting command,
is sent to
.ce
inform the DTE or DCE that the DCE or DTE, respectively, is still in the
.ce
disconnected phase and cannot execute the set mode command. No information
.ce
field is permitted with the DM response.
.ce
.parag
.ce
A DCE or DTE in a disconnected phase will monitor received commands
.ce
and will react to an SABM/SABME command as outlined in \(sc\ 2.4.4 below,
and will respond with a DM response with the F bit set to\ 1 to any other
command
.ce
received with the P\ bit set to\ 1.
.ce
.parag
.ce
2.3.4.9
.ce
\fIFrame reject (FRMR)\fR
.ce
\fI response\fR
.ce
.parag
.ce
The FRMR unnumbered response is used by the DCE or DTE to report an error
condition not recoverable by retransmission of the identical frame;
.ce
i.e.\ at least one of the following conditions, which results from the
receipt of a valid frame:
.ce
.parag
.ce
1)
.ce
the receipt of a command or response control field that is
.ce
undefined or not implemented;
.ce
.parag
.ce
2)
.ce
the receipt of an I frame with an information field which
.ce
exceeds the maximum established length;
.ce
.parag
.ce
3)
.ce
the receipt of an invalid N(R); or
.ce
.parag
.ce
4)
.ce
the receipt of a frame with an information field which is
.ce
not permitted or the receipt of a supervisory or unnumbered
.ce
frame with incorrect length.
.ce
.parag
.ce
An undefined or not implemented control field is any of the
.ce
control field encodings that are not identified in Tables\ 5/X.25 or\ 6/X.25.
.ce
.parag
.ce

.ce
A valid N(R) must be within the range from the lowest send sequence
.ce
number N(S) of the still unacknowledged frame(s) to the current DCE send
state variable inclusive (or to the current internal variable \fIx\fR
.ce
if the DCE is in
.ce
the timer recovery condition as described in \(sc\ 2.4.5.9).
.ce
.parag
.ce

.ce
An information field which immediately follows the control field, and consists
of\ 3 or 5\ octets [modulo\ 8 (basic) operation or modulo\ 128 (extended)
operation, respectively], is returned with this response and provides the
.ce
reason for the FRMR response. These formats are given in Tables\ 7/X.25
.ce
and\ 8/X.25.
.ce
.parag
.ce
2.3.5
.ce
\fIException condition reporting and recovery\fR
.ce
.parag
.ce
The error recovery procedures which are available to effect
.ce
recovery following the detection/occurrence of an
.ce
exception condition
.ce
 at the Data Link Layer are described below. Exception conditions described
are those situations which may occur as the result of transmission errors,
DCE or DTE malfunction, or operational situations.
.ce
.parag
.ce
2.3.5.1
.ce
\fIBusy condition\fR
.ce
.parag
.ce
The busy condition results when the DCE or DTE is temporarily
.ce
unable to continue to receive I frames due to internal constraints,
.ce
e.g.\ receive buffering limitations. In this case an RNR frame is transmitted
.ce
from the busy DCE or DTE. I\ frames pending transmission may be transmitted
from the busy DCE or DTE prior to or following the RNR\ frame.
.ce
.parag
.ce
An indication that the busy condition has cleared is communicated by the
transmission of a UA (only in response to a SABM/SABME command), RR, REJ
or SABM/SABME (modulo\ 8/modulo\ 128) frame.
.ce
.parag
.ce
.line
.ce
\fBTable 7/X.25 [T7.25], p.\fR
.ce
.line
.ce
\fBTable 8/X.25 [T8.25], p.\fR
.ce
2.3.5.2
.ce
\fIN(S) sequence error condition\fR
.ce
.parag
.ce
The information field of all I frames received whose N(S) does not equal
the receive state variable V(R) will be discarded.
.ce
.parag
.ce
An N(S) sequence error exception condition occurs in the receiver when
an I\ frame received contains an N(S) which is not equal to the receive
state
.ce
variable V(R) at the receiver. The receiver does not acknowledge (increment
its receive state variable) the I\ frame causing the sequence error, or
any I\ frame which may follow, until an I\ frame with the correct N(S)
is received.
.ce
.parag
.ce

.ce
A DCE or DTE which receives one or more valid I frames having sequence
errors or subsequent supervisory frames (RR, RNR and REJ) shall accept
the
.ce
control information contained in the N(R) field and the P or F bit to perform
data link control functions; e.g.\ to receive acknowledgement of previously
.ce
transmitted I\ frames and to cause the DCE or DTE to respond (P bit set to 1).
.ce
.parag
.ce

.ce
The means specified in \(sc\(sc\ 2.3.5.2.1 and 2.3.5.2.2 shall be available
for initiating the retransmission of lost or errored I\ frames following
the
.ce
occurrence of an N(S) sequence error condition.
.ce
.parag
.ce
2.3.5.2.1
.ce
\fIREJ recovery\fR
.ce
.parag
.ce
The REJ frame is used by a receiving DCE or DTE to initiate a
.ce
recovery (retransmission) following the detection of an N(S) sequence error.
.ce
.parag
.ce

.ce
With respect to each direction of transmission on the data link, only one
\*Qsent REJ\*U exception condition from a DCE or DTE, to a DTE or DCE,
is
.ce
established at a time. A \*Qsent REJ\*U exception condition is cleared when the
.ce
requested I\ frame is received.
.ce
.parag
.ce

.ce
A DCE or DTE receiving a REJ frame initiates sequential
.ce
(re\(hy)transmission of I\ frames starting with the I\ frame indicated
by the N(R) contained in the REJ frame. The retransmitted frames may contain
an N(R) and a P bit that are updated from, and therefore different from,
the ones contained in the originally transmitted I\ frames.
.ce
.parag
.ce
2.3.5.2.2
.ce
\fITime\(hyout recovery\fR
.ce
.parag
.ce
If a DCE or DTE, due to a transmission error, does not receive (or receives
and discards) a single I\ frame or the last I\ frame(s) in a sequence of
I\ frames, it will not detect an N(S) sequence error condition and, therefore,
will not transmit a REJ frame. The DTE or DCE which transmitted the
.ce
unacknowledged I\ frame(s) shall, following the completion of a system
specified time\(hyout period (see \(sc\(sc\ 2.4.5.1 and 2.4.5.9 below),
take appropriate recovery action to determine at which I\ frame retransmission
must begin. The
.ce
retransmitted frame(s) may contain an N(R) and a P bit that is updated from,
.ce
and therefore different from, the ones contained in the originally transmitted
frame(s).
.ce
.parag
.ce
2.3.5.3
.ce
\fIInvalid frame\fR
.ce
\fI condition\fR
.ce
.parag
.ce
Any frame which is invalid will be discarded, and no action is
.ce
taken as the result of that frame. An invalid frame is defined as one
.ce
which:
.ce
.parag
.ce
a)
.ce
is not properly bounded by two flags;
.ce
.parag
.ce
b)
.ce
in basic (modulo 8) operation, contains fewer than 32 bits
.ce
between flags; in extended (modulo\ 128) operation, contains
.ce
fewer than 40\ bits between flags of frames that contain sequence
.ce
numbers or 32\ bits between flags of frames that do not contain
.ce
sequence numbers;
.ce
.parag
.ce
c)
.ce
contains a Frame Check Sequence (FCS) error; or
.ce
.parag
.ce
d)
.ce
contains an address other than A or B (for single link
.ce
operation) or other than C or D (for multilink operation).
.ce
.parag
.ce

.ce
For those networks that are octet aligned, a detection of
.ce
non\(hyoctet alignment may be made at the Data Link Layer by adding a frame
.ce
validity check that requires the number of bits between the opening flag and
.ce
the closing flag, excluding bits inserted for transparency, to be an integral
number of octets in length, or the frame is considered invalid.
.ce
.parag
.ce
2.3.5.4
.ce
\fIFrame rejection\fR
.ce
\fI condition\fR
.ce
.parag
.ce
A frame rejection condition is established upon the receipt of an error\(hyfree
frame with one of the conditions listed in \(sc\ 2.3.4.9 above.
.ce
.parag
.ce

.ce
At the DCE or DTE, this frame rejection exception condition is
.ce
reported by an FRMR response for appropriate DTE or DCE action, respectively.
Once a DCE has established such an exception condition, no additional I\
frames are accepted until the condition is reset by the DTE, except for
examination of the P bit. The FRMR response may be repeated at each opportunity,
as specified in \(sc\ 2.4.7.3, until recovery is effected by the DTE, or
until the DCE initiates its own recovery in case the DTE does not respond.
.ce
.parag
.ce
2.3.5.5
.ce
\fIExcessive idle channel state condition on incoming\fR
.ce

.ce
\fIchannel\fR
.ce
.parag
.ce
Upon detection of an idle channel state condition (see \(sc\ 2.2.12.2 above)
on the incoming channel, the DCE shall wait for a period\ T3 (see
.ce
\(sc\ 2.4.8.3 below) without taking any specific action, waiting for detection
of a return to the active channel state (i.e.,\ detection of at least one
flag
.ce
sequence). After the period\ T3, the DCE shall notify the higher layer
(e.g.\ the Packet Layer or the MLP) of the excessive idle channel state
condition, but
.ce
shall not take any action that would preclude the DTE from establishing the
.ce
data link by normal data link set\(hyup procedures.
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ Other actions to be taken by the DCE at the Data Link Layer upon
expiration of period\ T3 is a subject for further study.
.ce
.parag
.ce

.ce
2.4
.ce
\fIDescription of the \fR
.ce
\fILAPB procedure\fR
.ce
.parag
.ce
2.4.1
.ce
\fILAPB basic and extended modes of operation\fR
.ce
.parag
.ce
In accordance with the system choice made by the DTE at
.ce
subscription time, the DCE will either support modulo\ 8 (basic) operation or
.ce
will support modulo\ 128 (extended) operation. Changing from basic operation
to extended operation, or vice versa, in the DCE requires resubscription
by the
.ce
DTE for the desired service, and is not supported dynamically.
.ce
.parag
.ce
Table 5/X.25 indicates the command and response control field formats used
with the basic (modulo\ 8) service. The mode\(hysetting command employed
to
.ce
initialize (set up) or reset the basic mode is the SABM command. Table
6/X.25 indicates the command and response control field formats used with
the extended (modulo\ 128) service. The mode\(hysetting command employed
to initialize (set up) or reset the extended mode is the SABME command.
.ce
.parag
.ce
2.4.2
.ce
\fILAPB procedure for addressing\fR
.ce
.parag
.ce
The address field identifies a frame as either a command or a
.ce
response. A command frame contains the address of the DCE or DTE to which
the command is being sent. A response frame contains the address of the
DCE or DTE sending the frame.
.ce
.parag
.ce
In order to allow differentiation between single link operation and
.ce
the optional multilink operation for diagnostic and/or maintenance reasons,
.ce
different address pair encodings are assigned to data links operating with
.ce
multilink procedure compared to data links operating with the single link
.ce
procedure.
.ce
.parag
.ce
Frames containing commands transferred from the DCE to the DTE will
.ce
contain the address\ A for the single link operation and address\ C for the
.ce
multilink operation.
.ce
.parag
.ce
Frames containing responses transferred from the DCE to the DTE will contain
the address\ B for the single link operation and address\ D for the
.ce
multilink operation.
.ce
.parag
.ce

.ce
Frames containing commands transferred from the DTE to the DCE shall contain
the address\ B for the single link operation and address\ D for the
.ce
multilink operation.
.ce
.parag
.ce
Frames containing responses transferred from the DTE to the DCE shall contain
the address\ A for the single link operation and address\ C for the
.ce
multilink operation.
.ce
.parag
.ce
These addresses are coded as follows:
.ce
.parag
.ce
Address
.ce
1\ 2\ 3\ 4\ 5\ 6\ 7\ 8
.ce
.parag
.ce
Single link operation
.ce
\ \ A
.ce
1\ 1\ 0\ 0\ 0\ 0\ 0\ 0
.ce
.line
.ce
\ \ B
.ce
1\ 0\ 0\ 0\ 0\ 0\ 0\ 0
.ce
.parag
.ce
Multilink operation
.ce
\ \ C
.ce
1\ 1\ 1\ 1\ 0\ 0\ 0\ 0
.ce
.line
.ce
\ \ D
.ce
1\ 1\ 1\ 0\ 0\ 0\ 0\ 0
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ The DCE will discard all frames received with an address other
than\ A or\ B (single link operation), or\ C or\ D (multilink operation).
.ce
.parag
.ce
2.4.3
.ce
\fILAPB procedure for the use of the P/F bit\fR
.ce
.parag
.ce
The DCE or DTE receiving an SABM/SABME, DISC, supervisory command or I\
frame with the P\ bit set to\ 1 will set the F\ bit to\ 1 in the next response
frame it transmits.
.ce
.parag
.ce
The response frame returned by the DCE to an SABM/SABME or DISC
.ce
command with the P\ bit set to\ 1 will be a UA or DM response with the
F\ bit set to\ 1. The response frame returned by the DCE to an I\ frame
with the P\ bit set to\ 1, received during the information transfer phase,
will be an RR, REJ, RNR or FRMR response with the F\ bit set to\ 1. The
response frame returned by the
.ce
DCE to a supervisory command with the P\ bit set to\ 1, received during the
.ce
information transfer phase, will be an RR, REJ, RNR or FRMR response with
the F\ bit set to\ 1. The response frame returned by the DCE to an I\ frame
or
.ce
supervisory frame with the P\ bit set to\ 1, received during the disconnected
.ce
phase, will be a DM response with the F\ bit set to\ 1.
.ce
.parag
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The P bit may be used by the DCE in conjunction with the timer
.ce
recovery condition (see \(sc\ 2.4.5.9 below).
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ Other use of the P bit by the DCE is a subject for further
.ce
study.
.ce
.parag
.ce
2.4.4
.ce
\fILAPB procedure for data link set\(hyup and disconnection\fR
.ce
.parag
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.ce
2.4.4.1
.ce
\fIData link set\(hyup\fR
.ce
.parag
.ce
The DCE will indicate that it is able to set up the data link by
.ce
transmitting contiguous flags (active channel state).
.ce
.parag
.ce
Either the DTE or the DCE may initiate data link set\(hyup. Prior to
.ce
initiation of data link set\(hyup, either the DCE or the DTE may initiate data
.ce
link
.ce
disconnection (see \(sc\ 2.4.4.3) for the purpose of insuring that the
DCE and the DTE are in the same phase. The DCE may also transmit an unsolicited
DM response to request the DTE to initiate data link set\(hyup.
.ce
.parag
.ce
The DTE shall initiate data link set\(hyup by transmitting an SABM/SABME
command to the DCE. If, upon receipt of the SABM/SABME command correctly,
the DCE determines that it can enter the information transfer phase, it
will return a UA response to the DTE, will reset its send and receive state
variables V(S) and V(R) to zero, and will consider that the data link is
set up. If, upon
.ce
receipt
.ce
of the SABM/SABME command correctly, the DCE determines that it cannot enter
.ce
the information transfer phase, it will return a DM response to the DTE as a
.ce
denial to the data link set\(hyup initialization and will consider that the
.ce
data link is
.ce
\fInot\fR
.ce
 set up. In order to avoid misinterpretation of the DM response received,
it is suggested that the DTE always sends its SABM/SABME command with the
P\ bit set to\ 1. Otherwise, it is not possible to differentiate a DM response
intended as a denial to data link set\(hyup from a DM response that is
issued in a separate unsolicited sense as a request for a mode\(hysetting
command (as described
.ce
in\ \(sc\ 2.4.4.4.2).
.ce
.parag
.ce
The DCE will initiate data link set\(hyup by transmitting an SABM/SABME
command to the DTE and starting its Timer\ T1 in order to determine when
too
.ce
much time has elapsed waiting for a reply (see \(sc\ 2.4.8.1 below). Upon
reception of a UA response from the DTE, the DCE will reset its send and
receive state
.ce
variables\ V(S) and\ V(R) to zero, will stop its Timer\ T1, and will consider
that the data link is set up. Upon reception of a DM response from the
DTE as a
.ce
denial
.ce
to the data link set\(hyup initialization, the DCE will stop its Timer\
T1 and will consider that the data link is \fInot\fR
.ce
set up.
.ce
.parag
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.ce
The DCE, having sent the SABM/SABME command, will ignore and discard any
frames except an SABM/SABME or DISC command, or a UA or DM response
.ce
received from the DTE. The receipt of an SABM/SABME or DISC command from the
.ce
DTE will result in a collision situation that is resolved per \(sc\ 2.4.4.5
below. Frames other than the UA and DM responses sent in response to a
received
.ce
SABM/SABME or DISC command will be sent only after the data link is set
up and if no outstanding SABM/SABME command exists.
.ce
.parag
.ce
After the DCE sends the SABM/SABME command, if a UA or DM response is not
received correctly, Timer\ T1 will run out in the DCE. The DCE will then
.ce
resend the SABM/SABME command and will restart Timer\ T1. After transmission
of the SABM/SABME command N2 times by the DCE, appropriate higher layer
recovery action will be initiated. The value of N2 is defined in \(sc\
2.4.8.4 below.
.ce
.parag
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2.4.4.2
.ce
\fIInformation transfer phase\fR
.ce
.parag
.ce
After having transmitted the UA response to the SABM/SABME command or having
received the UA response to a transmitted SABM/SABME command, the DCE will
accept and transmit I and supervisory frames according to the procedures
described in \(sc\ 2.4.5 below.
.ce
.parag
.ce

.ce
When receiving the SABM/SABME command while in the information
.ce
transfer phase, the DCE will conform to the data link resetting procedure
.ce
described in \(sc\ 2.4.7 below.
.ce
.parag
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2.4.4.3
.ce
\fIData link disconnection\fR
.ce
.parag
.ce
The DTE shall initiate a disconnect of the data link by
.ce
transmitting a DISC command to the DCE. On correctly receiving a DISC command
in the information transfer phase, the DCE will send a UA response and
enter
.ce
the disconnected phase. On correctly receiving a DISC command in the
.ce
disconnected phase, the DCE will send a DM response and remain in the
.ce
disconnected phase. In order to avoid misinterpretation of the DM response
.ce
received, it is suggested that the DTE always sends its DISC command with
the P\ bit set to\ 1. Otherwise, it is not possible to differentiate a
DM response
.ce
intended as an indication that the DCE is already in the disconnected phase
.ce
from a DM response that is issued in a separate unsolicited sense as a
request for a mode\(hysetting command (as described in \(sc\ 2.4.4.4.2).
.ce
.parag
.ce

.ce
The DCE will initiate a disconnect of the data link by transmitting a DISC
command to the DTE and starting its Timer\ T1 (see \(sc\ 2.4.8.1 below).
Upon reception of an UA response from the DTE, the DCE will stop its Timer\
T1 and
.ce
will enter the disconnected phase. Upon reception of a DM response from
the DTE as an indication that the DTE was already in the disconnected phase,
the DCE
.ce
will stop its Timer\ T1 and will enter the disconnected phase.
.ce
.parag
.ce
The DCE, having sent the DISC command, will ignore and discard any
.ce
frames except an SABM/SABME or DISC command, or a UA or DM response received
.ce
from the DTE. The receipt of an SABM/SABME or DISC command from the DTE will
.ce
result in a collision situation that is resolved per \(sc\ 2.4.4.5 below.
.ce
.parag
.ce

.ce
After the DCE sends the DISC command, if a UA or DM response is not
.ce
received correctly, Timer\ T1 will run out in the DCE. The DCE will then
resend the DISC command and will restart Timer\ T1. After transmission
of the DISC
.ce
command N2 times by the DCE, appropriate higher layer recovery action will
be initiated. The value of N2 is defined in \(sc\ 2.4.8.4 below.
.ce
.parag
.ce
2.4.4.4
.ce
\fIDisconnected phase\fR
.ce
.parag
.ce
2.4.4.4.1
.ce
After having received a DISC command from the DTE and
.ce
returned a UA response to the DTE, or having received the UA response to a
.ce
transmitted DISC command, the DCE will enter the disconnected phase.
.ce
.parag
.ce
In the disconnected phase, the DCE may initiate data link set\(hyup. In
the disconnected phase, the DCE will react to the receipt of an SABM/SABME
command as described in \(sc\ 2.4.4.1 above and will transmit a DM response
in
.ce
answer to a received DISC command. When receiving any other command (defined,
or undefined or not implemented) with the P\ bit set to\ 1, the DCE will
transmit a DM response with the F\ bit set to\ 1. Other frames received
in the
.ce
disconnected phase will be ignored by the DCE.
.ce
.parag
.ce
2.4.4.4.2
.ce
When the DCE enters the disconnected phase after
.ce
detecting error conditions as listed in \(sc\ 2.4.6 below, or after an internal
.ce
malfunction, it may indicate this by sending a DM response rather than
a DISC command. In these cases, the DCE will transmit a DM response and
start its
.ce
Timer\ T1 (see \(sc\ 2.4.8.1 below).
.ce
.parag
.ce
If Timer T1 runs out before the reception of an SABM/SABME or DISC command
from the DTE, the DCE will retransmit the DM response and restart
.ce
Timer\ T1. After transmission of the DM response N2 times, the DCE will
remain in the disconnected phase and appropriate recovery actions will
be initiated. The value of N2 is defined in \(sc\ 2.4.8.4 below.
.ce
.parag
.ce
Alternatively, after an internal malfunction, the DCE may either
.ce
initiate a data link resetting procedure (see \(sc\ 2.4.7 below) or disconnect
the data link (see \(sc\ 2.4.4.3 above) prior to initiating a data link
set\(hyup
.ce
procedure (see \(sc\ 2.4.4.1 above).
.ce
.parag
.ce
2.4.4.5
.ce
\fICollision of unnumbered commands\fR
.ce
.parag
.ce
Collision situations shall be resolved in the following way:
.ce
.parag
.ce
2.4.4.5.1
.ce
If the sent and received unnumbered commands are the
.ce
same, the DCE and the DTE shall each send the UA response at the earliest
.ce
possible opportunity. The DCE shall enter the indicated phase either,
.ce
.parag
.ce

.ce
1)
.ce
after receiving the UA response,
.ce
.parag
.ce
2)
.ce
after sending the UA response, or
.ce
.parag
.ce
3)
.ce
after timing out waiting for the UA response having sent a
.ce
UA response.
.ce
.parag
.ce
In the case of 2) above, the DCE will accept a subsequent UA
.ce
response to the mode\(hysetting command it issued without causing an exception
.ce
condition if received within the time\(hyout interval.
.ce
.parag
.ce

.ce
2.4.4.5.2
.ce
If the sent and received unnumbered commands are
.ce
different, the DCE and the DTE shall each enter the disconnected phase and
.ce
issue a DM response at the earliest possible opportunity.
.ce
.parag
.ce
2.4.4.6
.ce
\fICollision of DM response with SABM/SABME or DISC\fR
.ce

.ce
\fIcommand\fR
.ce
.parag
.ce
When a DM response is issued by the DCE or DTE as an unsolicited
.ce
response to request the DTE or DCE, respectively, to issue a mode\(hysetting
.ce
command as described in \(sc\ 2.4.4.4, a collision between an SABM/SABME
or DISC
.ce
command and the unsolicited DM response may occur. In order to avoid
.ce
misinterpretation of the DM response received, the DTE always sends its
.ce
SABM/SABME or DISC command with the P\ bit set to\ 1.
.ce
.parag
.ce
2.4.4.7
.ce
\fICollision of DM responses\fR
.ce
.parag
.ce
A contention situation may occur when both the DCE and the DTE
.ce
issue a DM response to request a mode\(hysetting command. In this case, the DTE
.ce
will issue an SABM/SABME command to resolve the contention situation.
.ce
.parag
.ce

.ce
2.4.5
.ce
\fILAPB procedures for information transfer\fR
.ce
.parag
.ce
The procedures which apply to the transmission of I\ frames in each direction
during the information transfer phase are described below.
.ce
.parag
.ce
In the following, \*Qnumber one higher\*U is in reference to a
.ce
continuously repeated sequence series, i.e., 7 is 1\ higher than 6 and 0 is
.ce
1\ higher than 7 for modulo\ 8 series, and 127 is 1\ higher than 126 and 0 is
.ce
1\ higher than 127 for modulo\ 128 series.
.ce
.parag
.ce
2.4.5.1
.ce
\fISending I frames\fR
.ce
.parag
.ce
When the DCE has an I frame to transmit (i.e. an I frame not
.ce
already transmitted, or having to be retransmitted as described in \(sc\
2.4.5.6
.ce
below), it will transmit it with an N(S) equal to its current send state
.ce
variable V(S), and an N(R) equal to its current receive state variable
V(R). At the end of the transmission of the I\ frame, the DCE will increment
its send
.ce
state variable V(S) by\ 1.
.ce
.parag
.ce
If Timer T1 is not running at the time of transmission of an I frame, it
will be started.
.ce
.parag
.ce
If the send state variable V(S) is equal to the last value of N(R)
.ce
received plus \fIk\fR
.ce
(where \fIk\fR
.ce
is the maximum number of outstanding I\ frames \(em
.ce
see \(sc\ 2.4.8.6 below), the DCE will not transmit any new I\ frames, but may
.ce
retransmit an I\ frame as described in \(sc\(sc\ 2.4.5.6 or 2.4.5.9 below.
.ce
.parag
.ce
When the DCE is in the busy condition, it may still transmit I frames,
provided that the DTE is not busy. When the DCE is in the frame rejection
.ce
condition, it will stop transmitting I\ frames.
.ce
.parag
.ce
2.4.5.2
.ce
\fIReceiving an I frame\fR
.ce
.parag
.ce
2.4.5.2.1
.ce
When the DCE is not in a busy condition and receives a valid I\ frame whose
send sequence number N(S) is equal to the DCE receive state
.ce
variable V(R), the DCE will accept the information field of this frame,
.ce
increment by one its receive state variable V(R), and act as follows:
.ce
.parag
.ce

.ce
a)
.ce
If the DCE is still not in a busy condition:
.ce
.parag
.ce
i)
.ce
If an I frame is available for transmission by the
.ce
DCE, it may act as in \(sc\ 2.4.5.1 above and acknowledge
.ce
the received I frame by setting N(R) in the control field
.ce
of the next transmitted I\ frame to the value of the DCE
.ce
receive state variable V(R). Alternatively, the DCE may
.ce
acknowledge the received I\ frame by transmitting an RR
.ce
frame with the N(R) equal to the value of the DCE receive
.ce
state variable V(R).
.ce
.parag
.ce
ii)
.ce
If no I frame is available for transmission by the
.ce
DCE, it will transmit an RR frame with N(R) equal to the
.ce
value of the DCE receive state variable V(R).
.ce
.parag
.ce

.ce
b)
.ce
If the DCE is now in a busy condition, it will transmit an
.ce
RNR frame with N(R) equal to the value of the DCE receive
.ce
state variable V(R) (see \(sc\ 2.4.5.8).
.ce
.parag
.ce

.ce
2.4.5.2.2
.ce
When the DCE is in a busy condition, it may ignore the
.ce
information field contained in any received I\ frame.
.ce
.parag
.ce
2.4.5.3
.ce
\fIReception of invalid frames\fR
.ce
.parag
.ce
When the DCE receives an invalid frame (see \(sc\ 2.3.5.3), this frame
will be discarded.
.ce
.parag
.ce
2.4.5.4
.ce
\fIReception of out\(hyof\(hysequence I frames\fR
.ce
.parag
.ce
When the DCE receives a valid I frame whose send sequence number
.ce
N(S) is incorrect, i.e.,\ not equal to the current DCE receive state variable
.ce
V(R), it will discard the information field of the I\ frame and transmit
an REJ frame with the N(R) set to one higher than the N(S) of the last
correctly
.ce
received I\ frame. The REJ frame will be a command frame with the P\ bit
set to\ 1 if an acknowledged transfer of the retransmission request is
required;
.ce
otherwise the REJ frame may be either a command or a response frame. The DCE
.ce
will then discard the information field of all I\ frames received until the
.ce
expected I\ frame is correctly received. When receiving the expected I\ frame,
.ce
the DCE will then acknowledge the I\ frame as described in \(sc\ 2.4.5.2
above. The DCE will use the N(R) and P\ bit information in the discarded
I\ frames as
.ce
described in \(sc\ 2.3.5.2 above.
.ce
.parag
.ce
2.4.5.5
.ce
\fIReceiving acknowledgement\fR
.ce
.parag
.ce
When correctly receiving an I frame or a supervisory frame (RR, RNR or
REJ), even in the busy condition, the DCE will consider the N(R) contained
in this frame as an acknowledgement for all I\ frames it has transmitted
with an N(S) up to and including the received N(R)\(em1. The DCE will stop
Timer\ T1 when it correctly receives an I\ frame or a supervisory frame
with the N(R) higher
.ce
than the last received N(R) (actually acknowledging some I\ frames), or
an REJ frame with an N(R) equal to the last received N(R).
.ce
.parag
.ce

.ce
If Timer T1 has been stopped by the receipt on an I, RR or RNR frame, and
if there are outstanding I\ frames still unacknowledged, the DCE will
.ce
restart Timer\ T1. If Timer\ T1 then runs out, the DCE will follow the
recovery procedure (\(sc\ 2.4.5.9 below) with respect to the unacknowledged
I\ frames. If
.ce
Timer\ T1 has been stopped by the receipt of an REJ frame, the DCE will
follow the retransmission procedures in \(sc\ 2.4.5.6 below.
.ce
.parag
.ce
2.4.5.6
.ce
\fIReceiving an REJ frame\fR
.ce
.parag
.ce
When receiving an REJ frame, the DCE will set its send state
.ce
variable V(S) to the N(R) received in the REJ control field. It will transmit
the corresponding I\ frame as soon as it is available or retransmit it
in
.ce
accordance with the procedures described in \(sc\ 2.4.5.1 above. (Re)transmission
will conform to the following procedure:
.ce
.parag
.ce
i)
.ce
if the DCE is transmitting a supervisory command or response
.ce
when it receives the REJ frame, it will complete that
.ce
transmission before commencing transmission of the requested
.ce
I\ frame;
.ce
.parag
.ce

.ce
ii)
.ce
if the DCE is transmitting an unnumbered command or
.ce
response when it receives the REJ frame, it will ignore the
.ce
request for retransmission;
.ce
.parag
.ce
iii)
.ce
if the DCE is transmitting an I frame when the REJ frame
.ce
is received, it may abort the I\ frame and commence transmission
.ce
of the requested I\ frame immediately after abortion;
.ce
.parag
.ce
iv)
.ce
if the DCE is not transmitting any frame when the REJ frame
.ce
is received, it will commence transmission of the requested
.ce
I\ frame immediately.
.ce
.parag
.ce
In all cases, if other unacknowledged I frames had already been
.ce
transmitted following the one indicated in the REJ frame, then those I
frames will be retransmitted by the DCE following the retransmission of
the requested I\ frame. Other I\ frames not yet transmitted may be transmitted
following the
.ce
retransmitted I\ frames.
.ce
.parag
.ce
If the REJ frame was received from the DTE as a command with the P bit
set to\ 1, the DCE will transmit an RR, RNR or REJ response with the F
bit set to\ 1 before transmitting or retransmitting the corresponding I\
frame.
.ce
.parag
.ce
2.4.5.7
.ce
\fIReceiving an RNR frame\fR
.ce
.parag
.ce
After receiving an RNR frame whose N(R) acknowledges all frames
.ce
previously transmitted, the DCE will stop Timer\ T1 and may then transmit
.ce
an I\ frame, with the P\ bit set to\ 0, whose send sequence number is
.ce
equal to the N(R) indicated in the RNR frame, restarting Timer\ T1 as it
does. After receiving an RNR frame whose N(R) indicates a previously transmitted
.ce
frame, the DCE will not transmit or retransmit any I\ frame, Timer\ T1 being
.ce
already running. In either case, if the Timer\ T1 runs out before receipt
of a busy clearance indication, the DCE will follow the procedure described
in
.ce
\(sc\ 2.4.5.9\ below. In any case, the DCE will not transmit any other
I\ frames
.ce
before receiving an RR or REJ frame, or before the completion of a link
.ce
resetting procedure.
.ce
.parag
.ce
Alternatively, after receiving an RNR frame, the DCE may wait for a
.ce
period of time (e.g.,\ the length of the Timer\ T1) and then transmit a
.ce
supervisory command frame (RR, RNR or REJ) with the P\ bit set to\ 1, and
start Timer\ T1, in order to determine if there is any change in the receive
status of the DTE. The DTE shall respond to the P\ bit set to\ 1 with a
supervisory
.ce
response frame (RR, RNR or REJ) with the F\ bit set to\ 1 indicating either
.ce
continuance of the busy condition (RNR) or clearance of the busy condition
(RR or REJ). Upon receipt of the DTE response, Timer\ T1 is stopped.
.ce
.parag
.ce

.ce
1)
.ce
If the response is the RR or REJ response, the busy
.ce
condition is cleared and the DCE may transmit I\ frames beginning
.ce
with the I\ frame identified by the N(R) in the received response
.ce
frame.
.ce
.parag
.ce
2)
.ce
If the response is the RNR response, the busy condition
.ce
still exists, and the DCE will after a period of time (e.g.\ the
.ce
length of Timer\ T1) repeat the enquiry of the DTE receive
.ce
status.
.ce
.parag
.ce
If Timer T1 runs out before a status response is received, the
.ce
enquiry process above is repeated. If N2 attempts to get a status response
fail (i.e.\ Timer\ T1 runs out N2\ times), the DCE will initiate a data
link resetting procedure as described in \(sc\ 2.4.7.2\ below or will transmit
a DM response to ask the DTE to initiate a data link set\(hyup procedure
as described in \(sc\ 2.4.4.1 and enter the disconnected phase. The value
of N2 is defined in \(sc\ 2.4.8.4\ below.
.ce
.parag
.ce

.ce
If, at any time during the enquiry process, an unsolicited RR or REJ frame
is received from the DTE, it will be considered to be an indication of
.ce
clearance of the busy condition. Should the unsolicited RR or REJ frame be a
.ce
command frame with the P bit set to\ 1, the appropriate response frame
with the F\ bit set to 1 must be transmitted before the DCE may resume
transmission of
.ce
I\ frames. If Timer\ T1 is running, the DCE will wait for the non\(hybusy
response with the F bit set to\ 1 or will wait for Timer\ T1 to run out
and then either
.ce
may reinitiate the enquiry process in order to realize a successful P/F bit
.ce
exchange or may resume transmission of I frames beginning with the I\ frame
.ce
identified by the N(R) in the received RR or REJ frame.
.ce
.parag
.ce
2.4.5.8
.ce
\fIDCE busy condition\fR
.ce
.parag
.ce
When the DCE enters a busy condition, it will transmit an RNR frame at
the earliest opportunity. The RNR frame will be a command frame with the
P bit set to\ 1 if an acknowledged transfer of the busy condition indication
is
.ce
required; otherwise the RNR frame may be either a command or a response
frame. While in the busy condition, the DCE will accept and process supervisory
.ce
frames, will accept and process the contents of the N(R) fields of I\ frames,
.ce
and will return an RNR response with the F bit set to\ 1 if it receives a
.ce
supervisory command or I command frame with the P bit set to\ 1. To clear the
.ce
busy condition, the DCE will transmit either an REJ frame or an RR frame, with
.ce
N(R) set to the current receive state variable V(R), depending on whether or
.ce
not it discarded information fields of correctly received I\ frames. The REJ
.ce
frame or the RR frame will be a command frame with the P bit set to\ 1 if an
.ce
acknowledged transfer of the busy\(hyto\(hynon\(hybusy transition is required,
otherwise the REJ frame or the RR frame may be either a command or a response
frame.
.ce
.parag
.ce

.ce
2.4.5.9
.ce
\fIWaiting acknowledgement\fR
.ce
.parag
.ce
The DCE maintains an internal transmission attempt variable which is set
to\ 0 when the DCE sends a UA response, when the DCE receives a UA
.ce
response or an RNR command or response, or when the DCE correctly receives
an I\ frame or supervisory frame with the N(R) higher than the last received
N(R) (actually acknowledging some outstanding I\ frames).
.ce
.parag
.ce
If Timer T1 runs out waiting for the acknowledgement from the DTE for an
I\ frame transmitted, the DCE will enter the timer recovery condition,
add
.ce
one to its transmission attempt variable and set an internal variable \fIx\fR
.ce
to
.ce
the current value of its send state variable V(S). The DCE will then restart
.ce
Timer T1, set its send state variable V(S) to the last value of N(R) received
from the DTE and retransmit the corresponding I\ frame with the P bit set
to\ 1, or transmit an appropriate supervisory command frame (RR, RNR or
REJ) with the P\ bit set to\ 1.
.ce
.parag
.ce

.ce
The timer recovery condition is cleared when the DCE receives a valid supervisory
frame with the F\ bit\ set to\ 1.
.ce
.parag
.ce
If, while in the timer recovery condition, the DCE correctly receives a
supervisory frame with the F bit set to\ 1 and with the N(R) within the
range from its current send state variable V(S) to \fIx\fR
.ce
included, it will clear the
.ce
timer recovery condition (including stopping Timer\ T1) and set its send
state variable V(S) to the value of the received N(R), and may then resume
with
.ce
I\ frame transmission or retransmission, as appropriate.
.ce
.parag
.ce
If, while in the timer recovery condition, the DCE correctly receives an
I or supervisory frame with the P/F bit set to\ 0 and with a valid N(R)
(see \(sc\ 2.3.4.9), it will not clear the timer recovery condition. The
value of the
.ce
received N(R) may be used to update the send state variable V(S). However,
the DCE may decide to keep the last transmitted I\ frame in store (even
if it is
.ce
acknowledged) in order to be able to retransmit it with the P bit set to\
1 when Timer\ T1 runs out at a later time.
.ce
.parag
.ce

.ce
If the received supervisory frame with the P/F bit set to\ 0 is an REJ
frame with a valid N(R), the DCE may either immediately initiate
.ce
(re)transmission from the value of the send state variable V(S), or it may
.ce
ignore the request for retransmission and wait until the supervisory frame
with the F bit set to\ 1 is received before initiating (re)transmission
of frames
.ce
from the value identified in the N(R) field of the supervisory frame with
the F\ bit set to\ 1. In the case of immediate retransmission, in order
to prevent
.ce
duplicate retransmissions following the clearance of the timer recovery
.ce
condition, the DCE shall inhibit retransmission of a specific I\ frame [same
.ce
N(R) in the same numbering cycle] if the DCE has retransmitted that I\
frame as the result of a received REJ frame with the P/F bit set to\ 0.
.ce
.parag
.ce
If, while in the timer recovery condition, the DCE receives a REJ
.ce
command with the P bit set to\ 1, the DCE will respond immediately with an
.ce
appropriate supervisory response with the F bit set to\ 1. The DCE may
then use the value of the N(R) in the REJ command to update the send state
variable
.ce
V(S), and may either immediately begin (re)transmission from the value N(R)
.ce
indicated in the REJ frame or ignore the request for retransmission and wait
.ce
until the supervisory frame with the F bit set to\ 1 is received before
.ce
initiating (re)transmission of I\ frames from the value identified in the
N(R) field of the supervisory frame with the F\ bit set to\ 1. In the case
of
.ce
immediate retransmission, in order to prevent duplicate retransmissions
.ce
following the clearance of the timer recovery condition, the DCE shall
inhibit retransmission of a specific I\ frame [same N(R) in the same numbering
cycle]
.ce
if the DCE has retransmitted that I\ frame as the result of the received REJ
.ce
command with the P\ bit set to\ 1.
.ce
.parag
.ce
If Timer T1 runs out in the timer recovery condition, and no I or
.ce
supervisory frame with the P/F bit set to 0 and with a valid N(R) has been
.ce
received, or no REJ command with the P bit set to\ 1 and with a valid N(R)
has been received, the DCE will add one to its transmission attempt variable,
.ce
restart Timer\ T1, and either retransmit the I frame sent with the P bit set
.ce
to\ 1 or transmit an appropriate supervisory command with the P bit set to\ 1.
.ce
.parag
.ce

.ce
If the transmission attempt variable is equal to N2, the DCE will
.ce
initiate a data link resetting procedure as described in \(sc\ 2.4.7.2
below, or
.ce
will transmit a DM response to ask the DTE to initiate a data link set\(hyup
.ce
procedure as described in \(sc\ 2.4.4.1 above and enter the disconnected
phase. N2 is a system parameter (see \(sc\ 2.4.8.4 below).
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ Although the DCE may implement the internal variable \fIx\fR
.ce
,
.ce
other mechanisms do exist that achieve the identical function.
.ce
.parag
.ce
2.4.6
.ce
\fILAPB conditions for \fR
.ce
\fIdata link resetting or data link\fR
.ce

.ce
\fIre\(hyinitialization\fR
.ce
\fI(data link set\(hyup)\fR
.ce
.parag
.ce
2.4.6.1
.ce
When the DCE receives, during the information transfer phase, a
.ce
frame which is not invalid (see \(sc\ 2.3.5.3) with one of the conditions
listed in \(sc\ 2.3.4.9 above, the DCE will request the DTE to initiate
a data link resetting procedure by transmitting an FRMR response to the
DTE as described in
.ce
\(sc\ 2.4.7.3.
.ce
.parag
.ce
2.4.6.2
.ce
When the DCE receives, during the information transfer phase, an FRMR response
from the DTE, the DCE will either initiate the data link
.ce
resetting procedures itself as described in \(sc\ 2.4.7.2 or return a DM
response to ask the DTE to initiate the data link set\(hyup (initialization)
procedure as described in \(sc\ 2.4.4.1. After transmitting a DM response,
the DCE will enter
.ce
the disconnected phase as described in \(sc\ 2.4.4.4.2.
.ce
.parag
.ce
2.4.6.3
.ce
When the DCE receives, during the information transfer phase, a UA response,
or an unsolicited response with the F bit set to\ 1, the DCE may
.ce
either initiate the data link resetting procedures itself as described in
.ce
\(sc\ 2.4.7.2, or return a DM response to ask the DTE to initiate the data link
.ce
set\(hyup (initialization) procedure as described in \(sc\ 2.4.4.1. After
transmitting a DM response, the DCE will enter the disconnected phase as
described in
.ce
\(sc\ 2.4.4.4.2.
.ce
.parag
.ce
2.4.6.4
.ce
When the DCE receives, during the information transfer phase, a DM response
from the DTE, the DCE will either initiate the data link set\(hyup
.ce
(initialization) procedures itself as described in \(sc\ 2.4.4.1, or return
a DM
.ce
response to ask the DTE to initiate the data link set\(hyup (initialization)
.ce
procedures as described in \(sc\ 2.4.4.1. After transmitting a DM response,
the DCE will enter the disconnected phase as described in \(sc\ 2.4.4.4.2.
.ce
.parag
.ce

.ce
2.4.7
.ce
\fILAPB procedure for data link resetting\fR
.ce
.parag
.ce
2.4.7.1
.ce
The data link resetting procedure is used to initialize both
.ce
directions of information transfer according to the procedure described
below. The data link resetting procedure only applies during the information
transfer phase.
.ce
.parag
.ce
2.4.7.2
.ce
Either the DTE or the DCE may initiate the data link resetting
.ce
procedure. The data link resetting procedure indicates a clearance of a DCE
.ce
and/or DTE busy condition, if present.
.ce
.parag
.ce
The DTE shall initiate a data link resetting by transmitting an
.ce
SABM/SABME command to the DCE. If, upon correct receipt of the SABM/SABME
.ce
command, the DCE determines that it can continue in the information transfer
.ce
phase, it will return a UA response to the DTE, will reset its send and
receive state variables V(S) and V(R) to zero, and will remain in the information
.ce
transfer phase. If, upon correct receipt of the SABM/SABME command, the DCE
.ce
determines that it cannot remain in the information transfer phase, it will
.ce
return a DM response as a denial to the resetting request and will enter the
.ce
disconnected phase.
.ce
.parag
.ce
The DCE will initiate a data link resetting by transmitting an
.ce
SABM/SABME command to the DTE and starting its Timer\ T1 (see \(sc\ 2.4.8.1
below). Upon reception of a UA response from the DTE, the DCE will reset
its send and receive state variables V(S) and V(R) to zero, will stop its
Timer\ T1, and will remain in the information transfer phase. Upon reception
of a DM response from the DTE as a denial to the data link resetting request,
the DCE will stop its Timer\ T1 and will enter the disconnected phase.
.ce
.parag
.ce
The DCE, having sent an SABM/SABME command, will ignore and discard
.ce
any frames received from the DTE except an SABM/SABME or DISC command,
or a UA or DM response. The receipt of an SABM/SABME or DISC command from
the DTE will result in a collision situation that is resolved per \(sc\
2.4.4.5 above. Frames
.ce
other than the UA or DM response sent in response to a received SABM/SABME
or DISC command will be sent only after the data link is reset and if no
.ce
outstanding SABM/SABME command exists.
.ce
.parag
.ce

.ce
After the DCE sends the SABM/SABME command, if a UA or DM response is not
received correctly, Timer\ T1 will run out in the DCE. The DCE will then
.ce
resend the SABM/SABME command and will restart Timer\ T1. After N2 attempts
to reset the data link, the DCE will initiate appropriate higher layer
recovery
.ce
action and will enter the disconnected phase. The value of N2 is defined in
.ce
\(sc\ 2.4.8.4 below.
.ce
.parag
.ce
2.4.7.3
.ce
The DCE may ask the DTE to reset the data link by transmitting an FRMR
response (see \(sc\ 2.4.6.1 above). After transmitting an FRMR response,
.ce
the DCE will enter the frame rejection condition.
.ce
.parag
.ce
The frame rejection condition is cleared when the DCE receives an SABM/SABME
command, a DISC command, a FRMR response, or a DM response; or if
.ce
the DCE transmits an SABM/SABME command, a DISC command, or a DM response.
.ce
Other commands received while in the frame rejection condition will cause
the DCE to retransmit the FRMR response with the same information field
as
.ce
originally transmitted.
.ce
.parag
.ce
The DCE may start Timer\ T1 on transmission of the FRMR response. If
.ce
Timer\ T1 runs out before the frame rejection condition is cleared, the
DCE may retransmit the FRMR response, and restart T1. After N2 attempts
(time outs) to get the DTE to reset the data link, the DCE may reset the
data link itself as described in \(sc\ 2.4.7.2 above. The value of N2 is
defined in \(sc\ 2.4.8.4 below.
.ce
.parag
.ce

.ce
In the frame rejection condition, I frames and supervisory frames will
not be transmitted by the DCE. Also, received I frames and supervisory
frames will be discarded by the DCE except for the observance of a P bit
set to\ 1.
.ce
When an additional FRMR response must be transmitted by the DCE as a result
of the receipt of a P\ bit set to 1 while Timer\ T1 is running, Timer\
T1 will
.ce
continue to run. Upon reception of an FRMR response (even during a frame
.ce
rejection condition), the DCE will initiate a resetting procedure by
.ce
transmitting an SABM/SABME command as described in \(sc\ 2.4.7.2 above, or will
.ce
transmit a DM response to ask the DTE to initiate the data link set\(hyup
.ce
procedure as described in \(sc\ 2.4.4.1 and enter the disconnected phase.
.ce
.parag
.ce
2.4.8
.ce
\fIList of \fR
.ce
\fILAPB system parameters\fR
.ce
.parag
.ce
The DCE and DTE system parameters are as follows:
.ce
.parag
.ce

.ce
2.4.8.1
.ce
\fITimer\fR
.ce
\fIT1\fR
.ce
.parag
.ce
The value of the DTE Timer T1 system parameter may be different
.ce
than the value of the DCE Timer T1 system parameter. These values shall
be made known to both the DTE and the DCE, and agreed to for a period of
time by both the DTE and the DCE.
.ce
.parag
.ce
The period of Timer T1, at the end of which retransmission of a frame may
be initiated (see \(sc\ 2.4.4 and \(sc\ 2.4.5 above for the DCE), shall
take into account whether T1 is started at the beginning or the end of
the transmission of a frame.
.ce
.parag
.ce
The proper operation of the procedure requires that the transmitter's (DCE
or DTE) Timer\ T1 be greater than the maximum time between transmission
of a frame (SABM/SABME, DISC, I\ or supervisory command, or DM or FRMR
response)
.ce
and the reception of the corresponding frame returned as an answer to that
.ce
frame (UA, DM or acknowledging frame). Therefore, the receiver (DCE or DTE)
.ce
should not delay the response or acknowledging frame returned to one of the
.ce
above frames by more than a value\ T2, where T2 is a system parameter (see
.ce
\(sc\ 2.4.8.2).
.ce
.parag
.ce

.ce
The DCE will not delay the response or acknowledging frame returned to
one of the above DTE frames by more than a period\ T2.
.ce
.parag
.ce
2.4.8.2
.ce
\fIParameter T2\fR
.ce
.parag
.ce
The value of the DTE parameter T2 may be different than the value of the
DCE parameter T2. These values shall be made known to both the DTE and
the DCE, and agreed to for a period of time by both the DTE and the DCE.
.ce
.parag
.ce

.ce
The period of parameter T2 shall indicate the amount of time
.ce
available at the DCE or DTE before the acknowledging frame must be initiated
in order to ensure its receipt by the DTE or DCE, respectively, prior to
Timer\ T1 running out at the DTE or DCE (parameter\ T2\ <\ Timer\ T1).
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ The period of parameter T2 shall take into account the
.ce
following timing factors: the transmission time of the acknowledging frame,
the propagation time over the access data link, the stated processing times
at the DCE and the DTE, and the time to complete the transmission of the
frame(s) in the DCE or DTE transmit queue that are neither displaceable
or modifiable in an orderly manner.
.ce
.parag
.ce

.ce
Given a value for Timer T1 for the DTE or DCE, the value of parameter T2
at the DCE or DTE, respectively, must be no larger than T1 minus\ 2 times
the propagation time over the access data link, minus the frame processing
time at the DCE, minus the frame processing time at the DTE, and minus
the transmission time of the acknowledging frame by the DCE or DTE, respectively.
.ce
.parag
.ce

.ce
2.4.8.3
.ce
\fITimer T3\fR
.ce
.parag
.ce
The DCE shall support a Timer T3 system parameter, the value of
.ce
which shall be made known to the DTE.
.ce
.parag
.ce
The period of Timer T3, at the end of which an indication of an
.ce
observed excessively long idle channel state condition is passed to the
Packet Layer, shall be sufficiently greater than the period of the DCE
Timer T1
.ce
(i.e.\ T3\ >\ T1) so that the expiration of T3 provides the desired level of
.ce
assurance that the data link channel is in a non\(hyactive, non\(hyoperational
state, and is in need of data link set\(hyup before normal data link operation
can
.ce
resume.
.ce
.parag
.ce
2.4.8.4
.ce
\fIMaximum number of attempts\fR
.ce
\fIto complete a\fR
.ce

.ce
\fItransmission N2\fR
.ce
.parag
.ce

.ce
The value of the DTE N2 system parameter may be different than the value
of the DCE N2 system parameter. These values shall be made known to both
the DTE and the DCE, and agreed to for a period of time by both the DTE
and the DCE.
.ce
.parag
.ce
The value of N2 shall indicate the maximum number of attempts made by the
DCE or DTE to complete the successful transmission of a frame to the DTE
or DCE, respectively.
.ce
.parag
.ce
2.4.8.5
.ce
\fIMaximum number of bits in an I frame N1\fR
.ce
.parag
.ce
The value of the DTE N1 system parameter may be different than the value
of the DCE N1 system parameter. These values shall be made known to both
the DTE and the DCE.
.ce
.parag
.ce
The values of N1 shall indicate the maximum number of bits in an
.ce
I\ frame (excluding flags and 0\ bits inserted for transparency) that the
DCE or DTE is willing to accept from the DTE or DCE, respectively.
.ce
.parag
.ce
In order to allow for universal operation, a DTE should support a
.ce
value of DTE N1 which is not less than 1080\ bits (135\ octets). DTEs should
be aware that the network may transmit longer packets (see \(sc\ 5.2),
that may result in a data link layer problem.
.ce
.parag
.ce

.ce
All networks shall offer to a DTE which requires it, a value of DCE N1
which is greater than or equal to 2072\ bits (259\ octets) plus the length
of the address, control and FCS fields at the DTE/DCE interface, and greater
than or equal to the maximum length of the data packets which may cross
the DTE/DCE
.ce
interface plus the length of the address, control and FCS fields at the
DTE/DCE interface.
.ce
.parag
.ce
Appendix II provides a description of how the values stated above are  derived.
.ce
.parag
.ce
2.4.8.6
.ce
\fIMaximum number of \fR
.ce
\fIoutstanding I frames k\fR
.ce
.parag
.ce
The value of the DTE k system parameter shall be the same as the
.ce
value of the DCE k system parameter. This value shall be agreed to for a
.ce
period of time by both the DTE and the DCE.
.ce
.parag
.ce
The value of k shall indicate the maximum number of sequentially
.ce
numbered I\ frames that the DTE or DCE may have outstanding
.ce
(i.e.\ unacknowledged) at any given time. The value of k shall never exceed
.ce
seven for modulo\ 8 operation, or one hundred and twenty\(hyseven for modulo\
128
.ce
operation. All networks (DCEs) shall support a value of seven. Other values
of k (less than and greater than seven) may also be supported by networks
.ce
(DCEs).
.ce
.parag
.ce
.line
.ce
\fBMONTAGE : \(sc 2.5 de cette Recommandation sur le reste de cette page\fR
.ce
.parag
.ce

.ce
.line
.ad r
\fBTable 2/X.25 [T2.25], p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
2.2.2
    \fIFlag sequence\fR
.sp 9p
.RT
.PP
All frames shall start and end with the flag sequence consisting of one
0\ bit followed by six contiguous 1\ bits and one 0\ bit. The DTE and DCE
shall only send complete eight\(hybit flag sequences when sending multiple flag
sequences (see \(sc\ 2.2.11). A single flag may be used as both the
closing
flag
for one frame and the
opening flag
for the next frame.
.RT
.sp 1P
.LP
2.2.3
    \fIAddress field\fR
.sp 9p
.RT
.PP
The address field shall consist of one octet. The address field
identifies the intended receiver of a command frame and the transmitter of a
response frame. The coding of the address field is described in \(sc\ 2.4.2
(LAPB) and in \(sc\ 2.7.1 (LAP) below.
.RT
.sp 1P
.LP
2.2.4
    \fIControl field\fR
.sp 9p
.RT
.PP
For modulo\ 8 (basic) operation, the control field shall consist of one
octet. For modulo\ 128 (extended) operation, the control field shall consist
of two octets for frame formats that contain sequence numbers, and one
octet
for frame formats that do not contain sequence numbers. The content of this
field is described in \(sc\ 2.3.2\ (LAPB) and in \(sc\ 2.6.2 (LAP) below.
.RT
.sp 1P
.LP
2.2.5
    \fIInformation field\fR
.sp 9p
.RT
.PP
The information field of a frame, when present, follows the control field
(see \(sc\ 2.2.4 above) and precedes the
frame check sequence field
(see \(sc\ 2.2.7 below).
.PP
See \(sc\(sc 2.3.4.9, 2.5.2, 2.6.4.8 and 5 for the various codings and
groupings of bits in the information field as used in this Recommendation.
.PP
See \(sc\(sc 2.3.4.9, 2.4.8.5, 2.6.4.8 and 2.7.7.5 below with regard to
the maximum information field length.
.RT
.sp 1P
.LP
2.2.6
    \fITransparency\fR
.sp 9p
.RT
.PP
The DCE or DTE, when transmitting, shall examine the frame content between
the two flag sequences including the address, control, information and
FCS fields and shall insert a 0 bit after all sequences of 5 contiguous
1\ bits (including the last 5\ bits of the FCS) to ensure that a flag sequence
is not
simulated. The DCE or DTE, when receiving, shall examine the frame content
and shall discard any 0\ bit which directly follows\ 5 contiguous 1\ bits.
.RT
.sp 1P
.LP
2.2.7
    \fIFrame check sequence (FCS)\fR \fI field\fR
.sp 9p
.RT
.PP
The notation used to describe the FCS is based on the property of cyclic
codes that a code vector such as 1000000100001 can be represented by a
polynomial \fIP\fR (\fIx\fR )\ =\ \fIx\fR \u1\d\u2\d\ +\ \fIx\fR \u5\d\
+\ 1. The elements of an \fIn\fR \(hyelement
code word are thus the coefficients of a polynomial of order \fIn\fR \
\(em\ 1. In this application, these coefficients can have the value\ 0
or\ 1 and the polynomial
operations are performed modulo\ 2. The polynomial representing the content
of a frame is generated using the first bit received after the frame opening
flag as the coefficient of the highest order term.
.PP
The FCS field shall be a 16\(hybit sequence. It shall be the ones
complement of the sum (modulo\ 2) of:
.RT
.LP
    1)
    the remainder of
\fIx\fR \uD\dlFk\fR (\fIx\fR \u1\d\u5\d\uD\dlF036+\ \fIx\fR \u1\d\u4\d\
+\ \fIx\fR \u1\d\u3\d\ +
\fIx\fR \u1\d\u2\d\ +\ \fIx\fR \u1\d\u1\d\ +\ \fIx\fR \u1\d\u0\d\ +\ \fIx\fR
\u9\d\ +\ \fIx\fR \u8\d\ +
\fIx\fR \u7\d\ +\ \fIx\fR \u6\d\ +\ \fIx\fR \u5\d\ +\ \fIx\fR \u4\d\uD\dlF036+\
\fIx\fR \u3\d\ +
+\ \fIx\fR \u2\d\ +\ \fIx\fR \ +\ 1) divided (modulo\ 2)
by the
generator polynomial \fIx\fR \u1\d\u6\d\ +\ \fIx\fR \u1\d\u2\d\ +\ \fIx\fR
\u5\d\ +\ 1,
where \fIk\fR is the number of bits in the frame existing between,
but not including, the final bit of the opening flag and the
first bit of the FCS, excluding bits inserted for transparency,
and
.LP
    2)
    the remainder of the division (modulo 2) by the generator
polynomial \fIx\fR \u1\d\u6\d\ +\ \fIx\fR \u1\d\u2\d\ +\ \fIx\fR \u5\d\
+\ 1 of the
product of \fIx\fR \u1\d\u6\d by the content of the frame, existing
between but not including, the final bit of the opening flag and
the first bit of the FCS, excluding bits inserted for
transparency.
.PP
As a typical implementation, at the transmitter, the initial
content of the register of the device computing the remainder of the division
is preset to all 1s and is then modified by division by the generator
polynomial (as described above) on the address, control and information
fields; the ones complement of the resulting remainder is transmitted as
the 16\(hybit
FCS.
.PP
At the receiver, the initial content of the register of the device
computing the remainder is preset to all 1s. The final remainder, after
multiplication by \fIx\fR \u1\d\u6\d and then division (modulo\ 2) by the
generator
polynomial \fIx\fR \u1\d\u6\d\ +\ \fIx\fR \u1\d\u2\d\ +\ \fIx\fR \u5\d\
+\ 1 of the serial incoming protected bits and the FCS, will be 0001110100001111
(\fIx\fR \u1\d\u5\d through
\fIx\fR \u0\d, respectively) in the absence of transmission errors.
.PP
\fINote\fR \ \(em\ Examples of transmitted bit patterns by the DCE and
the DTE illustrating application of the transparency mechanism and the
frame check
sequence to the SABM command and the UA response are given in Appendix\ I.
.bp
.RT
.sp 1P
.LP
2.2.8
    \fIOrder of bit transmission\fR
.sp 9p
.RT
.PP
Addresses, commands, responses and sequence numbers shall be
.PP
transmitted with the low\(hyorder bit first (for example, the first bit of the
sequence number that is transmitted shall have the weight 2\u0\d). The
order of transmitting bits within the information field is not specified
under \(sc\ 2 of
this Recommendation. The FCS shall be transmitted to the line commencing
with the coefficient of the highest term, which is found in bit position\
16 of the FCS\ field (see Tables\ 1/X.25 and 2/X.25).
.PP
\fINote\fR \ \(em\ In Tables 1/X.25 to 13/X.25, bit 1 is defined as the
low\(hyorder bit.
.RT
.sp 1P
.LP
2.2.9
    \fIInvalid frames\fR
.sp 9p
.RT
.PP
The definition of an invalid frame is described in \(sc\ 2.3.5.3 (LAPB)
and in \(sc\ 2.6.5.3 (LAP) below.
.RT
.sp 1P
.LP
2.2.10
    \fIFrame abortion\fR
.sp 9p
.RT
.PP
Aborting a frame is performed by transmitting at least seven
contiguous 1\ bits (with no inserted 0\ bits).
.RT
.sp 1P
.LP
2.2.11
    \fIInterframe time fill\fR
.sp 9p
.RT
.PP
Interframe time fill is accomplished by transmitting contiguous
flags between frames, i.e.\ multiple eight\(hybit flag sequences (see
\(sc\ 2.2.2).
.RT
.sp 1P
.LP
2.2.12
    \fILink channel states\fR
.sp 9p
.RT
.PP
A link channel as defined here is the means for transmission for
one direction.
.RT
.sp 1P
.LP
2.2.12.1
    \fIActive channel state\fR
.sp 9p
.RT
.PP
The DCE incoming or outgoing channel is defined to be in an active condition
when it is receiving or transmitting, respectively, a frame, an
abortion sequence or interframe time fill.
.RT
.sp 1P
.LP
2.2.12.2
    \fIIdle channel state\fR
.sp 9p
.RT
.PP
The DCE incoming or outgoing channel is defined to be in an idle
condition when it is receiving or transmitting, respectively, a continuous\
1s state for a period of at least 15\ bit times.
.PP
See \(sc\ 2.3.5.5 for a description of DCE action when an idle condition
exists on its incoming channel for an excessive period of time.
.RT
.sp 1P
.LP
2.3
    \fILAPB elements of procedures\fR \v'3p'
.sp 9p
.RT
.PP
2.3.1
The LAPB elements of procedures are defined in terms of actions
that occur on receipt of frames at the DCE or\ DTE.
.PP
The elements of procedures specified below contain the selection
.LP
of commands and responses relevant to the LAPB data link and system
configurations
described in \(sc\ 2.1 above. Together, \(sc\(sc\ 2.2 and 2.3 form the general
requirements for the proper management of a LAPB access data link.
.sp 2P
.LP
2.3.2
    \fILAPB control field formats and parameters\fR
.sp 1P
.RT
.sp 1P
.LP
2.3.2.1
    \fIControl field formats\fR
.sp 9p
.RT
.PP
The control field contains a command or a response, and sequence
numbers where applicable.
.PP
Three types of control field formats are used to perform numbered
information transfer (I\ format), numbered supervisory functions (S\ format)
and unnumbered control functions (U\ format).
.PP
The control field formats for basic (modulo\ 8) operation are depicted
in Table\ 3/X.25.
.PP
The control field formats for extended (modulo 128) operation are
depicted in Table\ 4/X.25.
.bp
.RT
.ce
.line
.ce
\fBTable 3/X.25 [T3.25], p.\fR
.ce
.line
.ce
\fBTable 4/X.25 [T4/X.25], p.\fR
.ce
2.3.2.1.1
.ce
\fIInformation transfer format\fR
.ce
\fI \(em I\fR
.ce
.parag
.ce
The I format is used to perform an information transfer. The
.ce
functions of N(S), N(R) and P are independent; i.e.,\ each I\ frame has
an N(S), an N(R) which may or may not acknowledge additional I\ frames
received by the
.ce
DCE or DTE, and a P\ bit that may be set to\ 0 or\ 1.
.ce
.parag
.ce
2.3.2.1.2
.ce
\fISupervisory format\fR
.ce
\fI \(em S\fR
.ce
.parag
.ce
The S format is used to perform data link supervisory control
.ce
functions such as acknowledge I\ frames, request retransmission of I\ frames,
and to request a temporary suspension of transmission of I\ frames. The
functions of N(R) and P/F are independent; i.e.,\ each supervisory frame
has an N(R) which
.ce
may or may not acknowledge additional I\ frames received by the DCE or\
DTE, and a P/F\ bit that may be set to\ 0 or\ 1.
.ce
.parag
.ce
2.3.2.1.3
.ce
\fIUnnumbered format\fR
.ce
\fI \(em U\fR
.ce
.parag
.ce
The U format is used to provide additional data link control
.ce
functions. This format contains no sequence numbers, but does include a
P/F\ bit that may be set to\ 0 or\ 1. The unnumbered frames have the same
control field
.ce
length (one octet) in both basic (modulo\ 8) operation and extended (modulo\
128) operation.
.ce
.parag
.ce
2.3.2.2
.ce
\fIControl field parameters\fR
.ce
.parag
.ce
The various parameters associated with the control field formats
.ce
are described below.
.ce
.parag
.ce
2.3.2.2.1
.ce
\fIModulus\fR
.ce
.parag
.ce
Each I frame is sequentially numbered and may have the value\ 0
.ce
through modulus minus\ 1 (where \*Qmodulus\*U is the modulus of the sequence
.ce
numbers). The modulus equals either\ 8 or\ 128 and the sequence numbers cycle
.ce
through the entire range.
.ce
.parag
.ce
2.3.2.2.2
.ce
\fISend state variable\fR
.ce
\fI V(S)\fR
.ce
.parag
.ce
The send state variable V(S) denotes the sequence number of the
.ce
next in\(hysequence I\ frame to be transmitted. V(S) can take on the values\ 0
.ce
through modulus minus\ 1. The value of V(S) is incremented by\ 1 with each
.ce
successive I\ frame transmission, but cannot exceed the N(R) of the last
.ce
received\ I or supervisory frame by more than the maximum number of outstanding
I\ frames\ (\fIk\fR
.ce
). The value of\ k is defined in \(sc\ 2.4.8.6 below.
.ce
.parag
.ce

.ce
2.3.2.2.3
.ce
\fISend sequence number\fR
.ce
\fI N(S)\fR
.ce
.parag
.ce
Only I frames contain N(S), the send sequence number of transmitted I\
frames. At the time that an in\(hysequence I\ frame is designated for
.ce
transmission, the value of N(S) is set equal to the value of the send state
.ce
variable\ V(S).
.ce
.parag
.ce
2.3.2.2.4
.ce
\fIReceive state variable\fR
.ce
\fIV(R)\fR
.ce
.parag
.ce
The receive state variable V(R) denotes the sequence number of the next
in\(hysequence I\ frame expected to be received. V(R) can take on the values
0 through modulus minus\ 1. The value of V(R) is incremented by\ 1 by the
receipt of an error\(hyfree, in\(hysequence I\ frame whose send sequence
number N(S) equals
.ce
the receive state variable V(R).
.ce
.parag
.ce
2.3.2.2.5
.ce
\fIReceive sequence number\fR
.ce
\fI N(R)\fR
.ce
.parag
.ce
All I frames and supervisory frames contain N(R), the expected send sequence
number of the next received I\ frame. At the time that a frame of the above
types is designated for transmission, the value of N(R) is set equal to
the current value of the receive state variable V(R). N(R) indicates that
the DCE or DTE transmitting the N(R) has received correctly all I\ frames
numbered up to and including N(R)\ \(em\ 1.
.ce
.parag
.ce
2.3.2.2.6
.ce
\fIPoll/Final bit\fR
.ce
\fI P/F\fR
.ce
.parag
.ce
All frames contain P/F, the Poll/Final bit. In command frames, the P/F
bit is referred to as the P bit. In response frames, it is referred to
as the F\ bit.
.ce
.parag
.ce
2.3.3
.ce
\fIFunctions of the Poll/Final bit\fR
.ce
.parag
.ce
The Poll bit set to 1 is used by the DCE or DTE to solicit (poll) a response
from the DTE or DCE, respectively. The Final bit set to\ 1 is used by the
DCE or DTE to indicate the response frame transmitted by the DTE or DCE,
.ce
respectively, as a result of the soliciting (poll) command.
.ce
.parag
.ce
The use of the P/F bit is described in \(sc\ 2.4.3 below.
.ce
.parag
.ce
2.3.4
.ce
\fICommands and responses\fR
.ce
.parag
.ce
For basic (modulo 8) operation, the commands and responses
.ce
represented in Table\ 5/X.25 will be supported by the DCE and the\ DTE.
.ce
.parag
.ce

.ce
For extended (modulo 128) operation, the commands and responses
.ce
represented in Table\ 6/X.25 will be supported by the DCE and the DTE.
.ce
.parag
.ce

.ce
For purposes of the LAPB procedures, the supervisory function bit encoding
\*Q11\*U and those encodings of the modifier function bits in
.ce
Tables\ 3/X.25 and 4/X.25 not identified in Tables\ 5/X.25 or 6/X.25 are
.ce
identified as \*Qundefined or not implemented\*U command and response control
.ce
fields.
.ce
.parag
.ce
The commands and responses in Tables 5/X.25 and 6/X.25 are defined as  follows:
.ce
.parag
.ce
2.3.4.1
.ce
\fIInformation (I)\fR
.ce
\fI command\fR
.ce
.parag
.ce
The function of the information (I) command is to transfer across a data
link a sequentially numbered frame containing an information field.
.ce
.parag
.ce
.line
.ce
\fBTable 5/X.25 [T5.25], p.\fR
.ce
.line
.ce
\fBTable 6/X.25 [T6.25], p.\fR
.ce
2.3.4.2
.ce
\fIReceive ready (RR)\fR
.ce
\fI command and response\fR
.ce
.parag
.ce
The receive ready (RR) supervisory frame is used by the DCE
.ce
or DTE to:
.ce
.parag
.ce

.ce
1)
.ce
indicate it is ready to receive an I frame; and
.ce
.parag
.ce
2)
.ce
acknowledge previously received I frames numbered up to and
.ce
including N(R)\ \(em\ 1.
.ce
.parag
.ce
An RR frame may be used to indicate the clearance of a busy
.ce
condition that was reported by the earlier transmission of an RNR frame
by that same station (DCE or\ DTE). In addition to indicating the DCE or
DTE status, the RR command with the P\ bit set to\ 1 may be used by the
DCE or DTE to ask for the status of the DTE or DCE, respectively.
.ce
.parag
.ce
2.3.4.3
.ce
\fIReceive not ready (RNR) command and response\fR
.ce
.parag
.ce
The receive not ready (RNR) supervisory frame is used by the DCE or DTE
to indicate a busy condition; i.e.\ temporary inability to accept
.ce
additional incoming I\ frames. I\ frames numbered up to and including N(R)\
\(em\ 1
.ce
are acknowledged. I\ frame N(R) and any subsequent I\ frames received, if any,
.ce
are not acknowledged; the acceptance status of these I\ frames will be
indicated in subsequent exchanges.
.ce
.parag
.ce
In addition to indicating the DCE or DTE status, the RNR command with the
P bit set to\ 1 may be used by an DCE or DTE to ask for the status of the
.ce
DTE or DCE, respectively.
.ce
.parag
.ce
2.3.4.4
.ce
\fIReject (REJ) command and response\fR
.ce
.parag
.ce
The reject (REJ) supervisory frame is used by the DCE or DTE to request
transmission of I\ frames starting with the frame numbered N(R).
.ce
I\ frames numbered N(R)\ \(em\ 1 and below are acknowledged. Additional
I\ frames
.ce
pending initial transmission may be transmitted following the retransmitted
.ce
I\ frame(s).
.ce
.parag
.ce

.ce
Only one REJ exception condition for a given direction of information transfer
may be established at any time. The REJ exception condition is cleared
(reset) upon the receipt of an I\ frame with an N(S) equal to the N(R)
of the
.ce
REJ\ frame.
.ce
.parag
.ce
An REJ frame may be used to indicate the clearance of a busy condition
that was reported by the earlier transmission of an RNR frame by that same
.ce
station (DCE or\ DTE). In addition to indicating the DCE or DTE status, the
.ce
REJ\ command with the P bit set to\ 1 may be used by the DCE or\ DTE to ask for
.ce
the status of the DTE or\ DCE, respectively.
.ce
.parag
.ce
\fR
.ce
2.3.4.5
.ce
\fISet asynchronous balanced mode (SABM)\fR
.ce
\fIcommand/\fR
.ce
\fISet asynchronous balanced mode extended (SABME)\fR
.ce

.ce
\fIcommand (subscription time option)\fR
.ce
.parag
.ce

.ce
The SABM unnumbered command is used to place the addressed DCE or DTE in
an asynchronous balanced mode (ABM) information transfer phase where all
command/response control fields will be one\ octet in length.
.ce
.parag
.ce
The SABME unnumbered command is used to place the addressed DCE or DTE
in an asynchronous balanced mode\ (ABM) information transfer phase where
.ce
numbered command/response control fields will be two octets in length, and
.ce
unnumbered command/response control fields will be one octet in length.
.ce
.parag
.ce

.ce
No information field is permitted with the SABM or SABME command. The transmission
of a SABM/SABME command indicates the clearance of a busy
.ce
condition that was reported by the earlier trans
.ce
mission\ of an RNR\ frame by   that
.ce
same station (DCE or\ DTE). The DCE or DTE confirms acceptance of SABM/SABME
.ce
[modulo\ 8 (basic) operation/modulo\ 128 (extended) operation] command by the
.ce
transmission, at the first opportunity, of a UA\ response. Upon acceptance of
.ce
this command, the DCE or DTE send state variable\ V(S) and receive state
.ce
variable V(R) are set to\ 0.
.ce
.parag
.ce
Previously transmitted I\ frames that are unacknowledged when this
.ce
command is actioned remain unac
.ce
knowledged.\ It is the responsibility of a
.ce
higher layer (e.g.\ Packet Layer or MLP) to recover from the possible loss of
.ce
the contents (e.g.\ packets) of such I\ frames.
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ The mode of operation of a data link [basic (modulo 8) or
.ce
extended (modulo\ 128)] is determined at subscription time and is only
changed by going through a new subscription process.
.ce
.parag
.ce
2.3.4.6
.ce
\fIDisconnect (DISC)\fR
.ce
\fI command\fR
.ce
.parag
.ce
The DISC unnumbered command is used to terminate the mode
.ce
previously set. It is used to inform the DCE or DTE receiving the DISC
command that the DTE or DCE sending the DISC command is suspending operation.
No
.ce
information field is permitted with the DISC command. Prior to actioning the
.ce
DISC command, the DCE or DTE receiving the DISC command confirms the acceptance
of the DISC command by the transmission of a UA response. The DTE or DCE
.ce
sending the DISC command enters the disconnected phase when it receives the
.ce
acknowledging UA response.
.ce
.parag
.ce
Previously transmitted I frames that are unacknowledged when this
.ce
command is actioned remain unacknowledged. It is the responsibility of
a higher layer (e.g.\ Packet Layer or MLP) to recover from the possible
loss of the
.ce
contents (e.g.,\ packets) of such I frames.
.ce
.parag
.ce
2.3.4.7
.ce
\fIUnnumbered acknowledgement (UA)\fR
.ce
\fI response\fR
.ce
.parag
.ce
The UA unnumbered response is used by the DCE or DTE to acknowledge the
receipt and acceptance of the mode\(hysetting commands. Received mode\(hysetting
commands are not actioned until the UA response is transmitted. The
.ce
transmission of a UA response indicates the clearance of a busy condition
that was reported by the earlier transmission of an RNR frame by that same
station (DCE or DTE). No information field is permitted with the UA response.
.ce
.parag
.ce

.ce
2.3.4.8
.ce
\fIDisconnected mode (DM)\fR
.ce
\fI response\fR
.ce
.parag
.ce
The DM unnumbered response is used to report a status where the DCE or
DTE is logically disconnected from the data link, and is in the disconnected
phase. The DM response may be sent to indicate that the DCE or DTE has
entered the disconnected phase without benefit of having received a DISC
command, or, if sent in response to the reception of a mode setting command,
is sent to
.ce
inform the DTE or DCE that the DCE or DTE, respectively, is still in the
.ce
disconnected phase and cannot execute the set mode command. No information
.ce
field is permitted with the DM response.
.ce
.parag
.ce
A DCE or DTE in a disconnected phase will monitor received commands
.ce
and will react to an SABM/SABME command as outlined in \(sc\ 2.4.4 below,
and will respond with a DM response with the F bit set to\ 1 to any other
command
.ce
received with the P\ bit set to\ 1.
.ce
.parag
.ce
2.3.4.9
.ce
\fIFrame reject (FRMR)\fR
.ce
\fI response\fR
.ce
.parag
.ce
The FRMR unnumbered response is used by the DCE or DTE to report an error
condition not recoverable by retransmission of the identical frame;
.ce
i.e.\ at least one of the following conditions, which results from the
receipt of a valid frame:
.ce
.parag
.ce
1)
.ce
the receipt of a command or response control field that is
.ce
undefined or not implemented;
.ce
.parag
.ce
2)
.ce
the receipt of an I frame with an information field which
.ce
exceeds the maximum established length;
.ce
.parag
.ce
3)
.ce
the receipt of an invalid N(R); or
.ce
.parag
.ce
4)
.ce
the receipt of a frame with an information field which is
.ce
not permitted or the receipt of a supervisory or unnumbered
.ce
frame with incorrect length.
.ce
.parag
.ce
An undefined or not implemented control field is any of the
.ce
control field encodings that are not identified in Tables\ 5/X.25 or\ 6/X.25.
.ce
.parag
.ce

.ce
A valid N(R) must be within the range from the lowest send sequence
.ce
number N(S) of the still unacknowledged frame(s) to the current DCE send
state variable inclusive (or to the current internal variable \fIx\fR
.ce
if the DCE is in
.ce
the timer recovery condition as described in \(sc\ 2.4.5.9).
.ce
.parag
.ce

.ce
An information field which immediately follows the control field, and consists
of\ 3 or 5\ octets [modulo\ 8 (basic) operation or modulo\ 128 (extended)
operation, respectively], is returned with this response and provides the
.ce
reason for the FRMR response. These formats are given in Tables\ 7/X.25
.ce
and\ 8/X.25.
.ce
.parag
.ce
2.3.5
.ce
\fIException condition reporting and recovery\fR
.ce
.parag
.ce
The error recovery procedures which are available to effect
.ce
recovery following the detection/occurrence of an
.ce
exception condition
.ce
 at the Data Link Layer are described below. Exception conditions described
are those situations which may occur as the result of transmission errors,
DCE or DTE malfunction, or operational situations.
.ce
.parag
.ce
2.3.5.1
.ce
\fIBusy condition\fR
.ce
.parag
.ce
The busy condition results when the DCE or DTE is temporarily
.ce
unable to continue to receive I frames due to internal constraints,
.ce
e.g.\ receive buffering limitations. In this case an RNR frame is transmitted
.ce
from the busy DCE or DTE. I\ frames pending transmission may be transmitted
from the busy DCE or DTE prior to or following the RNR\ frame.
.ce
.parag
.ce
An indication that the busy condition has cleared is communicated by the
transmission of a UA (only in response to a SABM/SABME command), RR, REJ
or SABM/SABME (modulo\ 8/modulo\ 128) frame.
.ce
.parag
.ce
.line
.ce
\fBTable 7/X.25 [T7.25], p.\fR
.ce
.line
.ce
\fBTable 8/X.25 [T8.25], p.\fR
.ce
2.3.5.2
.ce
\fIN(S) sequence error condition\fR
.ce
.parag
.ce
The information field of all I frames received whose N(S) does not equal
the receive state variable V(R) will be discarded.
.ce
.parag
.ce
An N(S) sequence error exception condition occurs in the receiver when
an I\ frame received contains an N(S) which is not equal to the receive
state
.ce
variable V(R) at the receiver. The receiver does not acknowledge (increment
its receive state variable) the I\ frame causing the sequence error, or
any I\ frame which may follow, until an I\ frame with the correct N(S)
is received.
.ce
.parag
.ce

.ce
A DCE or DTE which receives one or more valid I frames having sequence
errors or subsequent supervisory frames (RR, RNR and REJ) shall accept
the
.ce
control information contained in the N(R) field and the P or F bit to perform
data link control functions; e.g.\ to receive acknowledgement of previously
.ce
transmitted I\ frames and to cause the DCE or DTE to respond (P bit set to 1).
.ce
.parag
.ce

.ce
The means specified in \(sc\(sc\ 2.3.5.2.1 and 2.3.5.2.2 shall be available
for initiating the retransmission of lost or errored I\ frames following
the
.ce
occurrence of an N(S) sequence error condition.
.ce
.parag
.ce
2.3.5.2.1
.ce
\fIREJ recovery\fR
.ce
.parag
.ce
The REJ frame is used by a receiving DCE or DTE to initiate a
.ce
recovery (retransmission) following the detection of an N(S) sequence error.
.ce
.parag
.ce

.ce
With respect to each direction of transmission on the data link, only one
\*Qsent REJ\*U exception condition from a DCE or DTE, to a DTE or DCE,
is
.ce
established at a time. A \*Qsent REJ\*U exception condition is cleared when the
.ce
requested I\ frame is received.
.ce
.parag
.ce

.ce
A DCE or DTE receiving a REJ frame initiates sequential
.ce
(re\(hy)transmission of I\ frames starting with the I\ frame indicated
by the N(R) contained in the REJ frame. The retransmitted frames may contain
an N(R) and a P bit that are updated from, and therefore different from,
the ones contained in the originally transmitted I\ frames.
.ce
.parag
.ce
2.3.5.2.2
.ce
\fITime\(hyout recovery\fR
.ce
.parag
.ce
If a DCE or DTE, due to a transmission error, does not receive (or receives
and discards) a single I\ frame or the last I\ frame(s) in a sequence of
I\ frames, it will not detect an N(S) sequence error condition and, therefore,
will not transmit a REJ frame. The DTE or DCE which transmitted the
.ce
unacknowledged I\ frame(s) shall, following the completion of a system
specified time\(hyout period (see \(sc\(sc\ 2.4.5.1 and 2.4.5.9 below),
take appropriate recovery action to determine at which I\ frame retransmission
must begin. The
.ce
retransmitted frame(s) may contain an N(R) and a P bit that is updated from,
.ce
and therefore different from, the ones contained in the originally transmitted
frame(s).
.ce
.parag
.ce
2.3.5.3
.ce
\fIInvalid frame\fR
.ce
\fI condition\fR
.ce
.parag
.ce
Any frame which is invalid will be discarded, and no action is
.ce
taken as the result of that frame. An invalid frame is defined as one
.ce
which:
.ce
.parag
.ce
a)
.ce
is not properly bounded by two flags;
.ce
.parag
.ce
b)
.ce
in basic (modulo 8) operation, contains fewer than 32 bits
.ce
between flags; in extended (modulo\ 128) operation, contains
.ce
fewer than 40\ bits between flags of frames that contain sequence
.ce
numbers or 32\ bits between flags of frames that do not contain
.ce
sequence numbers;
.ce
.parag
.ce
c)
.ce
contains a Frame Check Sequence (FCS) error; or
.ce
.parag
.ce
d)
.ce
contains an address other than A or B (for single link
.ce
operation) or other than C or D (for multilink operation).
.ce
.parag
.ce

.ce
For those networks that are octet aligned, a detection of
.ce
non\(hyoctet alignment may be made at the Data Link Layer by adding a frame
.ce
validity check that requires the number of bits between the opening flag and
.ce
the closing flag, excluding bits inserted for transparency, to be an integral
number of octets in length, or the frame is considered invalid.
.ce
.parag
.ce
2.3.5.4
.ce
\fIFrame rejection\fR
.ce
\fI condition\fR
.ce
.parag
.ce
A frame rejection condition is established upon the receipt of an error\(hyfree
frame with one of the conditions listed in \(sc\ 2.3.4.9 above.
.ce
.parag
.ce

.ce
At the DCE or DTE, this frame rejection exception condition is
.ce
reported by an FRMR response for appropriate DTE or DCE action, respectively.
Once a DCE has established such an exception condition, no additional I\
frames are accepted until the condition is reset by the DTE, except for
examination of the P bit. The FRMR response may be repeated at each opportunity,
as specified in \(sc\ 2.4.7.3, until recovery is effected by the DTE, or
until the DCE initiates its own recovery in case the DTE does not respond.
.ce
.parag
.ce
2.3.5.5
.ce
\fIExcessive idle channel state condition on incoming\fR
.ce

.ce
\fIchannel\fR
.ce
.parag
.ce
Upon detection of an idle channel state condition (see \(sc\ 2.2.12.2 above)
on the incoming channel, the DCE shall wait for a period\ T3 (see
.ce
\(sc\ 2.4.8.3 below) without taking any specific action, waiting for detection
of a return to the active channel state (i.e.,\ detection of at least one
flag
.ce
sequence). After the period\ T3, the DCE shall notify the higher layer
(e.g.\ the Packet Layer or the MLP) of the excessive idle channel state
condition, but
.ce
shall not take any action that would preclude the DTE from establishing the
.ce
data link by normal data link set\(hyup procedures.
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ Other actions to be taken by the DCE at the Data Link Layer upon
expiration of period\ T3 is a subject for further study.
.ce
.parag
.ce

.ce
2.4
.ce
\fIDescription of the \fR
.ce
\fILAPB procedure\fR
.ce
.parag
.ce
2.4.1
.ce
\fILAPB basic and extended modes of operation\fR
.ce
.parag
.ce
In accordance with the system choice made by the DTE at
.ce
subscription time, the DCE will either support modulo\ 8 (basic) operation or
.ce
will support modulo\ 128 (extended) operation. Changing from basic operation
to extended operation, or vice versa, in the DCE requires resubscription
by the
.ce
DTE for the desired service, and is not supported dynamically.
.ce
.parag
.ce
Table 5/X.25 indicates the command and response control field formats used
with the basic (modulo\ 8) service. The mode\(hysetting command employed
to
.ce
initialize (set up) or reset the basic mode is the SABM command. Table
6/X.25 indicates the command and response control field formats used with
the extended (modulo\ 128) service. The mode\(hysetting command employed
to initialize (set up) or reset the extended mode is the SABME command.
.ce
.parag
.ce
2.4.2
.ce
\fILAPB procedure for addressing\fR
.ce
.parag
.ce
The address field identifies a frame as either a command or a
.ce
response. A command frame contains the address of the DCE or DTE to which
the command is being sent. A response frame contains the address of the
DCE or DTE sending the frame.
.ce
.parag
.ce
In order to allow differentiation between single link operation and
.ce
the optional multilink operation for diagnostic and/or maintenance reasons,
.ce
different address pair encodings are assigned to data links operating with
.ce
multilink procedure compared to data links operating with the single link
.ce
procedure.
.ce
.parag
.ce
Frames containing commands transferred from the DCE to the DTE will
.ce
contain the address\ A for the single link operation and address\ C for the
.ce
multilink operation.
.ce
.parag
.ce
Frames containing responses transferred from the DCE to the DTE will contain
the address\ B for the single link operation and address\ D for the
.ce
multilink operation.
.ce
.parag
.ce

.ce
Frames containing commands transferred from the DTE to the DCE shall contain
the address\ B for the single link operation and address\ D for the
.ce
multilink operation.
.ce
.parag
.ce
Frames containing responses transferred from the DTE to the DCE shall contain
the address\ A for the single link operation and address\ C for the
.ce
multilink operation.
.ce
.parag
.ce
These addresses are coded as follows:
.ce
.parag
.ce
Address
.ce
1\ 2\ 3\ 4\ 5\ 6\ 7\ 8
.ce
.parag
.ce
Single link operation
.ce
\ \ A
.ce
1\ 1\ 0\ 0\ 0\ 0\ 0\ 0
.ce
.line
.ce
\ \ B
.ce
1\ 0\ 0\ 0\ 0\ 0\ 0\ 0
.ce
.parag
.ce
Multilink operation
.ce
\ \ C
.ce
1\ 1\ 1\ 1\ 0\ 0\ 0\ 0
.ce
.line
.ce
\ \ D
.ce
1\ 1\ 1\ 0\ 0\ 0\ 0\ 0
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ The DCE will discard all frames received with an address other
than\ A or\ B (single link operation), or\ C or\ D (multilink operation).
.ce
.parag
.ce
2.4.3
.ce
\fILAPB procedure for the use of the P/F bit\fR
.ce
.parag
.ce
The DCE or DTE receiving an SABM/SABME, DISC, supervisory command or I\
frame with the P\ bit set to\ 1 will set the F\ bit to\ 1 in the next response
frame it transmits.
.ce
.parag
.ce
The response frame returned by the DCE to an SABM/SABME or DISC
.ce
command with the P\ bit set to\ 1 will be a UA or DM response with the
F\ bit set to\ 1. The response frame returned by the DCE to an I\ frame
with the P\ bit set to\ 1, received during the information transfer phase,
will be an RR, REJ, RNR or FRMR response with the F\ bit set to\ 1. The
response frame returned by the
.ce
DCE to a supervisory command with the P\ bit set to\ 1, received during the
.ce
information transfer phase, will be an RR, REJ, RNR or FRMR response with
the F\ bit set to\ 1. The response frame returned by the DCE to an I\ frame
or
.ce
supervisory frame with the P\ bit set to\ 1, received during the disconnected
.ce
phase, will be a DM response with the F\ bit set to\ 1.
.ce
.parag
.ce
The P bit may be used by the DCE in conjunction with the timer
.ce
recovery condition (see \(sc\ 2.4.5.9 below).
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ Other use of the P bit by the DCE is a subject for further
.ce
study.
.ce
.parag
.ce
2.4.4
.ce
\fILAPB procedure for data link set\(hyup and disconnection\fR
.ce
.parag
.ce

.ce
2.4.4.1
.ce
\fIData link set\(hyup\fR
.ce
.parag
.ce
The DCE will indicate that it is able to set up the data link by
.ce
transmitting contiguous flags (active channel state).
.ce
.parag
.ce
Either the DTE or the DCE may initiate data link set\(hyup. Prior to
.ce
initiation of data link set\(hyup, either the DCE or the DTE may initiate data
.ce
link
.ce
disconnection (see \(sc\ 2.4.4.3) for the purpose of insuring that the
DCE and the DTE are in the same phase. The DCE may also transmit an unsolicited
DM response to request the DTE to initiate data link set\(hyup.
.ce
.parag
.ce
The DTE shall initiate data link set\(hyup by transmitting an SABM/SABME
command to the DCE. If, upon receipt of the SABM/SABME command correctly,
the DCE determines that it can enter the information transfer phase, it
will return a UA response to the DTE, will reset its send and receive state
variables V(S) and V(R) to zero, and will consider that the data link is
set up. If, upon
.ce
receipt
.ce
of the SABM/SABME command correctly, the DCE determines that it cannot enter
.ce
the information transfer phase, it will return a DM response to the DTE as a
.ce
denial to the data link set\(hyup initialization and will consider that the
.ce
data link is
.ce
\fInot\fR
.ce
 set up. In order to avoid misinterpretation of the DM response received,
it is suggested that the DTE always sends its SABM/SABME command with the
P\ bit set to\ 1. Otherwise, it is not possible to differentiate a DM response
intended as a denial to data link set\(hyup from a DM response that is
issued in a separate unsolicited sense as a request for a mode\(hysetting
command (as described
.ce
in\ \(sc\ 2.4.4.4.2).
.ce
.parag
.ce
The DCE will initiate data link set\(hyup by transmitting an SABM/SABME
command to the DTE and starting its Timer\ T1 in order to determine when
too
.ce
much time has elapsed waiting for a reply (see \(sc\ 2.4.8.1 below). Upon
reception of a UA response from the DTE, the DCE will reset its send and
receive state
.ce
variables\ V(S) and\ V(R) to zero, will stop its Timer\ T1, and will consider
that the data link is set up. Upon reception of a DM response from the
DTE as a
.ce
denial
.ce
to the data link set\(hyup initialization, the DCE will stop its Timer\
T1 and will consider that the data link is \fInot\fR
.ce
set up.
.ce
.parag
.ce

.ce
The DCE, having sent the SABM/SABME command, will ignore and discard any
frames except an SABM/SABME or DISC command, or a UA or DM response
.ce
received from the DTE. The receipt of an SABM/SABME or DISC command from the
.ce
DTE will result in a collision situation that is resolved per \(sc\ 2.4.4.5
below. Frames other than the UA and DM responses sent in response to a
received
.ce
SABM/SABME or DISC command will be sent only after the data link is set
up and if no outstanding SABM/SABME command exists.
.ce
.parag
.ce
After the DCE sends the SABM/SABME command, if a UA or DM response is not
received correctly, Timer\ T1 will run out in the DCE. The DCE will then
.ce
resend the SABM/SABME command and will restart Timer\ T1. After transmission
of the SABM/SABME command N2 times by the DCE, appropriate higher layer
recovery action will be initiated. The value of N2 is defined in \(sc\
2.4.8.4 below.
.ce
.parag
.ce
2.4.4.2
.ce
\fIInformation transfer phase\fR
.ce
.parag
.ce
After having transmitted the UA response to the SABM/SABME command or having
received the UA response to a transmitted SABM/SABME command, the DCE will
accept and transmit I and supervisory frames according to the procedures
described in \(sc\ 2.4.5 below.
.ce
.parag
.ce

.ce
When receiving the SABM/SABME command while in the information
.ce
transfer phase, the DCE will conform to the data link resetting procedure
.ce
described in \(sc\ 2.4.7 below.
.ce
.parag
.ce
2.4.4.3
.ce
\fIData link disconnection\fR
.ce
.parag
.ce
The DTE shall initiate a disconnect of the data link by
.ce
transmitting a DISC command to the DCE. On correctly receiving a DISC command
in the information transfer phase, the DCE will send a UA response and
enter
.ce
the disconnected phase. On correctly receiving a DISC command in the
.ce
disconnected phase, the DCE will send a DM response and remain in the
.ce
disconnected phase. In order to avoid misinterpretation of the DM response
.ce
received, it is suggested that the DTE always sends its DISC command with
the P\ bit set to\ 1. Otherwise, it is not possible to differentiate a
DM response
.ce
intended as an indication that the DCE is already in the disconnected phase
.ce
from a DM response that is issued in a separate unsolicited sense as a
request for a mode\(hysetting command (as described in \(sc\ 2.4.4.4.2).
.ce
.parag
.ce

.ce
The DCE will initiate a disconnect of the data link by transmitting a DISC
command to the DTE and starting its Timer\ T1 (see \(sc\ 2.4.8.1 below).
Upon reception of an UA response from the DTE, the DCE will stop its Timer\
T1 and
.ce
will enter the disconnected phase. Upon reception of a DM response from
the DTE as an indication that the DTE was already in the disconnected phase,
the DCE
.ce
will stop its Timer\ T1 and will enter the disconnected phase.
.ce
.parag
.ce
The DCE, having sent the DISC command, will ignore and discard any
.ce
frames except an SABM/SABME or DISC command, or a UA or DM response received
.ce
from the DTE. The receipt of an SABM/SABME or DISC command from the DTE will
.ce
result in a collision situation that is resolved per \(sc\ 2.4.4.5 below.
.ce
.parag
.ce

.ce
After the DCE sends the DISC command, if a UA or DM response is not
.ce
received correctly, Timer\ T1 will run out in the DCE. The DCE will then
resend the DISC command and will restart Timer\ T1. After transmission
of the DISC
.ce
command N2 times by the DCE, appropriate higher layer recovery action will
be initiated. The value of N2 is defined in \(sc\ 2.4.8.4 below.
.ce
.parag
.ce
2.4.4.4
.ce
\fIDisconnected phase\fR
.ce
.parag
.ce
2.4.4.4.1
.ce
After having received a DISC command from the DTE and
.ce
returned a UA response to the DTE, or having received the UA response to a
.ce
transmitted DISC command, the DCE will enter the disconnected phase.
.ce
.parag
.ce
In the disconnected phase, the DCE may initiate data link set\(hyup. In
the disconnected phase, the DCE will react to the receipt of an SABM/SABME
command as described in \(sc\ 2.4.4.1 above and will transmit a DM response
in
.ce
answer to a received DISC command. When receiving any other command (defined,
or undefined or not implemented) with the P\ bit set to\ 1, the DCE will
transmit a DM response with the F\ bit set to\ 1. Other frames received
in the
.ce
disconnected phase will be ignored by the DCE.
.ce
.parag
.ce
2.4.4.4.2
.ce
When the DCE enters the disconnected phase after
.ce
detecting error conditions as listed in \(sc\ 2.4.6 below, or after an internal
.ce
malfunction, it may indicate this by sending a DM response rather than
a DISC command. In these cases, the DCE will transmit a DM response and
start its
.ce
Timer\ T1 (see \(sc\ 2.4.8.1 below).
.ce
.parag
.ce
If Timer T1 runs out before the reception of an SABM/SABME or DISC command
from the DTE, the DCE will retransmit the DM response and restart
.ce
Timer\ T1. After transmission of the DM response N2 times, the DCE will
remain in the disconnected phase and appropriate recovery actions will
be initiated. The value of N2 is defined in \(sc\ 2.4.8.4 below.
.ce
.parag
.ce
Alternatively, after an internal malfunction, the DCE may either
.ce
initiate a data link resetting procedure (see \(sc\ 2.4.7 below) or disconnect
the data link (see \(sc\ 2.4.4.3 above) prior to initiating a data link
set\(hyup
.ce
procedure (see \(sc\ 2.4.4.1 above).
.ce
.parag
.ce
2.4.4.5
.ce
\fICollision of unnumbered commands\fR
.ce
.parag
.ce
Collision situations shall be resolved in the following way:
.ce
.parag
.ce
2.4.4.5.1
.ce
If the sent and received unnumbered commands are the
.ce
same, the DCE and the DTE shall each send the UA response at the earliest
.ce
possible opportunity. The DCE shall enter the indicated phase either,
.ce
.parag
.ce

.ce
1)
.ce
after receiving the UA response,
.ce
.parag
.ce
2)
.ce
after sending the UA response, or
.ce
.parag
.ce
3)
.ce
after timing out waiting for the UA response having sent a
.ce
UA response.
.ce
.parag
.ce
In the case of 2) above, the DCE will accept a subsequent UA
.ce
response to the mode\(hysetting command it issued without causing an exception
.ce
condition if received within the time\(hyout interval.
.ce
.parag
.ce

.ce
2.4.4.5.2
.ce
If the sent and received unnumbered commands are
.ce
different, the DCE and the DTE shall each enter the disconnected phase and
.ce
issue a DM response at the earliest possible opportunity.
.ce
.parag
.ce
2.4.4.6
.ce
\fICollision of DM response with SABM/SABME or DISC\fR
.ce

.ce
\fIcommand\fR
.ce
.parag
.ce
When a DM response is issued by the DCE or DTE as an unsolicited
.ce
response to request the DTE or DCE, respectively, to issue a mode\(hysetting
.ce
command as described in \(sc\ 2.4.4.4, a collision between an SABM/SABME
or DISC
.ce
command and the unsolicited DM response may occur. In order to avoid
.ce
misinterpretation of the DM response received, the DTE always sends its
.ce
SABM/SABME or DISC command with the P\ bit set to\ 1.
.ce
.parag
.ce
2.4.4.7
.ce
\fICollision of DM responses\fR
.ce
.parag
.ce
A contention situation may occur when both the DCE and the DTE
.ce
issue a DM response to request a mode\(hysetting command. In this case, the DTE
.ce
will issue an SABM/SABME command to resolve the contention situation.
.ce
.parag
.ce

.ce
2.4.5
.ce
\fILAPB procedures for information transfer\fR
.ce
.parag
.ce
The procedures which apply to the transmission of I\ frames in each direction
during the information transfer phase are described below.
.ce
.parag
.ce
In the following, \*Qnumber one higher\*U is in reference to a
.ce
continuously repeated sequence series, i.e., 7 is 1\ higher than 6 and 0 is
.ce
1\ higher than 7 for modulo\ 8 series, and 127 is 1\ higher than 126 and 0 is
.ce
1\ higher than 127 for modulo\ 128 series.
.ce
.parag
.ce
2.4.5.1
.ce
\fISending I frames\fR
.ce
.parag
.ce
When the DCE has an I frame to transmit (i.e. an I frame not
.ce
already transmitted, or having to be retransmitted as described in \(sc\
2.4.5.6
.ce
below), it will transmit it with an N(S) equal to its current send state
.ce
variable V(S), and an N(R) equal to its current receive state variable
V(R). At the end of the transmission of the I\ frame, the DCE will increment
its send
.ce
state variable V(S) by\ 1.
.ce
.parag
.ce
If Timer T1 is not running at the time of transmission of an I frame, it
will be started.
.ce
.parag
.ce
If the send state variable V(S) is equal to the last value of N(R)
.ce
received plus \fIk\fR
.ce
(where \fIk\fR
.ce
is the maximum number of outstanding I\ frames \(em
.ce
see \(sc\ 2.4.8.6 below), the DCE will not transmit any new I\ frames, but may
.ce
retransmit an I\ frame as described in \(sc\(sc\ 2.4.5.6 or 2.4.5.9 below.
.ce
.parag
.ce
When the DCE is in the busy condition, it may still transmit I frames,
provided that the DTE is not busy. When the DCE is in the frame rejection
.ce
condition, it will stop transmitting I\ frames.
.ce
.parag
.ce
2.4.5.2
.ce
\fIReceiving an I frame\fR
.ce
.parag
.ce
2.4.5.2.1
.ce
When the DCE is not in a busy condition and receives a valid I\ frame whose
send sequence number N(S) is equal to the DCE receive state
.ce
variable V(R), the DCE will accept the information field of this frame,
.ce
increment by one its receive state variable V(R), and act as follows:
.ce
.parag
.ce

.ce
a)
.ce
If the DCE is still not in a busy condition:
.ce
.parag
.ce
i)
.ce
If an I frame is available for transmission by the
.ce
DCE, it may act as in \(sc\ 2.4.5.1 above and acknowledge
.ce
the received I frame by setting N(R) in the control field
.ce
of the next transmitted I\ frame to the value of the DCE
.ce
receive state variable V(R). Alternatively, the DCE may
.ce
acknowledge the received I\ frame by transmitting an RR
.ce
frame with the N(R) equal to the value of the DCE receive
.ce
state variable V(R).
.ce
.parag
.ce
ii)
.ce
If no I frame is available for transmission by the
.ce
DCE, it will transmit an RR frame with N(R) equal to the
.ce
value of the DCE receive state variable V(R).
.ce
.parag
.ce

.ce
b)
.ce
If the DCE is now in a busy condition, it will transmit an
.ce
RNR frame with N(R) equal to the value of the DCE receive
.ce
state variable V(R) (see \(sc\ 2.4.5.8).
.ce
.parag
.ce

.ce
2.4.5.2.2
.ce
When the DCE is in a busy condition, it may ignore the
.ce
information field contained in any received I\ frame.
.ce
.parag
.ce
2.4.5.3
.ce
\fIReception of invalid frames\fR
.ce
.parag
.ce
When the DCE receives an invalid frame (see \(sc\ 2.3.5.3), this frame
will be discarded.
.ce
.parag
.ce
2.4.5.4
.ce
\fIReception of out\(hyof\(hysequence I frames\fR
.ce
.parag
.ce
When the DCE receives a valid I frame whose send sequence number
.ce
N(S) is incorrect, i.e.,\ not equal to the current DCE receive state variable
.ce
V(R), it will discard the information field of the I\ frame and transmit
an REJ frame with the N(R) set to one higher than the N(S) of the last
correctly
.ce
received I\ frame. The REJ frame will be a command frame with the P\ bit
set to\ 1 if an acknowledged transfer of the retransmission request is
required;
.ce
otherwise the REJ frame may be either a command or a response frame. The DCE
.ce
will then discard the information field of all I\ frames received until the
.ce
expected I\ frame is correctly received. When receiving the expected I\ frame,
.ce
the DCE will then acknowledge the I\ frame as described in \(sc\ 2.4.5.2
above. The DCE will use the N(R) and P\ bit information in the discarded
I\ frames as
.ce
described in \(sc\ 2.3.5.2 above.
.ce
.parag
.ce
2.4.5.5
.ce
\fIReceiving acknowledgement\fR
.ce
.parag
.ce
When correctly receiving an I frame or a supervisory frame (RR, RNR or
REJ), even in the busy condition, the DCE will consider the N(R) contained
in this frame as an acknowledgement for all I\ frames it has transmitted
with an N(S) up to and including the received N(R)\(em1. The DCE will stop
Timer\ T1 when it correctly receives an I\ frame or a supervisory frame
with the N(R) higher
.ce
than the last received N(R) (actually acknowledging some I\ frames), or
an REJ frame with an N(R) equal to the last received N(R).
.ce
.parag
.ce

.ce
If Timer T1 has been stopped by the receipt on an I, RR or RNR frame, and
if there are outstanding I\ frames still unacknowledged, the DCE will
.ce
restart Timer\ T1. If Timer\ T1 then runs out, the DCE will follow the
recovery procedure (\(sc\ 2.4.5.9 below) with respect to the unacknowledged
I\ frames. If
.ce
Timer\ T1 has been stopped by the receipt of an REJ frame, the DCE will
follow the retransmission procedures in \(sc\ 2.4.5.6 below.
.ce
.parag
.ce
2.4.5.6
.ce
\fIReceiving an REJ frame\fR
.ce
.parag
.ce
When receiving an REJ frame, the DCE will set its send state
.ce
variable V(S) to the N(R) received in the REJ control field. It will transmit
the corresponding I\ frame as soon as it is available or retransmit it
in
.ce
accordance with the procedures described in \(sc\ 2.4.5.1 above. (Re)transmission
will conform to the following procedure:
.ce
.parag
.ce
i)
.ce
if the DCE is transmitting a supervisory command or response
.ce
when it receives the REJ frame, it will complete that
.ce
transmission before commencing transmission of the requested
.ce
I\ frame;
.ce
.parag
.ce

.ce
ii)
.ce
if the DCE is transmitting an unnumbered command or
.ce
response when it receives the REJ frame, it will ignore the
.ce
request for retransmission;
.ce
.parag
.ce
iii)
.ce
if the DCE is transmitting an I frame when the REJ frame
.ce
is received, it may abort the I\ frame and commence transmission
.ce
of the requested I\ frame immediately after abortion;
.ce
.parag
.ce
iv)
.ce
if the DCE is not transmitting any frame when the REJ frame
.ce
is received, it will commence transmission of the requested
.ce
I\ frame immediately.
.ce
.parag
.ce
In all cases, if other unacknowledged I frames had already been
.ce
transmitted following the one indicated in the REJ frame, then those I
frames will be retransmitted by the DCE following the retransmission of
the requested I\ frame. Other I\ frames not yet transmitted may be transmitted
following the
.ce
retransmitted I\ frames.
.ce
.parag
.ce
If the REJ frame was received from the DTE as a command with the P bit
set to\ 1, the DCE will transmit an RR, RNR or REJ response with the F
bit set to\ 1 before transmitting or retransmitting the corresponding I\
frame.
.ce
.parag
.ce
2.4.5.7
.ce
\fIReceiving an RNR frame\fR
.ce
.parag
.ce
After receiving an RNR frame whose N(R) acknowledges all frames
.ce
previously transmitted, the DCE will stop Timer\ T1 and may then transmit
.ce
an I\ frame, with the P\ bit set to\ 0, whose send sequence number is
.ce
equal to the N(R) indicated in the RNR frame, restarting Timer\ T1 as it
does. After receiving an RNR frame whose N(R) indicates a previously transmitted
.ce
frame, the DCE will not transmit or retransmit any I\ frame, Timer\ T1 being
.ce
already running. In either case, if the Timer\ T1 runs out before receipt
of a busy clearance indication, the DCE will follow the procedure described
in
.ce
\(sc\ 2.4.5.9\ below. In any case, the DCE will not transmit any other
I\ frames
.ce
before receiving an RR or REJ frame, or before the completion of a link
.ce
resetting procedure.
.ce
.parag
.ce
Alternatively, after receiving an RNR frame, the DCE may wait for a
.ce
period of time (e.g.,\ the length of the Timer\ T1) and then transmit a
.ce
supervisory command frame (RR, RNR or REJ) with the P\ bit set to\ 1, and
start Timer\ T1, in order to determine if there is any change in the receive
status of the DTE. The DTE shall respond to the P\ bit set to\ 1 with a
supervisory
.ce
response frame (RR, RNR or REJ) with the F\ bit set to\ 1 indicating either
.ce
continuance of the busy condition (RNR) or clearance of the busy condition
(RR or REJ). Upon receipt of the DTE response, Timer\ T1 is stopped.
.ce
.parag
.ce

.ce
1)
.ce
If the response is the RR or REJ response, the busy
.ce
condition is cleared and the DCE may transmit I\ frames beginning
.ce
with the I\ frame identified by the N(R) in the received response
.ce
frame.
.ce
.parag
.ce
2)
.ce
If the response is the RNR response, the busy condition
.ce
still exists, and the DCE will after a period of time (e.g.\ the
.ce
length of Timer\ T1) repeat the enquiry of the DTE receive
.ce
status.
.ce
.parag
.ce
If Timer T1 runs out before a status response is received, the
.ce
enquiry process above is repeated. If N2 attempts to get a status response
fail (i.e.\ Timer\ T1 runs out N2\ times), the DCE will initiate a data
link resetting procedure as described in \(sc\ 2.4.7.2\ below or will transmit
a DM response to ask the DTE to initiate a data link set\(hyup procedure
as described in \(sc\ 2.4.4.1 and enter the disconnected phase. The value
of N2 is defined in \(sc\ 2.4.8.4\ below.
.ce
.parag
.ce

.ce
If, at any time during the enquiry process, an unsolicited RR or REJ frame
is received from the DTE, it will be considered to be an indication of
.ce
clearance of the busy condition. Should the unsolicited RR or REJ frame be a
.ce
command frame with the P bit set to\ 1, the appropriate response frame
with the F\ bit set to 1 must be transmitted before the DCE may resume
transmission of
.ce
I\ frames. If Timer\ T1 is running, the DCE will wait for the non\(hybusy
response with the F bit set to\ 1 or will wait for Timer\ T1 to run out
and then either
.ce
may reinitiate the enquiry process in order to realize a successful P/F bit
.ce
exchange or may resume transmission of I frames beginning with the I\ frame
.ce
identified by the N(R) in the received RR or REJ frame.
.ce
.parag
.ce
2.4.5.8
.ce
\fIDCE busy condition\fR
.ce
.parag
.ce
When the DCE enters a busy condition, it will transmit an RNR frame at
the earliest opportunity. The RNR frame will be a command frame with the
P bit set to\ 1 if an acknowledged transfer of the busy condition indication
is
.ce
required; otherwise the RNR frame may be either a command or a response
frame. While in the busy condition, the DCE will accept and process supervisory
.ce
frames, will accept and process the contents of the N(R) fields of I\ frames,
.ce
and will return an RNR response with the F bit set to\ 1 if it receives a
.ce
supervisory command or I command frame with the P bit set to\ 1. To clear the
.ce
busy condition, the DCE will transmit either an REJ frame or an RR frame, with
.ce
N(R) set to the current receive state variable V(R), depending on whether or
.ce
not it discarded information fields of correctly received I\ frames. The REJ
.ce
frame or the RR frame will be a command frame with the P bit set to\ 1 if an
.ce
acknowledged transfer of the busy\(hyto\(hynon\(hybusy transition is required,
otherwise the REJ frame or the RR frame may be either a command or a response
frame.
.ce
.parag
.ce

.ce
2.4.5.9
.ce
\fIWaiting acknowledgement\fR
.ce
.parag
.ce
The DCE maintains an internal transmission attempt variable which is set
to\ 0 when the DCE sends a UA response, when the DCE receives a UA
.ce
response or an RNR command or response, or when the DCE correctly receives
an I\ frame or supervisory frame with the N(R) higher than the last received
N(R) (actually acknowledging some outstanding I\ frames).
.ce
.parag
.ce
If Timer T1 runs out waiting for the acknowledgement from the DTE for an
I\ frame transmitted, the DCE will enter the timer recovery condition,
add
.ce
one to its transmission attempt variable and set an internal variable \fIx\fR
.ce
to
.ce
the current value of its send state variable V(S). The DCE will then restart
.ce
Timer T1, set its send state variable V(S) to the last value of N(R) received
from the DTE and retransmit the corresponding I\ frame with the P bit set
to\ 1, or transmit an appropriate supervisory command frame (RR, RNR or
REJ) with the P\ bit set to\ 1.
.ce
.parag
.ce

.ce
The timer recovery condition is cleared when the DCE receives a valid supervisory
frame with the F\ bit\ set to\ 1.
.ce
.parag
.ce
If, while in the timer recovery condition, the DCE correctly receives a
supervisory frame with the F bit set to\ 1 and with the N(R) within the
range from its current send state variable V(S) to \fIx\fR
.ce
included, it will clear the
.ce
timer recovery condition (including stopping Timer\ T1) and set its send
state variable V(S) to the value of the received N(R), and may then resume
with
.ce
I\ frame transmission or retransmission, as appropriate.
.ce
.parag
.ce
If, while in the timer recovery condition, the DCE correctly receives an
I or supervisory frame with the P/F bit set to\ 0 and with a valid N(R)
(see \(sc\ 2.3.4.9), it will not clear the timer recovery condition. The
value of the
.ce
received N(R) may be used to update the send state variable V(S). However,
the DCE may decide to keep the last transmitted I\ frame in store (even
if it is
.ce
acknowledged) in order to be able to retransmit it with the P bit set to\
1 when Timer\ T1 runs out at a later time.
.ce
.parag
.ce

.ce
If the received supervisory frame with the P/F bit set to\ 0 is an REJ
frame with a valid N(R), the DCE may either immediately initiate
.ce
(re)transmission from the value of the send state variable V(S), or it may
.ce
ignore the request for retransmission and wait until the supervisory frame
with the F bit set to\ 1 is received before initiating (re)transmission
of frames
.ce
from the value identified in the N(R) field of the supervisory frame with
the F\ bit set to\ 1. In the case of immediate retransmission, in order
to prevent
.ce
duplicate retransmissions following the clearance of the timer recovery
.ce
condition, the DCE shall inhibit retransmission of a specific I\ frame [same
.ce
N(R) in the same numbering cycle] if the DCE has retransmitted that I\
frame as the result of a received REJ frame with the P/F bit set to\ 0.
.ce
.parag
.ce
If, while in the timer recovery condition, the DCE receives a REJ
.ce
command with the P bit set to\ 1, the DCE will respond immediately with an
.ce
appropriate supervisory response with the F bit set to\ 1. The DCE may
then use the value of the N(R) in the REJ command to update the send state
variable
.ce
V(S), and may either immediately begin (re)transmission from the value N(R)
.ce
indicated in the REJ frame or ignore the request for retransmission and wait
.ce
until the supervisory frame with the F bit set to\ 1 is received before
.ce
initiating (re)transmission of I\ frames from the value identified in the
N(R) field of the supervisory frame with the F\ bit set to\ 1. In the case
of
.ce
immediate retransmission, in order to prevent duplicate retransmissions
.ce
following the clearance of the timer recovery condition, the DCE shall
inhibit retransmission of a specific I\ frame [same N(R) in the same numbering
cycle]
.ce
if the DCE has retransmitted that I\ frame as the result of the received REJ
.ce
command with the P\ bit set to\ 1.
.ce
.parag
.ce
If Timer T1 runs out in the timer recovery condition, and no I or
.ce
supervisory frame with the P/F bit set to 0 and with a valid N(R) has been
.ce
received, or no REJ command with the P bit set to\ 1 and with a valid N(R)
has been received, the DCE will add one to its transmission attempt variable,
.ce
restart Timer\ T1, and either retransmit the I frame sent with the P bit set
.ce
to\ 1 or transmit an appropriate supervisory command with the P bit set to\ 1.
.ce
.parag
.ce

.ce
If the transmission attempt variable is equal to N2, the DCE will
.ce
initiate a data link resetting procedure as described in \(sc\ 2.4.7.2
below, or
.ce
will transmit a DM response to ask the DTE to initiate a data link set\(hyup
.ce
procedure as described in \(sc\ 2.4.4.1 above and enter the disconnected
phase. N2 is a system parameter (see \(sc\ 2.4.8.4 below).
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ Although the DCE may implement the internal variable \fIx\fR
.ce
,
.ce
other mechanisms do exist that achieve the identical function.
.ce
.parag
.ce
2.4.6
.ce
\fILAPB conditions for \fR
.ce
\fIdata link resetting or data link\fR
.ce

.ce
\fIre\(hyinitialization\fR
.ce
\fI(data link set\(hyup)\fR
.ce
.parag
.ce
2.4.6.1
.ce
When the DCE receives, during the information transfer phase, a
.ce
frame which is not invalid (see \(sc\ 2.3.5.3) with one of the conditions
listed in \(sc\ 2.3.4.9 above, the DCE will request the DTE to initiate
a data link resetting procedure by transmitting an FRMR response to the
DTE as described in
.ce
\(sc\ 2.4.7.3.
.ce
.parag
.ce
2.4.6.2
.ce
When the DCE receives, during the information transfer phase, an FRMR response
from the DTE, the DCE will either initiate the data link
.ce
resetting procedures itself as described in \(sc\ 2.4.7.2 or return a DM
response to ask the DTE to initiate the data link set\(hyup (initialization)
procedure as described in \(sc\ 2.4.4.1. After transmitting a DM response,
the DCE will enter
.ce
the disconnected phase as described in \(sc\ 2.4.4.4.2.
.ce
.parag
.ce
2.4.6.3
.ce
When the DCE receives, during the information transfer phase, a UA response,
or an unsolicited response with the F bit set to\ 1, the DCE may
.ce
either initiate the data link resetting procedures itself as described in
.ce
\(sc\ 2.4.7.2, or return a DM response to ask the DTE to initiate the data link
.ce
set\(hyup (initialization) procedure as described in \(sc\ 2.4.4.1. After
transmitting a DM response, the DCE will enter the disconnected phase as
described in
.ce
\(sc\ 2.4.4.4.2.
.ce
.parag
.ce
2.4.6.4
.ce
When the DCE receives, during the information transfer phase, a DM response
from the DTE, the DCE will either initiate the data link set\(hyup
.ce
(initialization) procedures itself as described in \(sc\ 2.4.4.1, or return
a DM
.ce
response to ask the DTE to initiate the data link set\(hyup (initialization)
.ce
procedures as described in \(sc\ 2.4.4.1. After transmitting a DM response,
the DCE will enter the disconnected phase as described in \(sc\ 2.4.4.4.2.
.ce
.parag
.ce

.ce
2.4.7
.ce
\fILAPB procedure for data link resetting\fR
.ce
.parag
.ce
2.4.7.1
.ce
The data link resetting procedure is used to initialize both
.ce
directions of information transfer according to the procedure described
below. The data link resetting procedure only applies during the information
transfer phase.
.ce
.parag
.ce
2.4.7.2
.ce
Either the DTE or the DCE may initiate the data link resetting
.ce
procedure. The data link resetting procedure indicates a clearance of a DCE
.ce
and/or DTE busy condition, if present.
.ce
.parag
.ce
The DTE shall initiate a data link resetting by transmitting an
.ce
SABM/SABME command to the DCE. If, upon correct receipt of the SABM/SABME
.ce
command, the DCE determines that it can continue in the information transfer
.ce
phase, it will return a UA response to the DTE, will reset its send and
receive state variables V(S) and V(R) to zero, and will remain in the information
.ce
transfer phase. If, upon correct receipt of the SABM/SABME command, the DCE
.ce
determines that it cannot remain in the information transfer phase, it will
.ce
return a DM response as a denial to the resetting request and will enter the
.ce
disconnected phase.
.ce
.parag
.ce
The DCE will initiate a data link resetting by transmitting an
.ce
SABM/SABME command to the DTE and starting its Timer\ T1 (see \(sc\ 2.4.8.1
below). Upon reception of a UA response from the DTE, the DCE will reset
its send and receive state variables V(S) and V(R) to zero, will stop its
Timer\ T1, and will remain in the information transfer phase. Upon reception
of a DM response from the DTE as a denial to the data link resetting request,
the DCE will stop its Timer\ T1 and will enter the disconnected phase.
.ce
.parag
.ce
The DCE, having sent an SABM/SABME command, will ignore and discard
.ce
any frames received from the DTE except an SABM/SABME or DISC command,
or a UA or DM response. The receipt of an SABM/SABME or DISC command from
the DTE will result in a collision situation that is resolved per \(sc\
2.4.4.5 above. Frames
.ce
other than the UA or DM response sent in response to a received SABM/SABME
or DISC command will be sent only after the data link is reset and if no
.ce
outstanding SABM/SABME command exists.
.ce
.parag
.ce

.ce
After the DCE sends the SABM/SABME command, if a UA or DM response is not
received correctly, Timer\ T1 will run out in the DCE. The DCE will then
.ce
resend the SABM/SABME command and will restart Timer\ T1. After N2 attempts
to reset the data link, the DCE will initiate appropriate higher layer
recovery
.ce
action and will enter the disconnected phase. The value of N2 is defined in
.ce
\(sc\ 2.4.8.4 below.
.ce
.parag
.ce
2.4.7.3
.ce
The DCE may ask the DTE to reset the data link by transmitting an FRMR
response (see \(sc\ 2.4.6.1 above). After transmitting an FRMR response,
.ce
the DCE will enter the frame rejection condition.
.ce
.parag
.ce
The frame rejection condition is cleared when the DCE receives an SABM/SABME
command, a DISC command, a FRMR response, or a DM response; or if
.ce
the DCE transmits an SABM/SABME command, a DISC command, or a DM response.
.ce
Other commands received while in the frame rejection condition will cause
the DCE to retransmit the FRMR response with the same information field
as
.ce
originally transmitted.
.ce
.parag
.ce
The DCE may start Timer\ T1 on transmission of the FRMR response. If
.ce
Timer\ T1 runs out before the frame rejection condition is cleared, the
DCE may retransmit the FRMR response, and restart T1. After N2 attempts
(time outs) to get the DTE to reset the data link, the DCE may reset the
data link itself as described in \(sc\ 2.4.7.2 above. The value of N2 is
defined in \(sc\ 2.4.8.4 below.
.ce
.parag
.ce

.ce
In the frame rejection condition, I frames and supervisory frames will
not be transmitted by the DCE. Also, received I frames and supervisory
frames will be discarded by the DCE except for the observance of a P bit
set to\ 1.
.ce
When an additional FRMR response must be transmitted by the DCE as a result
of the receipt of a P\ bit set to 1 while Timer\ T1 is running, Timer\
T1 will
.ce
continue to run. Upon reception of an FRMR response (even during a frame
.ce
rejection condition), the DCE will initiate a resetting procedure by
.ce
transmitting an SABM/SABME command as described in \(sc\ 2.4.7.2 above, or will
.ce
transmit a DM response to ask the DTE to initiate the data link set\(hyup
.ce
procedure as described in \(sc\ 2.4.4.1 and enter the disconnected phase.
.ce
.parag
.ce
2.4.8
.ce
\fIList of \fR
.ce
\fILAPB system parameters\fR
.ce
.parag
.ce
The DCE and DTE system parameters are as follows:
.ce
.parag
.ce

.ce
2.4.8.1
.ce
\fITimer\fR
.ce
\fIT1\fR
.ce
.parag
.ce
The value of the DTE Timer T1 system parameter may be different
.ce
than the value of the DCE Timer T1 system parameter. These values shall
be made known to both the DTE and the DCE, and agreed to for a period of
time by both the DTE and the DCE.
.ce
.parag
.ce
The period of Timer T1, at the end of which retransmission of a frame may
be initiated (see \(sc\ 2.4.4 and \(sc\ 2.4.5 above for the DCE), shall
take into account whether T1 is started at the beginning or the end of
the transmission of a frame.
.ce
.parag
.ce
The proper operation of the procedure requires that the transmitter's (DCE
or DTE) Timer\ T1 be greater than the maximum time between transmission
of a frame (SABM/SABME, DISC, I\ or supervisory command, or DM or FRMR
response)
.ce
and the reception of the corresponding frame returned as an answer to that
.ce
frame (UA, DM or acknowledging frame). Therefore, the receiver (DCE or DTE)
.ce
should not delay the response or acknowledging frame returned to one of the
.ce
above frames by more than a value\ T2, where T2 is a system parameter (see
.ce
\(sc\ 2.4.8.2).
.ce
.parag
.ce

.ce
The DCE will not delay the response or acknowledging frame returned to
one of the above DTE frames by more than a period\ T2.
.ce
.parag
.ce
2.4.8.2
.ce
\fIParameter T2\fR
.ce
.parag
.ce
The value of the DTE parameter T2 may be different than the value of the
DCE parameter T2. These values shall be made known to both the DTE and
the DCE, and agreed to for a period of time by both the DTE and the DCE.
.ce
.parag
.ce

.ce
The period of parameter T2 shall indicate the amount of time
.ce
available at the DCE or DTE before the acknowledging frame must be initiated
in order to ensure its receipt by the DTE or DCE, respectively, prior to
Timer\ T1 running out at the DTE or DCE (parameter\ T2\ <\ Timer\ T1).
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ The period of parameter T2 shall take into account the
.ce
following timing factors: the transmission time of the acknowledging frame,
the propagation time over the access data link, the stated processing times
at the DCE and the DTE, and the time to complete the transmission of the
frame(s) in the DCE or DTE transmit queue that are neither displaceable
or modifiable in an orderly manner.
.ce
.parag
.ce

.ce
Given a value for Timer T1 for the DTE or DCE, the value of parameter T2
at the DCE or DTE, respectively, must be no larger than T1 minus\ 2 times
the propagation time over the access data link, minus the frame processing
time at the DCE, minus the frame processing time at the DTE, and minus
the transmission time of the acknowledging frame by the DCE or DTE, respectively.
.ce
.parag
.ce

.ce
2.4.8.3
.ce
\fITimer T3\fR
.ce
.parag
.ce
The DCE shall support a Timer T3 system parameter, the value of
.ce
which shall be made known to the DTE.
.ce
.parag
.ce
The period of Timer T3, at the end of which an indication of an
.ce
observed excessively long idle channel state condition is passed to the
Packet Layer, shall be sufficiently greater than the period of the DCE
Timer T1
.ce
(i.e.\ T3\ >\ T1) so that the expiration of T3 provides the desired level of
.ce
assurance that the data link channel is in a non\(hyactive, non\(hyoperational
state, and is in need of data link set\(hyup before normal data link operation
can
.ce
resume.
.ce
.parag
.ce
2.4.8.4
.ce
\fIMaximum number of attempts\fR
.ce
\fIto complete a\fR
.ce

.ce
\fItransmission N2\fR
.ce
.parag
.ce

.ce
The value of the DTE N2 system parameter may be different than the value
of the DCE N2 system parameter. These values shall be made known to both
the DTE and the DCE, and agreed to for a period of time by both the DTE
and the DCE.
.ce
.parag
.ce
The value of N2 shall indicate the maximum number of attempts made by the
DCE or DTE to complete the successful transmission of a frame to the DTE
or DCE, respectively.
.ce
.parag
.ce
2.4.8.5
.ce
\fIMaximum number of bits in an I frame N1\fR
.ce
.parag
.ce
The value of the DTE N1 system parameter may be different than the value
of the DCE N1 system parameter. These values shall be made known to both
the DTE and the DCE.
.ce
.parag
.ce
The values of N1 shall indicate the maximum number of bits in an
.ce
I\ frame (excluding flags and 0\ bits inserted for transparency) that the
DCE or DTE is willing to accept from the DTE or DCE, respectively.
.ce
.parag
.ce
In order to allow for universal operation, a DTE should support a
.ce
value of DTE N1 which is not less than 1080\ bits (135\ octets). DTEs should
be aware that the network may transmit longer packets (see \(sc\ 5.2),
that may result in a data link layer problem.
.ce
.parag
.ce

.ce
All networks shall offer to a DTE which requires it, a value of DCE N1
which is greater than or equal to 2072\ bits (259\ octets) plus the length
of the address, control and FCS fields at the DTE/DCE interface, and greater
than or equal to the maximum length of the data packets which may cross
the DTE/DCE
.ce
interface plus the length of the address, control and FCS fields at the
DTE/DCE interface.
.ce
.parag
.ce
Appendix II provides a description of how the values stated above are  derived.
.ce
.parag
.ce
2.4.8.6
.ce
\fIMaximum number of \fR
.ce
\fIoutstanding I frames k\fR
.ce
.parag
.ce
The value of the DTE k system parameter shall be the same as the
.ce
value of the DCE k system parameter. This value shall be agreed to for a
.ce
period of time by both the DTE and the DCE.
.ce
.parag
.ce
The value of k shall indicate the maximum number of sequentially
.ce
numbered I\ frames that the DTE or DCE may have outstanding
.ce
(i.e.\ unacknowledged) at any given time. The value of k shall never exceed
.ce
seven for modulo\ 8 operation, or one hundred and twenty\(hyseven for modulo\
128
.ce
operation. All networks (DCEs) shall support a value of seven. Other values
of k (less than and greater than seven) may also be supported by networks
.ce
(DCEs).
.ce
.parag
.ce
.line
.ce
\fBMONTAGE : \(sc 2.5 de cette Recommandation sur le reste de cette page\fR
.ce
.parag
.ce

.ce
.line
.ad r
\fBTable 3/X.25 [T3.25], p.\fR
.sp 1P
.RT
.ad b
.RT
.ce
.line
.ce
\fBTable 4/X.25 [T4/X.25], p.\fR
.ce
2.3.2.1.1
.ce
\fIInformation transfer format\fR
.ce
\fI \(em I\fR
.ce
.parag
.ce
The I format is used to perform an information transfer. The
.ce
functions of N(S), N(R) and P are independent; i.e.,\ each I\ frame has
an N(S), an N(R) which may or may not acknowledge additional I\ frames
received by the
.ce
DCE or DTE, and a P\ bit that may be set to\ 0 or\ 1.
.ce
.parag
.ce
2.3.2.1.2
.ce
\fISupervisory format\fR
.ce
\fI \(em S\fR
.ce
.parag
.ce
The S format is used to perform data link supervisory control
.ce
functions such as acknowledge I\ frames, request retransmission of I\ frames,
and to request a temporary suspension of transmission of I\ frames. The
functions of N(R) and P/F are independent; i.e.,\ each supervisory frame
has an N(R) which
.ce
may or may not acknowledge additional I\ frames received by the DCE or\
DTE, and a P/F\ bit that may be set to\ 0 or\ 1.
.ce
.parag
.ce
2.3.2.1.3
.ce
\fIUnnumbered format\fR
.ce
\fI \(em U\fR
.ce
.parag
.ce
The U format is used to provide additional data link control
.ce
functions. This format contains no sequence numbers, but does include a
P/F\ bit that may be set to\ 0 or\ 1. The unnumbered frames have the same
control field
.ce
length (one octet) in both basic (modulo\ 8) operation and extended (modulo\
128) operation.
.ce
.parag
.ce
2.3.2.2
.ce
\fIControl field parameters\fR
.ce
.parag
.ce
The various parameters associated with the control field formats
.ce
are described below.
.ce
.parag
.ce
2.3.2.2.1
.ce
\fIModulus\fR
.ce
.parag
.ce
Each I frame is sequentially numbered and may have the value\ 0
.ce
through modulus minus\ 1 (where \*Qmodulus\*U is the modulus of the sequence
.ce
numbers). The modulus equals either\ 8 or\ 128 and the sequence numbers cycle
.ce
through the entire range.
.ce
.parag
.ce
2.3.2.2.2
.ce
\fISend state variable\fR
.ce
\fI V(S)\fR
.ce
.parag
.ce
The send state variable V(S) denotes the sequence number of the
.ce
next in\(hysequence I\ frame to be transmitted. V(S) can take on the values\ 0
.ce
through modulus minus\ 1. The value of V(S) is incremented by\ 1 with each
.ce
successive I\ frame transmission, but cannot exceed the N(R) of the last
.ce
received\ I or supervisory frame by more than the maximum number of outstanding
I\ frames\ (\fIk\fR
.ce
). The value of\ k is defined in \(sc\ 2.4.8.6 below.
.ce
.parag
.ce

.ce
2.3.2.2.3
.ce
\fISend sequence number\fR
.ce
\fI N(S)\fR
.ce
.parag
.ce
Only I frames contain N(S), the send sequence number of transmitted I\
frames. At the time that an in\(hysequence I\ frame is designated for
.ce
transmission, the value of N(S) is set equal to the value of the send state
.ce
variable\ V(S).
.ce
.parag
.ce
2.3.2.2.4
.ce
\fIReceive state variable\fR
.ce
\fIV(R)\fR
.ce
.parag
.ce
The receive state variable V(R) denotes the sequence number of the next
in\(hysequence I\ frame expected to be received. V(R) can take on the values
0 through modulus minus\ 1. The value of V(R) is incremented by\ 1 by the
receipt of an error\(hyfree, in\(hysequence I\ frame whose send sequence
number N(S) equals
.ce
the receive state variable V(R).
.ce
.parag
.ce
2.3.2.2.5
.ce
\fIReceive sequence number\fR
.ce
\fI N(R)\fR
.ce
.parag
.ce
All I frames and supervisory frames contain N(R), the expected send sequence
number of the next received I\ frame. At the time that a frame of the above
types is designated for transmission, the value of N(R) is set equal to
the current value of the receive state variable V(R). N(R) indicates that
the DCE or DTE transmitting the N(R) has received correctly all I\ frames
numbered up to and including N(R)\ \(em\ 1.
.ce
.parag
.ce
2.3.2.2.6
.ce
\fIPoll/Final bit\fR
.ce
\fI P/F\fR
.ce
.parag
.ce
All frames contain P/F, the Poll/Final bit. In command frames, the P/F
bit is referred to as the P bit. In response frames, it is referred to
as the F\ bit.
.ce
.parag
.ce
2.3.3
.ce
\fIFunctions of the Poll/Final bit\fR
.ce
.parag
.ce
The Poll bit set to 1 is used by the DCE or DTE to solicit (poll) a response
from the DTE or DCE, respectively. The Final bit set to\ 1 is used by the
DCE or DTE to indicate the response frame transmitted by the DTE or DCE,
.ce
respectively, as a result of the soliciting (poll) command.
.ce
.parag
.ce
The use of the P/F bit is described in \(sc\ 2.4.3 below.
.ce
.parag
.ce
2.3.4
.ce
\fICommands and responses\fR
.ce
.parag
.ce
For basic (modulo 8) operation, the commands and responses
.ce
represented in Table\ 5/X.25 will be supported by the DCE and the\ DTE.
.ce
.parag
.ce

.ce
For extended (modulo 128) operation, the commands and responses
.ce
represented in Table\ 6/X.25 will be supported by the DCE and the DTE.
.ce
.parag
.ce

.ce
For purposes of the LAPB procedures, the supervisory function bit encoding
\*Q11\*U and those encodings of the modifier function bits in
.ce
Tables\ 3/X.25 and 4/X.25 not identified in Tables\ 5/X.25 or 6/X.25 are
.ce
identified as \*Qundefined or not implemented\*U command and response control
.ce
fields.
.ce
.parag
.ce
The commands and responses in Tables 5/X.25 and 6/X.25 are defined as  follows:
.ce
.parag
.ce
2.3.4.1
.ce
\fIInformation (I)\fR
.ce
\fI command\fR
.ce
.parag
.ce
The function of the information (I) command is to transfer across a data
link a sequentially numbered frame containing an information field.
.ce
.parag
.ce
.line
.ce
\fBTable 5/X.25 [T5.25], p.\fR
.ce
.line
.ce
\fBTable 6/X.25 [T6.25], p.\fR
.ce
2.3.4.2
.ce
\fIReceive ready (RR)\fR
.ce
\fI command and response\fR
.ce
.parag
.ce
The receive ready (RR) supervisory frame is used by the DCE
.ce
or DTE to:
.ce
.parag
.ce

.ce
1)
.ce
indicate it is ready to receive an I frame; and
.ce
.parag
.ce
2)
.ce
acknowledge previously received I frames numbered up to and
.ce
including N(R)\ \(em\ 1.
.ce
.parag
.ce
An RR frame may be used to indicate the clearance of a busy
.ce
condition that was reported by the earlier transmission of an RNR frame
by that same station (DCE or\ DTE). In addition to indicating the DCE or
DTE status, the RR command with the P\ bit set to\ 1 may be used by the
DCE or DTE to ask for the status of the DTE or DCE, respectively.
.ce
.parag
.ce
2.3.4.3
.ce
\fIReceive not ready (RNR) command and response\fR
.ce
.parag
.ce
The receive not ready (RNR) supervisory frame is used by the DCE or DTE
to indicate a busy condition; i.e.\ temporary inability to accept
.ce
additional incoming I\ frames. I\ frames numbered up to and including N(R)\
\(em\ 1
.ce
are acknowledged. I\ frame N(R) and any subsequent I\ frames received, if any,
.ce
are not acknowledged; the acceptance status of these I\ frames will be
indicated in subsequent exchanges.
.ce
.parag
.ce
In addition to indicating the DCE or DTE status, the RNR command with the
P bit set to\ 1 may be used by an DCE or DTE to ask for the status of the
.ce
DTE or DCE, respectively.
.ce
.parag
.ce
2.3.4.4
.ce
\fIReject (REJ) command and response\fR
.ce
.parag
.ce
The reject (REJ) supervisory frame is used by the DCE or DTE to request
transmission of I\ frames starting with the frame numbered N(R).
.ce
I\ frames numbered N(R)\ \(em\ 1 and below are acknowledged. Additional
I\ frames
.ce
pending initial transmission may be transmitted following the retransmitted
.ce
I\ frame(s).
.ce
.parag
.ce

.ce
Only one REJ exception condition for a given direction of information transfer
may be established at any time. The REJ exception condition is cleared
(reset) upon the receipt of an I\ frame with an N(S) equal to the N(R)
of the
.ce
REJ\ frame.
.ce
.parag
.ce
An REJ frame may be used to indicate the clearance of a busy condition
that was reported by the earlier transmission of an RNR frame by that same
.ce
station (DCE or\ DTE). In addition to indicating the DCE or DTE status, the
.ce
REJ\ command with the P bit set to\ 1 may be used by the DCE or\ DTE to ask for
.ce
the status of the DTE or\ DCE, respectively.
.ce
.parag
.ce
\fR
.ce
2.3.4.5
.ce
\fISet asynchronous balanced mode (SABM)\fR
.ce
\fIcommand/\fR
.ce
\fISet asynchronous balanced mode extended (SABME)\fR
.ce

.ce
\fIcommand (subscription time option)\fR
.ce
.parag
.ce

.ce
The SABM unnumbered command is used to place the addressed DCE or DTE in
an asynchronous balanced mode (ABM) information transfer phase where all
command/response control fields will be one\ octet in length.
.ce
.parag
.ce
The SABME unnumbered command is used to place the addressed DCE or DTE
in an asynchronous balanced mode\ (ABM) information transfer phase where
.ce
numbered command/response control fields will be two octets in length, and
.ce
unnumbered command/response control fields will be one octet in length.
.ce
.parag
.ce

.ce
No information field is permitted with the SABM or SABME command. The transmission
of a SABM/SABME command indicates the clearance of a busy
.ce
condition that was reported by the earlier trans
.ce
mission\ of an RNR\ frame by   that
.ce
same station (DCE or\ DTE). The DCE or DTE confirms acceptance of SABM/SABME
.ce
[modulo\ 8 (basic) operation/modulo\ 128 (extended) operation] command by the
.ce
transmission, at the first opportunity, of a UA\ response. Upon acceptance of
.ce
this command, the DCE or DTE send state variable\ V(S) and receive state
.ce
variable V(R) are set to\ 0.
.ce
.parag
.ce
Previously transmitted I\ frames that are unacknowledged when this
.ce
command is actioned remain unac
.ce
knowledged.\ It is the responsibility of a
.ce
higher layer (e.g.\ Packet Layer or MLP) to recover from the possible loss of
.ce
the contents (e.g.\ packets) of such I\ frames.
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ The mode of operation of a data link [basic (modulo 8) or
.ce
extended (modulo\ 128)] is determined at subscription time and is only
changed by going through a new subscription process.
.ce
.parag
.ce
2.3.4.6
.ce
\fIDisconnect (DISC)\fR
.ce
\fI command\fR
.ce
.parag
.ce
The DISC unnumbered command is used to terminate the mode
.ce
previously set. It is used to inform the DCE or DTE receiving the DISC
command that the DTE or DCE sending the DISC command is suspending operation.
No
.ce
information field is permitted with the DISC command. Prior to actioning the
.ce
DISC command, the DCE or DTE receiving the DISC command confirms the acceptance
of the DISC command by the transmission of a UA response. The DTE or DCE
.ce
sending the DISC command enters the disconnected phase when it receives the
.ce
acknowledging UA response.
.ce
.parag
.ce
Previously transmitted I frames that are unacknowledged when this
.ce
command is actioned remain unacknowledged. It is the responsibility of
a higher layer (e.g.\ Packet Layer or MLP) to recover from the possible
loss of the
.ce
contents (e.g.,\ packets) of such I frames.
.ce
.parag
.ce
2.3.4.7
.ce
\fIUnnumbered acknowledgement (UA)\fR
.ce
\fI response\fR
.ce
.parag
.ce
The UA unnumbered response is used by the DCE or DTE to acknowledge the
receipt and acceptance of the mode\(hysetting commands. Received mode\(hysetting
commands are not actioned until the UA response is transmitted. The
.ce
transmission of a UA response indicates the clearance of a busy condition
that was reported by the earlier transmission of an RNR frame by that same
station (DCE or DTE). No information field is permitted with the UA response.
.ce
.parag
.ce

.ce
2.3.4.8
.ce
\fIDisconnected mode (DM)\fR
.ce
\fI response\fR
.ce
.parag
.ce
The DM unnumbered response is used to report a status where the DCE or
DTE is logically disconnected from the data link, and is in the disconnected
phase. The DM response may be sent to indicate that the DCE or DTE has
entered the disconnected phase without benefit of having received a DISC
command, or, if sent in response to the reception of a mode setting command,
is sent to
.ce
inform the DTE or DCE that the DCE or DTE, respectively, is still in the
.ce
disconnected phase and cannot execute the set mode command. No information
.ce
field is permitted with the DM response.
.ce
.parag
.ce
A DCE or DTE in a disconnected phase will monitor received commands
.ce
and will react to an SABM/SABME command as outlined in \(sc\ 2.4.4 below,
and will respond with a DM response with the F bit set to\ 1 to any other
command
.ce
received with the P\ bit set to\ 1.
.ce
.parag
.ce
2.3.4.9
.ce
\fIFrame reject (FRMR)\fR
.ce
\fI response\fR
.ce
.parag
.ce
The FRMR unnumbered response is used by the DCE or DTE to report an error
condition not recoverable by retransmission of the identical frame;
.ce
i.e.\ at least one of the following conditions, which results from the
receipt of a valid frame:
.ce
.parag
.ce
1)
.ce
the receipt of a command or response control field that is
.ce
undefined or not implemented;
.ce
.parag
.ce
2)
.ce
the receipt of an I frame with an information field which
.ce
exceeds the maximum established length;
.ce
.parag
.ce
3)
.ce
the receipt of an invalid N(R); or
.ce
.parag
.ce
4)
.ce
the receipt of a frame with an information field which is
.ce
not permitted or the receipt of a supervisory or unnumbered
.ce
frame with incorrect length.
.ce
.parag
.ce
An undefined or not implemented control field is any of the
.ce
control field encodings that are not identified in Tables\ 5/X.25 or\ 6/X.25.
.ce
.parag
.ce

.ce
A valid N(R) must be within the range from the lowest send sequence
.ce
number N(S) of the still unacknowledged frame(s) to the current DCE send
state variable inclusive (or to the current internal variable \fIx\fR
.ce
if the DCE is in
.ce
the timer recovery condition as described in \(sc\ 2.4.5.9).
.ce
.parag
.ce

.ce
An information field which immediately follows the control field, and consists
of\ 3 or 5\ octets [modulo\ 8 (basic) operation or modulo\ 128 (extended)
operation, respectively], is returned with this response and provides the
.ce
reason for the FRMR response. These formats are given in Tables\ 7/X.25
.ce
and\ 8/X.25.
.ce
.parag
.ce
2.3.5
.ce
\fIException condition reporting and recovery\fR
.ce
.parag
.ce
The error recovery procedures which are available to effect
.ce
recovery following the detection/occurrence of an
.ce
exception condition
.ce
 at the Data Link Layer are described below. Exception conditions described
are those situations which may occur as the result of transmission errors,
DCE or DTE malfunction, or operational situations.
.ce
.parag
.ce
2.3.5.1
.ce
\fIBusy condition\fR
.ce
.parag
.ce
The busy condition results when the DCE or DTE is temporarily
.ce
unable to continue to receive I frames due to internal constraints,
.ce
e.g.\ receive buffering limitations. In this case an RNR frame is transmitted
.ce
from the busy DCE or DTE. I\ frames pending transmission may be transmitted
from the busy DCE or DTE prior to or following the RNR\ frame.
.ce
.parag
.ce
An indication that the busy condition has cleared is communicated by the
transmission of a UA (only in response to a SABM/SABME command), RR, REJ
or SABM/SABME (modulo\ 8/modulo\ 128) frame.
.ce
.parag
.ce
.line
.ce
\fBTable 7/X.25 [T7.25], p.\fR
.ce
.line
.ce
\fBTable 8/X.25 [T8.25], p.\fR
.ce
2.3.5.2
.ce
\fIN(S) sequence error condition\fR
.ce
.parag
.ce
The information field of all I frames received whose N(S) does not equal
the receive state variable V(R) will be discarded.
.ce
.parag
.ce
An N(S) sequence error exception condition occurs in the receiver when
an I\ frame received contains an N(S) which is not equal to the receive
state
.ce
variable V(R) at the receiver. The receiver does not acknowledge (increment
its receive state variable) the I\ frame causing the sequence error, or
any I\ frame which may follow, until an I\ frame with the correct N(S)
is received.
.ce
.parag
.ce

.ce
A DCE or DTE which receives one or more valid I frames having sequence
errors or subsequent supervisory frames (RR, RNR and REJ) shall accept
the
.ce
control information contained in the N(R) field and the P or F bit to perform
data link control functions; e.g.\ to receive acknowledgement of previously
.ce
transmitted I\ frames and to cause the DCE or DTE to respond (P bit set to 1).
.ce
.parag
.ce

.ce
The means specified in \(sc\(sc\ 2.3.5.2.1 and 2.3.5.2.2 shall be available
for initiating the retransmission of lost or errored I\ frames following
the
.ce
occurrence of an N(S) sequence error condition.
.ce
.parag
.ce
2.3.5.2.1
.ce
\fIREJ recovery\fR
.ce
.parag
.ce
The REJ frame is used by a receiving DCE or DTE to initiate a
.ce
recovery (retransmission) following the detection of an N(S) sequence error.
.ce
.parag
.ce

.ce
With respect to each direction of transmission on the data link, only one
\*Qsent REJ\*U exception condition from a DCE or DTE, to a DTE or DCE,
is
.ce
established at a time. A \*Qsent REJ\*U exception condition is cleared when the
.ce
requested I\ frame is received.
.ce
.parag
.ce

.ce
A DCE or DTE receiving a REJ frame initiates sequential
.ce
(re\(hy)transmission of I\ frames starting with the I\ frame indicated
by the N(R) contained in the REJ frame. The retransmitted frames may contain
an N(R) and a P bit that are updated from, and therefore different from,
the ones contained in the originally transmitted I\ frames.
.ce
.parag
.ce
2.3.5.2.2
.ce
\fITime\(hyout recovery\fR
.ce
.parag
.ce
If a DCE or DTE, due to a transmission error, does not receive (or receives
and discards) a single I\ frame or the last I\ frame(s) in a sequence of
I\ frames, it will not detect an N(S) sequence error condition and, therefore,
will not transmit a REJ frame. The DTE or DCE which transmitted the
.ce
unacknowledged I\ frame(s) shall, following the completion of a system
specified time\(hyout period (see \(sc\(sc\ 2.4.5.1 and 2.4.5.9 below),
take appropriate recovery action to determine at which I\ frame retransmission
must begin. The
.ce
retransmitted frame(s) may contain an N(R) and a P bit that is updated from,
.ce
and therefore different from, the ones contained in the originally transmitted
frame(s).
.ce
.parag
.ce
2.3.5.3
.ce
\fIInvalid frame\fR
.ce
\fI condition\fR
.ce
.parag
.ce
Any frame which is invalid will be discarded, and no action is
.ce
taken as the result of that frame. An invalid frame is defined as one
.ce
which:
.ce
.parag
.ce
a)
.ce
is not properly bounded by two flags;
.ce
.parag
.ce
b)
.ce
in basic (modulo 8) operation, contains fewer than 32 bits
.ce
between flags; in extended (modulo\ 128) operation, contains
.ce
fewer than 40\ bits between flags of frames that contain sequence
.ce
numbers or 32\ bits between flags of frames that do not contain
.ce
sequence numbers;
.ce
.parag
.ce
c)
.ce
contains a Frame Check Sequence (FCS) error; or
.ce
.parag
.ce
d)
.ce
contains an address other than A or B (for single link
.ce
operation) or other than C or D (for multilink operation).
.ce
.parag
.ce

.ce
For those networks that are octet aligned, a detection of
.ce
non\(hyoctet alignment may be made at the Data Link Layer by adding a frame
.ce
validity check that requires the number of bits between the opening flag and
.ce
the closing flag, excluding bits inserted for transparency, to be an integral
number of octets in length, or the frame is considered invalid.
.ce
.parag
.ce
2.3.5.4
.ce
\fIFrame rejection\fR
.ce
\fI condition\fR
.ce
.parag
.ce
A frame rejection condition is established upon the receipt of an error\(hyfree
frame with one of the conditions listed in \(sc\ 2.3.4.9 above.
.ce
.parag
.ce

.ce
At the DCE or DTE, this frame rejection exception condition is
.ce
reported by an FRMR response for appropriate DTE or DCE action, respectively.
Once a DCE has established such an exception condition, no additional I\
frames are accepted until the condition is reset by the DTE, except for
examination of the P bit. The FRMR response may be repeated at each opportunity,
as specified in \(sc\ 2.4.7.3, until recovery is effected by the DTE, or
until the DCE initiates its own recovery in case the DTE does not respond.
.ce
.parag
.ce
2.3.5.5
.ce
\fIExcessive idle channel state condition on incoming\fR
.ce

.ce
\fIchannel\fR
.ce
.parag
.ce
Upon detection of an idle channel state condition (see \(sc\ 2.2.12.2 above)
on the incoming channel, the DCE shall wait for a period\ T3 (see
.ce
\(sc\ 2.4.8.3 below) without taking any specific action, waiting for detection
of a return to the active channel state (i.e.,\ detection of at least one
flag
.ce
sequence). After the period\ T3, the DCE shall notify the higher layer
(e.g.\ the Packet Layer or the MLP) of the excessive idle channel state
condition, but
.ce
shall not take any action that would preclude the DTE from establishing the
.ce
data link by normal data link set\(hyup procedures.
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ Other actions to be taken by the DCE at the Data Link Layer upon
expiration of period\ T3 is a subject for further study.
.ce
.parag
.ce

.ce
2.4
.ce
\fIDescription of the \fR
.ce
\fILAPB procedure\fR
.ce
.parag
.ce
2.4.1
.ce
\fILAPB basic and extended modes of operation\fR
.ce
.parag
.ce
In accordance with the system choice made by the DTE at
.ce
subscription time, the DCE will either support modulo\ 8 (basic) operation or
.ce
will support modulo\ 128 (extended) operation. Changing from basic operation
to extended operation, or vice versa, in the DCE requires resubscription
by the
.ce
DTE for the desired service, and is not supported dynamically.
.ce
.parag
.ce
Table 5/X.25 indicates the command and response control field formats used
with the basic (modulo\ 8) service. The mode\(hysetting command employed
to
.ce
initialize (set up) or reset the basic mode is the SABM command. Table
6/X.25 indicates the command and response control field formats used with
the extended (modulo\ 128) service. The mode\(hysetting command employed
to initialize (set up) or reset the extended mode is the SABME command.
.ce
.parag
.ce
2.4.2
.ce
\fILAPB procedure for addressing\fR
.ce
.parag
.ce
The address field identifies a frame as either a command or a
.ce
response. A command frame contains the address of the DCE or DTE to which
the command is being sent. A response frame contains the address of the
DCE or DTE sending the frame.
.ce
.parag
.ce
In order to allow differentiation between single link operation and
.ce
the optional multilink operation for diagnostic and/or maintenance reasons,
.ce
different address pair encodings are assigned to data links operating with
.ce
multilink procedure compared to data links operating with the single link
.ce
procedure.
.ce
.parag
.ce
Frames containing commands transferred from the DCE to the DTE will
.ce
contain the address\ A for the single link operation and address\ C for the
.ce
multilink operation.
.ce
.parag
.ce
Frames containing responses transferred from the DCE to the DTE will contain
the address\ B for the single link operation and address\ D for the
.ce
multilink operation.
.ce
.parag
.ce

.ce
Frames containing commands transferred from the DTE to the DCE shall contain
the address\ B for the single link operation and address\ D for the
.ce
multilink operation.
.ce
.parag
.ce
Frames containing responses transferred from the DTE to the DCE shall contain
the address\ A for the single link operation and address\ C for the
.ce
multilink operation.
.ce
.parag
.ce
These addresses are coded as follows:
.ce
.parag
.ce
Address
.ce
1\ 2\ 3\ 4\ 5\ 6\ 7\ 8
.ce
.parag
.ce
Single link operation
.ce
\ \ A
.ce
1\ 1\ 0\ 0\ 0\ 0\ 0\ 0
.ce
.line
.ce
\ \ B
.ce
1\ 0\ 0\ 0\ 0\ 0\ 0\ 0
.ce
.parag
.ce
Multilink operation
.ce
\ \ C
.ce
1\ 1\ 1\ 1\ 0\ 0\ 0\ 0
.ce
.line
.ce
\ \ D
.ce
1\ 1\ 1\ 0\ 0\ 0\ 0\ 0
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ The DCE will discard all frames received with an address other
than\ A or\ B (single link operation), or\ C or\ D (multilink operation).
.ce
.parag
.ce
2.4.3
.ce
\fILAPB procedure for the use of the P/F bit\fR
.ce
.parag
.ce
The DCE or DTE receiving an SABM/SABME, DISC, supervisory command or I\
frame with the P\ bit set to\ 1 will set the F\ bit to\ 1 in the next response
frame it transmits.
.ce
.parag
.ce
The response frame returned by the DCE to an SABM/SABME or DISC
.ce
command with the P\ bit set to\ 1 will be a UA or DM response with the
F\ bit set to\ 1. The response frame returned by the DCE to an I\ frame
with the P\ bit set to\ 1, received during the information transfer phase,
will be an RR, REJ, RNR or FRMR response with the F\ bit set to\ 1. The
response frame returned by the
.ce
DCE to a supervisory command with the P\ bit set to\ 1, received during the
.ce
information transfer phase, will be an RR, REJ, RNR or FRMR response with
the F\ bit set to\ 1. The response frame returned by the DCE to an I\ frame
or
.ce
supervisory frame with the P\ bit set to\ 1, received during the disconnected
.ce
phase, will be a DM response with the F\ bit set to\ 1.
.ce
.parag
.ce
The P bit may be used by the DCE in conjunction with the timer
.ce
recovery condition (see \(sc\ 2.4.5.9 below).
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ Other use of the P bit by the DCE is a subject for further
.ce
study.
.ce
.parag
.ce
2.4.4
.ce
\fILAPB procedure for data link set\(hyup and disconnection\fR
.ce
.parag
.ce

.ce
2.4.4.1
.ce
\fIData link set\(hyup\fR
.ce
.parag
.ce
The DCE will indicate that it is able to set up the data link by
.ce
transmitting contiguous flags (active channel state).
.ce
.parag
.ce
Either the DTE or the DCE may initiate data link set\(hyup. Prior to
.ce
initiation of data link set\(hyup, either the DCE or the DTE may initiate data
.ce
link
.ce
disconnection (see \(sc\ 2.4.4.3) for the purpose of insuring that the
DCE and the DTE are in the same phase. The DCE may also transmit an unsolicited
DM response to request the DTE to initiate data link set\(hyup.
.ce
.parag
.ce
The DTE shall initiate data link set\(hyup by transmitting an SABM/SABME
command to the DCE. If, upon receipt of the SABM/SABME command correctly,
the DCE determines that it can enter the information transfer phase, it
will return a UA response to the DTE, will reset its send and receive state
variables V(S) and V(R) to zero, and will consider that the data link is
set up. If, upon
.ce
receipt
.ce
of the SABM/SABME command correctly, the DCE determines that it cannot enter
.ce
the information transfer phase, it will return a DM response to the DTE as a
.ce
denial to the data link set\(hyup initialization and will consider that the
.ce
data link is
.ce
\fInot\fR
.ce
 set up. In order to avoid misinterpretation of the DM response received,
it is suggested that the DTE always sends its SABM/SABME command with the
P\ bit set to\ 1. Otherwise, it is not possible to differentiate a DM response
intended as a denial to data link set\(hyup from a DM response that is
issued in a separate unsolicited sense as a request for a mode\(hysetting
command (as described
.ce
in\ \(sc\ 2.4.4.4.2).
.ce
.parag
.ce
The DCE will initiate data link set\(hyup by transmitting an SABM/SABME
command to the DTE and starting its Timer\ T1 in order to determine when
too
.ce
much time has elapsed waiting for a reply (see \(sc\ 2.4.8.1 below). Upon
reception of a UA response from the DTE, the DCE will reset its send and
receive state
.ce
variables\ V(S) and\ V(R) to zero, will stop its Timer\ T1, and will consider
that the data link is set up. Upon reception of a DM response from the
DTE as a
.ce
denial
.ce
to the data link set\(hyup initialization, the DCE will stop its Timer\
T1 and will consider that the data link is \fInot\fR
.ce
set up.
.ce
.parag
.ce

.ce
The DCE, having sent the SABM/SABME command, will ignore and discard any
frames except an SABM/SABME or DISC command, or a UA or DM response
.ce
received from the DTE. The receipt of an SABM/SABME or DISC command from the
.ce
DTE will result in a collision situation that is resolved per \(sc\ 2.4.4.5
below. Frames other than the UA and DM responses sent in response to a
received
.ce
SABM/SABME or DISC command will be sent only after the data link is set
up and if no outstanding SABM/SABME command exists.
.ce
.parag
.ce
After the DCE sends the SABM/SABME command, if a UA or DM response is not
received correctly, Timer\ T1 will run out in the DCE. The DCE will then
.ce
resend the SABM/SABME command and will restart Timer\ T1. After transmission
of the SABM/SABME command N2 times by the DCE, appropriate higher layer
recovery action will be initiated. The value of N2 is defined in \(sc\
2.4.8.4 below.
.ce
.parag
.ce
2.4.4.2
.ce
\fIInformation transfer phase\fR
.ce
.parag
.ce
After having transmitted the UA response to the SABM/SABME command or having
received the UA response to a transmitted SABM/SABME command, the DCE will
accept and transmit I and supervisory frames according to the procedures
described in \(sc\ 2.4.5 below.
.ce
.parag
.ce

.ce
When receiving the SABM/SABME command while in the information
.ce
transfer phase, the DCE will conform to the data link resetting procedure
.ce
described in \(sc\ 2.4.7 below.
.ce
.parag
.ce
2.4.4.3
.ce
\fIData link disconnection\fR
.ce
.parag
.ce
The DTE shall initiate a disconnect of the data link by
.ce
transmitting a DISC command to the DCE. On correctly receiving a DISC command
in the information transfer phase, the DCE will send a UA response and
enter
.ce
the disconnected phase. On correctly receiving a DISC command in the
.ce
disconnected phase, the DCE will send a DM response and remain in the
.ce
disconnected phase. In order to avoid misinterpretation of the DM response
.ce
received, it is suggested that the DTE always sends its DISC command with
the P\ bit set to\ 1. Otherwise, it is not possible to differentiate a
DM response
.ce
intended as an indication that the DCE is already in the disconnected phase
.ce
from a DM response that is issued in a separate unsolicited sense as a
request for a mode\(hysetting command (as described in \(sc\ 2.4.4.4.2).
.ce
.parag
.ce

.ce
The DCE will initiate a disconnect of the data link by transmitting a DISC
command to the DTE and starting its Timer\ T1 (see \(sc\ 2.4.8.1 below).
Upon reception of an UA response from the DTE, the DCE will stop its Timer\
T1 and
.ce
will enter the disconnected phase. Upon reception of a DM response from
the DTE as an indication that the DTE was already in the disconnected phase,
the DCE
.ce
will stop its Timer\ T1 and will enter the disconnected phase.
.ce
.parag
.ce
The DCE, having sent the DISC command, will ignore and discard any
.ce
frames except an SABM/SABME or DISC command, or a UA or DM response received
.ce
from the DTE. The receipt of an SABM/SABME or DISC command from the DTE will
.ce
result in a collision situation that is resolved per \(sc\ 2.4.4.5 below.
.ce
.parag
.ce

.ce
After the DCE sends the DISC command, if a UA or DM response is not
.ce
received correctly, Timer\ T1 will run out in the DCE. The DCE will then
resend the DISC command and will restart Timer\ T1. After transmission
of the DISC
.ce
command N2 times by the DCE, appropriate higher layer recovery action will
be initiated. The value of N2 is defined in \(sc\ 2.4.8.4 below.
.ce
.parag
.ce
2.4.4.4
.ce
\fIDisconnected phase\fR
.ce
.parag
.ce
2.4.4.4.1
.ce
After having received a DISC command from the DTE and
.ce
returned a UA response to the DTE, or having received the UA response to a
.ce
transmitted DISC command, the DCE will enter the disconnected phase.
.ce
.parag
.ce
In the disconnected phase, the DCE may initiate data link set\(hyup. In
the disconnected phase, the DCE will react to the receipt of an SABM/SABME
command as described in \(sc\ 2.4.4.1 above and will transmit a DM response
in
.ce
answer to a received DISC command. When receiving any other command (defined,
or undefined or not implemented) with the P\ bit set to\ 1, the DCE will
transmit a DM response with the F\ bit set to\ 1. Other frames received
in the
.ce
disconnected phase will be ignored by the DCE.
.ce
.parag
.ce
2.4.4.4.2
.ce
When the DCE enters the disconnected phase after
.ce
detecting error conditions as listed in \(sc\ 2.4.6 below, or after an internal
.ce
malfunction, it may indicate this by sending a DM response rather than
a DISC command. In these cases, the DCE will transmit a DM response and
start its
.ce
Timer\ T1 (see \(sc\ 2.4.8.1 below).
.ce
.parag
.ce
If Timer T1 runs out before the reception of an SABM/SABME or DISC command
from the DTE, the DCE will retransmit the DM response and restart
.ce
Timer\ T1. After transmission of the DM response N2 times, the DCE will
remain in the disconnected phase and appropriate recovery actions will
be initiated. The value of N2 is defined in \(sc\ 2.4.8.4 below.
.ce
.parag
.ce
Alternatively, after an internal malfunction, the DCE may either
.ce
initiate a data link resetting procedure (see \(sc\ 2.4.7 below) or disconnect
the data link (see \(sc\ 2.4.4.3 above) prior to initiating a data link
set\(hyup
.ce
procedure (see \(sc\ 2.4.4.1 above).
.ce
.parag
.ce
2.4.4.5
.ce
\fICollision of unnumbered commands\fR
.ce
.parag
.ce
Collision situations shall be resolved in the following way:
.ce
.parag
.ce
2.4.4.5.1
.ce
If the sent and received unnumbered commands are the
.ce
same, the DCE and the DTE shall each send the UA response at the earliest
.ce
possible opportunity. The DCE shall enter the indicated phase either,
.ce
.parag
.ce

.ce
1)
.ce
after receiving the UA response,
.ce
.parag
.ce
2)
.ce
after sending the UA response, or
.ce
.parag
.ce
3)
.ce
after timing out waiting for the UA response having sent a
.ce
UA response.
.ce
.parag
.ce
In the case of 2) above, the DCE will accept a subsequent UA
.ce
response to the mode\(hysetting command it issued without causing an exception
.ce
condition if received within the time\(hyout interval.
.ce
.parag
.ce

.ce
2.4.4.5.2
.ce
If the sent and received unnumbered commands are
.ce
different, the DCE and the DTE shall each enter the disconnected phase and
.ce
issue a DM response at the earliest possible opportunity.
.ce
.parag
.ce
2.4.4.6
.ce
\fICollision of DM response with SABM/SABME or DISC\fR
.ce

.ce
\fIcommand\fR
.ce
.parag
.ce
When a DM response is issued by the DCE or DTE as an unsolicited
.ce
response to request the DTE or DCE, respectively, to issue a mode\(hysetting
.ce
command as described in \(sc\ 2.4.4.4, a collision between an SABM/SABME
or DISC
.ce
command and the unsolicited DM response may occur. In order to avoid
.ce
misinterpretation of the DM response received, the DTE always sends its
.ce
SABM/SABME or DISC command with the P\ bit set to\ 1.
.ce
.parag
.ce
2.4.4.7
.ce
\fICollision of DM responses\fR
.ce
.parag
.ce
A contention situation may occur when both the DCE and the DTE
.ce
issue a DM response to request a mode\(hysetting command. In this case, the DTE
.ce
will issue an SABM/SABME command to resolve the contention situation.
.ce
.parag
.ce

.ce
2.4.5
.ce
\fILAPB procedures for information transfer\fR
.ce
.parag
.ce
The procedures which apply to the transmission of I\ frames in each direction
during the information transfer phase are described below.
.ce
.parag
.ce
In the following, \*Qnumber one higher\*U is in reference to a
.ce
continuously repeated sequence series, i.e., 7 is 1\ higher than 6 and 0 is
.ce
1\ higher than 7 for modulo\ 8 series, and 127 is 1\ higher than 126 and 0 is
.ce
1\ higher than 127 for modulo\ 128 series.
.ce
.parag
.ce
2.4.5.1
.ce
\fISending I frames\fR
.ce
.parag
.ce
When the DCE has an I frame to transmit (i.e. an I frame not
.ce
already transmitted, or having to be retransmitted as described in \(sc\
2.4.5.6
.ce
below), it will transmit it with an N(S) equal to its current send state
.ce
variable V(S), and an N(R) equal to its current receive state variable
V(R). At the end of the transmission of the I\ frame, the DCE will increment
its send
.ce
state variable V(S) by\ 1.
.ce
.parag
.ce
If Timer T1 is not running at the time of transmission of an I frame, it
will be started.
.ce
.parag
.ce
If the send state variable V(S) is equal to the last value of N(R)
.ce
received plus \fIk\fR
.ce
(where \fIk\fR
.ce
is the maximum number of outstanding I\ frames \(em
.ce
see \(sc\ 2.4.8.6 below), the DCE will not transmit any new I\ frames, but may
.ce
retransmit an I\ frame as described in \(sc\(sc\ 2.4.5.6 or 2.4.5.9 below.
.ce
.parag
.ce
When the DCE is in the busy condition, it may still transmit I frames,
provided that the DTE is not busy. When the DCE is in the frame rejection
.ce
condition, it will stop transmitting I\ frames.
.ce
.parag
.ce
2.4.5.2
.ce
\fIReceiving an I frame\fR
.ce
.parag
.ce
2.4.5.2.1
.ce
When the DCE is not in a busy condition and receives a valid I\ frame whose
send sequence number N(S) is equal to the DCE receive state
.ce
variable V(R), the DCE will accept the information field of this frame,
.ce
increment by one its receive state variable V(R), and act as follows:
.ce
.parag
.ce

.ce
a)
.ce
If the DCE is still not in a busy condition:
.ce
.parag
.ce
i)
.ce
If an I frame is available for transmission by the
.ce
DCE, it may act as in \(sc\ 2.4.5.1 above and acknowledge
.ce
the received I frame by setting N(R) in the control field
.ce
of the next transmitted I\ frame to the value of the DCE
.ce
receive state variable V(R). Alternatively, the DCE may
.ce
acknowledge the received I\ frame by transmitting an RR
.ce
frame with the N(R) equal to the value of the DCE receive
.ce
state variable V(R).
.ce
.parag
.ce
ii)
.ce
If no I frame is available for transmission by the
.ce
DCE, it will transmit an RR frame with N(R) equal to the
.ce
value of the DCE receive state variable V(R).
.ce
.parag
.ce

.ce
b)
.ce
If the DCE is now in a busy condition, it will transmit an
.ce
RNR frame with N(R) equal to the value of the DCE receive
.ce
state variable V(R) (see \(sc\ 2.4.5.8).
.ce
.parag
.ce

.ce
2.4.5.2.2
.ce
When the DCE is in a busy condition, it may ignore the
.ce
information field contained in any received I\ frame.
.ce
.parag
.ce
2.4.5.3
.ce
\fIReception of invalid frames\fR
.ce
.parag
.ce
When the DCE receives an invalid frame (see \(sc\ 2.3.5.3), this frame
will be discarded.
.ce
.parag
.ce
2.4.5.4
.ce
\fIReception of out\(hyof\(hysequence I frames\fR
.ce
.parag
.ce
When the DCE receives a valid I frame whose send sequence number
.ce
N(S) is incorrect, i.e.,\ not equal to the current DCE receive state variable
.ce
V(R), it will discard the information field of the I\ frame and transmit
an REJ frame with the N(R) set to one higher than the N(S) of the last
correctly
.ce
received I\ frame. The REJ frame will be a command frame with the P\ bit
set to\ 1 if an acknowledged transfer of the retransmission request is
required;
.ce
otherwise the REJ frame may be either a command or a response frame. The DCE
.ce
will then discard the information field of all I\ frames received until the
.ce
expected I\ frame is correctly received. When receiving the expected I\ frame,
.ce
the DCE will then acknowledge the I\ frame as described in \(sc\ 2.4.5.2
above. The DCE will use the N(R) and P\ bit information in the discarded
I\ frames as
.ce
described in \(sc\ 2.3.5.2 above.
.ce
.parag
.ce
2.4.5.5
.ce
\fIReceiving acknowledgement\fR
.ce
.parag
.ce
When correctly receiving an I frame or a supervisory frame (RR, RNR or
REJ), even in the busy condition, the DCE will consider the N(R) contained
in this frame as an acknowledgement for all I\ frames it has transmitted
with an N(S) up to and including the received N(R)\(em1. The DCE will stop
Timer\ T1 when it correctly receives an I\ frame or a supervisory frame
with the N(R) higher
.ce
than the last received N(R) (actually acknowledging some I\ frames), or
an REJ frame with an N(R) equal to the last received N(R).
.ce
.parag
.ce

.ce
If Timer T1 has been stopped by the receipt on an I, RR or RNR frame, and
if there are outstanding I\ frames still unacknowledged, the DCE will
.ce
restart Timer\ T1. If Timer\ T1 then runs out, the DCE will follow the
recovery procedure (\(sc\ 2.4.5.9 below) with respect to the unacknowledged
I\ frames. If
.ce
Timer\ T1 has been stopped by the receipt of an REJ frame, the DCE will
follow the retransmission procedures in \(sc\ 2.4.5.6 below.
.ce
.parag
.ce
2.4.5.6
.ce
\fIReceiving an REJ frame\fR
.ce
.parag
.ce
When receiving an REJ frame, the DCE will set its send state
.ce
variable V(S) to the N(R) received in the REJ control field. It will transmit
the corresponding I\ frame as soon as it is available or retransmit it
in
.ce
accordance with the procedures described in \(sc\ 2.4.5.1 above. (Re)transmission
will conform to the following procedure:
.ce
.parag
.ce
i)
.ce
if the DCE is transmitting a supervisory command or response
.ce
when it receives the REJ frame, it will complete that
.ce
transmission before commencing transmission of the requested
.ce
I\ frame;
.ce
.parag
.ce

.ce
ii)
.ce
if the DCE is transmitting an unnumbered command or
.ce
response when it receives the REJ frame, it will ignore the
.ce
request for retransmission;
.ce
.parag
.ce
iii)
.ce
if the DCE is transmitting an I frame when the REJ frame
.ce
is received, it may abort the I\ frame and commence transmission
.ce
of the requested I\ frame immediately after abortion;
.ce
.parag
.ce
iv)
.ce
if the DCE is not transmitting any frame when the REJ frame
.ce
is received, it will commence transmission of the requested
.ce
I\ frame immediately.
.ce
.parag
.ce
In all cases, if other unacknowledged I frames had already been
.ce
transmitted following the one indicated in the REJ frame, then those I
frames will be retransmitted by the DCE following the retransmission of
the requested I\ frame. Other I\ frames not yet transmitted may be transmitted
following the
.ce
retransmitted I\ frames.
.ce
.parag
.ce
If the REJ frame was received from the DTE as a command with the P bit
set to\ 1, the DCE will transmit an RR, RNR or REJ response with the F
bit set to\ 1 before transmitting or retransmitting the corresponding I\
frame.
.ce
.parag
.ce
2.4.5.7
.ce
\fIReceiving an RNR frame\fR
.ce
.parag
.ce
After receiving an RNR frame whose N(R) acknowledges all frames
.ce
previously transmitted, the DCE will stop Timer\ T1 and may then transmit
.ce
an I\ frame, with the P\ bit set to\ 0, whose send sequence number is
.ce
equal to the N(R) indicated in the RNR frame, restarting Timer\ T1 as it
does. After receiving an RNR frame whose N(R) indicates a previously transmitted
.ce
frame, the DCE will not transmit or retransmit any I\ frame, Timer\ T1 being
.ce
already running. In either case, if the Timer\ T1 runs out before receipt
of a busy clearance indication, the DCE will follow the procedure described
in
.ce
\(sc\ 2.4.5.9\ below. In any case, the DCE will not transmit any other
I\ frames
.ce
before receiving an RR or REJ frame, or before the completion of a link
.ce
resetting procedure.
.ce
.parag
.ce
Alternatively, after receiving an RNR frame, the DCE may wait for a
.ce
period of time (e.g.,\ the length of the Timer\ T1) and then transmit a
.ce
supervisory command frame (RR, RNR or REJ) with the P\ bit set to\ 1, and
start Timer\ T1, in order to determine if there is any change in the receive
status of the DTE. The DTE shall respond to the P\ bit set to\ 1 with a
supervisory
.ce
response frame (RR, RNR or REJ) with the F\ bit set to\ 1 indicating either
.ce
continuance of the busy condition (RNR) or clearance of the busy condition
(RR or REJ). Upon receipt of the DTE response, Timer\ T1 is stopped.
.ce
.parag
.ce

.ce
1)
.ce
If the response is the RR or REJ response, the busy
.ce
condition is cleared and the DCE may transmit I\ frames beginning
.ce
with the I\ frame identified by the N(R) in the received response
.ce
frame.
.ce
.parag
.ce
2)
.ce
If the response is the RNR response, the busy condition
.ce
still exists, and the DCE will after a period of time (e.g.\ the
.ce
length of Timer\ T1) repeat the enquiry of the DTE receive
.ce
status.
.ce
.parag
.ce
If Timer T1 runs out before a status response is received, the
.ce
enquiry process above is repeated. If N2 attempts to get a status response
fail (i.e.\ Timer\ T1 runs out N2\ times), the DCE will initiate a data
link resetting procedure as described in \(sc\ 2.4.7.2\ below or will transmit
a DM response to ask the DTE to initiate a data link set\(hyup procedure
as described in \(sc\ 2.4.4.1 and enter the disconnected phase. The value
of N2 is defined in \(sc\ 2.4.8.4\ below.
.ce
.parag
.ce

.ce
If, at any time during the enquiry process, an unsolicited RR or REJ frame
is received from the DTE, it will be considered to be an indication of
.ce
clearance of the busy condition. Should the unsolicited RR or REJ frame be a
.ce
command frame with the P bit set to\ 1, the appropriate response frame
with the F\ bit set to 1 must be transmitted before the DCE may resume
transmission of
.ce
I\ frames. If Timer\ T1 is running, the DCE will wait for the non\(hybusy
response with the F bit set to\ 1 or will wait for Timer\ T1 to run out
and then either
.ce
may reinitiate the enquiry process in order to realize a successful P/F bit
.ce
exchange or may resume transmission of I frames beginning with the I\ frame
.ce
identified by the N(R) in the received RR or REJ frame.
.ce
.parag
.ce
2.4.5.8
.ce
\fIDCE busy condition\fR
.ce
.parag
.ce
When the DCE enters a busy condition, it will transmit an RNR frame at
the earliest opportunity. The RNR frame will be a command frame with the
P bit set to\ 1 if an acknowledged transfer of the busy condition indication
is
.ce
required; otherwise the RNR frame may be either a command or a response
frame. While in the busy condition, the DCE will accept and process supervisory
.ce
frames, will accept and process the contents of the N(R) fields of I\ frames,
.ce
and will return an RNR response with the F bit set to\ 1 if it receives a
.ce
supervisory command or I command frame with the P bit set to\ 1. To clear the
.ce
busy condition, the DCE will transmit either an REJ frame or an RR frame, with
.ce
N(R) set to the current receive state variable V(R), depending on whether or
.ce
not it discarded information fields of correctly received I\ frames. The REJ
.ce
frame or the RR frame will be a command frame with the P bit set to\ 1 if an
.ce
acknowledged transfer of the busy\(hyto\(hynon\(hybusy transition is required,
otherwise the REJ frame or the RR frame may be either a command or a response
frame.
.ce
.parag
.ce

.ce
2.4.5.9
.ce
\fIWaiting acknowledgement\fR
.ce
.parag
.ce
The DCE maintains an internal transmission attempt variable which is set
to\ 0 when the DCE sends a UA response, when the DCE receives a UA
.ce
response or an RNR command or response, or when the DCE correctly receives
an I\ frame or supervisory frame with the N(R) higher than the last received
N(R) (actually acknowledging some outstanding I\ frames).
.ce
.parag
.ce
If Timer T1 runs out waiting for the acknowledgement from the DTE for an
I\ frame transmitted, the DCE will enter the timer recovery condition,
add
.ce
one to its transmission attempt variable and set an internal variable \fIx\fR
.ce
to
.ce
the current value of its send state variable V(S). The DCE will then restart
.ce
Timer T1, set its send state variable V(S) to the last value of N(R) received
from the DTE and retransmit the corresponding I\ frame with the P bit set
to\ 1, or transmit an appropriate supervisory command frame (RR, RNR or
REJ) with the P\ bit set to\ 1.
.ce
.parag
.ce

.ce
The timer recovery condition is cleared when the DCE receives a valid supervisory
frame with the F\ bit\ set to\ 1.
.ce
.parag
.ce
If, while in the timer recovery condition, the DCE correctly receives a
supervisory frame with the F bit set to\ 1 and with the N(R) within the
range from its current send state variable V(S) to \fIx\fR
.ce
included, it will clear the
.ce
timer recovery condition (including stopping Timer\ T1) and set its send
state variable V(S) to the value of the received N(R), and may then resume
with
.ce
I\ frame transmission or retransmission, as appropriate.
.ce
.parag
.ce
If, while in the timer recovery condition, the DCE correctly receives an
I or supervisory frame with the P/F bit set to\ 0 and with a valid N(R)
(see \(sc\ 2.3.4.9), it will not clear the timer recovery condition. The
value of the
.ce
received N(R) may be used to update the send state variable V(S). However,
the DCE may decide to keep the last transmitted I\ frame in store (even
if it is
.ce
acknowledged) in order to be able to retransmit it with the P bit set to\
1 when Timer\ T1 runs out at a later time.
.ce
.parag
.ce

.ce
If the received supervisory frame with the P/F bit set to\ 0 is an REJ
frame with a valid N(R), the DCE may either immediately initiate
.ce
(re)transmission from the value of the send state variable V(S), or it may
.ce
ignore the request for retransmission and wait until the supervisory frame
with the F bit set to\ 1 is received before initiating (re)transmission
of frames
.ce
from the value identified in the N(R) field of the supervisory frame with
the F\ bit set to\ 1. In the case of immediate retransmission, in order
to prevent
.ce
duplicate retransmissions following the clearance of the timer recovery
.ce
condition, the DCE shall inhibit retransmission of a specific I\ frame [same
.ce
N(R) in the same numbering cycle] if the DCE has retransmitted that I\
frame as the result of a received REJ frame with the P/F bit set to\ 0.
.ce
.parag
.ce
If, while in the timer recovery condition, the DCE receives a REJ
.ce
command with the P bit set to\ 1, the DCE will respond immediately with an
.ce
appropriate supervisory response with the F bit set to\ 1. The DCE may
then use the value of the N(R) in the REJ command to update the send state
variable
.ce
V(S), and may either immediately begin (re)transmission from the value N(R)
.ce
indicated in the REJ frame or ignore the request for retransmission and wait
.ce
until the supervisory frame with the F bit set to\ 1 is received before
.ce
initiating (re)transmission of I\ frames from the value identified in the
N(R) field of the supervisory frame with the F\ bit set to\ 1. In the case
of
.ce
immediate retransmission, in order to prevent duplicate retransmissions
.ce
following the clearance of the timer recovery condition, the DCE shall
inhibit retransmission of a specific I\ frame [same N(R) in the same numbering
cycle]
.ce
if the DCE has retransmitted that I\ frame as the result of the received REJ
.ce
command with the P\ bit set to\ 1.
.ce
.parag
.ce
If Timer T1 runs out in the timer recovery condition, and no I or
.ce
supervisory frame with the P/F bit set to 0 and with a valid N(R) has been
.ce
received, or no REJ command with the P bit set to\ 1 and with a valid N(R)
has been received, the DCE will add one to its transmission attempt variable,
.ce
restart Timer\ T1, and either retransmit the I frame sent with the P bit set
.ce
to\ 1 or transmit an appropriate supervisory command with the P bit set to\ 1.
.ce
.parag
.ce

.ce
If the transmission attempt variable is equal to N2, the DCE will
.ce
initiate a data link resetting procedure as described in \(sc\ 2.4.7.2
below, or
.ce
will transmit a DM response to ask the DTE to initiate a data link set\(hyup
.ce
procedure as described in \(sc\ 2.4.4.1 above and enter the disconnected
phase. N2 is a system parameter (see \(sc\ 2.4.8.4 below).
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ Although the DCE may implement the internal variable \fIx\fR
.ce
,
.ce
other mechanisms do exist that achieve the identical function.
.ce
.parag
.ce
2.4.6
.ce
\fILAPB conditions for \fR
.ce
\fIdata link resetting or data link\fR
.ce

.ce
\fIre\(hyinitialization\fR
.ce
\fI(data link set\(hyup)\fR
.ce
.parag
.ce
2.4.6.1
.ce
When the DCE receives, during the information transfer phase, a
.ce
frame which is not invalid (see \(sc\ 2.3.5.3) with one of the conditions
listed in \(sc\ 2.3.4.9 above, the DCE will request the DTE to initiate
a data link resetting procedure by transmitting an FRMR response to the
DTE as described in
.ce
\(sc\ 2.4.7.3.
.ce
.parag
.ce
2.4.6.2
.ce
When the DCE receives, during the information transfer phase, an FRMR response
from the DTE, the DCE will either initiate the data link
.ce
resetting procedures itself as described in \(sc\ 2.4.7.2 or return a DM
response to ask the DTE to initiate the data link set\(hyup (initialization)
procedure as described in \(sc\ 2.4.4.1. After transmitting a DM response,
the DCE will enter
.ce
the disconnected phase as described in \(sc\ 2.4.4.4.2.
.ce
.parag
.ce
2.4.6.3
.ce
When the DCE receives, during the information transfer phase, a UA response,
or an unsolicited response with the F bit set to\ 1, the DCE may
.ce
either initiate the data link resetting procedures itself as described in
.ce
\(sc\ 2.4.7.2, or return a DM response to ask the DTE to initiate the data link
.ce
set\(hyup (initialization) procedure as described in \(sc\ 2.4.4.1. After
transmitting a DM response, the DCE will enter the disconnected phase as
described in
.ce
\(sc\ 2.4.4.4.2.
.ce
.parag
.ce
2.4.6.4
.ce
When the DCE receives, during the information transfer phase, a DM response
from the DTE, the DCE will either initiate the data link set\(hyup
.ce
(initialization) procedures itself as described in \(sc\ 2.4.4.1, or return
a DM
.ce
response to ask the DTE to initiate the data link set\(hyup (initialization)
.ce
procedures as described in \(sc\ 2.4.4.1. After transmitting a DM response,
the DCE will enter the disconnected phase as described in \(sc\ 2.4.4.4.2.
.ce
.parag
.ce

.ce
2.4.7
.ce
\fILAPB procedure for data link resetting\fR
.ce
.parag
.ce
2.4.7.1
.ce
The data link resetting procedure is used to initialize both
.ce
directions of information transfer according to the procedure described
below. The data link resetting procedure only applies during the information
transfer phase.
.ce
.parag
.ce
2.4.7.2
.ce
Either the DTE or the DCE may initiate the data link resetting
.ce
procedure. The data link resetting procedure indicates a clearance of a DCE
.ce
and/or DTE busy condition, if present.
.ce
.parag
.ce
The DTE shall initiate a data link resetting by transmitting an
.ce
SABM/SABME command to the DCE. If, upon correct receipt of the SABM/SABME
.ce
command, the DCE determines that it can continue in the information transfer
.ce
phase, it will return a UA response to the DTE, will reset its send and
receive state variables V(S) and V(R) to zero, and will remain in the information
.ce
transfer phase. If, upon correct receipt of the SABM/SABME command, the DCE
.ce
determines that it cannot remain in the information transfer phase, it will
.ce
return a DM response as a denial to the resetting request and will enter the
.ce
disconnected phase.
.ce
.parag
.ce
The DCE will initiate a data link resetting by transmitting an
.ce
SABM/SABME command to the DTE and starting its Timer\ T1 (see \(sc\ 2.4.8.1
below). Upon reception of a UA response from the DTE, the DCE will reset
its send and receive state variables V(S) and V(R) to zero, will stop its
Timer\ T1, and will remain in the information transfer phase. Upon reception
of a DM response from the DTE as a denial to the data link resetting request,
the DCE will stop its Timer\ T1 and will enter the disconnected phase.
.ce
.parag
.ce
The DCE, having sent an SABM/SABME command, will ignore and discard
.ce
any frames received from the DTE except an SABM/SABME or DISC command,
or a UA or DM response. The receipt of an SABM/SABME or DISC command from
the DTE will result in a collision situation that is resolved per \(sc\
2.4.4.5 above. Frames
.ce
other than the UA or DM response sent in response to a received SABM/SABME
or DISC command will be sent only after the data link is reset and if no
.ce
outstanding SABM/SABME command exists.
.ce
.parag
.ce

.ce
After the DCE sends the SABM/SABME command, if a UA or DM response is not
received correctly, Timer\ T1 will run out in the DCE. The DCE will then
.ce
resend the SABM/SABME command and will restart Timer\ T1. After N2 attempts
to reset the data link, the DCE will initiate appropriate higher layer
recovery
.ce
action and will enter the disconnected phase. The value of N2 is defined in
.ce
\(sc\ 2.4.8.4 below.
.ce
.parag
.ce
2.4.7.3
.ce
The DCE may ask the DTE to reset the data link by transmitting an FRMR
response (see \(sc\ 2.4.6.1 above). After transmitting an FRMR response,
.ce
the DCE will enter the frame rejection condition.
.ce
.parag
.ce
The frame rejection condition is cleared when the DCE receives an SABM/SABME
command, a DISC command, a FRMR response, or a DM response; or if
.ce
the DCE transmits an SABM/SABME command, a DISC command, or a DM response.
.ce
Other commands received while in the frame rejection condition will cause
the DCE to retransmit the FRMR response with the same information field
as
.ce
originally transmitted.
.ce
.parag
.ce
The DCE may start Timer\ T1 on transmission of the FRMR response. If
.ce
Timer\ T1 runs out before the frame rejection condition is cleared, the
DCE may retransmit the FRMR response, and restart T1. After N2 attempts
(time outs) to get the DTE to reset the data link, the DCE may reset the
data link itself as described in \(sc\ 2.4.7.2 above. The value of N2 is
defined in \(sc\ 2.4.8.4 below.
.ce
.parag
.ce

.ce
In the frame rejection condition, I frames and supervisory frames will
not be transmitted by the DCE. Also, received I frames and supervisory
frames will be discarded by the DCE except for the observance of a P bit
set to\ 1.
.ce
When an additional FRMR response must be transmitted by the DCE as a result
of the receipt of a P\ bit set to 1 while Timer\ T1 is running, Timer\
T1 will
.ce
continue to run. Upon reception of an FRMR response (even during a frame
.ce
rejection condition), the DCE will initiate a resetting procedure by
.ce
transmitting an SABM/SABME command as described in \(sc\ 2.4.7.2 above, or will
.ce
transmit a DM response to ask the DTE to initiate the data link set\(hyup
.ce
procedure as described in \(sc\ 2.4.4.1 and enter the disconnected phase.
.ce
.parag
.ce
2.4.8
.ce
\fIList of \fR
.ce
\fILAPB system parameters\fR
.ce
.parag
.ce
The DCE and DTE system parameters are as follows:
.ce
.parag
.ce

.ce
2.4.8.1
.ce
\fITimer\fR
.ce
\fIT1\fR
.ce
.parag
.ce
The value of the DTE Timer T1 system parameter may be different
.ce
than the value of the DCE Timer T1 system parameter. These values shall
be made known to both the DTE and the DCE, and agreed to for a period of
time by both the DTE and the DCE.
.ce
.parag
.ce
The period of Timer T1, at the end of which retransmission of a frame may
be initiated (see \(sc\ 2.4.4 and \(sc\ 2.4.5 above for the DCE), shall
take into account whether T1 is started at the beginning or the end of
the transmission of a frame.
.ce
.parag
.ce
The proper operation of the procedure requires that the transmitter's (DCE
or DTE) Timer\ T1 be greater than the maximum time between transmission
of a frame (SABM/SABME, DISC, I\ or supervisory command, or DM or FRMR
response)
.ce
and the reception of the corresponding frame returned as an answer to that
.ce
frame (UA, DM or acknowledging frame). Therefore, the receiver (DCE or DTE)
.ce
should not delay the response or acknowledging frame returned to one of the
.ce
above frames by more than a value\ T2, where T2 is a system parameter (see
.ce
\(sc\ 2.4.8.2).
.ce
.parag
.ce

.ce
The DCE will not delay the response or acknowledging frame returned to
one of the above DTE frames by more than a period\ T2.
.ce
.parag
.ce
2.4.8.2
.ce
\fIParameter T2\fR
.ce
.parag
.ce
The value of the DTE parameter T2 may be different than the value of the
DCE parameter T2. These values shall be made known to both the DTE and
the DCE, and agreed to for a period of time by both the DTE and the DCE.
.ce
.parag
.ce

.ce
The period of parameter T2 shall indicate the amount of time
.ce
available at the DCE or DTE before the acknowledging frame must be initiated
in order to ensure its receipt by the DTE or DCE, respectively, prior to
Timer\ T1 running out at the DTE or DCE (parameter\ T2\ <\ Timer\ T1).
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ The period of parameter T2 shall take into account the
.ce
following timing factors: the transmission time of the acknowledging frame,
the propagation time over the access data link, the stated processing times
at the DCE and the DTE, and the time to complete the transmission of the
frame(s) in the DCE or DTE transmit queue that are neither displaceable
or modifiable in an orderly manner.
.ce
.parag
.ce

.ce
Given a value for Timer T1 for the DTE or DCE, the value of parameter T2
at the DCE or DTE, respectively, must be no larger than T1 minus\ 2 times
the propagation time over the access data link, minus the frame processing
time at the DCE, minus the frame processing time at the DTE, and minus
the transmission time of the acknowledging frame by the DCE or DTE, respectively.
.ce
.parag
.ce

.ce
2.4.8.3
.ce
\fITimer T3\fR
.ce
.parag
.ce
The DCE shall support a Timer T3 system parameter, the value of
.ce
which shall be made known to the DTE.
.ce
.parag
.ce
The period of Timer T3, at the end of which an indication of an
.ce
observed excessively long idle channel state condition is passed to the
Packet Layer, shall be sufficiently greater than the period of the DCE
Timer T1
.ce
(i.e.\ T3\ >\ T1) so that the expiration of T3 provides the desired level of
.ce
assurance that the data link channel is in a non\(hyactive, non\(hyoperational
state, and is in need of data link set\(hyup before normal data link operation
can
.ce
resume.
.ce
.parag
.ce
2.4.8.4
.ce
\fIMaximum number of attempts\fR
.ce
\fIto complete a\fR
.ce

.ce
\fItransmission N2\fR
.ce
.parag
.ce

.ce
The value of the DTE N2 system parameter may be different than the value
of the DCE N2 system parameter. These values shall be made known to both
the DTE and the DCE, and agreed to for a period of time by both the DTE
and the DCE.
.ce
.parag
.ce
The value of N2 shall indicate the maximum number of attempts made by the
DCE or DTE to complete the successful transmission of a frame to the DTE
or DCE, respectively.
.ce
.parag
.ce
2.4.8.5
.ce
\fIMaximum number of bits in an I frame N1\fR
.ce
.parag
.ce
The value of the DTE N1 system parameter may be different than the value
of the DCE N1 system parameter. These values shall be made known to both
the DTE and the DCE.
.ce
.parag
.ce
The values of N1 shall indicate the maximum number of bits in an
.ce
I\ frame (excluding flags and 0\ bits inserted for transparency) that the
DCE or DTE is willing to accept from the DTE or DCE, respectively.
.ce
.parag
.ce
In order to allow for universal operation, a DTE should support a
.ce
value of DTE N1 which is not less than 1080\ bits (135\ octets). DTEs should
be aware that the network may transmit longer packets (see \(sc\ 5.2),
that may result in a data link layer problem.
.ce
.parag
.ce

.ce
All networks shall offer to a DTE which requires it, a value of DCE N1
which is greater than or equal to 2072\ bits (259\ octets) plus the length
of the address, control and FCS fields at the DTE/DCE interface, and greater
than or equal to the maximum length of the data packets which may cross
the DTE/DCE
.ce
interface plus the length of the address, control and FCS fields at the
DTE/DCE interface.
.ce
.parag
.ce
Appendix II provides a description of how the values stated above are  derived.
.ce
.parag
.ce
2.4.8.6
.ce
\fIMaximum number of \fR
.ce
\fIoutstanding I frames k\fR
.ce
.parag
.ce
The value of the DTE k system parameter shall be the same as the
.ce
value of the DCE k system parameter. This value shall be agreed to for a
.ce
period of time by both the DTE and the DCE.
.ce
.parag
.ce
The value of k shall indicate the maximum number of sequentially
.ce
numbered I\ frames that the DTE or DCE may have outstanding
.ce
(i.e.\ unacknowledged) at any given time. The value of k shall never exceed
.ce
seven for modulo\ 8 operation, or one hundred and twenty\(hyseven for modulo\
128
.ce
operation. All networks (DCEs) shall support a value of seven. Other values
of k (less than and greater than seven) may also be supported by networks
.ce
(DCEs).
.ce
.parag
.ce
.line
.ce
\fBMONTAGE : \(sc 2.5 de cette Recommandation sur le reste de cette page\fR
.ce
.parag
.ce

.ce
.line
.ad r
\fBTable 4/X.25 [T4/X.25], p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
2.3.2.1.1
    \fIInformation transfer format\fR \fI \(em I\fR
.sp 9p
.RT
.PP
The I format is used to perform an information transfer. The
functions of N(S), N(R) and P are independent; i.e.,\ each I\ frame has
an N(S), an N(R) which may or may not acknowledge additional I\ frames
received by the
DCE or DTE, and a P\ bit that may be set to\ 0 or\ 1.
.RT
.sp 1P
.LP
2.3.2.1.2
    \fISupervisory format\fR \fI \(em S\fR
.sp 9p
.RT
.PP
The S format is used to perform data link supervisory control
functions such as acknowledge I\ frames, request retransmission of I\ frames,
and to request a temporary suspension of transmission of I\ frames. The
functions of N(R) and P/F are independent; i.e.,\ each supervisory frame
has an N(R) which
may or may not acknowledge additional I\ frames received by the DCE or\
DTE, and a P/F\ bit that may be set to\ 0 or\ 1.
.RT
.sp 1P
.LP
2.3.2.1.3
    \fIUnnumbered format\fR \fI \(em U\fR
.sp 9p
.RT
.PP
The U format is used to provide additional data link control
functions. This format contains no sequence numbers, but does include a
P/F\ bit that may be set to\ 0 or\ 1. The unnumbered frames have the same
control field
length (one octet) in both basic (modulo\ 8) operation and extended (modulo\
128) operation.
.RT
.sp 1P
.LP
2.3.2.2
    \fIControl field parameters\fR
.sp 9p
.RT
.PP
The various parameters associated with the control field formats
are described below.
.RT
.sp 1P
.LP
2.3.2.2.1
    \fIModulus\fR
.sp 9p
.RT
.PP
Each I frame is sequentially numbered and may have the value\ 0
through modulus minus\ 1 (where \*Qmodulus\*U is the modulus of the sequence
numbers). The modulus equals either\ 8 or\ 128 and the sequence numbers cycle
through the entire range.
.RT
.sp 1P
.LP
2.3.2.2.2
    \fISend state variable\fR \fI V(S)\fR
.sp 9p
.RT
.PP
The send state variable V(S) denotes the sequence number of the
next in\(hysequence I\ frame to be transmitted. V(S) can take on the values\ 0
through modulus minus\ 1. The value of V(S) is incremented by\ 1 with each
successive I\ frame transmission, but cannot exceed the N(R) of the last
received\ I or supervisory frame by more than the maximum number of outstanding
I\ frames\ (\fIk\fR ). The value of\ k is defined in \(sc\ 2.4.8.6 below.
.RT
.sp 1P
.LP
2.3.2.2.3
    \fISend sequence number\fR \fI N(S)\fR
.sp 9p
.RT
.PP
Only I frames contain N(S), the send sequence number of transmitted I\
frames. At the time that an in\(hysequence I\ frame is designated for
transmission, the value of N(S) is set equal to the value of the send state
variable\ V(S).
.RT
.sp 1P
.LP
2.3.2.2.4
    \fIReceive state variable\fR \fIV(R)\fR
.sp 9p
.RT
.PP
The receive state variable V(R) denotes the sequence number of the next
in\(hysequence I\ frame expected to be received. V(R) can take on the values
0 through modulus minus\ 1. The value of V(R) is incremented by\ 1 by the
receipt of an error\(hyfree, in\(hysequence I\ frame whose send sequence
number N(S) equals
the receive state variable V(R).
.RT
.sp 1P
.LP
2.3.2.2.5
    \fIReceive sequence number\fR \fI N(R)\fR
.sp 9p
.RT
.PP
All I frames and supervisory frames contain N(R), the expected send sequence
number of the next received I\ frame. At the time that a frame of the above
types is designated for transmission, the value of N(R) is set equal to
the current value of the receive state variable V(R). N(R) indicates that
the DCE or DTE transmitting the N(R) has received correctly all I\ frames
numbered up to and including N(R)\ \(em\ 1.
.RT
.sp 1P
.LP
2.3.2.2.6
    \fIPoll/Final bit\fR \fI P/F\fR
.sp 9p
.RT
.PP
All frames contain P/F, the Poll/Final bit. In command frames, the P/F
bit is referred to as the P bit. In response frames, it is referred to
as the F\ bit.
.bp
.RT
.sp 1P
.LP
2.3.3
    \fIFunctions of the Poll/Final bit\fR
.sp 9p
.RT
.PP
The Poll bit set to 1 is used by the DCE or DTE to solicit (poll) a response
from the DTE or DCE, respectively. The Final bit set to\ 1 is used by the
DCE or DTE to indicate the response frame transmitted by the DTE or DCE,
respectively, as a result of the soliciting (poll) command.
.PP
The use of the P/F bit is described in \(sc\ 2.4.3 below.
.RT
.sp 1P
.LP
2.3.4
    \fICommands and responses\fR
.sp 9p
.RT
.PP
For basic (modulo 8) operation, the commands and responses
represented in Table\ 5/X.25 will be supported by the DCE and the\ DTE.
.PP
For extended (modulo 128) operation, the commands and responses
represented in Table\ 6/X.25 will be supported by the DCE and the DTE.
.RT
.PP
For purposes of the LAPB procedures, the supervisory function bit encoding
\*Q11\*U and those encodings of the modifier function bits in
Tables\ 3/X.25 and 4/X.25 not identified in Tables\ 5/X.25 or 6/X.25 are
identified as \*Qundefined or not implemented\*U command and response control
fields.
.PP
The commands and responses in Tables 5/X.25 and 6/X.25 are defined as  follows:
.RT
.sp 1P
.LP
2.3.4.1
    \fIInformation (I)\fR \fI command\fR
.sp 9p
.RT
.PP
The function of the information (I) command is to transfer across a data
link a sequentially numbered frame containing an information field.
.RT
.ce
.line
.ce
\fBTable 5/X.25 [T5.25], p.\fR
.ce
.line
.ce
\fBTable 6/X.25 [T6.25], p.\fR
.ce
2.3.4.2
.ce
\fIReceive ready (RR)\fR
.ce
\fI command and response\fR
.ce
.parag
.ce
The receive ready (RR) supervisory frame is used by the DCE
.ce
or DTE to:
.ce
.parag
.ce

.ce
1)
.ce
indicate it is ready to receive an I frame; and
.ce
.parag
.ce
2)
.ce
acknowledge previously received I frames numbered up to and
.ce
including N(R)\ \(em\ 1.
.ce
.parag
.ce
An RR frame may be used to indicate the clearance of a busy
.ce
condition that was reported by the earlier transmission of an RNR frame
by that same station (DCE or\ DTE). In addition to indicating the DCE or
DTE status, the RR command with the P\ bit set to\ 1 may be used by the
DCE or DTE to ask for the status of the DTE or DCE, respectively.
.ce
.parag
.ce
2.3.4.3
.ce
\fIReceive not ready (RNR) command and response\fR
.ce
.parag
.ce
The receive not ready (RNR) supervisory frame is used by the DCE or DTE
to indicate a busy condition; i.e.\ temporary inability to accept
.ce
additional incoming I\ frames. I\ frames numbered up to and including N(R)\
\(em\ 1
.ce
are acknowledged. I\ frame N(R) and any subsequent I\ frames received, if any,
.ce
are not acknowledged; the acceptance status of these I\ frames will be
indicated in subsequent exchanges.
.ce
.parag
.ce
In addition to indicating the DCE or DTE status, the RNR command with the
P bit set to\ 1 may be used by an DCE or DTE to ask for the status of the
.ce
DTE or DCE, respectively.
.ce
.parag
.ce
2.3.4.4
.ce
\fIReject (REJ) command and response\fR
.ce
.parag
.ce
The reject (REJ) supervisory frame is used by the DCE or DTE to request
transmission of I\ frames starting with the frame numbered N(R).
.ce
I\ frames numbered N(R)\ \(em\ 1 and below are acknowledged. Additional
I\ frames
.ce
pending initial transmission may be transmitted following the retransmitted
.ce
I\ frame(s).
.ce
.parag
.ce

.ce
Only one REJ exception condition for a given direction of information transfer
may be established at any time. The REJ exception condition is cleared
(reset) upon the receipt of an I\ frame with an N(S) equal to the N(R)
of the
.ce
REJ\ frame.
.ce
.parag
.ce
An REJ frame may be used to indicate the clearance of a busy condition
that was reported by the earlier transmission of an RNR frame by that same
.ce
station (DCE or\ DTE). In addition to indicating the DCE or DTE status, the
.ce
REJ\ command with the P bit set to\ 1 may be used by the DCE or\ DTE to ask for
.ce
the status of the DTE or\ DCE, respectively.
.ce
.parag
.ce
\fR
.ce
2.3.4.5
.ce
\fISet asynchronous balanced mode (SABM)\fR
.ce
\fIcommand/\fR
.ce
\fISet asynchronous balanced mode extended (SABME)\fR
.ce

.ce
\fIcommand (subscription time option)\fR
.ce
.parag
.ce

.ce
The SABM unnumbered command is used to place the addressed DCE or DTE in
an asynchronous balanced mode (ABM) information transfer phase where all
command/response control fields will be one\ octet in length.
.ce
.parag
.ce
The SABME unnumbered command is used to place the addressed DCE or DTE
in an asynchronous balanced mode\ (ABM) information transfer phase where
.ce
numbered command/response control fields will be two octets in length, and
.ce
unnumbered command/response control fields will be one octet in length.
.ce
.parag
.ce

.ce
No information field is permitted with the SABM or SABME command. The transmission
of a SABM/SABME command indicates the clearance of a busy
.ce
condition that was reported by the earlier trans
.ce
mission\ of an RNR\ frame by   that
.ce
same station (DCE or\ DTE). The DCE or DTE confirms acceptance of SABM/SABME
.ce
[modulo\ 8 (basic) operation/modulo\ 128 (extended) operation] command by the
.ce
transmission, at the first opportunity, of a UA\ response. Upon acceptance of
.ce
this command, the DCE or DTE send state variable\ V(S) and receive state
.ce
variable V(R) are set to\ 0.
.ce
.parag
.ce
Previously transmitted I\ frames that are unacknowledged when this
.ce
command is actioned remain unac
.ce
knowledged.\ It is the responsibility of a
.ce
higher layer (e.g.\ Packet Layer or MLP) to recover from the possible loss of
.ce
the contents (e.g.\ packets) of such I\ frames.
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ The mode of operation of a data link [basic (modulo 8) or
.ce
extended (modulo\ 128)] is determined at subscription time and is only
changed by going through a new subscription process.
.ce
.parag
.ce
2.3.4.6
.ce
\fIDisconnect (DISC)\fR
.ce
\fI command\fR
.ce
.parag
.ce
The DISC unnumbered command is used to terminate the mode
.ce
previously set. It is used to inform the DCE or DTE receiving the DISC
command that the DTE or DCE sending the DISC command is suspending operation.
No
.ce
information field is permitted with the DISC command. Prior to actioning the
.ce
DISC command, the DCE or DTE receiving the DISC command confirms the acceptance
of the DISC command by the transmission of a UA response. The DTE or DCE
.ce
sending the DISC command enters the disconnected phase when it receives the
.ce
acknowledging UA response.
.ce
.parag
.ce
Previously transmitted I frames that are unacknowledged when this
.ce
command is actioned remain unacknowledged. It is the responsibility of
a higher layer (e.g.\ Packet Layer or MLP) to recover from the possible
loss of the
.ce
contents (e.g.,\ packets) of such I frames.
.ce
.parag
.ce
2.3.4.7
.ce
\fIUnnumbered acknowledgement (UA)\fR
.ce
\fI response\fR
.ce
.parag
.ce
The UA unnumbered response is used by the DCE or DTE to acknowledge the
receipt and acceptance of the mode\(hysetting commands. Received mode\(hysetting
commands are not actioned until the UA response is transmitted. The
.ce
transmission of a UA response indicates the clearance of a busy condition
that was reported by the earlier transmission of an RNR frame by that same
station (DCE or DTE). No information field is permitted with the UA response.
.ce
.parag
.ce

.ce
2.3.4.8
.ce
\fIDisconnected mode (DM)\fR
.ce
\fI response\fR
.ce
.parag
.ce
The DM unnumbered response is used to report a status where the DCE or
DTE is logically disconnected from the data link, and is in the disconnected
phase. The DM response may be sent to indicate that the DCE or DTE has
entered the disconnected phase without benefit of having received a DISC
command, or, if sent in response to the reception of a mode setting command,
is sent to
.ce
inform the DTE or DCE that the DCE or DTE, respectively, is still in the
.ce
disconnected phase and cannot execute the set mode command. No information
.ce
field is permitted with the DM response.
.ce
.parag
.ce
A DCE or DTE in a disconnected phase will monitor received commands
.ce
and will react to an SABM/SABME command as outlined in \(sc\ 2.4.4 below,
and will respond with a DM response with the F bit set to\ 1 to any other
command
.ce
received with the P\ bit set to\ 1.
.ce
.parag
.ce
2.3.4.9
.ce
\fIFrame reject (FRMR)\fR
.ce
\fI response\fR
.ce
.parag
.ce
The FRMR unnumbered response is used by the DCE or DTE to report an error
condition not recoverable by retransmission of the identical frame;
.ce
i.e.\ at least one of the following conditions, which results from the
receipt of a valid frame:
.ce
.parag
.ce
1)
.ce
the receipt of a command or response control field that is
.ce
undefined or not implemented;
.ce
.parag
.ce
2)
.ce
the receipt of an I frame with an information field which
.ce
exceeds the maximum established length;
.ce
.parag
.ce
3)
.ce
the receipt of an invalid N(R); or
.ce
.parag
.ce
4)
.ce
the receipt of a frame with an information field which is
.ce
not permitted or the receipt of a supervisory or unnumbered
.ce
frame with incorrect length.
.ce
.parag
.ce
An undefined or not implemented control field is any of the
.ce
control field encodings that are not identified in Tables\ 5/X.25 or\ 6/X.25.
.ce
.parag
.ce

.ce
A valid N(R) must be within the range from the lowest send sequence
.ce
number N(S) of the still unacknowledged frame(s) to the current DCE send
state variable inclusive (or to the current internal variable \fIx\fR
.ce
if the DCE is in
.ce
the timer recovery condition as described in \(sc\ 2.4.5.9).
.ce
.parag
.ce

.ce
An information field which immediately follows the control field, and consists
of\ 3 or 5\ octets [modulo\ 8 (basic) operation or modulo\ 128 (extended)
operation, respectively], is returned with this response and provides the
.ce
reason for the FRMR response. These formats are given in Tables\ 7/X.25
.ce
and\ 8/X.25.
.ce
.parag
.ce
2.3.5
.ce
\fIException condition reporting and recovery\fR
.ce
.parag
.ce
The error recovery procedures which are available to effect
.ce
recovery following the detection/occurrence of an
.ce
exception condition
.ce
 at the Data Link Layer are described below. Exception conditions described
are those situations which may occur as the result of transmission errors,
DCE or DTE malfunction, or operational situations.
.ce
.parag
.ce
2.3.5.1
.ce
\fIBusy condition\fR
.ce
.parag
.ce
The busy condition results when the DCE or DTE is temporarily
.ce
unable to continue to receive I frames due to internal constraints,
.ce
e.g.\ receive buffering limitations. In this case an RNR frame is transmitted
.ce
from the busy DCE or DTE. I\ frames pending transmission may be transmitted
from the busy DCE or DTE prior to or following the RNR\ frame.
.ce
.parag
.ce
An indication that the busy condition has cleared is communicated by the
transmission of a UA (only in response to a SABM/SABME command), RR, REJ
or SABM/SABME (modulo\ 8/modulo\ 128) frame.
.ce
.parag
.ce
.line
.ce
\fBTable 7/X.25 [T7.25], p.\fR
.ce
.line
.ce
\fBTable 8/X.25 [T8.25], p.\fR
.ce
2.3.5.2
.ce
\fIN(S) sequence error condition\fR
.ce
.parag
.ce
The information field of all I frames received whose N(S) does not equal
the receive state variable V(R) will be discarded.
.ce
.parag
.ce
An N(S) sequence error exception condition occurs in the receiver when
an I\ frame received contains an N(S) which is not equal to the receive
state
.ce
variable V(R) at the receiver. The receiver does not acknowledge (increment
its receive state variable) the I\ frame causing the sequence error, or
any I\ frame which may follow, until an I\ frame with the correct N(S)
is received.
.ce
.parag
.ce

.ce
A DCE or DTE which receives one or more valid I frames having sequence
errors or subsequent supervisory frames (RR, RNR and REJ) shall accept
the
.ce
control information contained in the N(R) field and the P or F bit to perform
data link control functions; e.g.\ to receive acknowledgement of previously
.ce
transmitted I\ frames and to cause the DCE or DTE to respond (P bit set to 1).
.ce
.parag
.ce

.ce
The means specified in \(sc\(sc\ 2.3.5.2.1 and 2.3.5.2.2 shall be available
for initiating the retransmission of lost or errored I\ frames following
the
.ce
occurrence of an N(S) sequence error condition.
.ce
.parag
.ce
2.3.5.2.1
.ce
\fIREJ recovery\fR
.ce
.parag
.ce
The REJ frame is used by a receiving DCE or DTE to initiate a
.ce
recovery (retransmission) following the detection of an N(S) sequence error.
.ce
.parag
.ce

.ce
With respect to each direction of transmission on the data link, only one
\*Qsent REJ\*U exception condition from a DCE or DTE, to a DTE or DCE,
is
.ce
established at a time. A \*Qsent REJ\*U exception condition is cleared when the
.ce
requested I\ frame is received.
.ce
.parag
.ce

.ce
A DCE or DTE receiving a REJ frame initiates sequential
.ce
(re\(hy)transmission of I\ frames starting with the I\ frame indicated
by the N(R) contained in the REJ frame. The retransmitted frames may contain
an N(R) and a P bit that are updated from, and therefore different from,
the ones contained in the originally transmitted I\ frames.
.ce
.parag
.ce
2.3.5.2.2
.ce
\fITime\(hyout recovery\fR
.ce
.parag
.ce
If a DCE or DTE, due to a transmission error, does not receive (or receives
and discards) a single I\ frame or the last I\ frame(s) in a sequence of
I\ frames, it will not detect an N(S) sequence error condition and, therefore,
will not transmit a REJ frame. The DTE or DCE which transmitted the
.ce
unacknowledged I\ frame(s) shall, following the completion of a system
specified time\(hyout period (see \(sc\(sc\ 2.4.5.1 and 2.4.5.9 below),
take appropriate recovery action to determine at which I\ frame retransmission
must begin. The
.ce
retransmitted frame(s) may contain an N(R) and a P bit that is updated from,
.ce
and therefore different from, the ones contained in the originally transmitted
frame(s).
.ce
.parag
.ce
2.3.5.3
.ce
\fIInvalid frame\fR
.ce
\fI condition\fR
.ce
.parag
.ce
Any frame which is invalid will be discarded, and no action is
.ce
taken as the result of that frame. An invalid frame is defined as one
.ce
which:
.ce
.parag
.ce
a)
.ce
is not properly bounded by two flags;
.ce
.parag
.ce
b)
.ce
in basic (modulo 8) operation, contains fewer than 32 bits
.ce
between flags; in extended (modulo\ 128) operation, contains
.ce
fewer than 40\ bits between flags of frames that contain sequence
.ce
numbers or 32\ bits between flags of frames that do not contain
.ce
sequence numbers;
.ce
.parag
.ce
c)
.ce
contains a Frame Check Sequence (FCS) error; or
.ce
.parag
.ce
d)
.ce
contains an address other than A or B (for single link
.ce
operation) or other than C or D (for multilink operation).
.ce
.parag
.ce

.ce
For those networks that are octet aligned, a detection of
.ce
non\(hyoctet alignment may be made at the Data Link Layer by adding a frame
.ce
validity check that requires the number of bits between the opening flag and
.ce
the closing flag, excluding bits inserted for transparency, to be an integral
number of octets in length, or the frame is considered invalid.
.ce
.parag
.ce
2.3.5.4
.ce
\fIFrame rejection\fR
.ce
\fI condition\fR
.ce
.parag
.ce
A frame rejection condition is established upon the receipt of an error\(hyfree
frame with one of the conditions listed in \(sc\ 2.3.4.9 above.
.ce
.parag
.ce

.ce
At the DCE or DTE, this frame rejection exception condition is
.ce
reported by an FRMR response for appropriate DTE or DCE action, respectively.
Once a DCE has established such an exception condition, no additional I\
frames are accepted until the condition is reset by the DTE, except for
examination of the P bit. The FRMR response may be repeated at each opportunity,
as specified in \(sc\ 2.4.7.3, until recovery is effected by the DTE, or
until the DCE initiates its own recovery in case the DTE does not respond.
.ce
.parag
.ce
2.3.5.5
.ce
\fIExcessive idle channel state condition on incoming\fR
.ce

.ce
\fIchannel\fR
.ce
.parag
.ce
Upon detection of an idle channel state condition (see \(sc\ 2.2.12.2 above)
on the incoming channel, the DCE shall wait for a period\ T3 (see
.ce
\(sc\ 2.4.8.3 below) without taking any specific action, waiting for detection
of a return to the active channel state (i.e.,\ detection of at least one
flag
.ce
sequence). After the period\ T3, the DCE shall notify the higher layer
(e.g.\ the Packet Layer or the MLP) of the excessive idle channel state
condition, but
.ce
shall not take any action that would preclude the DTE from establishing the
.ce
data link by normal data link set\(hyup procedures.
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ Other actions to be taken by the DCE at the Data Link Layer upon
expiration of period\ T3 is a subject for further study.
.ce
.parag
.ce

.ce
2.4
.ce
\fIDescription of the \fR
.ce
\fILAPB procedure\fR
.ce
.parag
.ce
2.4.1
.ce
\fILAPB basic and extended modes of operation\fR
.ce
.parag
.ce
In accordance with the system choice made by the DTE at
.ce
subscription time, the DCE will either support modulo\ 8 (basic) operation or
.ce
will support modulo\ 128 (extended) operation. Changing from basic operation
to extended operation, or vice versa, in the DCE requires resubscription
by the
.ce
DTE for the desired service, and is not supported dynamically.
.ce
.parag
.ce
Table 5/X.25 indicates the command and response control field formats used
with the basic (modulo\ 8) service. The mode\(hysetting command employed
to
.ce
initialize (set up) or reset the basic mode is the SABM command. Table
6/X.25 indicates the command and response control field formats used with
the extended (modulo\ 128) service. The mode\(hysetting command employed
to initialize (set up) or reset the extended mode is the SABME command.
.ce
.parag
.ce
2.4.2
.ce
\fILAPB procedure for addressing\fR
.ce
.parag
.ce
The address field identifies a frame as either a command or a
.ce
response. A command frame contains the address of the DCE or DTE to which
the command is being sent. A response frame contains the address of the
DCE or DTE sending the frame.
.ce
.parag
.ce
In order to allow differentiation between single link operation and
.ce
the optional multilink operation for diagnostic and/or maintenance reasons,
.ce
different address pair encodings are assigned to data links operating with
.ce
multilink procedure compared to data links operating with the single link
.ce
procedure.
.ce
.parag
.ce
Frames containing commands transferred from the DCE to the DTE will
.ce
contain the address\ A for the single link operation and address\ C for the
.ce
multilink operation.
.ce
.parag
.ce
Frames containing responses transferred from the DCE to the DTE will contain
the address\ B for the single link operation and address\ D for the
.ce
multilink operation.
.ce
.parag
.ce

.ce
Frames containing commands transferred from the DTE to the DCE shall contain
the address\ B for the single link operation and address\ D for the
.ce
multilink operation.
.ce
.parag
.ce
Frames containing responses transferred from the DTE to the DCE shall contain
the address\ A for the single link operation and address\ C for the
.ce
multilink operation.
.ce
.parag
.ce
These addresses are coded as follows:
.ce
.parag
.ce
Address
.ce
1\ 2\ 3\ 4\ 5\ 6\ 7\ 8
.ce
.parag
.ce
Single link operation
.ce
\ \ A
.ce
1\ 1\ 0\ 0\ 0\ 0\ 0\ 0
.ce
.line
.ce
\ \ B
.ce
1\ 0\ 0\ 0\ 0\ 0\ 0\ 0
.ce
.parag
.ce
Multilink operation
.ce
\ \ C
.ce
1\ 1\ 1\ 1\ 0\ 0\ 0\ 0
.ce
.line
.ce
\ \ D
.ce
1\ 1\ 1\ 0\ 0\ 0\ 0\ 0
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ The DCE will discard all frames received with an address other
than\ A or\ B (single link operation), or\ C or\ D (multilink operation).
.ce
.parag
.ce
2.4.3
.ce
\fILAPB procedure for the use of the P/F bit\fR
.ce
.parag
.ce
The DCE or DTE receiving an SABM/SABME, DISC, supervisory command or I\
frame with the P\ bit set to\ 1 will set the F\ bit to\ 1 in the next response
frame it transmits.
.ce
.parag
.ce
The response frame returned by the DCE to an SABM/SABME or DISC
.ce
command with the P\ bit set to\ 1 will be a UA or DM response with the
F\ bit set to\ 1. The response frame returned by the DCE to an I\ frame
with the P\ bit set to\ 1, received during the information transfer phase,
will be an RR, REJ, RNR or FRMR response with the F\ bit set to\ 1. The
response frame returned by the
.ce
DCE to a supervisory command with the P\ bit set to\ 1, received during the
.ce
information transfer phase, will be an RR, REJ, RNR or FRMR response with
the F\ bit set to\ 1. The response frame returned by the DCE to an I\ frame
or
.ce
supervisory frame with the P\ bit set to\ 1, received during the disconnected
.ce
phase, will be a DM response with the F\ bit set to\ 1.
.ce
.parag
.ce
The P bit may be used by the DCE in conjunction with the timer
.ce
recovery condition (see \(sc\ 2.4.5.9 below).
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ Other use of the P bit by the DCE is a subject for further
.ce
study.
.ce
.parag
.ce
2.4.4
.ce
\fILAPB procedure for data link set\(hyup and disconnection\fR
.ce
.parag
.ce

.ce
2.4.4.1
.ce
\fIData link set\(hyup\fR
.ce
.parag
.ce
The DCE will indicate that it is able to set up the data link by
.ce
transmitting contiguous flags (active channel state).
.ce
.parag
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Either the DTE or the DCE may initiate data link set\(hyup. Prior to
.ce
initiation of data link set\(hyup, either the DCE or the DTE may initiate data
.ce
link
.ce
disconnection (see \(sc\ 2.4.4.3) for the purpose of insuring that the
DCE and the DTE are in the same phase. The DCE may also transmit an unsolicited
DM response to request the DTE to initiate data link set\(hyup.
.ce
.parag
.ce
The DTE shall initiate data link set\(hyup by transmitting an SABM/SABME
command to the DCE. If, upon receipt of the SABM/SABME command correctly,
the DCE determines that it can enter the information transfer phase, it
will return a UA response to the DTE, will reset its send and receive state
variables V(S) and V(R) to zero, and will consider that the data link is
set up. If, upon
.ce
receipt
.ce
of the SABM/SABME command correctly, the DCE determines that it cannot enter
.ce
the information transfer phase, it will return a DM response to the DTE as a
.ce
denial to the data link set\(hyup initialization and will consider that the
.ce
data link is
.ce
\fInot\fR
.ce
 set up. In order to avoid misinterpretation of the DM response received,
it is suggested that the DTE always sends its SABM/SABME command with the
P\ bit set to\ 1. Otherwise, it is not possible to differentiate a DM response
intended as a denial to data link set\(hyup from a DM response that is
issued in a separate unsolicited sense as a request for a mode\(hysetting
command (as described
.ce
in\ \(sc\ 2.4.4.4.2).
.ce
.parag
.ce
The DCE will initiate data link set\(hyup by transmitting an SABM/SABME
command to the DTE and starting its Timer\ T1 in order to determine when
too
.ce
much time has elapsed waiting for a reply (see \(sc\ 2.4.8.1 below). Upon
reception of a UA response from the DTE, the DCE will reset its send and
receive state
.ce
variables\ V(S) and\ V(R) to zero, will stop its Timer\ T1, and will consider
that the data link is set up. Upon reception of a DM response from the
DTE as a
.ce
denial
.ce
to the data link set\(hyup initialization, the DCE will stop its Timer\
T1 and will consider that the data link is \fInot\fR
.ce
set up.
.ce
.parag
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.ce
The DCE, having sent the SABM/SABME command, will ignore and discard any
frames except an SABM/SABME or DISC command, or a UA or DM response
.ce
received from the DTE. The receipt of an SABM/SABME or DISC command from the
.ce
DTE will result in a collision situation that is resolved per \(sc\ 2.4.4.5
below. Frames other than the UA and DM responses sent in response to a
received
.ce
SABM/SABME or DISC command will be sent only after the data link is set
up and if no outstanding SABM/SABME command exists.
.ce
.parag
.ce
After the DCE sends the SABM/SABME command, if a UA or DM response is not
received correctly, Timer\ T1 will run out in the DCE. The DCE will then
.ce
resend the SABM/SABME command and will restart Timer\ T1. After transmission
of the SABM/SABME command N2 times by the DCE, appropriate higher layer
recovery action will be initiated. The value of N2 is defined in \(sc\
2.4.8.4 below.
.ce
.parag
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2.4.4.2
.ce
\fIInformation transfer phase\fR
.ce
.parag
.ce
After having transmitted the UA response to the SABM/SABME command or having
received the UA response to a transmitted SABM/SABME command, the DCE will
accept and transmit I and supervisory frames according to the procedures
described in \(sc\ 2.4.5 below.
.ce
.parag
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.ce
When receiving the SABM/SABME command while in the information
.ce
transfer phase, the DCE will conform to the data link resetting procedure
.ce
described in \(sc\ 2.4.7 below.
.ce
.parag
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2.4.4.3
.ce
\fIData link disconnection\fR
.ce
.parag
.ce
The DTE shall initiate a disconnect of the data link by
.ce
transmitting a DISC command to the DCE. On correctly receiving a DISC command
in the information transfer phase, the DCE will send a UA response and
enter
.ce
the disconnected phase. On correctly receiving a DISC command in the
.ce
disconnected phase, the DCE will send a DM response and remain in the
.ce
disconnected phase. In order to avoid misinterpretation of the DM response
.ce
received, it is suggested that the DTE always sends its DISC command with
the P\ bit set to\ 1. Otherwise, it is not possible to differentiate a
DM response
.ce
intended as an indication that the DCE is already in the disconnected phase
.ce
from a DM response that is issued in a separate unsolicited sense as a
request for a mode\(hysetting command (as described in \(sc\ 2.4.4.4.2).
.ce
.parag
.ce

.ce
The DCE will initiate a disconnect of the data link by transmitting a DISC
command to the DTE and starting its Timer\ T1 (see \(sc\ 2.4.8.1 below).
Upon reception of an UA response from the DTE, the DCE will stop its Timer\
T1 and
.ce
will enter the disconnected phase. Upon reception of a DM response from
the DTE as an indication that the DTE was already in the disconnected phase,
the DCE
.ce
will stop its Timer\ T1 and will enter the disconnected phase.
.ce
.parag
.ce
The DCE, having sent the DISC command, will ignore and discard any
.ce
frames except an SABM/SABME or DISC command, or a UA or DM response received
.ce
from the DTE. The receipt of an SABM/SABME or DISC command from the DTE will
.ce
result in a collision situation that is resolved per \(sc\ 2.4.4.5 below.
.ce
.parag
.ce

.ce
After the DCE sends the DISC command, if a UA or DM response is not
.ce
received correctly, Timer\ T1 will run out in the DCE. The DCE will then
resend the DISC command and will restart Timer\ T1. After transmission
of the DISC
.ce
command N2 times by the DCE, appropriate higher layer recovery action will
be initiated. The value of N2 is defined in \(sc\ 2.4.8.4 below.
.ce
.parag
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2.4.4.4
.ce
\fIDisconnected phase\fR
.ce
.parag
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2.4.4.4.1
.ce
After having received a DISC command from the DTE and
.ce
returned a UA response to the DTE, or having received the UA response to a
.ce
transmitted DISC command, the DCE will enter the disconnected phase.
.ce
.parag
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In the disconnected phase, the DCE may initiate data link set\(hyup. In
the disconnected phase, the DCE will react to the receipt of an SABM/SABME
command as described in \(sc\ 2.4.4.1 above and will transmit a DM response
in
.ce
answer to a received DISC command. When receiving any other command (defined,
or undefined or not implemented) with the P\ bit set to\ 1, the DCE will
transmit a DM response with the F\ bit set to\ 1. Other frames received
in the
.ce
disconnected phase will be ignored by the DCE.
.ce
.parag
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2.4.4.4.2
.ce
When the DCE enters the disconnected phase after
.ce
detecting error conditions as listed in \(sc\ 2.4.6 below, or after an internal
.ce
malfunction, it may indicate this by sending a DM response rather than
a DISC command. In these cases, the DCE will transmit a DM response and
start its
.ce
Timer\ T1 (see \(sc\ 2.4.8.1 below).
.ce
.parag
.ce
If Timer T1 runs out before the reception of an SABM/SABME or DISC command
from the DTE, the DCE will retransmit the DM response and restart
.ce
Timer\ T1. After transmission of the DM response N2 times, the DCE will
remain in the disconnected phase and appropriate recovery actions will
be initiated. The value of N2 is defined in \(sc\ 2.4.8.4 below.
.ce
.parag
.ce
Alternatively, after an internal malfunction, the DCE may either
.ce
initiate a data link resetting procedure (see \(sc\ 2.4.7 below) or disconnect
the data link (see \(sc\ 2.4.4.3 above) prior to initiating a data link
set\(hyup
.ce
procedure (see \(sc\ 2.4.4.1 above).
.ce
.parag
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2.4.4.5
.ce
\fICollision of unnumbered commands\fR
.ce
.parag
.ce
Collision situations shall be resolved in the following way:
.ce
.parag
.ce
2.4.4.5.1
.ce
If the sent and received unnumbered commands are the
.ce
same, the DCE and the DTE shall each send the UA response at the earliest
.ce
possible opportunity. The DCE shall enter the indicated phase either,
.ce
.parag
.ce

.ce
1)
.ce
after receiving the UA response,
.ce
.parag
.ce
2)
.ce
after sending the UA response, or
.ce
.parag
.ce
3)
.ce
after timing out waiting for the UA response having sent a
.ce
UA response.
.ce
.parag
.ce
In the case of 2) above, the DCE will accept a subsequent UA
.ce
response to the mode\(hysetting command it issued without causing an exception
.ce
condition if received within the time\(hyout interval.
.ce
.parag
.ce

.ce
2.4.4.5.2
.ce
If the sent and received unnumbered commands are
.ce
different, the DCE and the DTE shall each enter the disconnected phase and
.ce
issue a DM response at the earliest possible opportunity.
.ce
.parag
.ce
2.4.4.6
.ce
\fICollision of DM response with SABM/SABME or DISC\fR
.ce

.ce
\fIcommand\fR
.ce
.parag
.ce
When a DM response is issued by the DCE or DTE as an unsolicited
.ce
response to request the DTE or DCE, respectively, to issue a mode\(hysetting
.ce
command as described in \(sc\ 2.4.4.4, a collision between an SABM/SABME
or DISC
.ce
command and the unsolicited DM response may occur. In order to avoid
.ce
misinterpretation of the DM response received, the DTE always sends its
.ce
SABM/SABME or DISC command with the P\ bit set to\ 1.
.ce
.parag
.ce
2.4.4.7
.ce
\fICollision of DM responses\fR
.ce
.parag
.ce
A contention situation may occur when both the DCE and the DTE
.ce
issue a DM response to request a mode\(hysetting command. In this case, the DTE
.ce
will issue an SABM/SABME command to resolve the contention situation.
.ce
.parag
.ce

.ce
2.4.5
.ce
\fILAPB procedures for information transfer\fR
.ce
.parag
.ce
The procedures which apply to the transmission of I\ frames in each direction
during the information transfer phase are described below.
.ce
.parag
.ce
In the following, \*Qnumber one higher\*U is in reference to a
.ce
continuously repeated sequence series, i.e., 7 is 1\ higher than 6 and 0 is
.ce
1\ higher than 7 for modulo\ 8 series, and 127 is 1\ higher than 126 and 0 is
.ce
1\ higher than 127 for modulo\ 128 series.
.ce
.parag
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2.4.5.1
.ce
\fISending I frames\fR
.ce
.parag
.ce
When the DCE has an I frame to transmit (i.e. an I frame not
.ce
already transmitted, or having to be retransmitted as described in \(sc\
2.4.5.6
.ce
below), it will transmit it with an N(S) equal to its current send state
.ce
variable V(S), and an N(R) equal to its current receive state variable
V(R). At the end of the transmission of the I\ frame, the DCE will increment
its send
.ce
state variable V(S) by\ 1.
.ce
.parag
.ce
If Timer T1 is not running at the time of transmission of an I frame, it
will be started.
.ce
.parag
.ce
If the send state variable V(S) is equal to the last value of N(R)
.ce
received plus \fIk\fR
.ce
(where \fIk\fR
.ce
is the maximum number of outstanding I\ frames \(em
.ce
see \(sc\ 2.4.8.6 below), the DCE will not transmit any new I\ frames, but may
.ce
retransmit an I\ frame as described in \(sc\(sc\ 2.4.5.6 or 2.4.5.9 below.
.ce
.parag
.ce
When the DCE is in the busy condition, it may still transmit I frames,
provided that the DTE is not busy. When the DCE is in the frame rejection
.ce
condition, it will stop transmitting I\ frames.
.ce
.parag
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2.4.5.2
.ce
\fIReceiving an I frame\fR
.ce
.parag
.ce
2.4.5.2.1
.ce
When the DCE is not in a busy condition and receives a valid I\ frame whose
send sequence number N(S) is equal to the DCE receive state
.ce
variable V(R), the DCE will accept the information field of this frame,
.ce
increment by one its receive state variable V(R), and act as follows:
.ce
.parag
.ce

.ce
a)
.ce
If the DCE is still not in a busy condition:
.ce
.parag
.ce
i)
.ce
If an I frame is available for transmission by the
.ce
DCE, it may act as in \(sc\ 2.4.5.1 above and acknowledge
.ce
the received I frame by setting N(R) in the control field
.ce
of the next transmitted I\ frame to the value of the DCE
.ce
receive state variable V(R). Alternatively, the DCE may
.ce
acknowledge the received I\ frame by transmitting an RR
.ce
frame with the N(R) equal to the value of the DCE receive
.ce
state variable V(R).
.ce
.parag
.ce
ii)
.ce
If no I frame is available for transmission by the
.ce
DCE, it will transmit an RR frame with N(R) equal to the
.ce
value of the DCE receive state variable V(R).
.ce
.parag
.ce

.ce
b)
.ce
If the DCE is now in a busy condition, it will transmit an
.ce
RNR frame with N(R) equal to the value of the DCE receive
.ce
state variable V(R) (see \(sc\ 2.4.5.8).
.ce
.parag
.ce

.ce
2.4.5.2.2
.ce
When the DCE is in a busy condition, it may ignore the
.ce
information field contained in any received I\ frame.
.ce
.parag
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2.4.5.3
.ce
\fIReception of invalid frames\fR
.ce
.parag
.ce
When the DCE receives an invalid frame (see \(sc\ 2.3.5.3), this frame
will be discarded.
.ce
.parag
.ce
2.4.5.4
.ce
\fIReception of out\(hyof\(hysequence I frames\fR
.ce
.parag
.ce
When the DCE receives a valid I frame whose send sequence number
.ce
N(S) is incorrect, i.e.,\ not equal to the current DCE receive state variable
.ce
V(R), it will discard the information field of the I\ frame and transmit
an REJ frame with the N(R) set to one higher than the N(S) of the last
correctly
.ce
received I\ frame. The REJ frame will be a command frame with the P\ bit
set to\ 1 if an acknowledged transfer of the retransmission request is
required;
.ce
otherwise the REJ frame may be either a command or a response frame. The DCE
.ce
will then discard the information field of all I\ frames received until the
.ce
expected I\ frame is correctly received. When receiving the expected I\ frame,
.ce
the DCE will then acknowledge the I\ frame as described in \(sc\ 2.4.5.2
above. The DCE will use the N(R) and P\ bit information in the discarded
I\ frames as
.ce
described in \(sc\ 2.3.5.2 above.
.ce
.parag
.ce
2.4.5.5
.ce
\fIReceiving acknowledgement\fR
.ce
.parag
.ce
When correctly receiving an I frame or a supervisory frame (RR, RNR or
REJ), even in the busy condition, the DCE will consider the N(R) contained
in this frame as an acknowledgement for all I\ frames it has transmitted
with an N(S) up to and including the received N(R)\(em1. The DCE will stop
Timer\ T1 when it correctly receives an I\ frame or a supervisory frame
with the N(R) higher
.ce
than the last received N(R) (actually acknowledging some I\ frames), or
an REJ frame with an N(R) equal to the last received N(R).
.ce
.parag
.ce

.ce
If Timer T1 has been stopped by the receipt on an I, RR or RNR frame, and
if there are outstanding I\ frames still unacknowledged, the DCE will
.ce
restart Timer\ T1. If Timer\ T1 then runs out, the DCE will follow the
recovery procedure (\(sc\ 2.4.5.9 below) with respect to the unacknowledged
I\ frames. If
.ce
Timer\ T1 has been stopped by the receipt of an REJ frame, the DCE will
follow the retransmission procedures in \(sc\ 2.4.5.6 below.
.ce
.parag
.ce
2.4.5.6
.ce
\fIReceiving an REJ frame\fR
.ce
.parag
.ce
When receiving an REJ frame, the DCE will set its send state
.ce
variable V(S) to the N(R) received in the REJ control field. It will transmit
the corresponding I\ frame as soon as it is available or retransmit it
in
.ce
accordance with the procedures described in \(sc\ 2.4.5.1 above. (Re)transmission
will conform to the following procedure:
.ce
.parag
.ce
i)
.ce
if the DCE is transmitting a supervisory command or response
.ce
when it receives the REJ frame, it will complete that
.ce
transmission before commencing transmission of the requested
.ce
I\ frame;
.ce
.parag
.ce

.ce
ii)
.ce
if the DCE is transmitting an unnumbered command or
.ce
response when it receives the REJ frame, it will ignore the
.ce
request for retransmission;
.ce
.parag
.ce
iii)
.ce
if the DCE is transmitting an I frame when the REJ frame
.ce
is received, it may abort the I\ frame and commence transmission
.ce
of the requested I\ frame immediately after abortion;
.ce
.parag
.ce
iv)
.ce
if the DCE is not transmitting any frame when the REJ frame
.ce
is received, it will commence transmission of the requested
.ce
I\ frame immediately.
.ce
.parag
.ce
In all cases, if other unacknowledged I frames had already been
.ce
transmitted following the one indicated in the REJ frame, then those I
frames will be retransmitted by the DCE following the retransmission of
the requested I\ frame. Other I\ frames not yet transmitted may be transmitted
following the
.ce
retransmitted I\ frames.
.ce
.parag
.ce
If the REJ frame was received from the DTE as a command with the P bit
set to\ 1, the DCE will transmit an RR, RNR or REJ response with the F
bit set to\ 1 before transmitting or retransmitting the corresponding I\
frame.
.ce
.parag
.ce
2.4.5.7
.ce
\fIReceiving an RNR frame\fR
.ce
.parag
.ce
After receiving an RNR frame whose N(R) acknowledges all frames
.ce
previously transmitted, the DCE will stop Timer\ T1 and may then transmit
.ce
an I\ frame, with the P\ bit set to\ 0, whose send sequence number is
.ce
equal to the N(R) indicated in the RNR frame, restarting Timer\ T1 as it
does. After receiving an RNR frame whose N(R) indicates a previously transmitted
.ce
frame, the DCE will not transmit or retransmit any I\ frame, Timer\ T1 being
.ce
already running. In either case, if the Timer\ T1 runs out before receipt
of a busy clearance indication, the DCE will follow the procedure described
in
.ce
\(sc\ 2.4.5.9\ below. In any case, the DCE will not transmit any other
I\ frames
.ce
before receiving an RR or REJ frame, or before the completion of a link
.ce
resetting procedure.
.ce
.parag
.ce
Alternatively, after receiving an RNR frame, the DCE may wait for a
.ce
period of time (e.g.,\ the length of the Timer\ T1) and then transmit a
.ce
supervisory command frame (RR, RNR or REJ) with the P\ bit set to\ 1, and
start Timer\ T1, in order to determine if there is any change in the receive
status of the DTE. The DTE shall respond to the P\ bit set to\ 1 with a
supervisory
.ce
response frame (RR, RNR or REJ) with the F\ bit set to\ 1 indicating either
.ce
continuance of the busy condition (RNR) or clearance of the busy condition
(RR or REJ). Upon receipt of the DTE response, Timer\ T1 is stopped.
.ce
.parag
.ce

.ce
1)
.ce
If the response is the RR or REJ response, the busy
.ce
condition is cleared and the DCE may transmit I\ frames beginning
.ce
with the I\ frame identified by the N(R) in the received response
.ce
frame.
.ce
.parag
.ce
2)
.ce
If the response is the RNR response, the busy condition
.ce
still exists, and the DCE will after a period of time (e.g.\ the
.ce
length of Timer\ T1) repeat the enquiry of the DTE receive
.ce
status.
.ce
.parag
.ce
If Timer T1 runs out before a status response is received, the
.ce
enquiry process above is repeated. If N2 attempts to get a status response
fail (i.e.\ Timer\ T1 runs out N2\ times), the DCE will initiate a data
link resetting procedure as described in \(sc\ 2.4.7.2\ below or will transmit
a DM response to ask the DTE to initiate a data link set\(hyup procedure
as described in \(sc\ 2.4.4.1 and enter the disconnected phase. The value
of N2 is defined in \(sc\ 2.4.8.4\ below.
.ce
.parag
.ce

.ce
If, at any time during the enquiry process, an unsolicited RR or REJ frame
is received from the DTE, it will be considered to be an indication of
.ce
clearance of the busy condition. Should the unsolicited RR or REJ frame be a
.ce
command frame with the P bit set to\ 1, the appropriate response frame
with the F\ bit set to 1 must be transmitted before the DCE may resume
transmission of
.ce
I\ frames. If Timer\ T1 is running, the DCE will wait for the non\(hybusy
response with the F bit set to\ 1 or will wait for Timer\ T1 to run out
and then either
.ce
may reinitiate the enquiry process in order to realize a successful P/F bit
.ce
exchange or may resume transmission of I frames beginning with the I\ frame
.ce
identified by the N(R) in the received RR or REJ frame.
.ce
.parag
.ce
2.4.5.8
.ce
\fIDCE busy condition\fR
.ce
.parag
.ce
When the DCE enters a busy condition, it will transmit an RNR frame at
the earliest opportunity. The RNR frame will be a command frame with the
P bit set to\ 1 if an acknowledged transfer of the busy condition indication
is
.ce
required; otherwise the RNR frame may be either a command or a response
frame. While in the busy condition, the DCE will accept and process supervisory
.ce
frames, will accept and process the contents of the N(R) fields of I\ frames,
.ce
and will return an RNR response with the F bit set to\ 1 if it receives a
.ce
supervisory command or I command frame with the P bit set to\ 1. To clear the
.ce
busy condition, the DCE will transmit either an REJ frame or an RR frame, with
.ce
N(R) set to the current receive state variable V(R), depending on whether or
.ce
not it discarded information fields of correctly received I\ frames. The REJ
.ce
frame or the RR frame will be a command frame with the P bit set to\ 1 if an
.ce
acknowledged transfer of the busy\(hyto\(hynon\(hybusy transition is required,
otherwise the REJ frame or the RR frame may be either a command or a response
frame.
.ce
.parag
.ce

.ce
2.4.5.9
.ce
\fIWaiting acknowledgement\fR
.ce
.parag
.ce
The DCE maintains an internal transmission attempt variable which is set
to\ 0 when the DCE sends a UA response, when the DCE receives a UA
.ce
response or an RNR command or response, or when the DCE correctly receives
an I\ frame or supervisory frame with the N(R) higher than the last received
N(R) (actually acknowledging some outstanding I\ frames).
.ce
.parag
.ce
If Timer T1 runs out waiting for the acknowledgement from the DTE for an
I\ frame transmitted, the DCE will enter the timer recovery condition,
add
.ce
one to its transmission attempt variable and set an internal variable \fIx\fR
.ce
to
.ce
the current value of its send state variable V(S). The DCE will then restart
.ce
Timer T1, set its send state variable V(S) to the last value of N(R) received
from the DTE and retransmit the corresponding I\ frame with the P bit set
to\ 1, or transmit an appropriate supervisory command frame (RR, RNR or
REJ) with the P\ bit set to\ 1.
.ce
.parag
.ce

.ce
The timer recovery condition is cleared when the DCE receives a valid supervisory
frame with the F\ bit\ set to\ 1.
.ce
.parag
.ce
If, while in the timer recovery condition, the DCE correctly receives a
supervisory frame with the F bit set to\ 1 and with the N(R) within the
range from its current send state variable V(S) to \fIx\fR
.ce
included, it will clear the
.ce
timer recovery condition (including stopping Timer\ T1) and set its send
state variable V(S) to the value of the received N(R), and may then resume
with
.ce
I\ frame transmission or retransmission, as appropriate.
.ce
.parag
.ce
If, while in the timer recovery condition, the DCE correctly receives an
I or supervisory frame with the P/F bit set to\ 0 and with a valid N(R)
(see \(sc\ 2.3.4.9), it will not clear the timer recovery condition. The
value of the
.ce
received N(R) may be used to update the send state variable V(S). However,
the DCE may decide to keep the last transmitted I\ frame in store (even
if it is
.ce
acknowledged) in order to be able to retransmit it with the P bit set to\
1 when Timer\ T1 runs out at a later time.
.ce
.parag
.ce

.ce
If the received supervisory frame with the P/F bit set to\ 0 is an REJ
frame with a valid N(R), the DCE may either immediately initiate
.ce
(re)transmission from the value of the send state variable V(S), or it may
.ce
ignore the request for retransmission and wait until the supervisory frame
with the F bit set to\ 1 is received before initiating (re)transmission
of frames
.ce
from the value identified in the N(R) field of the supervisory frame with
the F\ bit set to\ 1. In the case of immediate retransmission, in order
to prevent
.ce
duplicate retransmissions following the clearance of the timer recovery
.ce
condition, the DCE shall inhibit retransmission of a specific I\ frame [same
.ce
N(R) in the same numbering cycle] if the DCE has retransmitted that I\
frame as the result of a received REJ frame with the P/F bit set to\ 0.
.ce
.parag
.ce
If, while in the timer recovery condition, the DCE receives a REJ
.ce
command with the P bit set to\ 1, the DCE will respond immediately with an
.ce
appropriate supervisory response with the F bit set to\ 1. The DCE may
then use the value of the N(R) in the REJ command to update the send state
variable
.ce
V(S), and may either immediately begin (re)transmission from the value N(R)
.ce
indicated in the REJ frame or ignore the request for retransmission and wait
.ce
until the supervisory frame with the F bit set to\ 1 is received before
.ce
initiating (re)transmission of I\ frames from the value identified in the
N(R) field of the supervisory frame with the F\ bit set to\ 1. In the case
of
.ce
immediate retransmission, in order to prevent duplicate retransmissions
.ce
following the clearance of the timer recovery condition, the DCE shall
inhibit retransmission of a specific I\ frame [same N(R) in the same numbering
cycle]
.ce
if the DCE has retransmitted that I\ frame as the result of the received REJ
.ce
command with the P\ bit set to\ 1.
.ce
.parag
.ce
If Timer T1 runs out in the timer recovery condition, and no I or
.ce
supervisory frame with the P/F bit set to 0 and with a valid N(R) has been
.ce
received, or no REJ command with the P bit set to\ 1 and with a valid N(R)
has been received, the DCE will add one to its transmission attempt variable,
.ce
restart Timer\ T1, and either retransmit the I frame sent with the P bit set
.ce
to\ 1 or transmit an appropriate supervisory command with the P bit set to\ 1.
.ce
.parag
.ce

.ce
If the transmission attempt variable is equal to N2, the DCE will
.ce
initiate a data link resetting procedure as described in \(sc\ 2.4.7.2
below, or
.ce
will transmit a DM response to ask the DTE to initiate a data link set\(hyup
.ce
procedure as described in \(sc\ 2.4.4.1 above and enter the disconnected
phase. N2 is a system parameter (see \(sc\ 2.4.8.4 below).
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ Although the DCE may implement the internal variable \fIx\fR
.ce
,
.ce
other mechanisms do exist that achieve the identical function.
.ce
.parag
.ce
2.4.6
.ce
\fILAPB conditions for \fR
.ce
\fIdata link resetting or data link\fR
.ce

.ce
\fIre\(hyinitialization\fR
.ce
\fI(data link set\(hyup)\fR
.ce
.parag
.ce
2.4.6.1
.ce
When the DCE receives, during the information transfer phase, a
.ce
frame which is not invalid (see \(sc\ 2.3.5.3) with one of the conditions
listed in \(sc\ 2.3.4.9 above, the DCE will request the DTE to initiate
a data link resetting procedure by transmitting an FRMR response to the
DTE as described in
.ce
\(sc\ 2.4.7.3.
.ce
.parag
.ce
2.4.6.2
.ce
When the DCE receives, during the information transfer phase, an FRMR response
from the DTE, the DCE will either initiate the data link
.ce
resetting procedures itself as described in \(sc\ 2.4.7.2 or return a DM
response to ask the DTE to initiate the data link set\(hyup (initialization)
procedure as described in \(sc\ 2.4.4.1. After transmitting a DM response,
the DCE will enter
.ce
the disconnected phase as described in \(sc\ 2.4.4.4.2.
.ce
.parag
.ce
2.4.6.3
.ce
When the DCE receives, during the information transfer phase, a UA response,
or an unsolicited response with the F bit set to\ 1, the DCE may
.ce
either initiate the data link resetting procedures itself as described in
.ce
\(sc\ 2.4.7.2, or return a DM response to ask the DTE to initiate the data link
.ce
set\(hyup (initialization) procedure as described in \(sc\ 2.4.4.1. After
transmitting a DM response, the DCE will enter the disconnected phase as
described in
.ce
\(sc\ 2.4.4.4.2.
.ce
.parag
.ce
2.4.6.4
.ce
When the DCE receives, during the information transfer phase, a DM response
from the DTE, the DCE will either initiate the data link set\(hyup
.ce
(initialization) procedures itself as described in \(sc\ 2.4.4.1, or return
a DM
.ce
response to ask the DTE to initiate the data link set\(hyup (initialization)
.ce
procedures as described in \(sc\ 2.4.4.1. After transmitting a DM response,
the DCE will enter the disconnected phase as described in \(sc\ 2.4.4.4.2.
.ce
.parag
.ce

.ce
2.4.7
.ce
\fILAPB procedure for data link resetting\fR
.ce
.parag
.ce
2.4.7.1
.ce
The data link resetting procedure is used to initialize both
.ce
directions of information transfer according to the procedure described
below. The data link resetting procedure only applies during the information
transfer phase.
.ce
.parag
.ce
2.4.7.2
.ce
Either the DTE or the DCE may initiate the data link resetting
.ce
procedure. The data link resetting procedure indicates a clearance of a DCE
.ce
and/or DTE busy condition, if present.
.ce
.parag
.ce
The DTE shall initiate a data link resetting by transmitting an
.ce
SABM/SABME command to the DCE. If, upon correct receipt of the SABM/SABME
.ce
command, the DCE determines that it can continue in the information transfer
.ce
phase, it will return a UA response to the DTE, will reset its send and
receive state variables V(S) and V(R) to zero, and will remain in the information
.ce
transfer phase. If, upon correct receipt of the SABM/SABME command, the DCE
.ce
determines that it cannot remain in the information transfer phase, it will
.ce
return a DM response as a denial to the resetting request and will enter the
.ce
disconnected phase.
.ce
.parag
.ce
The DCE will initiate a data link resetting by transmitting an
.ce
SABM/SABME command to the DTE and starting its Timer\ T1 (see \(sc\ 2.4.8.1
below). Upon reception of a UA response from the DTE, the DCE will reset
its send and receive state variables V(S) and V(R) to zero, will stop its
Timer\ T1, and will remain in the information transfer phase. Upon reception
of a DM response from the DTE as a denial to the data link resetting request,
the DCE will stop its Timer\ T1 and will enter the disconnected phase.
.ce
.parag
.ce
The DCE, having sent an SABM/SABME command, will ignore and discard
.ce
any frames received from the DTE except an SABM/SABME or DISC command,
or a UA or DM response. The receipt of an SABM/SABME or DISC command from
the DTE will result in a collision situation that is resolved per \(sc\
2.4.4.5 above. Frames
.ce
other than the UA or DM response sent in response to a received SABM/SABME
or DISC command will be sent only after the data link is reset and if no
.ce
outstanding SABM/SABME command exists.
.ce
.parag
.ce

.ce
After the DCE sends the SABM/SABME command, if a UA or DM response is not
received correctly, Timer\ T1 will run out in the DCE. The DCE will then
.ce
resend the SABM/SABME command and will restart Timer\ T1. After N2 attempts
to reset the data link, the DCE will initiate appropriate higher layer
recovery
.ce
action and will enter the disconnected phase. The value of N2 is defined in
.ce
\(sc\ 2.4.8.4 below.
.ce
.parag
.ce
2.4.7.3
.ce
The DCE may ask the DTE to reset the data link by transmitting an FRMR
response (see \(sc\ 2.4.6.1 above). After transmitting an FRMR response,
.ce
the DCE will enter the frame rejection condition.
.ce
.parag
.ce
The frame rejection condition is cleared when the DCE receives an SABM/SABME
command, a DISC command, a FRMR response, or a DM response; or if
.ce
the DCE transmits an SABM/SABME command, a DISC command, or a DM response.
.ce
Other commands received while in the frame rejection condition will cause
the DCE to retransmit the FRMR response with the same information field
as
.ce
originally transmitted.
.ce
.parag
.ce
The DCE may start Timer\ T1 on transmission of the FRMR response. If
.ce
Timer\ T1 runs out before the frame rejection condition is cleared, the
DCE may retransmit the FRMR response, and restart T1. After N2 attempts
(time outs) to get the DTE to reset the data link, the DCE may reset the
data link itself as described in \(sc\ 2.4.7.2 above. The value of N2 is
defined in \(sc\ 2.4.8.4 below.
.ce
.parag
.ce

.ce
In the frame rejection condition, I frames and supervisory frames will
not be transmitted by the DCE. Also, received I frames and supervisory
frames will be discarded by the DCE except for the observance of a P bit
set to\ 1.
.ce
When an additional FRMR response must be transmitted by the DCE as a result
of the receipt of a P\ bit set to 1 while Timer\ T1 is running, Timer\
T1 will
.ce
continue to run. Upon reception of an FRMR response (even during a frame
.ce
rejection condition), the DCE will initiate a resetting procedure by
.ce
transmitting an SABM/SABME command as described in \(sc\ 2.4.7.2 above, or will
.ce
transmit a DM response to ask the DTE to initiate the data link set\(hyup
.ce
procedure as described in \(sc\ 2.4.4.1 and enter the disconnected phase.
.ce
.parag
.ce
2.4.8
.ce
\fIList of \fR
.ce
\fILAPB system parameters\fR
.ce
.parag
.ce
The DCE and DTE system parameters are as follows:
.ce
.parag
.ce

.ce
2.4.8.1
.ce
\fITimer\fR
.ce
\fIT1\fR
.ce
.parag
.ce
The value of the DTE Timer T1 system parameter may be different
.ce
than the value of the DCE Timer T1 system parameter. These values shall
be made known to both the DTE and the DCE, and agreed to for a period of
time by both the DTE and the DCE.
.ce
.parag
.ce
The period of Timer T1, at the end of which retransmission of a frame may
be initiated (see \(sc\ 2.4.4 and \(sc\ 2.4.5 above for the DCE), shall
take into account whether T1 is started at the beginning or the end of
the transmission of a frame.
.ce
.parag
.ce
The proper operation of the procedure requires that the transmitter's (DCE
or DTE) Timer\ T1 be greater than the maximum time between transmission
of a frame (SABM/SABME, DISC, I\ or supervisory command, or DM or FRMR
response)
.ce
and the reception of the corresponding frame returned as an answer to that
.ce
frame (UA, DM or acknowledging frame). Therefore, the receiver (DCE or DTE)
.ce
should not delay the response or acknowledging frame returned to one of the
.ce
above frames by more than a value\ T2, where T2 is a system parameter (see
.ce
\(sc\ 2.4.8.2).
.ce
.parag
.ce

.ce
The DCE will not delay the response or acknowledging frame returned to
one of the above DTE frames by more than a period\ T2.
.ce
.parag
.ce
2.4.8.2
.ce
\fIParameter T2\fR
.ce
.parag
.ce
The value of the DTE parameter T2 may be different than the value of the
DCE parameter T2. These values shall be made known to both the DTE and
the DCE, and agreed to for a period of time by both the DTE and the DCE.
.ce
.parag
.ce

.ce
The period of parameter T2 shall indicate the amount of time
.ce
available at the DCE or DTE before the acknowledging frame must be initiated
in order to ensure its receipt by the DTE or DCE, respectively, prior to
Timer\ T1 running out at the DTE or DCE (parameter\ T2\ <\ Timer\ T1).
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ The period of parameter T2 shall take into account the
.ce
following timing factors: the transmission time of the acknowledging frame,
the propagation time over the access data link, the stated processing times
at the DCE and the DTE, and the time to complete the transmission of the
frame(s) in the DCE or DTE transmit queue that are neither displaceable
or modifiable in an orderly manner.
.ce
.parag
.ce

.ce
Given a value for Timer T1 for the DTE or DCE, the value of parameter T2
at the DCE or DTE, respectively, must be no larger than T1 minus\ 2 times
the propagation time over the access data link, minus the frame processing
time at the DCE, minus the frame processing time at the DTE, and minus
the transmission time of the acknowledging frame by the DCE or DTE, respectively.
.ce
.parag
.ce

.ce
2.4.8.3
.ce
\fITimer T3\fR
.ce
.parag
.ce
The DCE shall support a Timer T3 system parameter, the value of
.ce
which shall be made known to the DTE.
.ce
.parag
.ce
The period of Timer T3, at the end of which an indication of an
.ce
observed excessively long idle channel state condition is passed to the
Packet Layer, shall be sufficiently greater than the period of the DCE
Timer T1
.ce
(i.e.\ T3\ >\ T1) so that the expiration of T3 provides the desired level of
.ce
assurance that the data link channel is in a non\(hyactive, non\(hyoperational
state, and is in need of data link set\(hyup before normal data link operation
can
.ce
resume.
.ce
.parag
.ce
2.4.8.4
.ce
\fIMaximum number of attempts\fR
.ce
\fIto complete a\fR
.ce

.ce
\fItransmission N2\fR
.ce
.parag
.ce

.ce
The value of the DTE N2 system parameter may be different than the value
of the DCE N2 system parameter. These values shall be made known to both
the DTE and the DCE, and agreed to for a period of time by both the DTE
and the DCE.
.ce
.parag
.ce
The value of N2 shall indicate the maximum number of attempts made by the
DCE or DTE to complete the successful transmission of a frame to the DTE
or DCE, respectively.
.ce
.parag
.ce
2.4.8.5
.ce
\fIMaximum number of bits in an I frame N1\fR
.ce
.parag
.ce
The value of the DTE N1 system parameter may be different than the value
of the DCE N1 system parameter. These values shall be made known to both
the DTE and the DCE.
.ce
.parag
.ce
The values of N1 shall indicate the maximum number of bits in an
.ce
I\ frame (excluding flags and 0\ bits inserted for transparency) that the
DCE or DTE is willing to accept from the DTE or DCE, respectively.
.ce
.parag
.ce
In order to allow for universal operation, a DTE should support a
.ce
value of DTE N1 which is not less than 1080\ bits (135\ octets). DTEs should
be aware that the network may transmit longer packets (see \(sc\ 5.2),
that may result in a data link layer problem.
.ce
.parag
.ce

.ce
All networks shall offer to a DTE which requires it, a value of DCE N1
which is greater than or equal to 2072\ bits (259\ octets) plus the length
of the address, control and FCS fields at the DTE/DCE interface, and greater
than or equal to the maximum length of the data packets which may cross
the DTE/DCE
.ce
interface plus the length of the address, control and FCS fields at the
DTE/DCE interface.
.ce
.parag
.ce
Appendix II provides a description of how the values stated above are  derived.
.ce
.parag
.ce
2.4.8.6
.ce
\fIMaximum number of \fR
.ce
\fIoutstanding I frames k\fR
.ce
.parag
.ce
The value of the DTE k system parameter shall be the same as the
.ce
value of the DCE k system parameter. This value shall be agreed to for a
.ce
period of time by both the DTE and the DCE.
.ce
.parag
.ce
The value of k shall indicate the maximum number of sequentially
.ce
numbered I\ frames that the DTE or DCE may have outstanding
.ce
(i.e.\ unacknowledged) at any given time. The value of k shall never exceed
.ce
seven for modulo\ 8 operation, or one hundred and twenty\(hyseven for modulo\
128
.ce
operation. All networks (DCEs) shall support a value of seven. Other values
of k (less than and greater than seven) may also be supported by networks
.ce
(DCEs).
.ce
.parag
.ce
.line
.ce
\fBMONTAGE : \(sc 2.5 de cette Recommandation sur le reste de cette page\fR
.ce
.parag
.ce

.ce
.line
.ad r
\fBTable 5/X.25 [T5.25], p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.ce
.line
.ce
\fBTable 6/X.25 [T6.25], p.\fR
.ce
2.3.4.2
.ce
\fIReceive ready (RR)\fR
.ce
\fI command and response\fR
.ce
.parag
.ce
The receive ready (RR) supervisory frame is used by the DCE
.ce
or DTE to:
.ce
.parag
.ce

.ce
1)
.ce
indicate it is ready to receive an I frame; and
.ce
.parag
.ce
2)
.ce
acknowledge previously received I frames numbered up to and
.ce
including N(R)\ \(em\ 1.
.ce
.parag
.ce
An RR frame may be used to indicate the clearance of a busy
.ce
condition that was reported by the earlier transmission of an RNR frame
by that same station (DCE or\ DTE). In addition to indicating the DCE or
DTE status, the RR command with the P\ bit set to\ 1 may be used by the
DCE or DTE to ask for the status of the DTE or DCE, respectively.
.ce
.parag
.ce
2.3.4.3
.ce
\fIReceive not ready (RNR) command and response\fR
.ce
.parag
.ce
The receive not ready (RNR) supervisory frame is used by the DCE or DTE
to indicate a busy condition; i.e.\ temporary inability to accept
.ce
additional incoming I\ frames. I\ frames numbered up to and including N(R)\
\(em\ 1
.ce
are acknowledged. I\ frame N(R) and any subsequent I\ frames received, if any,
.ce
are not acknowledged; the acceptance status of these I\ frames will be
indicated in subsequent exchanges.
.ce
.parag
.ce
In addition to indicating the DCE or DTE status, the RNR command with the
P bit set to\ 1 may be used by an DCE or DTE to ask for the status of the
.ce
DTE or DCE, respectively.
.ce
.parag
.ce
2.3.4.4
.ce
\fIReject (REJ) command and response\fR
.ce
.parag
.ce
The reject (REJ) supervisory frame is used by the DCE or DTE to request
transmission of I\ frames starting with the frame numbered N(R).
.ce
I\ frames numbered N(R)\ \(em\ 1 and below are acknowledged. Additional
I\ frames
.ce
pending initial transmission may be transmitted following the retransmitted
.ce
I\ frame(s).
.ce
.parag
.ce

.ce
Only one REJ exception condition for a given direction of information transfer
may be established at any time. The REJ exception condition is cleared
(reset) upon the receipt of an I\ frame with an N(S) equal to the N(R)
of the
.ce
REJ\ frame.
.ce
.parag
.ce
An REJ frame may be used to indicate the clearance of a busy condition
that was reported by the earlier transmission of an RNR frame by that same
.ce
station (DCE or\ DTE). In addition to indicating the DCE or DTE status, the
.ce
REJ\ command with the P bit set to\ 1 may be used by the DCE or\ DTE to ask for
.ce
the status of the DTE or\ DCE, respectively.
.ce
.parag
.ce
\fR
.ce
2.3.4.5
.ce
\fISet asynchronous balanced mode (SABM)\fR
.ce
\fIcommand/\fR
.ce
\fISet asynchronous balanced mode extended (SABME)\fR
.ce

.ce
\fIcommand (subscription time option)\fR
.ce
.parag
.ce

.ce
The SABM unnumbered command is used to place the addressed DCE or DTE in
an asynchronous balanced mode (ABM) information transfer phase where all
command/response control fields will be one\ octet in length.
.ce
.parag
.ce
The SABME unnumbered command is used to place the addressed DCE or DTE
in an asynchronous balanced mode\ (ABM) information transfer phase where
.ce
numbered command/response control fields will be two octets in length, and
.ce
unnumbered command/response control fields will be one octet in length.
.ce
.parag
.ce

.ce
No information field is permitted with the SABM or SABME command. The transmission
of a SABM/SABME command indicates the clearance of a busy
.ce
condition that was reported by the earlier trans
.ce
mission\ of an RNR\ frame by   that
.ce
same station (DCE or\ DTE). The DCE or DTE confirms acceptance of SABM/SABME
.ce
[modulo\ 8 (basic) operation/modulo\ 128 (extended) operation] command by the
.ce
transmission, at the first opportunity, of a UA\ response. Upon acceptance of
.ce
this command, the DCE or DTE send state variable\ V(S) and receive state
.ce
variable V(R) are set to\ 0.
.ce
.parag
.ce
Previously transmitted I\ frames that are unacknowledged when this
.ce
command is actioned remain unac
.ce
knowledged.\ It is the responsibility of a
.ce
higher layer (e.g.\ Packet Layer or MLP) to recover from the possible loss of
.ce
the contents (e.g.\ packets) of such I\ frames.
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ The mode of operation of a data link [basic (modulo 8) or
.ce
extended (modulo\ 128)] is determined at subscription time and is only
changed by going through a new subscription process.
.ce
.parag
.ce
2.3.4.6
.ce
\fIDisconnect (DISC)\fR
.ce
\fI command\fR
.ce
.parag
.ce
The DISC unnumbered command is used to terminate the mode
.ce
previously set. It is used to inform the DCE or DTE receiving the DISC
command that the DTE or DCE sending the DISC command is suspending operation.
No
.ce
information field is permitted with the DISC command. Prior to actioning the
.ce
DISC command, the DCE or DTE receiving the DISC command confirms the acceptance
of the DISC command by the transmission of a UA response. The DTE or DCE
.ce
sending the DISC command enters the disconnected phase when it receives the
.ce
acknowledging UA response.
.ce
.parag
.ce
Previously transmitted I frames that are unacknowledged when this
.ce
command is actioned remain unacknowledged. It is the responsibility of
a higher layer (e.g.\ Packet Layer or MLP) to recover from the possible
loss of the
.ce
contents (e.g.,\ packets) of such I frames.
.ce
.parag
.ce
2.3.4.7
.ce
\fIUnnumbered acknowledgement (UA)\fR
.ce
\fI response\fR
.ce
.parag
.ce
The UA unnumbered response is used by the DCE or DTE to acknowledge the
receipt and acceptance of the mode\(hysetting commands. Received mode\(hysetting
commands are not actioned until the UA response is transmitted. The
.ce
transmission of a UA response indicates the clearance of a busy condition
that was reported by the earlier transmission of an RNR frame by that same
station (DCE or DTE). No information field is permitted with the UA response.
.ce
.parag
.ce

.ce
2.3.4.8
.ce
\fIDisconnected mode (DM)\fR
.ce
\fI response\fR
.ce
.parag
.ce
The DM unnumbered response is used to report a status where the DCE or
DTE is logically disconnected from the data link, and is in the disconnected
phase. The DM response may be sent to indicate that the DCE or DTE has
entered the disconnected phase without benefit of having received a DISC
command, or, if sent in response to the reception of a mode setting command,
is sent to
.ce
inform the DTE or DCE that the DCE or DTE, respectively, is still in the
.ce
disconnected phase and cannot execute the set mode command. No information
.ce
field is permitted with the DM response.
.ce
.parag
.ce
A DCE or DTE in a disconnected phase will monitor received commands
.ce
and will react to an SABM/SABME command as outlined in \(sc\ 2.4.4 below,
and will respond with a DM response with the F bit set to\ 1 to any other
command
.ce
received with the P\ bit set to\ 1.
.ce
.parag
.ce
2.3.4.9
.ce
\fIFrame reject (FRMR)\fR
.ce
\fI response\fR
.ce
.parag
.ce
The FRMR unnumbered response is used by the DCE or DTE to report an error
condition not recoverable by retransmission of the identical frame;
.ce
i.e.\ at least one of the following conditions, which results from the
receipt of a valid frame:
.ce
.parag
.ce
1)
.ce
the receipt of a command or response control field that is
.ce
undefined or not implemented;
.ce
.parag
.ce
2)
.ce
the receipt of an I frame with an information field which
.ce
exceeds the maximum established length;
.ce
.parag
.ce
3)
.ce
the receipt of an invalid N(R); or
.ce
.parag
.ce
4)
.ce
the receipt of a frame with an information field which is
.ce
not permitted or the receipt of a supervisory or unnumbered
.ce
frame with incorrect length.
.ce
.parag
.ce
An undefined or not implemented control field is any of the
.ce
control field encodings that are not identified in Tables\ 5/X.25 or\ 6/X.25.
.ce
.parag
.ce

.ce
A valid N(R) must be within the range from the lowest send sequence
.ce
number N(S) of the still unacknowledged frame(s) to the current DCE send
state variable inclusive (or to the current internal variable \fIx\fR
.ce
if the DCE is in
.ce
the timer recovery condition as described in \(sc\ 2.4.5.9).
.ce
.parag
.ce

.ce
An information field which immediately follows the control field, and consists
of\ 3 or 5\ octets [modulo\ 8 (basic) operation or modulo\ 128 (extended)
operation, respectively], is returned with this response and provides the
.ce
reason for the FRMR response. These formats are given in Tables\ 7/X.25
.ce
and\ 8/X.25.
.ce
.parag
.ce
2.3.5
.ce
\fIException condition reporting and recovery\fR
.ce
.parag
.ce
The error recovery procedures which are available to effect
.ce
recovery following the detection/occurrence of an
.ce
exception condition
.ce
 at the Data Link Layer are described below. Exception conditions described
are those situations which may occur as the result of transmission errors,
DCE or DTE malfunction, or operational situations.
.ce
.parag
.ce
2.3.5.1
.ce
\fIBusy condition\fR
.ce
.parag
.ce
The busy condition results when the DCE or DTE is temporarily
.ce
unable to continue to receive I frames due to internal constraints,
.ce
e.g.\ receive buffering limitations. In this case an RNR frame is transmitted
.ce
from the busy DCE or DTE. I\ frames pending transmission may be transmitted
from the busy DCE or DTE prior to or following the RNR\ frame.
.ce
.parag
.ce
An indication that the busy condition has cleared is communicated by the
transmission of a UA (only in response to a SABM/SABME command), RR, REJ
or SABM/SABME (modulo\ 8/modulo\ 128) frame.
.ce
.parag
.ce
.line
.ce
\fBTable 7/X.25 [T7.25], p.\fR
.ce
.line
.ce
\fBTable 8/X.25 [T8.25], p.\fR
.ce
2.3.5.2
.ce
\fIN(S) sequence error condition\fR
.ce
.parag
.ce
The information field of all I frames received whose N(S) does not equal
the receive state variable V(R) will be discarded.
.ce
.parag
.ce
An N(S) sequence error exception condition occurs in the receiver when
an I\ frame received contains an N(S) which is not equal to the receive
state
.ce
variable V(R) at the receiver. The receiver does not acknowledge (increment
its receive state variable) the I\ frame causing the sequence error, or
any I\ frame which may follow, until an I\ frame with the correct N(S)
is received.
.ce
.parag
.ce

.ce
A DCE or DTE which receives one or more valid I frames having sequence
errors or subsequent supervisory frames (RR, RNR and REJ) shall accept
the
.ce
control information contained in the N(R) field and the P or F bit to perform
data link control functions; e.g.\ to receive acknowledgement of previously
.ce
transmitted I\ frames and to cause the DCE or DTE to respond (P bit set to 1).
.ce
.parag
.ce

.ce
The means specified in \(sc\(sc\ 2.3.5.2.1 and 2.3.5.2.2 shall be available
for initiating the retransmission of lost or errored I\ frames following
the
.ce
occurrence of an N(S) sequence error condition.
.ce
.parag
.ce
2.3.5.2.1
.ce
\fIREJ recovery\fR
.ce
.parag
.ce
The REJ frame is used by a receiving DCE or DTE to initiate a
.ce
recovery (retransmission) following the detection of an N(S) sequence error.
.ce
.parag
.ce

.ce
With respect to each direction of transmission on the data link, only one
\*Qsent REJ\*U exception condition from a DCE or DTE, to a DTE or DCE,
is
.ce
established at a time. A \*Qsent REJ\*U exception condition is cleared when the
.ce
requested I\ frame is received.
.ce
.parag
.ce

.ce
A DCE or DTE receiving a REJ frame initiates sequential
.ce
(re\(hy)transmission of I\ frames starting with the I\ frame indicated
by the N(R) contained in the REJ frame. The retransmitted frames may contain
an N(R) and a P bit that are updated from, and therefore different from,
the ones contained in the originally transmitted I\ frames.
.ce
.parag
.ce
2.3.5.2.2
.ce
\fITime\(hyout recovery\fR
.ce
.parag
.ce
If a DCE or DTE, due to a transmission error, does not receive (or receives
and discards) a single I\ frame or the last I\ frame(s) in a sequence of
I\ frames, it will not detect an N(S) sequence error condition and, therefore,
will not transmit a REJ frame. The DTE or DCE which transmitted the
.ce
unacknowledged I\ frame(s) shall, following the completion of a system
specified time\(hyout period (see \(sc\(sc\ 2.4.5.1 and 2.4.5.9 below),
take appropriate recovery action to determine at which I\ frame retransmission
must begin. The
.ce
retransmitted frame(s) may contain an N(R) and a P bit that is updated from,
.ce
and therefore different from, the ones contained in the originally transmitted
frame(s).
.ce
.parag
.ce
2.3.5.3
.ce
\fIInvalid frame\fR
.ce
\fI condition\fR
.ce
.parag
.ce
Any frame which is invalid will be discarded, and no action is
.ce
taken as the result of that frame. An invalid frame is defined as one
.ce
which:
.ce
.parag
.ce
a)
.ce
is not properly bounded by two flags;
.ce
.parag
.ce
b)
.ce
in basic (modulo 8) operation, contains fewer than 32 bits
.ce
between flags; in extended (modulo\ 128) operation, contains
.ce
fewer than 40\ bits between flags of frames that contain sequence
.ce
numbers or 32\ bits between flags of frames that do not contain
.ce
sequence numbers;
.ce
.parag
.ce
c)
.ce
contains a Frame Check Sequence (FCS) error; or
.ce
.parag
.ce
d)
.ce
contains an address other than A or B (for single link
.ce
operation) or other than C or D (for multilink operation).
.ce
.parag
.ce

.ce
For those networks that are octet aligned, a detection of
.ce
non\(hyoctet alignment may be made at the Data Link Layer by adding a frame
.ce
validity check that requires the number of bits between the opening flag and
.ce
the closing flag, excluding bits inserted for transparency, to be an integral
number of octets in length, or the frame is considered invalid.
.ce
.parag
.ce
2.3.5.4
.ce
\fIFrame rejection\fR
.ce
\fI condition\fR
.ce
.parag
.ce
A frame rejection condition is established upon the receipt of an error\(hyfree
frame with one of the conditions listed in \(sc\ 2.3.4.9 above.
.ce
.parag
.ce

.ce
At the DCE or DTE, this frame rejection exception condition is
.ce
reported by an FRMR response for appropriate DTE or DCE action, respectively.
Once a DCE has established such an exception condition, no additional I\
frames are accepted until the condition is reset by the DTE, except for
examination of the P bit. The FRMR response may be repeated at each opportunity,
as specified in \(sc\ 2.4.7.3, until recovery is effected by the DTE, or
until the DCE initiates its own recovery in case the DTE does not respond.
.ce
.parag
.ce
2.3.5.5
.ce
\fIExcessive idle channel state condition on incoming\fR
.ce

.ce
\fIchannel\fR
.ce
.parag
.ce
Upon detection of an idle channel state condition (see \(sc\ 2.2.12.2 above)
on the incoming channel, the DCE shall wait for a period\ T3 (see
.ce
\(sc\ 2.4.8.3 below) without taking any specific action, waiting for detection
of a return to the active channel state (i.e.,\ detection of at least one
flag
.ce
sequence). After the period\ T3, the DCE shall notify the higher layer
(e.g.\ the Packet Layer or the MLP) of the excessive idle channel state
condition, but
.ce
shall not take any action that would preclude the DTE from establishing the
.ce
data link by normal data link set\(hyup procedures.
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ Other actions to be taken by the DCE at the Data Link Layer upon
expiration of period\ T3 is a subject for further study.
.ce
.parag
.ce

.ce
2.4
.ce
\fIDescription of the \fR
.ce
\fILAPB procedure\fR
.ce
.parag
.ce
2.4.1
.ce
\fILAPB basic and extended modes of operation\fR
.ce
.parag
.ce
In accordance with the system choice made by the DTE at
.ce
subscription time, the DCE will either support modulo\ 8 (basic) operation or
.ce
will support modulo\ 128 (extended) operation. Changing from basic operation
to extended operation, or vice versa, in the DCE requires resubscription
by the
.ce
DTE for the desired service, and is not supported dynamically.
.ce
.parag
.ce
Table 5/X.25 indicates the command and response control field formats used
with the basic (modulo\ 8) service. The mode\(hysetting command employed
to
.ce
initialize (set up) or reset the basic mode is the SABM command. Table
6/X.25 indicates the command and response control field formats used with
the extended (modulo\ 128) service. The mode\(hysetting command employed
to initialize (set up) or reset the extended mode is the SABME command.
.ce
.parag
.ce
2.4.2
.ce
\fILAPB procedure for addressing\fR
.ce
.parag
.ce
The address field identifies a frame as either a command or a
.ce
response. A command frame contains the address of the DCE or DTE to which
the command is being sent. A response frame contains the address of the
DCE or DTE sending the frame.
.ce
.parag
.ce
In order to allow differentiation between single link operation and
.ce
the optional multilink operation for diagnostic and/or maintenance reasons,
.ce
different address pair encodings are assigned to data links operating with
.ce
multilink procedure compared to data links operating with the single link
.ce
procedure.
.ce
.parag
.ce
Frames containing commands transferred from the DCE to the DTE will
.ce
contain the address\ A for the single link operation and address\ C for the
.ce
multilink operation.
.ce
.parag
.ce
Frames containing responses transferred from the DCE to the DTE will contain
the address\ B for the single link operation and address\ D for the
.ce
multilink operation.
.ce
.parag
.ce

.ce
Frames containing commands transferred from the DTE to the DCE shall contain
the address\ B for the single link operation and address\ D for the
.ce
multilink operation.
.ce
.parag
.ce
Frames containing responses transferred from the DTE to the DCE shall contain
the address\ A for the single link operation and address\ C for the
.ce
multilink operation.
.ce
.parag
.ce
These addresses are coded as follows:
.ce
.parag
.ce
Address
.ce
1\ 2\ 3\ 4\ 5\ 6\ 7\ 8
.ce
.parag
.ce
Single link operation
.ce
\ \ A
.ce
1\ 1\ 0\ 0\ 0\ 0\ 0\ 0
.ce
.line
.ce
\ \ B
.ce
1\ 0\ 0\ 0\ 0\ 0\ 0\ 0
.ce
.parag
.ce
Multilink operation
.ce
\ \ C
.ce
1\ 1\ 1\ 1\ 0\ 0\ 0\ 0
.ce
.line
.ce
\ \ D
.ce
1\ 1\ 1\ 0\ 0\ 0\ 0\ 0
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ The DCE will discard all frames received with an address other
than\ A or\ B (single link operation), or\ C or\ D (multilink operation).
.ce
.parag
.ce
2.4.3
.ce
\fILAPB procedure for the use of the P/F bit\fR
.ce
.parag
.ce
The DCE or DTE receiving an SABM/SABME, DISC, supervisory command or I\
frame with the P\ bit set to\ 1 will set the F\ bit to\ 1 in the next response
frame it transmits.
.ce
.parag
.ce
The response frame returned by the DCE to an SABM/SABME or DISC
.ce
command with the P\ bit set to\ 1 will be a UA or DM response with the
F\ bit set to\ 1. The response frame returned by the DCE to an I\ frame
with the P\ bit set to\ 1, received during the information transfer phase,
will be an RR, REJ, RNR or FRMR response with the F\ bit set to\ 1. The
response frame returned by the
.ce
DCE to a supervisory command with the P\ bit set to\ 1, received during the
.ce
information transfer phase, will be an RR, REJ, RNR or FRMR response with
the F\ bit set to\ 1. The response frame returned by the DCE to an I\ frame
or
.ce
supervisory frame with the P\ bit set to\ 1, received during the disconnected
.ce
phase, will be a DM response with the F\ bit set to\ 1.
.ce
.parag
.ce
The P bit may be used by the DCE in conjunction with the timer
.ce
recovery condition (see \(sc\ 2.4.5.9 below).
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ Other use of the P bit by the DCE is a subject for further
.ce
study.
.ce
.parag
.ce
2.4.4
.ce
\fILAPB procedure for data link set\(hyup and disconnection\fR
.ce
.parag
.ce

.ce
2.4.4.1
.ce
\fIData link set\(hyup\fR
.ce
.parag
.ce
The DCE will indicate that it is able to set up the data link by
.ce
transmitting contiguous flags (active channel state).
.ce
.parag
.ce
Either the DTE or the DCE may initiate data link set\(hyup. Prior to
.ce
initiation of data link set\(hyup, either the DCE or the DTE may initiate data
.ce
link
.ce
disconnection (see \(sc\ 2.4.4.3) for the purpose of insuring that the
DCE and the DTE are in the same phase. The DCE may also transmit an unsolicited
DM response to request the DTE to initiate data link set\(hyup.
.ce
.parag
.ce
The DTE shall initiate data link set\(hyup by transmitting an SABM/SABME
command to the DCE. If, upon receipt of the SABM/SABME command correctly,
the DCE determines that it can enter the information transfer phase, it
will return a UA response to the DTE, will reset its send and receive state
variables V(S) and V(R) to zero, and will consider that the data link is
set up. If, upon
.ce
receipt
.ce
of the SABM/SABME command correctly, the DCE determines that it cannot enter
.ce
the information transfer phase, it will return a DM response to the DTE as a
.ce
denial to the data link set\(hyup initialization and will consider that the
.ce
data link is
.ce
\fInot\fR
.ce
 set up. In order to avoid misinterpretation of the DM response received,
it is suggested that the DTE always sends its SABM/SABME command with the
P\ bit set to\ 1. Otherwise, it is not possible to differentiate a DM response
intended as a denial to data link set\(hyup from a DM response that is
issued in a separate unsolicited sense as a request for a mode\(hysetting
command (as described
.ce
in\ \(sc\ 2.4.4.4.2).
.ce
.parag
.ce
The DCE will initiate data link set\(hyup by transmitting an SABM/SABME
command to the DTE and starting its Timer\ T1 in order to determine when
too
.ce
much time has elapsed waiting for a reply (see \(sc\ 2.4.8.1 below). Upon
reception of a UA response from the DTE, the DCE will reset its send and
receive state
.ce
variables\ V(S) and\ V(R) to zero, will stop its Timer\ T1, and will consider
that the data link is set up. Upon reception of a DM response from the
DTE as a
.ce
denial
.ce
to the data link set\(hyup initialization, the DCE will stop its Timer\
T1 and will consider that the data link is \fInot\fR
.ce
set up.
.ce
.parag
.ce

.ce
The DCE, having sent the SABM/SABME command, will ignore and discard any
frames except an SABM/SABME or DISC command, or a UA or DM response
.ce
received from the DTE. The receipt of an SABM/SABME or DISC command from the
.ce
DTE will result in a collision situation that is resolved per \(sc\ 2.4.4.5
below. Frames other than the UA and DM responses sent in response to a
received
.ce
SABM/SABME or DISC command will be sent only after the data link is set
up and if no outstanding SABM/SABME command exists.
.ce
.parag
.ce
After the DCE sends the SABM/SABME command, if a UA or DM response is not
received correctly, Timer\ T1 will run out in the DCE. The DCE will then
.ce
resend the SABM/SABME command and will restart Timer\ T1. After transmission
of the SABM/SABME command N2 times by the DCE, appropriate higher layer
recovery action will be initiated. The value of N2 is defined in \(sc\
2.4.8.4 below.
.ce
.parag
.ce
2.4.4.2
.ce
\fIInformation transfer phase\fR
.ce
.parag
.ce
After having transmitted the UA response to the SABM/SABME command or having
received the UA response to a transmitted SABM/SABME command, the DCE will
accept and transmit I and supervisory frames according to the procedures
described in \(sc\ 2.4.5 below.
.ce
.parag
.ce

.ce
When receiving the SABM/SABME command while in the information
.ce
transfer phase, the DCE will conform to the data link resetting procedure
.ce
described in \(sc\ 2.4.7 below.
.ce
.parag
.ce
2.4.4.3
.ce
\fIData link disconnection\fR
.ce
.parag
.ce
The DTE shall initiate a disconnect of the data link by
.ce
transmitting a DISC command to the DCE. On correctly receiving a DISC command
in the information transfer phase, the DCE will send a UA response and
enter
.ce
the disconnected phase. On correctly receiving a DISC command in the
.ce
disconnected phase, the DCE will send a DM response and remain in the
.ce
disconnected phase. In order to avoid misinterpretation of the DM response
.ce
received, it is suggested that the DTE always sends its DISC command with
the P\ bit set to\ 1. Otherwise, it is not possible to differentiate a
DM response
.ce
intended as an indication that the DCE is already in the disconnected phase
.ce
from a DM response that is issued in a separate unsolicited sense as a
request for a mode\(hysetting command (as described in \(sc\ 2.4.4.4.2).
.ce
.parag
.ce

.ce
The DCE will initiate a disconnect of the data link by transmitting a DISC
command to the DTE and starting its Timer\ T1 (see \(sc\ 2.4.8.1 below).
Upon reception of an UA response from the DTE, the DCE will stop its Timer\
T1 and
.ce
will enter the disconnected phase. Upon reception of a DM response from
the DTE as an indication that the DTE was already in the disconnected phase,
the DCE
.ce
will stop its Timer\ T1 and will enter the disconnected phase.
.ce
.parag
.ce
The DCE, having sent the DISC command, will ignore and discard any
.ce
frames except an SABM/SABME or DISC command, or a UA or DM response received
.ce
from the DTE. The receipt of an SABM/SABME or DISC command from the DTE will
.ce
result in a collision situation that is resolved per \(sc\ 2.4.4.5 below.
.ce
.parag
.ce

.ce
After the DCE sends the DISC command, if a UA or DM response is not
.ce
received correctly, Timer\ T1 will run out in the DCE. The DCE will then
resend the DISC command and will restart Timer\ T1. After transmission
of the DISC
.ce
command N2 times by the DCE, appropriate higher layer recovery action will
be initiated. The value of N2 is defined in \(sc\ 2.4.8.4 below.
.ce
.parag
.ce
2.4.4.4
.ce
\fIDisconnected phase\fR
.ce
.parag
.ce
2.4.4.4.1
.ce
After having received a DISC command from the DTE and
.ce
returned a UA response to the DTE, or having received the UA response to a
.ce
transmitted DISC command, the DCE will enter the disconnected phase.
.ce
.parag
.ce
In the disconnected phase, the DCE may initiate data link set\(hyup. In
the disconnected phase, the DCE will react to the receipt of an SABM/SABME
command as described in \(sc\ 2.4.4.1 above and will transmit a DM response
in
.ce
answer to a received DISC command. When receiving any other command (defined,
or undefined or not implemented) with the P\ bit set to\ 1, the DCE will
transmit a DM response with the F\ bit set to\ 1. Other frames received
in the
.ce
disconnected phase will be ignored by the DCE.
.ce
.parag
.ce
2.4.4.4.2
.ce
When the DCE enters the disconnected phase after
.ce
detecting error conditions as listed in \(sc\ 2.4.6 below, or after an internal
.ce
malfunction, it may indicate this by sending a DM response rather than
a DISC command. In these cases, the DCE will transmit a DM response and
start its
.ce
Timer\ T1 (see \(sc\ 2.4.8.1 below).
.ce
.parag
.ce
If Timer T1 runs out before the reception of an SABM/SABME or DISC command
from the DTE, the DCE will retransmit the DM response and restart
.ce
Timer\ T1. After transmission of the DM response N2 times, the DCE will
remain in the disconnected phase and appropriate recovery actions will
be initiated. The value of N2 is defined in \(sc\ 2.4.8.4 below.
.ce
.parag
.ce
Alternatively, after an internal malfunction, the DCE may either
.ce
initiate a data link resetting procedure (see \(sc\ 2.4.7 below) or disconnect
the data link (see \(sc\ 2.4.4.3 above) prior to initiating a data link
set\(hyup
.ce
procedure (see \(sc\ 2.4.4.1 above).
.ce
.parag
.ce
2.4.4.5
.ce
\fICollision of unnumbered commands\fR
.ce
.parag
.ce
Collision situations shall be resolved in the following way:
.ce
.parag
.ce
2.4.4.5.1
.ce
If the sent and received unnumbered commands are the
.ce
same, the DCE and the DTE shall each send the UA response at the earliest
.ce
possible opportunity. The DCE shall enter the indicated phase either,
.ce
.parag
.ce

.ce
1)
.ce
after receiving the UA response,
.ce
.parag
.ce
2)
.ce
after sending the UA response, or
.ce
.parag
.ce
3)
.ce
after timing out waiting for the UA response having sent a
.ce
UA response.
.ce
.parag
.ce
In the case of 2) above, the DCE will accept a subsequent UA
.ce
response to the mode\(hysetting command it issued without causing an exception
.ce
condition if received within the time\(hyout interval.
.ce
.parag
.ce

.ce
2.4.4.5.2
.ce
If the sent and received unnumbered commands are
.ce
different, the DCE and the DTE shall each enter the disconnected phase and
.ce
issue a DM response at the earliest possible opportunity.
.ce
.parag
.ce
2.4.4.6
.ce
\fICollision of DM response with SABM/SABME or DISC\fR
.ce

.ce
\fIcommand\fR
.ce
.parag
.ce
When a DM response is issued by the DCE or DTE as an unsolicited
.ce
response to request the DTE or DCE, respectively, to issue a mode\(hysetting
.ce
command as described in \(sc\ 2.4.4.4, a collision between an SABM/SABME
or DISC
.ce
command and the unsolicited DM response may occur. In order to avoid
.ce
misinterpretation of the DM response received, the DTE always sends its
.ce
SABM/SABME or DISC command with the P\ bit set to\ 1.
.ce
.parag
.ce
2.4.4.7
.ce
\fICollision of DM responses\fR
.ce
.parag
.ce
A contention situation may occur when both the DCE and the DTE
.ce
issue a DM response to request a mode\(hysetting command. In this case, the DTE
.ce
will issue an SABM/SABME command to resolve the contention situation.
.ce
.parag
.ce

.ce
2.4.5
.ce
\fILAPB procedures for information transfer\fR
.ce
.parag
.ce
The procedures which apply to the transmission of I\ frames in each direction
during the information transfer phase are described below.
.ce
.parag
.ce
In the following, \*Qnumber one higher\*U is in reference to a
.ce
continuously repeated sequence series, i.e., 7 is 1\ higher than 6 and 0 is
.ce
1\ higher than 7 for modulo\ 8 series, and 127 is 1\ higher than 126 and 0 is
.ce
1\ higher than 127 for modulo\ 128 series.
.ce
.parag
.ce
2.4.5.1
.ce
\fISending I frames\fR
.ce
.parag
.ce
When the DCE has an I frame to transmit (i.e. an I frame not
.ce
already transmitted, or having to be retransmitted as described in \(sc\
2.4.5.6
.ce
below), it will transmit it with an N(S) equal to its current send state
.ce
variable V(S), and an N(R) equal to its current receive state variable
V(R). At the end of the transmission of the I\ frame, the DCE will increment
its send
.ce
state variable V(S) by\ 1.
.ce
.parag
.ce
If Timer T1 is not running at the time of transmission of an I frame, it
will be started.
.ce
.parag
.ce
If the send state variable V(S) is equal to the last value of N(R)
.ce
received plus \fIk\fR
.ce
(where \fIk\fR
.ce
is the maximum number of outstanding I\ frames \(em
.ce
see \(sc\ 2.4.8.6 below), the DCE will not transmit any new I\ frames, but may
.ce
retransmit an I\ frame as described in \(sc\(sc\ 2.4.5.6 or 2.4.5.9 below.
.ce
.parag
.ce
When the DCE is in the busy condition, it may still transmit I frames,
provided that the DTE is not busy. When the DCE is in the frame rejection
.ce
condition, it will stop transmitting I\ frames.
.ce
.parag
.ce
2.4.5.2
.ce
\fIReceiving an I frame\fR
.ce
.parag
.ce
2.4.5.2.1
.ce
When the DCE is not in a busy condition and receives a valid I\ frame whose
send sequence number N(S) is equal to the DCE receive state
.ce
variable V(R), the DCE will accept the information field of this frame,
.ce
increment by one its receive state variable V(R), and act as follows:
.ce
.parag
.ce

.ce
a)
.ce
If the DCE is still not in a busy condition:
.ce
.parag
.ce
i)
.ce
If an I frame is available for transmission by the
.ce
DCE, it may act as in \(sc\ 2.4.5.1 above and acknowledge
.ce
the received I frame by setting N(R) in the control field
.ce
of the next transmitted I\ frame to the value of the DCE
.ce
receive state variable V(R). Alternatively, the DCE may
.ce
acknowledge the received I\ frame by transmitting an RR
.ce
frame with the N(R) equal to the value of the DCE receive
.ce
state variable V(R).
.ce
.parag
.ce
ii)
.ce
If no I frame is available for transmission by the
.ce
DCE, it will transmit an RR frame with N(R) equal to the
.ce
value of the DCE receive state variable V(R).
.ce
.parag
.ce

.ce
b)
.ce
If the DCE is now in a busy condition, it will transmit an
.ce
RNR frame with N(R) equal to the value of the DCE receive
.ce
state variable V(R) (see \(sc\ 2.4.5.8).
.ce
.parag
.ce

.ce
2.4.5.2.2
.ce
When the DCE is in a busy condition, it may ignore the
.ce
information field contained in any received I\ frame.
.ce
.parag
.ce
2.4.5.3
.ce
\fIReception of invalid frames\fR
.ce
.parag
.ce
When the DCE receives an invalid frame (see \(sc\ 2.3.5.3), this frame
will be discarded.
.ce
.parag
.ce
2.4.5.4
.ce
\fIReception of out\(hyof\(hysequence I frames\fR
.ce
.parag
.ce
When the DCE receives a valid I frame whose send sequence number
.ce
N(S) is incorrect, i.e.,\ not equal to the current DCE receive state variable
.ce
V(R), it will discard the information field of the I\ frame and transmit
an REJ frame with the N(R) set to one higher than the N(S) of the last
correctly
.ce
received I\ frame. The REJ frame will be a command frame with the P\ bit
set to\ 1 if an acknowledged transfer of the retransmission request is
required;
.ce
otherwise the REJ frame may be either a command or a response frame. The DCE
.ce
will then discard the information field of all I\ frames received until the
.ce
expected I\ frame is correctly received. When receiving the expected I\ frame,
.ce
the DCE will then acknowledge the I\ frame as described in \(sc\ 2.4.5.2
above. The DCE will use the N(R) and P\ bit information in the discarded
I\ frames as
.ce
described in \(sc\ 2.3.5.2 above.
.ce
.parag
.ce
2.4.5.5
.ce
\fIReceiving acknowledgement\fR
.ce
.parag
.ce
When correctly receiving an I frame or a supervisory frame (RR, RNR or
REJ), even in the busy condition, the DCE will consider the N(R) contained
in this frame as an acknowledgement for all I\ frames it has transmitted
with an N(S) up to and including the received N(R)\(em1. The DCE will stop
Timer\ T1 when it correctly receives an I\ frame or a supervisory frame
with the N(R) higher
.ce
than the last received N(R) (actually acknowledging some I\ frames), or
an REJ frame with an N(R) equal to the last received N(R).
.ce
.parag
.ce

.ce
If Timer T1 has been stopped by the receipt on an I, RR or RNR frame, and
if there are outstanding I\ frames still unacknowledged, the DCE will
.ce
restart Timer\ T1. If Timer\ T1 then runs out, the DCE will follow the
recovery procedure (\(sc\ 2.4.5.9 below) with respect to the unacknowledged
I\ frames. If
.ce
Timer\ T1 has been stopped by the receipt of an REJ frame, the DCE will
follow the retransmission procedures in \(sc\ 2.4.5.6 below.
.ce
.parag
.ce
2.4.5.6
.ce
\fIReceiving an REJ frame\fR
.ce
.parag
.ce
When receiving an REJ frame, the DCE will set its send state
.ce
variable V(S) to the N(R) received in the REJ control field. It will transmit
the corresponding I\ frame as soon as it is available or retransmit it
in
.ce
accordance with the procedures described in \(sc\ 2.4.5.1 above. (Re)transmission
will conform to the following procedure:
.ce
.parag
.ce
i)
.ce
if the DCE is transmitting a supervisory command or response
.ce
when it receives the REJ frame, it will complete that
.ce
transmission before commencing transmission of the requested
.ce
I\ frame;
.ce
.parag
.ce

.ce
ii)
.ce
if the DCE is transmitting an unnumbered command or
.ce
response when it receives the REJ frame, it will ignore the
.ce
request for retransmission;
.ce
.parag
.ce
iii)
.ce
if the DCE is transmitting an I frame when the REJ frame
.ce
is received, it may abort the I\ frame and commence transmission
.ce
of the requested I\ frame immediately after abortion;
.ce
.parag
.ce
iv)
.ce
if the DCE is not transmitting any frame when the REJ frame
.ce
is received, it will commence transmission of the requested
.ce
I\ frame immediately.
.ce
.parag
.ce
In all cases, if other unacknowledged I frames had already been
.ce
transmitted following the one indicated in the REJ frame, then those I
frames will be retransmitted by the DCE following the retransmission of
the requested I\ frame. Other I\ frames not yet transmitted may be transmitted
following the
.ce
retransmitted I\ frames.
.ce
.parag
.ce
If the REJ frame was received from the DTE as a command with the P bit
set to\ 1, the DCE will transmit an RR, RNR or REJ response with the F
bit set to\ 1 before transmitting or retransmitting the corresponding I\
frame.
.ce
.parag
.ce
2.4.5.7
.ce
\fIReceiving an RNR frame\fR
.ce
.parag
.ce
After receiving an RNR frame whose N(R) acknowledges all frames
.ce
previously transmitted, the DCE will stop Timer\ T1 and may then transmit
.ce
an I\ frame, with the P\ bit set to\ 0, whose send sequence number is
.ce
equal to the N(R) indicated in the RNR frame, restarting Timer\ T1 as it
does. After receiving an RNR frame whose N(R) indicates a previously transmitted
.ce
frame, the DCE will not transmit or retransmit any I\ frame, Timer\ T1 being
.ce
already running. In either case, if the Timer\ T1 runs out before receipt
of a busy clearance indication, the DCE will follow the procedure described
in
.ce
\(sc\ 2.4.5.9\ below. In any case, the DCE will not transmit any other
I\ frames
.ce
before receiving an RR or REJ frame, or before the completion of a link
.ce
resetting procedure.
.ce
.parag
.ce
Alternatively, after receiving an RNR frame, the DCE may wait for a
.ce
period of time (e.g.,\ the length of the Timer\ T1) and then transmit a
.ce
supervisory command frame (RR, RNR or REJ) with the P\ bit set to\ 1, and
start Timer\ T1, in order to determine if there is any change in the receive
status of the DTE. The DTE shall respond to the P\ bit set to\ 1 with a
supervisory
.ce
response frame (RR, RNR or REJ) with the F\ bit set to\ 1 indicating either
.ce
continuance of the busy condition (RNR) or clearance of the busy condition
(RR or REJ). Upon receipt of the DTE response, Timer\ T1 is stopped.
.ce
.parag
.ce

.ce
1)
.ce
If the response is the RR or REJ response, the busy
.ce
condition is cleared and the DCE may transmit I\ frames beginning
.ce
with the I\ frame identified by the N(R) in the received response
.ce
frame.
.ce
.parag
.ce
2)
.ce
If the response is the RNR response, the busy condition
.ce
still exists, and the DCE will after a period of time (e.g.\ the
.ce
length of Timer\ T1) repeat the enquiry of the DTE receive
.ce
status.
.ce
.parag
.ce
If Timer T1 runs out before a status response is received, the
.ce
enquiry process above is repeated. If N2 attempts to get a status response
fail (i.e.\ Timer\ T1 runs out N2\ times), the DCE will initiate a data
link resetting procedure as described in \(sc\ 2.4.7.2\ below or will transmit
a DM response to ask the DTE to initiate a data link set\(hyup procedure
as described in \(sc\ 2.4.4.1 and enter the disconnected phase. The value
of N2 is defined in \(sc\ 2.4.8.4\ below.
.ce
.parag
.ce

.ce
If, at any time during the enquiry process, an unsolicited RR or REJ frame
is received from the DTE, it will be considered to be an indication of
.ce
clearance of the busy condition. Should the unsolicited RR or REJ frame be a
.ce
command frame with the P bit set to\ 1, the appropriate response frame
with the F\ bit set to 1 must be transmitted before the DCE may resume
transmission of
.ce
I\ frames. If Timer\ T1 is running, the DCE will wait for the non\(hybusy
response with the F bit set to\ 1 or will wait for Timer\ T1 to run out
and then either
.ce
may reinitiate the enquiry process in order to realize a successful P/F bit
.ce
exchange or may resume transmission of I frames beginning with the I\ frame
.ce
identified by the N(R) in the received RR or REJ frame.
.ce
.parag
.ce
2.4.5.8
.ce
\fIDCE busy condition\fR
.ce
.parag
.ce
When the DCE enters a busy condition, it will transmit an RNR frame at
the earliest opportunity. The RNR frame will be a command frame with the
P bit set to\ 1 if an acknowledged transfer of the busy condition indication
is
.ce
required; otherwise the RNR frame may be either a command or a response
frame. While in the busy condition, the DCE will accept and process supervisory
.ce
frames, will accept and process the contents of the N(R) fields of I\ frames,
.ce
and will return an RNR response with the F bit set to\ 1 if it receives a
.ce
supervisory command or I command frame with the P bit set to\ 1. To clear the
.ce
busy condition, the DCE will transmit either an REJ frame or an RR frame, with
.ce
N(R) set to the current receive state variable V(R), depending on whether or
.ce
not it discarded information fields of correctly received I\ frames. The REJ
.ce
frame or the RR frame will be a command frame with the P bit set to\ 1 if an
.ce
acknowledged transfer of the busy\(hyto\(hynon\(hybusy transition is required,
otherwise the REJ frame or the RR frame may be either a command or a response
frame.
.ce
.parag
.ce

.ce
2.4.5.9
.ce
\fIWaiting acknowledgement\fR
.ce
.parag
.ce
The DCE maintains an internal transmission attempt variable which is set
to\ 0 when the DCE sends a UA response, when the DCE receives a UA
.ce
response or an RNR command or response, or when the DCE correctly receives
an I\ frame or supervisory frame with the N(R) higher than the last received
N(R) (actually acknowledging some outstanding I\ frames).
.ce
.parag
.ce
If Timer T1 runs out waiting for the acknowledgement from the DTE for an
I\ frame transmitted, the DCE will enter the timer recovery condition,
add
.ce
one to its transmission attempt variable and set an internal variable \fIx\fR
.ce
to
.ce
the current value of its send state variable V(S). The DCE will then restart
.ce
Timer T1, set its send state variable V(S) to the last value of N(R) received
from the DTE and retransmit the corresponding I\ frame with the P bit set
to\ 1, or transmit an appropriate supervisory command frame (RR, RNR or
REJ) with the P\ bit set to\ 1.
.ce
.parag
.ce

.ce
The timer recovery condition is cleared when the DCE receives a valid supervisory
frame with the F\ bit\ set to\ 1.
.ce
.parag
.ce
If, while in the timer recovery condition, the DCE correctly receives a
supervisory frame with the F bit set to\ 1 and with the N(R) within the
range from its current send state variable V(S) to \fIx\fR
.ce
included, it will clear the
.ce
timer recovery condition (including stopping Timer\ T1) and set its send
state variable V(S) to the value of the received N(R), and may then resume
with
.ce
I\ frame transmission or retransmission, as appropriate.
.ce
.parag
.ce
If, while in the timer recovery condition, the DCE correctly receives an
I or supervisory frame with the P/F bit set to\ 0 and with a valid N(R)
(see \(sc\ 2.3.4.9), it will not clear the timer recovery condition. The
value of the
.ce
received N(R) may be used to update the send state variable V(S). However,
the DCE may decide to keep the last transmitted I\ frame in store (even
if it is
.ce
acknowledged) in order to be able to retransmit it with the P bit set to\
1 when Timer\ T1 runs out at a later time.
.ce
.parag
.ce

.ce
If the received supervisory frame with the P/F bit set to\ 0 is an REJ
frame with a valid N(R), the DCE may either immediately initiate
.ce
(re)transmission from the value of the send state variable V(S), or it may
.ce
ignore the request for retransmission and wait until the supervisory frame
with the F bit set to\ 1 is received before initiating (re)transmission
of frames
.ce
from the value identified in the N(R) field of the supervisory frame with
the F\ bit set to\ 1. In the case of immediate retransmission, in order
to prevent
.ce
duplicate retransmissions following the clearance of the timer recovery
.ce
condition, the DCE shall inhibit retransmission of a specific I\ frame [same
.ce
N(R) in the same numbering cycle] if the DCE has retransmitted that I\
frame as the result of a received REJ frame with the P/F bit set to\ 0.
.ce
.parag
.ce
If, while in the timer recovery condition, the DCE receives a REJ
.ce
command with the P bit set to\ 1, the DCE will respond immediately with an
.ce
appropriate supervisory response with the F bit set to\ 1. The DCE may
then use the value of the N(R) in the REJ command to update the send state
variable
.ce
V(S), and may either immediately begin (re)transmission from the value N(R)
.ce
indicated in the REJ frame or ignore the request for retransmission and wait
.ce
until the supervisory frame with the F bit set to\ 1 is received before
.ce
initiating (re)transmission of I\ frames from the value identified in the
N(R) field of the supervisory frame with the F\ bit set to\ 1. In the case
of
.ce
immediate retransmission, in order to prevent duplicate retransmissions
.ce
following the clearance of the timer recovery condition, the DCE shall
inhibit retransmission of a specific I\ frame [same N(R) in the same numbering
cycle]
.ce
if the DCE has retransmitted that I\ frame as the result of the received REJ
.ce
command with the P\ bit set to\ 1.
.ce
.parag
.ce
If Timer T1 runs out in the timer recovery condition, and no I or
.ce
supervisory frame with the P/F bit set to 0 and with a valid N(R) has been
.ce
received, or no REJ command with the P bit set to\ 1 and with a valid N(R)
has been received, the DCE will add one to its transmission attempt variable,
.ce
restart Timer\ T1, and either retransmit the I frame sent with the P bit set
.ce
to\ 1 or transmit an appropriate supervisory command with the P bit set to\ 1.
.ce
.parag
.ce

.ce
If the transmission attempt variable is equal to N2, the DCE will
.ce
initiate a data link resetting procedure as described in \(sc\ 2.4.7.2
below, or
.ce
will transmit a DM response to ask the DTE to initiate a data link set\(hyup
.ce
procedure as described in \(sc\ 2.4.4.1 above and enter the disconnected
phase. N2 is a system parameter (see \(sc\ 2.4.8.4 below).
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ Although the DCE may implement the internal variable \fIx\fR
.ce
,
.ce
other mechanisms do exist that achieve the identical function.
.ce
.parag
.ce
2.4.6
.ce
\fILAPB conditions for \fR
.ce
\fIdata link resetting or data link\fR
.ce

.ce
\fIre\(hyinitialization\fR
.ce
\fI(data link set\(hyup)\fR
.ce
.parag
.ce
2.4.6.1
.ce
When the DCE receives, during the information transfer phase, a
.ce
frame which is not invalid (see \(sc\ 2.3.5.3) with one of the conditions
listed in \(sc\ 2.3.4.9 above, the DCE will request the DTE to initiate
a data link resetting procedure by transmitting an FRMR response to the
DTE as described in
.ce
\(sc\ 2.4.7.3.
.ce
.parag
.ce
2.4.6.2
.ce
When the DCE receives, during the information transfer phase, an FRMR response
from the DTE, the DCE will either initiate the data link
.ce
resetting procedures itself as described in \(sc\ 2.4.7.2 or return a DM
response to ask the DTE to initiate the data link set\(hyup (initialization)
procedure as described in \(sc\ 2.4.4.1. After transmitting a DM response,
the DCE will enter
.ce
the disconnected phase as described in \(sc\ 2.4.4.4.2.
.ce
.parag
.ce
2.4.6.3
.ce
When the DCE receives, during the information transfer phase, a UA response,
or an unsolicited response with the F bit set to\ 1, the DCE may
.ce
either initiate the data link resetting procedures itself as described in
.ce
\(sc\ 2.4.7.2, or return a DM response to ask the DTE to initiate the data link
.ce
set\(hyup (initialization) procedure as described in \(sc\ 2.4.4.1. After
transmitting a DM response, the DCE will enter the disconnected phase as
described in
.ce
\(sc\ 2.4.4.4.2.
.ce
.parag
.ce
2.4.6.4
.ce
When the DCE receives, during the information transfer phase, a DM response
from the DTE, the DCE will either initiate the data link set\(hyup
.ce
(initialization) procedures itself as described in \(sc\ 2.4.4.1, or return
a DM
.ce
response to ask the DTE to initiate the data link set\(hyup (initialization)
.ce
procedures as described in \(sc\ 2.4.4.1. After transmitting a DM response,
the DCE will enter the disconnected phase as described in \(sc\ 2.4.4.4.2.
.ce
.parag
.ce

.ce
2.4.7
.ce
\fILAPB procedure for data link resetting\fR
.ce
.parag
.ce
2.4.7.1
.ce
The data link resetting procedure is used to initialize both
.ce
directions of information transfer according to the procedure described
below. The data link resetting procedure only applies during the information
transfer phase.
.ce
.parag
.ce
2.4.7.2
.ce
Either the DTE or the DCE may initiate the data link resetting
.ce
procedure. The data link resetting procedure indicates a clearance of a DCE
.ce
and/or DTE busy condition, if present.
.ce
.parag
.ce
The DTE shall initiate a data link resetting by transmitting an
.ce
SABM/SABME command to the DCE. If, upon correct receipt of the SABM/SABME
.ce
command, the DCE determines that it can continue in the information transfer
.ce
phase, it will return a UA response to the DTE, will reset its send and
receive state variables V(S) and V(R) to zero, and will remain in the information
.ce
transfer phase. If, upon correct receipt of the SABM/SABME command, the DCE
.ce
determines that it cannot remain in the information transfer phase, it will
.ce
return a DM response as a denial to the resetting request and will enter the
.ce
disconnected phase.
.ce
.parag
.ce
The DCE will initiate a data link resetting by transmitting an
.ce
SABM/SABME command to the DTE and starting its Timer\ T1 (see \(sc\ 2.4.8.1
below). Upon reception of a UA response from the DTE, the DCE will reset
its send and receive state variables V(S) and V(R) to zero, will stop its
Timer\ T1, and will remain in the information transfer phase. Upon reception
of a DM response from the DTE as a denial to the data link resetting request,
the DCE will stop its Timer\ T1 and will enter the disconnected phase.
.ce
.parag
.ce
The DCE, having sent an SABM/SABME command, will ignore and discard
.ce
any frames received from the DTE except an SABM/SABME or DISC command,
or a UA or DM response. The receipt of an SABM/SABME or DISC command from
the DTE will result in a collision situation that is resolved per \(sc\
2.4.4.5 above. Frames
.ce
other than the UA or DM response sent in response to a received SABM/SABME
or DISC command will be sent only after the data link is reset and if no
.ce
outstanding SABM/SABME command exists.
.ce
.parag
.ce

.ce
After the DCE sends the SABM/SABME command, if a UA or DM response is not
received correctly, Timer\ T1 will run out in the DCE. The DCE will then
.ce
resend the SABM/SABME command and will restart Timer\ T1. After N2 attempts
to reset the data link, the DCE will initiate appropriate higher layer
recovery
.ce
action and will enter the disconnected phase. The value of N2 is defined in
.ce
\(sc\ 2.4.8.4 below.
.ce
.parag
.ce
2.4.7.3
.ce
The DCE may ask the DTE to reset the data link by transmitting an FRMR
response (see \(sc\ 2.4.6.1 above). After transmitting an FRMR response,
.ce
the DCE will enter the frame rejection condition.
.ce
.parag
.ce
The frame rejection condition is cleared when the DCE receives an SABM/SABME
command, a DISC command, a FRMR response, or a DM response; or if
.ce
the DCE transmits an SABM/SABME command, a DISC command, or a DM response.
.ce
Other commands received while in the frame rejection condition will cause
the DCE to retransmit the FRMR response with the same information field
as
.ce
originally transmitted.
.ce
.parag
.ce
The DCE may start Timer\ T1 on transmission of the FRMR response. If
.ce
Timer\ T1 runs out before the frame rejection condition is cleared, the
DCE may retransmit the FRMR response, and restart T1. After N2 attempts
(time outs) to get the DTE to reset the data link, the DCE may reset the
data link itself as described in \(sc\ 2.4.7.2 above. The value of N2 is
defined in \(sc\ 2.4.8.4 below.
.ce
.parag
.ce

.ce
In the frame rejection condition, I frames and supervisory frames will
not be transmitted by the DCE. Also, received I frames and supervisory
frames will be discarded by the DCE except for the observance of a P bit
set to\ 1.
.ce
When an additional FRMR response must be transmitted by the DCE as a result
of the receipt of a P\ bit set to 1 while Timer\ T1 is running, Timer\
T1 will
.ce
continue to run. Upon reception of an FRMR response (even during a frame
.ce
rejection condition), the DCE will initiate a resetting procedure by
.ce
transmitting an SABM/SABME command as described in \(sc\ 2.4.7.2 above, or will
.ce
transmit a DM response to ask the DTE to initiate the data link set\(hyup
.ce
procedure as described in \(sc\ 2.4.4.1 and enter the disconnected phase.
.ce
.parag
.ce
2.4.8
.ce
\fIList of \fR
.ce
\fILAPB system parameters\fR
.ce
.parag
.ce
The DCE and DTE system parameters are as follows:
.ce
.parag
.ce

.ce
2.4.8.1
.ce
\fITimer\fR
.ce
\fIT1\fR
.ce
.parag
.ce
The value of the DTE Timer T1 system parameter may be different
.ce
than the value of the DCE Timer T1 system parameter. These values shall
be made known to both the DTE and the DCE, and agreed to for a period of
time by both the DTE and the DCE.
.ce
.parag
.ce
The period of Timer T1, at the end of which retransmission of a frame may
be initiated (see \(sc\ 2.4.4 and \(sc\ 2.4.5 above for the DCE), shall
take into account whether T1 is started at the beginning or the end of
the transmission of a frame.
.ce
.parag
.ce
The proper operation of the procedure requires that the transmitter's (DCE
or DTE) Timer\ T1 be greater than the maximum time between transmission
of a frame (SABM/SABME, DISC, I\ or supervisory command, or DM or FRMR
response)
.ce
and the reception of the corresponding frame returned as an answer to that
.ce
frame (UA, DM or acknowledging frame). Therefore, the receiver (DCE or DTE)
.ce
should not delay the response or acknowledging frame returned to one of the
.ce
above frames by more than a value\ T2, where T2 is a system parameter (see
.ce
\(sc\ 2.4.8.2).
.ce
.parag
.ce

.ce
The DCE will not delay the response or acknowledging frame returned to
one of the above DTE frames by more than a period\ T2.
.ce
.parag
.ce
2.4.8.2
.ce
\fIParameter T2\fR
.ce
.parag
.ce
The value of the DTE parameter T2 may be different than the value of the
DCE parameter T2. These values shall be made known to both the DTE and
the DCE, and agreed to for a period of time by both the DTE and the DCE.
.ce
.parag
.ce

.ce
The period of parameter T2 shall indicate the amount of time
.ce
available at the DCE or DTE before the acknowledging frame must be initiated
in order to ensure its receipt by the DTE or DCE, respectively, prior to
Timer\ T1 running out at the DTE or DCE (parameter\ T2\ <\ Timer\ T1).
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ The period of parameter T2 shall take into account the
.ce
following timing factors: the transmission time of the acknowledging frame,
the propagation time over the access data link, the stated processing times
at the DCE and the DTE, and the time to complete the transmission of the
frame(s) in the DCE or DTE transmit queue that are neither displaceable
or modifiable in an orderly manner.
.ce
.parag
.ce

.ce
Given a value for Timer T1 for the DTE or DCE, the value of parameter T2
at the DCE or DTE, respectively, must be no larger than T1 minus\ 2 times
the propagation time over the access data link, minus the frame processing
time at the DCE, minus the frame processing time at the DTE, and minus
the transmission time of the acknowledging frame by the DCE or DTE, respectively.
.ce
.parag
.ce

.ce
2.4.8.3
.ce
\fITimer T3\fR
.ce
.parag
.ce
The DCE shall support a Timer T3 system parameter, the value of
.ce
which shall be made known to the DTE.
.ce
.parag
.ce
The period of Timer T3, at the end of which an indication of an
.ce
observed excessively long idle channel state condition is passed to the
Packet Layer, shall be sufficiently greater than the period of the DCE
Timer T1
.ce
(i.e.\ T3\ >\ T1) so that the expiration of T3 provides the desired level of
.ce
assurance that the data link channel is in a non\(hyactive, non\(hyoperational
state, and is in need of data link set\(hyup before normal data link operation
can
.ce
resume.
.ce
.parag
.ce
2.4.8.4
.ce
\fIMaximum number of attempts\fR
.ce
\fIto complete a\fR
.ce

.ce
\fItransmission N2\fR
.ce
.parag
.ce

.ce
The value of the DTE N2 system parameter may be different than the value
of the DCE N2 system parameter. These values shall be made known to both
the DTE and the DCE, and agreed to for a period of time by both the DTE
and the DCE.
.ce
.parag
.ce
The value of N2 shall indicate the maximum number of attempts made by the
DCE or DTE to complete the successful transmission of a frame to the DTE
or DCE, respectively.
.ce
.parag
.ce
2.4.8.5
.ce
\fIMaximum number of bits in an I frame N1\fR
.ce
.parag
.ce
The value of the DTE N1 system parameter may be different than the value
of the DCE N1 system parameter. These values shall be made known to both
the DTE and the DCE.
.ce
.parag
.ce
The values of N1 shall indicate the maximum number of bits in an
.ce
I\ frame (excluding flags and 0\ bits inserted for transparency) that the
DCE or DTE is willing to accept from the DTE or DCE, respectively.
.ce
.parag
.ce
In order to allow for universal operation, a DTE should support a
.ce
value of DTE N1 which is not less than 1080\ bits (135\ octets). DTEs should
be aware that the network may transmit longer packets (see \(sc\ 5.2),
that may result in a data link layer problem.
.ce
.parag
.ce

.ce
All networks shall offer to a DTE which requires it, a value of DCE N1
which is greater than or equal to 2072\ bits (259\ octets) plus the length
of the address, control and FCS fields at the DTE/DCE interface, and greater
than or equal to the maximum length of the data packets which may cross
the DTE/DCE
.ce
interface plus the length of the address, control and FCS fields at the
DTE/DCE interface.
.ce
.parag
.ce
Appendix II provides a description of how the values stated above are  derived.
.ce
.parag
.ce
2.4.8.6
.ce
\fIMaximum number of \fR
.ce
\fIoutstanding I frames k\fR
.ce
.parag
.ce
The value of the DTE k system parameter shall be the same as the
.ce
value of the DCE k system parameter. This value shall be agreed to for a
.ce
period of time by both the DTE and the DCE.
.ce
.parag
.ce
The value of k shall indicate the maximum number of sequentially
.ce
numbered I\ frames that the DTE or DCE may have outstanding
.ce
(i.e.\ unacknowledged) at any given time. The value of k shall never exceed
.ce
seven for modulo\ 8 operation, or one hundred and twenty\(hyseven for modulo\
128
.ce
operation. All networks (DCEs) shall support a value of seven. Other values
of k (less than and greater than seven) may also be supported by networks
.ce
(DCEs).
.ce
.parag
.ce
.line
.ce
\fBMONTAGE : \(sc 2.5 de cette Recommandation sur le reste de cette page\fR
.ce
.parag
.ce

.ce
.line
.ad r
\fBTable 6/X.25 [T6.25], p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
.sp 5
2.3.4.2
    \fIReceive ready (RR)\fR \fI command and response\fR
.sp 9p
.RT
.PP
The receive ready (RR) supervisory frame is used by the DCE
or DTE to:
.RT
.LP
    1)
    indicate it is ready to receive an I frame; and
.LP
    2)
    acknowledge previously received I frames numbered up to and
including N(R)\ \(em\ 1.
.PP
An RR frame may be used to indicate the clearance of a busy
condition that was reported by the earlier transmission of an RNR frame
by that same station (DCE or\ DTE). In addition to indicating the DCE or
DTE status, the RR command with the P\ bit set to\ 1 may be used by the
DCE or DTE to ask for the status of the DTE or DCE, respectively.
.bp
.sp 1P
.LP
2.3.4.3
    \fIReceive not ready (RNR) command and response\fR
.sp 9p
.RT
.PP
The receive not ready (RNR) supervisory frame is used by the DCE or DTE
to indicate a busy condition; i.e.\ temporary inability to accept
additional incoming I\ frames. I\ frames numbered up to and including N(R)\
\(em\ 1
are acknowledged. I\ frame N(R) and any subsequent I\ frames received, if any,
are not acknowledged; the acceptance status of these I\ frames will be
indicated in subsequent exchanges.
.PP
In addition to indicating the DCE or DTE status, the RNR command with the
P bit set to\ 1 may be used by an DCE or DTE to ask for the status of the
DTE or DCE, respectively.
.RT
.sp 1P
.LP
2.3.4.4
    \fIReject (REJ) command and response\fR
.sp 9p
.RT
.PP
The reject (REJ) supervisory frame is used by the DCE or DTE to request
transmission of I\ frames starting with the frame numbered N(R).
I\ frames numbered N(R)\ \(em\ 1 and below are acknowledged. Additional
I\ frames
pending initial transmission may be transmitted following the retransmitted
I\ frame(s).
.PP
Only one REJ exception condition for a given direction of information transfer
may be established at any time. The REJ exception condition is cleared
(reset) upon the receipt of an I\ frame with an N(S) equal to the N(R)
of the
REJ\ frame.
.PP
An REJ frame may be used to indicate the clearance of a busy condition
that was reported by the earlier transmission of an RNR frame by that same
station (DCE or\ DTE). In addition to indicating the DCE or DTE status, the
REJ\ command with the P bit set to\ 1 may be used by the DCE or\ DTE to ask for
the status of the DTE or\ DCE, respectively.
.RT
.sp 1P
.LP
\fR 2.3.4.5
     \fISet asynchronous balanced mode (SABM)\fR \fIcommand/\fR \fISet asynchronous
balanced mode extended (SABME)\fR
\fIcommand (subscription time option)\fR
.sp 9p
.RT
.PP
The SABM unnumbered command is used to place the addressed DCE or DTE in
an asynchronous balanced mode (ABM) information transfer phase where all
command/response control fields will be one\ octet in length.
.PP
The SABME unnumbered command is used to place the addressed DCE or DTE
in an asynchronous balanced mode\ (ABM) information transfer phase where
numbered command/response control fields will be two octets in length, and
unnumbered command/response control fields will be one octet in length.
.PP
No information field is permitted with the SABM or SABME command. The transmission
of a SABM/SABME command indicates the clearance of a busy
condition that was reported by the earlier trans
mission\ of an RNR\ frame by   that
same station (DCE or\ DTE). The DCE or DTE confirms acceptance of SABM/SABME
[modulo\ 8 (basic) operation/modulo\ 128 (extended) operation] command by the
transmission, at the first opportunity, of a UA\ response. Upon acceptance of
this command, the DCE or DTE send state variable\ V(S) and receive state
variable V(R) are set to\ 0.
.PP
Previously transmitted I\ frames that are unacknowledged when this
command is actioned remain unac
knowledged.\ It is the responsibility of a
higher layer (e.g.\ Packet Layer or MLP) to recover from the possible loss of
.PP
the contents (e.g.\ packets) of such I\ frames.
.PP
\fINote\fR \ \(em\ The mode of operation of a data link [basic (modulo 8) or
extended (modulo\ 128)] is determined at subscription time and is only
changed by going through a new subscription process.
.RT
.sp 1P
.LP
2.3.4.6
    \fIDisconnect (DISC)\fR \fI command\fR
.sp 9p
.RT
.PP
The DISC unnumbered command is used to terminate the mode
previously set. It is used to inform the DCE or DTE receiving the DISC
command that the DTE or DCE sending the DISC command is suspending operation.
No
information field is permitted with the DISC command. Prior to actioning the
DISC command, the DCE or DTE receiving the DISC command confirms the acceptance
of the DISC command by the transmission of a UA response. The DTE or DCE
sending the DISC command enters the disconnected phase when it receives the
acknowledging UA response.
.bp
.PP
Previously transmitted I frames that are unacknowledged when this
command is actioned remain unacknowledged. It is the responsibility of
a higher layer (e.g.\ Packet Layer or MLP) to recover from the possible
loss of the
contents (e.g.,\ packets) of such I frames.
.RT
.sp 1P
.LP
2.3.4.7
    \fIUnnumbered acknowledgement (UA)\fR \fI response\fR
.sp 9p
.RT
.PP
The UA unnumbered response is used by the DCE or DTE to acknowledge the
receipt and acceptance of the mode\(hysetting commands. Received mode\(hysetting
commands are not actioned until the UA response is transmitted. The
transmission of a UA response indicates the clearance of a busy condition
that was reported by the earlier transmission of an RNR frame by that same
station (DCE or DTE). No information field is permitted with the UA response.
.RT
.sp 1P
.LP
2.3.4.8
    \fIDisconnected mode (DM)\fR \fI response\fR
.sp 9p
.RT
.PP
The DM unnumbered response is used to report a status where the DCE or
DTE is logically disconnected from the data link, and is in the disconnected
phase. The DM response may be sent to indicate that the DCE or DTE has
entered the disconnected phase without benefit of having received a DISC
command, or, if sent in response to the reception of a mode setting command,
is sent to
inform the DTE or DCE that the DCE or DTE, respectively, is still in the
disconnected phase and cannot execute the set mode command. No information
field is permitted with the DM response.
.PP
A DCE or DTE in a disconnected phase will monitor received commands
and will react to an SABM/SABME command as outlined in \(sc\ 2.4.4 below,
and will respond with a DM response with the F bit set to\ 1 to any other
command
received with the P\ bit set to\ 1.
.RT
.sp 1P
.LP
2.3.4.9
    \fIFrame reject (FRMR)\fR \fI response\fR
.sp 9p
.RT
.PP
The FRMR unnumbered response is used by the DCE or DTE to report an error
condition not recoverable by retransmission of the identical frame;
i.e.\ at least one of the following conditions, which results from the
receipt of a valid frame:
.RT
.LP
    1)
    the receipt of a command or response control field that is
undefined or not implemented;
.LP
    2)
    the receipt of an I frame with an information field which
exceeds the maximum established length;
.LP
    3)
    the receipt of an invalid N(R); or
.LP
    4)
    the receipt of a frame with an information field which is
not permitted or the receipt of a supervisory or unnumbered
frame with incorrect length.
.PP
An undefined or not implemented control field is any of the
control field encodings that are not identified in Tables\ 5/X.25 or\ 6/X.25.
.PP
A valid N(R) must be within the range from the lowest send sequence
number N(S) of the still unacknowledged frame(s) to the current DCE send
state variable inclusive (or to the current internal variable \fIx\fR if
the DCE is in
the timer recovery condition as described in \(sc\ 2.4.5.9).
.PP
An information field which immediately follows the control field, and consists
of\ 3 or 5\ octets [modulo\ 8 (basic) operation or modulo\ 128 (extended)
operation, respectively], is returned with this response and provides the
reason for the FRMR response. These formats are given in Tables\ 7/X.25
and\ 8/X.25.
.RT
.sp 1P
.LP
2.3.5
    \fIException condition reporting and recovery\fR
.sp 9p
.RT
.PP
The error recovery procedures which are available to effect
recovery following the detection/occurrence of an
exception condition
at the Data Link Layer are described below. Exception conditions described
are those situations which may occur as the result of transmission errors,
DCE or DTE malfunction, or operational situations.
.RT
.sp 1P
.LP
2.3.5.1
    \fIBusy condition\fR
.sp 9p
.RT
.PP
The busy condition results when the DCE or DTE is temporarily
unable to continue to receive I frames due to internal constraints,
e.g.\ receive buffering limitations. In this case an RNR frame is transmitted
from the busy DCE or DTE. I\ frames pending transmission may be transmitted
from the busy DCE or DTE prior to or following the RNR\ frame.
.PP
An indication that the busy condition has cleared is communicated by the
transmission of a UA (only in response to a SABM/SABME command), RR, REJ
or SABM/SABME (modulo\ 8/modulo\ 128) frame.
.bp
.RT
.ce
.line
.ce
\fBTable 7/X.25 [T7.25], p.\fR
.ce
.line
.ce
\fBTable 8/X.25 [T8.25], p.\fR
.ce
2.3.5.2
.ce
\fIN(S) sequence error condition\fR
.ce
.parag
.ce
The information field of all I frames received whose N(S) does not equal
the receive state variable V(R) will be discarded.
.ce
.parag
.ce
An N(S) sequence error exception condition occurs in the receiver when
an I\ frame received contains an N(S) which is not equal to the receive
state
.ce
variable V(R) at the receiver. The receiver does not acknowledge (increment
its receive state variable) the I\ frame causing the sequence error, or
any I\ frame which may follow, until an I\ frame with the correct N(S)
is received.
.ce
.parag
.ce

.ce
A DCE or DTE which receives one or more valid I frames having sequence
errors or subsequent supervisory frames (RR, RNR and REJ) shall accept
the
.ce
control information contained in the N(R) field and the P or F bit to perform
data link control functions; e.g.\ to receive acknowledgement of previously
.ce
transmitted I\ frames and to cause the DCE or DTE to respond (P bit set to 1).
.ce
.parag
.ce

.ce
The means specified in \(sc\(sc\ 2.3.5.2.1 and 2.3.5.2.2 shall be available
for initiating the retransmission of lost or errored I\ frames following
the
.ce
occurrence of an N(S) sequence error condition.
.ce
.parag
.ce
2.3.5.2.1
.ce
\fIREJ recovery\fR
.ce
.parag
.ce
The REJ frame is used by a receiving DCE or DTE to initiate a
.ce
recovery (retransmission) following the detection of an N(S) sequence error.
.ce
.parag
.ce

.ce
With respect to each direction of transmission on the data link, only one
\*Qsent REJ\*U exception condition from a DCE or DTE, to a DTE or DCE,
is
.ce
established at a time. A \*Qsent REJ\*U exception condition is cleared when the
.ce
requested I\ frame is received.
.ce
.parag
.ce

.ce
A DCE or DTE receiving a REJ frame initiates sequential
.ce
(re\(hy)transmission of I\ frames starting with the I\ frame indicated
by the N(R) contained in the REJ frame. The retransmitted frames may contain
an N(R) and a P bit that are updated from, and therefore different from,
the ones contained in the originally transmitted I\ frames.
.ce
.parag
.ce
2.3.5.2.2
.ce
\fITime\(hyout recovery\fR
.ce
.parag
.ce
If a DCE or DTE, due to a transmission error, does not receive (or receives
and discards) a single I\ frame or the last I\ frame(s) in a sequence of
I\ frames, it will not detect an N(S) sequence error condition and, therefore,
will not transmit a REJ frame. The DTE or DCE which transmitted the
.ce
unacknowledged I\ frame(s) shall, following the completion of a system
specified time\(hyout period (see \(sc\(sc\ 2.4.5.1 and 2.4.5.9 below),
take appropriate recovery action to determine at which I\ frame retransmission
must begin. The
.ce
retransmitted frame(s) may contain an N(R) and a P bit that is updated from,
.ce
and therefore different from, the ones contained in the originally transmitted
frame(s).
.ce
.parag
.ce
2.3.5.3
.ce
\fIInvalid frame\fR
.ce
\fI condition\fR
.ce
.parag
.ce
Any frame which is invalid will be discarded, and no action is
.ce
taken as the result of that frame. An invalid frame is defined as one
.ce
which:
.ce
.parag
.ce
a)
.ce
is not properly bounded by two flags;
.ce
.parag
.ce
b)
.ce
in basic (modulo 8) operation, contains fewer than 32 bits
.ce
between flags; in extended (modulo\ 128) operation, contains
.ce
fewer than 40\ bits between flags of frames that contain sequence
.ce
numbers or 32\ bits between flags of frames that do not contain
.ce
sequence numbers;
.ce
.parag
.ce
c)
.ce
contains a Frame Check Sequence (FCS) error; or
.ce
.parag
.ce
d)
.ce
contains an address other than A or B (for single link
.ce
operation) or other than C or D (for multilink operation).
.ce
.parag
.ce

.ce
For those networks that are octet aligned, a detection of
.ce
non\(hyoctet alignment may be made at the Data Link Layer by adding a frame
.ce
validity check that requires the number of bits between the opening flag and
.ce
the closing flag, excluding bits inserted for transparency, to be an integral
number of octets in length, or the frame is considered invalid.
.ce
.parag
.ce
2.3.5.4
.ce
\fIFrame rejection\fR
.ce
\fI condition\fR
.ce
.parag
.ce
A frame rejection condition is established upon the receipt of an error\(hyfree
frame with one of the conditions listed in \(sc\ 2.3.4.9 above.
.ce
.parag
.ce

.ce
At the DCE or DTE, this frame rejection exception condition is
.ce
reported by an FRMR response for appropriate DTE or DCE action, respectively.
Once a DCE has established such an exception condition, no additional I\
frames are accepted until the condition is reset by the DTE, except for
examination of the P bit. The FRMR response may be repeated at each opportunity,
as specified in \(sc\ 2.4.7.3, until recovery is effected by the DTE, or
until the DCE initiates its own recovery in case the DTE does not respond.
.ce
.parag
.ce
2.3.5.5
.ce
\fIExcessive idle channel state condition on incoming\fR
.ce

.ce
\fIchannel\fR
.ce
.parag
.ce
Upon detection of an idle channel state condition (see \(sc\ 2.2.12.2 above)
on the incoming channel, the DCE shall wait for a period\ T3 (see
.ce
\(sc\ 2.4.8.3 below) without taking any specific action, waiting for detection
of a return to the active channel state (i.e.,\ detection of at least one
flag
.ce
sequence). After the period\ T3, the DCE shall notify the higher layer
(e.g.\ the Packet Layer or the MLP) of the excessive idle channel state
condition, but
.ce
shall not take any action that would preclude the DTE from establishing the
.ce
data link by normal data link set\(hyup procedures.
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ Other actions to be taken by the DCE at the Data Link Layer upon
expiration of period\ T3 is a subject for further study.
.ce
.parag
.ce

.ce
2.4
.ce
\fIDescription of the \fR
.ce
\fILAPB procedure\fR
.ce
.parag
.ce
2.4.1
.ce
\fILAPB basic and extended modes of operation\fR
.ce
.parag
.ce
In accordance with the system choice made by the DTE at
.ce
subscription time, the DCE will either support modulo\ 8 (basic) operation or
.ce
will support modulo\ 128 (extended) operation. Changing from basic operation
to extended operation, or vice versa, in the DCE requires resubscription
by the
.ce
DTE for the desired service, and is not supported dynamically.
.ce
.parag
.ce
Table 5/X.25 indicates the command and response control field formats used
with the basic (modulo\ 8) service. The mode\(hysetting command employed
to
.ce
initialize (set up) or reset the basic mode is the SABM command. Table
6/X.25 indicates the command and response control field formats used with
the extended (modulo\ 128) service. The mode\(hysetting command employed
to initialize (set up) or reset the extended mode is the SABME command.
.ce
.parag
.ce
2.4.2
.ce
\fILAPB procedure for addressing\fR
.ce
.parag
.ce
The address field identifies a frame as either a command or a
.ce
response. A command frame contains the address of the DCE or DTE to which
the command is being sent. A response frame contains the address of the
DCE or DTE sending the frame.
.ce
.parag
.ce
In order to allow differentiation between single link operation and
.ce
the optional multilink operation for diagnostic and/or maintenance reasons,
.ce
different address pair encodings are assigned to data links operating with
.ce
multilink procedure compared to data links operating with the single link
.ce
procedure.
.ce
.parag
.ce
Frames containing commands transferred from the DCE to the DTE will
.ce
contain the address\ A for the single link operation and address\ C for the
.ce
multilink operation.
.ce
.parag
.ce
Frames containing responses transferred from the DCE to the DTE will contain
the address\ B for the single link operation and address\ D for the
.ce
multilink operation.
.ce
.parag
.ce

.ce
Frames containing commands transferred from the DTE to the DCE shall contain
the address\ B for the single link operation and address\ D for the
.ce
multilink operation.
.ce
.parag
.ce
Frames containing responses transferred from the DTE to the DCE shall contain
the address\ A for the single link operation and address\ C for the
.ce
multilink operation.
.ce
.parag
.ce
These addresses are coded as follows:
.ce
.parag
.ce
Address
.ce
1\ 2\ 3\ 4\ 5\ 6\ 7\ 8
.ce
.parag
.ce
Single link operation
.ce
\ \ A
.ce
1\ 1\ 0\ 0\ 0\ 0\ 0\ 0
.ce
.line
.ce
\ \ B
.ce
1\ 0\ 0\ 0\ 0\ 0\ 0\ 0
.ce
.parag
.ce
Multilink operation
.ce
\ \ C
.ce
1\ 1\ 1\ 1\ 0\ 0\ 0\ 0
.ce
.line
.ce
\ \ D
.ce
1\ 1\ 1\ 0\ 0\ 0\ 0\ 0
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ The DCE will discard all frames received with an address other
than\ A or\ B (single link operation), or\ C or\ D (multilink operation).
.ce
.parag
.ce
2.4.3
.ce
\fILAPB procedure for the use of the P/F bit\fR
.ce
.parag
.ce
The DCE or DTE receiving an SABM/SABME, DISC, supervisory command or I\
frame with the P\ bit set to\ 1 will set the F\ bit to\ 1 in the next response
frame it transmits.
.ce
.parag
.ce
The response frame returned by the DCE to an SABM/SABME or DISC
.ce
command with the P\ bit set to\ 1 will be a UA or DM response with the
F\ bit set to\ 1. The response frame returned by the DCE to an I\ frame
with the P\ bit set to\ 1, received during the information transfer phase,
will be an RR, REJ, RNR or FRMR response with the F\ bit set to\ 1. The
response frame returned by the
.ce
DCE to a supervisory command with the P\ bit set to\ 1, received during the
.ce
information transfer phase, will be an RR, REJ, RNR or FRMR response with
the F\ bit set to\ 1. The response frame returned by the DCE to an I\ frame
or
.ce
supervisory frame with the P\ bit set to\ 1, received during the disconnected
.ce
phase, will be a DM response with the F\ bit set to\ 1.
.ce
.parag
.ce
The P bit may be used by the DCE in conjunction with the timer
.ce
recovery condition (see \(sc\ 2.4.5.9 below).
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ Other use of the P bit by the DCE is a subject for further
.ce
study.
.ce
.parag
.ce
2.4.4
.ce
\fILAPB procedure for data link set\(hyup and disconnection\fR
.ce
.parag
.ce

.ce
2.4.4.1
.ce
\fIData link set\(hyup\fR
.ce
.parag
.ce
The DCE will indicate that it is able to set up the data link by
.ce
transmitting contiguous flags (active channel state).
.ce
.parag
.ce
Either the DTE or the DCE may initiate data link set\(hyup. Prior to
.ce
initiation of data link set\(hyup, either the DCE or the DTE may initiate data
.ce
link
.ce
disconnection (see \(sc\ 2.4.4.3) for the purpose of insuring that the
DCE and the DTE are in the same phase. The DCE may also transmit an unsolicited
DM response to request the DTE to initiate data link set\(hyup.
.ce
.parag
.ce
The DTE shall initiate data link set\(hyup by transmitting an SABM/SABME
command to the DCE. If, upon receipt of the SABM/SABME command correctly,
the DCE determines that it can enter the information transfer phase, it
will return a UA response to the DTE, will reset its send and receive state
variables V(S) and V(R) to zero, and will consider that the data link is
set up. If, upon
.ce
receipt
.ce
of the SABM/SABME command correctly, the DCE determines that it cannot enter
.ce
the information transfer phase, it will return a DM response to the DTE as a
.ce
denial to the data link set\(hyup initialization and will consider that the
.ce
data link is
.ce
\fInot\fR
.ce
 set up. In order to avoid misinterpretation of the DM response received,
it is suggested that the DTE always sends its SABM/SABME command with the
P\ bit set to\ 1. Otherwise, it is not possible to differentiate a DM response
intended as a denial to data link set\(hyup from a DM response that is
issued in a separate unsolicited sense as a request for a mode\(hysetting
command (as described
.ce
in\ \(sc\ 2.4.4.4.2).
.ce
.parag
.ce
The DCE will initiate data link set\(hyup by transmitting an SABM/SABME
command to the DTE and starting its Timer\ T1 in order to determine when
too
.ce
much time has elapsed waiting for a reply (see \(sc\ 2.4.8.1 below). Upon
reception of a UA response from the DTE, the DCE will reset its send and
receive state
.ce
variables\ V(S) and\ V(R) to zero, will stop its Timer\ T1, and will consider
that the data link is set up. Upon reception of a DM response from the
DTE as a
.ce
denial
.ce
to the data link set\(hyup initialization, the DCE will stop its Timer\
T1 and will consider that the data link is \fInot\fR
.ce
set up.
.ce
.parag
.ce

.ce
The DCE, having sent the SABM/SABME command, will ignore and discard any
frames except an SABM/SABME or DISC command, or a UA or DM response
.ce
received from the DTE. The receipt of an SABM/SABME or DISC command from the
.ce
DTE will result in a collision situation that is resolved per \(sc\ 2.4.4.5
below. Frames other than the UA and DM responses sent in response to a
received
.ce
SABM/SABME or DISC command will be sent only after the data link is set
up and if no outstanding SABM/SABME command exists.
.ce
.parag
.ce
After the DCE sends the SABM/SABME command, if a UA or DM response is not
received correctly, Timer\ T1 will run out in the DCE. The DCE will then
.ce
resend the SABM/SABME command and will restart Timer\ T1. After transmission
of the SABM/SABME command N2 times by the DCE, appropriate higher layer
recovery action will be initiated. The value of N2 is defined in \(sc\
2.4.8.4 below.
.ce
.parag
.ce
2.4.4.2
.ce
\fIInformation transfer phase\fR
.ce
.parag
.ce
After having transmitted the UA response to the SABM/SABME command or having
received the UA response to a transmitted SABM/SABME command, the DCE will
accept and transmit I and supervisory frames according to the procedures
described in \(sc\ 2.4.5 below.
.ce
.parag
.ce

.ce
When receiving the SABM/SABME command while in the information
.ce
transfer phase, the DCE will conform to the data link resetting procedure
.ce
described in \(sc\ 2.4.7 below.
.ce
.parag
.ce
2.4.4.3
.ce
\fIData link disconnection\fR
.ce
.parag
.ce
The DTE shall initiate a disconnect of the data link by
.ce
transmitting a DISC command to the DCE. On correctly receiving a DISC command
in the information transfer phase, the DCE will send a UA response and
enter
.ce
the disconnected phase. On correctly receiving a DISC command in the
.ce
disconnected phase, the DCE will send a DM response and remain in the
.ce
disconnected phase. In order to avoid misinterpretation of the DM response
.ce
received, it is suggested that the DTE always sends its DISC command with
the P\ bit set to\ 1. Otherwise, it is not possible to differentiate a
DM response
.ce
intended as an indication that the DCE is already in the disconnected phase
.ce
from a DM response that is issued in a separate unsolicited sense as a
request for a mode\(hysetting command (as described in \(sc\ 2.4.4.4.2).
.ce
.parag
.ce

.ce
The DCE will initiate a disconnect of the data link by transmitting a DISC
command to the DTE and starting its Timer\ T1 (see \(sc\ 2.4.8.1 below).
Upon reception of an UA response from the DTE, the DCE will stop its Timer\
T1 and
.ce
will enter the disconnected phase. Upon reception of a DM response from
the DTE as an indication that the DTE was already in the disconnected phase,
the DCE
.ce
will stop its Timer\ T1 and will enter the disconnected phase.
.ce
.parag
.ce
The DCE, having sent the DISC command, will ignore and discard any
.ce
frames except an SABM/SABME or DISC command, or a UA or DM response received
.ce
from the DTE. The receipt of an SABM/SABME or DISC command from the DTE will
.ce
result in a collision situation that is resolved per \(sc\ 2.4.4.5 below.
.ce
.parag
.ce

.ce
After the DCE sends the DISC command, if a UA or DM response is not
.ce
received correctly, Timer\ T1 will run out in the DCE. The DCE will then
resend the DISC command and will restart Timer\ T1. After transmission
of the DISC
.ce
command N2 times by the DCE, appropriate higher layer recovery action will
be initiated. The value of N2 is defined in \(sc\ 2.4.8.4 below.
.ce
.parag
.ce
2.4.4.4
.ce
\fIDisconnected phase\fR
.ce
.parag
.ce
2.4.4.4.1
.ce
After having received a DISC command from the DTE and
.ce
returned a UA response to the DTE, or having received the UA response to a
.ce
transmitted DISC command, the DCE will enter the disconnected phase.
.ce
.parag
.ce
In the disconnected phase, the DCE may initiate data link set\(hyup. In
the disconnected phase, the DCE will react to the receipt of an SABM/SABME
command as described in \(sc\ 2.4.4.1 above and will transmit a DM response
in
.ce
answer to a received DISC command. When receiving any other command (defined,
or undefined or not implemented) with the P\ bit set to\ 1, the DCE will
transmit a DM response with the F\ bit set to\ 1. Other frames received
in the
.ce
disconnected phase will be ignored by the DCE.
.ce
.parag
.ce
2.4.4.4.2
.ce
When the DCE enters the disconnected phase after
.ce
detecting error conditions as listed in \(sc\ 2.4.6 below, or after an internal
.ce
malfunction, it may indicate this by sending a DM response rather than
a DISC command. In these cases, the DCE will transmit a DM response and
start its
.ce
Timer\ T1 (see \(sc\ 2.4.8.1 below).
.ce
.parag
.ce
If Timer T1 runs out before the reception of an SABM/SABME or DISC command
from the DTE, the DCE will retransmit the DM response and restart
.ce
Timer\ T1. After transmission of the DM response N2 times, the DCE will
remain in the disconnected phase and appropriate recovery actions will
be initiated. The value of N2 is defined in \(sc\ 2.4.8.4 below.
.ce
.parag
.ce
Alternatively, after an internal malfunction, the DCE may either
.ce
initiate a data link resetting procedure (see \(sc\ 2.4.7 below) or disconnect
the data link (see \(sc\ 2.4.4.3 above) prior to initiating a data link
set\(hyup
.ce
procedure (see \(sc\ 2.4.4.1 above).
.ce
.parag
.ce
2.4.4.5
.ce
\fICollision of unnumbered commands\fR
.ce
.parag
.ce
Collision situations shall be resolved in the following way:
.ce
.parag
.ce
2.4.4.5.1
.ce
If the sent and received unnumbered commands are the
.ce
same, the DCE and the DTE shall each send the UA response at the earliest
.ce
possible opportunity. The DCE shall enter the indicated phase either,
.ce
.parag
.ce

.ce
1)
.ce
after receiving the UA response,
.ce
.parag
.ce
2)
.ce
after sending the UA response, or
.ce
.parag
.ce
3)
.ce
after timing out waiting for the UA response having sent a
.ce
UA response.
.ce
.parag
.ce
In the case of 2) above, the DCE will accept a subsequent UA
.ce
response to the mode\(hysetting command it issued without causing an exception
.ce
condition if received within the time\(hyout interval.
.ce
.parag
.ce

.ce
2.4.4.5.2
.ce
If the sent and received unnumbered commands are
.ce
different, the DCE and the DTE shall each enter the disconnected phase and
.ce
issue a DM response at the earliest possible opportunity.
.ce
.parag
.ce
2.4.4.6
.ce
\fICollision of DM response with SABM/SABME or DISC\fR
.ce

.ce
\fIcommand\fR
.ce
.parag
.ce
When a DM response is issued by the DCE or DTE as an unsolicited
.ce
response to request the DTE or DCE, respectively, to issue a mode\(hysetting
.ce
command as described in \(sc\ 2.4.4.4, a collision between an SABM/SABME
or DISC
.ce
command and the unsolicited DM response may occur. In order to avoid
.ce
misinterpretation of the DM response received, the DTE always sends its
.ce
SABM/SABME or DISC command with the P\ bit set to\ 1.
.ce
.parag
.ce
2.4.4.7
.ce
\fICollision of DM responses\fR
.ce
.parag
.ce
A contention situation may occur when both the DCE and the DTE
.ce
issue a DM response to request a mode\(hysetting command. In this case, the DTE
.ce
will issue an SABM/SABME command to resolve the contention situation.
.ce
.parag
.ce

.ce
2.4.5
.ce
\fILAPB procedures for information transfer\fR
.ce
.parag
.ce
The procedures which apply to the transmission of I\ frames in each direction
during the information transfer phase are described below.
.ce
.parag
.ce
In the following, \*Qnumber one higher\*U is in reference to a
.ce
continuously repeated sequence series, i.e., 7 is 1\ higher than 6 and 0 is
.ce
1\ higher than 7 for modulo\ 8 series, and 127 is 1\ higher than 126 and 0 is
.ce
1\ higher than 127 for modulo\ 128 series.
.ce
.parag
.ce
2.4.5.1
.ce
\fISending I frames\fR
.ce
.parag
.ce
When the DCE has an I frame to transmit (i.e. an I frame not
.ce
already transmitted, or having to be retransmitted as described in \(sc\
2.4.5.6
.ce
below), it will transmit it with an N(S) equal to its current send state
.ce
variable V(S), and an N(R) equal to its current receive state variable
V(R). At the end of the transmission of the I\ frame, the DCE will increment
its send
.ce
state variable V(S) by\ 1.
.ce
.parag
.ce
If Timer T1 is not running at the time of transmission of an I frame, it
will be started.
.ce
.parag
.ce
If the send state variable V(S) is equal to the last value of N(R)
.ce
received plus \fIk\fR
.ce
(where \fIk\fR
.ce
is the maximum number of outstanding I\ frames \(em
.ce
see \(sc\ 2.4.8.6 below), the DCE will not transmit any new I\ frames, but may
.ce
retransmit an I\ frame as described in \(sc\(sc\ 2.4.5.6 or 2.4.5.9 below.
.ce
.parag
.ce
When the DCE is in the busy condition, it may still transmit I frames,
provided that the DTE is not busy. When the DCE is in the frame rejection
.ce
condition, it will stop transmitting I\ frames.
.ce
.parag
.ce
2.4.5.2
.ce
\fIReceiving an I frame\fR
.ce
.parag
.ce
2.4.5.2.1
.ce
When the DCE is not in a busy condition and receives a valid I\ frame whose
send sequence number N(S) is equal to the DCE receive state
.ce
variable V(R), the DCE will accept the information field of this frame,
.ce
increment by one its receive state variable V(R), and act as follows:
.ce
.parag
.ce

.ce
a)
.ce
If the DCE is still not in a busy condition:
.ce
.parag
.ce
i)
.ce
If an I frame is available for transmission by the
.ce
DCE, it may act as in \(sc\ 2.4.5.1 above and acknowledge
.ce
the received I frame by setting N(R) in the control field
.ce
of the next transmitted I\ frame to the value of the DCE
.ce
receive state variable V(R). Alternatively, the DCE may
.ce
acknowledge the received I\ frame by transmitting an RR
.ce
frame with the N(R) equal to the value of the DCE receive
.ce
state variable V(R).
.ce
.parag
.ce
ii)
.ce
If no I frame is available for transmission by the
.ce
DCE, it will transmit an RR frame with N(R) equal to the
.ce
value of the DCE receive state variable V(R).
.ce
.parag
.ce

.ce
b)
.ce
If the DCE is now in a busy condition, it will transmit an
.ce
RNR frame with N(R) equal to the value of the DCE receive
.ce
state variable V(R) (see \(sc\ 2.4.5.8).
.ce
.parag
.ce

.ce
2.4.5.2.2
.ce
When the DCE is in a busy condition, it may ignore the
.ce
information field contained in any received I\ frame.
.ce
.parag
.ce
2.4.5.3
.ce
\fIReception of invalid frames\fR
.ce
.parag
.ce
When the DCE receives an invalid frame (see \(sc\ 2.3.5.3), this frame
will be discarded.
.ce
.parag
.ce
2.4.5.4
.ce
\fIReception of out\(hyof\(hysequence I frames\fR
.ce
.parag
.ce
When the DCE receives a valid I frame whose send sequence number
.ce
N(S) is incorrect, i.e.,\ not equal to the current DCE receive state variable
.ce
V(R), it will discard the information field of the I\ frame and transmit
an REJ frame with the N(R) set to one higher than the N(S) of the last
correctly
.ce
received I\ frame. The REJ frame will be a command frame with the P\ bit
set to\ 1 if an acknowledged transfer of the retransmission request is
required;
.ce
otherwise the REJ frame may be either a command or a response frame. The DCE
.ce
will then discard the information field of all I\ frames received until the
.ce
expected I\ frame is correctly received. When receiving the expected I\ frame,
.ce
the DCE will then acknowledge the I\ frame as described in \(sc\ 2.4.5.2
above. The DCE will use the N(R) and P\ bit information in the discarded
I\ frames as
.ce
described in \(sc\ 2.3.5.2 above.
.ce
.parag
.ce
2.4.5.5
.ce
\fIReceiving acknowledgement\fR
.ce
.parag
.ce
When correctly receiving an I frame or a supervisory frame (RR, RNR or
REJ), even in the busy condition, the DCE will consider the N(R) contained
in this frame as an acknowledgement for all I\ frames it has transmitted
with an N(S) up to and including the received N(R)\(em1. The DCE will stop
Timer\ T1 when it correctly receives an I\ frame or a supervisory frame
with the N(R) higher
.ce
than the last received N(R) (actually acknowledging some I\ frames), or
an REJ frame with an N(R) equal to the last received N(R).
.ce
.parag
.ce

.ce
If Timer T1 has been stopped by the receipt on an I, RR or RNR frame, and
if there are outstanding I\ frames still unacknowledged, the DCE will
.ce
restart Timer\ T1. If Timer\ T1 then runs out, the DCE will follow the
recovery procedure (\(sc\ 2.4.5.9 below) with respect to the unacknowledged
I\ frames. If
.ce
Timer\ T1 has been stopped by the receipt of an REJ frame, the DCE will
follow the retransmission procedures in \(sc\ 2.4.5.6 below.
.ce
.parag
.ce
2.4.5.6
.ce
\fIReceiving an REJ frame\fR
.ce
.parag
.ce
When receiving an REJ frame, the DCE will set its send state
.ce
variable V(S) to the N(R) received in the REJ control field. It will transmit
the corresponding I\ frame as soon as it is available or retransmit it
in
.ce
accordance with the procedures described in \(sc\ 2.4.5.1 above. (Re)transmission
will conform to the following procedure:
.ce
.parag
.ce
i)
.ce
if the DCE is transmitting a supervisory command or response
.ce
when it receives the REJ frame, it will complete that
.ce
transmission before commencing transmission of the requested
.ce
I\ frame;
.ce
.parag
.ce

.ce
ii)
.ce
if the DCE is transmitting an unnumbered command or
.ce
response when it receives the REJ frame, it will ignore the
.ce
request for retransmission;
.ce
.parag
.ce
iii)
.ce
if the DCE is transmitting an I frame when the REJ frame
.ce
is received, it may abort the I\ frame and commence transmission
.ce
of the requested I\ frame immediately after abortion;
.ce
.parag
.ce
iv)
.ce
if the DCE is not transmitting any frame when the REJ frame
.ce
is received, it will commence transmission of the requested
.ce
I\ frame immediately.
.ce
.parag
.ce
In all cases, if other unacknowledged I frames had already been
.ce
transmitted following the one indicated in the REJ frame, then those I
frames will be retransmitted by the DCE following the retransmission of
the requested I\ frame. Other I\ frames not yet transmitted may be transmitted
following the
.ce
retransmitted I\ frames.
.ce
.parag
.ce
If the REJ frame was received from the DTE as a command with the P bit
set to\ 1, the DCE will transmit an RR, RNR or REJ response with the F
bit set to\ 1 before transmitting or retransmitting the corresponding I\
frame.
.ce
.parag
.ce
2.4.5.7
.ce
\fIReceiving an RNR frame\fR
.ce
.parag
.ce
After receiving an RNR frame whose N(R) acknowledges all frames
.ce
previously transmitted, the DCE will stop Timer\ T1 and may then transmit
.ce
an I\ frame, with the P\ bit set to\ 0, whose send sequence number is
.ce
equal to the N(R) indicated in the RNR frame, restarting Timer\ T1 as it
does. After receiving an RNR frame whose N(R) indicates a previously transmitted
.ce
frame, the DCE will not transmit or retransmit any I\ frame, Timer\ T1 being
.ce
already running. In either case, if the Timer\ T1 runs out before receipt
of a busy clearance indication, the DCE will follow the procedure described
in
.ce
\(sc\ 2.4.5.9\ below. In any case, the DCE will not transmit any other
I\ frames
.ce
before receiving an RR or REJ frame, or before the completion of a link
.ce
resetting procedure.
.ce
.parag
.ce
Alternatively, after receiving an RNR frame, the DCE may wait for a
.ce
period of time (e.g.,\ the length of the Timer\ T1) and then transmit a
.ce
supervisory command frame (RR, RNR or REJ) with the P\ bit set to\ 1, and
start Timer\ T1, in order to determine if there is any change in the receive
status of the DTE. The DTE shall respond to the P\ bit set to\ 1 with a
supervisory
.ce
response frame (RR, RNR or REJ) with the F\ bit set to\ 1 indicating either
.ce
continuance of the busy condition (RNR) or clearance of the busy condition
(RR or REJ). Upon receipt of the DTE response, Timer\ T1 is stopped.
.ce
.parag
.ce

.ce
1)
.ce
If the response is the RR or REJ response, the busy
.ce
condition is cleared and the DCE may transmit I\ frames beginning
.ce
with the I\ frame identified by the N(R) in the received response
.ce
frame.
.ce
.parag
.ce
2)
.ce
If the response is the RNR response, the busy condition
.ce
still exists, and the DCE will after a period of time (e.g.\ the
.ce
length of Timer\ T1) repeat the enquiry of the DTE receive
.ce
status.
.ce
.parag
.ce
If Timer T1 runs out before a status response is received, the
.ce
enquiry process above is repeated. If N2 attempts to get a status response
fail (i.e.\ Timer\ T1 runs out N2\ times), the DCE will initiate a data
link resetting procedure as described in \(sc\ 2.4.7.2\ below or will transmit
a DM response to ask the DTE to initiate a data link set\(hyup procedure
as described in \(sc\ 2.4.4.1 and enter the disconnected phase. The value
of N2 is defined in \(sc\ 2.4.8.4\ below.
.ce
.parag
.ce

.ce
If, at any time during the enquiry process, an unsolicited RR or REJ frame
is received from the DTE, it will be considered to be an indication of
.ce
clearance of the busy condition. Should the unsolicited RR or REJ frame be a
.ce
command frame with the P bit set to\ 1, the appropriate response frame
with the F\ bit set to 1 must be transmitted before the DCE may resume
transmission of
.ce
I\ frames. If Timer\ T1 is running, the DCE will wait for the non\(hybusy
response with the F bit set to\ 1 or will wait for Timer\ T1 to run out
and then either
.ce
may reinitiate the enquiry process in order to realize a successful P/F bit
.ce
exchange or may resume transmission of I frames beginning with the I\ frame
.ce
identified by the N(R) in the received RR or REJ frame.
.ce
.parag
.ce
2.4.5.8
.ce
\fIDCE busy condition\fR
.ce
.parag
.ce
When the DCE enters a busy condition, it will transmit an RNR frame at
the earliest opportunity. The RNR frame will be a command frame with the
P bit set to\ 1 if an acknowledged transfer of the busy condition indication
is
.ce
required; otherwise the RNR frame may be either a command or a response
frame. While in the busy condition, the DCE will accept and process supervisory
.ce
frames, will accept and process the contents of the N(R) fields of I\ frames,
.ce
and will return an RNR response with the F bit set to\ 1 if it receives a
.ce
supervisory command or I command frame with the P bit set to\ 1. To clear the
.ce
busy condition, the DCE will transmit either an REJ frame or an RR frame, with
.ce
N(R) set to the current receive state variable V(R), depending on whether or
.ce
not it discarded information fields of correctly received I\ frames. The REJ
.ce
frame or the RR frame will be a command frame with the P bit set to\ 1 if an
.ce
acknowledged transfer of the busy\(hyto\(hynon\(hybusy transition is required,
otherwise the REJ frame or the RR frame may be either a command or a response
frame.
.ce
.parag
.ce

.ce
2.4.5.9
.ce
\fIWaiting acknowledgement\fR
.ce
.parag
.ce
The DCE maintains an internal transmission attempt variable which is set
to\ 0 when the DCE sends a UA response, when the DCE receives a UA
.ce
response or an RNR command or response, or when the DCE correctly receives
an I\ frame or supervisory frame with the N(R) higher than the last received
N(R) (actually acknowledging some outstanding I\ frames).
.ce
.parag
.ce
If Timer T1 runs out waiting for the acknowledgement from the DTE for an
I\ frame transmitted, the DCE will enter the timer recovery condition,
add
.ce
one to its transmission attempt variable and set an internal variable \fIx\fR
.ce
to
.ce
the current value of its send state variable V(S). The DCE will then restart
.ce
Timer T1, set its send state variable V(S) to the last value of N(R) received
from the DTE and retransmit the corresponding I\ frame with the P bit set
to\ 1, or transmit an appropriate supervisory command frame (RR, RNR or
REJ) with the P\ bit set to\ 1.
.ce
.parag
.ce

.ce
The timer recovery condition is cleared when the DCE receives a valid supervisory
frame with the F\ bit\ set to\ 1.
.ce
.parag
.ce
If, while in the timer recovery condition, the DCE correctly receives a
supervisory frame with the F bit set to\ 1 and with the N(R) within the
range from its current send state variable V(S) to \fIx\fR
.ce
included, it will clear the
.ce
timer recovery condition (including stopping Timer\ T1) and set its send
state variable V(S) to the value of the received N(R), and may then resume
with
.ce
I\ frame transmission or retransmission, as appropriate.
.ce
.parag
.ce
If, while in the timer recovery condition, the DCE correctly receives an
I or supervisory frame with the P/F bit set to\ 0 and with a valid N(R)
(see \(sc\ 2.3.4.9), it will not clear the timer recovery condition. The
value of the
.ce
received N(R) may be used to update the send state variable V(S). However,
the DCE may decide to keep the last transmitted I\ frame in store (even
if it is
.ce
acknowledged) in order to be able to retransmit it with the P bit set to\
1 when Timer\ T1 runs out at a later time.
.ce
.parag
.ce

.ce
If the received supervisory frame with the P/F bit set to\ 0 is an REJ
frame with a valid N(R), the DCE may either immediately initiate
.ce
(re)transmission from the value of the send state variable V(S), or it may
.ce
ignore the request for retransmission and wait until the supervisory frame
with the F bit set to\ 1 is received before initiating (re)transmission
of frames
.ce
from the value identified in the N(R) field of the supervisory frame with
the F\ bit set to\ 1. In the case of immediate retransmission, in order
to prevent
.ce
duplicate retransmissions following the clearance of the timer recovery
.ce
condition, the DCE shall inhibit retransmission of a specific I\ frame [same
.ce
N(R) in the same numbering cycle] if the DCE has retransmitted that I\
frame as the result of a received REJ frame with the P/F bit set to\ 0.
.ce
.parag
.ce
If, while in the timer recovery condition, the DCE receives a REJ
.ce
command with the P bit set to\ 1, the DCE will respond immediately with an
.ce
appropriate supervisory response with the F bit set to\ 1. The DCE may
then use the value of the N(R) in the REJ command to update the send state
variable
.ce
V(S), and may either immediately begin (re)transmission from the value N(R)
.ce
indicated in the REJ frame or ignore the request for retransmission and wait
.ce
until the supervisory frame with the F bit set to\ 1 is received before
.ce
initiating (re)transmission of I\ frames from the value identified in the
N(R) field of the supervisory frame with the F\ bit set to\ 1. In the case
of
.ce
immediate retransmission, in order to prevent duplicate retransmissions
.ce
following the clearance of the timer recovery condition, the DCE shall
inhibit retransmission of a specific I\ frame [same N(R) in the same numbering
cycle]
.ce
if the DCE has retransmitted that I\ frame as the result of the received REJ
.ce
command with the P\ bit set to\ 1.
.ce
.parag
.ce
If Timer T1 runs out in the timer recovery condition, and no I or
.ce
supervisory frame with the P/F bit set to 0 and with a valid N(R) has been
.ce
received, or no REJ command with the P bit set to\ 1 and with a valid N(R)
has been received, the DCE will add one to its transmission attempt variable,
.ce
restart Timer\ T1, and either retransmit the I frame sent with the P bit set
.ce
to\ 1 or transmit an appropriate supervisory command with the P bit set to\ 1.
.ce
.parag
.ce

.ce
If the transmission attempt variable is equal to N2, the DCE will
.ce
initiate a data link resetting procedure as described in \(sc\ 2.4.7.2
below, or
.ce
will transmit a DM response to ask the DTE to initiate a data link set\(hyup
.ce
procedure as described in \(sc\ 2.4.4.1 above and enter the disconnected
phase. N2 is a system parameter (see \(sc\ 2.4.8.4 below).
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ Although the DCE may implement the internal variable \fIx\fR
.ce
,
.ce
other mechanisms do exist that achieve the identical function.
.ce
.parag
.ce
2.4.6
.ce
\fILAPB conditions for \fR
.ce
\fIdata link resetting or data link\fR
.ce

.ce
\fIre\(hyinitialization\fR
.ce
\fI(data link set\(hyup)\fR
.ce
.parag
.ce
2.4.6.1
.ce
When the DCE receives, during the information transfer phase, a
.ce
frame which is not invalid (see \(sc\ 2.3.5.3) with one of the conditions
listed in \(sc\ 2.3.4.9 above, the DCE will request the DTE to initiate
a data link resetting procedure by transmitting an FRMR response to the
DTE as described in
.ce
\(sc\ 2.4.7.3.
.ce
.parag
.ce
2.4.6.2
.ce
When the DCE receives, during the information transfer phase, an FRMR response
from the DTE, the DCE will either initiate the data link
.ce
resetting procedures itself as described in \(sc\ 2.4.7.2 or return a DM
response to ask the DTE to initiate the data link set\(hyup (initialization)
procedure as described in \(sc\ 2.4.4.1. After transmitting a DM response,
the DCE will enter
.ce
the disconnected phase as described in \(sc\ 2.4.4.4.2.
.ce
.parag
.ce
2.4.6.3
.ce
When the DCE receives, during the information transfer phase, a UA response,
or an unsolicited response with the F bit set to\ 1, the DCE may
.ce
either initiate the data link resetting procedures itself as described in
.ce
\(sc\ 2.4.7.2, or return a DM response to ask the DTE to initiate the data link
.ce
set\(hyup (initialization) procedure as described in \(sc\ 2.4.4.1. After
transmitting a DM response, the DCE will enter the disconnected phase as
described in
.ce
\(sc\ 2.4.4.4.2.
.ce
.parag
.ce
2.4.6.4
.ce
When the DCE receives, during the information transfer phase, a DM response
from the DTE, the DCE will either initiate the data link set\(hyup
.ce
(initialization) procedures itself as described in \(sc\ 2.4.4.1, or return
a DM
.ce
response to ask the DTE to initiate the data link set\(hyup (initialization)
.ce
procedures as described in \(sc\ 2.4.4.1. After transmitting a DM response,
the DCE will enter the disconnected phase as described in \(sc\ 2.4.4.4.2.
.ce
.parag
.ce

.ce
2.4.7
.ce
\fILAPB procedure for data link resetting\fR
.ce
.parag
.ce
2.4.7.1
.ce
The data link resetting procedure is used to initialize both
.ce
directions of information transfer according to the procedure described
below. The data link resetting procedure only applies during the information
transfer phase.
.ce
.parag
.ce
2.4.7.2
.ce
Either the DTE or the DCE may initiate the data link resetting
.ce
procedure. The data link resetting procedure indicates a clearance of a DCE
.ce
and/or DTE busy condition, if present.
.ce
.parag
.ce
The DTE shall initiate a data link resetting by transmitting an
.ce
SABM/SABME command to the DCE. If, upon correct receipt of the SABM/SABME
.ce
command, the DCE determines that it can continue in the information transfer
.ce
phase, it will return a UA response to the DTE, will reset its send and
receive state variables V(S) and V(R) to zero, and will remain in the information
.ce
transfer phase. If, upon correct receipt of the SABM/SABME command, the DCE
.ce
determines that it cannot remain in the information transfer phase, it will
.ce
return a DM response as a denial to the resetting request and will enter the
.ce
disconnected phase.
.ce
.parag
.ce
The DCE will initiate a data link resetting by transmitting an
.ce
SABM/SABME command to the DTE and starting its Timer\ T1 (see \(sc\ 2.4.8.1
below). Upon reception of a UA response from the DTE, the DCE will reset
its send and receive state variables V(S) and V(R) to zero, will stop its
Timer\ T1, and will remain in the information transfer phase. Upon reception
of a DM response from the DTE as a denial to the data link resetting request,
the DCE will stop its Timer\ T1 and will enter the disconnected phase.
.ce
.parag
.ce
The DCE, having sent an SABM/SABME command, will ignore and discard
.ce
any frames received from the DTE except an SABM/SABME or DISC command,
or a UA or DM response. The receipt of an SABM/SABME or DISC command from
the DTE will result in a collision situation that is resolved per \(sc\
2.4.4.5 above. Frames
.ce
other than the UA or DM response sent in response to a received SABM/SABME
or DISC command will be sent only after the data link is reset and if no
.ce
outstanding SABM/SABME command exists.
.ce
.parag
.ce

.ce
After the DCE sends the SABM/SABME command, if a UA or DM response is not
received correctly, Timer\ T1 will run out in the DCE. The DCE will then
.ce
resend the SABM/SABME command and will restart Timer\ T1. After N2 attempts
to reset the data link, the DCE will initiate appropriate higher layer
recovery
.ce
action and will enter the disconnected phase. The value of N2 is defined in
.ce
\(sc\ 2.4.8.4 below.
.ce
.parag
.ce
2.4.7.3
.ce
The DCE may ask the DTE to reset the data link by transmitting an FRMR
response (see \(sc\ 2.4.6.1 above). After transmitting an FRMR response,
.ce
the DCE will enter the frame rejection condition.
.ce
.parag
.ce
The frame rejection condition is cleared when the DCE receives an SABM/SABME
command, a DISC command, a FRMR response, or a DM response; or if
.ce
the DCE transmits an SABM/SABME command, a DISC command, or a DM response.
.ce
Other commands received while in the frame rejection condition will cause
the DCE to retransmit the FRMR response with the same information field
as
.ce
originally transmitted.
.ce
.parag
.ce
The DCE may start Timer\ T1 on transmission of the FRMR response. If
.ce
Timer\ T1 runs out before the frame rejection condition is cleared, the
DCE may retransmit the FRMR response, and restart T1. After N2 attempts
(time outs) to get the DTE to reset the data link, the DCE may reset the
data link itself as described in \(sc\ 2.4.7.2 above. The value of N2 is
defined in \(sc\ 2.4.8.4 below.
.ce
.parag
.ce

.ce
In the frame rejection condition, I frames and supervisory frames will
not be transmitted by the DCE. Also, received I frames and supervisory
frames will be discarded by the DCE except for the observance of a P bit
set to\ 1.
.ce
When an additional FRMR response must be transmitted by the DCE as a result
of the receipt of a P\ bit set to 1 while Timer\ T1 is running, Timer\
T1 will
.ce
continue to run. Upon reception of an FRMR response (even during a frame
.ce
rejection condition), the DCE will initiate a resetting procedure by
.ce
transmitting an SABM/SABME command as described in \(sc\ 2.4.7.2 above, or will
.ce
transmit a DM response to ask the DTE to initiate the data link set\(hyup
.ce
procedure as described in \(sc\ 2.4.4.1 and enter the disconnected phase.
.ce
.parag
.ce
2.4.8
.ce
\fIList of \fR
.ce
\fILAPB system parameters\fR
.ce
.parag
.ce
The DCE and DTE system parameters are as follows:
.ce
.parag
.ce

.ce
2.4.8.1
.ce
\fITimer\fR
.ce
\fIT1\fR
.ce
.parag
.ce
The value of the DTE Timer T1 system parameter may be different
.ce
than the value of the DCE Timer T1 system parameter. These values shall
be made known to both the DTE and the DCE, and agreed to for a period of
time by both the DTE and the DCE.
.ce
.parag
.ce
The period of Timer T1, at the end of which retransmission of a frame may
be initiated (see \(sc\ 2.4.4 and \(sc\ 2.4.5 above for the DCE), shall
take into account whether T1 is started at the beginning or the end of
the transmission of a frame.
.ce
.parag
.ce
The proper operation of the procedure requires that the transmitter's (DCE
or DTE) Timer\ T1 be greater than the maximum time between transmission
of a frame (SABM/SABME, DISC, I\ or supervisory command, or DM or FRMR
response)
.ce
and the reception of the corresponding frame returned as an answer to that
.ce
frame (UA, DM or acknowledging frame). Therefore, the receiver (DCE or DTE)
.ce
should not delay the response or acknowledging frame returned to one of the
.ce
above frames by more than a value\ T2, where T2 is a system parameter (see
.ce
\(sc\ 2.4.8.2).
.ce
.parag
.ce

.ce
The DCE will not delay the response or acknowledging frame returned to
one of the above DTE frames by more than a period\ T2.
.ce
.parag
.ce
2.4.8.2
.ce
\fIParameter T2\fR
.ce
.parag
.ce
The value of the DTE parameter T2 may be different than the value of the
DCE parameter T2. These values shall be made known to both the DTE and
the DCE, and agreed to for a period of time by both the DTE and the DCE.
.ce
.parag
.ce

.ce
The period of parameter T2 shall indicate the amount of time
.ce
available at the DCE or DTE before the acknowledging frame must be initiated
in order to ensure its receipt by the DTE or DCE, respectively, prior to
Timer\ T1 running out at the DTE or DCE (parameter\ T2\ <\ Timer\ T1).
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ The period of parameter T2 shall take into account the
.ce
following timing factors: the transmission time of the acknowledging frame,
the propagation time over the access data link, the stated processing times
at the DCE and the DTE, and the time to complete the transmission of the
frame(s) in the DCE or DTE transmit queue that are neither displaceable
or modifiable in an orderly manner.
.ce
.parag
.ce

.ce
Given a value for Timer T1 for the DTE or DCE, the value of parameter T2
at the DCE or DTE, respectively, must be no larger than T1 minus\ 2 times
the propagation time over the access data link, minus the frame processing
time at the DCE, minus the frame processing time at the DTE, and minus
the transmission time of the acknowledging frame by the DCE or DTE, respectively.
.ce
.parag
.ce

.ce
2.4.8.3
.ce
\fITimer T3\fR
.ce
.parag
.ce
The DCE shall support a Timer T3 system parameter, the value of
.ce
which shall be made known to the DTE.
.ce
.parag
.ce
The period of Timer T3, at the end of which an indication of an
.ce
observed excessively long idle channel state condition is passed to the
Packet Layer, shall be sufficiently greater than the period of the DCE
Timer T1
.ce
(i.e.\ T3\ >\ T1) so that the expiration of T3 provides the desired level of
.ce
assurance that the data link channel is in a non\(hyactive, non\(hyoperational
state, and is in need of data link set\(hyup before normal data link operation
can
.ce
resume.
.ce
.parag
.ce
2.4.8.4
.ce
\fIMaximum number of attempts\fR
.ce
\fIto complete a\fR
.ce

.ce
\fItransmission N2\fR
.ce
.parag
.ce

.ce
The value of the DTE N2 system parameter may be different than the value
of the DCE N2 system parameter. These values shall be made known to both
the DTE and the DCE, and agreed to for a period of time by both the DTE
and the DCE.
.ce
.parag
.ce
The value of N2 shall indicate the maximum number of attempts made by the
DCE or DTE to complete the successful transmission of a frame to the DTE
or DCE, respectively.
.ce
.parag
.ce
2.4.8.5
.ce
\fIMaximum number of bits in an I frame N1\fR
.ce
.parag
.ce
The value of the DTE N1 system parameter may be different than the value
of the DCE N1 system parameter. These values shall be made known to both
the DTE and the DCE.
.ce
.parag
.ce
The values of N1 shall indicate the maximum number of bits in an
.ce
I\ frame (excluding flags and 0\ bits inserted for transparency) that the
DCE or DTE is willing to accept from the DTE or DCE, respectively.
.ce
.parag
.ce
In order to allow for universal operation, a DTE should support a
.ce
value of DTE N1 which is not less than 1080\ bits (135\ octets). DTEs should
be aware that the network may transmit longer packets (see \(sc\ 5.2),
that may result in a data link layer problem.
.ce
.parag
.ce

.ce
All networks shall offer to a DTE which requires it, a value of DCE N1
which is greater than or equal to 2072\ bits (259\ octets) plus the length
of the address, control and FCS fields at the DTE/DCE interface, and greater
than or equal to the maximum length of the data packets which may cross
the DTE/DCE
.ce
interface plus the length of the address, control and FCS fields at the
DTE/DCE interface.
.ce
.parag
.ce
Appendix II provides a description of how the values stated above are  derived.
.ce
.parag
.ce
2.4.8.6
.ce
\fIMaximum number of \fR
.ce
\fIoutstanding I frames k\fR
.ce
.parag
.ce
The value of the DTE k system parameter shall be the same as the
.ce
value of the DCE k system parameter. This value shall be agreed to for a
.ce
period of time by both the DTE and the DCE.
.ce
.parag
.ce
The value of k shall indicate the maximum number of sequentially
.ce
numbered I\ frames that the DTE or DCE may have outstanding
.ce
(i.e.\ unacknowledged) at any given time. The value of k shall never exceed
.ce
seven for modulo\ 8 operation, or one hundred and twenty\(hyseven for modulo\
128
.ce
operation. All networks (DCEs) shall support a value of seven. Other values
of k (less than and greater than seven) may also be supported by networks
.ce
(DCEs).
.ce
.parag
.ce
.line
.ce
\fBMONTAGE : \(sc 2.5 de cette Recommandation sur le reste de cette page\fR
.ce
.parag
.ce

.ce
.line
.ad r
\fBTable 7/X.25 [T7.25], p.\fR
.sp 1P
.RT
.ad b
.RT
.ce
.line
.ce
\fBTable 8/X.25 [T8.25], p.\fR
.ce
2.3.5.2
.ce
\fIN(S) sequence error condition\fR
.ce
.parag
.ce
The information field of all I frames received whose N(S) does not equal
the receive state variable V(R) will be discarded.
.ce
.parag
.ce
An N(S) sequence error exception condition occurs in the receiver when
an I\ frame received contains an N(S) which is not equal to the receive
state
.ce
variable V(R) at the receiver. The receiver does not acknowledge (increment
its receive state variable) the I\ frame causing the sequence error, or
any I\ frame which may follow, until an I\ frame with the correct N(S)
is received.
.ce
.parag
.ce

.ce
A DCE or DTE which receives one or more valid I frames having sequence
errors or subsequent supervisory frames (RR, RNR and REJ) shall accept
the
.ce
control information contained in the N(R) field and the P or F bit to perform
data link control functions; e.g.\ to receive acknowledgement of previously
.ce
transmitted I\ frames and to cause the DCE or DTE to respond (P bit set to 1).
.ce
.parag
.ce

.ce
The means specified in \(sc\(sc\ 2.3.5.2.1 and 2.3.5.2.2 shall be available
for initiating the retransmission of lost or errored I\ frames following
the
.ce
occurrence of an N(S) sequence error condition.
.ce
.parag
.ce
2.3.5.2.1
.ce
\fIREJ recovery\fR
.ce
.parag
.ce
The REJ frame is used by a receiving DCE or DTE to initiate a
.ce
recovery (retransmission) following the detection of an N(S) sequence error.
.ce
.parag
.ce

.ce
With respect to each direction of transmission on the data link, only one
\*Qsent REJ\*U exception condition from a DCE or DTE, to a DTE or DCE,
is
.ce
established at a time. A \*Qsent REJ\*U exception condition is cleared when the
.ce
requested I\ frame is received.
.ce
.parag
.ce

.ce
A DCE or DTE receiving a REJ frame initiates sequential
.ce
(re\(hy)transmission of I\ frames starting with the I\ frame indicated
by the N(R) contained in the REJ frame. The retransmitted frames may contain
an N(R) and a P bit that are updated from, and therefore different from,
the ones contained in the originally transmitted I\ frames.
.ce
.parag
.ce
2.3.5.2.2
.ce
\fITime\(hyout recovery\fR
.ce
.parag
.ce
If a DCE or DTE, due to a transmission error, does not receive (or receives
and discards) a single I\ frame or the last I\ frame(s) in a sequence of
I\ frames, it will not detect an N(S) sequence error condition and, therefore,
will not transmit a REJ frame. The DTE or DCE which transmitted the
.ce
unacknowledged I\ frame(s) shall, following the completion of a system
specified time\(hyout period (see \(sc\(sc\ 2.4.5.1 and 2.4.5.9 below),
take appropriate recovery action to determine at which I\ frame retransmission
must begin. The
.ce
retransmitted frame(s) may contain an N(R) and a P bit that is updated from,
.ce
and therefore different from, the ones contained in the originally transmitted
frame(s).
.ce
.parag
.ce
2.3.5.3
.ce
\fIInvalid frame\fR
.ce
\fI condition\fR
.ce
.parag
.ce
Any frame which is invalid will be discarded, and no action is
.ce
taken as the result of that frame. An invalid frame is defined as one
.ce
which:
.ce
.parag
.ce
a)
.ce
is not properly bounded by two flags;
.ce
.parag
.ce
b)
.ce
in basic (modulo 8) operation, contains fewer than 32 bits
.ce
between flags; in extended (modulo\ 128) operation, contains
.ce
fewer than 40\ bits between flags of frames that contain sequence
.ce
numbers or 32\ bits between flags of frames that do not contain
.ce
sequence numbers;
.ce
.parag
.ce
c)
.ce
contains a Frame Check Sequence (FCS) error; or
.ce
.parag
.ce
d)
.ce
contains an address other than A or B (for single link
.ce
operation) or other than C or D (for multilink operation).
.ce
.parag
.ce

.ce
For those networks that are octet aligned, a detection of
.ce
non\(hyoctet alignment may be made at the Data Link Layer by adding a frame
.ce
validity check that requires the number of bits between the opening flag and
.ce
the closing flag, excluding bits inserted for transparency, to be an integral
number of octets in length, or the frame is considered invalid.
.ce
.parag
.ce
2.3.5.4
.ce
\fIFrame rejection\fR
.ce
\fI condition\fR
.ce
.parag
.ce
A frame rejection condition is established upon the receipt of an error\(hyfree
frame with one of the conditions listed in \(sc\ 2.3.4.9 above.
.ce
.parag
.ce

.ce
At the DCE or DTE, this frame rejection exception condition is
.ce
reported by an FRMR response for appropriate DTE or DCE action, respectively.
Once a DCE has established such an exception condition, no additional I\
frames are accepted until the condition is reset by the DTE, except for
examination of the P bit. The FRMR response may be repeated at each opportunity,
as specified in \(sc\ 2.4.7.3, until recovery is effected by the DTE, or
until the DCE initiates its own recovery in case the DTE does not respond.
.ce
.parag
.ce
2.3.5.5
.ce
\fIExcessive idle channel state condition on incoming\fR
.ce

.ce
\fIchannel\fR
.ce
.parag
.ce
Upon detection of an idle channel state condition (see \(sc\ 2.2.12.2 above)
on the incoming channel, the DCE shall wait for a period\ T3 (see
.ce
\(sc\ 2.4.8.3 below) without taking any specific action, waiting for detection
of a return to the active channel state (i.e.,\ detection of at least one
flag
.ce
sequence). After the period\ T3, the DCE shall notify the higher layer
(e.g.\ the Packet Layer or the MLP) of the excessive idle channel state
condition, but
.ce
shall not take any action that would preclude the DTE from establishing the
.ce
data link by normal data link set\(hyup procedures.
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ Other actions to be taken by the DCE at the Data Link Layer upon
expiration of period\ T3 is a subject for further study.
.ce
.parag
.ce

.ce
2.4
.ce
\fIDescription of the \fR
.ce
\fILAPB procedure\fR
.ce
.parag
.ce
2.4.1
.ce
\fILAPB basic and extended modes of operation\fR
.ce
.parag
.ce
In accordance with the system choice made by the DTE at
.ce
subscription time, the DCE will either support modulo\ 8 (basic) operation or
.ce
will support modulo\ 128 (extended) operation. Changing from basic operation
to extended operation, or vice versa, in the DCE requires resubscription
by the
.ce
DTE for the desired service, and is not supported dynamically.
.ce
.parag
.ce
Table 5/X.25 indicates the command and response control field formats used
with the basic (modulo\ 8) service. The mode\(hysetting command employed
to
.ce
initialize (set up) or reset the basic mode is the SABM command. Table
6/X.25 indicates the command and response control field formats used with
the extended (modulo\ 128) service. The mode\(hysetting command employed
to initialize (set up) or reset the extended mode is the SABME command.
.ce
.parag
.ce
2.4.2
.ce
\fILAPB procedure for addressing\fR
.ce
.parag
.ce
The address field identifies a frame as either a command or a
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response. A command frame contains the address of the DCE or DTE to which
the command is being sent. A response frame contains the address of the
DCE or DTE sending the frame.
.ce
.parag
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In order to allow differentiation between single link operation and
.ce
the optional multilink operation for diagnostic and/or maintenance reasons,
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different address pair encodings are assigned to data links operating with
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multilink procedure compared to data links operating with the single link
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procedure.
.ce
.parag
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Frames containing commands transferred from the DCE to the DTE will
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contain the address\ A for the single link operation and address\ C for the
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multilink operation.
.ce
.parag
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Frames containing responses transferred from the DCE to the DTE will contain
the address\ B for the single link operation and address\ D for the
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multilink operation.
.ce
.parag
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Frames containing commands transferred from the DTE to the DCE shall contain
the address\ B for the single link operation and address\ D for the
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multilink operation.
.ce
.parag
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Frames containing responses transferred from the DTE to the DCE shall contain
the address\ A for the single link operation and address\ C for the
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multilink operation.
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.parag
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These addresses are coded as follows:
.ce
.parag
.ce
Address
.ce
1\ 2\ 3\ 4\ 5\ 6\ 7\ 8
.ce
.parag
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Single link operation
.ce
\ \ A
.ce
1\ 1\ 0\ 0\ 0\ 0\ 0\ 0
.ce
.line
.ce
\ \ B
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1\ 0\ 0\ 0\ 0\ 0\ 0\ 0
.ce
.parag
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Multilink operation
.ce
\ \ C
.ce
1\ 1\ 1\ 1\ 0\ 0\ 0\ 0
.ce
.line
.ce
\ \ D
.ce
1\ 1\ 1\ 0\ 0\ 0\ 0\ 0
.ce
.parag
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\fINote\fR
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\ \(em\ The DCE will discard all frames received with an address other
than\ A or\ B (single link operation), or\ C or\ D (multilink operation).
.ce
.parag
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2.4.3
.ce
\fILAPB procedure for the use of the P/F bit\fR
.ce
.parag
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The DCE or DTE receiving an SABM/SABME, DISC, supervisory command or I\
frame with the P\ bit set to\ 1 will set the F\ bit to\ 1 in the next response
frame it transmits.
.ce
.parag
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The response frame returned by the DCE to an SABM/SABME or DISC
.ce
command with the P\ bit set to\ 1 will be a UA or DM response with the
F\ bit set to\ 1. The response frame returned by the DCE to an I\ frame
with the P\ bit set to\ 1, received during the information transfer phase,
will be an RR, REJ, RNR or FRMR response with the F\ bit set to\ 1. The
response frame returned by the
.ce
DCE to a supervisory command with the P\ bit set to\ 1, received during the
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information transfer phase, will be an RR, REJ, RNR or FRMR response with
the F\ bit set to\ 1. The response frame returned by the DCE to an I\ frame
or
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supervisory frame with the P\ bit set to\ 1, received during the disconnected
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phase, will be a DM response with the F\ bit set to\ 1.
.ce
.parag
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The P bit may be used by the DCE in conjunction with the timer
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recovery condition (see \(sc\ 2.4.5.9 below).
.ce
.parag
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\fINote\fR
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\ \(em\ Other use of the P bit by the DCE is a subject for further
.ce
study.
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.parag
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2.4.4
.ce
\fILAPB procedure for data link set\(hyup and disconnection\fR
.ce
.parag
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.ce
2.4.4.1
.ce
\fIData link set\(hyup\fR
.ce
.parag
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The DCE will indicate that it is able to set up the data link by
.ce
transmitting contiguous flags (active channel state).
.ce
.parag
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Either the DTE or the DCE may initiate data link set\(hyup. Prior to
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initiation of data link set\(hyup, either the DCE or the DTE may initiate data
.ce
link
.ce
disconnection (see \(sc\ 2.4.4.3) for the purpose of insuring that the
DCE and the DTE are in the same phase. The DCE may also transmit an unsolicited
DM response to request the DTE to initiate data link set\(hyup.
.ce
.parag
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The DTE shall initiate data link set\(hyup by transmitting an SABM/SABME
command to the DCE. If, upon receipt of the SABM/SABME command correctly,
the DCE determines that it can enter the information transfer phase, it
will return a UA response to the DTE, will reset its send and receive state
variables V(S) and V(R) to zero, and will consider that the data link is
set up. If, upon
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receipt
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of the SABM/SABME command correctly, the DCE determines that it cannot enter
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the information transfer phase, it will return a DM response to the DTE as a
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denial to the data link set\(hyup initialization and will consider that the
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data link is
.ce
\fInot\fR
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 set up. In order to avoid misinterpretation of the DM response received,
it is suggested that the DTE always sends its SABM/SABME command with the
P\ bit set to\ 1. Otherwise, it is not possible to differentiate a DM response
intended as a denial to data link set\(hyup from a DM response that is
issued in a separate unsolicited sense as a request for a mode\(hysetting
command (as described
.ce
in\ \(sc\ 2.4.4.4.2).
.ce
.parag
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The DCE will initiate data link set\(hyup by transmitting an SABM/SABME
command to the DTE and starting its Timer\ T1 in order to determine when
too
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much time has elapsed waiting for a reply (see \(sc\ 2.4.8.1 below). Upon
reception of a UA response from the DTE, the DCE will reset its send and
receive state
.ce
variables\ V(S) and\ V(R) to zero, will stop its Timer\ T1, and will consider
that the data link is set up. Upon reception of a DM response from the
DTE as a
.ce
denial
.ce
to the data link set\(hyup initialization, the DCE will stop its Timer\
T1 and will consider that the data link is \fInot\fR
.ce
set up.
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.parag
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.ce
The DCE, having sent the SABM/SABME command, will ignore and discard any
frames except an SABM/SABME or DISC command, or a UA or DM response
.ce
received from the DTE. The receipt of an SABM/SABME or DISC command from the
.ce
DTE will result in a collision situation that is resolved per \(sc\ 2.4.4.5
below. Frames other than the UA and DM responses sent in response to a
received
.ce
SABM/SABME or DISC command will be sent only after the data link is set
up and if no outstanding SABM/SABME command exists.
.ce
.parag
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After the DCE sends the SABM/SABME command, if a UA or DM response is not
received correctly, Timer\ T1 will run out in the DCE. The DCE will then
.ce
resend the SABM/SABME command and will restart Timer\ T1. After transmission
of the SABM/SABME command N2 times by the DCE, appropriate higher layer
recovery action will be initiated. The value of N2 is defined in \(sc\
2.4.8.4 below.
.ce
.parag
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2.4.4.2
.ce
\fIInformation transfer phase\fR
.ce
.parag
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After having transmitted the UA response to the SABM/SABME command or having
received the UA response to a transmitted SABM/SABME command, the DCE will
accept and transmit I and supervisory frames according to the procedures
described in \(sc\ 2.4.5 below.
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.parag
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.ce
When receiving the SABM/SABME command while in the information
.ce
transfer phase, the DCE will conform to the data link resetting procedure
.ce
described in \(sc\ 2.4.7 below.
.ce
.parag
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2.4.4.3
.ce
\fIData link disconnection\fR
.ce
.parag
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The DTE shall initiate a disconnect of the data link by
.ce
transmitting a DISC command to the DCE. On correctly receiving a DISC command
in the information transfer phase, the DCE will send a UA response and
enter
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the disconnected phase. On correctly receiving a DISC command in the
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disconnected phase, the DCE will send a DM response and remain in the
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disconnected phase. In order to avoid misinterpretation of the DM response
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received, it is suggested that the DTE always sends its DISC command with
the P\ bit set to\ 1. Otherwise, it is not possible to differentiate a
DM response
.ce
intended as an indication that the DCE is already in the disconnected phase
.ce
from a DM response that is issued in a separate unsolicited sense as a
request for a mode\(hysetting command (as described in \(sc\ 2.4.4.4.2).
.ce
.parag
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.ce
The DCE will initiate a disconnect of the data link by transmitting a DISC
command to the DTE and starting its Timer\ T1 (see \(sc\ 2.4.8.1 below).
Upon reception of an UA response from the DTE, the DCE will stop its Timer\
T1 and
.ce
will enter the disconnected phase. Upon reception of a DM response from
the DTE as an indication that the DTE was already in the disconnected phase,
the DCE
.ce
will stop its Timer\ T1 and will enter the disconnected phase.
.ce
.parag
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The DCE, having sent the DISC command, will ignore and discard any
.ce
frames except an SABM/SABME or DISC command, or a UA or DM response received
.ce
from the DTE. The receipt of an SABM/SABME or DISC command from the DTE will
.ce
result in a collision situation that is resolved per \(sc\ 2.4.4.5 below.
.ce
.parag
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.ce
After the DCE sends the DISC command, if a UA or DM response is not
.ce
received correctly, Timer\ T1 will run out in the DCE. The DCE will then
resend the DISC command and will restart Timer\ T1. After transmission
of the DISC
.ce
command N2 times by the DCE, appropriate higher layer recovery action will
be initiated. The value of N2 is defined in \(sc\ 2.4.8.4 below.
.ce
.parag
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2.4.4.4
.ce
\fIDisconnected phase\fR
.ce
.parag
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2.4.4.4.1
.ce
After having received a DISC command from the DTE and
.ce
returned a UA response to the DTE, or having received the UA response to a
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transmitted DISC command, the DCE will enter the disconnected phase.
.ce
.parag
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In the disconnected phase, the DCE may initiate data link set\(hyup. In
the disconnected phase, the DCE will react to the receipt of an SABM/SABME
command as described in \(sc\ 2.4.4.1 above and will transmit a DM response
in
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answer to a received DISC command. When receiving any other command (defined,
or undefined or not implemented) with the P\ bit set to\ 1, the DCE will
transmit a DM response with the F\ bit set to\ 1. Other frames received
in the
.ce
disconnected phase will be ignored by the DCE.
.ce
.parag
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2.4.4.4.2
.ce
When the DCE enters the disconnected phase after
.ce
detecting error conditions as listed in \(sc\ 2.4.6 below, or after an internal
.ce
malfunction, it may indicate this by sending a DM response rather than
a DISC command. In these cases, the DCE will transmit a DM response and
start its
.ce
Timer\ T1 (see \(sc\ 2.4.8.1 below).
.ce
.parag
.ce
If Timer T1 runs out before the reception of an SABM/SABME or DISC command
from the DTE, the DCE will retransmit the DM response and restart
.ce
Timer\ T1. After transmission of the DM response N2 times, the DCE will
remain in the disconnected phase and appropriate recovery actions will
be initiated. The value of N2 is defined in \(sc\ 2.4.8.4 below.
.ce
.parag
.ce
Alternatively, after an internal malfunction, the DCE may either
.ce
initiate a data link resetting procedure (see \(sc\ 2.4.7 below) or disconnect
the data link (see \(sc\ 2.4.4.3 above) prior to initiating a data link
set\(hyup
.ce
procedure (see \(sc\ 2.4.4.1 above).
.ce
.parag
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2.4.4.5
.ce
\fICollision of unnumbered commands\fR
.ce
.parag
.ce
Collision situations shall be resolved in the following way:
.ce
.parag
.ce
2.4.4.5.1
.ce
If the sent and received unnumbered commands are the
.ce
same, the DCE and the DTE shall each send the UA response at the earliest
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possible opportunity. The DCE shall enter the indicated phase either,
.ce
.parag
.ce

.ce
1)
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after receiving the UA response,
.ce
.parag
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2)
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after sending the UA response, or
.ce
.parag
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3)
.ce
after timing out waiting for the UA response having sent a
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UA response.
.ce
.parag
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In the case of 2) above, the DCE will accept a subsequent UA
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response to the mode\(hysetting command it issued without causing an exception
.ce
condition if received within the time\(hyout interval.
.ce
.parag
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.ce
2.4.4.5.2
.ce
If the sent and received unnumbered commands are
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different, the DCE and the DTE shall each enter the disconnected phase and
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issue a DM response at the earliest possible opportunity.
.ce
.parag
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2.4.4.6
.ce
\fICollision of DM response with SABM/SABME or DISC\fR
.ce

.ce
\fIcommand\fR
.ce
.parag
.ce
When a DM response is issued by the DCE or DTE as an unsolicited
.ce
response to request the DTE or DCE, respectively, to issue a mode\(hysetting
.ce
command as described in \(sc\ 2.4.4.4, a collision between an SABM/SABME
or DISC
.ce
command and the unsolicited DM response may occur. In order to avoid
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misinterpretation of the DM response received, the DTE always sends its
.ce
SABM/SABME or DISC command with the P\ bit set to\ 1.
.ce
.parag
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2.4.4.7
.ce
\fICollision of DM responses\fR
.ce
.parag
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A contention situation may occur when both the DCE and the DTE
.ce
issue a DM response to request a mode\(hysetting command. In this case, the DTE
.ce
will issue an SABM/SABME command to resolve the contention situation.
.ce
.parag
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.ce
2.4.5
.ce
\fILAPB procedures for information transfer\fR
.ce
.parag
.ce
The procedures which apply to the transmission of I\ frames in each direction
during the information transfer phase are described below.
.ce
.parag
.ce
In the following, \*Qnumber one higher\*U is in reference to a
.ce
continuously repeated sequence series, i.e., 7 is 1\ higher than 6 and 0 is
.ce
1\ higher than 7 for modulo\ 8 series, and 127 is 1\ higher than 126 and 0 is
.ce
1\ higher than 127 for modulo\ 128 series.
.ce
.parag
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2.4.5.1
.ce
\fISending I frames\fR
.ce
.parag
.ce
When the DCE has an I frame to transmit (i.e. an I frame not
.ce
already transmitted, or having to be retransmitted as described in \(sc\
2.4.5.6
.ce
below), it will transmit it with an N(S) equal to its current send state
.ce
variable V(S), and an N(R) equal to its current receive state variable
V(R). At the end of the transmission of the I\ frame, the DCE will increment
its send
.ce
state variable V(S) by\ 1.
.ce
.parag
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If Timer T1 is not running at the time of transmission of an I frame, it
will be started.
.ce
.parag
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If the send state variable V(S) is equal to the last value of N(R)
.ce
received plus \fIk\fR
.ce
(where \fIk\fR
.ce
is the maximum number of outstanding I\ frames \(em
.ce
see \(sc\ 2.4.8.6 below), the DCE will not transmit any new I\ frames, but may
.ce
retransmit an I\ frame as described in \(sc\(sc\ 2.4.5.6 or 2.4.5.9 below.
.ce
.parag
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When the DCE is in the busy condition, it may still transmit I frames,
provided that the DTE is not busy. When the DCE is in the frame rejection
.ce
condition, it will stop transmitting I\ frames.
.ce
.parag
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2.4.5.2
.ce
\fIReceiving an I frame\fR
.ce
.parag
.ce
2.4.5.2.1
.ce
When the DCE is not in a busy condition and receives a valid I\ frame whose
send sequence number N(S) is equal to the DCE receive state
.ce
variable V(R), the DCE will accept the information field of this frame,
.ce
increment by one its receive state variable V(R), and act as follows:
.ce
.parag
.ce

.ce
a)
.ce
If the DCE is still not in a busy condition:
.ce
.parag
.ce
i)
.ce
If an I frame is available for transmission by the
.ce
DCE, it may act as in \(sc\ 2.4.5.1 above and acknowledge
.ce
the received I frame by setting N(R) in the control field
.ce
of the next transmitted I\ frame to the value of the DCE
.ce
receive state variable V(R). Alternatively, the DCE may
.ce
acknowledge the received I\ frame by transmitting an RR
.ce
frame with the N(R) equal to the value of the DCE receive
.ce
state variable V(R).
.ce
.parag
.ce
ii)
.ce
If no I frame is available for transmission by the
.ce
DCE, it will transmit an RR frame with N(R) equal to the
.ce
value of the DCE receive state variable V(R).
.ce
.parag
.ce

.ce
b)
.ce
If the DCE is now in a busy condition, it will transmit an
.ce
RNR frame with N(R) equal to the value of the DCE receive
.ce
state variable V(R) (see \(sc\ 2.4.5.8).
.ce
.parag
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.ce
2.4.5.2.2
.ce
When the DCE is in a busy condition, it may ignore the
.ce
information field contained in any received I\ frame.
.ce
.parag
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2.4.5.3
.ce
\fIReception of invalid frames\fR
.ce
.parag
.ce
When the DCE receives an invalid frame (see \(sc\ 2.3.5.3), this frame
will be discarded.
.ce
.parag
.ce
2.4.5.4
.ce
\fIReception of out\(hyof\(hysequence I frames\fR
.ce
.parag
.ce
When the DCE receives a valid I frame whose send sequence number
.ce
N(S) is incorrect, i.e.,\ not equal to the current DCE receive state variable
.ce
V(R), it will discard the information field of the I\ frame and transmit
an REJ frame with the N(R) set to one higher than the N(S) of the last
correctly
.ce
received I\ frame. The REJ frame will be a command frame with the P\ bit
set to\ 1 if an acknowledged transfer of the retransmission request is
required;
.ce
otherwise the REJ frame may be either a command or a response frame. The DCE
.ce
will then discard the information field of all I\ frames received until the
.ce
expected I\ frame is correctly received. When receiving the expected I\ frame,
.ce
the DCE will then acknowledge the I\ frame as described in \(sc\ 2.4.5.2
above. The DCE will use the N(R) and P\ bit information in the discarded
I\ frames as
.ce
described in \(sc\ 2.3.5.2 above.
.ce
.parag
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2.4.5.5
.ce
\fIReceiving acknowledgement\fR
.ce
.parag
.ce
When correctly receiving an I frame or a supervisory frame (RR, RNR or
REJ), even in the busy condition, the DCE will consider the N(R) contained
in this frame as an acknowledgement for all I\ frames it has transmitted
with an N(S) up to and including the received N(R)\(em1. The DCE will stop
Timer\ T1 when it correctly receives an I\ frame or a supervisory frame
with the N(R) higher
.ce
than the last received N(R) (actually acknowledging some I\ frames), or
an REJ frame with an N(R) equal to the last received N(R).
.ce
.parag
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.ce
If Timer T1 has been stopped by the receipt on an I, RR or RNR frame, and
if there are outstanding I\ frames still unacknowledged, the DCE will
.ce
restart Timer\ T1. If Timer\ T1 then runs out, the DCE will follow the
recovery procedure (\(sc\ 2.4.5.9 below) with respect to the unacknowledged
I\ frames. If
.ce
Timer\ T1 has been stopped by the receipt of an REJ frame, the DCE will
follow the retransmission procedures in \(sc\ 2.4.5.6 below.
.ce
.parag
.ce
2.4.5.6
.ce
\fIReceiving an REJ frame\fR
.ce
.parag
.ce
When receiving an REJ frame, the DCE will set its send state
.ce
variable V(S) to the N(R) received in the REJ control field. It will transmit
the corresponding I\ frame as soon as it is available or retransmit it
in
.ce
accordance with the procedures described in \(sc\ 2.4.5.1 above. (Re)transmission
will conform to the following procedure:
.ce
.parag
.ce
i)
.ce
if the DCE is transmitting a supervisory command or response
.ce
when it receives the REJ frame, it will complete that
.ce
transmission before commencing transmission of the requested
.ce
I\ frame;
.ce
.parag
.ce

.ce
ii)
.ce
if the DCE is transmitting an unnumbered command or
.ce
response when it receives the REJ frame, it will ignore the
.ce
request for retransmission;
.ce
.parag
.ce
iii)
.ce
if the DCE is transmitting an I frame when the REJ frame
.ce
is received, it may abort the I\ frame and commence transmission
.ce
of the requested I\ frame immediately after abortion;
.ce
.parag
.ce
iv)
.ce
if the DCE is not transmitting any frame when the REJ frame
.ce
is received, it will commence transmission of the requested
.ce
I\ frame immediately.
.ce
.parag
.ce
In all cases, if other unacknowledged I frames had already been
.ce
transmitted following the one indicated in the REJ frame, then those I
frames will be retransmitted by the DCE following the retransmission of
the requested I\ frame. Other I\ frames not yet transmitted may be transmitted
following the
.ce
retransmitted I\ frames.
.ce
.parag
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If the REJ frame was received from the DTE as a command with the P bit
set to\ 1, the DCE will transmit an RR, RNR or REJ response with the F
bit set to\ 1 before transmitting or retransmitting the corresponding I\
frame.
.ce
.parag
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2.4.5.7
.ce
\fIReceiving an RNR frame\fR
.ce
.parag
.ce
After receiving an RNR frame whose N(R) acknowledges all frames
.ce
previously transmitted, the DCE will stop Timer\ T1 and may then transmit
.ce
an I\ frame, with the P\ bit set to\ 0, whose send sequence number is
.ce
equal to the N(R) indicated in the RNR frame, restarting Timer\ T1 as it
does. After receiving an RNR frame whose N(R) indicates a previously transmitted
.ce
frame, the DCE will not transmit or retransmit any I\ frame, Timer\ T1 being
.ce
already running. In either case, if the Timer\ T1 runs out before receipt
of a busy clearance indication, the DCE will follow the procedure described
in
.ce
\(sc\ 2.4.5.9\ below. In any case, the DCE will not transmit any other
I\ frames
.ce
before receiving an RR or REJ frame, or before the completion of a link
.ce
resetting procedure.
.ce
.parag
.ce
Alternatively, after receiving an RNR frame, the DCE may wait for a
.ce
period of time (e.g.,\ the length of the Timer\ T1) and then transmit a
.ce
supervisory command frame (RR, RNR or REJ) with the P\ bit set to\ 1, and
start Timer\ T1, in order to determine if there is any change in the receive
status of the DTE. The DTE shall respond to the P\ bit set to\ 1 with a
supervisory
.ce
response frame (RR, RNR or REJ) with the F\ bit set to\ 1 indicating either
.ce
continuance of the busy condition (RNR) or clearance of the busy condition
(RR or REJ). Upon receipt of the DTE response, Timer\ T1 is stopped.
.ce
.parag
.ce

.ce
1)
.ce
If the response is the RR or REJ response, the busy
.ce
condition is cleared and the DCE may transmit I\ frames beginning
.ce
with the I\ frame identified by the N(R) in the received response
.ce
frame.
.ce
.parag
.ce
2)
.ce
If the response is the RNR response, the busy condition
.ce
still exists, and the DCE will after a period of time (e.g.\ the
.ce
length of Timer\ T1) repeat the enquiry of the DTE receive
.ce
status.
.ce
.parag
.ce
If Timer T1 runs out before a status response is received, the
.ce
enquiry process above is repeated. If N2 attempts to get a status response
fail (i.e.\ Timer\ T1 runs out N2\ times), the DCE will initiate a data
link resetting procedure as described in \(sc\ 2.4.7.2\ below or will transmit
a DM response to ask the DTE to initiate a data link set\(hyup procedure
as described in \(sc\ 2.4.4.1 and enter the disconnected phase. The value
of N2 is defined in \(sc\ 2.4.8.4\ below.
.ce
.parag
.ce

.ce
If, at any time during the enquiry process, an unsolicited RR or REJ frame
is received from the DTE, it will be considered to be an indication of
.ce
clearance of the busy condition. Should the unsolicited RR or REJ frame be a
.ce
command frame with the P bit set to\ 1, the appropriate response frame
with the F\ bit set to 1 must be transmitted before the DCE may resume
transmission of
.ce
I\ frames. If Timer\ T1 is running, the DCE will wait for the non\(hybusy
response with the F bit set to\ 1 or will wait for Timer\ T1 to run out
and then either
.ce
may reinitiate the enquiry process in order to realize a successful P/F bit
.ce
exchange or may resume transmission of I frames beginning with the I\ frame
.ce
identified by the N(R) in the received RR or REJ frame.
.ce
.parag
.ce
2.4.5.8
.ce
\fIDCE busy condition\fR
.ce
.parag
.ce
When the DCE enters a busy condition, it will transmit an RNR frame at
the earliest opportunity. The RNR frame will be a command frame with the
P bit set to\ 1 if an acknowledged transfer of the busy condition indication
is
.ce
required; otherwise the RNR frame may be either a command or a response
frame. While in the busy condition, the DCE will accept and process supervisory
.ce
frames, will accept and process the contents of the N(R) fields of I\ frames,
.ce
and will return an RNR response with the F bit set to\ 1 if it receives a
.ce
supervisory command or I command frame with the P bit set to\ 1. To clear the
.ce
busy condition, the DCE will transmit either an REJ frame or an RR frame, with
.ce
N(R) set to the current receive state variable V(R), depending on whether or
.ce
not it discarded information fields of correctly received I\ frames. The REJ
.ce
frame or the RR frame will be a command frame with the P bit set to\ 1 if an
.ce
acknowledged transfer of the busy\(hyto\(hynon\(hybusy transition is required,
otherwise the REJ frame or the RR frame may be either a command or a response
frame.
.ce
.parag
.ce

.ce
2.4.5.9
.ce
\fIWaiting acknowledgement\fR
.ce
.parag
.ce
The DCE maintains an internal transmission attempt variable which is set
to\ 0 when the DCE sends a UA response, when the DCE receives a UA
.ce
response or an RNR command or response, or when the DCE correctly receives
an I\ frame or supervisory frame with the N(R) higher than the last received
N(R) (actually acknowledging some outstanding I\ frames).
.ce
.parag
.ce
If Timer T1 runs out waiting for the acknowledgement from the DTE for an
I\ frame transmitted, the DCE will enter the timer recovery condition,
add
.ce
one to its transmission attempt variable and set an internal variable \fIx\fR
.ce
to
.ce
the current value of its send state variable V(S). The DCE will then restart
.ce
Timer T1, set its send state variable V(S) to the last value of N(R) received
from the DTE and retransmit the corresponding I\ frame with the P bit set
to\ 1, or transmit an appropriate supervisory command frame (RR, RNR or
REJ) with the P\ bit set to\ 1.
.ce
.parag
.ce

.ce
The timer recovery condition is cleared when the DCE receives a valid supervisory
frame with the F\ bit\ set to\ 1.
.ce
.parag
.ce
If, while in the timer recovery condition, the DCE correctly receives a
supervisory frame with the F bit set to\ 1 and with the N(R) within the
range from its current send state variable V(S) to \fIx\fR
.ce
included, it will clear the
.ce
timer recovery condition (including stopping Timer\ T1) and set its send
state variable V(S) to the value of the received N(R), and may then resume
with
.ce
I\ frame transmission or retransmission, as appropriate.
.ce
.parag
.ce
If, while in the timer recovery condition, the DCE correctly receives an
I or supervisory frame with the P/F bit set to\ 0 and with a valid N(R)
(see \(sc\ 2.3.4.9), it will not clear the timer recovery condition. The
value of the
.ce
received N(R) may be used to update the send state variable V(S). However,
the DCE may decide to keep the last transmitted I\ frame in store (even
if it is
.ce
acknowledged) in order to be able to retransmit it with the P bit set to\
1 when Timer\ T1 runs out at a later time.
.ce
.parag
.ce

.ce
If the received supervisory frame with the P/F bit set to\ 0 is an REJ
frame with a valid N(R), the DCE may either immediately initiate
.ce
(re)transmission from the value of the send state variable V(S), or it may
.ce
ignore the request for retransmission and wait until the supervisory frame
with the F bit set to\ 1 is received before initiating (re)transmission
of frames
.ce
from the value identified in the N(R) field of the supervisory frame with
the F\ bit set to\ 1. In the case of immediate retransmission, in order
to prevent
.ce
duplicate retransmissions following the clearance of the timer recovery
.ce
condition, the DCE shall inhibit retransmission of a specific I\ frame [same
.ce
N(R) in the same numbering cycle] if the DCE has retransmitted that I\
frame as the result of a received REJ frame with the P/F bit set to\ 0.
.ce
.parag
.ce
If, while in the timer recovery condition, the DCE receives a REJ
.ce
command with the P bit set to\ 1, the DCE will respond immediately with an
.ce
appropriate supervisory response with the F bit set to\ 1. The DCE may
then use the value of the N(R) in the REJ command to update the send state
variable
.ce
V(S), and may either immediately begin (re)transmission from the value N(R)
.ce
indicated in the REJ frame or ignore the request for retransmission and wait
.ce
until the supervisory frame with the F bit set to\ 1 is received before
.ce
initiating (re)transmission of I\ frames from the value identified in the
N(R) field of the supervisory frame with the F\ bit set to\ 1. In the case
of
.ce
immediate retransmission, in order to prevent duplicate retransmissions
.ce
following the clearance of the timer recovery condition, the DCE shall
inhibit retransmission of a specific I\ frame [same N(R) in the same numbering
cycle]
.ce
if the DCE has retransmitted that I\ frame as the result of the received REJ
.ce
command with the P\ bit set to\ 1.
.ce
.parag
.ce
If Timer T1 runs out in the timer recovery condition, and no I or
.ce
supervisory frame with the P/F bit set to 0 and with a valid N(R) has been
.ce
received, or no REJ command with the P bit set to\ 1 and with a valid N(R)
has been received, the DCE will add one to its transmission attempt variable,
.ce
restart Timer\ T1, and either retransmit the I frame sent with the P bit set
.ce
to\ 1 or transmit an appropriate supervisory command with the P bit set to\ 1.
.ce
.parag
.ce

.ce
If the transmission attempt variable is equal to N2, the DCE will
.ce
initiate a data link resetting procedure as described in \(sc\ 2.4.7.2
below, or
.ce
will transmit a DM response to ask the DTE to initiate a data link set\(hyup
.ce
procedure as described in \(sc\ 2.4.4.1 above and enter the disconnected
phase. N2 is a system parameter (see \(sc\ 2.4.8.4 below).
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ Although the DCE may implement the internal variable \fIx\fR
.ce
,
.ce
other mechanisms do exist that achieve the identical function.
.ce
.parag
.ce
2.4.6
.ce
\fILAPB conditions for \fR
.ce
\fIdata link resetting or data link\fR
.ce

.ce
\fIre\(hyinitialization\fR
.ce
\fI(data link set\(hyup)\fR
.ce
.parag
.ce
2.4.6.1
.ce
When the DCE receives, during the information transfer phase, a
.ce
frame which is not invalid (see \(sc\ 2.3.5.3) with one of the conditions
listed in \(sc\ 2.3.4.9 above, the DCE will request the DTE to initiate
a data link resetting procedure by transmitting an FRMR response to the
DTE as described in
.ce
\(sc\ 2.4.7.3.
.ce
.parag
.ce
2.4.6.2
.ce
When the DCE receives, during the information transfer phase, an FRMR response
from the DTE, the DCE will either initiate the data link
.ce
resetting procedures itself as described in \(sc\ 2.4.7.2 or return a DM
response to ask the DTE to initiate the data link set\(hyup (initialization)
procedure as described in \(sc\ 2.4.4.1. After transmitting a DM response,
the DCE will enter
.ce
the disconnected phase as described in \(sc\ 2.4.4.4.2.
.ce
.parag
.ce
2.4.6.3
.ce
When the DCE receives, during the information transfer phase, a UA response,
or an unsolicited response with the F bit set to\ 1, the DCE may
.ce
either initiate the data link resetting procedures itself as described in
.ce
\(sc\ 2.4.7.2, or return a DM response to ask the DTE to initiate the data link
.ce
set\(hyup (initialization) procedure as described in \(sc\ 2.4.4.1. After
transmitting a DM response, the DCE will enter the disconnected phase as
described in
.ce
\(sc\ 2.4.4.4.2.
.ce
.parag
.ce
2.4.6.4
.ce
When the DCE receives, during the information transfer phase, a DM response
from the DTE, the DCE will either initiate the data link set\(hyup
.ce
(initialization) procedures itself as described in \(sc\ 2.4.4.1, or return
a DM
.ce
response to ask the DTE to initiate the data link set\(hyup (initialization)
.ce
procedures as described in \(sc\ 2.4.4.1. After transmitting a DM response,
the DCE will enter the disconnected phase as described in \(sc\ 2.4.4.4.2.
.ce
.parag
.ce

.ce
2.4.7
.ce
\fILAPB procedure for data link resetting\fR
.ce
.parag
.ce
2.4.7.1
.ce
The data link resetting procedure is used to initialize both
.ce
directions of information transfer according to the procedure described
below. The data link resetting procedure only applies during the information
transfer phase.
.ce
.parag
.ce
2.4.7.2
.ce
Either the DTE or the DCE may initiate the data link resetting
.ce
procedure. The data link resetting procedure indicates a clearance of a DCE
.ce
and/or DTE busy condition, if present.
.ce
.parag
.ce
The DTE shall initiate a data link resetting by transmitting an
.ce
SABM/SABME command to the DCE. If, upon correct receipt of the SABM/SABME
.ce
command, the DCE determines that it can continue in the information transfer
.ce
phase, it will return a UA response to the DTE, will reset its send and
receive state variables V(S) and V(R) to zero, and will remain in the information
.ce
transfer phase. If, upon correct receipt of the SABM/SABME command, the DCE
.ce
determines that it cannot remain in the information transfer phase, it will
.ce
return a DM response as a denial to the resetting request and will enter the
.ce
disconnected phase.
.ce
.parag
.ce
The DCE will initiate a data link resetting by transmitting an
.ce
SABM/SABME command to the DTE and starting its Timer\ T1 (see \(sc\ 2.4.8.1
below). Upon reception of a UA response from the DTE, the DCE will reset
its send and receive state variables V(S) and V(R) to zero, will stop its
Timer\ T1, and will remain in the information transfer phase. Upon reception
of a DM response from the DTE as a denial to the data link resetting request,
the DCE will stop its Timer\ T1 and will enter the disconnected phase.
.ce
.parag
.ce
The DCE, having sent an SABM/SABME command, will ignore and discard
.ce
any frames received from the DTE except an SABM/SABME or DISC command,
or a UA or DM response. The receipt of an SABM/SABME or DISC command from
the DTE will result in a collision situation that is resolved per \(sc\
2.4.4.5 above. Frames
.ce
other than the UA or DM response sent in response to a received SABM/SABME
or DISC command will be sent only after the data link is reset and if no
.ce
outstanding SABM/SABME command exists.
.ce
.parag
.ce

.ce
After the DCE sends the SABM/SABME command, if a UA or DM response is not
received correctly, Timer\ T1 will run out in the DCE. The DCE will then
.ce
resend the SABM/SABME command and will restart Timer\ T1. After N2 attempts
to reset the data link, the DCE will initiate appropriate higher layer
recovery
.ce
action and will enter the disconnected phase. The value of N2 is defined in
.ce
\(sc\ 2.4.8.4 below.
.ce
.parag
.ce
2.4.7.3
.ce
The DCE may ask the DTE to reset the data link by transmitting an FRMR
response (see \(sc\ 2.4.6.1 above). After transmitting an FRMR response,
.ce
the DCE will enter the frame rejection condition.
.ce
.parag
.ce
The frame rejection condition is cleared when the DCE receives an SABM/SABME
command, a DISC command, a FRMR response, or a DM response; or if
.ce
the DCE transmits an SABM/SABME command, a DISC command, or a DM response.
.ce
Other commands received while in the frame rejection condition will cause
the DCE to retransmit the FRMR response with the same information field
as
.ce
originally transmitted.
.ce
.parag
.ce
The DCE may start Timer\ T1 on transmission of the FRMR response. If
.ce
Timer\ T1 runs out before the frame rejection condition is cleared, the
DCE may retransmit the FRMR response, and restart T1. After N2 attempts
(time outs) to get the DTE to reset the data link, the DCE may reset the
data link itself as described in \(sc\ 2.4.7.2 above. The value of N2 is
defined in \(sc\ 2.4.8.4 below.
.ce
.parag
.ce

.ce
In the frame rejection condition, I frames and supervisory frames will
not be transmitted by the DCE. Also, received I frames and supervisory
frames will be discarded by the DCE except for the observance of a P bit
set to\ 1.
.ce
When an additional FRMR response must be transmitted by the DCE as a result
of the receipt of a P\ bit set to 1 while Timer\ T1 is running, Timer\
T1 will
.ce
continue to run. Upon reception of an FRMR response (even during a frame
.ce
rejection condition), the DCE will initiate a resetting procedure by
.ce
transmitting an SABM/SABME command as described in \(sc\ 2.4.7.2 above, or will
.ce
transmit a DM response to ask the DTE to initiate the data link set\(hyup
.ce
procedure as described in \(sc\ 2.4.4.1 and enter the disconnected phase.
.ce
.parag
.ce
2.4.8
.ce
\fIList of \fR
.ce
\fILAPB system parameters\fR
.ce
.parag
.ce
The DCE and DTE system parameters are as follows:
.ce
.parag
.ce

.ce
2.4.8.1
.ce
\fITimer\fR
.ce
\fIT1\fR
.ce
.parag
.ce
The value of the DTE Timer T1 system parameter may be different
.ce
than the value of the DCE Timer T1 system parameter. These values shall
be made known to both the DTE and the DCE, and agreed to for a period of
time by both the DTE and the DCE.
.ce
.parag
.ce
The period of Timer T1, at the end of which retransmission of a frame may
be initiated (see \(sc\ 2.4.4 and \(sc\ 2.4.5 above for the DCE), shall
take into account whether T1 is started at the beginning or the end of
the transmission of a frame.
.ce
.parag
.ce
The proper operation of the procedure requires that the transmitter's (DCE
or DTE) Timer\ T1 be greater than the maximum time between transmission
of a frame (SABM/SABME, DISC, I\ or supervisory command, or DM or FRMR
response)
.ce
and the reception of the corresponding frame returned as an answer to that
.ce
frame (UA, DM or acknowledging frame). Therefore, the receiver (DCE or DTE)
.ce
should not delay the response or acknowledging frame returned to one of the
.ce
above frames by more than a value\ T2, where T2 is a system parameter (see
.ce
\(sc\ 2.4.8.2).
.ce
.parag
.ce

.ce
The DCE will not delay the response or acknowledging frame returned to
one of the above DTE frames by more than a period\ T2.
.ce
.parag
.ce
2.4.8.2
.ce
\fIParameter T2\fR
.ce
.parag
.ce
The value of the DTE parameter T2 may be different than the value of the
DCE parameter T2. These values shall be made known to both the DTE and
the DCE, and agreed to for a period of time by both the DTE and the DCE.
.ce
.parag
.ce

.ce
The period of parameter T2 shall indicate the amount of time
.ce
available at the DCE or DTE before the acknowledging frame must be initiated
in order to ensure its receipt by the DTE or DCE, respectively, prior to
Timer\ T1 running out at the DTE or DCE (parameter\ T2\ <\ Timer\ T1).
.ce
.parag
.ce
\fINote\fR
.ce
\ \(em\ The period of parameter T2 shall take into account the
.ce
following timing factors: the transmission time of the acknowledging frame,
the propagation time over the access data link, the stated processing times
at the DCE and the DTE, and the time to complete the transmission of the
frame(s) in the DCE or DTE transmit queue that are neither displaceable
or modifiable in an orderly manner.
.ce
.parag
.ce

.ce
Given a value for Timer T1 for the DTE or DCE, the value of parameter T2
at the DCE or DTE, respectively, must be no larger than T1 minus\ 2 times
the propagation time over the access data link, minus the frame processing
time at the DCE, minus the frame processing time at the DTE, and minus
the transmission time of the acknowledging frame by the DCE or DTE, respectively.
.ce
.parag
.ce

.ce
2.4.8.3
.ce
\fITimer T3\fR
.ce
.parag
.ce
The DCE shall support a Timer T3 system parameter, the value of
.ce
which shall be made known to the DTE.
.ce
.parag
.ce
The period of Timer T3, at the end of which an indication of an
.ce
observed excessively long idle channel state condition is passed to the
Packet Layer, shall be sufficiently greater than the period of the DCE
Timer T1
.ce
(i.e.\ T3\ >\ T1) so that the expiration of T3 provides the desired level of
.ce
assurance that the data link channel is in a non\(hyactive, non\(hyoperational
state, and is in need of data link set\(hyup before normal data link operation
can
.ce
resume.
.ce
.parag
.ce
2.4.8.4
.ce
\fIMaximum number of attempts\fR
.ce
\fIto complete a\fR
.ce

.ce
\fItransmission N2\fR
.ce
.parag
.ce

.ce
The value of the DTE N2 system parameter may be different than the value
of the DCE N2 system parameter. These values shall be made known to both
the DTE and the DCE, and agreed to for a period of time by both the DTE
and the DCE.
.ce
.parag
.ce
The value of N2 shall indicate the maximum number of attempts made by the
DCE or DTE to complete the successful transmission of a frame to the DTE
or DCE, respectively.
.ce
.parag
.ce
2.4.8.5
.ce
\fIMaximum number of bits in an I frame N1\fR
.ce
.parag
.ce
The value of the DTE N1 system parameter may be different than the value
of the DCE N1 system parameter. These values shall be made known to both
the DTE and the DCE.
.ce
.parag
.ce
The values of N1 shall indicate the maximum number of bits in an
.ce
I\ frame (excluding flags and 0\ bits inserted for transparency) that the
DCE or DTE is willing to accept from the DTE or DCE, respectively.
.ce
.parag
.ce
In order to allow for universal operation, a DTE should support a
.ce
value of DTE N1 which is not less than 1080\ bits (135\ octets). DTEs should
be aware that the network may transmit longer packets (see \(sc\ 5.2),
that may result in a data link layer problem.
.ce
.parag
.ce

.ce
All networks shall offer to a DTE which requires it, a value of DCE N1
which is greater than or equal to 2072\ bits (259\ octets) plus the length
of the address, control and FCS fields at the DTE/DCE interface, and greater
than or equal to the maximum length of the data packets which may cross
the DTE/DCE
.ce
interface plus the length of the address, control and FCS fields at the
DTE/DCE interface.
.ce
.parag
.ce
Appendix II provides a description of how the values stated above are  derived.
.ce
.parag
.ce
2.4.8.6
.ce
\fIMaximum number of \fR
.ce
\fIoutstanding I frames k\fR
.ce
.parag
.ce
The value of the DTE k system parameter shall be the same as the
.ce
value of the DCE k system parameter. This value shall be agreed to for a
.ce
period of time by both the DTE and the DCE.
.ce
.parag
.ce
The value of k shall indicate the maximum number of sequentially
.ce
numbered I\ frames that the DTE or DCE may have outstanding
.ce
(i.e.\ unacknowledged) at any given time. The value of k shall never exceed
.ce
seven for modulo\ 8 operation, or one hundred and twenty\(hyseven for modulo\
128
.ce
operation. All networks (DCEs) shall support a value of seven. Other values
of k (less than and greater than seven) may also be supported by networks
.ce
(DCEs).
.ce
.parag
.ce
.line
.ce
\fBMONTAGE : \(sc 2.5 de cette Recommandation sur le reste de cette page\fR
.ce
.parag
.ce

.ce
.line
.ad r
\fBTable 8/X.25 [T8.25], p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
2.3.5.2
    \fIN(S) sequence error condition\fR
.sp 9p
.RT
.PP
The information field of all I frames received whose N(S) does not equal
the receive state variable V(R) will be discarded.
.PP
An N(S) sequence error exception condition occurs in the receiver when
an I\ frame received contains an N(S) which is not equal to the receive
state
variable V(R) at the receiver. The receiver does not acknowledge (increment
its receive state variable) the I\ frame causing the sequence error, or
any I\ frame which may follow, until an I\ frame with the correct N(S)
is received.
.PP
A DCE or DTE which receives one or more valid I frames having sequence
errors or subsequent supervisory frames (RR, RNR and REJ) shall accept
the
control information contained in the N(R) field and the P or F bit to perform
data link control functions; e.g.\ to receive acknowledgement of previously
transmitted I\ frames and to cause the DCE or DTE to respond (P bit set to 1).
.PP
The means specified in \(sc\(sc\ 2.3.5.2.1 and 2.3.5.2.2 shall be available
for initiating the retransmission of lost or errored I\ frames following
the
occurrence of an N(S) sequence error condition.
.RT
.sp 1P
.LP
2.3.5.2.1
    \fIREJ recovery\fR
.sp 9p
.RT
.PP
The REJ frame is used by a receiving DCE or DTE to initiate a
recovery (retransmission) following the detection of an N(S) sequence error.
.PP
With respect to each direction of transmission on the data link, only one
\*Qsent REJ\*U exception condition from a DCE or DTE, to a DTE or DCE,
is
established at a time. A \*Qsent REJ\*U exception condition is cleared when the
requested I\ frame is received.
.PP
A DCE or DTE receiving a REJ frame initiates sequential
(re\(hy)transmission of I\ frames starting with the I\ frame indicated
by the N(R) contained in the REJ frame. The retransmitted frames may contain
an N(R) and a P bit that are updated from, and therefore different from,
the ones contained in the originally transmitted I\ frames.
.RT
.sp 1P
.LP
2.3.5.2.2
    \fITime\(hyout recovery\fR
.sp 9p
.RT
.PP
If a DCE or DTE, due to a transmission error, does not receive (or receives
and discards) a single I\ frame or the last I\ frame(s) in a sequence of
I\ frames, it will not detect an N(S) sequence error condition and, therefore,
will not transmit a REJ frame. The DTE or DCE which transmitted the
unacknowledged I\ frame(s) shall, following the completion of a system
specified time\(hyout period (see \(sc\(sc\ 2.4.5.1 and 2.4.5.9 below),
take appropriate recovery action to determine at which I\ frame retransmission
must begin. The
retransmitted frame(s) may contain an N(R) and a P bit that is updated from,
and therefore different from, the ones contained in the originally transmitted
frame(s).
.RT
.sp 1P
.LP
2.3.5.3
    \fIInvalid frame\fR \fI condition\fR
.sp 9p
.RT
.PP
Any frame which is invalid will be discarded, and no action is
taken as the result of that frame. An invalid frame is defined as one
which:
.RT
.LP
    a)
    is not properly bounded by two flags;
.LP
    b)
    in basic (modulo 8) operation, contains fewer than 32 bits
between flags; in extended (modulo\ 128) operation, contains
fewer than 40\ bits between flags of frames that contain sequence
numbers or 32\ bits between flags of frames that do not contain
sequence numbers;
.LP
    c)
    contains a Frame Check Sequence (FCS) error; or
.LP
    d)
    contains an address other than A or B (for single link
operation) or other than C or D (for multilink operation).
.PP
For those networks that are octet aligned, a detection of
non\(hyoctet alignment may be made at the Data Link Layer by adding a frame
validity check that requires the number of bits between the opening flag and
the closing flag, excluding bits inserted for transparency, to be an integral
number of octets in length, or the frame is considered invalid.
.sp 1P
.LP
2.3.5.4
    \fIFrame rejection\fR \fI condition\fR
.sp 9p
.RT
.PP
A frame rejection condition is established upon the receipt of an error\(hyfree
frame with one of the conditions listed in \(sc\ 2.3.4.9 above.
.bp
.PP
At the DCE or DTE, this frame rejection exception condition is
reported by an FRMR response for appropriate DTE or DCE action, respectively.
Once a DCE has established such an exception condition, no additional I\
frames are accepted until the condition is reset by the DTE, except for
examination of the P bit. The FRMR response may be repeated at each opportunity,
as specified in \(sc\ 2.4.7.3, until recovery is effected by the DTE, or
until the DCE initiates its own recovery in case the DTE does not respond.
.RT
.sp 1P
.LP
2.3.5.5
    \fIExcessive idle channel state condition on incoming\fR
\fIchannel\fR
.sp 9p
.RT
.PP
Upon detection of an idle channel state condition (see \(sc\ 2.2.12.2 above)
on the incoming channel, the DCE shall wait for a period\ T3 (see
\(sc\ 2.4.8.3 below) without taking any specific action, waiting for detection
of a return to the active channel state (i.e.,\ detection of at least one
flag
.PP
sequence). After the period\ T3, the DCE shall notify the higher layer
(e.g.\ the Packet Layer or the MLP) of the excessive idle channel state
condition, but
shall not take any action that would preclude the DTE from establishing the
data link by normal data link set\(hyup procedures.
.PP
\fINote\fR \ \(em\ Other actions to be taken by the DCE at the Data Link
Layer upon expiration of period\ T3 is a subject for further study.
.RT
.sp 2P
.LP
2.4
    \fIDescription of the \fR \fILAPB procedure\fR
.sp 1P
.RT
.sp 1P
.LP
2.4.1
    \fILAPB basic and extended modes of operation\fR
.sp 9p
.RT
.PP
In accordance with the system choice made by the DTE at
subscription time, the DCE will either support modulo\ 8 (basic) operation or
will support modulo\ 128 (extended) operation. Changing from basic operation
to extended operation, or vice versa, in the DCE requires resubscription
by the
DTE for the desired service, and is not supported dynamically.
.PP
Table 5/X.25 indicates the command and response control field formats used
with the basic (modulo\ 8) service. The mode\(hysetting command employed
to
initialize (set up) or reset the basic mode is the SABM command. Table
6/X.25 indicates the command and response control field formats used with
the extended (modulo\ 128) service. The mode\(hysetting command employed
to initialize (set up) or reset the extended mode is the SABME command.
.RT
.sp 1P
.LP
2.4.2
    \fILAPB procedure for addressing\fR
.sp 9p
.RT
.PP
The address field identifies a frame as either a command or a
response. A command frame contains the address of the DCE or DTE to which
the command is being sent. A response frame contains the address of the
DCE or DTE sending the frame.
.PP
In order to allow differentiation between single link operation and
the optional multilink operation for diagnostic and/or maintenance reasons,
different address pair encodings are assigned to data links operating with
multilink procedure compared to data links operating with the single link
procedure.
.PP
Frames containing commands transferred from the DCE to the DTE will
contain the address\ A for the single link operation and address\ C for the
multilink operation.
.PP
Frames containing responses transferred from the DCE to the DTE will contain
the address\ B for the single link operation and address\ D for the
multilink operation.
.PP
Frames containing commands transferred from the DTE to the DCE shall contain
the address\ B for the single link operation and address\ D for the
multilink operation.
.PP
Frames containing responses transferred from the DTE to the DCE shall contain
the address\ A for the single link operation and address\ C for the
multilink operation.
.PP
These addresses are coded as follows:
.RT
.LP
    Address
    1\ 2\ 3\ 4\ 5\ 6\ 7\ 8
.LP
    Single link operation
    \ \ A
    1\ 1\ 0\ 0\ 0\ 0\ 0\ 0
.LP
    \ \ B
    1\ 0\ 0\ 0\ 0\ 0\ 0\ 0
.LP
    Multilink operation
    \ \ C
    1\ 1\ 1\ 1\ 0\ 0\ 0\ 0
.LP
    \ \ D
    1\ 1\ 1\ 0\ 0\ 0\ 0\ 0
.PP
\fINote\fR \ \(em\ The DCE will discard all frames received with an address
other than\ A or\ B (single link operation), or\ C or\ D (multilink operation).
.bp
.sp 1P
.LP
2.4.3
    \fILAPB procedure for the use of the P/F bit\fR
.sp 9p
.RT
.PP
The DCE or DTE receiving an SABM/SABME, DISC, supervisory command or I\
frame with the P\ bit set to\ 1 will set the F\ bit to\ 1 in the next response
frame it transmits.
.PP
The response frame returned by the DCE to an SABM/SABME or DISC
command with the P\ bit set to\ 1 will be a UA or DM response with the
F\ bit set to\ 1. The response frame returned by the DCE to an I\ frame
with the P\ bit set to\ 1, received during the information transfer phase,
will be an RR, REJ, RNR or FRMR response with the F\ bit set to\ 1. The
response frame returned by the
DCE to a supervisory command with the P\ bit set to\ 1, received during the
information transfer phase, will be an RR, REJ, RNR or FRMR response with
the F\ bit set to\ 1. The response frame returned by the DCE to an I\ frame
or
supervisory frame with the P\ bit set to\ 1, received during the disconnected
phase, will be a DM response with the F\ bit set to\ 1.
.PP
The P bit may be used by the DCE in conjunction with the timer
recovery condition (see \(sc\ 2.4.5.9 below).
.PP
\fINote\fR \ \(em\ Other use of the P bit by the DCE is a subject for further
study.
.RT
.sp 2P
.LP
2.4.4
    \fILAPB procedure for data link set\(hyup and disconnection\fR
.sp 1P
.RT
.sp 1P
.LP
2.4.4.1
    \fIData link set\(hyup\fR
.sp 9p
.RT
.PP
The DCE will indicate that it is able to set up the data link by
transmitting contiguous flags (active channel state).
.PP
Either the DTE or the DCE may initiate data link set\(hyup. Prior to
initiation of data link set\(hyup, either the DCE or the DTE may initiate data
link
disconnection (see \(sc\ 2.4.4.3) for the purpose of insuring that the
DCE and the DTE are in the same phase. The DCE may also transmit an unsolicited
DM response to request the DTE to initiate data link set\(hyup.
.PP
The DTE shall initiate data link set\(hyup by transmitting an SABM/SABME
command to the DCE. If, upon receipt of the SABM/SABME command correctly,
the DCE determines that it can enter the information transfer phase, it
will return a UA response to the DTE, will reset its send and receive state
variables V(S) and V(R) to zero, and will consider that the data link is
set up. If, upon
receipt
of the SABM/SABME command correctly, the DCE determines that it cannot enter
the information transfer phase, it will return a DM response to the DTE as a
.PP
denial to the data link set\(hyup initialization and will consider that the
data link is
\fInot\fR set up. In order to avoid misinterpretation of the DM response
received, it is suggested that the DTE always sends its SABM/SABME command
with the P\ bit set to\ 1. Otherwise, it is not possible to differentiate
a DM response intended as a denial to data link set\(hyup from a DM response
that is issued in a separate unsolicited sense as a request for a mode\(hysetting
command (as described
in\ \(sc\ 2.4.4.4.2).
.PP
The DCE will initiate data link set\(hyup by transmitting an SABM/SABME
command to the DTE and starting its Timer\ T1 in order to determine when
too
much time has elapsed waiting for a reply (see \(sc\ 2.4.8.1 below). Upon
reception of a UA response from the DTE, the DCE will reset its send and
receive state
variables\ V(S) and\ V(R) to zero, will stop its Timer\ T1, and will consider
that the data link is set up. Upon reception of a DM response from the
DTE as a
denial
to the data link set\(hyup initialization, the DCE will stop its Timer\
T1 and will consider that the data link is \fInot\fR set up.
.PP
The DCE, having sent the SABM/SABME command, will ignore and discard any
frames except an SABM/SABME or DISC command, or a UA or DM response
received from the DTE. The receipt of an SABM/SABME or DISC command from the
DTE will result in a collision situation that is resolved per \(sc\ 2.4.4.5
below. Frames other than the UA and DM responses sent in response to a
received
SABM/SABME or DISC command will be sent only after the data link is set
up and if no outstanding SABM/SABME command exists.
.PP
After the DCE sends the SABM/SABME command, if a UA or DM response is not
received correctly, Timer\ T1 will run out in the DCE. The DCE will then
resend the SABM/SABME command and will restart Timer\ T1. After transmission
of the SABM/SABME command N2 times by the DCE, appropriate higher layer
recovery action will be initiated. The value of N2 is defined in \(sc\
2.4.8.4 below.
.RT
.sp 1P
.LP
2.4.4.2
    \fIInformation transfer phase\fR
.sp 9p
.RT
.PP
After having transmitted the UA response to the SABM/SABME command or having
received the UA response to a transmitted SABM/SABME command, the DCE will
accept and transmit I and supervisory frames according to the procedures
described in \(sc\ 2.4.5 below.
.PP
When receiving the SABM/SABME command while in the information
transfer phase, the DCE will conform to the data link resetting procedure
described in \(sc\ 2.4.7 below.
.bp
.RT
.sp 1P
.LP
2.4.4.3
    \fIData link disconnection\fR
.sp 9p
.RT
.PP
The DTE shall initiate a disconnect of the data link by
transmitting a DISC command to the DCE. On correctly receiving a DISC command
in the information transfer phase, the DCE will send a UA response and
enter
the disconnected phase. On correctly receiving a DISC command in the
disconnected phase, the DCE will send a DM response and remain in the
disconnected phase. In order to avoid misinterpretation of the DM response
received, it is suggested that the DTE always sends its DISC command with
the P\ bit set to\ 1. Otherwise, it is not possible to differentiate a
DM response
intended as an indication that the DCE is already in the disconnected phase
from a DM response that is issued in a separate unsolicited sense as a
request for a mode\(hysetting command (as described in \(sc\ 2.4.4.4.2).
.PP
The DCE will initiate a disconnect of the data link by transmitting a DISC
command to the DTE and starting its Timer\ T1 (see \(sc\ 2.4.8.1 below).
Upon reception of an UA response from the DTE, the DCE will stop its Timer\
T1 and
will enter the disconnected phase. Upon reception of a DM response from
the DTE as an indication that the DTE was already in the disconnected phase,
the DCE
will stop its Timer\ T1 and will enter the disconnected phase.
.PP
The DCE, having sent the DISC command, will ignore and discard any
frames except an SABM/SABME or DISC command, or a UA or DM response received
from the DTE. The receipt of an SABM/SABME or DISC command from the DTE will
result in a collision situation that is resolved per \(sc\ 2.4.4.5 below.
.PP
After the DCE sends the DISC command, if a UA or DM response is not
received correctly, Timer\ T1 will run out in the DCE. The DCE will then
resend the DISC command and will restart Timer\ T1. After transmission
of the DISC
command N2 times by the DCE, appropriate higher layer recovery action will
be initiated. The value of N2 is defined in \(sc\ 2.4.8.4 below.
.RT
.sp 1P
.LP
2.4.4.4
    \fIDisconnected phase\fR \v'3p'
.sp 9p
.RT
.PP
2.4.4.4.1
After having received a DISC command from the DTE and
returned a UA response to the DTE, or having received the UA response to a
transmitted DISC command, the DCE will enter the disconnected phase.
.PP
In the disconnected phase, the DCE may initiate data link set\(hyup. In
the disconnected phase, the DCE will react to the receipt of an SABM/SABME
command as described in \(sc\ 2.4.4.1 above and will transmit a DM response
in
answer to a received DISC command. When receiving any other command (defined,
or undefined or not implemented) with the P\ bit set to\ 1, the DCE will
transmit a DM response with the F\ bit set to\ 1. Other frames received
in the
disconnected phase will be ignored by the DCE.
.PP
2.4.4.4.2
When the DCE enters the disconnected phase after
detecting error conditions as listed in \(sc\ 2.4.6 below, or after an internal
malfunction, it may indicate this by sending a DM response rather than
a DISC command. In these cases, the DCE will transmit a DM response and
start its
Timer\ T1 (see \(sc\ 2.4.8.1 below).
.sp 9p
.RT
.PP
If Timer T1 runs out before the reception of an SABM/SABME or DISC command
from the DTE, the DCE will retransmit the DM response and restart
Timer\ T1. After transmission of the DM response N2 times, the DCE will
remain in the disconnected phase and appropriate recovery actions will
be initiated. The value of N2 is defined in \(sc\ 2.4.8.4 below.
.PP
Alternatively, after an internal malfunction, the DCE may either
initiate a data link resetting procedure (see \(sc\ 2.4.7 below) or disconnect
the data link (see \(sc\ 2.4.4.3 above) prior to initiating a data link
set\(hyup
procedure (see \(sc\ 2.4.4.1 above).
.RT
.sp 1P
.LP
2.4.4.5
    \fICollision of unnumbered commands\fR
.sp 9p
.RT
.PP
Collision situations shall be resolved in the following way:
.RT
.PP
2.4.4.5.1
If the sent and received unnumbered commands are the
same, the DCE and the DTE shall each send the UA response at the earliest
possible opportunity. The DCE shall enter the indicated phase either,
.sp 9p
.RT
.LP
    1)
    after receiving the UA response,
.LP
    2)
    after sending the UA response, or
.LP
    3)
    after timing out waiting for the UA response having sent a
UA response.
.LP
In the case of 2) above, the DCE will accept a subsequent UA
response to the mode\(hysetting command it issued without causing an exception
condition if received within the time\(hyout interval.
.bp
.PP
2.4.4.5.2
If the sent and received unnumbered commands are
different, the DCE and the DTE shall each enter the disconnected phase and
issue a DM response at the earliest possible opportunity.
.sp 9p
.RT
.sp 1P
.LP
2.4.4.6
    \fICollision of DM response with SABM/SABME or DISC\fR
\fIcommand\fR
.sp 9p
.RT
.PP
When a DM response is issued by the DCE or DTE as an unsolicited
response to request the DTE or DCE, respectively, to issue a mode\(hysetting
command as described in \(sc\ 2.4.4.4, a collision between an SABM/SABME
or DISC
command and the unsolicited DM response may occur. In order to avoid
misinterpretation of the DM response received, the DTE always sends its
SABM/SABME or DISC command with the P\ bit set to\ 1.
.RT
.sp 1P
.LP
2.4.4.7
    \fICollision of DM responses\fR
.sp 9p
.RT
.PP
A contention situation may occur when both the DCE and the DTE
issue a DM response to request a mode\(hysetting command. In this case, the DTE
will issue an SABM/SABME command to resolve the contention situation.
.RT
.sp 1P
.LP
2.4.5
    \fILAPB procedures for information transfer\fR
.sp 9p
.RT
.PP
The procedures which apply to the transmission of I\ frames in each direction
during the information transfer phase are described below.
.PP
In the following, \*Qnumber one higher\*U is in reference to a
continuously repeated sequence series, i.e., 7 is 1\ higher than 6 and 0 is
1\ higher than 7 for modulo\ 8 series, and 127 is 1\ higher than 126 and 0 is
1\ higher than 127 for modulo\ 128 series.
.RT
.sp 1P
.LP
2.4.5.1
    \fISending I frames\fR
.sp 9p
.RT
.PP
When the DCE has an I frame to transmit (i.e. an I frame not
already transmitted, or having to be retransmitted as described in \(sc\
2.4.5.6
below), it will transmit it with an N(S) equal to its current send state
variable V(S), and an N(R) equal to its current receive state variable
V(R). At the end of the transmission of the I\ frame, the DCE will increment
its send
state variable V(S) by\ 1.
.PP
If Timer T1 is not running at the time of transmission of an I frame, it
will be started.
.PP
If the send state variable V(S) is equal to the last value of N(R)
received plus \fIk\fR (where \fIk\fR is the maximum number of outstanding
I\ frames \(em
see \(sc\ 2.4.8.6 below), the DCE will not transmit any new I\ frames, but may
retransmit an I\ frame as described in \(sc\(sc\ 2.4.5.6 or 2.4.5.9 below.
.PP
When the DCE is in the busy condition, it may still transmit I frames,
provided that the DTE is not busy. When the DCE is in the frame rejection
condition, it will stop transmitting I\ frames.
.RT
.sp 1P
.LP
2.4.5.2
    \fIReceiving an I frame\fR \v'3p'
.sp 9p
.RT
.PP
2.4.5.2.1
When the DCE is not in a busy condition and receives a valid I\ frame whose
send sequence number N(S) is equal to the DCE receive state
variable V(R), the DCE will accept the information field of this frame,
increment by one its receive state variable V(R), and act as follows:
.LP
    a)
    If the DCE is still not in a busy condition:
.LP
    i)
    If an I frame is available for transmission by the
DCE, it may act as in \(sc\ 2.4.5.1 above and acknowledge
the received I frame by setting N(R) in the control field
of the next transmitted I\ frame to the value of the DCE
receive state variable V(R). Alternatively, the DCE may
acknowledge the received I\ frame by transmitting an RR
frame with the N(R) equal to the value of the DCE receive
state variable V(R).
.LP
    ii)
    If no I frame is available for transmission by the
DCE, it will transmit an RR frame with N(R) equal to the
value of the DCE receive state variable V(R).
.LP
    b)
    If the DCE is now in a busy condition, it will transmit an
RNR frame with N(R) equal to the value of the DCE receive
state variable V(R) (see \(sc\ 2.4.5.8).
.PP
2.4.5.2.2
When the DCE is in a busy condition, it may ignore the
information field contained in any received I\ frame.
.sp 9p
.RT
.sp 1P
.LP
2.4.5.3
    \fIReception of invalid frames\fR
.sp 9p
.RT
.PP
When the DCE receives an invalid frame (see \(sc\ 2.3.5.3), this frame
will be discarded.
.bp
.RT
.sp 1P
.LP
2.4.5.4
    \fIReception of out\(hyof\(hysequence I frames\fR
.sp 9p
.RT
.PP
When the DCE receives a valid I frame whose send sequence number
N(S) is incorrect, i.e.,\ not equal to the current DCE receive state variable
V(R), it will discard the information field of the I\ frame and transmit
an REJ frame with the N(R) set to one higher than the N(S) of the last
correctly
received I\ frame. The REJ frame will be a command frame with the P\ bit
set to\ 1 if an acknowledged transfer of the retransmission request is
required;
otherwise the REJ frame may be either a command or a response frame. The DCE
.PP
will then discard the information field of all I\ frames received until the
expected I\ frame is correctly received. When receiving the expected I\ frame,
the DCE will then acknowledge the I\ frame as described in \(sc\ 2.4.5.2
above. The DCE will use the N(R) and P\ bit information in the discarded
I\ frames as
described in \(sc\ 2.3.5.2 above.
.RT
.sp 1P
.LP
2.4.5.5
    \fIReceiving acknowledgement\fR
.sp 9p
.RT
.PP
When correctly receiving an I frame or a supervisory frame (RR, RNR or
REJ), even in the busy condition, the DCE will consider the N(R) contained
in this frame as an acknowledgement for all I\ frames it has transmitted
with an N(S) up to and including the received N(R)\(em1. The DCE will stop
Timer\ T1 when it correctly receives an I\ frame or a supervisory frame
with the N(R) higher
than the last received N(R) (actually acknowledging some I\ frames), or
an REJ frame with an N(R) equal to the last received N(R).
.PP
If Timer T1 has been stopped by the receipt on an I, RR or RNR frame, and
if there are outstanding I\ frames still unacknowledged, the DCE will
restart Timer\ T1. If Timer\ T1 then runs out, the DCE will follow the
recovery procedure (\(sc\ 2.4.5.9 below) with respect to the unacknowledged
I\ frames. If
Timer\ T1 has been stopped by the receipt of an REJ frame, the DCE will
follow the retransmission procedures in \(sc\ 2.4.5.6 below.
.RT
.sp 1P
.LP
2.4.5.6
    \fIReceiving an REJ frame\fR
.sp 9p
.RT
.PP
When receiving an REJ frame, the DCE will set its send state
variable V(S) to the N(R) received in the REJ control field. It will transmit
the corresponding I\ frame as soon as it is available or retransmit it
in
accordance with the procedures described in \(sc\ 2.4.5.1 above. (Re)transmission
will conform to the following procedure:
.RT
.LP
    i)
    if the DCE is transmitting a supervisory command or response
when it receives the REJ frame, it will complete that
transmission before commencing transmission of the requested
I\ frame;
.LP
    ii)
    if the DCE is transmitting an unnumbered command or
response when it receives the REJ frame, it will ignore the
request for retransmission;
.LP
    iii)
    if the DCE is transmitting an I frame when the REJ frame
is received, it may abort the I\ frame and commence transmission
of the requested I\ frame immediately after abortion;
.LP
    iv)
    if the DCE is not transmitting any frame when the REJ frame
is received, it will commence transmission of the requested
I\ frame immediately.
.PP
In all cases, if other unacknowledged I frames had already been
transmitted following the one indicated in the REJ frame, then those I
frames will be retransmitted by the DCE following the retransmission of
the requested I\ frame. Other I\ frames not yet transmitted may be transmitted
following the
retransmitted I\ frames.
.PP
If the REJ frame was received from the DTE as a command with the P bit
set to\ 1, the DCE will transmit an RR, RNR or REJ response with the F
bit set to\ 1 before transmitting or retransmitting the corresponding I\
frame.
.RT
.sp 1P
.LP
2.4.5.7
    \fIReceiving an RNR frame\fR
.sp 9p
.RT
.PP
After receiving an RNR frame whose N(R) acknowledges all frames
previously transmitted, the DCE will stop Timer\ T1 and may then transmit
an I\ frame, with the P\ bit set to\ 0, whose send sequence number is
equal to the N(R) indicated in the RNR frame, restarting Timer\ T1 as it
does. After receiving an RNR frame whose N(R) indicates a previously transmitted
frame, the DCE will not transmit or retransmit any I\ frame, Timer\ T1 being
already running. In either case, if the Timer\ T1 runs out before receipt
of a busy clearance indication, the DCE will follow the procedure described
in
\(sc\ 2.4.5.9\ below. In any case, the DCE will not transmit any other
I\ frames
before receiving an RR or REJ frame, or before the completion of a link
resetting procedure.
.bp
.PP
Alternatively, after receiving an RNR frame, the DCE may wait for a
period of time (e.g.,\ the length of the Timer\ T1) and then transmit a
supervisory command frame (RR, RNR or REJ) with the P\ bit set to\ 1, and
start Timer\ T1, in order to determine if there is any change in the receive
status of the DTE. The DTE shall respond to the P\ bit set to\ 1 with a
supervisory
response frame (RR, RNR or REJ) with the F\ bit set to\ 1 indicating either
continuance of the busy condition (RNR) or clearance of the busy condition
(RR or REJ). Upon receipt of the DTE response, Timer\ T1 is stopped.
.RT
.LP
    1)
    If the response is the RR or REJ response, the busy
condition is cleared and the DCE may transmit I\ frames beginning
with the I\ frame identified by the N(R) in the received response
frame.
.LP
    2)
    If the response is the RNR response, the busy condition
still exists, and the DCE will after a period of time (e.g.\ the
length of Timer\ T1) repeat the enquiry of the DTE receive
status.
.PP
If Timer T1 runs out before a status response is received, the
enquiry process above is repeated. If N2 attempts to get a status response
fail (i.e.\ Timer\ T1 runs out N2\ times), the DCE will initiate a data
link resetting procedure as described in \(sc\ 2.4.7.2\ below or will transmit
a DM response to ask the DTE to initiate a data link set\(hyup procedure
as described in \(sc\ 2.4.4.1 and enter the disconnected phase. The value
of N2 is defined in \(sc\ 2.4.8.4\ below.
.PP
If, at any time during the enquiry process, an unsolicited RR or REJ frame
is received from the DTE, it will be considered to be an indication of
clearance of the busy condition. Should the unsolicited RR or REJ frame be a
command frame with the P bit set to\ 1, the appropriate response frame
with the F\ bit set to 1 must be transmitted before the DCE may resume
transmission of
I\ frames. If Timer\ T1 is running, the DCE will wait for the non\(hybusy
response with the F bit set to\ 1 or will wait for Timer\ T1 to run out
and then either
may reinitiate the enquiry process in order to realize a successful P/F bit
exchange or may resume transmission of I frames beginning with the I\ frame
identified by the N(R) in the received RR or REJ frame.
.RT
.sp 1P
.LP
2.4.5.8
    \fIDCE busy condition\fR
.sp 9p
.RT
.PP
When the DCE enters a busy condition, it will transmit an RNR frame at
the earliest opportunity. The RNR frame will be a command frame with the
P bit set to\ 1 if an acknowledged transfer of the busy condition indication
is
required; otherwise the RNR frame may be either a command or a response
frame. While in the busy condition, the DCE will accept and process supervisory
frames, will accept and process the contents of the N(R) fields of I\ frames,
and will return an RNR response with the F bit set to\ 1 if it receives a
supervisory command or I command frame with the P bit set to\ 1. To clear the
busy condition, the DCE will transmit either an REJ frame or an RR frame, with
.PP
N(R) set to the current receive state variable V(R), depending on whether or
not it discarded information fields of correctly received I\ frames. The REJ
frame or the RR frame will be a command frame with the P bit set to\ 1 if an
acknowledged transfer of the busy\(hyto\(hynon\(hybusy transition is required,
otherwise the REJ frame or the RR frame may be either a command or a response
frame.
.RT
.sp 1P
.LP
2.4.5.9
    \fIWaiting acknowledgement\fR
.sp 9p
.RT
.PP
The DCE maintains an internal transmission attempt variable which is set
to\ 0 when the DCE sends a UA response, when the DCE receives a UA
response or an RNR command or response, or when the DCE correctly receives
an I\ frame or supervisory frame with the N(R) higher than the last received
N(R) (actually acknowledging some outstanding I\ frames).
.PP
If Timer T1 runs out waiting for the acknowledgement from the DTE for an
I\ frame transmitted, the DCE will enter the timer recovery condition,
add
one to its transmission attempt variable and set an internal variable \fIx\fR
to
the current value of its send state variable V(S). The DCE will then restart
Timer T1, set its send state variable V(S) to the last value of N(R) received
from the DTE and retransmit the corresponding I\ frame with the P bit set
to\ 1, or transmit an appropriate supervisory command frame (RR, RNR or
REJ) with the P\ bit set to\ 1.
.PP
The timer recovery condition is cleared when the DCE receives a valid supervisory
frame with the F\ bit\ set to\ 1.
.PP
If, while in the timer recovery condition, the DCE correctly receives a
supervisory frame with the F bit set to\ 1 and with the N(R) within the
range from its current send state variable V(S) to \fIx\fR included, it
will clear the
timer recovery condition (including stopping Timer\ T1) and set its send
state variable V(S) to the value of the received N(R), and may then resume
with
I\ frame transmission or retransmission, as appropriate.
.bp
.PP
If, while in the timer recovery condition, the DCE correctly receives an
I or supervisory frame with the P/F bit set to\ 0 and with a valid N(R)
(see \(sc\ 2.3.4.9), it will not clear the timer recovery condition. The
value of the
received N(R) may be used to update the send state variable V(S). However,
the DCE may decide to keep the last transmitted I\ frame in store (even
if it is
acknowledged) in order to be able to retransmit it with the P bit set to\
1 when Timer\ T1 runs out at a later time.
.PP
If the received supervisory frame with the P/F bit set to\ 0 is an REJ
frame with a valid N(R), the DCE may either immediately initiate
(re)transmission from the value of the send state variable V(S), or it may
ignore the request for retransmission and wait until the supervisory frame
with the F bit set to\ 1 is received before initiating (re)transmission
of frames
from the value identified in the N(R) field of the supervisory frame with
the F\ bit set to\ 1. In the case of immediate retransmission, in order
to prevent
duplicate retransmissions following the clearance of the timer recovery
condition, the DCE shall inhibit retransmission of a specific I\ frame [same
N(R) in the same numbering cycle] if the DCE has retransmitted that I\
frame as the result of a received REJ frame with the P/F bit set to\ 0.
.PP
If, while in the timer recovery condition, the DCE receives a REJ
command with the P bit set to\ 1, the DCE will respond immediately with an
appropriate supervisory response with the F bit set to\ 1. The DCE may
then use the value of the N(R) in the REJ command to update the send state
variable
V(S), and may either immediately begin (re)transmission from the value N(R)
indicated in the REJ frame or ignore the request for retransmission and wait
until the supervisory frame with the F bit set to\ 1 is received before
initiating (re)transmission of I\ frames from the value identified in the
N(R) field of the supervisory frame with the F\ bit set to\ 1. In the case
of
immediate retransmission, in order to prevent duplicate retransmissions
.PP
following the clearance of the timer recovery condition, the DCE shall
inhibit retransmission of a specific I\ frame [same N(R) in the same numbering
cycle]
if the DCE has retransmitted that I\ frame as the result of the received REJ
command with the P\ bit set to\ 1.
.PP
If Timer T1 runs out in the timer recovery condition, and no I or
supervisory frame with the P/F bit set to 0 and with a valid N(R) has been
received, or no REJ command with the P bit set to\ 1 and with a valid N(R)
has been received, the DCE will add one to its transmission attempt variable,
restart Timer\ T1, and either retransmit the I frame sent with the P bit set
to\ 1 or transmit an appropriate supervisory command with the P bit set to\ 1.
.PP
If the transmission attempt variable is equal to N2, the DCE will
initiate a data link resetting procedure as described in \(sc\ 2.4.7.2
below, or
will transmit a DM response to ask the DTE to initiate a data link set\(hyup
procedure as described in \(sc\ 2.4.4.1 above and enter the disconnected
phase. N2 is a system parameter (see \(sc\ 2.4.8.4 below).
.PP
\fINote\fR \ \(em\ Although the DCE may implement the internal variable
\fIx\fR ,
other mechanisms do exist that achieve the identical function.
.RT
.sp 1P
.LP
2.4.6
    \fILAPB conditions for \fR \fIdata link resetting or data link\fR
\fIre\(hyinitialization\fR \fI(data link set\(hyup)\fR \v'3p'
.sp 9p
.RT
.PP
2.4.6.1
When the DCE receives, during the information transfer phase, a
frame which is not invalid (see \(sc\ 2.3.5.3) with one of the conditions
listed in \(sc\ 2.3.4.9 above, the DCE will request the DTE to initiate
a data link resetting procedure by transmitting an FRMR response to the
DTE as described in
\(sc\ 2.4.7.3.
.PP
2.4.6.2
When the DCE receives, during the information transfer phase, an FRMR response
from the DTE, the DCE will either initiate the data link
resetting procedures itself as described in \(sc\ 2.4.7.2 or return a DM
response to ask the DTE to initiate the data link set\(hyup (initialization)
procedure as described in \(sc\ 2.4.4.1. After transmitting a DM response,
the DCE will enter
the disconnected phase as described in \(sc\ 2.4.4.4.2.
.PP
2.4.6.3
When the DCE receives, during the information transfer phase, a UA response,
or an unsolicited response with the F bit set to\ 1, the DCE may
either initiate the data link resetting procedures itself as described in
\(sc\ 2.4.7.2, or return a DM response to ask the DTE to initiate the data link
set\(hyup (initialization) procedure as described in \(sc\ 2.4.4.1. After
transmitting a DM response, the DCE will enter the disconnected phase as
described in
\(sc\ 2.4.4.4.2.
.PP
2.4.6.4
When the DCE receives, during the information transfer phase, a DM response
from the DTE, the DCE will either initiate the data link set\(hyup
(initialization) procedures itself as described in \(sc\ 2.4.4.1, or return
a DM
response to ask the DTE to initiate the data link set\(hyup (initialization)
procedures as described in \(sc\ 2.4.4.1. After transmitting a DM response,
the DCE will enter the disconnected phase as described in \(sc\ 2.4.4.4.2.
.bp
.sp 1P
.LP
2.4.7
    \fILAPB procedure for data link resetting\fR \v'3p'
.sp 9p
.RT
.PP
2.4.7.1
The data link resetting procedure is used to initialize both
directions of information transfer according to the procedure described
below. The data link resetting procedure only applies during the information
transfer phase.
.PP
2.4.7.2
Either the DTE or the DCE may initiate the data link resetting
procedure. The data link resetting procedure indicates a clearance of a DCE
and/or DTE busy condition, if present.
.PP
The DTE shall initiate a data link resetting by transmitting an
SABM/SABME command to the DCE. If, upon correct receipt of the SABM/SABME
command, the DCE determines that it can continue in the information transfer
phase, it will return a UA response to the DTE, will reset its send and
receive state variables V(S) and V(R) to zero, and will remain in the information
.PP
transfer phase. If, upon correct receipt of the SABM/SABME command, the DCE
determines that it cannot remain in the information transfer phase, it will
return a DM response as a denial to the resetting request and will enter the
disconnected phase.
.PP
The DCE will initiate a data link resetting by transmitting an
SABM/SABME command to the DTE and starting its Timer\ T1 (see \(sc\ 2.4.8.1
below). Upon reception of a UA response from the DTE, the DCE will reset
its send and receive state variables V(S) and V(R) to zero, will stop its
Timer\ T1, and will remain in the information transfer phase. Upon reception
of a DM response from the DTE as a denial to the data link resetting request,
the DCE will stop its Timer\ T1 and will enter the disconnected phase.
.PP
The DCE, having sent an SABM/SABME command, will ignore and discard
any frames received from the DTE except an SABM/SABME or DISC command,
or a UA or DM response. The receipt of an SABM/SABME or DISC command from
the DTE will result in a collision situation that is resolved per \(sc\
2.4.4.5 above. Frames
other than the UA or DM response sent in response to a received SABM/SABME
or DISC command will be sent only after the data link is reset and if no
outstanding SABM/SABME command exists.
.PP
After the DCE sends the SABM/SABME command, if a UA or DM response is not
received correctly, Timer\ T1 will run out in the DCE. The DCE will then
resend the SABM/SABME command and will restart Timer\ T1. After N2 attempts
to reset the data link, the DCE will initiate appropriate higher layer
recovery
action and will enter the disconnected phase. The value of N2 is defined in
\(sc\ 2.4.8.4 below.
.RT
.PP
2.4.7.3
The DCE may ask the DTE to reset the data link by transmitting an FRMR
response (see \(sc\ 2.4.6.1 above). After transmitting an FRMR response,
the DCE will enter the frame rejection condition.
.sp 9p
.RT
.PP
The frame rejection condition is cleared when the DCE receives an SABM/SABME
command, a DISC command, a FRMR response, or a DM response; or if
the DCE transmits an SABM/SABME command, a DISC command, or a DM response.
Other commands received while in the frame rejection condition will cause
the DCE to retransmit the FRMR response with the same information field
as
originally transmitted.
.PP
The DCE may start Timer\ T1 on transmission of the FRMR response. If
.PP
Timer\ T1 runs out before the frame rejection condition is cleared, the
DCE may retransmit the FRMR response, and restart T1. After N2 attempts
(time outs) to get the DTE to reset the data link, the DCE may reset the
data link itself as described in \(sc\ 2.4.7.2 above. The value of N2 is
defined in \(sc\ 2.4.8.4 below.
.PP
In the frame rejection condition, I frames and supervisory frames will
not be transmitted by the DCE. Also, received I frames and supervisory
frames will be discarded by the DCE except for the observance of a P bit
set to\ 1.
When an additional FRMR response must be transmitted by the DCE as a result
of the receipt of a P\ bit set to 1 while Timer\ T1 is running, Timer\
T1 will
continue to run. Upon reception of an FRMR response (even during a frame
rejection condition), the DCE will initiate a resetting procedure by
transmitting an SABM/SABME command as described in \(sc\ 2.4.7.2 above, or will
transmit a DM response to ask the DTE to initiate the data link set\(hyup
procedure as described in \(sc\ 2.4.4.1 and enter the disconnected phase.
.RT
.sp 1P
.LP
2.4.8
    \fIList of \fR \fILAPB system parameters\fR
.sp 9p
.RT
.PP
The DCE and DTE system parameters are as follows:
.RT
.sp 1P
.LP
2.4.8.1
    \fITimer\fR \fIT1\fR
.sp 9p
.RT
.PP
The value of the DTE Timer T1 system parameter may be different
than the value of the DCE Timer T1 system parameter. These values shall
be made known to both the DTE and the DCE, and agreed to for a period of
time by both the DTE and the DCE.
.bp
.PP
The period of Timer T1, at the end of which retransmission of a frame may
be initiated (see \(sc\ 2.4.4 and \(sc\ 2.4.5 above for the DCE), shall
take into account whether T1 is started at the beginning or the end of
the transmission of a frame.
.PP
The proper operation of the procedure requires that the transmitter's (DCE
or DTE) Timer\ T1 be greater than the maximum time between transmission
of a frame (SABM/SABME, DISC, I\ or supervisory command, or DM or FRMR
response)
and the reception of the corresponding frame returned as an answer to that
frame (UA, DM or acknowledging frame). Therefore, the receiver (DCE or DTE)
should not delay the response or acknowledging frame returned to one of the
above frames by more than a value\ T2, where T2 is a system parameter (see
\(sc\ 2.4.8.2).
.PP
The DCE will not delay the response or acknowledging frame returned to
one of the above DTE frames by more than a period\ T2.
.RT
.sp 1P
.LP
2.4.8.2
    \fIParameter T2\fR
.sp 9p
.RT
.PP
The value of the DTE parameter T2 may be different than the value of the
DCE parameter T2. These values shall be made known to both the DTE and
the DCE, and agreed to for a period of time by both the DTE and the DCE.
.PP
The period of parameter T2 shall indicate the amount of time
available at the DCE or DTE before the acknowledging frame must be initiated
in order to ensure its receipt by the DTE or DCE, respectively, prior to
Timer\ T1 running out at the DTE or DCE (parameter\ T2\ <\ Timer\ T1).
.PP
\fINote\fR \ \(em\ The period of parameter T2 shall take into account the
following timing factors: the transmission time of the acknowledging frame,
the propagation time over the access data link, the stated processing times
at the DCE and the DTE, and the time to complete the transmission of the
frame(s) in the DCE or DTE transmit queue that are neither displaceable
or modifiable in an orderly manner.
.PP
Given a value for Timer T1 for the DTE or DCE, the value of parameter T2
at the DCE or DTE, respectively, must be no larger than T1 minus\ 2 times
the propagation time over the access data link, minus the frame processing
time at the DCE, minus the frame processing time at the DTE, and minus
the transmission time of the acknowledging frame by the DCE or DTE, respectively.
.RT
.sp 1P
.LP
2.4.8.3
    \fITimer T3\fR
.sp 9p
.RT
.PP
The DCE shall support a Timer T3 system parameter, the value of
which shall be made known to the DTE.
.PP
The period of Timer T3, at the end of which an indication of an
observed excessively long idle channel state condition is passed to the
Packet Layer, shall be sufficiently greater than the period of the DCE
Timer T1
(i.e.\ T3\ >\ T1) so that the expiration of T3 provides the desired level of
assurance that the data link channel is in a non\(hyactive, non\(hyoperational
state, and is in need of data link set\(hyup before normal data link operation
can
resume.
.RT
.sp 1P
.LP
2.4.8.4
    \fIMaximum number of attempts\fR \fIto complete a\fR
\fItransmission N2\fR
.sp 9p
.RT
.PP
The value of the DTE N2 system parameter may be different than the value
of the DCE N2 system parameter. These values shall be made known to both
the DTE and the DCE, and agreed to for a period of time by both the DTE
and the DCE.
.PP
The value of N2 shall indicate the maximum number of attempts made by the
DCE or DTE to complete the successful transmission of a frame to the DTE
or DCE, respectively.
.RT
.sp 1P
.LP
2.4.8.5
    \fIMaximum number of bits in an I frame N1\fR
.sp 9p
.RT
.PP
The value of the DTE N1 system parameter may be different than the value
of the DCE N1 system parameter. These values shall be made known to both
the DTE and the DCE.
.PP
The values of N1 shall indicate the maximum number of bits in an
I\ frame (excluding flags and 0\ bits inserted for transparency) that the
DCE or DTE is willing to accept from the DTE or DCE, respectively.
.PP
In order to allow for universal operation, a DTE should support a
value of DTE N1 which is not less than 1080\ bits (135\ octets). DTEs should
be aware that the network may transmit longer packets (see \(sc\ 5.2),
that may result in a data link layer problem.
.bp
.PP
All networks shall offer to a DTE which requires it, a value of DCE N1
which is greater than or equal to 2072\ bits (259\ octets) plus the length
of the address, control and FCS fields at the DTE/DCE interface, and greater
than or equal to the maximum length of the data packets which may cross
the DTE/DCE
interface plus the length of the address, control and FCS fields at the
DTE/DCE interface.
.PP
Appendix II provides a description of how the values stated above are  derived.
.RT
.sp 1P
.LP
2.4.8.6
    \fIMaximum number of \fR \fIoutstanding I frames k\fR
.sp 9p
.RT
.PP
The value of the DTE k system parameter shall be the same as the
value of the DCE k system parameter. This value shall be agreed to for a
period of time by both the DTE and the DCE.
.PP
The value of k shall indicate the maximum number of sequentially
numbered I\ frames that the DTE or DCE may have outstanding
(i.e.\ unacknowledged) at any given time. The value of k shall never exceed
seven for modulo\ 8 operation, or one hundred and twenty\(hyseven for modulo\
128
operation. All networks (DCEs) shall support a value of seven. Other values
of k (less than and greater than seven) may also be supported by networks
(DCEs).
.RT
.LP
.rs
.sp 39P
.sp 2P
.LP
\fBMONTAGE : \(sc 2.5 de cette Recommandation sur le reste de cette page\fR
.sp 1P
.RT
.LP
.bp