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InfoMagic Standards 1994 January
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6_10_02.tro
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.rs
.\" Troff code generated by TPS Convert from ITU Original Files
.\" Not Copyright ( c) 1991
.\"
.\" Assumes tbl, eqn, MS macros, and lots of luck.
.TA 1c 2c 3c 4c 5c 6c 7c 8c
.ds CH
.ds CF
.EQ
delim @@
.EN
.nr LL 40.5P
.nr ll 40.5P
.nr HM 3P
.nr FM 6P
.nr PO 4P
.nr PD 9p
.po 4P
.rs
\v | 5i'
.sp 2P
.LP
4.2
\fIPrimitive procedures\fR
.sp 1P
.RT
.sp 1P
.LP
4.2.1
\fIGeneral\fR
.EF '% Fascicle\ VI.10\ \(em\ Rec.\ Q.921''
.OF '''Fascicle\ VI.10\ \(em\ Rec.\ Q.921 %'
.sp 9p
.RT
.PP
Primitive procedures specify the interactions between adjacent
layers to invoke and provide a service. The service primitives represent the
elements of the procedures.
.PP
In the scope of this Recommendation the interactions between layer\ 3 and
the data link layer are specified.
.RT
.sp 1P
.LP
4.2.2
\fILayer 3 \(em\fR
\fIData link layer interactions\fR
.sp 9p
.RT
.PP
The states of a data link connection endpoint may be derived from the internal
states of the data link layer entity supporting this type of a
data link connection.
.PP
Data link connection endpoint states are defined as follows:
.RT
.LP
a)
Broadcast data link connection endpoint:
.LP
\(em
\fIinformation transfer\fR state.
.LP
b)
Point\(hyto\(hypoint data link connection endpoint:
.LP
\(em
\fIlink connection released\fR state;
.LP
\(em
\fIawaiting establish\fR state;
.LP
\(em
\fIawaiting release\fR state;
.LP
\(em
\fIlink connection established\fR state.
.PP
The primitives provide the procedural means to specify
conceptually how a data link service user can invoke a service.
.PP
This section defines the constraints on the sequences in which the
primitives may occur. The sequences are related to the states at one
point\(hyto\(hypoint data link connection endpoint.
.PP
The possible overall sequences of primitives at a point\(hyto\(hypoint
data link connection endpoint are defined in the state transition diagram,
Figure\ 8/Q.921. The \fIlink connection released\fR and \fIlink connection\fR
\fIestablished\fR states are stable states whilst the \fIawaiting establish\fR
and \fIawaiting release\fR states are transition states.
.PP
The model illustrates the behaviour of layer 2 as seen by layer 3.
This model assumes that the primitives passed between layers is implemented
by a first in first out queue. In this model, \*Qcollisions\*U of REQUEST
and
INDICATION primitives can occur thereby illustrating actions that seem
to be in conflict with the actual layer\ 2 protocol description. In some
implementations, these collisions could occur.
.RT
.LP
.rs
.sp 19P
.ad r
Blanc
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 47P
.ad r
\fBFigure 8/Q.921, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 24P
.ad r
\fBFigure 8/Q.921 (Notes), p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 2P
.LP
\fB5\fR \fBDefinition of the\fR
\fBpeer\(hyto\(hypeer procedures of the data link\fR \fBlayer\fR
.sp 1P
.RT
.PP
The procedures for use by the data link layer are specified in the following
sections.
.PP
The elements of procedure (frame types) which apply are:
.RT
.LP
a)
for unacknowledged information transfer (\(sc\ 5.2):
.LP
UI\(hycommand;
.LP
b)
for multiple frame acknowledged information transfer
(\(sc\(sc\ 5.5 to 5.8):
.LP
SABME\(hycommand,
.LP
UA\(hyresponse,
.LP
DM\(hyresponse,
.LP
DISC\(hycommand,
.LP
RR\(hycommand/response,
.LP
RNR\(hycommand/response,
.LP
REJ\(hycommand/response,
.LP
I\(hycommand,
.LP
FRMR\(hyresponse;
.LP
c)
for connection management entity information transfer:
.LP
XID\(hycommand/response.
.sp 2P
.LP
5.1
\fIProcedure for the use of the\fR
\fIP/F bit\fR
.sp 1P
.RT
.sp 1P
.LP
5.1.1
\fIUnacknowledged information transfer\fR
.sp 9p
.RT
.PP
For unacknowledged information transfer the P/F bit is not used and shall
be set to\ 0.
.bp
.RT
.sp 1P
.LP
5.1.2
\fIAcknowledged multiple frame information transfer\fR
.sp 9p
.RT
.PP
A data link layer entity receiving an SABME, DISC, RR, RNR, REJ
or I\ frame, with the P bit set to 1, shall set the F bit to 1 in the next
response frame it transmits, as defined in Table\ 7/Q.921.
.RT
.ce
\fBH.T. [T13.921]\fR
.ce
TABLE\ 7/Q.921
.ce
\fBImmediate response operation of P/F bit\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(114p) | cw(114p) .
{
Command received with P\ bit\ =\ 1
} {
Response transmitted with F\ bit\ =\ 1
}
_
.T&
cw(114p) | cw(114p) .
SABME, DISC UA, DM
_
.T&
cw(114p) | cw(114p) .
I, RR, RNR, REJ {
RR, RNR, REJ (note)
}
.TE
.LP
\fINote\ \(em\ \fR
A LAPB data link layer entity may transmit an FRMR or DM response
with the F\ bit set to 1 in response to an I\ frame or supervisory command
with the P\ bit set to\ 1.
.nr PS 9
.RT
.ad r
\fBTable 7/Q.921 [T13.921] + Note, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.sp 5
.sp 2P
.LP
5.2
\fIProcedures for unacknowledged information transfer\fR
.sp 1P
.RT
.sp 1P
.LP
5.2.1
\fIGeneral\fR
.sp 9p
.RT
.PP
The procedures which apply to the transmission of information in
unacknowledged operation are defined below.
.PP
No data link layer error recovery procedures are defined for
unacknowledged operation.
.RT
.sp 1P
.LP
5.2.2
\fITransmission of unacknowledged information\fR
.sp 9p
.RT
.PP
\fINote\fR \ \(em\ The term \*Qtransmission of a UI frame\*U refers to the
delivery of a UI frame by the data link layer to the physical layer.
.PP
Unacknowledged information is passed to the data link layer by
layer 3 or management entities using the primitives DL\(hyUNIT DATA\(hyREQUEST
or
MDL\(hyUNIT DATA\(hyREQUEST, respectively. The layer\ 3 or management message
unit
shall be transmitted in a UI command frame.
.PP
For broadcast operation, the TEI value in the UI command address field
shall be set to 127 (binary
111\ 1111, the group value).
.PP
For point\(hyto\(hypoint operation, the appropriate TEI value shall be
used.
.PP
The P bit shall be set to 0.
.PP
In the case of persistent layer 1 deactivation, the data link layer
will be informed by an appropriate indication. Upon receipt of this indication,
all UI transmission queues shall be discarded.
.PP
\fINote\fR \ \(em\ The network side system management deactivation procedures
should ensure that layer\ 1 is not deactivated before all UI data transfer is
completed.
.RT
.sp 1P
.LP
5.2.3
\fIReceipt of unacknowledged information\fR
.sp 9p
.RT
.PP
On receipt of a UI command frame with a SAPI and TEI which are
supported by the receiver, the contents of the information field shall be
passed to the layer\ 3 or management entity using the data link layer to
layer\ 3 primitive DL\(hyUNIT DATA\(hyINDICATION or the data link layer
to management
primitive MDL\(hyUNIT DATA\(hyINDICATION, respectively. Otherwise, the
UI command
frame shall be discarded.
.bp
.RT
.sp 2P
.LP
5.3
\fITerminal endpoint identifier (TEI) management procedures\fR
.sp 1P
.RT
.sp 1P
.LP
5.3.1
\fIGeneral\fR
.sp 9p
.RT
.PP
TEI management is based on the following procedural
means:
.RT
.LP
\(em
TEI assignment procedures (see \(sc\ 5.3.2);
.LP
\(em
TEI check procedures (see \(sc\ 5.3.3);
.LP
\(em
TEI removal procedures (see \(sc\ 5.3.4);
.LP
\(em
optional user equipment initiated TEI Identity verify
procedures (see \(sc\ 5.3.5).
.PP
A user equipment in the \fITEI\(hyunassigned\fR state shall use the TEI
assignment procedures to enter the \fITEI\(hyassigned\fR state. Conceptually,
these
procedures exist in the layer management entity. The layer management entity
on the network side is referred to as the Assignment Source Point (ASP)
in this Recommendation.
.PP
The purpose of these procedures is to:
.RT
.LP
a)
allow automatic TEI equipment to request the network to
assign a TEI value that the data link layer entities within
the requesting user equipment will use in their subsequent
communications;
.LP
b)
allow a network to remove a previously assigned TEI value
from specific or all user equipments;
.LP
c)
allow a network to check:
.LP
\(em
whether or not a TEI value is in use, or
.LP
\(em
whether multiple TEI assignment has occurred;
.LP
d)
allow user equipment the option to request that the network
invoke TEI check procedures.
.PP
The user side layer management entity shall instruct the user data link
layer entities to remove all TEI values when it is notified that the
terminal is disconnected at the interface (as defined in
Recommendation\ I.430).
.PP
Additionally, the user side layer management entity should instruct
the user data link layer entity to remove a TEI value for its own internal
reasons (for example, losing the ability to communicate with the network).
The layer management entity shall use the MDL\(hyREMOVE\(hyREQUEST primitive
for these
purposes.
.PP
\(sc\ 5.3.4.1 includes the actions taken by a data link layer entity
receiving an MDL\(hyREMOVE\(hyREQUEST primitive.
.PP
Typically, one TEI value would be used by the user equipment (for
example, a data link layer entity which has been assigned a TEI value could
use that value for all SAPs which it supports). If required, a number of
TEI values may be requested by multiple use of the procedures defined in
\(sc\ 5.3.2. It shall be the responsibility of the user to maintain the
association between TEI and SAPI values.
.PP
The initiation of TEI assignment procedures occurs on the receipt of a
request for establishment or unacknowledged information transfer while
in the TEI\(hyunassigned state. The data link layer entity shall inform
the layer
management entity using the MDL\(hyASSIGN\(hyINDICATION primitive. Alternatively,
the user side layer management entity may initiate the TEI assignment procedures
for its own reasons.
.PP
\fINote\fR \ \(em\ In the case of initialization from a no power condition,
the user equipment should postpone the start of the TEI assignment procedure
until a layer 2\ service that needs a TEI is to be provided.
.PP
All layer management entity messages used for these TEI management
procedures are transmitted to, or received from, the data link layer entity
using the MDL\(hyUNIT DATA\(hyREQUEST primitive, or the MDL\(hyUNIT DATA\(hyINDICATION
primitive, respectively. The data link layer entity shall transmit management
entity messages in UI command frames. The SAPI value shall be\ 63. The
TEI
value shall be\ 127.
.RT
.sp 1P
.LP
5.3.2
\fITEI assignment procedure\fR
.sp 9p
.RT
.PP
If the user equipment is of the non\(hyautomatic TEI assignment
category, the user side layer management entity shall deliver the TEI value
to be used to the data link layer entity(s) via the MDL\(hyASSIGN\(hyREQUEST
primitive.
.bp
.PP
If the user equipment is of the automatic TEI assignment category,
upon initiation of the automatic TEI assignment procedure, the user side
layer management entity shall transmit to its peer a message containing
the following elements:
.RT
.LP
a)
message type = Identity request;
.LP
b)
Reference number (Ri); and
.LP
c)
Action indicator (Ai).
.PP
The Reference number, Ri, shall be used to differentiate between a number
of user equipments which may simultaneously request assignment of
a TEI value. The Ri shall be 2\ octets in length and shall be randomly
generated for each request message by the user equipments.
.PP
All values in the range 0 to 65535 shall be available from the random number
generator.
.PP
\fINote\fR \ \(em\ The design of the random number generator should minimize
the probability of identical reference numbers being generated by terminals
which initiate their TEI assignment procedures simultaneously.
.PP
The single\(hyoctet Action indicator, Ai, shall be used to indicate a
request to the ASP for the assignment of any TEI value available.
.PP
The coding of the Ai shall be Ai = Group address TEI = 127. This Ai
value requests the ASP to assign any TEI value.
.PP
A timer T202 shall be started.
.PP
The ASP, on receipt of the Identity request message, shall
either:
.RT
.LP
\(em
select a TEI value;
.LP
\(em
deny Identity requests with Ai values in the range 64\(hy126,
or ignore Identity requests with the Ai value in the range 0\(hy63;
or
.LP
\(em
ignore the Identity request message if a previous Identity
request message that contains an identical Ri has been received
and no response has been issued. In this case, the ASP shall not
assign a TEI\ value to either request.
.PP
Selection of a TEI value shall be on the basis of information
stored at the ASP. This may consist of:
.LP
\(em
a map of the full range of automatic TEI values; or
.LP
\(em
an updated list of all automatic TEI values available for
assignment, or a smaller subset.
.PP
The ASP, after having selected the TEI value, shall inform the
network data link entities by means of the MDL\(hyASSIGN\(hyREQUEST primitive
and
transmit to its peer a message containing the following elements:
.LP
a)
message type = Identity assigned;
.LP
b)
Reference number (Ri); and
.LP
c)
the assigned TEI value in the Ai field.
.PP
If the available TEI information/resources are exhausted, a TEI
check procedure should be initiated.
.PP
A user side layer management entity receiving this
Identity
assigned message
shall compare the TEI value to its own to see if it is
already allocated if an
Identity request message
is outstanding.
Additionally, the TEI value may be compared on the receipt of all Identity
assigned messages.
.PP
If there is a match, the management entity shall either:
.RT
.LP
\(em
initiate TEI removal; or
.LP
\(em
initiate the TEI identity verify procedures.
.PP
If there is no match, the user side layer management entity
shall:
.LP
\(em
compare the Ri value with any outstanding Identity request
message and if it matches, consider the TEI value assigned to
the user equipment, discard the value of Ri, inform the user
side data link layer entities by means of the MDL\(hyASSIGN\(hyREQUEST
primitive and stop timer T202;
.LP
\(em
compare the Ri value with any outstanding Identity request
message and if there is no match, do nothing;
.LP
\(em
if there is no outstanding Identity request message, do
nothing.
.bp
.PP
When the data link layer receives the MDL\(hyASSIGN\(hyREQUEST primitive
from the layer management entity, the data link layer entity shall:
.LP
\(em
enter the TEI\(hyassigned state; and
.LP
\(em
proceed with data link establishment procedures if a
DL\(hyESTABLISH\(hyREQUEST primitive is outstanding, or proceed with
the transmission of a UI command frame if a DL\(hyUNIT
DATA\(hyREQUEST primitive is outstanding.
.PP
To deny an Identity request message, the ASP shall transmit to its peer
a message containing the following elements:
.LP
a)
message type = Identity denied;
.LP
b)
Reference number (Ri); and
.LP
c)
the value of TEI which is denied in the Ai field (a value of
127\ indicates that no TEI values are available).
.sp 1P
.LP
5.3.2.1
\fIExpiry of timer T202\fR
.sp 9p
.RT
.PP
If the user receives either no response or an
Identity denied message
to its Identity request message, then on expiry of timer\ T202, the timer
shall be restarted and the Identity request message shall be
retransmitted with a new value of\ Ri.
.PP
After N202 unsuccessful attempts to acquire a TEI\ value, the layer
management entity shall inform the data link layer entity using the
MDL\(hyERROR\(hyRESPONSE primitive. The data link layer entity receiving the
MDL\(hyERROR\(hyRESPONSE primitive shall respond with the DL\(hyRELEASE\(hyINDICATION
primitive if a request for establishment had previously occurred, and shall
discard all unserviced DL\(hyUNIT DATA\(hyREQUEST primitives.
.PP
The values of T202 and N202 are specified in \(sc\ 5.9.
.PP
The TEI assignment procedure is illustrated in Figure\ 9/Q.921.
.RT
.LP
.rs
.sp 25P
.ad r
\fBFigure 9/Q.921, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 2P
.LP
5.3.3
\fITEI check procedure\fR
.sp 1P
.RT
.sp 1P
.LP
5.3.3.1
\fIUse of the TEI check procedure\fR
.sp 9p
.RT
.PP
The TEI check procedure shall be used in the TEI audit and
recovery procedures. The TEI check procedure allows the network side layer
management entity either:
.RT
.LP
\(em
to establish that a TEI value is in use; or
.LP
\(em
to verify
multiple TEI assignment
.
.PP
The TEI check procedure for verifying multiple TEI assignment may also
optionally be invoked as a response to an
Identity verify
request message
from the user equipment.
.sp 1P
.LP
5.3.3.2
\fIOperation of the TEI check procedure\fR
.sp 9p
.RT
.PP
The
TEI check procedure
is illustrated in
Figure\ 10/Q.921.
.RT
.LP
.rs
.sp 15P
.ad r
\fBFigure 10/Q.921, p. \fR
.sp 1P
.RT
.ad b
.RT
.PP
The ASP shall transmit a message containing the following
elements:
.LP
a)
message type = Identity check request; and
.LP
b)
Ai field which contains the TEI value to be checked or the
value\ 127 when all TEI values are to be checked.
.PP
Timer T201 shall be started.
.PP
If any user equipment has been assigned the TEI value specified in
the identity check request message, it shall respond by transmitting a
message containing the following elements:
.RT
.LP
a)
message type = Identity check response;
.LP
b)
the TEI value in the Ai field; and
.LP
c)
Reference number (Ri).
.PP
\fINote\fR \ \(em\ The randomly\(hygenerated Ri is present in the Identity
check response to ensure that in the case where more than one user equipment
happens to commence transmission of the Identity check response at precisely
the same time (i.e.,\ the first \*Q0\*U\ bit of the opening flag coincides)
due to
different Ri values a collision at layer\ 1 (see ISDN user\(hynetwork interfaces;
layer\ 1 Recommendations [I.43x\ series] for clarification) occurs. The
resolution of this collision results in multiple Identity check responses.
.PP
When the TEI check procedure is used to verify multiple TEI
assignment:
.RT
.LP
\(em
if more than one
Identity check response
is received
within T201, then multiple TEI assignment shall be considered
present; otherwise the request shall be repeated once and timer
T201 restarted;
.LP
\(em
if more than one Identity check response is received within
the second T201 period, multiple TEI assignment shall be
considered present;
.bp
.LP
\(em
if no Identity check response is received after both T201
periods, the TEI value shall be assumed to be free and available
for (re)assignment;
.LP
\(em
if one Identity check response is received in one or both
T201 periods, the TEI value shall be assumed to be in
use.
.PP
When the TEI check procedure is used to test whether a TEI
value is in use, it is completed upon the receipt of the first
TEI Identity check response message
, and the TEI value is assumed to be in use.
Otherwise:
.LP
\(em
if no Identity check response is received within T201, the
identity check request shall be repeated once and timer T201
restarted;
.LP
\(em
if no Identity check response is received after the second
Identity check request, the TEI value shall be assumed to be
free and available for reassignment.
.PP
If the Ai value in the Identity check request is equal to 127, it is preferred
that the receiving user side layer management entity respond with a single
Identity check response message that contains all of the TEI values in
use within that user equipment (see \(sc\ 5.3.5.5). If an Identity check
request
with Ai equal to\ 127 is transmitted and an Identity check response is
received making use of the extension facility, each Ai variable in the
Ai field shall be processed as if received in separate Identity check responses
for parallel
Identity check requests.
.sp 1P
.LP
5.3.4
\fITEI removal procedure\fR
.sp 9p
.RT
.PP
When the network side layer management entity determines that the removal
of a TEI value (see \(sc\ 5.3.4.2) is necessary, the ASP shall transmit
a message containing the following elements and issue an MDL\(hyREMOVE\(hyREQUEST
primitive:
.RT
.LP
a)
message type = Identity remove; and
.LP
b)
TEI value which is to be removed, as indicated in the Ai
field (the value 127 indicates that all user equipments should
remove their TEI values; otherwise, the specific TEI value
should be removed).
.PP
The
Identity remove message
shall be sent twice in
succession, to overcome possible message loss.
.PP
When the user side layer management entity determines that the removal
of a TEI value is necessary (see \(sc\ 5.3.4.2), it shall instruct the
data link
layer entity to enter the \fITEI\(hyunassigned\fR state, using the MDL\(hyREMOVE\(hyREQUEST
primitive. This action would also be taken for all TEI values when the
Ai field contains the value of\ 127.
.PP
Further action to be taken shall be either initiation of automatic TEI
assignment for a new TEI value or notification to the equipment user for
the
need for corrective action (that is, when equipment uses a non\(hyautomatic TEI
value and does not support the automatic TEI assignment procedure.)
.RT
.sp 1P
.LP
5.3.4.1
\fIAction taken by the data link layer entity receiving the\fR
\fIMDL\(hyREMOVE\(hyREQUEST primitive\fR
.sp 9p
.RT
.PP
A data link layer entity receiving an MDL\(hyREMOVE\(hyREQUEST primitive
shall:
.RT
.LP
a)
if no DL\(hyRELEASE\(hyREQUEST primitive is outstanding and the
user equipment is not in the \fITEI\(hyassigned\fR state, issue a
DL\(hyRELEASE\(hyINDICATION primitive; or
.LP
b)
if a DL\(hyRELEASE\(hyREQUEST primitive is outstanding, issue a
DL\(hyRELEASE\(hyCONFIRM primitive.
.PP
The data link layer entity shall then enter the \fITEI\(hyunassigned\fR
state after discarding the contents of both UI and I queues.
.sp 1P
.LP
5.3.4.2
\fIConditions for TEI removal\fR
.sp 9p
.RT
.PP
At the user equipment, automatic TEI values shall be removed, and in the
case of non\(hyautomatic TEI values, an appropriate indication shall be
made to the user under the following conditions:
.RT
.LP
\(em
on request from the ASP by an Identity remove message;
.LP
\(em
on receipt of an MPH\(hyINFORMATION\(hyINDICATION (disconnected)
primitive;
.LP
\(em
on receipt of an MDL\(hyERROR\(hyINDICATION primitive indicating
that the data link layer entity has assumed possible multiple
assignment of a TEI value, rather than requesting a TEI check
procedure by the transmission of an Identity verify request
message; or
.LP
\(em
optionally on receipt of an Identity assigned message
containing a TEI value in the Ai field, which is already in use
within the user equipment (see \(sc\ 5.3.2).
.bp
.PP
At the network side, TEI values should be removed:
.LP
\(em
following a TEI audit procedure showing that a TEI value is
no longer in use or that
multiple TEI assignment
has
occurred; or
.LP
\(em
on receipt of an MDL\(hyERROR\(hyINDICATION primitive indicating a
possible multiple TEI assignment, which may be confirmed by the
invocation of the TEI check procedures.
.sp 2P
.LP
5.3.5
\fITEI identity verify procedure\fR
.sp 1P
.RT
.sp 1P
.LP
5.3.5.1
\fIGeneral\fR
.sp 9p
.RT
.PP
The TEI identity verify procedure allows the user side layer
management entity to have the capability to request that the network invoke
the identity check procedure for verification of multiple TEI assignment.
.PP
The TEI identity verify procedure is optional for both the network and
user equipment.
.RT
.sp 1P
.LP
5.3.5.2
\fIOperation of the TEI identity verify procedure\fR
.sp 9p
.RT
.PP
The TEI identity verify procedure is illustrated in
Figure\ 11/Q.921.
.RT
.LP
.rs
.sp 28P
.ad r
\fBFigure 11/Q.921 + Notes, p.\fR
.sp 1P
.RT
.ad b
.RT
.PP
The user equipment shall transmit an Identity verify message
containing the following elements:
.LP
a)
message type = Identity verify request;
.LP
b)
the TEI value to be checked in the Ai field; and
.LP
c)
the Ri field, which is not processed by the network and is
coded\ 0.
.PP
Timer T202 is started.
.bp
.PP
The ASP, on receipt of the TEI Identity verify message shall, if
implemented, invoke the TEI check procedure as defined in \(sc\ 5.3.3. This
will result in the ASP sending an Identity check request message to the user
equipment.
.RT
.sp 1P
.LP
5.3.5.3
\fIExpiry of Timer T202\fR
.sp 9p
.RT
.PP
If the user equipment receives no Identity check request message
with an A\di\uequal to its TEI or an A\di\uequal to 127 before the expiry of
timer T202, the user side layer management entity shall restart the timer
and the Identity verify message shall be retransmitted. If no Identity
check
request message is received from the ASP after the second Identity verify
request, the TEI shall be removed.
.RT
.sp 2P
.LP
5.3.6
\fIFormats and codes\fR
.sp 1P
.RT
.sp 1P
.LP
5.3.6.1
\fIGeneral\fR
.sp 9p
.RT
.PP
All messages used for TEI management procedures are carried in the information
field of UI command frames with a SAPI value set to 63 (binary
11\ 1111) and TEI value set to\ 127 (binary 111\ 1111).
.PP
All messages have the structure shown in Figure 12/Q.921.
.RT
.LP
.rs
.sp 27P
.ad r
\fBFigure 12/Q.921 [T14.921], p. \ \
(A traiter comme tableau MEP)\fR
.sp 1P
.RT
.ad b
.RT
.PP
.sp 3
Fields that are not used in a specific message are coded all
zeros, and are not to be processed by either side.
.PP
The coding of each field for the various messages is specified in
Table\ 8/Q.921.
.PP
E is the Action indicator field extension bit (see
\(sc\ 5.3.6.5).
.bp
.RT
.ce
\fBH.T. [T15.921]\fR
.ce
TABLE\ 8/Q.921
.ce
\fBCodes for messages concerning TEI management procedures\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(48p) | cw(30p) | cw(42p) | cw(24p) | cw(84p) .
Message name {
Management
entity
identifier
} Reference number Ri Message type Action indicator Ai
_
.T&
lw(48p) | cw(30p) | lw(42p) | cw(24p) | lw(84p) .
{
Identity request
(user to network)
} 0000\ 1111 0\(hy65535 0000\ 0001 {
Ai\ =\ 127,
Any TEI value acceptable
}
_
.T&
lw(48p) | cw(30p) | lw(42p) | cw(24p) | lw(84p) .
{
Identity assigned
(network to user)
} 0000\ 1111 0\(hy65535 0000\ 0010 {
Ai\ =\ 64\(hy126,
Assigned TEI value
}
_
.T&
lw(48p) | cw(30p) | lw(42p) | cw(24p) | lw(84p) , ^ | ^ | ^ | ^ | l.
{
Identity denied
(network to user)
} 0000\ 1111 0\(hy65535 0000\ 0011 {
Ai\ =\ 64\(hy126,
Denied TEI value
}
{
Ai\ =\ 127,
No TEI value available
}
_
.T&
lw(48p) | cw(30p) | lw(42p) | cw(24p) | lw(84p) , ^ | ^ | ^ | ^ | l.
{
Identity check request
(network to user)
} 0000\ 1111 Not used (coded\ 0) 0000\ 0100 {
Ai\ =\ 127,
Check all TEI values
}
{
Ai\ =\ 0\(hy126,
TEI value to be checked
}
_
.T&
lw(48p) | cw(30p) | lw(42p) | cw(24p) | lw(84p) .
{
Identity check response
(user to network)
} 0000\ 1111 0\(hy65535 0000\ 0101 {
Ai\ =\ 0\(hy126,
TEI value in use
}
_
.T&
lw(48p) | cw(30p) | lw(42p) | cw(24p) | lw(84p) , ^ | ^ | ^ | ^ | l.
{
Identity remove
(network to user)
} 0000\ 1111 Not used (coded\ 0) 0000\ 0110 {
Ai\ =\ 127,
Request for removal of all TEI values
}
{
Ai\ =\ 0\(hy126,
TEI value to be removed
}
_
.T&
lw(48p) | cw(30p) | lw(42p) | cw(24p) | lw(84p) .
{
Identity verify
(user to network)
} 0000\ 1111 Not used (coded\ 0) 0000\ 0111 {
Ai\ =\ 0\(hy126,
TEI value to be checked
}
_
.TE
.nr PS 9
.RT
.ad r
\fBTable 8/Q.921 [T15.921], p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.sp 1
.sp 1P
.LP
5.3.6.2
\fILayer management entity identifier\fR
.sp 9p
.RT
.PP
For TEI administration procedures, the layer management entity
identifier octet is 0000 | 111. Other values are reserved for further
standardization.
.RT
.sp 1P
.LP
5.3.6.3
\fIReference number (Ri)\fR
.sp 9p
.RT
.PP
Octets 2 and 3 contain Ri. When used, it can assume any value
between 0 and 65535.
.RT
.sp 1P
.LP
5.3.6.4
\fIMessage type\fR
.sp 9p
.RT
.PP
Octet 4 contains the message type. The purpose of the message type is to
identify the function of the message being sent.
.RT
.sp 1P
.LP
5.3.6.5
\fIAction indicator (Ai)\fR
.sp 9p
.RT
.PP
The Ai field is extended by reserving the first transmitted bit of the
Ai field octets to indicate the final octet of the Ai field.
.PP
Ai variables in the Ai field are coded as follows:
.RT
.LP
a)
bit 1 is the
extension bit
and is coded as
follows:
.LP
\(em
0 to indicate an extension, and
.LP
\(em
1 to indicate the final octet;
.LP
b)
bits 2 to 8 contain the Action indicator.
.PP
The purpose of the Action indicator is to identify the concerned TEI value(s).
.bp
.sp 1P
.LP
5.4
\fIAutomatic negotiation of data link layer parameters\fR
.sp 9p
.RT
.PP
This procedure is defined in Appendix IV.
.RT
.sp 2P
.LP
5.5
\fIProcedures for establishment and release of multiple frame\fR
\fIoperation\fR
.sp 1P
.RT
.sp 1P
.LP
5.5.1
\fIEstablishment of multiple frame operation\fR
.sp 9p
.RT
.PP
The provision of extended multiple frame operation (modulo 128
sequencing) is recommended.
.RT
.sp 1P
.LP
5.5.1.1
\fIGeneral\fR
.sp 9p
.RT
.PP
These procedures shall be used to establish multiple frame
operation between the network and a designated user entity.
.PP
Layer 3 will request establishment of the multiple frame operation by the
use of the DL\(hyESTABLISH\(hyREQUEST primitives. Re\(hyestablishment may
be
initiated as a result of the data link layer procedures defined in \(sc\
5.7. All frames other than unnumbered frame formats received during the
establishment
procedures shall be ignored.
.RT
.sp 1P
.LP
5.5.1.2
\fIEstablishment procedures\fR
.sp 9p
.RT
.PP
A data link layer entity shall initiate a request for the multiple frame
operation to be set by transmitting the SABME command. All existing
exception conditions shall be cleared, the retransmission counter shall be
reset, and timer\ T200 shall then be started (timer\ T200 is defined in
\(sc\ 5.9.1). All
mode setting commands
shall be transmitted with the P\ bit set
to\ 1.
.PP
Layer 3 initiated establishment procedures imply the discard of all
outstanding DL\(hyDATA\(hyREQUEST primitives and all I frames in queue.
.PP
A data link layer entity receiving an SABME command, if it is able
to enter the
\fImultiple\(hyframe\(hyestablished\fR state
, shall:
.RT
.LP
\(em
respond with a UA response with the F bit set to the same
binary value as the P bit in the received SABME command;
.LP
\(em
set V(S), V(R) and V(A) to\ 0;
.LP
\(em
enter the \fImultiple\(hyframe\(hyestablished\fR state and inform
layer\ 3 using the DL\(hyESTABLISH\(hyINDICATION primitive;
.LP
\(em
clear all existing exception conditions;
.LP
\(em
clear any existing peer receiver busy condition; and
.LP
\(em
start timer T203 (timer T203 is defined in \(sc\ 5.9.8), if
implemented.
.PP
If the data link layer entity is unable to enter the
\fImultiple\(hyframe\(hyestablished\fR state, it shall respond to the SABME
command with a DM response with the F bit set to the same binary value
as the P bit in the received SABME command.
.PP
Upon reception of the UA response with the F bit set to 1, the
originator of the SABME command shall:
.RT
.LP
\(em
reset timer T200;
.LP
\(em
start timer T203 if implemented;
.LP
\(em
set V(S), V(R), and V(A) to\ 0; and
.LP
\(em
enter the \fImultiple\(hyframe\(hyestablished\fR state and inform
layer\ 3 using the DL\(hyESTABLISH\(hyCONFIRM primitive.
.PP
Upon reception of a DM response with the F bit set to 1, the
originator of the SABME command shall indicate this to layer\ 3 by means of
the DL\(hyRELEASE\(hyINDICATION primitive, and reset timer T200. It shall
then enter the \fITEI\(hyassigned\fR state. DM responses with the F bit
set to 0 shall be ignored in this case.
.PP
A DL\(hyRELEASE\(hyREQUEST primitive received during data link layer
initiated re\(hyestablishment shall be serviced on completion of the establishment
mode\(hysetting operation.
.bp
.RT
.sp 1P
.LP
5.5.1.3
\fIProcedure on expiry of timer T200\fR
.sp 9p
.RT
.PP
If timer T200 expires before the UA or DM response with the F bit set to\
1 is received, the data link layer entity shall:
.RT
.LP
\(em
retransmit the SABME command as above;
.LP
\(em
restart timer T200; and
.LP
\(em
increment the
retransmission counter
.
.PP
After retransmission of the SABME command N200 times, the data
link layer entity shall indicate this to layer\ 3 and the connection management
entity by means of the DL\(hyRELEASE\(hyINDICATION and MDL\(hyERROR\(hyINDICATION
primitives, respectively, and enter the \fITEI\(hyassigned\fR state, after
discarding all outstanding DL\(hyDATA\(hyREQUEST primitives and all I frames
in queue.
.PP
The value of N200 is defined in \(sc\ 5.9.2.
.RT
.sp 1P
.LP
5.5.2
\fIInformation transfer\fR
.sp 9p
.RT
.PP
Having either transmitted the UA response to a received SABME
command or received the UA response to a transmitted SABME command, I frames
and supervisory frames shall be transmitted and received according to the
procedures described in \(sc\ 5.6.
.PP
If an SABME command is received while in the
\fImultiple\(hyframe\(hyestablished\fR state, the data link layer entity
shall conform to the re\(hyestablishment procedure described in \(sc\ 5.7.
.PP
On receipt of a UI command, the procedures defined in \(sc\ 5.2 shall be
followed.
.RT
.sp 2P
.LP
5.5.3
\fITermination of multiple frame operation\fR
.sp 1P
.RT
.sp 1P
.LP
5.5.3.1
\fIGeneral\fR
.sp 9p
.RT
.PP
These procedures shall be used to terminate the multiple frame
operation between the network and a designated user entity.
.PP
Layer 3 will request termination of the multiple frame operation by
use of the DL\(hyRELEASE\(hyREQUEST primitive.
.PP
All frames other than unnumbered frames received during the release
procedures shall be ignored.
.PP
All outstanding DL\(hyDATA\(hyREQUEST primitives and all I frames in queue
shall be discarded.
.PP
In the case of persistant layer 1 deactivation the data link layer
entity shall discard all I queues and deliver to layer\ 3 a DL\(hyRELEASE\(hyCONFIRM
primitive if a DL\(hyRELEASE\(hyREQUEST primitive is outstanding, or otherwise
a
DL\(hyRELEASE\(hyINDICATION primitive.
.RT
.sp 1P
.LP
5.5.3.2
\fIRelease procedure\fR
.sp 9p
.RT
.PP
A data link layer entity shall initiate a request for release of
the multiple frame operation by transmitting the Disconnect (DISC) command
with the P bit set to 1. Timer T200 shall then be started and the retransmission
counter reset.
.PP
A data link layer entity receiving a DISC command while in the
\fImultiple\(hyframe\(hyestablished\fR or \fItimer recovery\fR state shall
transmit a UA
response with the F bit set to the same binary value as the P bit in the
received DISC command. A DL\(hyRELEASE\(hyINDICATION primitive shall be
passed to
layer\ 3, and the \fITEI\(hyassigned\fR state shall be entered.
.PP
If the originator of the DISC command receives either:
.RT
.LP
\(em
a UA response with the F bit set to 1; or
.LP
\(em
a DM response with the F bit set to 1, indicating that the
peer data link layer entity is already in the \fITEI\(hyassigned\fR state,
.LP
it shall enter the \fITEI\(hyassigned\fR state and reset timer T200.
.bp
.PP
The data link layer entity which issued the DISC command is now in the
\fITEI\(hyassigned\fR state and will notify layer\ 3 by means of the
DL\(hyRELEASE\(hyCONFIRM primitive. The conditions relating to this state
are defined in \(sc\ 5.5.4.
.sp 1P
.LP
5.5.3.3
\fIProcedure on expiry of timer T200\fR
.sp 9p
.RT
.PP
If timer T200 expires before a UA or DM response with the F bit set to\
1 is received, the originator of the DISC command shall:
.RT
.LP
\(em
retransmit the DISC command as defined in \(sc\ 5.5.3.2;
.LP
\(em
restart timer T200; and
.LP
\(em
increment the retransmission counter.
.PP
If the data link layer entity has not received the correct
response as defined in \(sc\ 5.5.3.2, after N200 attempts to recover, the
data link layer entity shall indicate this to the connection management
entity by means of the MDL\(hyERROR\(hyINDICATION primitive, enter the
\fITEI\(hyassigned\fR state and
notify layer\ 3 by means of the DL\(hyRELEASE\(hyCONFIRM primitive.
.sp 1P
.LP
5.5.4
\fITEI\(hyassigned state\fR
.sp 9p
.RT
.PP
While in the
\fITEI\(hyassigned\fR state:
.RT
.LP
\(em
the receipt of a DISC command shall result in the
transmission of a DM response with F\ bit set to the value of the
received P\ bit;
.LP
\(em
on receipt of an SABME command, the procedures defined in
\(sc\ 5.5.1 shall be followed;
.LP
\(em
on receipt of an unsolicited DM response with the F bit set
to 0, the data link layer entity shall, if it is able to,
initiate the establishment procedures by the transmission of an SABME (see
\(sc\ 5.5.1.2). Otherwise, the DM shall be ignored;
.LP
\(em
on receipt of UI commands, the procedures defined in \(sc\ 5.2
shall be followed; and
.LP
\(em
on receipt of any unsolicited UA response an
MDL\(hyERROR\(hyINDICATION primitive indicating a possible double
assignment of a TEI value shall be issued; and
.LP
\(em
all other frame types shall be discarded.
.sp 2P
.LP
5.5.5
\fICollision\fR \fIof unnumbered commands and responses\fR
.sp 1P
.RT
.sp 1P
.LP
5.5.5.1
\fIIdentical transmitted and received commands\fR
.sp 9p
.RT
.PP
If the transmitted and received unnumbered commands (SABME or
DISC) are the same, the data link layer entities shall send the UA response
at the earliest possible opportunity. The indicated state shall be entered
after receiving the UA response. The data link layer entity shall notify
layer\ 3 by means of the appropriate confirm primitive.
.RT
.sp 1P
.LP
5.5.5.2
\fIDifferent transmitted and received commands\fR
.sp 9p
.RT
.PP
If the transmitted and received unnumbered commands (SABME or
DISC) are different, the data link layer entities shall issue a DM response
at the earliest possible opportunity. Upon receipt of a DM response with
the F bit set to 1, the data link layer shall enter the \fITEI\(hyassigned\fR
state and notify layer\ 3 by means of the appropriate primitive. The entity
receiving the DISC
command will issue a DL\(hyRELEASE\(hyINDICATION primitive, while the other
entity
will issue a DL\(hyRELEASE\(hyCONFIRM primitive.
.RT
.sp 1P
.LP
5.5.6
\fIUnsolicited DM response and SABME or DISC command\fR
.sp 9p
.RT
.PP
When a DM response with the F bit set to 0 is received by a data
link layer entity, a collision between a transmitted SABME or DISC command
and the unsolicited DM response may have occurred. This is typically caused
by a user equipment applying a protocol procedure according to\ X.25 LAPB\
[9] to
ask for a mode\(hysetting command.
.PP
In order to avoid misinterpretation of the DM response received, a
data link layer entity shall always send its SABME or DISC command with the
P\ bit set to\ 1.
.PP
A DM response with the F bit set to 0 colliding with an SABME or
DISC command shall be ignored.
.bp
.RT
.sp 1P
.LP
5.6
\fIProcedures for information transfer in multiple frame operation\fR
.sp 9p
.RT
.PP
The procedures which apply to the transmission of I frames are
defined below.
.PP
\fINote\fR \ \(em\ The term \*Qtransmission of an I frame\*U refers to
the delivery of an I frame by the data link layer to the physical layer.
.RT
.sp 1P
.LP
5.6.1
\fITransmitting I frames\fR
.sp 9p
.RT
.PP
Information received by the data link layer entity from layer 3 by means
of a DL\(hyDATA\(hyREQUEST primitive shall be transmitted in an I\ frame.
The
control field parameters\ N(S) and\ N(R) shall be assigned the values of
V(S) and\ V(R), respectively. V(S) shall be incremented by\ 1 at the end
of the transmission of the I frame.
.PP
If timer T200 is not running at the time of transmission of an I
frame, it shall be started. If time T200 expires, the procedures defined in
\(sc\ 5.6.7 shall be followed.
.PP
If V(S) is equal to V(A) plus \fIk\fR (where \fIk\fR is the maximum number
of outstanding I frames\ \(em\ see \(sc\ 5.9.5), the data link layer entity
shall not
transmit any new I frames, but may retransmit an I frame as a result of the
error recovery procedures as described in \(sc\(sc\ 5.6.4 and 5.6.7.
.PP
When the network side or user side is in the own receiver busy
condition, it may still transmit I frames, provided that a
peer receiver
busy condition
does not exist.
.PP
\fINote\fR \ \(em\ Any DL\(hyDATA\(hyREQUEST primitives received whilst
in the timer recovery condition shall be queued.
.RT
.sp 1P
.LP
5.6.2
\fIReceiving I frames\fR
.sp 9p
.RT
.PP
Independant of a timer recovery condition, when a data link layer entity
is not in an own receiver busy condition and receives a valid I frame
whose N(S) is equal to the current V(R), the data link layer entity
shall:
.RT
.LP
\(em
pass the information field of this frame to layer 3 using the
DL\(hyDATA\(hyINDICATION primitive;
.LP
\(em
increment by 1 its V(R) and act as indicated below.
.sp 1P
.LP
5.6.2.1
\fIP bit set to 1\fR
.sp 9p
.RT
.PP
If the P bit of the received I frame was set to 1, the data link
layer entity shall respond to its peer in one of the following ways:
.RT
.LP
\(em
if the data link layer entity receiving the I frame is still
not in an
own receiver busy condition
, it shall send an
RR response with the F\ bit set to\ 1;
.LP
\(em
if the data link layer entity receiving the I frame enters
the own receiver busy condition upon receipt of the I frame,
it shall send an RNR response with the F bit set to\ 1.
.sp 1P
.LP
5.6.2.2
\fIP bit set to 0\fR
.sp 9p
.RT
.PP
If the P bit of the received I frame was set to 0 and:
.RT
.LP
a)
if the data link layer entity is still not in an own
receiver busy condition:
.LP
\(em
if no I frame is available for transmission or if an I
frame is available for transmission but a peer receiver
busy condition exists, the data link layer entity shall
transmit an RR response with the F bit set to 0; or
.LP
\(em
if an I frame is available for transmission and no peer
receiver busy condition exists, the data link layer entity
shall transmit the I frame with the value of N(R) set to
the current value of V(R) as defined in \(sc\ 5.6.1; or
.LP
b)
if, on receipt of this I frame, the data link layer entity
is now in an own receiver busy condition, it shall transmit an
RNR response with the F\ bit set to\ 0.
.PP
When the data link layer entity is in an own receiver busy
condition, it shall process any received I frame according to
\(sc\ 5.6.6.
.bp
.sp 2P
.LP
5.6.3
\fISending and receiving acknowledgements\fR
.sp 1P
.RT
.sp 1P
.LP
5.6.3.1
\fISending acknowledgements\fR
.sp 9p
.RT
.PP
Whenever a data link layer entity transmits an I frame or a
supervisory frame, N(R) shall be set equal to V(R).
.RT
.sp 1P
.LP
5.6.3.2
\fIReceiving acknowledgements\fR
.sp 9p
.RT
.PP
On receipt of a valid I frame or supervisory frame (RR, RNR,
or\ REJ), even in the own receiver busy, or timer recovery conditions,
the data link layer entity shall treat the N(R) contained in this frame
as an
acknowledgement for all the I frames it has transmitted with an N(S) up
to and including the received N(R)\ \(em\ 1. V(A) shall be set to N(R).
The data link
layer entity shall reset the timer T200 on receipt of a valid I frame or
supervisory frame with the N(R) higher than V(A) (actually acknowledging
some I frames), or an REJ frame with an N(R) equal to V(A).
.PP
\fINote\ 1\fR \ \(em\ If a supervisory frame with the P bit set to 1 has been
transmitted and not acknowledged, timer T200 shall not be reset.
.PP
\fINote\ 2\fR \ \(em\ Upon receipt of a valid I frame, timer T200 shall not be
reset if the data link layer entity is in the peer receiver busy condition.
.PP
If timer T200 has been reset by the receipt of an I, RR, or RNR frame,
and if there are outstanding I frames still unacknowledged, the data link
layer entity shall restart timer T200. If timer T200 then expires, the
data link
layer entity shall follow the recovery procedure as defined in \(sc\ 5.6.7 with
respect to the unacknowledged I frames.
.PP
If timer T200 has been reset by the receipt of an REJ frame, the data link
layer entity shall follow the retransmission procedures in \(sc\ 5.6.4.
.RT
.sp 1P
.LP
5.6.4
\fIReceiving REJ frames\fR
.sp 9p
.RT
.PP
On receipt of a valid REJ frame, the data link layer entity shall act as
follows:
.RT
.LP
a)
if it is not in the timer recovery condition:
.LP
\(em
clear an existing peer receiver busy condition;
.LP
\(em
set its V(S) and its V(A) to the value of the N(R)
contained in the REJ frame control field;
.LP
\(em
stop timer T200;
.LP
\(em
start timer T203 if implemented;
.LP
\(em
if it was an REJ command frame with the P\ bit set to 1,
transmit an appropriate supervisory response frame (see
Note\ 2, \(sc\ 5.6.5) with the F\ bit set to\ 1.
.LP
\(em
transmit the corresponding I frame as soon as possible,
as defined in \(sc\ 5.6.1, taking into account the items\ 1)
to\ 3) below and the paragraph following items\ 1) to\ 3)
and
.LP
\(em
notify a protocol violation to the connection
management entity by means of the MDL\(hyERROR\(hyINDICATION
primitive, if it was an REJ response frame with the F\ bit
set to\ 1.
.LP
b)
if it is in the timer recovery condition and it was an REJ
response frame with the F\ bit set to\ 1:
.LP
\(em
clear an existing peer receiver busy condition;
.LP
\(em
set its V(S) and its V(A) to the value N(R) contained
in the REJ frame control field;
.LP
\(em
stop timer T200;
.LP
\(em
start timer T203 if implemented;
.LP
\(em
enter the multiple\(hyframe\(hyestablished state; and
.LP
\(em
transmit the corresponding I frame as soon as possible,
as defined in \(sc\ 5.6.1, taking into account the items\ 1)
to\ 3) below and the paragraph following items\ 1) to\ 3).
.LP
c)
if it is in the timer recovery condition and it was an REJ
frame other than an REJ response frame with the F bit set
to\ 1:
.LP
\(em
clear an existing peer receiver busy condition;
.LP
\(em
set its V(A) to the value of the N(R) contained in the
REJ frame control field; and
.LP
\(em
if it was an REJ command frame with the P bit set
to\ 1, transmit an appropriate supervisory response frame
with the F\ bit set to\ 1 (see Note\ 2 in
\(sc\ 5.6.5).
.bp
.PP
Transmission of I frames shall take account of the
following:
.LP
1)
if the data link layer entity is transmitting a supervisory
frame when it receives the REJ frame, it shall complete that
transmission before commencing transmission of the requested I
frame;
.LP
2)
if the data link layer entity is transmitting an SABME
command, a DISC command, a UA response or a DM response when it
receives the REJ frame, it shall ignore the request for
retransmission; and
.LP
3)
if the data link layer entity is not transmitting a frame
when the REJ is received, it shall immediately commence
transmission of the requested I\ frame.
.PP
All outstanding unacknowledged I frames, commencing with the I
frame identified in the received REJ frame, shall be transmitted. Other I
frames not yet transmitted may be transmitted following the retransmitted I
frames.
.sp 1P
.LP
5.6.5
\fIReceiving RNR frames\fR
.sp 9p
.RT
.PP
After receiving a valid RNR command or response, if the data link layer
entity is not engaged in a mode\(hysetting operation, it shall set a peer
receiver busy condition and then:
.RT
.LP
\(em
if it was an RNR command with the P bit set to 1, it shall
respond with an RR response with the F bit set to 1 if the
data link layer entity is not in an own receiver busy condition,
and shall respond with an RNR response with the F bit set to 1
if the data link layer entity is in an own receiver busy
condition; and
.LP
\(em
if it was an RNR response with the F bit set to 1, an
existing timer recovery condition shall be cleared and the N(R)
contained in this RNR response shall be used to update
V(S).
.PP
The data link layer entity shall take note of the peer receiver
busy condition and not transmit any I frames to the peer which has indicated
the busy condition.
.PP
\fINote\ 1\fR \ \(em\ The N(R) in any RR or RNR command frame (irrespective of
the setting of the P\ bit) will not be used to update the\ V(S).
.PP
The data link layer entity shall then:
.RT
.LP
\(em
treat the N(R) contained in the received RNR frame as an
acknowledgement for all the I frames that have been
(re)transmitted with an N(S) up to and including N(R) minus\ 1,
and set its V(A) to the value of the N(R) contained in the RNR
frame; and
.LP
\(em
restart timer T200 unless a supervisory response frame with
the F bit set to 1 is still expected.
.PP
If timer T200 expires, the data link layer entity shall:
.LP
\(em
if it is not yet in a timer recovery condition, enter the
timer recovery condition and reset the retransmission count
variable; or
.LP
\(em
if it is already in a timer recovery condition, add one to
its retransmission count variable.
.PP
The data link layer entity shall then:
.LP
a)
if the value of the retransmission count variable is less
than\ N200:
.LP
\(em
transmit an appropriate supervisory command (see
Note\ 2) with a P\ bit set to\ 1;
.LP
\(em
restart timer T200; and
.LP
b)
if the value of the retransmission count variable is equal
to N200, initiate a re\(hyestablishment procedure as defined in
\(sc\ 5.7, and indicate this by means of the MDL\(hyERROR\(hyINDICATION
primitive to the connection management entity.
.PP
The data link layer entity receiving the supervisory frame with
the P bit set to 1 shall respond, at the earliest opportunity, with a
supervisory response frame (see Note\ 2) with the F bit set to 1, to indicate
whether or not its own receiver busy condition still exists.
.PP
Upon receipt of the supervisory response with the F bit set to 1,
the data link layer entity shall reset timer T200, and:
.RT
.LP
\(em
if the response is an RR or REJ response, the peer receiver
busy condition is cleared and the data link layer entity may
transmit new I frames or retransmit I frames as defined in
\(sc\(sc\ 5.6.1 or\ 5.6.4, respectively; or
.bp
.LP
\(em
if the response is an RNR response, the data link layer
entity receiving the response shall proceed according to this
\(sc\ 5.6.5, first paragraph.
.PP
If a supervisory command (RR, RNR, or REJ) with the P bit set to 0 or 1,
or a supervisory response frame (RR, RNR, or REJ) with the F bit
set to 0 is received during the
enquiry process
, the data link layer
entity shall:
.LP
\(em
if the supervisory frame is an RR or REJ command frame or an
RR or REJ response frame with the F bit set to 0, clear the
peer receiver busy condition and if the supervisory frame
received was a command with the P bit set to 1, transmit the
appropriate supervisory response frame (see Note\ 2) with the F
bit set to 1. However, the transmission or retransmission of
I frames shall not be undertaken until the appropriate
supervisory response frame with the F bit set to 1 is
received or until expiry of timer T200; or
.LP
\(em
if the supervisory frame is an RNR command frame or an RNR
response frame with the F bit set to 0, retain the peer
receiver busy condition and if the supervisory frame received
was an RNR command with P bit set to 1, transmit the
appropriate supervisory response frame (see Note\ 2) with the
F\ bit set to\ 1.
.PP
Upon receipt of an SABME command, the data link layer entity
shall clear the peer receiver busy condition.
.PP
\fINote\ 2\fR \ \(em\ If the data link layer entity is not in an own receiver
busy condition and is in a Reject exception condition [that is, an N(S)
sequence error has been received, and an REJ frame has been transmitted, but
the requested I frame has not been received], the appropriate supervisory
frame is the RR frame.
.PP
If the data link layer entity is not in an own receiver busy condition
but is in an N(S) sequence error exception condition [that is, an N(S)
sequence error has been received but an REJ frame has not been transmitted],
the appropriate supervisory frame is the REJ frame.
.PP
If the data link layer entity is in its own receiver busy condition, the
appropriate supervisory frame is the RNR frame.
.PP
Otherwise, the appropriate supervisory frame is the RR frame.
.RT
.sp 1P
.LP
5.6.6
\fIData link layer\fR
\fIown receiver busy condition\fR
.sp 9p
.RT
.PP
When the data link layer entity enters an own receiver busy
condition, it shall transmit an RNR frame at the earliest opportunity.
.PP
The RNR frame may be either:
.RT
.LP
\(em
an RNR response with the F bit set to 0; or
.LP
\(em
if this condition is entered on receiving a command frame
with the P\ bit set to\ 1, an RNR response with the F\ bit set
to\ 1; or
.LP
\(em
if this condition is entered on expiry of timer T200, an RNR
command with the P\ bit set to\ 1.
.PP
All received I frames with the P bit set to 0 shall be
discarded, after updating\ V(A).
.PP
All received supervisory frames with the P/F bit set to 0 shall be
processed, including updating\ V(A).
.PP
All received I frames with the P bit set to 1 shall be discarded,
after updating\ V(A). However, an RNR response frame with the F\ bit set to\ 1
shall be transmitted.
.PP
All received supervisory frames with the P bit set to 1 shall be
processed including updating\ V(A). An RNR response with the F\ bit set to\ 1
shall be transmitted.
.PP
To indicate to the peer data link layer entity the clearance of the
own receiver busy condition, the data link layer entity shall transmit an
RR\ frame or, if a previously detected\ N(S) sequence error has not yet been
reported, an REJ frame with the N(R) set to the current value of\ V(R).
.PP
The transmission of an SABME command or a UA response (in reply to
an SABME command) also indicates to the peer data link layer entity the
clearance of the own receiver busy condition.
.bp
.RT
.sp 1P
.LP
5.6.7
\fIWaiting acknowledgement\fR
.sp 9p
.RT
.PP
The data link layer entity shall maintain an internal
retransmission count variable.
.PP
If timer T200 expires, the data link layer entity shall:
.RT
.LP
\(em
if it is not yet in the timer recovery condition, enter the
timer recovery condition and reset the retransmission count
variable; or
.LP
\(em
if it is already in the timer recovery condition, add one to
its retransmission count variable.
.PP
The data link layer entity shall then:
.LP
a)
if the value of the retransmission count variable is less
than\ N200:
.LP
\(em
restart timer T200; and either
.LP
\(em
transmit an appropriate supervisory command (see Note\ 2
in \(sc\ 5.6.5) with the P\ bit set to\ 1; or
.LP
\(em
retransmit the last transmitted I frame [V(S)\ \(em\ 1] with
the P\ bit set to\ 1; or
.LP
b)
if the value of the retransmission count variable is equal
to N200, initiate a re\(hyestablishment procedure as defined in
\(sc\ 5.7 and indicate this by means of the MDL\(hyERROR\(hyINDICATION
primitive to the connection management entity.
.PP
The timer recovery condition is cleared when the data link layer entity
receives a valid supervisory frame response with the F bit set to 1. If
the received supervisory frame N(R) is within the range from its current
V(A) to its current V(S) inclusive, it shall set its V(S) to the value
of the
received N(R). Timer T200 shall be reset if the received supervisory frame
response is an RR or REJ response, and then the data link layer entity shall
resume with I frame transmission or retransmission, as appropriate. Timer
T200 shall be reset and restarted if the received supervisory response
is an RNR
response, to proceed with the enquiry process according to \(sc\ 5.6.5.
.sp 2P
.LP
5.7
\fIRe\(hyestablishment of multiple frame operation\fR
.sp 1P
.RT
.sp 1P
.LP
5.7.1
\fICriteria for re\(hyestablishment\fR
.sp 9p
.RT
.PP
The criteria for re\(hyestablishing the multiple frame mode of
operation are defined in this section by the following conditions:
.RT
.LP
\(em
the receipt, while in the multiple\(hyframe mode of operation,
of an SABME;
.LP
\(em
the receipt of a DL\(hyESTABLISH\(hyREQUEST primitive from layer\ 3
(see \(sc\ 5.5.1.1);
.LP
\(em
the occurrence of N200 retransmission failures while in the
timer recovery condition (see \(sc\ 5.6.7);
.LP
\(em
the occurrence of a frame rejection condition as
identified in \(sc\ 5.8.5;
.LP
\(em
the receipt, while in the multiple\(hyframe mode of operation,
of an FRMR response frame (see \(sc\ 5.8.6);
.LP
\(em
the receipt, while in the multiple\(hyframe mode of operation,
of an unsolicited DM response with the F bit set to 0
(see \(sc\ 5.8.7);
.LP
\(em
the receipt, while in the timer\(hyrecovery condition, of a
DM response with the F\ bit set to\ 1.
.sp 1P
.LP
5.7.2
\fIProcedures\fR
.sp 9p
.RT
.PP
In all re\(hyestablishment situations, the data link layer entity
shall follow the procedures defined in \(sc\ 5.5.1. All locally generated
conditions for re\(hyestablishment will cause the transmission of the SABME.
.PP
In the case of data link layer and peer initiated re\(hyestablishment,
the data link layer entity shall also:
.RT
.LP
\(em
issue an MDL\(hyERROR\(hyINDICATION primitive to the connection
management entity; and
.LP
\(em
if V(S) > V(A) prior to re\(hyestablishment, issue a
DL\(hyESTABLISH\(hyINDICATION primitive to layer\ 3, and discard all I
queues.
.PP
In case of layer 3 initiated re\(hyestablishment or if a
DL\(hyESTABLISH\(hyREQUEST primitive occurs pending re\(hyestablishment, the
DL\(hyESTABLISH\(hyCONFIRM primitive shall be used.
.bp
.sp 1P
.LP
5.8
\fIException condition reporting and recovery\fR
.sp 9p
.RT
.PP
Exception conditions may occur as the result of physical layer
errors or data link layer procedural errors.
.PP
The error recovery procedures which are available to effect recovery following
the detection of an exception condition at the data link layer are
defined in this section.
.PP
The actions to be taken by the connection management entity on receipt
of an MDL\(hyERROR\(hyINDICATION primitive are defined in Appendix\ II.
.RT
.sp 1P
.LP
5.8.1
\fIN(S) sequence error\fR
.sp 9p
.RT
.PP
An N(S) sequence error exception condition occurs in the receiver when
a valid I frame is received which contains an N(S) value which is not
equal to the V(R) at the receiver. The information field of all I frames
whose N(S) does not equal V(R) shall be discarded.
.PP
The receiver shall not acknowledge [nor increment its V(R)] the I
frame causing the sequence error, nor any I frames which may follow, until
an I frame with the correct N(S) is received.
.PP
A data link layer entity which receives one or more I frames having
sequence errors but otherwise error\(hyfree, or subsequent supervisory
frames (RR, RNR, and\ REJ), shall use the control field information contained
in the N(R)
field and the P or F bit to perform data link control functions; for example,
to receive acknowledgement of previously transmitted I frames and to cause
the data link layer entity to respond if the P bit is set to 1. Therefore,
the
retransmitted I frame may contain an N(R) field value and P bit that are
updated from, and therefore different from, the ones contained in the
originally transmitted I\ frame.
.PP
The REJ frame is used by a receiving data link layer entity to
initiate an exception condition recovery (retransmission) following the
detection of an N(S) sequence error.
.PP
Only one REJ exception condition for a given direction of information transfer
shall be established at a time.
.PP
A data link layer entity receiving an REJ command or response shall
initiate sequential transmission (retransmission) of I frames starting
with the I frame indicated by the N(R) contained in the REJ frame.
.PP
An REJ exception condition is cleared when the requested I frame is
received or when an SABME or DISC command is received.
.PP
An optional procedure for the retransmission of an REJ response frame is
described in Appendix\ I.
.RT
.sp 1P
.LP
5.8.2
\fIN(R) sequence error\fR
.sp 9p
.RT
.PP
An N(R) sequence error exception condition occurs in the
transmitter when a valid supervisory frame or I frame is received which
contains an invalid N(R) value.
.PP
A valid N(R) is one that is in the range V(A) \(= N(R) \(= V(S).
.PP
The information field contained in an I frame which is correct in
sequence and format may be delivered to layer\ 3 by means of the
DL\(hyDATA\(hyINDICATION primitive.
.PP
The data link layer entity shall inform the connection management
entity of this exception condition by means of the MDL\(hyERROR\(hyINDICATION
primitive, and initiate re\(hyestablishment according to \(sc\ 5.7.2.
.RT
.sp 1P
.LP
5.8.3
\fITimer recovery condition\fR
.sp 9p
.RT
.PP
If a data link layer entity, due to a transmission error, does not receive
a single I frame or the last I frame(s) in a sequence of I frames, it will
not detect an
out\(hyof\(hysequence exception condition
and therefore
will not transmit an REJ frame.
.PP
The data link layer entity which transmitted the unacknowledged I
frame(s) shall, on the expiry of timer T200, take appropriate recovery
action as defined in \(sc\ 5.6.7 to determine at which I frame retransmission
must
begin.
.bp
.RT
.sp 1P
.LP
5.8.4
\fIInvalid frame condition\fR
.sp 9p
.RT
.PP
Any frame received which is invalid (as defined in \(sc\ 2.9) shall be
discarded, and no action shall be taken as a result of that frame.
.RT
.sp 1P
.LP
5.8.5
\fIFrame rejection condition\fR
.sp 9p
.RT
.PP
A frame rejection condition results from one of the conditions
described in \(sc\ 3.6.1 (third paragraph) or \(sc\ 3.6.11, items\ b, c and\ d.
.PP
Upon occurrence of a frame rejection condition whilst in the multiple frame
operation,the data link layer entity shall:
.RT
.LP
\(em
issue an MDL\(hyERROR\(hyINDICATION primitive; and
.LP
\(em
initiate re\(hyestablishment (see \(sc\ 5.7.2).
.PP
\fINote\fR \ \(em\ For satisfactory operation it is essential that a
receiver is able
to discriminate between invalid frames, as defined in \(sc\ 2.9, and frames
with an I\(hyfield which exceeds the maximum established length [see\ d)
of \(sc\ 3.6.11]. An unbounded frame may be assumed, and thus discarded,
if two times the longest
permissible frame plus two octets are received without a flag detection.
.sp 1P
.LP
5.8.6
\fIReceipt of an FRMR response frame\fR
.sp 9p
.RT
.PP
Upon receipt of an FRMR response frame in the multiple\(hyframe mode of
operation, the data link layer entity shall:
.RT
.LP
\(em
issue an MDL\(hyERROR\(hyINDICATION primitive; and
.LP
\(em
initiate re\(hyestablishment (see \(sc\ 5.7.2).
.sp 1P
.LP
5.8.7
\fIUnsolicited response frames\fR
.sp 9p
.RT
.PP
The action to be taken on the receipt of an unsolicited response
frame is defined in Table\ 9/Q.921.
.PP
The data link layer entity shall assume possible multiple\(hyTEI
assignment on the receipt of an unsolicited UA response and shall inform
layer management.
.RT
.ce
\fBH.T. [T16.921]\fR
.ce
TABLE\ 9/Q.921
.ce
\fBActions taken on receipt of unsolicited response frames\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(42p) | cw(40p) | cw(40p) | cw(34p) | cw(36p) sw(36p) , ^ | ^ | ^ | ^ | c | c.
Unsolicited response frame TEI\(hyassigned Awaiting Establishment Awaiting Release {
Multiple frame modes
of operation
}
Established mode Time recovery condition
_
.T&
cw(42p) | cw(40p) | cw(40p) | cw(34p) | cw(36p) | cw(36p) .
UA response F = 1 MDL\(hyError Indication Solicited Solicited MDL\(hyError Indication MDL\(hyError Indication
_
.T&
cw(42p) | cw(40p) | cw(40p) | cw(34p) | cw(36p) | cw(36p) .
UA response F = 0 MDL\(hyError Indication MDL\(hyError Indication MDL\(hyError Indication MDL\(hyError Indication MDL\(hyError Indication
_
.T&
cw(42p) | cw(40p) | cw(40p) | cw(34p) | cw(36p) | cw(36p) .
DM response F = 1 Ignore Solicited Solicited MDL\(hyError Indication {
Re\(hyestablish
MDL\(hyError
Indication
}
_
.T&
cw(42p) | cw(40p) | cw(40p) | cw(34p) | cw(36p) | cw(36p) .
DM response F = 0 Establish Ignore Ignore {
Re\(hyestablish
MDL\(hyError
Indication
} {
Re\(hyestablish
MDL\(hyError
Indication
}
_
.T&
cw(42p) | cw(40p) | cw(40p) | cw(34p) | cw(36p) | cw(36p) .
Supervisory Response F\ =\ 1 Ignore Ignore Ignore MDL\(hyError Indication Solicited
_
.T&
cw(42p) | cw(40p) | cw(40p) | cw(34p) | cw(36p) | cw(36p) .
Supervisory Response F\ =\ 0 Ignore Ignore Ignore Solicited Solicited
_
.TE
.nr PS 9
.RT
.ad r
\fBTable 9/Q.921 [T16.921], p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
5.8.8
\fIMultiple assignment of a TEI value\fR
.sp 9p
.RT
.PP
A data link layer entity shall assume multiple assignment of a TEI value
and initiate recovery as specified below by:
.RT
.LP
a)
the receipt of a UA response frame whilst in the
\fImultiple\(hyframe\(hyestablished\fR state;
.LP
b)
the receipt of a UA response frame whilst in the \fItimer\fR \fIrecovery\fR
state;
.LP
c)
the receipt of a UA response frame whilst in the
\fITEI\(hyassigned\fR state.
.PP
A data link layer entity, after assuming multiple assignment of a TEI value
shall inform the connection management entity by means of the
MDL\(hyERROR\(hyINDICATION primitive.
.sp 1P
.LP
5.9
\fIList of system parameters\fR
.sp 9p
.RT
.PP
The system parameters listed below are associated with each
individual SAP.
.PP
A method of assigning these parameters is defined in \(sc\ 5.4.
.PP
The term default implies that the value defined should be used in the absence
of any assignment or negotiation of alternative values.
.RT
.sp 1P
.LP
5.9.1
\fITimer T200\fR
.sp 9p
.RT
.PP
The default value for timer T200 at the end of which transmission of a
frame may be initiated according to the procedures described in \(sc\ 5.6
shall be one second.
.PP
\fINote\ 1\fR \ \(em\ The proper operation of the procedure requires that
timer T200 be greater than the maximum time between transmission of command
frames
and the reception of their corresponding response or acknowledgement frames.
.PP
\fINote\ 2\fR \ \(em\ When an implementation includes multiple terminals
on the user side together with a satellite connection in the transmission
path, a
value of T200 greater than 1\ second may be necessary. A value of 2.5\ seconds
is suggested.
.RT
.sp 1P
.LP
5.9.2
\fIMaximum number of retransmissions (N200)\fR
.sp 9p
.RT
.PP
The maximum number of retransmissions of a frame (N200) is a system parameter.
The default value of N200 shall be\ 3.
.RT
.sp 1P
.LP
5.9.3
\fIMaximum number of octets in an information field (N201)\fR
.sp 9p
.RT
.PP
The maximum number of octets in an information field (N201) is a
system parameter. (See also \(sc\ 2.5.)
.RT
.LP
\(em
For an SAP supporting signalling, the default value shall be
260\ octets.
.LP
\(em
For SAPs supporting packet information, the default value
shall be 260\ octets.
.sp 1P
.LP
5.9.4
\fIMaximum number of transmissions of the TEI Identity request\fR
\fImessage (N202)\fR
.sp 9p
.RT
.PP
The maximum number of transmissions of a TEI Identity request
message (when the user requests a TEI) is a system parameter. The default
value of N202 shall be\ 3.
.RT
.sp 1P
.LP
5.9.5
\fIMaximum number of outstanding I frames (k)\fR
.sp 9p
.RT
.PP
The maximum number (\fIk\fR ) of sequentially numbered I frames that may
be outstanding (that is, unacknowledged) at any given time is a system
parameter which shall not exceed\ 127, for extended (modulo\ 128)
operation.
.RT
.LP
\(em
For an SAP supporting basic access (16 kbit/s) signalling,
the default value shall be\ 1.
.LP
\(em
For an SAP supporting primary rate (64 kbit/s) signalling,
the default value shall be\ 7.
.LP
\(em
For an SAP supporting basic access (16 kbit/s) packet
information, the default value shall be\ 3.
.LP
\(em
For an SAP supporting primary rate (64 kbit/s) packet
information, the default value shall be\ 7.
.bp
.sp 1P
.LP
5.9.6
\fITimer T201\fR
.sp 9p
.RT
.PP
The minimum time between retransmission of the TEI Identity check messages
(T201) is a system parameter which shall be set to T200\ seconds.
.RT
.sp 1P
.LP
5.9.7
\fITimer T202\fR
.sp 9p
.RT
.PP
The minimum time between the transmission of TEI Identity request messages
is a system parameter (T202) which shall be set to 2\ seconds.
.RT
.sp 1P
.LP
5.9.8
\fITimer T203\fR
.sp 9p
.RT
.PP
Timer T203 represents the maximum time allowed without frames being exchanged.
The default value of timer T203 shall be 10\ seconds.
.RT
.sp 2P
.LP
5.10
\fIData link layer monitor function\fR
.sp 1P
.RT
.sp 1P
.LP
5.10.1
\fIGeneral\fR
.sp 9p
.RT
.PP
The procedural elements defined in \(sc\ 5 allow for the supervision of
the data link layer resource. This section describes procedures which may
be
used to provide this supervision function. The use of this function is
optional.
.RT
.sp 1P
.LP
5.10.2
\fIData link layer supervision in the multiple\(hyframe\(hyestablished\fR
\fIstate\fR
.sp 9p
.RT
.PP
The procedures specified herein propose a solution which is already identified
in the HDLC classes of procedures. The connection verification is a service
provided by data link layer to layer\ 3. This implies that layer\ 3 is
informed in case of a failure only. Furthermore, the procedure may be
incorporated in the \*Qnormal\*U exchange of information and may become more
efficient than a procedure based on the involvement of layer\ 3.
.PP
The procedure is based on supervisory command frames (RR command,
RNR\ command) and timer T203, and operates in the multiple\(hyframe\(hyestablished
state as follows.
.PP
If there are no frames being exchanged on the data link connection
(neither new nor outstanding I frames, nor supervisory frames with a P
bit set to 1), there is no means to detect a faulty data link connection
condition, or a user equipment having been unplugged. Timer T203 represents
the maximum time allowed without frames being exchanged.
.PP
If timer T203 expires, a supervisory command with a P bit set to 1
is transmitted. Such a procedure is protected against transmission errors by
making use of the normal timer T200 procedure including retransmission count
and N200 attempts.
.RT
.sp 2P
.LP
5.10.3
\fIConnection verification procedures\fR
.sp 1P
.RT
.sp 1P
.LP
5.10.3.1
\fIStart timer T203\fR
.sp 9p
.RT
.PP
The timer T203 is started:
.RT
.LP
\(em
when the \fImultiple\(hyframe\(hyestablished\fR state is entered; and
.LP
\(em
in the \fImultiple\(hyframe\(hyestablished\fR state whenever T200 is
stopped. (See Note in \(sc\ 5.10.3.2.)
.PP
Upon receiving an I or supervisory frame, timer T203 will be
restarted if timer T200 is not to be started.
.sp 1P
.LP
5.10.3.2
\fIStop timer T203\fR
.sp 9p
.RT
.PP
The timer T203 is stopped:
.RT
.LP
\(em
when, in the \fImultiple\(hyframe\(hyestablished\fR state, the timer
T200 is started (see note); and
.LP
\(em
upon leaving the \fImultiple\(hyframe\(hyestablished\fR state.
.PP
\fINote\fR \ \(em\ These two conditions mean that timer T203 is only started
whenever T200 is stopped and not restarted.
.bp
.sp 1P
.LP
5.10.3.3
\fIExpiry of timer T203\fR
.sp 9p
.RT
.PP
If timer T203 expires, the data link layer entity will act as
follows (it should be noted that timer T200 is neither running nor
expired):
.RT
.LP
a)
set the retransmission count variable to 0;
.LP
b)
enter \fItimer recovery\fR state;
.LP
c)
transmit a supervisory command with the P bit set to 1 as
follows:
.LP
\(em
if there is not a receiver busy condition (own receiver
not busy), transmit an RR command; or
.LP
\(em
if there is a receiver busy condition (own receiver
busy), transmit an RNR command; and
.LP
d)
start timer T200; and
.LP
e)
send MDL\(hyERROR\(hyINDICATION primitive to connection management
after N200 retransmissions.
.LP
.sp 5
.ce 1000
ANNEX\ A
.ce 0
.ce 1000
(to Recommendation Q.921)
.sp 9p
.RT
.ce 0
.ce 1000
\fBProvision of point\(hyto\(hypoint signalling connections\fR
.sp 1P
.RT
.ce 0
.PP
In certain applications it may be advantageous to have a
single point\(hyto\(hypoint signalling connection at layer\ 3; the allocation
of the value\ 0 as a preferred TEI for that purpose is a network option.
Use of the
value\ 0 in such applications does not preclude using that value in other
applications or networks.
.sp 1P
.RT
.LP
.rs
.sp 20P
.ad r
Blanc
.ad b
.RT
.LP
.bp
.ce 1000
ANNEX\ B
.ce 0
.ce 1000
(to Recommendation Q.921)
.sp 9p
.RT
.ce 0
.ce 1000
\fBSDL for point\(hyto\(hypoint procedures\fR
.sp 1P
.RT
.ce 0
.LP
B.1
\fIGeneral\fR
.sp 1P
.RT
.PP
The purpose of this annex is to provide one example of an SDL
representation of the point\(hyto\(hypoint procedures of the data link
layer, to
assist in the understanding of this Recommendation. This representation does
not describe all of the possible actions of the data link layer entity, as a
non\(hypartitioned representation was selected in order to minimize its
complexity. The SDL representation does not therefore constrain implementations
from exploiting the full scope of the procedures as presented within the
text of this Recommendation. The text description of the procedures is
definitive.
.PP
The representation is a peer\(hyto\(hypeer model of the point\(hyto\(hypoint
procedures of the data link layer and is applicable to the data link layer
entities at both the user and network sides for all ranges of TEI values.
See Figure\ B\(hy1/Q.921.
.RT
.LP
.rs
.sp 20P
.ad r
\fBFigure B\(hy1/Q.921, p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
B.2
\fIAn overview of the states of the point\(hyto\(hypoint data link\fR
\fIlayer entity\fR
.sp 9p
.RT
.PP
The SDL representation of the point\(hyto\(hypoint procedures are based
on an expansion of the three basic states identified in \(sc\ 3.4.2/Q.920\
(I.440) to the following 8\ states:
.RT
.LP
State 1
\fITEI unassigned\fR
.LP
State 2
\fIAssign awaiting TEI\fR
.LP
State 3
\fIEstablish awaiting TEI\fR
.LP
State 4
\fITEI assigned\fR
.LP
State 5
\fIAwaiting establishment\fR
.LP
State 6
\fIAwaiting release\fR
.LP
State 7
\fIMultiple frame established\fR
.LP
State 8
\fITimer recovery\fR .bp
.PP
An overview of the inter\(hyrelationship of these states is provided in
Figure\ B\(hy2/Q.921. This overview is incomplete, and serves only as an
introduction to the SDL representation. All data link layer entities are
conceptually initiated in the \fITEI unassigned\fR state (state\ 1), and will
interact with the layer management in order to request a TEI value. TEI
assignment initiated by a Unit data request will cause the data link layer
entity to move to the \fITEI assigned\fR state (state\ 4) via the \fIassign
awaiting\fR \fITEI\fR state (state\ 2). Initiation by an Establishment
request will cause a
transition to the \fIawaiting establishment\fR state (state\ 5) via the
\fIestablish\fR \fIawaiting TEI\fR state (state\ 3). Direct TEI assignment
will cause an immediate transition to the \fITEI assigned\fR state (state\
4). In states\ 4\(hy8, Unit data
requests can be directly serviced by the data link layer entity. The receipt
of an Establish request in the \fITEI assigned\fR state (state\ 4) will
cause the
initiation of the establishment procedures and the transition to the \fIawaiting\fR
\fIestablishment\fR state (state\ 5). Completion of the LAP establishment
procedures takes the data link layer entity into the \fImultiple frame
established\fR state
(state\ 7). Peer initiated establishment causes a direct transition from the
\fITEI assigned\fR state (state\ 4) to the \fImultiple frame established\fR
state
(state\ 7). In the \fImultiple frame established\fR state (state\ 7), Acknowledged
.LP
data transfer requests can be serviced directly subject to the restrictions
of the procedures. Expiry of timer T200, which is used in both the flow
control
and data transfer aspects of the data link layer entity's procedures initiates
the transition to the \fItimer recovery\fR state (state\ 8). Completion
of the timer recovery procedures will return the data link layer entity
to the \fImultiple\fR \fIframe established\fR state (state\ 7). In states\
7 and\ 8, of the SDL
representation the following conditions which are identified within the
Recommendation are observed:
.LP
a)
peer receiver busy,
.LP
b)
reject exception,
.LP
c)
own receiver busy.
.PP
In addition other conditions are used in order to avoid
identification of additional states. The complete combination of both of
these categories of conditions with the 8\ states of the SDL representation
is the
basis for the state transition table description of the data link layer
entity. A peer initiated LAP release will take the data link layer entity
directly into the \fITEI assigned\fR state (state\ 4), whilst a Release
request will be via the
\fIawaiting release\fR state (state\ 6). TEI removal will cause a transition
to the \fITEI unassigned\fR state (state\ 1).
.LP
.rs
.sp 31P
.ad r
Blanc
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 47P
.ad r
\fBFigure B\(hy2/Q.921, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
B.3
\fICover notes\fR
.sp 9p
.RT
.PP
The following symbols and abbreviations are used within this
description. A full description of the symbols and their meaning and
application can be found in the Series\ Z\ Recommendations (Fascicles\ X.1
to\ X.5).
.RT
.LP
.rs
.sp 38P
.ad r
\fBDiagrams, p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
B.4
\fIThe use of\fR
\fIqueues\fR
.sp 9p
.RT
.PP
To enable a satisfactory representation of the data link layer
entity,
conceptual queues
for the UI frame and I frame transmission
have been explicitly brought out. These conceptual queues are finite but
unbounded and should in no way restrict the implementation of the
point\(hyto\(hypoint procedures. Two additional signals have been provided
in order to cause the servicing of these queues to be initiated \(em\
UI\ frame queued
up
and
I\ frame queued up
.
.bp
.RT
.sp 1P
.LP
B.5
\fISDL representation\fR
.sp 9p
.RT
.LP
.rs
.sp 47P
.ad r
\fBFigure B\(hy3/Q.921 (1 of 3) + Notes, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 22P
.ad r
\fBFigure B\(hy3/Q.921 (2 of 3), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.rs
.sp 27P
.ad r
\fBFigure B\(hy3/Q.921 (3 of 3), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 47P
.ad r
\fBFigure B\(hy4/Q.921 (1 of 2), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 24P
.ad r
\fBFigure B\(hy4/Q.921 (2 of 2), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.rs
.sp 24P
.ad r
\fBFigure B\(hy5/Q.921 (1 of 3), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 47P
.ad r
\fBFigure B\(hy5/Q.921 (2 of 3), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 24P
.ad r
\fBFigure B\(hy5/Q.921 (3 of 3), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.rs
.sp 23P
.ad r
\fBFigure B\(hy6/Q.921 (1 of 2), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 47P
.ad r
\fBFigure B\(hy6/Q.921 (2 of 2), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 47P
.ad r
\fBFigure B\(hy7/Q.921 (1 of 10), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 47P
.ad r
\fBFigure B\(hy7/Q.921 (2 of 10), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 47P
.ad r
\fBFigure B\(hy7/Q.921 (3 of 10), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 47P
.ad r
\fBFigure B\(hy7/Q.921 (4 of 10), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 47P
.ad r
\fBFigure B\(hy7/Q.921 (5 of 10), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 47P
.ad r
\fBFigure B\(hy7/Q.921 (6 of 10), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 47P
.ad r
\fBFigure B\(hy7/Q.921 (7 of 10), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 47P
.ad r
\fBFigure B\(hy7/Q.921 (8 of 10), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 47P
.ad r
\fBFigure B\(hy7/Q.921 (9 of 10), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 24P
.ad r
\fBFigure B\(hy7/Q.921 (10 of 10), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.rs
.sp 24P
.ad r
\fBFigure B\(hy8/Q.921 (1 of 9), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 47P
.ad r
\fBFigure B\(hy8/Q.921 (2 of 9), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 47P
.ad r
\fBFigure B\(hy8/Q.921 (3 of 9), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 47P
.ad r
\fBFigure B\(hy8/Q.921 (4 of 9), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 47P
.ad r
\fBFigure B\(hy8/Q.921 (5 of 9), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 47P
.ad r
\fBFigure B\(hy8/Q.921 (6 of 9), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 47P
.ad r
\fBFigure B\(hy8/Q.921 (7 of 9), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 47P
.ad r
\fBFigures B\(hy8/Q.921 (8 of 9) et (9 of 9).\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 47P
.ad r
\fBFigure B\(hy9/Q.921 (1 of 5), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 24P
.ad r
\fBFigure B\(hy9/Q.921 (2 of 5), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.rs
.sp 23P
.ad r
\fBFigure B\(hy9/Q.921 (3 of 5), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 24P
.ad r
\fBFigure B\(hy9/Q.921 (4 of 5), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.rs
.sp 24P
.ad r
\fBFigure B\(hy9/Q.921 (5 of 5), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.ce 1000
ANNEX\ C
.ce 0
.ce 1000
(to Recommendation Q.921)
.sp 9p
.RT
.ce 0
.ce 1000
\fBSDL representation of the broadcast procedures\fR
.sp 1P
.RT
.ce 0
.LP
.rs
.sp 49P
.ad r
\fBFigure C\(hy1/Q.921, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp