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InfoMagic Standards 1994 January
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1988
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1991-12-22
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.\" 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
\fBRecommendation\ X.29\fR
.RT
.sp 2P
.ce 1000
\fBPROCEDURES\ FOR\ THE\ EXCHANGE\ OF\ CONTROL\ INFORMATION\fR
.EF '% Fascicle\ VIII.2\ \(em\ Rec.\ X.29''
.OF '''Fascicle\ VIII.2\ \(em\ Rec.\ X.29 %'
.ce 0
.ce 1000
\fBAND\ USER\ DATA\ BETWEEN\ A\ PACKET\fR
.ce 0
.ce 1000
\fBASSEMBLY/DISASSEMBLY\ (PAD)\ FACILITY\fR
.ce 0
.sp 1P
.ce 1000
\fBAND\ A\ PACKET\ MODE\ DTE\ OR\ ANOTHER\ PAD\fR
.ce 0
.sp 1P
.ce 1000
\fI(provisional, Geneva, 1977; amended, Geneva, 1980,\fR
.sp 9p
.RT
.ce 0
.sp 1P
.ce 1000
\fIMalaga\(hyTorremolinos, 1984 and Melbourne, 1988)\fR
.ce 0
.sp 1P
.LP
\fBPreface\fR
.sp 1P
.RT
.PP
The establishment in various countries of public data networks
providing packet\(hyswitched data transmission services creates a need
to produce standards to facilitate international interworking.
.RT
.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 user classes of service and
facilities in a public data network, and Recommendation\ X.96
defines call progress signals;
.PP
(b)
that Recommendation X.3 defines the PAD in a public data network;
.PP
(c)
that Recommendation X.28 defines the DTE/DCE interface for a start\(hystop
mode DTE accessing the PAD in a public data
network;
.PP
(d)
that Recommendation X.25 defines the interface between the DTE and the
DCE for DTEs operating in the packet mode in public data
networks;
.PP
(e)
the need to allow interworking between a packet
mode DTE and a non\(hypacket mode DTE in the packet\(hyswitched transmission
service;
.PP
(f
)
the urgent need to allow interworking between a
start\(hystop mode DTE in a public switched telephone network, public switched
data network or a leased line and a packet mode DTE using the virtual call
facility of the packet\(hyswitched transmission service;
.PP
(g)
the need to allow interworking between PADs;
.PP
(h)
that the packet mode DTE shall not be obliged to use the control procedures
for PAD functions, but that some packet mode DTEs may wish to control specific
functions of the PAD,
.sp 1P
.LP
\fIunanimously recommends that\fR
.sp 9p
.RT
.PP
(1)
the Recommendation X.29 procedures shall apply to the
Recommendation\ X.25 interface between the DCE and the packet mode DTE;
.PP
(2)
the Recommendation X.29 procedures may be applied for
interworking between PADs;
.PP
(3)
the procedures be as specified below in \(sc\ 1 \fIProcedures\fR \fIfor
the exchange of PAD control information and user data\fR ;
.PP
(4)\fR the manner in which user data is transferred be as
specified below in \(sc\ 2 \fIUser data transfer\fR ;
.PP
(5)
the procedures for the control of the PAD via PAD
messages be as specified below in \(sc\ 3 \fIProcedures for the use of PAD\fR
\fImessages\fR ;
.PP
(6)
the formats of the data fields which are transferable on a virtual call
be as specified below in \(sc\ 4 \fIFormats\fR .
.PP
\fINote\ 1\fR \ \(em\ For ease of understanding, this Recommendation refers
to specific packet types and procedures of Recommendation\ X.25. When PAD
to PAD interworking is considered within a national network these packet
types or
procedures may have a different form from those used in Recommendation\ X.25
but will have the same operational meaning.
.bp
.PP
\fINote\ 2\fR \ \(em\ The following items are for further study:
.RT
.LP
\(em
the use of the permanent virtual circuit service;
.LP
\(em
interworking between DTEs having interfaces to different data
transmission services;
.LP
\(em
operation of non\(hypacket mode DTEs in other than start\(hystop
mode.
.sp 2P
.LP
\fB1\fR \fBProcedures for the exchange of PAD control information
and user data\fR
.sp 1P
.RT
.PP
1.1
The exchange of control information and user data between a PAD and a packet
mode DTE or between PADs is performed by using user data fields
defined in Recommendation\ X.25.
.sp 9p
.RT
.PP
1.2
Annex A describes some of the characteristics of virtual calls
as defined in Recommendation\ X.25, as related to the PAD representation of
a start\(hystop mode DTE to a packet mode DTE. The characteristics described in
Annex\ A also apply for interworking between PADs.
.sp 1P
.LP
1.3
\fICall user data\fR
.sp 9p
.RT
.PP
The
call user data field
of \fIincoming call\fR \| or \fIcall\fR
\fIrequest\fR \| packets to or from the packet mode DTE or the PAD is comprised
of
two fields:
.RT
.LP
a)
the
protocol identifier field
, and
.LP
b)
the
call data field
.
.PP
The protocol identifier field is used for protocol identification purposes
and the call data field contains user data.
.PP
A \fIcall request\fR \| packet received by the PAD, containing no call
user data field, will be accepted by the PAD.
.PP
If a call data field is present, the PAD will send it, unchanged, to the
start\(hystop mode DTE, using the call data block of the \fIincoming call
PAD\fR \fIservice\fR signal (see \(sc\ 3.5.22, Recommendation\ X.28).
.RT
.sp 2P
.LP
1.4
\fIUser sequences\fR
.sp 1P
.RT
.PP
1.4.1
User sequences are used to exchange user data between the PAD and the packet
mode DTE or a PAD.
.sp 9p
.RT
.PP
1.4.2
User sequences are conveyed in the user data fields of complete
packet sequences with Q\ =\ 0, and in both directions on a virtual call. (See
Recommendation\ X.25.)
.PP
1.4.3
There will be only one user sequence in a complete packet
sequence.
.PP
1.4.4
The PAD will transmit all \fIdata\fR \| packets with the D bit set to
0.
.PP
On reception of a \fIdata\fR \| packet with the D bit set to 1, the PAD
will transmit the corresponding acknowledgement as soon as possible.
.PP
If the PAD does not support the D bit procedure, the PAD may reset the
virtual call.
.PP
As no error correction procedure is in place from the PAD to the
start\(hystop mode DTE, no guarantee of delivery can be implied from the
acknowledgement.
.RT
.sp 2P
.LP
1.5
\fIPAD messages\fR
.sp 1P
.RT
.PP
1.5.1
\fIPAD\fR \| messages are used to exchange control information
between the PAD and the packet mode DTE (or remote PAD). A \fIPAD\fR message
consists of a
control identifier field
and a
message code field
possibly followed by a
parameter field
(see \(sc\ 4.4 below).
.sp 9p
.RT
.PP
1.5.2
\fIPAD\fR \| messages are conveyed in the user data fields of complete
packet sequences with Q\ =\ 1 and in both directions on a virtual call.
(See Recommendation\ X.25.)
.PP
1.5.3
There will be only one \fIPAD\fR \| message in a complete packet
sequence.
.PP
1.5.4
The PAD will take into consideration a \fIPAD\fR \| message only when it
has been completely received.
.PP
1.5.5
In the case where a parameter reference (see \(sc\ 3 below) appears
more than once in a \fIPAD\fR \| message, only the last appearance is taken
into
account.
.bp
.PP
1.5.6
The PAD will transmit all \fIdata\fR \| packets with the D bit set
to 0.
.PP
On reception of a \fIdata\fR \| packet with both the Q bit and the D bit
set to 1, the PAD will transmit the corresponding acknowledgement as soon
as
possible.
.PP
If the PAD does not support the D bit procedure, the PAD may reset the
virtual call.
.RT
.sp 2P
.LP
\fB2\fR \fBUser data transfer\fR
.sp 1P
.RT
.PP
2.1
\fIData\fR \| packets will be forwarded by the PAD when a \fIset\fR ,
\fIread\fR , or \fIset and read PAD\fR \| message is received, or under
any of the other data forwarding conditions provided by the PAD (see Recommendation\
X.28,
\(sc\ 4.4).
.sp 9p
.RT
.PP
2.2
The occurrence of a data forwarding condition will not cause the PAD to
transmit empty data packets.
.sp 2P
.LP
\fB3\fR \fBProcedures for the use of PAD messages\fR
.sp 1P
.RT
.sp 1P
.LP
3.1
\fIProcedures for reading, setting, and reading and setting of PAD\fR
\fIparameters\fR \v'3p'
.sp 9p
.RT
.PP
3.1.1
The current values of PAD parameters may be changed and read
by transmitting to the PAD a \fIset\fR , \fIread\fR , or \fIset and read
PAD\fR
message.
.PP
3.1.2
When the PAD receives a \fIset\fR , \fIread\fR \| or \fIset and read PAD\fR \|
message, any data previously received will be delivered to the start\(hystop
mode DTE before taking action on the \fIPAD\fR message. The PAD will also
consider the arrival of such a \fIPAD\fR message as a data forwarding condition.
.PP
3.1.3
The PAD will respond to a valid \fIread\fR \| or \fIset and read PAD\fR \|
message by transmitting a \fIparameter indication PAD\fR message. This
\fIPAD\fR
message will have a parameter field containing a list of parameter references
and current values (after any necessary modification) of the PAD parameters
to which the received \fIPAD\fR message referred.
.PP
3.1.4
The PAD will not return a \fIparameter indication PAD\fR \| message in
response to a valid \fIset PAD\fR \| message received.
.PP
3.1.5
Table 1/X.29 specifies the PAD's response of the PAD to \fIset\fR ,
\fIset and read\fR , and \fIread PAD\fR \| messages.
.PP
3.1.6
If the function of a character is duplicated by the selection of parameter
values by use of the \fIset\fR \| or \fIset and read PAD\fR message, the
PAD
will consider these parameter changes as valid, and will respond as described
in this Recommendation. After these changes are invoked, the PAD will follow
the procedure described in Recommendation\ X.28, \(sc\ 3.3.2.
.sp 2P
.LP
3.2
\fIProcedures for inviting the PAD to clear\fR
.sp 1P
.RT
.PP
3.2.1
The
\fIinvitation to clear PAD\fR \| message
is used to
request that the PAD clears the virtual call, after transmission of all data
previously transmitted to the start\(hystop mode DTE.
.sp 9p
.RT
.PP
\fINote\fR \ \(em\ The \fIclear indication\fR \| packet, which is transmitted
by the PAD after delivery of the last character to the start\(hystop mode
DTE, will have a clearing cause field set to \fIDTE clearing\fR .
.sp 2P
.LP
3.3
\fIInterrupt and \fR \fIdiscard\fR \fIprocedures\fR
.sp 1P
.RT
.PP
3.3.1
If parameter 7 is set to 21, the PAD will transmit an
\fIinterrupt\fR \| packet with all bits of the interrupt user data field
set to\ 0
followed by an \fIindication of break PAD\fR message to indicate that the
PAD, at the request of the start\(hystop mode DTE, is discarding the user
sequences
received. The \fIPAD\fR message will contain an indication in its parameter
field that parameter\ 8 has been set to\ 1 (\fIdiscard output\fR ).
.sp 9p
.RT
.PP
3.3.2
Before resuming data transmission to the PAD, the response to the
\fIindication of break PAD\fR \| message
shall be a \fIset\fR or \fIset and read\fR \fIPAD\fR message, indicating
that parameter\ 8 should be set to\ 0 (\fInormal data\fR \fIdelivery\fR
).
.PP
Prior to sending this PAD message, any in\(hyprogress complete packet sequence
being transmitted to the PAD must be terminated (with a packet that
will be discarded by the PAD) in accordance with Recommendation\ X.25
procedures.
.bp
.ce
\fBH.T. [T1.29]\fR
.ce
TABLE\ 1/X.29
.ce
\fBPAD messages transmitted by the PAD in response to set, set\fR
.ce
\fBand read, and read PAD messages\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(24p) sw(48p) | cw(78p) | cw(78p) , c | c | ^ | ^ .
T{
PAD message received by the PAD
T} Action upon PAD parameters T{
Corresponding \fIparameter indication PAD\fR
message transmitted
to the packet mode \fIDTE\fR
T}
Type Parameter field
_
.T&
cw(24p) | lw(48p) | lw(78p) | lw(78p) , ^ | l | l | l.
Set None T{
Reset all implemented Recommendation\ X.3 parameters to their
initial values corresponding to the initial profile
T} None
T{
List of selected parameters with the desired values
T} T{
Set the selected parameters to the given values:
a)
if no error is encountered
b)
if the PAD fails to modify the values of some parameters
T} T{
Positionner les param\*`etres choisis aux valeurs
indiqu\*'ees:
a)
None
b)
List of these invalid parameters
(see\ Note)
T}
_
.T&
cw(24p) | lw(48p) | lw(78p) | lw(78p) , ^ | l | l | l.
Set and read None T{
Reset all implemented Recommendation\ X.3 parameters to their
initial values corresponding to the initial profile
T} T{
List all implemented Recommendation\ X.3 parameters, and their
initial values
T}
T{
List of selected parameters with the desired values
T} T{
Set the selected parameters to the given values
T} T{
List of these parameters with their new current values
(see Note)
T}
_
.T&
cw(24p) | lw(48p) | lw(78p) | lw(78p) , ^ | l | l | l.
Read None None T{
List all implemented Recommendation\ X.3 parameters with their
current values
T}
List of selected parameters None T{
List of these parameters with their current
values (see Note)
\fINote\fR
\ \(em\ If any of the parameters contain an error, then the error bit is set and the value field is coded as described in Table\ 3/X.29.
.parag
T}
_
.TE
.nr PS 9
.RT
.ad r
\fBTableau 1/X.29 [T1.29], p. 1\fR
.sp 1P
.RT
.ad b
.RT
.PP
3.3.3
If a PAD receives an \fIindication of break PAD\fR \| message which
contains a parameter field as described in \(sc\ 3.3.1 above, it will respond
by
transmitting a \fIset PAD\fR message as described in \(sc\ 3.3.2 above and will
transmit a \fIbreak\fR signal to the start\(hystop mode DTE. If a PAD receives
an
\fIindication\fR \fIof break PAD\fR message which does not contain a parameter
field, it will not respond to the packet mode DTE or PAD but it will transmit
a
\fIbreak\fR signal to the start\(hystop mode DTE.
.PP
3.3.4
When the PAD transmits an \fIinterrupt\fR \| packet after the receipt
from the start\(hystop mode DTE of an \fIinterrupt PAD command\fR signal
or a \fIbreak\fR signal, when parameter\ 7 is set to\ 1, the interrupt
user data field is
coded in bits\ 8 to\ 1 as\ 00000001.
.PP
3.3.5
If the PAD receives an \fIinterrupt\fR \| packet it will confirm it in
accordance with Recommendation\ X.25 procedures. The PAD will not transmit
the contents of the interrupt user data field to the start\(hystop DTE.
The PAD will ignore the values of the interrupt user data field. It is
for further study
whether the coding of this field given in \(sc\ 3.3.4 above causes a different
response.
.PP
3.3.6
If parameter 7 is set to 5, the PAD will transmit an \fIinterrupt\fR \|
packet with all bits of the \fIinterrupt\fR \| packet set to 0, followed
by an
\fIindication of break PAD\fR message. The \fIPAD\fR message will not contain a
parameter field as described in \(sc\ 4.4.7.
.PP
3.3.7
Some PADs may always send the break signal to the start\(hystop mode DTE
upon receipt of an \fIinterrupt\fR \| packet rather than upon receipt of
an
\fIindication of break PAD\fR message.
.bp
.sp 1P
.LP
3.4
\fIProcedure for resets\fR
.sp 9p
.RT
.PP
Virtual calls may be reset according to the procedures defined in Recommendation\
X.25. The effect of the resetting procedure on the value of PAD parameter\
8 is to reset its value to\ 0 (\fInormal data delivery\fR ). The current
values of all other PAD parameters are not affected.
.RT
.sp 2P
.LP
3.5
\fIError handling procedures by the PAD\fR
.sp 1P
.RT
.PP
3.5.1
If the PAD receives a \fIset\fR , \fIread\fR \| or \fIset and read PAD\fR \|
message containing an invalid reference to a PAD parameter, the parameter
field within the \fIparameter indication PAD\fR message transmitted by
the PAD will
contain an indication that this has occurred. The remaining valid references
to PAD parameters are processed by the PAD.
.sp 9p
.RT
.PP
Possible reasons for an invalid access to a PAD parameter
are:
.LP
a)
the parameter reference has not been implemented in the
PAD;
.LP
b)
the parameter value has not been implemented in the PAD or
cannot be altered from the current setting;
.LP
c)
the parameter is a read\(hyonly one (\fIset\fR and \fIset and read\fR
\fIPAD\fR messages only);
.LP
d)
the parameter follows an invalid parameter separator (see
\(sc\ 4.4.5.4 below).
.PP
3.5.2
The PAD will transmit an \fIerror PAD\fR \| message containing the
message code of an invalid \fIPAD\fR \| message received under the following
conditions:
.LP
a)
if the PAD receives an unrecognizable message code;
.LP
b)
if the parameter field following a recognizable message code
is incorrect or incompatible with the message code;
.LP
c)
if the parameter field following a recognizable message code
has an invalid format;
.LP
d)
if the PAD receives an unsolicited \fIparameter indication\fR
\fIPAD\fR \| message;
.LP
e)
if the PAD receives a PAD message that is too
long.
.PP
3.5.3
The PAD will transmit an
\fIerror PAD\fR \| message
if a
\fIPAD\fR \| message containing less than 8\ bits is received.
.PP
3.5.4
If the PAD receives an \fIerror PAD\fR \| message it will not respond
with a \fIPAD\fR \| message of any type. Subsequent action is for further
study.
.sp 1P
.LP
3.6
\fIProcedures for inviting the PAD to reselect the called DTE\fR
.sp 9p
.RT
.PP
The \fIreselection or reselection with \fR \fITOA/NPI PAD\fR message (Type
of address/Numbering Plan Indicator) used by a packet mode DTE to request
that the PAD clear the virtual call, after transmission to the start\(hystop
mode DTE of all the previously transmitted data. Then, the PAD will establish
a call to the reselected DTE.
.PP
\fINote\fR \ \(em\ The TAO/NPI address subscription facility is designated in
Recommendation X.2 for further study.
.PP
When the \fIreselection PAD message\fR \| is received, the PAD will transmit
an \fIerror PAD message\fR \| with an error type \fIunauthorized reselection
PAD\fR
\fImessage\fR (00000110) under the following conditions:
.RT
.LP
a)
the virtual call has been established by the packet\(hymode
DTE;
.LP
b)
the \fIcalled DTE reselection prevention facility\fR \| has been requested
by the start\(hystop mode DTE;
.LP
c)
the \fIreselection PAD message\fR has been already received more than
N times (where N is for further study).
.PP
The format of the \fIreselection PAD\fR \| message is given in \(sc\ 4.4.9
below. The format of the \fIreselection with TOA/NPI\ PAD\fR message is
given in
\(sc\ 4.4.10 below. These messages contain the information needed by the PAD to
establish the new virtual call.
.PP
Upon receipt of the \fIreselection\fR or \fIreselection with TOA/NPI PAD\fR
\| message, the PAD will:
.RT
.LP
\(em
transmit to the start\(hystop mode DTE all previously received
data;
.LP
\(em
clear the virtual call that is established;
.bp
.LP
\(em
after having made the appropriate state changes as described
in Recommendation\ X.28, \(sc\ 3.2.5, establish a virtual call to the
reselected DTE. The \fIcall request packet\fR sent by the PAD, will
contain only the facilities subscribed by the start\(hystop mode
DTE and/or assigned by default. Any other facilities contained
in the \fIreselection PAD message\fR will be ignored. In
particular:
.LP
i)
\fIClosed User Group Signals\fR \ \(em\ Independently by the CUG indicated
in the \fIreselection PAD message\fR , the PAD will use the same CUG of
the original call.
.LP
ii)
\fIReverse Charging\fR \ \(em\ If the start\(hystop mode DTE was not
charged for the original call the reselected call will not be charged to the
start\(hystop mode DTE, independently of the indication in the \fIreselection
PAD\fR \fImessage\fR (i.e.,\ the PAD will use the \fIreverse charging\fR
facility in the
\fIcall request packet\fR ). If the start\(hystop mode DTE was charged
for the original call, the reselected call will be charged to the reselected
DTE if the
\fIreselection PAD message\fR contains the \fIreverse charging\fR facility.
.LP
iii)
\fICharging information\fR :
.LP
\(em
facility assigned for an agreed contractual
period: The information will be sent to the start\(hystop mode DTE at
the clearing of each call (original and reselected), or at the clearing
of the last reselected call. If the later procedure was selected, the PAD
will send
the total charging information, without sending the charge of the individual
calls (original and reselected).
.LP
\(em
facility on a per call basis: The PAD follows
the procedure indicated above, starting from the first
\fIcharging information facility request\fR (by the start\(hystop mode
DTE or packet mode DTE).
.LP
iv)
\fIRPOA selection:\fR \ for further study
.LP
\fINote\fR \ \(em\ The other facilities indicated in Table 4/X.28
with \fINote 2\fR \| are for further study.
.PP
\fINote\fR \ \(em\ This procedure is an optional feature of the PAD. PADs
which do not implement this feature will consider \fIreselection\fR and
\fIreselection with TOA/NPI\ PAD\fR messages as invalid. PADs may implement
this
feature either by accepting (1)\ \fIreselection PAD\fR messages or (2)\
\fIreselection\fR and \fIreselection with\fR \fITOA/NPI\ PAD\fR messages.
The sending of
\fIreselection or reselection with\fR \fITOA/NPI\ PAD\fR messages by a
PAD is for
further study.
.sp 2P
.LP
\fB4\fR \fBFormats\fR
.sp 1P
.RT
.sp 1P
.LP
4.1
\fIIntroduction\fR
.sp 9p
.RT
.PP
Bits of octets are numbered 8 to 1 where bit 1 is the low order
bit and is transmitted first. Octets of the call user data, of user sequences,
of \fIPAD\fR messages and of interrupt user data are consecutively numbered
starting from\ 1 and are transmitted in this order.
.RT
.sp 1P
.LP
4.2
\fICall user data format\fR \|(see Figure 1/X.29)
.sp 9p
.RT
.sp 1P
.LP
4.2.1
\fIProtocol identifier format\fR
.sp 9p
.RT
.PP
The
protocol identifier field
standardized by CCITT
consists of four octets.
.PP
The first octet is coded as follows:
.RT
.LP
bits 8 and 7
=\ 00 for CCITT use
.LP
=\ 01 for national use
.LP
=\ 10 reserved for international user bodies
.LP
=\ 11 for DTE\(hyDTE use.
.PP
When bits 8 and 7 are equal to 00, bits 6 to 1 are equal to\ 000001 for
indicating \fIPAD\fR \| messages relating to the \fIpacket assembly/disassembly\fR
facility for the start\(hystop mode DTE. Other coding of bits\ 6 to\ 1
is reserved for future standardization by the CCITT, subject to the rules
of
Recommendation\ X.244. All bits of octets\ 2, 3 and 4 are set to\ 0. These
octets are reserved as a future mechanism for providing the called PAD
or packet mode DTE with additional information pertinent to the calling
party.
.bp
.LP
.rs
.sp 26P
.ad r
\fBFigure\ 1/X.29, (MC), p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
4.2.2
\fICall data format\fR
.sp 9p
.RT
.PP
Octets of the
call data field
will contain the user
characters received by the PAD from the start\(hystop mode DTE during the call
establishment phase. The coding of these octets is similar to that of user
sequences (see \(sc\ 4.3 below). The call data field is limited to 12\
octets (see Figure\ 1/X.29).
.RT
.sp 2P
.LP
4.3
\fIUser sequence format\fR
.sp 1P
.RT
.PP
4.3.1
The order of bit transmission from the PAD is the same as the order that
bits are received from the start\(hystop mode DTE. The order of bit
transmission to the start\(hystop mode DTE is the same as the order that
bits are received.
.sp 9p
.RT
.PP
4.3.2
No maximum is specified for the length of a user sequence.
.sp 2P
.LP
4.4
\fIControl message format\fR
.sp 1P
.RT
.PP
4.4.1
Bits 8, 7, 6, 5 of octet 1 of a user data field of complete
packet sequences with Q\ =\ 1 are defined as the \fIcontrol identifier
field\fR , used to identify the facility, such as PAD, to be controlled.
The
control
identifier field
coding for \fIPAD\fR messages to control a PAD for a
start\(hystop mode DTE is\ 0000. Other codings of the control identifier
field are reserved for future standardization.
.sp 9p
.RT
.PP
\fINote\fR \ \(em\ The possibility of extending the control identifier
field is for further study.
.PP
4.4.2
When the control identifier field (see \(sc\ 4.4.1 above) is set
to\ 0000, bits\ 4, 3, 2, 1 of octet\ 1 are defined as the
message code
field
. The \fImessage code\fR field is used to identify specific types of \fIPAD\fR
messages, as given in Table\ 2/X.29.
.bp
.ce
\fBH.T. [T2.29]\fR
.ce
TABLE\ 2/X.29
.ce
\fBType and coding of octet 1 of PAD messages\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(144p) | cw(30p) sw(15p) sw(15p) sw(15p) sw(9p) , ^ | c | c | c | c | c.
Type Message code
Bits 4 3 2 1
_
.T&
lw(144p) | lw(30p) | cw(15p) | cw(15p) | cw(15p) | cw(9p) .
Set PAD message 0 0 1 0
.T&
lw(144p) | lw(30p) | cw(15p) | cw(15p) | cw(15p) | cw(9p) .
Read PAD message 0 1 0 0
.T&
lw(144p) | lw(30p) | cw(15p) | cw(15p) | cw(15p) | cw(9p) .
Set and read PAD message 0 1 1 0
.T&
lw(144p) | lw(30p) | cw(15p) | cw(15p) | cw(15p) | cw(9p) .
T{
Parameter indication PAD message
T} 0 0 0 0
.T&
lw(144p) | lw(30p) | cw(15p) | cw(15p) | cw(15p) | cw(9p) .
T{
Invitation to clear PAD message
T} 0 0 0 1
.T&
lw(144p) | lw(30p) | cw(15p) | cw(15p) | cw(15p) | cw(9p) .
T{
Indication of break PAD message
T} 0 0 1 1
.T&
lw(144p) | lw(30p) | cw(15p) | cw(15p) | cw(15p) | cw(9p) .
Reselection PAD message 0 1 1 1
.T&
lw(144p) | lw(30p) | cw(15p) | cw(15p) | cw(15p) | cw(9p) .
Error PAD message 0 1 0 1
.T&
lw(144p) | lw(30p) | cw(15p) | cw(15p) | cw(15p) | cw(9p) .
Reselection with TOA/NPI 1 0 0 T{
0
\fINote\fR
\ \(em\ The possibility of extending the message code field is for further
study.
.parag
T}
_
.TE
.nr PS 9
.RT
.ad r
\fBTable 2/X.29 [T2.29], p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.sp 1
.PP
4.4.3
All \fIPAD\fR \| messages consist of a control identifier field
(bits 8, 7, 6, 5 of octet\ 1 equal to\ 0000) and a message code field (bits\
4, 3, 2, 1 of octet\ 1).
.PP
\fISet\fR , \fIread\fR , \fIset and read\fR and \fIparameter indication
PAD\fR \|
messages consist of octet\ 1 which may be followed by one or more parameter
fields. Each parameter field consists of a
parameter reference octet
and a
parameter value octet
.
.PP
The parameter value octets of the \fIread PAD\fR \| message contain the
value\ 0.
.PP
The \fIerror PAD\fR \| message consists of octet 1 and one or two octets
giving the reason for the error.
.PP
The \fIindication of break PAD\fR \| message consists of octet 1 which
may be followed by a parameter field.
.PP
The \fIinvitation to clear PAD\fR \| message consists of octet 1
only.
.RT
.PP
4.4.4
The maximum length of PAD message is network dependent, but
will be at least 128\ octets.
.sp 1P
.LP
4.4.5
\fIParameter field\fR \fIfor \fR \fIset, read, set and read, and\fR \fIparameter
indication PAD messages\fR \| (see Figure\ 2/X.29)
.sp 9p
.RT
.PP
A parameter field contained in one of these \fIPAD\fR messages
consists of a
reference field
and a
value field
. A parameter
field is two octets in length, except when the extension mechanism is used
(see \(sc\ 4.4.5.1 below).
.RT
.PP
4.4.5.1
A reference field consists of a parameter reference, identified
as a decimal number in Recommendation\ X.3, and is binary coded in bits\ 7
to\ 1, where bit\ 1 is the low order bit. Reference fields need not be
ordered by increasing parameter reference numbers.
.PP
The code 1111111 (decimal 127) in bits 7 to 1 of the reference
field will be used for the extension of this field. Such coding will indicate
that there is another octet following. The following octet is coded with
the
parameter reference of Recommendation\ X.3 minus 127.
.PP
4.4.5.2
In \fIPAD\fR \| messages received by the PAD, bit\ 8 of each octet
will be ignored. In \fIparameter indication PAD\fR messages, bit\ 8 of each
reference field set to\ 1 will indicate an invalid access to the referred
parameter as described in \(sc\ 3.5 above.
.bp
.LP
.rs
.sp 32P
.ad r
\fBFigure 2/X.29, (M), p. 4\fR
.sp 1P
.RT
.ad b
.RT
.PP
4.4.5.3
A parameter value field consists of a value of the parameter
reference, identified as a decimal number in Recommendation\ X.3, and is
binary coded in bits\ 8 to\ 1, where bit\ 1 is the low order bit. Value
fields in \fIread PAD\fR messages are coded as all binary\ 0s. In \fIset\fR
and \fIset and read\fR
\fIPAD\fR messages, they will indicate the requested values of parameters. In
\fIparameter indication PAD\fR messages, they will indicate the current
values of PAD parameters, after modification if any. If bit\ 8 (error bit)
is set to\ 1 in the preceding octet (i.e.\ the parameter reference field),
the parameter value field will indicate the reason for the error, as given
in Table\ 3/X.29.
.PP
4.4.5.4
Parameters not standardized by CCITT may be supported. The
parameter separator is used in PAD messages to indicate the separation
between parameters specified in Recommendation\ X.3 and any others implemented
nationally or locally.
.PP
The parameter separator consists of a parameter field which
contains a reference field set to 00000000 and a value field set to
00000000.
.PP
When present, the parameter separator and the national or local
parameter fields must be placed after any CCITT standardized parameter
fields in PAD messages.
.PP
\fINote\fR \ \(em\ It is recommended that only the parameters defined in
Recommendation\ X.3 are used when communicating with a PAD in a different
country or network.
.bp
.RT
.ce
\fBH.T. [T3.29]\fR
.ce
TABLE\ 3/X.29
.ce
\fBCoding of parameter value field in case of error\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(120p) | cw(12p) sw(6p) sw(12p) sw(6p) sw(12p) sw(6p) sw(12p) sw(6p) sw(24p) , ^ | c s s s s s s s
^ | c | c | c | c | c | c | c | c | c.
Error type Parameter value field code
Bits 8 7 6 5 4 3 2 1 Decimal
_
.T&
lw(120p) | cw(12p) | cw(6p) | cw(12p) | cw(6p) | cw(12p) | cw(6p) | cw(12p) | cw(6p) | cw(24p) .
No additional information 0 0 0 0 0 0 0 0 0
.T&
lw(120p) | cw(12p) | cw(6p) | cw(12p) | cw(6p) | cw(12p) | cw(6p) | cw(12p) | cw(6p) | cw(24p) .
T{
The parameter reference does not exist or has not been implemented in
the PAD
T} 0 0 0 0 0 0 0 1 1
.T&
lw(120p) | cw(12p) | cw(6p) | cw(12p) | cw(6p) | cw(12p) | cw(6p) | cw(12p) | cw(6p) | cw(24p) .
T{
The parameter value is invalid or has not been implemented in the PAD
T} 0 0 0 0 0 0 1 0 2
.T&
lw(120p) | cw(12p) | cw(6p) | cw(12p) | cw(6p) | cw(12p) | cw(6p) | cw(12p) | cw(6p) | cw(24p) .
T{
The parameter value cannot be altered from the current setting
T} 0 0 0 0 0 0 1 1 3
.T&
lw(120p) | cw(12p) | cw(6p) | cw(12p) | cw(6p) | cw(12p) | cw(6p) | cw(12p) | cw(6p) | cw(24p) .
The parameter is read\(hyonly 0 0 0 0 0 1 0 0 4
.T&
lw(120p) | cw(12p) | cw(6p) | cw(12p) | cw(6p) | cw(12p) | cw(6p) | cw(12p) | cw(6p) | cw(24p) .
T{
The parameter follows an invalid parameter separator
T} 0 0 0 0 0 1 0 1 T{
5
\fINote\fR
\ \(em\ The value 0 is mandatory. Other values are optional.
.parag
T}
_
.TE
.nr PS 9
.RT
.ad r
\fBTable 3/X.29 [T3.29], p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
4.4.6
\fIFormat of error PAD messages\fR \| (see Figure 3/X.29)\fR
.sp 9p
.RT
.LP
.rs
.sp 13P
.ad r
\fBFigure 3/X.29, (M), p.\fR
.sp 1P
.RT
.ad b
.RT
.PP
4.4.6.1
Octet 2 of the \fIerror PAD\fR \| message will be coded as shown in
Table\ 4/X.29.
.PP
4.4.6.2
In cases b, c, d, e and f in Table 4/X.29, octet 3 of
an \fIerror PAD\fR \| message will contain the message code of the received
\fIPAD\fR
message.
.sp 1P
.LP
4.4.7
\fIParameter field for indication of break PAD messages\fR \|
(see Figure\ 4/X.29)
.sp 9p
.RT
.PP
This \fIPAD\fR \| message may either not contain a parameter field, or
contain a parameter field consisting of 2\ octets (i.e.\ one reference
field and one value field) coded as follows: the reference field will be
coded 00001000 (indicating parameter\ 8) and the value field will be coded
00000001 (indicating decimal\ 1).
.bp
.RT
.LP
.rs
.sp 27P
.ad r
\fBCuadro 4/X.29 [T4.29] p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.rs
.sp 10P
.ad r
\fBFigure 4/X.29, (M), p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
4.4.8
\fIParameter field for invitation to clear PAD message\fR \|
(see Figure\ 5/X.29)
.sp 9p
.RT
.LP
.rs
.sp 6P
.ad r
\fBFigure 5/X.29, (M), p.\fR
.sp 1P
.RT
.ad b
.RT
.PP
This \fIPAD\fR \| message will not contain a parameter field.
.bp
.sp 1P
.LP
4.4.9
\fI\fIReselection PAD message format\fR
.sp 9p
.RT
.PP
The format of this message is given in Figure\ 6/X.29.
.RT
.LP
.rs
.sp 19P
.ad r
\fBFigure 6/X.29, (MC), p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
4.4.9.1
\fIReselected DTE address length field\fR
.sp 9p
.RT
.PP
Bits 4, 3, 2 and 1 of the reselected DTE address length field
indicate the length of the reselected DTE address in semi\(hyoctets. The
address length is binary coded and bit\ 1 is the low order bit of the
indicator.
.RT
.sp 1P
.LP
4.4.9.2
\fIAddress field\fR
.sp 9p
.RT
.PP
Octet 3 and the following octets consist of the reselected DTE
address. Each digit of the address is coded in a semi\(hyoctet in binary coded
decimal with bit\ 5 or\ 1 being the low order bit of the digit.
.PP
Starting from the high order digit, the address is coded in octet\ 3
and consecutive octets with two digits per octet. In each octet, the higher
order digit is coded in bits\ 8, 7, 6 and\ 5.
.PP
The address field shall be rounded up to an integral number of octets by
inserting zeros in bits\ 4, 3, 2 and\ 1 of the last octet of the field
when
necessary.
.PP
The reselected DTE address field should contain the
\fIinternational data number\fR (DNIC\ +\ Network terminal
number).
.RT
.sp 1P
.LP
4.4.9.3
\fIFacility length field\fR
.sp 9p
.RT
.PP
The octet following the reselected DTE address field indicates the length
of the facility field, in octets. The facility length indicator is
binary coded and bit\ 1 is the low order bit of the indicator.
.RT
.sp 1P
.LP
4.4.9.4
\fIFacility field\fR
.sp 9p
.RT
.PP
The facility field is present only when optional user facilities
are included by the DTE. This field indicates the facilities that must be
included in the facility field of the \fIincoming call\fR packet received
by the
reselected DTE (see \(sc\ 3.6).
.PP
The coding of the facility field is defined in \(sc\ 7 of
Recommendation\ X.25.
.PP
The facility field contains an integral number of octets, the
maximum length of the complete PAD message is restricted, as described in
\(sc\ 4.4.4 above.
.bp
.RT
.sp 1P
.LP
4.4.9.5
\fICall user data field\fR
.sp 9p
.RT
.PP
Following the facility field, the call user data field may be
present and has a maximum length of 12 octets.
.PP
Call user data when present in the call user data field of the
\fIreselection PAD\fR message is included in the call user data field of the
\fIincoming call\fR packet received by the reselected DTE.
.RT
.sp 1P
.LP
4.4.10
\fIReselection with TOA/NPI PAD message format\fR
.sp 9p
.RT
.PP
The format of this message is given in Figure 7/X.29.
.PP
\fINote\fR \ \(em\ The TOA/NPI address subscription facility is designated in
Recommendation\ X.2 for further study.
.RT
.LP
.rs
.sp 16P
.ad r
\fBFigure 7/X.29, (N), p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
4.4.10.1
\fIReselected DTE address length field\fR
.sp 9p
.RT
.PP
Octet 2 indicates the length of the reselected DTE address in
semi\(hyoctets. The address length is binary coded and bit\ 1 is the low
order bit of the indicator. The maximum value of the reselected DTE address
length field is 17.
.RT
.sp 1P
.LP
4.4.10.2
\fIReselected DTE address field\fR
.sp 9p
.RT
.PP
Octet 3 will consist of the TOA/NPI indication as described in
Recommendation X.25. The following octets consist of the reselected DTE
address. Each digit of the address is coded in a semi\(hyoctet in binary coded
decimal with bit\ 5 or\ 1 being the low order bit of the digit. Starting
from the high order digit, the address digits are coded in consecutive
semi\(hyoctets. In each octet, the higher order digit is coded in bits\
8, 7, 6 and\ 5.
.RT
.sp 1P
.LP
4.4.10.3
\fIFacility length field\fR
.sp 9p
.RT
.PP
The octet following the address field indicates the length of the facility
field, in octets. The facility length indicator is binary coded and
bit\ 1 is the low order bit of the indicator.
.RT
.sp 1P
.LP
4.4.10.4
\fIFacility field\fR
.sp 9p
.RT
.PP
(See \(sc 4.4.9.4.)
.RT
.sp 1P
.LP
4.4.10.5
\fICall user data field\fR
.sp 9p
.RT
.PP
(See \(sc 4.4.9.5.)
.bp
.RT
.ce 1000
ANNEX\ A
.ce 0
.ce 1000
(to Recommendation X.29)
.sp 9p
.RT
.ce 0
.ce 1000
\fBCharacteristics of \fR \fBvirtual calls\fR \fBand Recommendation X.25\fR
.sp 1P
.RT
.ce 0
.ce 1000
\fBas related to the PAD representation of\fR
.ce 0
.ce 1000
\fBa start\(hystop mode DTE to a packet mode DTE\fR
.ce 0
.LP
A.1
\fIGeneral interface characteristics\fR
.sp 1P
.RT
.PP
A.1.1
The mechanical, electrical, functional and procedural
characteristics to activate, maintain and deactivate the physical access
path between the DTE and the DCE will be in accordance with the physical
level procedures of Recommendation\ X.25.
.sp 9p
.RT
.PP
A.1.2
The link access procedure for data interchange across the link
between the DTE and DCE will be in accordance with the link level procedures
of Recommendation\ X.25.
.PP
A.1.3
The packet format and control procedures for the exchange of
packets containing control information and user data between the DTE and the
DCE will be in accordance with the packet level procedures of
Recommendation\ X.25.
.sp 2P
.LP
A.2
\fIInterface procedures for virtual call control\fR
.sp 1P
.RT
.PP
A.2.1
Incoming calls are indicated to the packet mode DTE as
specified in Recommendation\ X.25. Call requests are indicated by the packet
mode DTE as specified in Recommendation\ X.25. Any use of optional user
facilities are indicated in accordance with \(sc\(sc\ 6 and\ 7 of
Recommendation\ X.25.
.sp 9p
.RT
.PP
A.2.2
The default throughput classes used by the PAD are determined by the data
rates of the start\(hystop mode DTE (where exact correspondence is not
obtained, the next higher throughput class is used).
.PP
A.2.3
The PAD and the packet mode DTE will use the clearing procedures specified
in \(sc\(sc\ 4.1.7, 4.1.8 and\ 4.1.9 of Recommendation\ X.25.
.sp 2P
.LP
A.3
\fIInterface procedures for data transfer\fR
.sp 1P
.RT
.PP
A.3.1
Data transfer on a virtual call can only take place in the
\fIdata transfer\fR \| state and when flow control permits (see \(sc\ 4.4 of
Recommendation\ X.25). The same is true for the transfer of \fIinterrupt\fR
packets (see \(sc\ 4.3 of Recommendation\ X.25).
.sp 9p
.RT
.PP
A.3.2
\fIInterrupt\fR \| packets transmitted by the packet mode DTE will be
confirmed by the PAD following the procedures in Recommendation\ X.25.
.PP
A.3.3
The reset procedure may be used by the packet mode DTE or the PAD to re\(hyinitialize
the virtual call and will conform to the procedures described in \(sc\
4.4.3 of Recommendation\ X.25.
.PP
A.3.4
A reset of the virtual call originated by the packet mode DTE or due to
network congestion may be indicated by the PAD to the start\(hystop mode
DTE.
.PP
A.3.5
A reset procedure initiated by the PAD may be due either to:
.LP
a)
the receipt at the PAD of a request to reset from the
non\(hypacket mode DTE. The resetting cause contained in the
\fIreset indication\fR packet will be \fIDTE reset\fR ; or
.LP
b)
a PAD or network failure.
.PP
A.3.6
For calls received by the PAD with bit 7 of octet 1 in the
\fIincoming call\fR \| packet set to\ 0, the PAD will set bit\ 7 of octet\
1 in the
\fIcall accepted\fR packet to\ 0 and will set the D\ bit in transmitted
\fIdata\fR
packets to\ 0.
.PP
Pending further study, and in the absence of bilateral agreement between
Administrations (used in conjunction with the D\ bit modification
facility), the following applies:
.PP
If the \fIincoming call\fR \| packet received by the PAD has bit 7 of
octet\ 1 set to\ 1, the PAD may set bit\ 7 of octet\ 1 of the \fIcall accepted\fR
packet to\ 1.
.PP
Calls originated by the PAD will set bit\ 7 of octet\ 1 in \fIcall\fR
\fIrequest\fR \| packets to\ 0. The called DTE can indicate if it requires
the support of the D\ bit procedure by setting bit\ 7 of octet\ 1 of \fIcall
accepted\fR packets to\ 1.
.PP
PAD procedures associated with the Delivery Confirmation (D) bit in
data packets (see \(sc\ 4.3.3 of Recommendation\ X.25) are described in
\(sc\(sc\ 1.4.4
and\ 1.5.6.
.bp
.RT
.sp 2P
.LP
A.4\fR \fIVirtual call characteristics\fR
.sp 1P
.RT
.sp 1P
.LP
A.4.1
\fIResetting\fR \v'3p'
.sp 9p
.RT
.PP
A.4.1.1
There may be a loss of data characters in any case of reset, as
stated in Recommendation\ X.25. Characters generated by either of the DTEs
prior to the \fIreset\fR indication or confirmation will not be delivered
to the other
DTE after the \fIreset\fR indication or confirmation.
.sp 2P
.LP
A.4.2
\fIInterrupt transfer\fR
.sp 1P
.RT
.PP
A.4.2.1
An \fIinterrupt\fR \| packet is always delivered at or before the
point in the data packet stream at which it was generated.
.sp 9p
.RT
.sp 1P
.LP
A.4.3
\fICall clearing\fR
.sp 9p
.RT
.PP
\fIData\fR \| packets transmitted immediately before a \fIclear request\fR
\| packet is sent, may be overtaken within the network by the \fIclear
request\fR
packet and subsequently be destroyed, as described in \(sc\ 4.5 of
Recommendation\ X.25.
.RT
.sp 2P
.LP
\fBRecommendation X.30\fR
.FS
This Recommendation is also included in the Recommendations of the I Series
under the number I.461.
.FE
.RT
.sp 2P
.ce 1000
\fBSUPPORT\ OF\ X.21,\ X.21\|\fIbis\fR \fB\ AND\ X.20\|\fR \fIbis\fR \fB\
BASED\ DATA\ TERMINAL\fR \|
\fBEQUIPMENTS\ (DTEs)\fR
.EF '% Fascicle\ VIII.2\ \(em\ Rec.\ X.30''
.OF '''Fascicle\ VIII.2\ \(em\ Rec.\ X.30 %'
.ce 0
.sp 1P
.ce 1000
\fBBY\ AN\ INTEGRATED\ SERVICES\ DIGITAL\ NETWORK\ (ISDN)\fR
.ce 0
.sp 1P
.ce 1000
\fI(Malaga\(hyTorremolinos, 1984; 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 the Integrated Services Digital Network (ISDN)
will offer the universal interfaces to connect subscriber terminals according
to the reference configurations described in Recommendation\ I.411;
.PP
(b)
that during the evolution of ISDN, however, there will exist for a considerable
period DTEs conforming to Recommendations\ X.21,
X.21\|\fIbis\fR and X.20\|\fIbis\fR which have to be connected to the ISDN;
.PP
(c)
that the
D\(hychannel signalling protocol
is
described in Recommendations\ I.430, IQ.920, Q.921,
Q.930 and Q.931;
.PP
(d)
that the X.21\|\fIbis\fR DTEs are an evolution of V series
DTEs, which also provide interworking capability with X.21 DTEs over PDN
services, and which use the network provided signal element timing and may
have specific call control features to comply with the X.21 calling
protocol
.FS
See Recommendation V.110.
.FE
;
.PP
(e)
that the X.20\|\fIbis\fR based DTEs are an evolution of
V\ series DTEs, which are operating in the asynchronous mode and which
may have call control features to comply with the X.20 calling protocol,
.sp 1P
.LP
\fIunanimously declares\fR
.sp 9p
.RT
.PP
(1)
that the scope of this Recommendation covers the
connection of X.21 and X.21\|\fIbis\fR based terminals of user classes
of service\ 3 to\ 7 and\ 19 to the ISDN operating in accordance with circuit\(hyswitched
or leased circuit services;
.PP
(2)
that the scope of this Recommendation also covers the
connection of X.20\|\fIbis\fR based terminals of user classes of service\
1 and\ 2 and of asynchronous data rates of 600, 1200, 2400, 4800 and\ 9600
to the ISDN
operating in accordance with circuit\(hyswitched or leased circuit services;
.bp
.PP
(3)
that the reference configurations of \(sc\ 1 of this
Recommendation shall apply;
.PP
(4)
that the terminal adaptor (TA) functions to support
X.21, X.21\|\fIbis\fR and/or X.20\|\fIbis\fR based DTEs including:
.LP
\(em
rate adaption functions,
.LP
\(em
call establishment functions,
.LP
\(em
mapping functions,
.LP
\(em
ready for data alignment,
.LP
shall be performed as outlined in \(sc\ 2;
.PP
(5)
that the scope of this Recommendation covers the rate
adaption requirements which are caused by the connection of existing terminals
to the ISDN user/network interface, but does not cover the requirements
on bit rate conversion caused by the inter\(hyoperation of terminals with
different bit rates (ISDN\(hyCSPDN interworking).
.sp 1P
.ce 1000
CONTENTS
.ce 0
.sp 1P
.sp 2P
.LP
1
\fIReference configurations\fR
.sp 1P
.RT
.LP
1.1
Customer access configuration
.LP
1.2
Network configuration
.LP
1.3
Interworking situation
.sp 1P
.LP
2
\fITerminal adaption functions\fR
.sp 9p
.RT
.LP
2.1
Terminal adaption functions for DTEs conforming to X.1 user classes of
service 3 to 6
.LP
2.2
Terminal adaption functions for DTEs conforming to X.1 user class of service 7
.LP
2.3
Terminal adaption functions for DTEs conforming to X.1 user class of
service 19
.LP
2.4
Terminal adaption functions for DTEs conforming to X.1 user class of
service 1 and 2
.sp 1P
.LP
3
\fITest loops\fR
.sp 9p
.RT
.sp 1P
.LP
\fIAnnex\ A\fR \(em
SDL diagrams
.sp 9p
.RT
.sp 1P
.LP
\fIAppendix\ I\fR \(em
Universal terminal adaptor
.sp 9p
.RT
.sp 1P
.LP
\fIAppendix\ II\fR \(em
Inslot identification of intermediate bit rate
.sp 9p
.RT
.sp 2P
.LP
\fB1\fR \fBReference configurations\fR
.sp 1P
.RT
.PP
Figures 1\(hy1/X.30 and 1\(hy2/X.30 show examples of possible
configurations and are included simply as an aid to \(sc\ 2 describing the
TA functions.
.RT
.sp 1P
.LP
1.1
\fICustomer access configuration\fR
.sp 9p
.RT
.PP
For the connection of X.21, X.21\|\fIbis\fR \| or X.20\|\fIbis\fR \| based
DTEs to the ISDN, Figure\ 1\(hy1/X.30 shows a possible reference
configuration.
.RT
.sp 1P
.LP
1.2
\fINetwork configuration\fR
.sp 9p
.RT
.PP
The specification of terminal adaption functions takes account of the network
configuration and the end\(hyto\(hyend connection types shown in
Figure\ 1\(hy2/X.30 in which the associated terminal equipment TE1 and
TE2 may be involved.
.PP
The TA functions for this scenario are described in \(sc\ 2.
.bp
.RT
.LP
.rs
.sp 22P
.ad r
\fBFigure 1\(hy1/X.30, (N), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.rs
.sp 25P
.ad r
\fBFigure 1\(hy2/X.30, (N), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.PP
The terminals TE1 and TE2 are physically and logically connected to the
ISDN where the call is handled.
.PP
The TA performs the necessary rate adaption, the signalling conversion
from the X.21 signalling to the Q.931 signalling and vice\(hyversa (X.21
mapping) and ready for data alignment. Interworking with dedicated networks,
e.g.\ a
CSPDN, will be provided on the basis of trunk lines interconnection by using
Interworking Functions (IWF).
.PP
The following principles shall apply:
.RT
.LP
i)
The non\(hyvoice services within the ISDN should basically not diverge
from what is being developed in X\ Series Recommendations. This refers
to the various aspects concerning quality of service, user facilities,
call
progress signals (see the X\ Series Recommendations, e.g.,\ X.2 and\ X.96).
However, existing features would be enhanced and additional features would
also be developed if account were taken of the new ISDN customer capabilities
(e.g.,\ multi\(hyterminal arrangement, user rate at 64\ kbit/s simultaneous
multi\(hymedia access as well as the possible solution of compatibility
checking).
.LP
ii)
Integration of X.21 based services into the ISDN is
applicable to user classes of services\ 3 to\ 7, and\ 19. Integration of
X.20\|\fIbis\fR based services into the ISDN is applicable to user classes of
service\ 1 and\ 2.
.LP
iii)
Terminals TE1 and TE2 connected to an ISDN shall use
the ISDN numbering scheme (see Recommendation\ E.164).
.sp 1P
.LP
1.3
\fIInterworking situation\fR
.sp 9p
.RT
.PP
Bearing in mind that this Recommendation defines the functions
performed by the X.21 terminal adaptors (TA\ X.21) and X.21\|\fIbis\fR terminal
adaptors (TA X.20\|\fIbis\fR ) and X.20\|\fIbis\fR terminal adaptors (TA\
X.20\|\fIbis\fR ), the following cases of interworking between these terminal
adaptors and DTEs
connected to CSPDN and PSTN may appear:
.RT
.LP
a)
For user classes of service 3 to 7:
.LP
(1)\ TA X.21\ \(hy\|\(hy\|\(hy\ TA X.21
.LP
(2)\ TA X.21\ \(hy\|\(hy\|\(hy\ TA X.21\|\fIbis\fR
.LP
(3)\ TA X.21\ \fIbis\fR \ \(hy\|\(hy\|\(hy\ TA X.21\|\fIbis\fR
.LP
(4)\ TA X.21\ \(hy\|\(hy\|\(hy\ DTE X.21
.LP
(5)\ TA X.21\ \(hy\|\(hy\|\(hy\ DTE X.21\|\fIbis\fR
.LP
(6)\ TA X.21\ \(hy\|\(hy\|\(hy\ V series DTE
.LP
(7)\ TA X.21\ \fIbis\fR \ \(hy\|\(hy\|\(hy\ DTE X.21
.LP
(8)\ TA X.21\ \fIbis\fR \ \(hy\|\(hy\|\(hy\ DTE X.21\|\fIbis\fR
.LP
(9)\ TA X.21\ \fIbis\fR \ \(hy\|\(hy\|\(hy\ V series DTE
.LP
b)
For user class of service 19:
.LP
(10)\ TA X.21\ \(hy\|\(hy\|\(hy\ TA X.21
.LP
(11)\ TA X.21\ \(hy\|\(hy\|\(hy\ TA X.21\|\fIbis\fR
.LP
(12)\ TA X.21\ \fIbis\fR \ \(hy\|\(hy\|\(hy\ TA X.21\|\fIbis\fR
.LP
(13)\ TA X.21\ \(hy\|\(hy\|\(hy\ TE1 (S/T reference point)
.LP
(14)\ TA X.21\ \fIbis\fR \ \(hy\|\(hy\|\(hy\ TE1 (S/T reference
point)
.LP
c)
For user classes of service 1 and 2:
.LP
(15)\ TA X.20\ \fIbis\fR \ \(hy\|\(hy\|\(hy\ TA X.20\|\fIbis\fR
.LP
(16)\ TA X.20\ \fIbis\fR \ \(hy\|\(hy\|\(hy\ DTE X.20\|\fIbis\fR
.LP
(17)\ TA X.20\ \fIbis\fR \ \(hy\|\(hy\|\(hy\ V series DTE
.PP
\fINote\ 1\fR \ \(em\ This Recommendation is intended to cover all
TA\(hyfunctions necessary to allow interworking as listed above. Currently,
this Recommendation covers all TA\(hyfunctions necessary to allow interworking
between DTEs connected to ISDN and to CSPDN with the following exceptions:
.LP
1)
for X.21\|\fIbis\fR \| and X.20\|\fIbis\fR \| only, the call control
procedure with direct call has been explicitly covered, but
other interface arrangements of X.21\|\fIbis\fR and X.20\|\fIbis\fR are not
precluded;
.LP
2)
for X.21\|\fIbis\fR , the half\(hyduplex mode of operation is for
further study.
.bp
.PP
This applies to all the cases listed above, where at least one
X.21\|\fIbis\fR \| or X.20\|\fIbis\fR \| terminal is involved. Alignment
with the
interworking functions may be necessary when the relevant Recommendations
are available.
.PP
\fINote\ 2\fR \ \(em\ Within the interworking cases 1\(hy17 mentioned above,
the
functions provided by TA\ X.21\|\fIbis\fR , TA\ X.20\|\fIbis\fR and the
functions provided by TA\ V.110 should be compatible.
.RT
.sp 2P
.LP
\fB2\fR \fBTerminal adaption functions\fR
.sp 1P
.RT
.PP
The terminal adaption functions to support X.21, X.21\|\fIbis\fR \| and/or
X.20\|\fIbis\fR \| based DTEs can be subdivided into three areas, namely:
.RT
.LP
\(em
rate adaption functions;
.LP
\(em
X.21/Q.931 mapping functions for call control;
.LP
\(em
ready for data alignment.
.PP
Some Administrations may provide separate TAs either for each
Recommendation\ X.1 user class of service or for a group of user classes of
service. Other Administrations may provide a universal TA for all user
classes of service\ 3 to\ 7 or\ 19 or\ 1 and\ 2. Within the Recommendation only
such functions are described which refer to single rate TAs. Additional
functions necessary for universal TAs (e.g.\ user rate identification) are
outlined in Appendix\ I.
.LP
2.1
\fITerminal adaption functions for DTEs conforming to X.1\fR
\fIuser\fR \fIclasses of service\fR \fI3 to\ 6\fR
.sp 1P
.RT
.sp 2P
.LP
2.1.1
\fIRate adaption\fR \fIfunctions\fR
.sp 1P
.RT
.sp 1P
.LP
2.1.1.1
\fIGeneral approach\fR
.sp 9p
.RT
.PP
The rate adaption functions within the TA are shown in
Figure\ 2\(hy1/X.30. The function RA1 adapts the X.1 user rate to the next
higher rate expressed by 2\fI\fI
\u\fIk\fR\dtimes 8\ kbit/s (where \fIk\fR \ =\ 0 or\ 1). RA2 performs a
second conversion to 64\ kbit/s.
.RT
.LP
.rs
.sp 11P
.ad r
\fBFigure 2\(hy1/X.30, (M), p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 2P
.LP
2.1.1.2
\fIFirst step of\fR
\fIrate adaption\fR \fI(RA1) of X.1 rates to\fR \fI\fIthe intermediate
rates of 8/16\ kbit/s\fR
.sp 1P
.RT
.sp 1P
.LP
2.1.1.2.1
\fIFrame structure\fR
.sp 9p
.RT
.PP
The conversion of the X.1 rates for user classes 3, 4 and 5 to
8\ kbit/s, and for user class of service\ 6 to 16\ kbit/s, shall be implemented
by means of the 40\ bit frame structure shown in Figure\ 2\(hy2/X.30.
.bp
.RT
.LP
.rs
.sp 20P
.ad r
\fBFigure 2\(hy2/X.30 (comme tableau) [T1.30], p.\fR
.sp 1P
.RT
.ad b
.RT
.PP
Figure 2\(hy2/X.30 shows that, in addition to the basic frame, a two frame
multiframe is employed. In odd frames, octet\ 0 contains all zeros, whilst
in even frames octet\ 0 consists of a one followed by seven E\ bits (see
\(sc\ 2.1.1.2.4). The order of bit transmission of the 40\ bit frame is from
left\(hyto\(hyright and from top\(hyto\(hybottom.
.sp 1P
.LP
2.1.1.2.2
\fIFrame synchronization\fR
.sp 9p
.RT
.PP
The 17 bit frame alignment pattern consists of all 8 bits (set to zero)
of octet\ 0 in odd frames and bit\ 1 (set to\ 1) of the following
consecutive 9\ octets of the 80\ bit long multiframe (see also \(sc\ 2.1.1.4.2).
The first bit of octet\ 0 alternates between one and zero in consecutive
frames and therefore provides a multiframe synchronization bit.
.RT
.sp 1P
.LP
2.1.1.2.3
\fIStatus bits\fR \fISP, SQ, SR\fR
.sp 9p
.RT
.PP
The bits SP, SQ and SR are used to convey channel associated status information.
The mapping of the information on circuit C of the\ X.21 interface to the
S bits and the circuit\ I in the distant interface should be done in such
a way that the SP, SQ and SR bits are associated with the bit groups P,
Q
and\ R. To assure proper and secure operation the mapping scheme has to be
consistent with Recommendations\ X.21 and\ X.24.
.PP
The mechanism for mapping is as follows:
.RT
.LP
\(em
In all cases where X.21 byte timing interchange circuit B is
not provided, the status bits SP, SQ and SR of the bit groups P,
Q and R are evaluated by sampling the C\ lead in the middle of
the 8th\ bit of the respective preceding bit group. On the other
hand, the conditions of the status bits SP, SQ and SR are
adopted by the I\ lead beginning with the transition of the
respective 8th\ bit of a bit group\ R, P and Q to the 1st\ bit of
the consecutive bit group\ P,Q and R on the R\ lead (see
Figure\ 2\(hy3/X.30).
.LP
\(em
In the case where X.21\(hybyte timing interchange circuit B is
provided for character alignment, circuit\ C is sampled
together with the bit\ 8 of the preceding character and
circuit\ I is changing its state at the boundaries between old
and new characters at circuit\ R. This operation is defined
in Recommendation\ X.24.
.bp
.LP
.rs
.sp 47P
.ad r
\fBFigure 2\(hy3/X.30, (MC), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.PP
\fINote\ 1\fR \ \(em\ According to Recommendation X.21 the provision of
the byte timing interchange circuit\ B is not mandatory.
.PP
\fINote\ 2\fR \ \(em\ The status bits may be used to transfer, during the data
transfer phase, information for half\(hyduplex operation between TA\ X.21\|\fIbis\fR
and TA\ X.21 or TA\ X.21\|\fIbis\fR (i.e.\ mapping of the condition of
the C\ lead of the
TA\ X.21, of the 105\ lead of the TA\ X.21\|\fIbis\fR , to the condition
on the 109\ lead of the remote TA\ X.21\|\fIbis\fR , and mapping of the
condition of the 105\ lead of
the TA\ X.21\ \fIbis\fR to the condition of the I\ lead on the remote TA\
X.21).
.PP
\fINote\ 3\fR \ \(em\ For bits SP, SQ, SR and X, a ZERO corresponds to the ON
condition, a ONE to the OFF condition.
.RT
.sp 1P
.LP
2.1.1.2.4
\fIAdditional signalling capacity\fR
(E bits)
.sp 9p
.RT
.PP
The E bits provide the additional signalling capacity for the
conveyance of information relating to the user rate. The coding of these
bits is shown in Table\ 2\(hy1/X.30.
.RT
.LP
.sp 1
.ce
\fBH.T. [T2.30]\fR
.ce
TABLE\ 2\(hy1/X.30
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
lw(66p) | lw(18p) | lw(18p) | lw(18p) | lw(18p) | lw(18p) | lw(18p) | lw(54p) .
.TE
.nr PS 9
.RT
.ad r
\fBTable 2\(hy1/X.30 [T2.30], p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.sp 1
.sp 1P
.LP
2.1.1.2.5
\fIData bits\fR
.sp 9p
.RT
.PP
Data is conveyed in P, Q and R bits, i.e. 24 bits per
frame.
.RT
.sp 1P
.LP
2.1.1.2.6
\fIRepetition strategy\fR
.sp 9p
.RT
.PP
For the adaption of user rates 600, 2400, 4800 bit/s to the
8\ kbit/s intermediate rate and of the 9600\ bit/s user rate to the 16\ kbit/s
intermediate rate, the sequence of even and odd octet\ 0 shall be maintained
as defined in Figure\ 2\(hy4/X.30. In order to achieve both short frame
synchronization as well as short transfer delay times, a user\(hybit\(hyrepetition
method is proposed. Figures 2\(hy4a/X.30 and\ 2.4b/X.30 contain a scheme
for the
adaption of the 600\ bit/s user rate and of the 2400\ bit/s user rate
respectively into the 8\ kbit/s bearer rate. Figures\ 2\(hy4c/X.30 and\
2\(hy4d/X.30
show the adaption of the 4800\ bit/s user rate to the 8\ kbit/s bearer
rate and of the 9600\ bit/s user rate to the 16\ kbit/s bearer rate.
.bp
.PP
In the case of a 600 bit/s user rate, an explicit frame group
synchronization pattern using bit E7 is provided to ensure preservation of
user octet boundaries and associated status bit. The coding for the E7 bit
shall be as follows:
\v'6p'
.RT
.sp 1P
.ce 1000
.\|.\|.\ 1110111011101\ .\|.\|.
.ce 0
.sp 1P
.LP
.sp 1
where the value 0 is marking the last 40 bit frame of each 8\ \(mu\ 40\
bit frame
group which contains three integer user octets.
.LP
.sp 1
.rs
.sp 39P
.ad r
\fBFigures 2.4a\(hyd/X.30 (comme tableau) [T3.30], p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
2.1.1.3
\fISecond step of\fR
\fIrate adaption\fR \fI(RA2)\fR
.sp 9p
.RT
.PP
As rate adaption of a single substream (8/16\ kbit/s) to 64\ kbit/s and
multiplexing of several substreams to 64\ kbit/s have to be compatible
to
enable interworking, a common approach is needed for second step rate adaption
and for subchannel multiplexing. It is described in
Recommendation\ I.460.
.RT
.sp 1P
.LP
2.1.1.4
\fIFraming/reframing method\fR \fIand\fR
\fIuser rate\fR
\fIidentification\fR
.sp 9p
.RT
.PP
For framing/reframing and user rate identification the following
strategies shall be applied.
.RT
.sp 1P
.LP
2.1.1.4.1
\fISearch for frame alignment\fR
.sp 9p
.RT
.PP
The following 17 bit alignment pattern is searched for:
.RT
.LP
0
0
0
0
0
0
0
0
1XXXXXXX
1XXXXXXX
1XXXXXXX
1XXXXXXX
1
X
X
X
X
X
X
X
1XXXXXXX
1XXXXXXX
1XXXXXXX
1XXXXXXX
.PP
No errors shall be tolerated in the defined bit positions
(i.e.\ all bit positions excluding those denoted
by\ \*QX\*U).
.PP
It is assumed that the error rate will be sufficiently low to expect alignment
following the detection of one 80\ bit multiframe.
.PP
In the case of X.1 user class of service 3 (600\ bit/s), a further
search for the frame group synchronization pattern contained in bit position
E7 shall be performed.
.RT
.sp 1P
.LP
2.1.1.4.2
\fIAlignment monitoring/recovery\fR
.sp 9p
.RT
.PP
The monitoring of the alignment shall be a continuous
process. The alignment is assumed to be correct if there is no error in
the 17 bit alignment pattern of the 80\ bit multiframe.
.PP
Loss of alignment is assumed following the detection of N
(provisional value:\ 3) consecutive multiframes each with at least one
alignment bit error.
.PP
Following a loss of alignment the TA shall enter a recovery state,
which
is indicated at the X.21 interface by r\ =\ 1 and i\ =\ ON. In the transmitted
frame, bit\ X, if used for the indication of the frame synchronizion to
the far end, shall be set to OFF.
.PP
If the recovery of alignment is achieved, r and i present again the data
and the status information respectively from the received frames. Bit\
X in the transmitted frames must be in the ON condition.
.PP
If recovery of alignment is not achieved within a fixed period, the TA
shall indicate \*QDCE not ready\*U (state\ 22) by signalling r\ =\ 0, i\
=\ OFF. The
duration of this period is network dependent (as in Recommendation\ X.21,
\(sc\ 2.6.2). In case of a circuit\(hyswitched service this leads to a
clearing of the connection.
.PP
In the case of a X.21\|\fIbis\fR \| TA, the signalling procedure in
Recommendation\ V.110, \(sc\ 4.1.5 should be used at the R\(hyreference
point.
.RT
.sp 1P
.LP
2.1.1.4.3
\fIIdentification of intermediate bit rate\fR
.sp 9p
.RT
.PP
As a basic approach the intermediate bit rate is derived from the X.1 user
rate contained in the Q.931 SETUP message.
.PP
As an alternative solution the intermediate bit rate may optionally be
identified by relying solely on B\(hychannel information
(see Appendix\ II).
.RT
.sp 1P
.LP
2.1.2
\fIX.21/X.21\|bis to Q.931 protocol mapping\fR
.sp 9p
.RT
.PP
The D\(hychannel signalling capabilities of the ISDN\(hycustomer\(hyaccess
as defined in Recommendation\ Q.931 have to include the requirements arising
from the mapping of the X.21 and X.21\|\fIbis\fR interface signalling procedures
to the Q.931 protocol at the S/T reference point.
.PP
The logical representation of these mapping functions is shown in
Figure\ 2\(hy5/X.30.
.bp
.RT
.LP
.rs
.sp 16P
.ad r
\fBFigure 2\(hy5/X.30, (M), p.\fR
.sp 1P
.RT
.ad b
.RT
.PP
The D\(hychannel signalling capabilities provided to X.21 and
X.21\|\fIbis\fR \| based terminals shall comprise the signalling messages
as defined in Recommendation\ Q.931.
.PP
The following description and drawings depict examples of X.21 and
X.21\|\fIbis\fR \| mapping to the ISDN call control procedures. It is recognized
that other possibilities and user options exist but this section is intended
to
provide general guidelines on\ X.21 and\ X.21\|\fIbis\fR support. Only
the normal call establishment and clearing procedures are shown.
.PP
\fINote\ 1\fR \ \(em\ Annex A contains an SDL description of the mapping
of the procedures at the R\(hyreference point to procedures at the S/T
reference point
and vice versa. However, the TA internal processes and states contained
in the SDL diagrams are understood not to be binding for implementation.
.PP
\fINote\ 2\fR \ \(em\ Manual direct or address calls and manual disconnection
from the TA should also be possible through the mapping of standard DTE/TA
interface procedures with manual operations at the TA. In addition, automatic
address calls may also be possible by the DTE employing a V.25 interface
between the DTE and TA (see Recommendation\ V.110).
.RT
.sp 1P
.LP
2.1.2.1
\fIQ.931/X.21\fR
\fImapping\fR \| (see Figures 2\(hy6/X.30 and
2\(hy7/X.30)
.sp 9p
.RT
.PP
The following sections are titled with the names of the Q.931
signalling messages at the S/T reference point.
.RT
.sp 1P
.LP
2.1.2.1.1
\fISETUP (from TA)\fR
.sp 9p
.RT
.PP
In ready state (state\ 1) both DTE and TA transmit r\ =\ 1, i\ =\ OFF
via the X.21 interface.
.PP
When the calling DTE indicates a call request (state 2, r\ =\ 0, i\ =\
ON) at the X.21 interface, the TA transmits a proceed to select signal
to the DTE (state\ 3, +,\ OFF). The DTE begins to send selection signals
to the TA
(state\ 4, r\ =\ +, i\ =\ OFF).
.PP
When an end of selection signal (r\ =\ +, i\ =\ ON) is received at the
X.21 interface, the TA transmits a SETUP message via the D\(hychannel at
the S/T reference point.
.PP
The Bearer capability information element included in the SETUP
message shall be coded with:
.RT
.LP
\(em
information transfer capability set to either:
.LP
a)
\*Qunrestricted digital information\*U, or
.LP
b)
\*Qrestricted digital information\*U;
.LP
\(em
transfer mode set to \*Qcircuit mode\*U;
.LP
\(em
information transfer rate set to \*Q64 kbit/s\*U.
.bp
.PP
\fINote\fR \ \(em\ Bearer capability information element octets 4a and 4b
shall not be included.
.PP
The user may also specify the layer\ 1 (e.g. rate adaption), layer 2
(e.g.\ LAPB) and layer\ 3 (e.g.\ X.25) information transfer protocols in
the Low layer compatibility information element in the SETUP message. (See
Q.931, annex entitled, \*QLow layer information coding principles\*U).
.PP
The Called party address information element shall be encoded en\(hybloc
i.e.,\ with the complete address of the called party as received from the
X.21 interface.
.PP
Afterwards, the state DCE waiting (state 6A, r = SYN, i = OFF) is
entered at the X.21 interface.
.RT
.sp 1P
.LP
2.1.2.1.2
\fISETUP ACKNOWLEDGE/CALL PROCEEDING (from ET)\fR
.sp 9p
.RT
.PP
The network reaction on the SETUP message received from the TA can be either:
.RT
.LP
a)
sending of a CALL PROCEEDING message to the TA; when the
CALL PROCEEDING message is received on the D\(hychannel at the
S/T reference point, the B\(hychannel will be allocated and the
TA transmits r\ =\ 1, i\ =\ OFF (within 80\ bit multiframes in the
case of user classes\ 3\(hy6) via the B\(hychannel at the S/T
reference point; or
.LP
b)
sending of a SETUP ACKNOWLEDGE message to the TA; when the
SETUP ACKNOWLEDGE message is received on the D\(hychannel at the
S/T reference point, the B\(hychannel will be allocated and
and the TA transmits 1, OFF (within 80\ bit multiframes in the
case of user classes\ 3\(hy6) via the B\ channel at the S/T
reference point.
.LP
In this case subsequent reception of CALL PROCEEDING does
not entail any further actions in the\ TA.
.sp 1P
.LP
2.1.2.1.3
\fIALERTING (from ET)\fR
.sp 9p
.RT
.PP
ALERTING message is only used with manual answering.
.PP
When an ALERTING message is received on the D\(hychannel at the
S/T reference point, the TA transmits the call progress signal (state\ 7,
r\ =\ IA5,\ i\ =\ OFF) to the calling DTE.
.PP
Afterwards the state DCE waiting (state 6A, r\ =\ SYN, i\ =\ OFF) is
entered at the X.21 interface.
.RT
.sp 1P
.LP
2.1.2.1.4
\fICONNECT (from ET)\fR
.sp 9p
.RT
.PP
When a CONNECT is received on the D\(hychannel at the S/T reference
point, the TA transmits any DCE provided information (state\ 10, r\ =\ IA5,
i\ =\ OFF) to the calling DTE. Afterwards the state connection in progress
(state\ 11) is entered at the X.21 interface.
.PP
When the frame alignment pattern of the 80 bit multiframe (in the case
of Recommendation\ X.1 user classes\ 3\(hy6) is received on the B\(hychannel
at the
S/T reference point, the TA performs switch\(hythrough.
.PP
When the calling DTE receives (1,\ ON) via the through\(hyconnected
B\(hychannel at the X.21 interface, the calling DTE enters the state ready for
data (state\ 12) and data transfer (state\ 13) can begin.
.RT
.sp 1P
.LP
2.1.2.1.5
\fISETUP (from ET)\fR
.sp 9p
.RT
.PP
The TA shall not accept a SETUP message unless the X.21 interface is in
the ready state (state\ 1). When a SETUP message is received on the
D\(hychannel at the S/T reference point, the TA shall follow the procedures for
determining compatibility checking (e.g.,\ data signalling rate) found in
Recommendation\ Q.931. If the TA determines that it can respond to the
incoming call, it follows the procedures of Recommendation\ Q.931. It is
expected that
ALERTING message would only be used with terminals that answer manually.
.PP
The TA transmits an incoming call (r\ =\ Bell, i\ =\ OFF) via the X.21
interface to the called DTE, and the incoming call state (state\ 8, r\ =\ BEL,
i\ =\ OFF) entered.
.PP
Call offering procedure in a multiterminal configuration is described in
\(sc\ 2.1.3.
.RT
.sp 1P
.LP
2.1.2.1.6
\fICONNECT (from TA)\fR
.sp 9p
.RT
.PP
When a call accepted (state 9, t\ =\ 1, c\ =\ ON) is received from
the called DTE, the TA transmits a CONNECT message via the D\(hychannel of the
S\(hyinterface.
.bp
.RT
.sp 1P
.LP
2.1.2.1.7
\fICONNECT ACKNOWLEDGE (from ET)\fR
.sp 9p
.RT
.PP
When a CONNECT ACKNOWLEDGE message is received on the D\(hychannel at the
reference point, the TA, selected by this message, transmits 1/OFF via
the allocated B\(hychannel and signals connection in progress (state\ 11,
r\ =\ 1,
i\ =\ OFF) to the DTE after delivering DCE provided information if any.
.PP
The TA performs switch\(hythrough after the frame alignment pattern
(80\ bit multiframe in the case of user classes\ 3\(hy6) has been received
via the B\(hychannel at the S/T reference point.
.PP
When the called DTE receives 1, ON via the switched through B\(hychannel
on the X.21 interface, the ready for data state (state\ 12, r\ =\ 1, i\
=\ ON) is
entered and data transfer (state\ 13, r\ =\ D, i\ =\ ON) can begin.
.RT
.sp 1P
.LP
2.1.2.1.8
\fIRELEASE (from ET)\fR
.sp 9p
.RT
.PP
In the case of a multiterminal configuration, the exchange
termination sends a RELEASE message to each TA that had signalled CALL
PROCEEDING, ALERTING or CONNECT but which was not selected for the call.
Subsequently the TA performs the DCE clear indication procedure at the X.21
interface and sends a RELEASE COMPLETE message to the exchange.
.RT
.sp 1P
.LP
2.1.2.1.9
\fIDISCONNECT (from TA)\fR
.sp 9p
.RT
.PP
A DTE clear request (state 16, t = 0, c = OFF) is transmitted via the B\(hychannel
from the clearing to the cleared DTE.
.PP
The TA at the clearing DTE recognizes the state 16 at the
X.21\(hyinterface, separates the R\(hy and I\(hyleads from the B\(hychannel
and transmits a DCE clear confirmation (state\ 17\ =\ 0, OFF) to the clearing
DTE. It transmits
also a DISCONNECT message via the D\(hychannel of the S/T reference point (see
Figure\ 2\(hy6/X.30).
.PP
After reception of the RELEASE message on the D\(hychannel, the TA tears
down the B\(hychannel, sends RELEASE COMPLETE to the exchange, transmits
DCE ready (r\ =\ 1, i\ =\ OFF) to the DTE, and the DTE enters the state
DTE ready (state\ 1, t\ =\ 1, c\ =\ OFF).
.RT
.sp 1P
.LP
2.1.2.1.10\ \ \fIDISCONNECT (between TA)\fR
.sp 9p
.RT
.PP
When the DTE initiates DTE clear request (t\ =\ 0,
c\ =\ OFF) this status is transmitted inslot within the B\(hychannel and
received as DCE clear indication (r\ =\ 0, i\ =\ OFF) in the DTE (see
Figure\ 2\(hy7/X.30).
.PP
The TA recognizes the clear request received inslot via the B\(hychannel
at the S/T reference point, separates the R\(hy and I\(hyleads from the
B\(hychannel and transmits a DCE clear indication (state\ 19\ =\ 0, OFF)
to the DTE to be cleared.
.PP
After the TA to be cleared has received DTE clear confirmation
(t\ =\ 0, c\ =\ OFF) from the DTE, it transmits the DISCONNECT message via the
D\(hychannel, and clears the B\(hychannel.
.PP
After reception of a RELEASE message on the D\(hychannel, the TA releases
the call reference, sends RELEASE COMPLETE message to the exchange, transmits
DCE ready (state\ 2, r\ =\ 1, i\ =\ OFF) to the DTE, and the DTE enters
the state
DTE ready (t\ =\ 1, c\ =\ OFF).
.RT
.LP
.rs
.sp 11P
.sp 2P
.LP
\fBMONTAGE : \(sc 2.1.2.1.11 SUR LE RESTE DE CETTE PAGE\fR
.sp 1P
.RT
.LP
.bp