ISO/IEC 9574:1992

Information technology — Provision of the OSI connection-mode network service by packet mode terminal equipment to an integrated services digital network (ISDN)

Specifies the method of providing in accordance with the procedures described in Recommendation X.31. This is done by specifying the mapping of the CONS primitives and parameters to and from the elements of the protocols used by two types of packet mode terminal equipment: an X.25 DTE (TE2) and a packet mode ISDN terminal (TE1).

Technologies de l'information — Fourniture du service de réseau OSI en mode connexion par un terminal en mode paquet raccordé à un réseau numérique avec intégration de services (RNIS)

General Information

• Status: Published
• Publication Date: 29-Dec-1992
• ICS: 35.100.30 - Network layer
• Technical Committee: ISO/IEC JTC 1/SC 6 - Telecommunications and information exchange between systems
• Drafting Committee: ISO/IEC JTC 1/SC 6/WG 7 - Network, transport and future network
• Current Stage: 9093 - International Standard confirmed
• Completion Date: 16-Aug-2002

Relations

• Revises: ISO/IEC 9574:1989 - Information technology — Telecommunications and information exchange between systems — Provision of the OSI connection-mode network service by packet mode terminal equipment connected to an integrated services digital network (ISDN)
• Effective Date: 15-Apr-2008

Standards Content (Sample)

INTERNATIONAL
ISO/IEC
STANDARD
9574
Second edition
1992-12-15
Information technology - Provision of the
OSI connection-mode network service by
packet mode terminal equipment to an
integrated services digital network (ISDN)
Technologies de I’informa tion - Fourniture du service de rgseau OSI en
mode connexion par un terminal en mode paquet raccord6 2 un rbseau
numkrique avec integration de service (RN/S)
Reference number
&O/l EC 9574: 1992(E)

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ISOIKEC 9574 : 1992 (E)
Contents
1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*. 1
2 Normative references . . . . . . . . . . . . . . . . .~. 2
3 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4 Abbreviations .,.,.,.~,.,. 5
5 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*.*. 5
6 Provision of the CONS in systems attached at the S/T reference point . . . . . . . . . 6
7 Provision of the CONS in systems attached at the R reference point . . . . . . . . . . . . . 16
Annex
A Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
0 ISO/lEC 1992
All rights reserved. No part of this publication may be reproduced or utilized in any form or
by any means, electronic or mechanical, including photocopying and microfilm, without per-
mission in writing from the publisher.
l Case Postale 56 l CH-1211 Geneve 20 l Switzerland
lSO/IEC Copyright Office
Printed in Switzerland
ii

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ISOLKEC 9574 : 1992 (E)
Foreword
IS0 (the International Organization for Standardization) and IEC (the Inter-
national Electrotechnical Commission) form the specialized system for
worldwide standardization. National bodies that are members of IS0 or
IEC participate in the development of International Standards through
technical committees established by the respective organization to deal
with particular fields of technical activity. IS0 and IEC technical com-
mittees collaborate in fields of mutual interest. Other international organ-
izations, governmental and non-governmental, in liaison with IS0 and IEC,
also take part in the work.
In the field of information technology, IS0 and IEC have established a joint
technical committee, lSO/IEC JTC 1. Draft International Standards adopted
by the joint technical committee are circulated to national bodies for vot-
ing. Publication as an International Standard requires approval by at least
75 % of the national bodies casting a vote.
International Standard lSO/IEC 9574 was prepared by Joint Technical
Committee ISO/IEC JTC 1, Information technology, in collaboration with
the CCITT. The identical text is published as CCITT Recommendation
X.61 2
This second edition cancels and replaces the first edition
(IS0 9574:1989), of which it constitutes a technical revision.
Annex A of this International Standard is for information only.
. . .
III

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ISO/IEC 9574 : 1992 (E)
INTERNATIONAL STANDARD
CCITT RECOMMENDATION
INFORIUATION TECHNOLOGY -
PROVISION OF THE OS1 CONNECTION-MODE NETWORK
SERVICE BY PACKET-MODE TERMINAL EQUIPMENT
CONNECTED TO AN INTEGRATED SERVICES DIGITAL
NETWORK (ISDN)
1
Scope
This Recommendation 1 International Standard specifies the method of providing the OS1 Connection-mode Network
Service (CONS) by packet mode terminal equipment connected to an Integrated Services Digital Network (ISDN) in
accordance with the procedures described in Recommendation X.3 1. This is done by specifying the mapping of the
CONS primitives and parameters to and from the elements of the protocols used by two types of packet mode terminal
equipment:
an X.25 DTE (TE2) connected to an R reference point and accessing an ISDN; and
a)
b) a packet mode ISDN terminal (TEl) operating ISO/IEC 8208 packet layer protocol (PLP) and connected
to an ISDN at either the S or T reference point.
This Recommendation 1 International Standard is applicable:
when operating according to Recommendation X.31, either a TEl or a TE2 is connected to a packet
a)
handler in an ISDN or an access unit to a packet-switched data network via an ISDN;
when using an ISDN circuit-switched channel, either TEls and/or TE2EAs are connected directly to
b)
each other (i.e. the terminals operate in DTE/DTE mode).
This Recommendation 1 International Standard does not address TE2s using TAs (at the R reference point) when using
tion X.6 13 I ISO/IEC 10588).
NOTES
1 The definitions of TEl, TEZ and TA equipment, and R, S, and T reference points are given in Recomruencla-
tion I.41 1.
2 This Recommendation 1 International Standard applies to a TEl or TE2/TA (i.e. an OS1 End System) regardless of
whether it is a physically separate system or embedded in other equipment such as a PBX.
This Recommendation 1 International Standard addresses the provision of the CONS using Virtual Calls as described in
Recommendation X.25. It does not address the use of X.25 Permanent Virtual Circuits. The extension of this
Recommendation 1 International Standard to include the use of X.25 PVCs is for further study.
NOTE - This Recommendation 1 International Standard uses numbers to identify layers, rather than their names.
This is
done to align the terminology of this document with the terminology of the related ISDN Recommendations, and does not imply
any change in the functionality of the layers from that defined in the reference model of open systems interconnection.
CCITT Rec. X.612 (1992 E)
1

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ISOLIEC 9574 : 1992 (E)
2 Normative references
The following CUM Recommendations and ISO/IEC International Standards contain provisions which, through
reference in this text, constitute provisions of this Recommendation 1 International Standard. At the time of
publication, the editions indicated were valid. All Recommendations and Standards are subject to revision. Parties to
agreements based on this Recommendation 1 International Standard are encouraged to investigate the possibility of
applying the most recent editions-of the Recommendations and Standards listed below. Members of IEC ,uld IS0
maintain registers of currently valid International Standards. The CCITT Secretariat maintains a list of currently valid
CCITT Recommendations.
21 . Identical Recommendations 1 International Standards
-
CCITT Recommendation X.213 (1992) 1 ISO/IEC 8348: 1992, Information technology - Network
service definition for Open Systems Interconnection
22 . Paired Recommendations 1 International Standards equivalent in technical content
-
CCITT Recommendation X.200 (1988), Reference model of open systems interconnection for CCITT
applications.
IS0 ‘7498:1984, Information processing systems - Open Systems Interconnection - Basic Reference
Model.
-
CCITT Recommendation X.210 (1988), Open systems interconnection Zayer service definition
conventions.
ISOITR 8509: 1987, Information processing systems - Open Systems Interconnection - Service
conventions.
-
CCITT Recommendation X.223 (1988), Use of X.25 to provide the OSI connection-mode network
service for CCITT applications.
IS0 8878: 1987, Information processing systems - Data communications - Use of X.25 to provide the
OSI connection-mode network service.
23 . Additional references
-
CCITT Recommendation I.23 1 (1988), Circuit-mode bearer service categories.
-
CCITT Recommendation I.232 (1988), Packet-mode bearer service categories.
-
CCITT Recommendation 1.430 (1988), Basic User-Network Interface Layer 1 Specification.
-
CCITT Recommendation I.43 1 (1988), Primary Rate User-Network Interface Layer 1 Spectfication.
>
-
CCITT Recommendation Q.921 (1.441) (1988), ISDN User-Network Inter$ace Data Link Layer
Specification.
-
CCITT Recommendation Q.93 1 (I.45 1) (1988), ISDN User-Network Interface Layer 3 Specification for
Basic Call Control.
-
CCITT Recommendation V.25 bis (1988), Automatic Answering Equipment and/or Parallel Automatic
Calling Equipment on the General Switched Telephone Network Including Procedures for Disabling of
Echo Control Devices for Both Manually and Automatically Established Calls.
-
CCITT Recommendation X.21 (1988), Interface Between Data Temzinal Equipment (DTE) and Data
Circuit-terminating Equipment (DCE) for Synchronous Operation on Public Data Networks.
-
CCI?T Recommendation X.21 bis (1988), Use on Public Data Networks of Data Terminal Equipment
(DTE) which is Designed for Interfacing to Synchronous V-series Modems.
CCI’IT Rec. X.612 (1992 E)
2

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ISOiIEC 9574 : 1992 (E)
-
CCITT Recommendation X.25 (1988), Interface between data terminal equipment (DTE) cvld data
circuit-terminating equipment (DCE) for terminals operating in the packet mode and connected to public data
networks by dedicated circuit.
-
CCITT Recommendation X.30 (1.462) (1988), Support of X.21, X.21 bis and X.20 bis based data
terminal equipments (DTEs) by an integrated services digital network (ISDN).
-
CCITT Recommendation X.3 1 (1.462) (1988), Support of Packet Mode Terminal Equipment by an
ISDN.
-
CCITT Recommendation X.32 (1988), Inter$ace between data terminal equipment (DTE) and data
circuit-terminating equipment (DCE) for terminals operating in the packet mode and accessing a packet
switched public data network through a public switched telephone network or an ISDN or a circuit
switched public data network.
-
IS0 7716: 1986, Information processing systems - Data communications - High-level data link control
procedures - Description of the X.25 LAPB-compatible DTE data link procedures.
-
ISO/IEC 8208: 1990, Information technology - Data communications - X.25 Packet Luyer Protocol for
Data Terminal Equipment.
3 Definitions
For the purposes of this Recommendation 1 International Standard, the following definitions apply.
. Basic reference model definitions
31
This Recommendation 1 International Standard makes use of the following terms defined in CCITT
Recommendation X.200 I IS0 7498:
a) Network Connection;
b) Network Layer;
c) Network Service.
32 . Service conventions definitions
This Recommendation 1 International Standard makes use of the following terms defined in CCITT
Recommendation X.210 I ISO/TR 8509:
a) Network Service provider;
b) Network Service user.
33 . Network service definitions
This Recommendation I International Standard makes use of the following terms defined in CCITT
Recommendation X.2 13 I ISO/IEC 8348:
a) N-CONNECT request;
b) N-DISCONNECT indication.
34 . X.25 definitions
This Recommendation I International Standard makes use of the following terms defined in CCITT
Recommendation X.25 I ISO/IEC 8208:
a) DATA packet;
b) Data terminal equipment;
C) INCOMING CALL packet;
CCITT Rec. X.612 (1992 E)
3

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ISO/IEC 9574 : 1992 (E)
d) Throughput class;
e) User data field;
35 . X.31 definitions
This Recommendation I International Standard makes use of the following terms defined in CCITT
Recommendation X.3 1:
a) Conditional notification class of service;
b) ISDN Virtual Circuit Bearer Service;
c) No notification class of service;
d) Packet handling function;
e) PSPDN Service;
f) Unconditional class of service;
Semi-permanent-access (see Note);
h) Demand access.
NOTE - Some HSDN Recommendations use the term ‘permanent’ to describe this feature.
36 . 1.112 definitions
This Recommendation I International Standard makes use of the following terms defined in CCITT
Recommendation I. 112:
terminal equipment;
a)
b) reference point.
37 . 1.411 definitions
This Recommendation I International Standard makes use of the following terms defined in CCITT
Recommendation I.4 11:
a) R reference point;
b) S reference point;
c) T reference point;
d) Terminal adaptor.
38 . I.412 definitions
This Recommendatiorr 1 International Standard makes use of the following terms defined in CCITT
Recommendation 1.412:
a) B channel;
b) D channel.
CCITI’ Rec. X.612 (1992 E)

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ISOiIEC 9574 : 1992 (E)
39 .
X.121 definitions
use of the following terms defined in CCITT
This Recommendation 1 International Standard makes
Recommendation X.121:
a) prefix;
b) escape.
4 Abbreviations
41 . Reference model abbreviations
OS1 Open Systems Interconnection
NS Network Service
42 . Network service abbreviations
CONS Connection-mode Network Service
Quality of service
QOS
43 . X.25 abbreviations
Data circuit-terminating equipment
DCE
DTE Data terminal equipment
LAP link access procedure
PLP Packet layer protocol
PSDN Packet switched data network
RPOA Recognized private operating agency
44 . ISDN abbreviations
AU Access unit
ISDN Integrated services digital network
PH Packet h,uldling function
SAP1 Service access point identifier
Terminal adaptor
TA
TE Terminal equipment
Higher layer compatibility
I-EC
Overview
5
ISDN environment
51 .
The ISDN environment is characterised by two configurations. In the first, a packet mode terminal is connected to a
packet mode service, as described in Recommendation X.31. In the second configuration, two packet mode terminals
are connected directly by an ISDN circuit-switched B or H channel.
CCITT Rec. X.612 (1992 E) 5

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ISO/IEC 9574 : 1992 (E)
5.1.1 Use of a packet mode service
The support of the packet mode terminal equipment by an ISDN when a packet mode service is used is described in
Recommendation X.31. In this case, references to Recommendation Q.93 1 procedures indicate their use as described
in Recommendation X.3 1. Two cases for ISDN support of packet mode terminal equipment are defined in
Recommendation X.3 1: one, the case where the support is via the ISDN Virtual Circuit service [see Figure la)], and
the other where the support is via access to PSDN services [see Figure lb)], respectively referred to as “case B” cvld
‘bse A”.
In case A an ISDN transparent circuit connection, either semi-permanent (i.e., non-switched) or demand
(i.e., switched), is used. The corresponding ISDN bearer service is a 64 kbit/s service as described in
Recommendation 1.231. The sub-network functions available to the user are those of the PSDN described in
Recommendation X.25 (semi-permanent access) and Recommendation X.32 (demand access), as well as in other
X-Series Recommendations (e.g., X.2, X.121).
In case B the ISDN virtual is used, as described in Recommendation 1.232. The sub-network
circuit bearer service
functions available are those -Series Recommendations.
described in the 1.2~~
In case A only B channels may be used to access the PSDN, while in case B both B and D ch,ulnels may be used to
access the ISDN packet handling function.
5.1.2 Direct circuit-switched connection
Two packet mode terminal equipment may be connected directly using an ISDN B or H channel. The channel used is
either semi-permanent (i.e. non-switched) or demand (i.e. switched), see Figure Ic).
52 . CONS in this ISDN environment
Recommendation X.223 I IS0 8878 specifies the method for providing the OS1 Connection-mode Network Service
(CONS) through the use of the X.25 packet layer protocol. When operating in an ISDN environment, a few
requirements additional to those contained in Recommendation X.223 I IS0 8878 are necesscuy.
The requirements for providing CONS by terminal equipment connected to an ISDN at the S or T reference points are
specified in clause 6.
The requirements for providing CONS in X.25 DTEs presenting an X.21, X.21 his, or V.25 his interface at the R
reference point and connected to an ISDN through a TA are specified in clause 7.
This Recommendation 1 International Standard uses the X.25 PLP to convey all elements, of all three phases, of the
OS1 Connection-mode Network Service.
6 Provision of the CONS in systems attachedi at the S/T reference point
61 . Procedures for TEls or TE2/TAs to provide CONS
This clause of this Recommendation 1 IntemationaI Standard covers the five cases that exist taking into account the
various types of underlying connections that can be available to the packet mode TEl or TE2/TA (see Table 1). The
protocol layers applicable to these cases are given in Figures 2 and 3 and as referenced in Table 1.
The mapping of the elements of the CONS to the protocol and procedures of ISO/IEC 8208 shall be as requimi by
Recommendation X.223 I IS0 8878. The remaining clauses in this section specify the provisions required in addition
to these mappings, by systems attached to an interface at the S/T reference point.
CCITT Rec. X.612 (1992 E)
6

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ISWIEC 9574 : 1992 (E)
a) Configurations for case 8 access
- SorT
I
TEl r
R R - -
- - SorT SorT
BB
I I I I
9 9 TA TA
TE2 TE2 * *
Demand access
b) Configurations for case A access
SorT -
S or T
I
I .
TEl
TEl
.
. SorT S or T
R
R ’
BorH p
I
I I
TE2 1 TA p
TA i 1 TE2
Demand access
4 .
L 4
T1 SOOOflO-92
c) Configurations for direct connection by 1SDiV circuit-stitched B or H channel
TE Terminal equipment
TA Terminal adaptor
PH Packet handling function
AU Access unit
Figure 1 - Cases covered by clause 6
Table 1 - Cases covered by clause 6
Underlying connection perceived
Fi PIWP. .Cllhrl~llce
Access to u U”YlG4.U””
- ‘b”‘-
by the terminal equipment
D channel
Packet Mode service
2 6.2
B channel : Semi-permanent
Packet Mode service 3 6.3.1
B channel : Demand
Packet Mode service ,
6.3.2 ,
I
I 3 I
B or H channel : Semi-permanent
Remote terminal
3 6.4.1
I I I I
B or H channel : Demand
Remote terminal 3 6.4.2
CCITT Rec. X.612 (1992 E)
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ISWIEC 9574 : 1992 (E)
OSI layer
4
I ------I
This Recommendation I International Standard
in conjunction with Recommendation X.223 I IS0 8878
1
I
3
I I
I
Q.931 (I .451)
ISO/lEC 8208
(see Note)
(X.25 PLP)
I
I t-
Q.921 (1.441)
2
(LAW
D channel 1
I.430 or 1.431
TlS00090-92
Additional signalling Packet switched
for incoming signalling and information
packet cal I s transfer
NOTE - This protocol need not be present in all systems.
Figure 2 - Protocol layers at S and T reference points
when D channel is used in ISDN
OSI layer
4
I
I
This Recommendation I International Standard
in conjunction with Recommendation X.223 I IS0 8878
Q.931 (1.451) ISO/IEC 8208
(see Note) (X.25 PLP)
Q.921 (1.441)
IS0 7776
WV
(X.25 LAPB)
(see Note)
1
D channel B or f-l channel
I.430 or 1.431
TI SO01 00-92
Packet switched
Signalling
signalling and information
for circuit switched
transfer
access
Additional signalling
for incoming
packet calls
NOTE - These two protocols need not be present in all systems.
Figure 3 - Protocol layers at S and T reference points
when B channel or H channel is used in ISDN
CCITT Rec. X.612 (1992 E)

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ISOI-IEC 9574 : 1992 (E)
6 . Additional procedure for a TEl or TE2/TA to provide the CONS when using the ISDN
D channel
TEls and TE2/TAs supporting packet operation on the ISDN
This clause is only applicable in case B access.
D ch International Standards indicated in Figure 2. At the physical layer, 1.430 shall be used if the ISDN access is a basic
interface and 1.431 shall be used if the ISDN access is a primary rate interface. At the data link layer, Q.921 shall be
used to provide the LAPD procedures. At the network layer, ISOKEC 8208 shall be used to provide the packet layer
protocol and Q.931 may be used to provide the “call-offering procedure” which, when present, takes place before the
conveyance of X.25 incoming call packets. The Q.931 procedures enable terminal identification (basic access) and a
determination of which channel (D or B) a specific incoming call packet is to be conveyed on. The following
limitations shall apply :
a) the maximum user data field length of ISOAEC 8208 data packets shall not exceed 256 octets;
b) the ISO/IEC 8208 throughput class used shall not exceed 16 kbits/sec on a basic interface.
The subscription and negotiated facilities in the X.25 call establi t packets shall be used, as necessary, to indicate
values consistent with these restrictions.
6.2.1
Outgoing calls
ISO/IEC 8208 procedures shall be operated over the ISDN layer 2 procedures using SAPI= as defined in
Recommendation X.3 1. No additional procedures are required.
NOTE - The address fields of the ISO/IEC 8208 packets contain sub-network addresses that are valid for the ISDN
(e.g. Recommendation E. 164 for public ISDNs, where a valid E.164 address could be Escape Code + Recommendation X.121
number. A network dependent prefix may also be required).
6.2.2 Incoming calls
The procedures applying to incoming calls vary according to whether the ISDN call offering procedures are used.
NOTE - The address fields of the ISO/IEC 8208 packets contain sub-network addresses that are valid for the ISDN.
6.2.2.1 Without call offering procedures
These procedures are used by the sub-network if
a) the interface to which the TEl or TE2/TA is connected has subscribed to the “conditional notification
class of service”, and the network does not use the call offering procedures for this incoming call; or
b) the interface to which the ‘El or TE2/TA is connected has subscribed to the “no notification class of
service”.
ISO/IEC 8208 procedures shall be operated over the ISDN layer 2 procedures using a SAM=16 as defmed in
Recommendation X.3 I. No additional procedures are required.
6.2.2.2 With call offering procedures
These procedures are used if
a) the interface to which the TEl or TE2/TA is connected has subscribed to the “unconditional notification
class of service”, or
b) the interface to which the TEI or TE2/TA is connected has subscribed to the “conditional notification
class of service”, and the network uses the call offering procedures for this incoming call.
CCITT Rec. X.612 (1992 E)
9

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ISWIEC 9574 : 1992 (E)
ISO/IEC 8208 protocol procedures shall be used over the ISDN layer 2 procedures using SAPI= 16. In addition t&e
ISDN call offering procedures are also operated over the ISDN layer 2 procedures, using either SAPI=O or SAPI=l6.
Systems conforming to this Recommendation 1 International Standard, that operate these ISDN call offering
procedures, shall implement these procedures using SAPI=O and may also implement these procedures using SAPI=16
(see Notes 1 and 2). The ISDN c‘all offering procedures (see Notes 3 and 4) determine which TEl or TE2/TA is to
receive the call and are not visible to the NS user. The ISO/IEC 8208 procedures are then operated after the TSDN call
offering procedures have been completed.
NOTES
1 Recommendation X.31 states (Note 4 to 6.2.2.3.1): “Networks providing packet mode call offering shall provide
Q.931 signalling procedures for packet mode calls on SAPI=O. For an interim period, some networks, by subscription agreement,
may offer SAPI= 6 broadcast call offering procedures for providing 4.931 signalling. This option will use all Q.931 procedures
“D-channel exclusive” and will not provide
for packet mode calls with the following restriction: All calls will be offered as
channel negotiation procedures. Terminals implementing SAPI=M procedures shall also implement SAPI=O procedures for
portability .”
2 In order to maximise terminal portability during the interim period identified in Note 1, systems should implement
the call offering procedures on both SAPI=O and SAPI=l6.
These procedures may use, for example, Recommendation 4.931 addressing, sub-addressing, direct-dialling-in,
3
multiple subscriber number and compatibility checking information elements, to determine which TFl or TE2/TA is to receive the
call. In order to avoid unnecessary connection failures, it is suggested that incoming calls are not rejected on the basis of
compatibility information unless this information identifies as required, a functionality that the terminal is not capable of (e.g. a
call should not be rejected because the HLC information element is not present).
4 These procedures may require D-channel selection, or may offer the terminal a choice of D or B channels. No
distinction is made here between these ways of selecting the D channel. Sub-clause 6.3 covers the case where a B channel is
selected.
63 . Additional procedures for a TEl or TE2/TA to provide the CONS when using the ISDN
B channel and accessing a packet mode service.
TI5ls and TE2/TAs supporting packet operation on the ISDN B channel present stacks of protocols at the S or T
reference point in accordance with the Recommendations and International Standards indicated in Figure 3. One stack,
which may be null, is used to support signaIling on SAPI=O for circuit switched access to the packet handling function
and the “call offering procedure”, and the other is used to support packet switched signalling and information transfer.
At the physical layer, 1.430 shall be used if the ISDN access is a basic interface, and 1.431 shall be used if the ISDN
access is a primary rate interface. At the data link layer, Q.921 shall be used over the D channel (signalling) and
IS0 7776 shall be used over the B channel (information). At the network layer, Q.931 shall be used over the D chcvlnel
to convey circuit switched signalling and for the ISDN call offering procedure. Also at the network layer,
ISO/IEC 8208 shall be used over the B channel for the packet layer protocol (see Note).
The following sub-clauses specify the requirements in addition to those specified in 6.1 for the cases of an underlying
semi-permanent connection, and an underlying demand access connection between the TEl or TE2/TA h‘andling function.
NOTE - In case B, the address fields of the ISO/IEC 8208 packets contain sub-network addresses that are valid for the
ISDN. In case A, the address fields of the ISO/IEC 8208 packets contain sub-network addresses that are valid for the PSPDN.
6.3.1 Semi-permanent B-channel connection
Both the basic interface and the primary rate interface provide for semi-permanent connection of a B channel between
the S or T reference point and the packet handling function.
6.3.1.1 Virtual call originated by the TEl or TE2/TA
No additional procedures are required.
10 CCITT Rec. X.612 (1992 E)

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ISOLIEC 9574 : 1992 (E)
6.3.1.2 Virtual call originated toward the TE1 or TE2/TA
The procedures applying to incoming calls vary according to whether the ISDN call offering procedures Lare used. The
use of call offering procedures cannot result in the selection of a semi-permanent B channel in case A access.
Therefore no additional procedures are required for case A.
6.3.1.2.1 Without call offering procedures
No additional procedures are required for case B access if
a) the interface to which the TEl or TE2LT.A is connected has subscribed to the “conditional notification
class of service”, and the network does not use the call offering procedures for this incoming call; or
b) the interface to which the TEl or TE2E.A is connected has subscribed to the “no notification class of
service”.
6.3.1.2.2 With calf offering procedures
These procedures are used if
a) the interface to which the TEl or TE2/TA is connected has subscribed to the “unconditional notification
class of service”, or
b) the interface to which the El or TE2fTA is connected has subscribed to the “conditional notification
class of service”, and the network uses the call offering procedures for this incoming call.
The ISDN call offering procedures (see Note) determine which B channel is to be used. These procedures are not
visible to the NS user. Following successful channel selection, the procedures specified in IS0 7776 and ISOKEC 8208
shall apply.
NOTE - These procedures may offer the terminal a choice of B channels with, or without, the choice of the D channel.
No distinction is made here between these ways of selecting a semi-permanent B channel. Subclause 6.3.2 covers the case where a
demand access B channel is selected. Subclause 6.2 covers the case where a D channel is selected.
6.3.2 Demand access B-channel connection
channel between
Both the basic interface and the primary rate interface provide for demand access connection ofaB
theS or T reference point ‘and the packet handling function.
These additional procedures shall only be used if a B channel is not already established between the TEl or TIWTA
and the packet handling function, if an additional B channel is needed to support the additional traffic, or if
noti
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