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INTERNATIONAL TELECOMMUNICATION UNION
CCITT I.121
THE INTERNATIONAL
TELEGRAPH AND TELEPHONE
CONSULTATIVE COMMITTEE
INTEGRATED SERVICES DIGITAL NETWORK
(ISDN) GENERAL STRUCTURE AND
SERVICE CAPABILITIES
BROADBAND ASPECTS OF ISDN
Recommendation I.121
Geneva, 1991
Printed in Switzerland
FOREWORD
The CCITT (the International Telegraph and Telephone
Consultative Committee) is a permanent organ of the International
Telecommunication Union (ITU). CCITT is responsible for studying
technical, operating and tariff questions and issuing
Recommendations on them with a view to standardizing
telecommunications on a worldwide basis.
The Plenary Assembly of CCITT which meets every four years,
establishes the topics for study and approves Recommendations
prepared by its Study Groups. The approval of Recommendations by
the members of CCITT between Plenary Assemblies is covered by the
procedure laid down in CCITT Resolution No. 2 (Melbourne, 1988).
Recommendation I.121 was prepared by Study Group XVIII and was
approved under the Resolution No. 2 procedure on the 5 of April
1991.
___________________
CCITT NOTES
1) In this Recommendation, the expression "Administration" is
used for conciseness to indicate both a telecommunication
Administration and a recognized private operating agency.
2) A list of abbreviations used in this Recommendation can be
found in Annex A.
a ITU 1991
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 permission in writing
from the ITU.
Preamble to B-ISDN Recommendations
In 1990, CCITT SG XVIII approved a first set of
Recommendations on B-ISDN. These are:
I.113 - Vocabulary of terms for broadband aspects of ISDN
I.121 - Broadband aspects of ISDN
I.150 - B-ISDN asynchronous transfer mode functional
characteristics
I.211 - B-ISDN service aspects
I.311 - B-ISDN general network aspects
I.321 - B-ISDN Protocol Reference Model and its application
I.327 - B-ISDN functional architecture
I.361 - B-ISDN ATM Layer specification
I.362 - B-ISDN ATM Adaptation Layer (AAL) functional
description
I.363 - B-ISDN ATM Adaptation Layer (AAL) specification
I.413 - B-ISDN user-network interface
I.432 - B-ISDN user-network interface - Physical Layer
specification
I.610 - Operation and maintenance principles of B-ISDN access
These Recommendations address general B-ISDN aspects as well
as specific service- and network-oriented issues, the fundamental
characteristics of the asynchronous transfer mode (ATM), a first
set of relevant ATM oriented parameters and their application at
the user-network interface as well as impact on operation and
maintenance of the B-ISDN access. They are an integral part of the
well established I-Series Recommendations. The set of
Recommendations are intended to serve as a consolidated basis for
ongoing work relative to B-ISDN both within CCITT and in other
organizations. They may also be used as a first basis towards the
development of network elements.
CCITT will continue to further develop and complete these
Recommendations in areas where there are unresolved issues and
develop additional Recommendations on B-ISDN in the I-Series and
other series in the future.
PAGE BLANCHE
Recommendation I.121
Recommendation I.121
BROADBAND ASPECTS OF ISDN
(revised 1990)
0 Foreword
This Recommendation states the basic principles of broadband
aspects of integrated services digital network (B-ISDN) and
indicates further developments of the ISDN network capabilities in
order to support more advanced services and applications.
The B-ISDN Recommendations were written taking into account
the following:
- the emerging demand for broadband services;
- the availability of high speed transmission, switching and
signal processing technologies;
- the improved data and image processing capabilities
available to the user;
- the advances in software application processing in the
computer and telecommunication industries;
- the need to integrate both interactive and distribution
services;
- the need to integrate both circuit and packet transfer mode
into one universal broadband network;
- the need to provide flexibility in satisfying the
requirements of both user and operator;
- the need to cover broadband aspects of ISDN in CCITT
Recommendations.
1 General
1.1 The main feature of the ISDN concept is the support of a wide
range of audio, video and data applications in the same network. A
key element of service integration for an ISDN is the provision of
a wide range of services to a broad variety of users utilizing a
limited set of connection types and multipurpose user/network
interfaces.
1.2 In the context of this Recommendation, the term B-ISDN is used
for convenience in order to refer to and emphasize the broadband
aspects of ISDN. The intent, however, is that there be one
comprehensive notion of an ISDN which provides broadband and other
ISDN services.
2 Principles of B-ISDN
2.1 Asynchronous transfer mode (ATM) is the transfer mode for
implementing B-ISDN and is independent of the means of transport at
the Physical Layer.
2.2 B-ISDN supports switched, semi-permanent and permanent, point-
to-point and point-to-multipoint connections and provides on
demand, reserved and permanent services. Connections in B-ISDN
support both circuit mode and packet mode services of a mono-
and/or multimedia type and of a connectionless or connection-
oriented nature and in a bidirectional or unidirectional
configuration.
2.3 The B-ISDN architecture is detailed in functional terms and
is, therefore, technology and implementation independent.
2.4 A B-ISDN will contain intelligent capabilities for the purpose
of providing advanced service characteristics, supporting powerful
operation and maintenance tools, network control and management.
Further inclusion of additional intelligent features has to be
considered in an overall context and may be allocated to different
network/terminal elements.
2.5 Since the B-ISDN is based on overall ISDN concepts, the ISDN
access reference configuration is also the basis for the B-ISDN
access reference configuration.
2.6 A layered structure approach, as used in established ISDN
protocols, is also appropriate for similar studies in B-ISDN. This
approach should also be used for studies on other overall aspects
of B-ISDN including information transfer, control, intelligence and
management.
2.7 Any extension of network capabilities or change in network
performance parameters will not degrade the quality of service of
existing services.
2.8 The evolution to B-ISDN should ensure the continued support of
existing interfaces and services.
2.9 New network capabilities will be incorporated into B-ISDN in
evolutionary steps to meet new user requirements and accommodate
advances in network developments and progress in technology.
2.10 It is recognized that B-ISDN may be implemented in a variety
of ways according to specific national situations.
3 Characteristics of an ATM-based B-ISDN
The underlaying asynchronous transfer mode (ATM) of the B-ISDN
provides some specific, advantageous facilities:
- high flexibility of network access due to the cell
transport concept and specific cell transfer principles;
- dynamic bandwidth allocation on demand with a fine degree
of granularity;
- flexible bearer capability allocation and easy provision of
semipermanent connections due to the virtual path concept;
- independence of the means of transport at the Physical
Layer.
4 Evolution
4.1 B-ISDN will be based on the concepts developed for ISDN and
may evolve by progressively incorporating directly into the network
additional B-ISDN functions enabling new and advanced services.
4.2 The deployment of B-ISDN may require a period of time
extending over one, or more, decade(s) as operators seek to find
the most economic means of evolving to the B-ISDN. These
evolutionary phases (e.g. deployment of metropolitan area networks,
passive optical networks, local area networks and also satellite
based networks) will need to be harmonized with the overall B-ISDN
concepts ensuring the continued support of existing interfaces and
services and be eventually integrated with the B-ISDN. In these
evolutionary phases appropriate arrangements must be developed for
the interworking of services on B-ISDN and services on other
networks.
4.3 In the evolution towards a B-ISDN, some digital end-to-end
connections may in part use digital transmission and switching
equipment that has not been optimized for the B-ISDN, e.g. G.702
plesiochronous digital hierarchy transmission systems. Such
connections may provide a reduced maximum service bit rate at the
user-network interface (UNI).
ANNEX A
(to Recommendation I.121)
Alphabetical list of abbreviations contained in this Recommendation
ATM Asynchronous transfer mode
B-ISDN Broadband aspects of integrated
services digital network