Table Of ContentISDN Technology
ISDN Technology
R.L. BREWSTER
Senior Lecturer in Telecommunications
Aston University
Birmingham
UK
SPRINGER-SCIENCE+BUSINESS MEDIA, B.V.
First edition 1993
Reprinted 1994
© 1993 Springer Science+Business Media Oordrecht
Originally published by Chapman & Hali in 1993
Softcover reprint of the hardcover 1s t edition 1993
ISBN 978-94-010-4693-0 ISBN 978-94-011-1592-6 (eBook)
DOI 10.1007/978-94-011-1592-6
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Contents
Preface IX
1 In trod uction 1
1.1 Preamble
2 Pulse code modulation 5
2.1 Introduction 5
2.2 Sampling 5
2.3 Quantization 6
2.4 Companding 7
2.5 PCM frame structures 10
2.6 Signalling and synchronization I I
2.7 Choice of line code I 3
2.8 PCM digital multiplexing hierarchy I 7
3 Digital switching 20
3.1 Introduction 20
3.2 The cross-point switching matrix 2 1
3.3 Two-stage switching networks 22
3.4 Three-stage and non-blocking switching networks 24
3.5 Digital switching 27
4 Packet switched networks 32
4.1 Introduction 32
4.2 OSI protocol model 34
4.3 X25 and HDLC 36
5 Integrated Digital Network 42
5.1 In trod uc ti on 42
5.2 Signalling system No.7 44
VI Contents
6 Digital Local Access 5 2
6.1 Introduction 52
6.2 Burst mode 53
6.3 Hybrid multiplex with echo cancellation 54
6.4 The need for scrambling 57
6.5 Pseudo-random binary sequences 57
6.6 Self-synchronizing scramb1ers/descramblers 59
6.7 The 2B 1Q scrambler descrambler 60
7 ISDN services and protocols 62
7.1 ISDN standards 62
7.2 The user-network interface 63
7.3 Basic rate access 64
7.4 LAP-D 69
7.5 Network (layer 3) protocol 71
7.6 Primary rate access 73
7.7 Supplementary services 75
8 Broadband ISDN 77
8.1 Introduction 77
8.2 Synchronous digital hierarchy 77
8.3 User access to broadband ISDN 80
9 ATM 84
9.1 Frame relay 84
9.2 Fast packet switching and ATD 88
9.3 ATM 89
9.4 Adaptation layer 94
9.5 Quality of Service 95
9.6 Call acceptance control 97
9.7 Admission control 97
9.8 Source policing 98
9.9 Congestion control 98
9.10 A TM network resource allocation 99
9.11 A TM transmission over SDH 102
1 0 Integrated network services 103
10.1 Introduction 103
10.2 Services over narrowband ISDN 103
10.3 Services over broadband ISDN 106
Contents vii
Appendix A CCITT Recommendations for ISDN 111
Appendix B Glossary of acronyms 115
Further reading 123
Index 124
Preface
The Integrated Services Digital Network (ISDN) represents the
current position in about a hundred years of evolutionary growth
of the worldwide telecommunications infrastructure. This
evolution is by no means complete and the next few years will see
the emergence of a "Broad-band" ISDN as the next stage of
evolutionary development. It is important to appreciate the
evolutionary nature of the telecommunications infrastructure if
one is to properly understand much of the thinking that lies
behind the current ISDN proposals.
This book therefore begins with a number of chapters devoted
to a study of the various developments which have eventually led
to the concept of an integrated digital network. These include the
development of digital transmission of speech using PCM and the
development of digital switching techniques based on stored
program control. The book then turns to a consideration of those
features of the existing telecommunications network which need
to be modified in order to make ISDN a realizable practicality. Of
particular importance is the digitization of transmission over the
links between the user and the local exchange. Next we look at
the current practice and proposals for ISDN based on the
technology presently in use in the telephone network. Finally, we
look at the proposals for a broadband ISDN likely to become
widely available by the turn of the century. Although a number
of standards are already emerging, most of these can only be
regarded as tentative as much research and development is still in
progress in this area, much of it funded as part of the "RACE"
programme. However, a pattern of development is becoming
apparent and A TM is emerging as the likely basis of future
networks, at least well into the 21st Century.
Because of the rapid evolution that is taking place, it is almost
impossible to keep up to date with all the current developments
in technology and in standards. Even in the time between
completion of the manuscript and this book actually appearing in
print, much of what has been written may well already appear
x Preface
somewhat out-dated. However, I have had to draw the line at
some point, otherwise this book would never have actually
reached the printers. I apologise to anyone who is disappointed
by any apparent vagueness that may appear as a result of this
necessity.
This book is intended to put the current discussion of ISDN into
context so that the modern telecommunications engineer can
understand why there are apparent anomalies in the way in
which ISDN operates. Most of these arise because of the
inevitable need to operate alongside, and interwork with, the
existing telecommunications network. The task of universally
introducing ISDN throughout the whole international
telecommunications operation is a task that will take as long, if
not longer, than the effect of the technology on which the current
ISDN is based.
I hope this book will prove to be of interest to both students of
telecommunications in universities and polytechnics as well as
those already engaged on work within the whole
telecommunications industry, both manufacturing and operational.
Finally, a word of appreciation to my own research students and
colleagues who have done much to keep me in touch with
developments in ISDN and whose internal memoranda have given
me much inspiration, especially for the later chapters on broad
band and ATM. Without their input into the pool of information
available to me, this book would not have been possible.
R.L.Brewster
Birmingham, March 1992.
1
Introduction
1.1 PREAMBLE
The preamble to the CCITT I series recommendations succinctly
defines an ISDN as follows:
"An ISDN is a network, in general evolving from a telephony
Integrated Digital Network (IDN) , that provides end-to-end digital
connectivity to support a wide range of services, including voice
and non-voice services, to which users have access by a limited
set of standard mUlti-purpose user-network interfaces".
An Integrated Services Digital Network (ISDN) is, as its name
implies, a network which operates on digital principles and is
designed to carry a wide range of services using common
techniques. Thus the various services are integrated to use a
single general-purpose network.
ISDNs are not designed from scratch. If they had been they
would no doubt have looked very different from that being
introduced at the present time. Rather the concept has evolved
from various developments to the existing Public Switched
Telephone Network (PSTN) to improve its performance by the
introduction of digital techniques. These techniques themselves
have evolved as modern digital electronic technology has
developed, and many of the things done at their introduction
would have been done differently given the technology available
today. However, it is not possible to go back on many of the
decisions taken over the last few decades. To replace the existing
plant at this time is not economically viable and, in the meantime,
it is necessary to maintain compatibility between any new plant
and that already in place.
To understand the current proposals for an ISDN, it is therefore
necessary first to look at the evolution that has taken place in the
last few years in the PSTN. This will involve us firstly in a look at
2 Introduction
Pulse-Code-Modulation (PCM), introduced in the 1940s to
overcome the limitations imposed by noise in long-distance
transmiSSlOn circuits. After that we must look at digital switching
techniques, exemplified by System X, introduced to make use of
newly emerging digital logic technologies to obtain more reliable
and flexible switching functions based on the PCM digital format
used in speech transmission.
In the meantime, there has been an ever-increasing demand
for data transmission facilities in the public domain. Initially this
was provided by the use of modems transmitting analogue-type
signals over the PSTN to represent the digital data symbols. This
technology is still in use, although nowadays much more use is
made of the digital capability within the PSTN in the form of
services such as the British Telecom KILO STREAM (64 kbits/s)
and MEGASTREAM (2 Mbits/s) services. There are, at present,
considerable limitations on these services, since access to the
digital service is, to date, only available at the local exchange
access to the trunk telephone network.
In the conventional "circuit-switched" operation of the
telephone network, a circuit is made available to a user
throughout the whole duration of a communication session
whether or not he is actually transmitting information. This can
lead to gross under-utilization of the available transmission
capability when used for many data applications. To overcome
this, packet-switched networks were introduced. This technique
allows a transmission resource to be shared among a number of
users on a statistical basis and therefore results in much more
efficient use of the resource. However, as we shall see later, the
technique can lead to some packet delay, which can, in some
circumstances, vary from packet to packet. Packet switching is
therefore not appropriate for digital speech transmission, unless
quite complex priority techniques are introduced. Packet
switching has enjoyed varied popularity and has not been greatly
used in the public domain in the UK. Nevertheless it has been
fairly widely used in other parts of Europe and has been
particularly popular in the United States, where it was introduced
fairly early on in the use of digital communication for computer
applications. The impact of packet switching on the initial ISDN
proposals is quite small, although the use of techniques based on
packet switching will almost certainly form the basis of the
Broadband ISDN (B-ISDN) now being developed for introduction
into general use around the turn of the century.
The initial ISDN service is based on the general provision of 64
kbits/s capacity channels. At the time of writing ISDN is only
available on a limited basis because all exchanges have not yet
been converted to digital switching technology. However, the
