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`THE FACTS ON FILE
`DICTIONARY OF TELECOMPIUNICATIONS
`
`Copyright © John Graham, 1983
`
`All rights reserved. No part of this book may be reproduced
`or utilized in any form or by any means, electronic or
`mechanical, including photocopying, recording or by any
`information storage and retrieval system, without permission
`in writing from the Publisher.
`
`Published in the United States of America in 1983 by Facts
`On File, Inc., 460 Park Avenue South, New York, N.Y.
`
`10016, by arrangement with Penguin Books Ltd., Harmonds-
`worth, Middlesex, England.
`
`Library of Congress Cataloging in Publication Data
`
`Graham, John, 1936-
`
`Dictionary of telecommunications.
`
`Bibliography: p.
`I. Title.
`l. Telecommunication—Dictionaries.
`TK5l02.G73
`1983
`384’.03’2l
`82—15675
`ISBN 0—87196—[20-2
`
`ISBN 0—87196-876—2 (pbk)
`Printed in the United States of America
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`PREFACE
`
`Our world has become an increasingly complex place in which, as individuals,
`we are very dependent on other people and upon organizations. An event in
`some distant part-of the globe can rapidly and significantly affect the quality
`of life in our home country.
`This increasing interdependence, on both" a national and international
`scale, has led us to create systems which can respond immediately to dangers,
`enabling appropriate defensive or offensive actions to be taken. These systems
`are operating all around us in military, civil, commercial and industrial fields.
`The electronic computer is at the heart of many such systems, but the
`role of telecommunications is no less important. As we proceed through the
`19805, there will be a further convergence between the technologies of com-
`puting and telecommunications. The changes will be dramatic:
`
`the paperless office
`the database society
`the cashless society
`the office at home
`
`We cannot doubt that the economic and social impact of these concepts will
`be very significant. Already, advanced systems of communication are affecting
`both the layman and the technician. Complex functions are being performed by
`people'using advanced terminals which are intended to be as easy to use as the
`conventional telephone.
`
`The aim of the book
`
`Telecommunications principles are becoming increasingly important in educa-
`tion at undergraduate and graduate levels. All those engaged in the fast-
`growing systems industries are finding that a knowledge of telecommunications
`is essential to solve today’s problems; this knowledge is also vital to those who
`are responsible for managing distributed organizations.
`It is the aim of this book to help people with the terminology of this subject
`and, at the same time, to provide a convenient source of reference to basic tele-
`communication principles.
`In common with many other modern technologies, the subject of telecom-
`munications has developed a language of its own. At first sight, the words
`appear to be intelligible English but, of course, with the inevitable sprinkling of
`initials such as FSK (frequency shift keying). The close association with the
`computer industry gives rise to some major new fields of technical complexity,
`e. g. packet switching.
`This book sets out to-define such terms and, as far as possible, to adhere to
`definitions in keeping with international practice. The scope of the publication
`is wide, dealing with fundamental concepts in telephone and telegraph com-
`munications, switched communications systems, broadcast systems, and narrow
`and broad bandwidth systems.
`The intention is to make the material intelligible to the layman as well as
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` equivalent to the range of sounds perceived
`
`by human ear. From 20 cycles per second
`to 20,000 cycles per second (20 kilocycles,
`written as 20 kHz).
`The radio frequency spectrum includes
`a great range of frequencies, among which
`the more useful are in the range from 20,000
`to 20,000,000,000 cycles per second. All
`forms of electromagnetic and light waves
`travel through space at the same speed of
`300,000,000 metres per second. In radio
`broadcasting and communications,
`it
`is
`useful to consider the following frequency
`bands:
`low frequency (LF). A range of signals
`from 30 kHz to 300 kHz. They are suitable
`for
`long-distance radio communication
`and are often used for military or trans-
`oceanic services. At the lower frequencies,
`a large antenna is needed to propagate and
`receive signals.
`high frequency (HF). A range of signals
`from 3 million cycles per second to 30
`million cycles per second, 3 to 30 Mega—
`Hertz (MHz). Used for long—distance com-
`munication, but the quality is dependent
`upon ionization in the upper atmosphere.
`very high frequency (VHF). A range of
`signals from 30 MHz to 300 MHz. Used for
`short-distance radio communication.
`ultra high frequency (UHF). A range of
`signals from 300 MHz to 3,000 MHz. Often
`used for television broadcasts, and cover—
`ing frequency bands from 470 MHz to 940
`MHz.
`'
`super high frequency (SHF). A range of
`signals from 3,000 MHz to 30,000 MHz,
`otherwise expressed as from 3 GigaHertz
`to 30 GigaHertz (GHz). :1»frequency band
`and bandwidth.
`
`front—end processor A computer subsys-
`tem used mainly to interface a main com-
`puter or host processor to a communication
`network.
`It
`takes responsibility for the
`communication control activity rather than
`the application programs which are run in
`the host system. Ii» communications con—
`troller.
`
`FS CI) file separator.
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`functional compatibility
`
`F SK Ci> frequency shift keying.
`
`It is an ob-
`full availability transposition
`jective in designing exchanges to minimize
`the number of crosspoints, whilst at the
`same time reducing the probability of call
`blocking. A method to achieve this optim-
`ization consists .of arranging the cross-
`points in stages in which a number of
`matrixes known as switching groups occur.
`Full availability transposition is said to
`exist when the pattern of interconnection
`is such that every group in one stage has
`a connection to every group in the previous
`stage.
`
`in
`full duplex A transmission channel
`which simultaneous two—way transmission
`is available. at?) half duplex and simplex.
`
`full duplex error control A system of error
`correction, used in data transmission over
`links which have a long transmission delay
`time, such that a transmitting terminal
`transmits a whole series of blocks without
`
`waiting for the receiving station to acknow-
`ledge correct acceptance of each block
`separately. If an error is detected, indi-
`vidual error blocks are retransmitted or all
`blocks commencing with an error block
`are retransmitted. These techniques require
`blocks to be numbered, and the latter
`method is also referred to as go back to N
`technique.
`
`full echo suppressor An echo suppressor
`in which the speech signals on each path
`are used to control the suppression loss
`in the other path of a four-wire circuit used
`for long-distance communication. Con-
`trast with half echo suppressor.
`
`fully provided route A transmission path
`designed to handle all the offered traffic
`without relying on any alternative route in
`times of peak load. 1:» high-usage route.
`
`compatibility Most modern
`functional
`communication systems . are of modular
`design;
`i.e.
`it
`is possible to extend the
`capacity of the system by adding additional
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`signal bandwidth
`
`process in setting up, maintaining or clear—
`ing a call. ti), for example, signalling and
`signals.
`2.
`In the general case, any electrical
`pulses
`transmitted in
`a network to
`represent message information, or control
`information in handling the process of com-
`munication.
`
`fre-
`range of
`bandwidth The
`signal
`quencies required to convey a particular
`message signal accurately over a channel.
`For example, a speech channel allows up to
`4 kHz for transmission of electrical signals
`representing voices, whereas a television
`channel may require up to 6 MHz to con—
`tain all the information necessary in a video
`signal.
`
`signal element A discrete pulse forming
`part of a digital signal and having a value
`determined by the pulse amplitude.
`
`signal message A message made up of a
`number of signal units and transmitted on
`a signalling channel to control the set—up,
`maintenance or termination of a call in a
`data network.
`
`signal phase
`
`E> phase.
`
`signal power A measure of the strength of
`any electrical signal and expressed in Bels
`or decibels~(dB).
`
`signal-to—listene‘r echo ratio The ratio of
`signal power to the power of echo signals
`reflected back to the transmitting station
`and caused by changes in the electrical
`characteristics of a circuit. A form of signal
`impairment
`in transmission over
`long-
`distance transmission paths; e. g. interconti—
`nental telephone circuits.
`
`In any radio trans—
`signal-to-noise ratio
`mission, or in any communication link, a
`certain amount of noise is generated, and is
`carried as background interference to the
`desired 'message signal. If the signal-to—
`noise ratio is high,
`then the message is
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`ARR|S883|PR|0000960
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`unlikely to be impaired; if it is low, the
`signal may well be severely impaired.
`The problem is accelerated by the use of
`amplifiers which will amplify both noise
`and signal. Repeaters containing amplifiers
`are spaced at intervals along transmission
`cables to maintain an appropriate signal—
`to-noise ratio. These amplifiers are de—
`signed to amplify signals occurring in a
`critical frequency bandwidth in which the
`message signal arises; the bandwidth of the
`amplifier is restricted so that noise outside
`the critical bandwidth is rejected.
`
`signal-to-quantizing noise ratio Quantiz—
`ing noise occurs in pulse code modulation
`(P CM) systems and arises as part of the
`process of converting an audio signal to
`digital form and vice versa. If the signal-to-
`noise ratio is high, the message signal is not
`seriously impaired, but if the ratio is low
`the signal may be impaired.
`
`signal unit (S U) A unit of information in
`a signalling system for a public data network
`and consisting of a defined number of bits
`which provide information to control the
`progress of calls. :11» acknowledgement sig-
`nal unit,
`initial signal unit,
`[one signal
`unit, multi—block synchronization unit, sub-
`sequent signal unit, synchronization signal
`unit, and system control signal unit.
`
`signalling This term is given to the pro—
`cedures concerned with the establishment,
`maintenance and termination of calls in a
`
`network; Signalling systems have been
`evolved to cater for the development of all
`classes of communication systems, includ—
`ing telephony,
`telegraphy and data com—
`munication.
`
`The simplest form of signalling occurs in
`a local telephone network in which various
`signals are transferred between the tele—
`phone instrument and the local exchange.
`For example, when a subscriber lifts the
`handset from the switch hook, an automatic
`call request signal is sent to the line. The
`response from the exchange is a proceed-to—
`send signal which is recognizable as a dial
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`signals
`
`out digital processing and are connected by
`a digital data link.
`the C C I TT system,
`An example is
`recommended for international exchanges,
`known as signalling system No. 6.
`
`signalling interworking A concept which
`arises when an existing network has to be
`extended or gradually replaced by a more
`modern network which uses different tech-
`
`nology and therefore different signalling
`standards. There is a need during the
`period of change-over to provide sub—
`systems capable of recognizing the new and
`old signalling systems.
`
`signalling rate The rate at which a device
`can transmit or receive information, usually
`expressed in bits per second. For example,
`1200 bits per second implies the ability to
`send/receive 1200 pulses per second.
`
`signalling system No. 6 A common chan-
`nel signalling system defined by the C CITT
`for use on international networks, in which
`each signal is represented by 28 bits includ-
`ing the signal itself, the channel identifica—
`tion and error control bits. Exchanges
`operating to this method have computer—
`like facilities and are linked by a data
`channel over which signals are exchanged
`as messages. :1) general article on signalling.
`
`Information transmitted within a
`signals
`network to control the handling of mess-
`ages and the set-up, clearing and mainten-
`ance of connections. The signals listed
`below are defined in this book. The prin-
`ciples concerned are described under the
`general article entitled signalling.
`
`access-barred signal
`address-complete signal
`address—incomplete signal
`address signal
`answer signal
`blocking signal
`call accepted signal
`call answered signal
`call connected signal
`callfailure signal
`
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`
`tone. Individual address digits dialled by
`the subscriber are also signals and other
`signals include: called terminalfree — ring—
`ing tone; called terminal answered — ringing
`tone stops; clear — a party replaces handset.
`This illustration is over—simplified, and the
`signalling process includes complexities
`concerned with multi—exchange connections
`and inter-exchange signalling.
`The methods of effecting signalling vary
`considerably with the evolution of switch—
`ing technology, and in any particular
`telephone network various forms may be
`encountered.
`
`Direct current signalling is dependent
`upon a DC path being created for signals
`through the transmission path used for each
`call. It has tended to be replaced by alter—
`nating current signalling in which different
`signals are represented by AC tones ap-
`plied to the transmission path. This form of
`signalling can itself be classified into in—
`band signalling and out—band signalling.
`In the former case, the tones are con—
`veyed within the bandwidth allocated to
`each speech channel, and in the latter case,
`conveyed in a gap of 900 Hz between
`speech channels. Signals can be distinctly
`conveyed as specific tones, and the dura-
`tion of the tone can also serve to define the
`
`,
`specific meaning.
`It has been usual for signalling to be
`carried on the actual transmission channel
`
`or on a subsidiary channel directly associ—
`ated with it. Various names are given to this
`technique,
`including: associated channel
`signalling, channel associated signalling, de-
`centralized signalling.
`.
`tech—
`With the use of modern digital
`niques,
`specific signalling channels are
`designated to control a large number of
`transmission paths. This is particularly
`used between exchanges. Terms applied to
`this technique include: common channel
`signalling, centralized control signalling,
`separate channel signalling, non—associated
`signalling. In such systems, each signal is
`represented by a group of bits which in—
`clude the channel
`identification and the
`specific signal
`intended. The exchanges
`concerned in such a system are able to carry
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