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`Cisco v. TQ Delta
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`Page 1 of 3
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`CSCO-1020
`Cisco v. TQ Delta
`Page 1 of 3
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`Library of Congress Cataloging in Publication Data
`Tanenbaum, Andrew S. 1944-.
`Computer networks / Andrew S. Tanenbaum. -— 3rd ed.
`p.
`cm.
`Includes bibliographical references and index.
`ISBN 0-13-349945—6
`l.Computer networks.
`TK5105.5.T36 1996
`004.6-—dc20
`
`I. Title.
`
`96-4121
`CIP
`
`Editorial/production manager: Camille Trentaeoste
`Interior design and composition: Andrew S. Tanenbaum
`Cover design director: Jerry Vorta
`Cover designer: Don Martinetti, DM Graphics, Inc.
`Cover concept; Andrew S. Tanenbaum, from an idea by Marilyn Tremaine
`Interior graphics: Hadel Studio
`Manufacturing manager: Alexis R. Heydt
`Acquisitions editor: Mary Franz
`Editorial Assistant: Noreen Regina
`
`© 1996 by Prentice Hall PTR
`Prentice—Hall, Inc.
`’
`A Simon & Schuster Company
`Upper Saddle River, New Jersey 07458
`The publisher offers discounts on this book when ordered in bulk quantities. For more information,
`contact:
`Corporate Sales Department, Prentice Hall PTR, One Lake Street, Upper Saddle River, NJ 07458.
`Phone: (800) 3823419; Fax: (201) 236-7141. E-mail: corpsales@prenhall.com
`
`All rights reserved. No part of this book may be reproduced, in any form or by any means, without
`permission in writing from the publisher.
`
`All product names mentioned herein are the trademarks of their respective owners.
`
`Printed in the United States of America
`10
`9
`8
`7
`6
`5
`
`ISBN 0-13-349945-6
`
`Prentice-Hall International (UK) Limited, London
`Prentice—Hall of Australia Pty. Limited, Sydney
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`Page 2 of 3
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`

`
`IHAP. 2
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`r opera-
`uld then
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`lephone,
`1 service
`the U.S.
`ng.
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`g signal-
`dvanced
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`t first be
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`i back to
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`igital by
`:ment
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`ipment
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`)1‘ leased
`e and are
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`.h analog
`
`SEC. 2.4
`
`THE TELEPHONE SYSTEM
`
`109
`
`transmission and examine how modems make it possible to transmit digital data
`over analog circuits. We will also look at two common modem interfaces, RS-
`232—C and RS-449.
`
`Transmission Impairments
`
`Analog signaling consists of varying a voltage with time to represent an infor-
`mation stream.
`If transmission media were perfect, the receiver would receive
`exactly the same signal that the transmitter sent. Unfortunately, media are not
`perfect, so the received signal is not the same as the transmitted signal. For digi-
`tal data, this difference can lead to errors.
`Transmission lines suffer from three major problems: attenuation, delay dis-
`tortion, and noise. Attenuation is the loss of energy as the signal propagates out-
`ward. On guided media (e.g., wires and optical fibers), the signal falls off loga-
`rithmically with the distance. The loss is expressed in decibels per kilometer.
`The amount of energy lost depends on the frequency. To see the effect of this fre-
`quency dependence, imagine a signal not as a simple waveform, but as a series of
`Fourier components. Each component is attenuated by a different amount, which
`results in a different Fourier spectrum at the receiver, and hence a different signal.
`If the attenuation is too much, the receiver may not be able to detect the signal
`at all, or the signal may fall below the noise level.
`In many cases, the attenuation
`properties of a medium are known, so amplifiers can be put in to try to compen-
`sate for the frequency-dependent attenuation. The approach helps but can never
`restore the signal exactly back to its original shape.
`It is caused by the
`The second transmission impairment is delay distortion.
`fact that different Fourier components travel at different speeds. For digital data,
`fast components from one bit may catch up and overtake slow components from
`the bit ahead, mixing the two bits and increasing the probability of incorrect
`reception.
`The third impainnent is noise, which is unwanted energy from sources other
`than the transmitter. Thermal noise is caused by the random motion of the elec-
`trons in a wire and is unavoidable. Cross talk is caused by inductive coupling
`between two wires that are close to each other. Sometimes when talking on the
`telephone, you can hear another conversation in the background. That is cross
`talk, Finally, there is impulse noise, caused by spikes on the power line or other
`causes. For digital data, impulse noise can wipe out one or more bits.
`
`Modems
`
`Due to the problems just discussed, especially the fact that both attenuation
`and propagation speed are frequency dependent, it is undesirable to have a wide
`range of frequencies in the signal. Unfortunately, square waves, as in digital data,
`
`Page 3 of 3