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`Second Edition
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`George Abe
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`Residential Broadband,
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`Indianapolis, IN 46290 USA
`
`Clscu Smms
`Mfi)
`CIsco PRESS
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`Cisco Press
`201 West 103rd Street
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`All rights reserved. No part of this book may be reproduced or transmitted in any form or by any means, electronic or
`mechanical, including photocopying, recording, or by any information storage and retrieval system, without written
`permission from the publisher, except for the inclusion of brief quotations in a review.
`Printed in the United States of America 1 2 3 4 5 6 7 8 9 0
`Library of Congress Catalogingein—Publication Number: 99-64088
`ISBN: 1-57870-177-5
`
`Warning and Disclaimer
`
`Trademark Acknowledgments
`
`All terms mentioned in this book that are known to be trademarks or service marks have been appropriately capitalized.
`Cisco Press or Cisco Systems, Inc. cannot attest to the accuracy of this information. Use of a term in this book should
`
`
`
`Residential Broadband, Second Edition
`George Abe
`Copyright © 2000 Cisco Press
`
`Cisco Press logo is a trademark of Cisco Systems, lnc.
`Published by:
`Cisco Press
`201 West 103rd Street
`Indianapolis, TN 46290 USA
`
`This book is designed to provide information about residential broadband. Every effort has been made to make this book
`as complete and as accurate as possible, but no warranty or fitness is implied.
`The information is provided on an “as is” basis. The author, Cisco Press, and Cisco Systems, Inc., shall have neither
`liability nor responsibility to any person or entity with respect to any loss or damages arising from the information
`contained in this book or from the use of the discs or programs that may accompany it.
`The opinions expressed in this book belong to the author and are not necessarily those of Cisco Systems, Inc.
`
`not be regarded as affecting the validity of any trademark or service mark.
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`0 MHZ
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`Frequency
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`Multicarrier techniques have a latency penalty (time delay to transmit a digital bit) compared
`with single carrier. In the DMT case for ADSL, there are 256 subbands of 4 kHz each. So no
`bit can travel faster than allowed by 4 kHz, even if the line was perfectly clean.
`-
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`Considerations in Selecting Modulation Techniques
`Selection of modulation technique for each Access Network has been highly contentious, partly
`because there’s a lot of money at stake. Standards organizations for cable TV, XDSL, and HDTV
`have spent years arguing the requirements of modulation, let alone the choice. While
`commercial self-interest, academic background, national pride, embedded base and personal
`ego play a role, there are engineering and cost tradeoffs to consider as well.
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` Modulation Techniques 69
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`Figure 2-7 Multicarrier Modulation
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`One of the noisiest debates about modulation techniques is between proponents of DMT and
`proponents of CAP for use in ADSL. DMT for ADSL uses 256 subbands, whereas CAP uses a
`single carrier with amplitude modulation, very similar to QAM. At the time of this writing, CAP
`has an advantage over DMT in that it consumes less power (thereby generating less heat) and
`costs less because it is more mature (more units in the field, greater integration). It is easy to see
`how DMT scales and why DMT has been selected by ANSI T1E1.4 and the International
`Telecommunications Union (ITU). Furthermore, a number of US. telephone companies have
`selected DMT. Because of these factors (and because of commercial issues with respect to the
`licensing of CAP), it appears DMT is gaining the upper hand for ADSL.
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`US. Digital Over—the—Air Broadcast Vestigial Sideband (VSB)
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`US. Digital Cable Forward Channels QAM—64, QAM-256
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`US. Digital Cable Return Channels QPSK
`European Digital Over-the-Air Broadcast OFDM
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`High Bit Rate Digital Subscriber Line (HDSL) 2B1Q
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`Asymmetric Digital Subscriber Line (ADSL) DMT, CAP
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`reception. Either way, the results are market confusion and additional costs.
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`70 Chapter 2: Technical Foundations of Residential Broadband
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`Some of the majors engineering considerations are listed here:
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`'
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`Scale——Will the modulation support large systems and fast bit rates?
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`° Noise immunity—Can the modulation scheme operate reliably in with real-world
`impairments?
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`Packaging—Can Application Specific Integrated Circuits (ASICs) be built? Can
`implementations be used in a variety of environments, such as different Access Networks?
`How large are the components, and how much power does the technique consume?
`' Performance—What is the spectral efficiency? What is the latency?
`' Cost—This is the dominant factor when dealing with consumer markets.
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`Table 2-2
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`The modulation schemes described in this chapter are likely to be residential broadband
`alternatives. Table 2-2 lists services and their respective modulation schemes, current as of this
`writing.
`Modulation Techniques for Current Services
`Modulation Technique
`Service
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`2B1Q
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`ISDN (United States)
`US. Direct Broadcast Satellite QPSK
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`Viewpoint: Interoperability of Modulation Techniques
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`The proliferation of modulation techniques raises interoperability problems for most consumer
`electronics devices. For digital TV, for example, the likelihood now exists that a television built
`for over—the—air digital broadcasts will not be capable of receiving a cable TV digital
`transmission without a separate box.
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`The consumer could end up with three set tops: an analog NTSC descrambler for analog cable,
`a VSB MPEG decoder for digital over-the—air reception, and a QAM MPEG decoder for digital
`cable reception. Or there will be new generations of TV with input jacks for all three types of
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`176 Chapter 4: xDSL Access Networks
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`ADSL Modulation
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`Although CAP modulation is well-understood and relatively inexpensive, some argue that it is
`difficult to scale because it is a single-carrier modulation technique and is susceptible to
`narrowband interference. DMT uses multiple carriers and is standardized by the ANSI
`committee T1E1.4 (document T1.413) and ITU G.992.1, or G.dmt.
`
`This standard calls for 256 subbands of 4 kHz each, thereby occupying 1,024 GHz. Each
`subband can be modulated with QAM 64 for clean subbands, down to QPSK. If each of the
`subbands can support QAM—64 modulation, then the forward channel supports 6.1 Mbps. On
`the return path are 32 subbands, with a potential for 1.5 Mbps.
`
`Adaptive Equalization
`Adaptive equalizers are amplifiers that shape frequency response to compensate for attenuation
`and phase error. Adaptive equalization requires that the modems learn line characteristics—and
`do so by sending probes and looking at the return signals. The equalizer then knows how it must
`amplify signals to get a nice flat frequency response. The greater the dynamic range, the more
`complex the equalization. ADSL requires 50 dB of dynamic range, making adaptive equali—
`zation complicated. Only with recent advances in digital signal processing (number crunching)
`has it become possible to have such equalization in relatively small packaging.
`
`chips. Numbers are mostly proprietary at this point, but it is estimated that a single transceiver
`
`CAP and DMT Compared
`CAP is a single—carrier technique that uses a wide passband. DMT is a multiple-carrier
`technique that uses many narrowband pas sbands as individual carriers. The two have a number
`of engineering differences, even though they ultimately can offer similar service to the network
`layers discussed previously.
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`Adaptive equalization is required for CAP because noise characteristics vary significantly
`across the frequency passband. Adaptive equalization is not needed for DMT because noise
`characteristics do not vary across any given 4 kHz subband. A major issue in comparing DMT
`with CAP is determining the point at which the complexity of adaptive equalization surpasses
`the complexity of DMT’s multiple Fourier transform calculations. This is determined by further
`implementation experience.
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`Power Consumption
`Although DMT clearly scales to RBB and does not need adaptive equalization, other factors
`must be considered. First, with 256 channels, DMT has a disadvantage regarding power
`consumption (and, therefore, cost) when compared with CAP. DMT has a high peak to average
`power ratio because the multiple carriers can constructively interfere to yield a strong signal.
`DMT has higher computational requirements, resulting in more transistor in the transceiver
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`Asymmetric DSL 177
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`will consume 5 watts of poWer, even with further advances. Power consumption is important
`because hundreds or thousands (as carriers dearly hope) of transceivers might be at the CO. This
`would require much more heat dissipation than CAP requires.
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`DMT appears to have the speed advantage over CAP, Because narrow carriers have relatively
`few equalization problems, more aggressive modulation techniques can be used on each
`channel. For CAP to achieve comparable bit rates, it might be necessary to use more bandwidth,
`far above 1 MHz. This creates new problems associated with high frequencies on wires and
`would reduce CAP’s current advantage in power consumption.
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`Higher peak/average, but will
`Lower, fewer gates
`Power consumption
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`likely narrow gap
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`Forward carriers 2561
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`32
`Return carriers
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`Increment
`32 Kb
`20 Kb
`Needed
`Adaptive equalizers
`None
`Licensing Globespan
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`Standardization In process
`Key competitors
`Globespan, Paradyne, Westell
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`Licensing
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`DMT is a public, open standard. Globespan Technologies (formerly ATT Paradyne) is the
`licensing agent for CAP. As of this writing, 20 companies have been issued licenses. Among the
`licensees are Bellcore, Westell, Nokia (Finland), and NEC (Japan). One of the marketing
`difficulties of CAP is that system providers are reluctant to license the intellectual property
`from a single source; it makes them feel vulnerable. If there were more licensors of the
`technology, perhaps CAP would have fared better.
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`Overview of CAP Versus DMT Summary
`This discussion has tried to fairly represent the CAP/DMT debate. Without a doubt, advances
`will be made in both technologies, which will narrow the various technical gaps. Table 4-3
`summarizes the important differences as they exist at the time of this writing.
`Comparison of CAP and DMTfor ADSL
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`CAP
`DMT
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`1 3
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`Many sources
`ITU and ANSI
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`Conexant, Cisco, Alcatel, Amati
`(now Texas Instruments),
`Westell, Efficient Networks
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