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`Cisco v. TQ Delta
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`CSCO-1016
`Cisco v. TQ Delta
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`Library of Congress Cataloging-in-Publication Data
`
`Goralski, Walter
`ADSL and DSL technologies I Walter Goralski.
`p.
`cm.
`Includes index.
`ISBN 0-07-024679-3
`1. Telephone switching systems, Electronic. 2. Digital
`I. Title.
`communications. 3. Telecommunication-Standards.
`TK6397.G65 1998
`621.387-dc21
`
`98-5178
`CIP
`
`McGraw-Hill
`A Division ofTheMcGraw·HiUCompanies ~
`
`Copyright © 1998 by The McGraw-Hill Companies, Inc. All rights reserved.
`Printed in the United States of America. Except as permitted under the United
`States Copyright Act of 1976, no part of this publication may be reproduced or
`distributed in any form or by any means, or stored in a data base or retrieval
`system, without the prior written permission of the publisher.
`
`5 6 7 8 9 0 DOC/DOC 9 0 3 2 1 0 9
`
`ISBN 0-07-024679-3
`
`The sponsoring editor for this book was Steven Elliot and the production
`supervisor was Sherri Souffrance. It was set in Century Schoolbook by Douglas
`& Gayle, Limited.
`
`Printed and bound by R. R. Donnelley & Sons Company.
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`neither McGraw-Hill nor its authors guarantees the accuracy or completeness
`of any information published herein and neither McGraw-Hill nor its authors
`shall be responsible for any errors, omissions, or damages arising out of use of
`this information. This work is published with the understanding that
`McGraw-Hill and its authors are supplying information but are not attempt(cid:173)
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`
`Page 2 of 5
`
`

`
`The Asymetric Digital Subscriber Line (ADSL) Architecture
`
`187
`
`-
`
`nple QAM example
`ves. If the four am(cid:173)
`ent signal types are
`ides combined with
`\1 sin (Ft) + Al cos
`Y signal types. This
`duced in all modem· ·
`ne). A sample "con(cid:173)
`'.luch more common
`>ws have been used
`ices.
`12 different phase
`;ine components (a
`signal, in this case
`3 greater (or fewer)
`ie number of phase
`:illation points and
`ke all multibit en(cid:173)
`ls that can be dis-
`
`ital video informa-
`into 4-bit nibbles
`~AM/CAP with 16
`'he nibbles are fed
`amplitudes for the
`
`. modulator, which
`:i.tor actually com-
`
`1al
`bits)
`
`bines the appropriate amplitudes of the sine and cosine of the carrier fre(cid:173)
`quency, thus creating the phase and amplitude shifts associated with the
`correct constellation point. Finally, the signal is filtered to assure that it
`does not interfere with other channels. Note that the resulting signal con(cid:173)
`tains substantial energy at the carrier frequency; this should not be sur(cid:173)
`prising.
`
`DMTforADSL
`
`ADSL devices (the ATU-C and ATU-R) have been built that use QAM,
`CAP, and DMT technology as line codes. However, the official standard
`line code for ADSL is DMT, as defined by the American National Stan(cid:173)
`dards Institute ANSlTl.413 standard in 1995 for ADSL. Although DMT
`is often said to be "newer" than CAP or QAM, DMT was actually invented
`years ago by Bell Labs. DMT was never implemented until recently for
`many reasons, not the least of which was that CAP and QAM were suffi(cid:173)
`cient for all telecommunications purposes common at the time.
`DMT works by first dividing the entire b,andwidth range on the for(cid:173)
`merly analog passband limited local loop into a large number of equally
`spaced subchannels. Technically, they are called subcarriers, but many
`people still call them subchannels. Above the preserved baseband analog
`signaling range, this bandwidth usually extends to 1.1 MHz. The entire
`1.1 MHZ bandwidth is divided into 256 subchannels, starting at 0 Hz.
`Each subchannel occupies 4.3125 kHz, giving a total bandwidth of 1.104
`MHz on the loop. Some of the subchannels are special, and others are not
`used at all. For example, channel #64 at 276 kHz is reserved for a pilot
`signal.
`Most DMT systems use only 250 or 249 subchannels for information.
`The lower subchannels, #1 through #6 in most cases, are reserved for 4
`kHz passband analog voice. Because 6 times 4.3125 Hz is 25.875 kHz, it
`is common to see 25 kHZ as the starting point for ADSL services. Note
`that a wide guardband is used between the analog voice and the DMT sig(cid:173)
`nals. In addition, the signal loss at the upper subchannels, such as #250
`and above, is so great that it is difficult to use them for information trans(cid:173)
`fer on long loop at all.
`There are 32 upstream channels, usually starting at channel #7, and
`250 downstream channels, which gives ADSL its distinct asymmetric
`bandwidth. Each of the subchannels is 4.3125 kHz wide, of course, and
`only when echo cancellation is used are there actually 250 downstream
`
`Page 3 of 5
`
`

`
`TheAsymetI --(cid:173)
`
`Figure 8-6
`DMT in operatior
`
`-
`
`188
`
`Chapter Eight
`
`subchannels. When only FDM is used for echo control, there are typically
`32 upstream c~annels and only 218 or less downstream channels because
`they no longer overlap. The upstream channels occupy the lower end of
`the spectrum for two reasons. First, the signal attenuation is less here,
`and customer transmitters are typcially lower-powered than local ex(cid:173)
`change transmitters, which is a concern. Second, there is more noise at
`the local exchange with the possibility of crosstalk, so it only makes sense
`to use the lower portions of the frequency range for the upstream signals.
`When ADSL devices that employ DMT are activated, each of the sub(cid:173)
`channels is "tested" by the end devices for attenuation. In actual practice,
`the "testing" is a complex kind of handshaking procedure, and the para(cid:173)
`meter used is gain (the reciprocal of the attenuation). The noise present
`in each of the subchannels is measured as well.
`Not all of the subchannels are used for information transfer, as men(cid:173)
`tioned above. Some are reserved for network management and perfor(cid:173)
`mance measurement functions. For instance, in the downstream direction,
`only 249 of the 256 subchannels available downstream are typically used
`for information. transfer.
`Usually, each of the numerous subchannels employs its own coding
`technique based on QAM. This may strike some as odd, given the fervor
`that vendors have when seeking to distinguish CAP/QAM and DMT. Nev(cid:173)
`ertheless, there obviously is at some of QAM in DMT. The real attraction
`of DMT is not so much that it is different than CAP and QAM, but rather
`that based on DMT's performance monitoring, some subchannels will
`carry more bits per baud than others. The total throughput is the sum of
`all the QAM bits sent on all the active subchannels (some may be com(cid:173)
`pletely "turned off").
`Moreover, all of the subchannels are constantly monitored for perfor(cid:173)
`mance and errors. The speed of an individual subchannel or group of sub(cid:173)
`channels can actually vary, giving DMT a granularity of 32 kbps. In other
`words, a DMT device might function at 768 kbps or 736 kbps (that is, 32
`kbps less), depending on operational and environmental conditions. Just
`by way of comparison, CAP devices usually offer 340 kbps granularity
`(768 kbps or 428 kbps), but pure QAM can offer granularity as fine as 1
`bps, which means that there is nothing that technically limits CAP/QAM
`to one level of granularity but not another. In fact, some vendors of CAP(cid:173)
`based ADSL equipment have claimed 32 kbps granularity, and even
`RADSL capabilities, for their latest products. It should be noted that
`these CAP RADSL products modify their spectrum when the rate
`changes, and now become a real issue to manage with regard to spectral
`compatibility.
`
`Page 4 of 5
`
`

`
`---(cid:173)
`
`Figure 8-6
`DMT in operation
`
`tpter Eight
`
`' are typically
`mels because
`lower end of
`L is less here,
`:ian local ex(cid:173)
`nore noise at
`makes sense
`ream signals.
`:h of the sub(cid:173)
`tual practice,
`md the para(cid:173)
`:ioise present
`
`sfer, as men(cid:173)
`G and perfor(cid:173)
`am direction,
`ypically used
`
`; own coding
`en the fervor
`LdDMT. Nev(cid:173)
`'al attraction
`v.I, but rather
`hannels will
`is the sum of
`may be com-
`
`'d for perfor-
`51"ou p of sub(cid:173)
`bps. In other
`s (that is, 32
`.ditions. Just
`granularity
`·as fine as 1
`;s CAP/QAM
`dors ofCAP(cid:173)
`;y, and even
`"noted that
`en the rate
`d to spectral
`
`The Asymetric Digital Subscriber Line (ADSL) Architecture
`
`189
`
`Ideal bits/channel
`
`Typical Loop Gain
`
`Actual bits/channel
`
`Frequency Range
`
`Frequency Range
`
`Frequency Range
`
`Ideal bits/channel
`
`Real Loop Gain
`
`Typical bits/channel
`
`Frequency Range
`
`Frequency Range
`
`Frequency Range
`
`("Notch" from bridged tap
`Noise from AM radio station)
`
`Experts generally concede that finer granularity is a benefit that can
`maximize user acceptance and deployment situations.
`
`Discrete Multitone (DMT)
`Operation
`
`Figure 8-6 shows discrete multitone technology in operation in an ADSL
`device on a typical local loop. The figure actually has two parts. The up(cid:173)
`per shows a kind of ideal situation, such as that found in a straight run
`of 24 gauge copper wire less than 18,000 feet without a lot of outside noise
`(good luck finding one of those). The only real attenuation effects come
`from the distances and frequencies involved. The lower part of the figure
`shows a typical local loop in the real world.
`Consider the design ideal first.Across the frequency range, on the left,
`there exists a targeted maximum number of bits per second per subcar(cid:173)
`rier (channel) that the device would like to send and receive. However, the
`middle figure shows the situation on a typical loop. The gain (the recip(cid:173)
`rocal of the attenuation) is better or worse depending on frequency. At
`higher frequencies, distance effects domi:riate; at lower frequencies, im(cid:173)
`pulse noise and crosstalk dominate. This leaves a broad middle range
`(about 25 kHz to 1.1 MHz) for signals, with the gain slowly dropping off
`with increasing frequency.
`
`Page 5 of 5