xDSL Local Loop Access Technology

T e c h n i c a l P a p e r

Delivering Broadband over Copper Wires

11

xDSL Local Loop Access Technology

Delivering Broadband over Copper Wires

Contents

What Are Digital Subscriber Line (xDSL) Services? 2

Development History 2

Different Types of xDSL and How They Work 2

Asymmetric Digital Subscriber Line (ADSL) 3

Rate-Adaptive Digital Subscriber Line (R-ADSL) 3

ADSL Lite 3

How ADSL Modems Work (sidebar) 4

ISDN Digital Subscriber Line (IDSL) 4

High Bit-Rate Digital Subscriber Line (HDSL) 4

Single-Line Digital Subscriber Line (SDSL) 5

Very High Bit-Rate Digital Subscriber Line (VDSL) 5

xDSL Delivers Broadband over Copper 5

Technology and Applications Comparison 5

DSL Modulation Schemes (box) 6

56 Kbps Analog Modems 6

ISDN 7

Cable Modems 7

xDSL 8

ADSL Development and Deployment Progress 8

Getting Started with ADSL 8

ADSL Suppliers 9

Network Design: What’s Needed 9

Upgrading Digital Loop Carriers (DLCs) (sidebar) 9

Conclusion 10

Glossary of Related Terms 10

xDSL Local Loop AccessTechnology

Delivering Broadband over Copper Wires

By Robyn Aber

Today’s environment is ripe for the emergence ofdigital subscriber line (xDSL) technologies. Theuse of more multimedia information on theInternet and World Wide Web by business andresidential users is a major growth factor.Another is the availability of affordable net-working equipment that enables larger numbersof users to access corporate information fromremote sites.

The opening of the telecommunicationsindustry in the United States and throughout theworld is sparking the entry into new servicedelivery by incumbent local exchange carriers(ILECs), interexchange carriers (IECs), Internetservice providers (ISPs), competitive localexchange carriers (CLECs), and satellite andcable companies. Mixed media networking, theneed for affordable broadband transmissionrates, and a competitive telecom service environ-ment all contribute to making xDSL the righttechnology at the right time. xDSL servicespromise to dramatically increase the speed of cop-per wire–based transmission systems withoutrequiring expensive upgrades to the local loopinfrastructure. New xDSL services are beingreadied to join the bandwidth race.

This paper describes the different xDSLtechnologies in development today and comparesthem to other current and emerging WAN servicetechnologies. It also reports on current and futureworldwide xDSL deployments and gives somemarket introduction projections. Finally, thepaper describes 3Com’s strategic direction withrespect to the emerging xDSL technology market.

What Are Digital Subscriber Line (xDSL)

Services?

xDSL services are dedicated, point-to-point,public network access technologies that allowmultiple forms of data, voice, and video to becarried over twisted-pair copper wire on thelocal loop (“last mile”) between a network ser-vice provider’s (NSP’s) central office and the

customer site, or on local loops created eitherintra-building or intra-campus. xDSL isexpected to have a significant impact in thenext three years by supporting high-speedInternet/intranet access, online services, video-on-demand, TV signal delivery, interactiveentertainment, and voice transmission toenterprise, small office, home office, and, ulti-mately, consumer markets. The major advan-tage of high-speed xDSL services is that theycan all be supported on ordinary copper tele-phone lines already installed in most commer-cial and residential buildings.

Development History

xDSL was designed initially to provide video-on-demand and interactive TV applicationsover twisted-pair wires. Interest in copper-based digital subscriber line services wasspurred when fiber-based broadband loopsproved to be too costly for widespread deploy-ment. Another boost came with the passage ofthe Telecommunications Reform Act of 1996,which allows local phone companies, long-dis-tance carriers, cable companies, radio/televi-sion broadcasters, Internet/online serviceproviders, and telecommunications equipmentmanufacturers in the United States to competein one another’s markets. The race to providebroadband bandwidth was on.

In xDSL, telecommunications companiessee an opportunity to leverage customerdemand for faster data access that has resultedfrom the explosive growth of the Internet andthe advent of IP telephony. xDSL has thepotential to deliver high-speed data access andmuch more. xDSL technology is in the earlystages of commercial availability. The keyplayers have agreed on standards and continueto work out interoperability, provisioning, andoperations issues.

Different Types of xDSL and How They Work

The “x” in xDSL stands for the various kindsof digital subscriber line technologies, includ-ing ADSL, R-ADSL, HDSL, SDSL, andVDSL. To fully grasp the significance of thesetechnologies and the applications for whicheach is best suited, it is important to under-stand how they differ. Key points to keep in

22

mind are the trade-offs between signal dis-tance and speed, and the differences in sym-metry of upstream and downstream traffic.Figure 1 shows that xDSL is used only in thelocal loop in an end-to-end remote accessarchitecture.

Table 1 on page 4 compares the differenttypes of xDSL technologies along with com-peting technologies, including 56 Kbps analogdial-up, cable modems, and Integrated Ser-vices Digital Network (ISDN).

Asymmetric Digital Subscriber Line (ADSL)

ADSL technology is asymmetric. It allowsmore bandwidth downstream—from an NSP’scentral office to the customer site—thanupstream from the subscriber to the centraloffice. This asymmetry, combined with “alwayson” access (which eliminates call setup), makesADSL ideal for Internet/intranet surfing,video-on-demand, and remote local area net-work (LAN) access. Users of these applicationstypically download much more informationthan they send. Downstream, ADSL supportsspeeds between 1.5 and 8 Mbps; upstream,the rate is between 640 Kbps and 1.54 Mbps.ADSL can provide 1.54 Mbps transmissionrates at distances of up to 18,000 feet over onewire pair. Optimal speeds of 6 to 8 Mbps canbe achieved at distances of 10,000 to 12,000feet using standard 24-gauge wire.

Rate-Adaptive Digital Subscriber Line (R-ADSL)

R-ADSL operates within the same transmissionrates as ADSL, but adjusts dynamically tovarying lengths and qualities of twisted-pairlocal access lines. With R-ADSL, it is possibleto connect over different lines at varying speeds.Connection speed can be selected when theline synchs up, during a connection, or as theresult of a signal from the central office.

ADSL Lite

ADSL Lite is a lower-speed version of ADSLthat will eliminate the need for the telco toinstall and maintain a premises-based POTSsplitter. Elimination of the POTS splitter isintended to simplify DSL installation andreduce the costs of DSL for NSPs. ADSL Liteis also supposed to work over longer distancesthan full-rate ADSL, making it more widelyavailable to mass market consumers. It willsupport both data and voice and provide anevolution path to full-rate ADSL.

The effort to introduce ADSL Lite hasbeen spearheaded by the Universal ADSLWorking Group, an industry group that workedto develop a worldwide G.Lite standard withinthe International Telecommunications Union(ITU) Study Group 15. An ITU standard(G.992.2) was approved in October, 1998.Additional standards work can be expected inANSI TIE1.4, the ATM Forum, and the

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Corporate

network

Internet

NSPs

≤1.54 Mbps

Enterprise users

Residential users

≤45 Mbps Regional broadband

network

xDSL access

network

xDSL line

xDSL line

xDSL LAN CPE

xDSL CPE

NSPs

Figure 1. xDSL in the End-to-End Network

ADSL Forum to address issues such as com-patibility with home wiring and networkinterfaces. 3Com is an active participant inthese standards bodies working on the devel-opment of ADSL Lite.

ISDN Digital Subscriber Line (IDSL)

IDSL provides full duplex throughput atspeeds up to 144 Kbps. Unlike ADSL, IDSLis restricted to carrying data only. While IDSLuses the same 2B1Q modulation code asISDN to deliver service without special lineconditioning, it differs from ISDN in a num-ber of ways. Unlike ISDN, IDSL is a non-switched service, so it does not cause switch

congestion at the service provider’s CO.ISDN also requires call setup, while IDSLdoes not (DSL is an “always on” service).

High Bit-Rate Digital Subscriber Line (HDSL)

HDSL technology is symmetric, providing thesame amount of bandwidth upstream asdownstream. HDSL is the most mature of thexDSL technologies, and has already beenimplemented in telco feeder plants (lines thatextend from central offices to remote nodes)and also in campus environments. Due to itsspeed—1.544 Mbps over two copper pairsand 2.048 Mbps over three pairs—telcos com-monly deploy HDSL as an alternative to

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How ADSL Modems Work

To create multiple channels,

ADSL modems divide the

available bandwidth of a

telephone line using one of

two methods: frequency

division multiplexing (FDM)

or echo cancellation.

FDM assigns one band for

upstream data and another

band for downstream data.

The downstream path is

then divided by time

division multiplexing (TDM)

into one or more high-speed

channels and one or more

low-speed channels. The

upstream path is also multi-

plexed into corresponding

low-speed channels.

Echo cancellation assigns

the upstream band to

overlap the downstream

band and separates the two

by means of local echo can-

cellation, the same

technique used by V.32 and

V.34 modems. Echo cancel-

lation uses bandwidth more

efficiently, but increases

complexity and cost. For

both FDM and echo cancel-

lation, a filter called a POTS

splitter front-ends an ADSL

modem to split off 4 kHz for

voice service (referred to as

plain old telephone service,

or POTS). This means that

both POTS and ADSL can be

transmitted on the same

wire, eliminating the need

to have a separate POTS

line for voice communi-

cation.

Table 1. Technology and Application Comparison

Distance Limitation

Technology Speed (24-gauge wire) Applications

56 Kbps analog 56 Kbps downstream None E-mail, remote LANmodems 28.8 or 33.6 Kbps upstream access, Internet/intranet access

ISDN Up to 128 Kbps 18,000 feet (additional Video conferencing, disaster(uncompressed) equipment can extend the recovery, leased line backup,Full duplex distance) transaction processing, call

center services, Internet/intranet access

Cable modem 10–30 Mbps downstream 30 miles over coaxial Internet access128 Kbps–10 Mbps (additional equipment canupstream (shared, not extend the distance todedicated, bandwidth) 200 miles)

ADSL Lite Up to 1 Mbps downstream 18,000 feet Internet/intranet access, WebUp to 512 Kbps upstream browsing, IP telephony, video

telephony

ADSL/R-ADSL 1.5–8 Mbps downstream 18,000 feet Internet/intranet access, video-Up to 1.544 Mbps upstream (12,000 feet for fastest on-demand, remote LAN

speeds) access, VPNs, VoIP

IDSL Up to 144 Kbps full duplex 18,000 feet (additional Internet/intranet access, Webequipment can extend the browsing, IP telephony, videodistance) telephony

HDSL 1.544 Mbps full duplex (T1) 12,000–15,000 feet Local, repeatered T1/E1 trunk 2.048 Mbps full duplex (E1) replacement, PBX interconnection,(uses 2–3 wire pairs) Frame Relay traffic aggregator,

LAN interconnect

SDSL 1.544 Mbps full duplex (T1) 10,000 feet Local, repeatered T1/E1 trunk2.048 Mbps full duplex (E1) replacement, collaborative(uses 1 wire pair) computing, LAN interconnect

VDSL 13–52 Mbps downstream 1,000–4,500 feet Multimedia Internet access,1.5–2.3 Mbps upstream (depending on speed) high-definition television(up to 34 Mbps if symmetric) program delivery

Source: 3Com, March 1998.

repeatered T1/E1. (T1 lines, used in NorthAmerica, have a data rate of 1.544 Mbps; E1lines, used in Europe, have a data rate of2.048 Mbps.) Although HDSL’s 12,000 to15,000-foot operating distance is shorter thanADSL’s, phone companies can install signalrepeaters to cost-effectively extend its usefulrange. HDSL’s reliance on two and threetwisted-pair wires makes it ideal for connect-ing PBX systems, digital local loops, IECpoints of presence (POPs), Internet servers,and campus-based networks. HDSL II is pro-posed as the next-generation HDSL withinANSI and ETSI. It will offer the same perfor-mance as HDSL, but over a single pair.

Single-Line Digital Subscriber Line (SDSL)

Like HDSL, SDSL supports symmetricalTI/E1 transmissions, but SDSL differs fromHDSL in two important ways: it uses a singlecopper-pair wire, and it has a maximum oper-ating range of 10,000 feet. Within its distancelimitation, SDSL is capable of accommodat-ing applications that require identical down-stream and upstream speeds, such as videoconferencing or collaborative computing.SDSL is a precursor to HDSL II.

Very High Bit-Rate Digital Subscriber

Line (VDSL)

VDSL technology is the fastest xDSL technol-ogy, supporting a downstream rate of 13 to 52Mbps and an upstream rate of 1.5 to 2.3Mbps over a single copper-pair wire. VDSLcan be viewed as a cost-effective alternative tofiber to the home. However, the maximumoperating distance for this asymmetric tech-nology is only 1,000 to 4,500 feet from thecentral office; this distance can be extended byrunning fiber optic cable from the CO to anoptical network unit and copper from thatpoint to the user location up to 4,500 feetaway. In addition to supporting the sameapplications as ADSL, VDSL’s additionalbandwidth could potentially enable NSPs todeliver high-definition television (HDTV),video-on-demand, and switched digital video,as well as legacy LAN extension symmetricalservices. VDSL is in the requirements andstandards definition stage.

xDSL Delivers Broadband over Copper

The best thing about xDSL technologies istheir ability to transport large amounts ofinformation across existing copper telephonelines. This is possible because xDSL modemsleverage signal processing techniques thatinsert and extract more digital data onto ana-log lines. The key is modulation, a process inwhich one signal modifies the property ofanother (see box on page 6).

In the case of digital subscriber lines, themodulating message signal from a sendingmodem alters the high-frequency carrier signalso that a composite wave, called a modulatedwave, is formed (Figure 2). Because this high-frequency carrier signal can be modified, alarge digital data payload can be carried in themodulated wave over greater distances than onordinary copper pairs. When the transmissionreaches its destination, the modulating mes-sage signal is recovered, or demodulated, bythe receiving modem.

Technology and Applications Comparison

There has been a lot of speculation in theindustry about which remote access technolo-gies will succeed and which will fail. As newlocal access technologies are rolled out, theydo not displace others; actually, the reverse istrue. Technologies like analog dial-up,dedicated leased lines, Frame Relay, and ISDNall coexist successfully in the market based ondifferences in service availability and on theirability to generate incremental revenue byserving different applications.

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Originalwaveform

Modulatedwaveform

Voltage

Time

Figure 2. An Amplitude-Shifted Sine Wave

There are many ways to alter the high-frequencycarrier signal that results in a modulated wave.For ADSL, the most talked-about xDSL tech-nology, there are two competing modulationschemes: carrierless amplitude phase (CAP)modulation and discrete multi-tone (DMT)modulation. CAP and DMT use the same fun-damental modulation technique—quadratureamplitude modulation (QAM)—but differ in theway they apply it.

QAM, a bandwidth conservation process rou-tinely used in modems, enables two digitalcarrier signals to occupy the same transmissionbandwidth. With QAM, two independentmessage signals are used to modulate twocarrier signals that have identical frequencies,but differ in amplitude and phase. QAMreceivers are able to discern whether to uselower or higher numbers of amplitude andphase states to overcome noise and inter-ference on the wire pair.

Carrierless Amplitude Phase (CAP)

Modulation

Generating a modulated wave that carriesamplitude and phase state changes is not easy.To overcome this challenge, the CAP version ofQAM stores parts of a modulated messagesignal in memory and then reassembles theparts in the modulated wave. The carrier signalis suppressed before transmission because itcontains no information and is reassembled atthe receiving modem (hence the word “carri-erless” in CAP). At start-up, CAP also tests thequality of the access line and implements themost efficient version of QAM to ensure satis-factory performance for individual signaltransmissions. CAP is normally FDM based.

CAP, a single carrier system, has severaladvantages: it is available today at 1.544Mbps (T1) speeds, and it is low on the costcurve due to its simplicity. It has the disad-vantage that it is not a bona fide AmericanNational Standards Institute (ANSI) orEuropean Telecom Standards Institute (ETSI)standard.

Discrete Multi-Tone (DMT) Modulation

DMT offers a multicarrier alternative to QAM.Because high-frequency signals on copperlines suffer more loss in the presence of noise,DMT discretely divides the available fre-quencies into 256 subchannels, or tones. Aswith CAP, a test occurs at startup to determinethe carrying capacity of each subchannel.Incoming data is then broken down into avariety of bits and distributed to a specificcombination of subchannels based on theirability to carry the transmission. To rise abovenoise, more data resides in the lower fre-quencies and less in the upper ones.

DMT’s main advantage is the fact that it is theANSI, ETSI, and ITU standard. But DMT alsohas drawbacks: it will initially be more costlythan CAP, and it is very complex. A variant ofDMT, discrete wavelet multi-tone (DWMT),goes a step further in complexity and per-formance by creating even more isolationbetween subchannels. When fully developed,DWMT could become the ADSL protocol ofchoice for long-distance transmission in envi-ronments with high interference. Otherversions of DMT, including Synchronized DMTand “Zipper” are being proposed for use withVDSL.

The fact that so many WAN services con-tinue to coexist often leads to confusion andcomplexity for enterprise network managersand planners. The range of services will cer-tainly continue into the next century. Factorsthat will determine the success of one technol-ogy versus another include availability, pric-ing, ease of installation and use, and relevanceto users’ applications. Some of the key issues

surrounding xDSL and competing technolo-gies are summarized in this section.

56 Kbps Analog Modems

56 Kbps analog modems (ITU V.90 standard)provide a range of midband (28.8 to 56 Kbps)access to the Internet, intranets, and remoteLANs.

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DSL Modulation Schemes

In order to realize 56 Kbps throughput,there must be a 56 Kbps modem using com-patible modulation techniques at each end ofthe connection. Therefore, NSPs and ISPsmust have V.90 modems at their points ofpresence. A single 56 Kbps modem at theuser’s site will deliver the next highest speedwith which it can synch up. Even when 56Kbps modems are installed at both the carrierand user sites, these modems achieve topspeeds only if the connection has just a singleanalog/digital conversion, and actual through-put is determined by line quality.

Another important fact to keep in mind isthat this technology is asymmetric. The 56Kbps rate is only achieved downstream on adigital line from the network to the user. Theupstream connection is analog and operates inthe 28.8 to 33.3 Kbps range.

ISDN

ISDN is also considered a digital subscriberline service. ISDN and xDSL technologiesshare some common technical characteristics:use of the existing telephone company coppercabling infrastructure; digital quality-of-ser-vice capabilities such as low noise, less inter-ference, and clearer voice transmission; andthe security of digital communications, whichis inherently more difficult to tap than tradi-tional analog systems.

However, ISDN differs from xDSL tech-nologies in that it is a switched service in whichboth ends must support ISDN, whereas xDSLis a point-to-point access service. ISDN alsorequires external power for operation. To ensurecontinuous operation, customers need either abackup power system or a redundant POTSline. In contrast, xDSL carries its own poweron the line. Voice and data transmission issplit (multiplexed) on the wire: voice is carriedunder 4 kHz; data is carried above 4 kHz. If apower failure occurs, xDSL data transmissionis lost, but lifeline POTS still operates.

Another key difference is that ISDN iswidely available now and has momentum inthe marketplace. Telcos, competitive accessproviders, and ISPs are investing the resourcesand building out the infrastructure to developit further. As ISDN modems and terminal

adapters become easier for users to configure,customer premises equipment (CPE) pricescontinue to drop, and tariffs are reduced, ISDNis gaining broader appeal among telecommutersand small office and retail users who requireInternet and intranet access, remote LAN access,credit authorization, or database connectivity.

Cable Modems

Designed to provide broadband Internetaccess, cable modems are primarily targeted atconsumers for residential use. Cable modemsoffer the potential of broadband (up to 30Mbps) information delivery downstream tousers and midband (128 Kbps) to broadband(up to 10 Mbps) connections back upstreamto the cable headend. Unlike xDSL andISDN, cable modems are a shared—not dedi-cated—access technology. The total availablebandwidth is shared among users in a neigh-borhood as if they were on a LAN. Given thatdesign, not everyone on the network will getthe top speeds of 10 to 30 Mbps that arequoted for downstream throughput. Actualrates will vary according to the number ofusers on the system at any given time and thetype of modem that is being used. Security isalso an issue on these shared access systems.

The multimedia cable network system(MCNS ) standard for the delivery of dataover cable has been defined and is beingadopted by major multiple system operators(MSOs) and cable modem manufacturers. Itsadoption adds more stability to cable as a datatransmission technology. However, the wide-spread introduction of cable modems is stillcontingent upon the development and imple-mentation of complex, two-way transmissionsystems and operations systems for manage-ment and billing. Today’s systems are primar-ily telco return, in which phone lines are usedto provide upstream transmission.

Another hurdle that cable modems mustovercome is negative perceptions about thequality of service delivered by cable systems.Some users are approaching the use of cablemodems for data transfer with caution. Forcable modem access providers to be successful,they must be able to compete not only onprice, but also on reliability of service.

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xDSL

For all intents and purposes, xDSL modemscan be considered “next-generation” modems,initially targeted for business users. xDSLtechnologies are being positioned for a widerange of data dialtone, video dialtone, voice,and PBX interconnect applications. For thenear term, however, the trend continues to betoward data applications, with voice-over-IPemerging as a new application.

While xDSL technologies hold a lot ofpromise, there are a number of critical issuesto be resolved before they can achieve wide-spread commercial deployment. Standards arenow agreed upon. During 1996–1997, stan-dards bodies split along the partisan lines ofDMT versus CAP modulation schemes (seebox on page 6). In January 1998, ANSI re-rat-ified DMT as the standard of choice, and theITU adopted it in February 1998.

Other ongoing issues for xDSL technolo-gies include interoperability, spectral compati-bility (e.g., interference between differentservices carried in the same cable binder),near-end crosstalk associated with reverseADSL provisioning, and loop qualification. Anontechnical but critical factor will be howsuccessfully NSPs move from xDSL technol-ogy and market trials to commercial rollout.

Sometime in the next three to five years,xDSL technology could potentially be used todeliver Asynchronous Transfer Mode (ATM)to the home over the existing copper infra-structure or via a hybrid fiber/copper network.Efforts to define the standards for doing thisare now under way in ANSI, ETSI, the ADSLForum, the ATM Forum, and the Full ServiceAccess Network (FSAN) Council. While jointdevelopment efforts are proceeding, consider-ably more cooperative work is needed beforethese organizations can agree upon a set ofstandards that will enable the delivery of low-cost, end-to-end ATM to the desktop overxDSL.

ADSL Development and Deployment

Progress

Of all the emerging xDSL technologies,ADSL is receiving the most attention becausethere is a standard (DMT) for it, and its capa-

bilities provide NSPs with a competitive offer-ing to cable modems. But there is increasinginterest in symmetrical xDSL offerings such asHDSL and SDSL.

As a local access service, ADSL’s imple-mentation has no critical drawbacks. It can bedeployed as an overlay network where there issubscriber demand, eliminating the need forNSPs to risk building out their infrastructureunnecessarily in the hope that the technologywill catch on.

ADSL development and deployment isfocused primarily in North America, followedby northern Europe and the Pacific Rim. InNorth America, US West, GTE, Ameritech,SBC, BellSouth, and Edmonton Tel (Canada)are the service providers leading the currentwave of ADSL/xDSL deployment. Covad,Northpoint, and a handful of other CLECsare entering high-density metropolitan areas—typically offering a portfolio of xDSL offeringsat different classes of service and price points,and competing with incumbent localexchange carriers. Chicago-based InterAccesswas the first ISP to offer ADSL. Telia (Swe-den), Telenor (Norway), British Telecom(UK), and Telfonica (Spain) are leading xDSLproponents in Europe. In the Pacific Rim, Tel-stra (Australia), Hong Kong Telecom, andSingtel (Singapore) are deploying xDSL fordata and video applications.

ADSL modems have been tested success-fully by more than 40 telephone companies,and close to 50,000 lines have been installedin various technology trials and commercialdeployments. Increasingly, alternative serviceproviders such as enterprises, multi-tenantbuilding owners, hospitality businesses (hotelsand resorts), and office park developers areoffering or considering offering ADSL to theirusers as private network operators.

Getting Started with ADSL

ADSL is not yet generally available. It is anemerging technology that is predominantly inthe early commercial deployment stage. NSPsstill must put in place the overlay networks tohandle commercial service offerings, and net-work equipment vendors must build produc-tion-level DMT systems. Users can expect to

88

see ADSL products and services introducedthroughout 1998, followed by more wide-spread deployment in 1999 and 2000.

ADSL Suppliers

xDSL suppliers generally fall into three cate-gories:• Component manufacturers• Systems providers• Service providers

Component manufacturers provide thechips, modems, and POTS splitters used atboth ends of a line to receive, send, and pro-cess digital data. Systems providers offer end-to-end solutions that include modems,splitters, and multiplexers as well as opera-tions, administration, management, and tech-nical support capabilities. Service providersoffer xDSL access services and may or may notbundle products from component manufac-turers or systems providers to offer their sub-scribers turnkey solutions.

Prospective users of ADSL need to deter-mine whether their local service provideroffers a turnkey solution, or whether theymust work directly with equipment manufac-turers, value-added resellers, or systems inte-grators. It is possible that ADSL modems willbe available at retail outlets during 1999 in anumber of markets where service is deployed.

Network Design: What’s Needed

Figure 3 illustrates the various components ofan ADSL network. It includes both NSP com-ponents (central office) and user components

(including branch offices, small offices, andhome offices).

Potential users of ADSL will need the fol-lowing:• An ADSL modem (compatible with the one

at the NSP’s point of presence)• A POTS splitter to separate voice and data

transmissions (unless using ADSL Lite)Since the ADSL modem essentially front-endsa LAN (or is capable of doing so), branchoffice or small business users will need a routeror hub; home users will need a computerinterface.

Providers of ADSL services will needmodems and POTS splitters in their digitalsubscriber line access multiplexer (DSLAM)to terminate and aggregate incoming ADSLlines and redirect voice traffic to the publicswitched telephone network (PSTN) and datato a high-speed digital line (DS3, OC-3, orOC-12). The DSLAM is the major intelli-gence component in the ADSL system. It con-sists of central site modems and a serviceaccess multiplexer (SAM) that interfaces to theNSP’s ATM or Frame Relay backbone. TheADSL service provisioning model includestwo types of DSLAM: the central officeDSLAM is built for high density and concen-tration, while the remote DSLAM sits in theremote DLC system. Service providers willalso need billing systems, testing and diagnos-tic functionality, and network managementcapabilities.

Significant development work is stillneeded by NSPs and equipment manufacturers

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Upgrading Digital LoopCarriers (DLCs)

The DLC system is the

carrier’s local loop infra-

structure that connects end

users located more than

18,000 feet, or 3.5 miles,

from the central office. DLC

systems consist of physical

pedestals containing line

cards that concentrate resi-

dential traffic onto digital

circuits. To provide end

users with ADSL capability,

NSPs will simply retrofit the

line cards in the DLC

systems. This is a very cost-

effective solution for NSPs,

because they are not

required to update their

infrastructure to provide

ADSL services. It is esti-

mated that 30 percent of

U. S. telephone customers

are on DLC systems. These

systems tend to be concen-

trated in the suburbs, where

more affluent people reside;

the initial residential target

audience for ADSL service

will be this suburban popu-

lation.

Figure 3. xDSL Network Elements

Business

Residence

DSLAM

R-DSLAM

Main distribution

frame

Copper or

fiber

Copper pair

Copper pair

POTS

splitter

xDSL

modem

LAN

Telco remote terminalTelco central office

POTS splittersxDSL modems

Data switch

Voice switchPOTS

splitter

xDSL

modem

alike to develop more affordable, scalable,interoperable, and easily provisioned ADSLsystems. But this is an exciting emerging tech-nology that will initially provide high-band-width local access for enterprise networks andteleworkers.

Conclusion

xDSL technology—with its ability to supportvoice, content-rich data, and video applica-tions over the installed base of twisted-paircopper wires—is inherently suited to meetuser demands for broadband, multimediacommunications. The most promising of thexDSL technologies for integrated Internetaccess, intranet access, remote LAN access,video-on-demand, and lifeline POTS applica-tions in the near term is ADSL or R-ADSL (a

rate-adaptive version of ADSL). During thepast year, ADSL has concluded trials by morethan 40 network service providers throughoutthe world, primarily in North America andnorthern Europe.

Service introduction began in 1997, butADSL service is still being rolled out in manyareas. In the meantime, xDSL technologiesand standards will continue to evolve, as willuser demand for these emerging services rela-tive to other local access service alternatives.

3Com has been shipping its end-to-end,standards-based xDSL solutions since June1997. With 3Com as a partner, telecom andenterprise NSPs can provision xDSL in a waythat leverages their existing infrastructure andservice delivery model, adding xDSL equip-ment as demand grows.

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Glossary of Related Terms

ADSL. Asymmetric digital subscriber line. AnxDSL technology in which modems attachedto twisted-pair copper wires transmit from 1.5to 8 Mbps downstream (to the subscriber) andfrom 16 to 640 Kbps upstream, depending onthe line distance.

amplitude. The maximum value of varyingwave forms.

ANSI. American National Standards Institute.The principal standards development body inthe United States. It consists of voluntarymembers that represent the U.S. in the Inter-national Standards Organization (ISO). Mem-bership includes manufacturers, commoncarriers, and other national standards organi-zations, such as the Institute of Electrical andElectronics Engineers (IEEE).

ATM. Asynchronous Transfer Mode. Aswitching technology that allows voice, data,image, and video traffic to be combined intoevenly sized cells for high-speed transmissionover one access circuit. Each 53 byte cell con-tains 48 bytes of payload and 5 bytes of con-trol information.

AWG. American Wire Gauge. A wire diame-ter specification; the lower the AWG number,the larger the wire diameter.

backbone network. The major transmissionpath for network interconnection.

broadband. A communication channel with abandwidth in excess of 1.54 Mbps.

CAP. Carrierless amplitude phase modulation.A version of quadrature amplitude modula-tion (QAM) that stores parts of a modulatedmessage signal in memory and then reassem-bles the parts in the modulated wave. The car-rier signal is suppressed before transmissionbecause it contains no information and isreassembled at the receiving modem (hencethe word “carrierless” in CAP).

CLEC. Competitive local exchange carrier. Analternative access provider that competes withincumbent local carriers.

CO. Central office. A facility that contains thelowest node in the hierarchy of switches thatcomprise the public telephone network.

core network. A combination of switchingoffices and transmission plant that connectsswitching offices together. In the U.S. localexchange, core networks are linked by severalcompeting interexchange networks. In the restof the world, core networks extend to nationalboundaries.

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CPE. Customer premises equipment.

dial up. A type of communications that isestablished by a switched circuit connectionusing the public telephone network.

DLC. Digital loop carrier. The carrier’s localloop infrastructure that connects end userslocated more than 18,000 feet or 3.5 milesaway from the central office. DLC systemsconsist of physical pedestals containing linecards that concentrate residential links ontodigital circuits.

DMT. Discrete multi-tone modulation. Awave modulation scheme that discretelydivides the available frequencies into 256 sub-channels or tones to avoid high-frequency sig-nal loss caused by noise on copper lines.

DSL. Digital subscriber line. A local loopaccess technology that calls for modems oneither end of copper twisted-pair wire todeliver data, voice, and video information overa dedicated digital network.

DSLAM. Digital subscriber line access multi-plexer. Multiplexing equipment that containsa high concentration of central office splitters,xDSL modems, and other electronics to con-nect traffic to the wide area network (WAN).

DWMT. Discrete wavelet multi-tone. A vari-ant of DMT modulation, DWMT goes a stepfurther in complexity and performance by cre-ating even more isolation between subchannels.

E1. The European basic multiplex rate thatcarries 30 voice channels in a 256-bit frametransmitted at 2.048 Mbps.

echo cancellation. A technique used byADSL, V.32, and V.34 modems that isolatesand filters unwanted signal energy fromechoes caused by the main transmitted signal.

ETSI. European Telecom Standards Institute.A consortium of manufacturers, service carri-ers, and others responsible for setting technicalstandards in the European telecommunica-tions industry.

FDM. Frequency division multiplexing. Atechnique that divides the available bandwidthof a channel into a number of separate channels.

frequency. The rate of signal oscillation inhertz (Hz).

FSAN. Full Service Access Network Council.A consortium of European service providers(PTTs) responsible for defining access net-work requirements.

HDSL. High bit-rate digital subscriber line.An xDSL technology in which modems oneither end of two or more twisted-pair linesdeliver symmetric T1 or E1 speeds. Currently,T1 requires two lines and E1 requires three.

HDTV. High-definition television. A systemof transmitting television signals at 24 Mbps,which increases the horizontal lines of resolu-tion from 480 to 560 lines per display.

IDSL. ISDN digital subscriber line. An xDSLtechnology that provides full duplex through-put at speeds up to 144 Kbps based on the2B1Q ISDN modulation code.

IEC. Interexchange carrier. A long-distanceservice provider.

IEEE. Institute of Electrical and ElectronicsEngineers.

ILEC. Incumbent local exchange carrier.

ISDN. Integrated Services Digital Network. Adigital subscriber line network with circuitand packet switching capabilities for voice anddata communications at data rates of up to1.544 or 2.048 Mbps.

ISO. International Standards Organization.

ISP. Internet service provider.

ITU. International TelecommunicationsUnion. An international standards body, for-merly called the CCITT.

Kbps. Kilobits per second.

LAN. Local area network. A type of broadcastnetwork, covering a limited area, in whichcomputers and other devices are attached to acommon transmission medium.

local loop. The line from a subscriber to thetelephone company central office.

Mbps. Megabits per second.

MCNS. Multimedia cable network system. Astandard for the delivery of data over cable.

midband. A communication channel with abandwidth range of 56 Kbps to 1 Mbps.

modem. Contraction for modulator/demodu-lator. A modem converts the serial digital data

from a transmitting device into a form suit-able for transmission over the analog tele-phone channel.

modulation. The process in which the char-acteristics of one wave or signal are varied inaccordance with another wave or signal. Mod-ulation can alter frequency, phase, or ampli-tude characteristics.

MSO. Multiple system operator. Cable serviceproviders owning two or more cable systems.

multiplex. Combining signals of multiplechannels into one channel. This process pro-vides multiple users with access to a singleconductor or medium by transmitting in mul-tiple distinct frequency bands (frequency division multiplexing, or FDM) or by assign-ing the same channel to different users at dif-ferent times (time division multiplexing, orTDM).

multiplexer. Equipment that divides a datachannel into two or more independent, fixeddata channels of lower speed.

narrowband. A communications channelwith a bandwidth of less than 56 Kbps.

NSP. Network service provider.

phase modulation. A technique that changesthe characteristics of a generated sine wave orsignal so that it will carry information.

POP. Point of presence. Physical access pointto an IEC network.

POTS. Plain old telephone service.

POTS splitter. A passive filter that separatesvoice traffic from data traffic.

PSTN. Public switched telephone network. A telephone system through which users canbe connected by dialing specific telephonenumbers.

QAM. Quadrature amplitude modulation. Abandwidth conservation process routinelyused in modems, QAM enables two digitalcarrier signals to occupy the same transmissionbandwidth.

R-ADSL. Rate-adaptive digital subscriberline. An emerging variation of CAP; it dividesthe transmission spectrum into discrete sub-channels and adjusts each signal transmissionaccording to line quality.

SAM. Service access multiplexer. A compo-nent of the DSLAM.

SDMT. Synchronized DMT. A multicarriermodulation scheme that adds time divisionduplexing on top of DMT systems and per-mits transmit and receive in discrete timeslots. Proposed for use with VDSL.

SDSL. Single-line digital subscriber line.SDSL is essentially HDSL over a singletwisted pair.

SMDS. Switched Multimegabit Data Service.A connectionless, high-speed, packet-switchedWAN technology offered by telephone com-panies.

SNMP. Simple Network Management Protocol.

T1. A 1.544 Mbps line; the same as DS1.

TDM. Time division multiplexing. A digitaltransmission method that combines signalsfrom multiple sources on a common path.This common path is divided into a numberof time slots and each signal or channel isassigned its own intermittent time slot, allow-ing the path to be shared by multiple channels.

telco. American jargon for telephone company.

twisted-pair. Telephone system cabling thatconsists of copper wires loosely twisted aroundeach other to help cancel out any inducednoise in balanced circuits.

UAWG. Universal ADSL Working Group. Anindustry group that supports the developmentof a worldwide G.Lite standard within theITU Study Group 15.

VDSL. Very high bit-rate digital subscriberline. A technology in which modems enableaccess and communications over twisted-pairlines at a data rate from 1.54 Mbps to 52Mbps. VDSL has a maximum operating rangefrom 1,000 feet to 4,500 feet on 24-gauge wire.

WAN. Wide area network. A geographicallydispersed network.

xDSL. The “x” represents the various forms ofdigital subscriber line (DSL) technologies:ADSL, R-ADSL, HDSL, SDSL, or VDSL.

Zipper. A DMT-based modulation schemeusing frequency division multiplexing. Itrequires synchronization of systems within thesame bundle. Proposed for use with VDSL.

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About 3Com Corporation

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