Asymmetric DigitalSubscriber Line

A 10-billion unit m arket induced byInternet!

D riving force: The A D SL ForumG etting 10M B /s to hom es,using telephone linesForm al education is oftentim es m isleading!?

W ho w ins: D iscrete M ultiTone (D M T)orC arrierless A m plitude/Phase(C A P)?

Success story: B ay N etw orks

BROADBAND APPLICATIONS

Entertainment Broadcast TVVODInternet—Text 0.014–6 MbpsInternet—Graphics 0.5–6 MbpsGambling 0.014–2 MbpsGames 0.014–16 Mbps

Consumer Shopping 0.5–6 MbpsEducation 0.5–6 MbpsEducation 1.5–6 Mbps

Professional Work at Home 0.014–6 MbpsSOHO 0.014–6 MbpsVideo Conf 0.128–1.5 Mbps

NT = Network Terminal (aka Gutless PC)

PC HOUSEHOLDS

0102030405060708090

100

1995 1996 1997 1998 1999 2000

PC Households WorldwideConnected PC Households Worldwide

ADSL

ADSL CONFIGURATION

SwitchedTelephone

NetworkModemModem

Line card filtersdetermine bandwidth

4 kHz Vioce ChannelData limited to 33 kbps

ATMBroadband

NetworkServiceADSL

Local loopdetermines bandwidth

<1 MHz bandwidthrates to 52 Mbps,

depending on length

ADSL

SPEED SHRINKS WITH DISTANCE

10

20

30

40

50

3 6 9 12 15 18

VDSL

ADSL

Dominant impairment—AttenuationAttenuation increases with distance and frequencySecondary impairments—Cross Talk, Impulse noise, Bridged TapsSpectrum used— DSL 100 kHz

HDSL 250 kHzADSL 1 MHzVDSL 10–30 MHz

Distance in kft of 24 (.5 mm) ga wire

DSL BROADBAND TECHNOLOGIESHDSL 1.5/2.0 Mbps symmetric

T1/E1 service only (2/3 lines, no FEC)

SDSL 1.5/2.0 Mbps symmetricT1/E1 rates on single line over POTSSuitable for residential services

ADSL 1.5–9.0 Mbps, asymmetric (640 max upstream)Single line over POTSFEC, multiple premises interfacesCircuit, packet and ATM multiplexing

VDSL Current View 13–52 Mbps, asymmetric (3 Mbos upstream)Emerging View 16–26 Mbps down, 6 Mbps up (PC symmetric)Single line over POTS and ISDNRest in the air

DSL = Digital Subscriber Line, H = High, S = Single line, A = Asymmetric, V = Very high rate

LINE CODES

Two general alternatives in marketplace today

QAM/CAP—single carrier (like voice band modems)

10,000 in field trialsHelped prove ADSL concept

DMT—Discrete MultiTone

ANSI standard1000 in field trialsHelped prove ADSL to 6 Mbps

DSL BROADBAND TECHNOLOGIES

ADSL 1.5–9.0 Mbps, asymmetric (640 maxupstream)Single line over POTSFEC, multiple premises interfacesCircuit, packet and ATM multiplexing

VDSL Current View 13–52 Mbps, asymmetric(3 Mbos upstream)Emerging View 16–26 Mbps down, 6Mbps up (PC symmetric)Single line over POTS and ISDNRest in the air

DSL = Digital Subscriber Line, H = High, S = Single line, A =Asymmetric, V = Very high rate

LINE CODES

Two general alternatives in marketplace today

QAM/CAP—single carrier (like voice band modems)

10,000 in field trialsHelped prove ADSL concept

DMT—Discrete MultiTone

ANSI standard1000 in field trialsHelped prove ADSL to 6 Mbps

DMT OR CAP?Both will work.But cannot be made to interoperate

DMT Benefits: 32 kbps rate granularity (CAP at 320 kbps)Probably works on more linesGreater immunity to impulse noiseSpectral management toolANSI standard (ergo, 5 silicon efforts underway)

CAP Benefits: Dominated field trials to dateHigher level of integration, nowWell understoodSupported by some major playersInteroperable with QAM

With equal levels of integration, complexity and power appear comparable.

Time is of the essence in the market

Stay tuned!

DSL: A POINT-TO-POINT SOLUTION

Point-to point DSL LinkAcross the Local Loop (i.e.

from the customer premises tothe CO)

Local Area Domain (i.e. fromthe ATU-R to either corporate

LAN or residential user)

Network Access Provider'sDomain (i.e. from the ATU-Cat the CO to various network

services—Frame Relay, ATM,Internet, etc.)

Customer'sNetwork

AccessProvider'sNetwork

ATU-C

ATU-C

ATU-R

ATU-R

NetworkInterfaces*

NetworkInterfaces*

ResidentialTerminal

ATU-C = DSL modem in the COATU-R = DSL modem at the service subscriber’s premises

* Network interface: DSL equipment on both sides of the link (i.e., both the ATU-R andATU-C) have network interfaces to connect to the access provider’s and customer’s

networks. These interfaces may include Ethernet, Fast Ethernet, ATM, T-1/E-1, ISDN,Frame Relay, or others

TWO-DIMENSIONAL LINE ENCODING

0

1

One-dimensionalBinary

Line Code

00

Two-dimensionalBinary

Line Code

0111

10

x

y

Amplitude

–T/2 +T/2 –T/2

+T/2

Amplitude

Signal x Signal y

SymbolPeriod

B IT M A P P IN G C O N S T E L L A T IO N SF O R A T W O -D IM E N S IO N A L L IN E C O D EA N D 6 4 -C A P

Bit MappingConstellation for aTwo-Dimensional,Binary Line Code

Bit MappingConstellation for 64-CAP

(a Two-Dimensional,Multilevel Line Code)

x

y

1

x

y

UPSTREAM AND DOWNSTREAMCHANNELS IN CAP SYSTEMS

Two WireTwisted Pair

AmplitudeSpectra

CAP

ExistingTelephone

Service

DuplexTransmission

and Control Channel

SimplexTransmission

Control Channel

4 kHz f1(low)

f1(high)

f2(low)

f2(high)

frequency (kHz)

CAP TRANSCEIVER ARCHITECTURE

Reed-SolomonEncoder w/Interleaving

ScramblerTrellis

EncoderChannelPrecoder

CAPTransmitter

D/A Tx FilterLine

Driver

Rx FilterEqualizer A/D AGCViterbi

Decoder

Symbol-to-BitMap

De-Scrambler

DownstreamData

UpstreamData

CAP Receiver Architecture

CAP Transmitter ArchitectureDigital Signal Processing Analog Front End

Hybrid Line

DMT’S UTILIZATION

FREQUENCY SPECTRUM AVAILABLE ON ASINGLE TWISTED PAIR WIRE(I.E., TELEPHONE CABLE)

AmplitudeSpectra ofSingle PairUTP Cable

DMT

ExistingTelephone

Service

256 Sub-Channels

4 kHz

frequency (kHz)4 kHz Intervals

1 MHz

DMT TRANSMITTER AND RECEIVER

ARCHITECTURES

BitBuffer

andEncoder

2n timedomainsamples

ADC S/P

FastFourier

Transform(FFT)

LPFn QAM

SymbolsReceiveSignal

ReceiveData

BitBuffer

andEncoder

InverseFast

FourierTransform

(IFFT)

P/S DAC LPFTransmit

SignalInput Data

2n timedomainsamples

n QAMSymbols

2n/T

2n/T

DMT Transmitter Architecture

DMT Receiver Architecture

MYTH VS. REALITY

Myth Reality

1. DMT has higher consumption than CAP. 1. For equivalent rates CAP requires more power than DMT.2. DMT was intended for Video on Demand (VoD) and has

been made obsolete by Internet access.2. DMT is very well suited for Internet support. The ANSI

standard explicitly addressed data access. The upstreamrate was chosen to reflect the 10:1 ratio that is optimum forInternet traffic.

3. CAP invented rate adaptation. 3. DMT has always been rate adaptive, takes it for granted, andimplements it in a highly flexible and elegant way. Indeed, thecoarse granularity of CAP (steps of ~300 kbps and nodownstream rates of less than 640 kbps) renders its rateadaptation essentially useless for rural low-rate/long-reachapplications. In contrast DMT steadily adapts in 32 kbpssteps to support optimum rates on all loops.

MYTH VS. REALITY

1. Performance is equivalent. 1. DMT is demonstrably more robust and has much betterperformance—delivering higher rates, much longer reach, orboth.

2. DMT is heavily patented or inaccessible. 2. DMT is defined in an open international standard mandatingfair access. Many manufacturers have independentlydeveloped their own solutions. It is the CAP technology hasremained proprietary and with only one supplier.

3. DMT is less available than CAP. 3. Solutions designed to the ANSI standard are available nowfrom several suppliers: there is only source of CAS chipsets,and this will ot comply with proposed future definition.

ADSL modems versus cable modems:Cable - 3Mbps (popular rate)ADSL - 9Mbps/downstream and 1Mbps/upstream (popular rates)

Issues in cable technology (speed and price):Getting faster and less expensive (better SNR, economies of scale, ...)IEEE 802.14 (cable TV metropolitan area network standard)

Issues in ADSL technology (very high speeed versus universal):Orckit Communications offers very high speed DSL (52Mbps vs 13Mbps)Microsoft (2000) Comcast $1B investment for UADSL (1.5Mbps vs 512Kbps)

Comparative dissadventages of ADSL:a. A 4KHz splitter needed to separate voice from ADSL (not if cable used)b. I-structure additions less widespread (modifications needed in PBX)c. More expensive ($40 to $80 vs $40 = analog modem + second line)d. More attractive for business (traditional experiences of phone companies)

Comparative adventages of ADSL:a. Sending a fixed number of packets in time (not when bandwidth available)b. Less security risks (interference can be an issue in cable technology)c. All ADSL i-structure can transmit upstream (only 20% cable-i-structure)d. All homes and businesses have phone lines (cable: H=60% and B=20%)

Reference:Lawton, G., Paving the Information Superhighway's Last Mile,IEEE Computer, April 1998, pp. 10-14

Research at UB/IFACT

The VLSI point-of-view based on past experience:

1. DMT and VLSIAlgorithm versus algorithm implementation

2. The story of HFMGetting repeated once every decade! IEEE Trans/ASSP

3. The story of GaAsRanking changes when technology changes! IEEE Trans/Computers

Acknowledgments:Dejan Rašković, …