Asymmetric Digital
Subscriber Line
A 10-billion unit market induced by
Internet!
Driving force: The ADSL Forum
Getting 10MB/s to homes,
using telephone lines
Formal education is often
times misleading!?
Who wins: Discrete MultiTone (DMT)
or
Carrierless Amplitude/Phase
(CAP)?
Success story: Bay Networks
BROADBAND
APPLICATIONS
Entertainment
Broadcast TV
VOD
Internet—Text
0.014–6 Mbps
Internet—Graphics
0.5–6 Mbps
Gambling
0.014–2 Mbps
Games
0.014–16 Mbps
Consumer
Shopping
Education
Education
Professional
Work at Home
SOHO
Video Conf
NT = Network Terminal (aka Gutless PC)
0.5–6 Mbps
0.5–6 Mbps
1.5–6 Mbps
0.014–6 Mbps
0.014–6 Mbps
0.128–1.5 Mbps
PC HOUSEHOLDS
PC Households Worldwide
Connected PC Households Worldwide
100
90
80
70
60
50
40
30
20
10
0
ADSL
1995
1996
1997
1998
1999
2000
ADSL CONFIGURATION
Line card f ilters
determine bandw idth
Sw itched
Telephone
Netw ork
Modem
Modem
4 kHz Vioce Channel
Data limited to 33 kbps
Local loop
determines bandw idth
ADSL
ADSL
<1 MHz bandw idth
rates to 52 Mbps,
depending on length
ATM
Broadband
Netw ork
Service
SPEED SHRINKS WITH
DISTANCE
50
VDSL
ADSL
40
Dominant impairment—Attenuation
Attenuation increases w ith distance and frequency
Secondary impairments—Cross Talk, Impulse noise, Bridged Taps
Spectrum used—
DSL
100 kHz
HDSL
250 kHz
ADSL
1 MHz
VDSL
10–30 MHz
30
20
10
3
6
9
12
Distance in kft of 24 (.5 mm) ga w ire
15
18
DSL BROADBAND TECHNOLOGIES
HDSL
1.5/2.0 Mbps symmetric
T1/E1 service only (2/3 lines, no FEC)
SDSL
1.5/2.0 Mbps symmetric
T1/E1 rates on single line over POTS
Suitable for residential services
ADSL
1.5–9.0 Mbps, asymmetric (640 max upstream)
Single line over POTS
FEC, multiple premises interfaces
Circuit, 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 ISDN
Rest 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 trials
Helped prove ADSL concept
DMT—Discrete MultiTone
ANSI standard
1000 in field trials
Helped prove ADSL to 6 Mbps
DSL BROADBAND
TECHNOLOGIES
ADSL
1.5–9.0 Mbps, asymmetric (640 max
upstream)
Single line over POTS
FEC, multiple premises interfaces
Circuit, 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 ISDN
Rest 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 trials
Helped prove ADSL concept
DMT—Discrete MultiTone
ANSI standard
1000 in field trials
Helped 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 lines
Greater immunity to impulse noise
Spectral management tool
ANSI standard (ergo, 5 silicon efforts underway)
CAP Benefits:
Dominated field trials to date
Higher level of integration, now
Well understood
Supported by some major players
Interoperable 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
Netw ork Access Provider's
Point-to point DSL Link
Local Area Domain (i.e. f rom
Domain (i.e. f rom the ATU-C
Across the Local Loop (i.e. the ATU-R to either corporate
at the CO to various netw ork from the customer premises to
LAN or residential user)
services—Frame Relay, ATM,
the CO)
Internet, etc.)
Customer's
Netw ork
Access
ATU-C
ATU-R
Provider's
Netw ork
ATU-C
ATU-R
Netw ork
Interfaces*
Netw ork
Interfaces*
Residential
Terminal
ATU-C = DSL modem in the CO
ATU-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 and
ATU-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
One-dimensional
Binary
Line Code
0
Tw o-dimensional
Binary
Line Code
y
10
00
x
1
11
Amplitude
–T/2
+T/2
01
Amplitude
–T/2
+T/2
Signal x
Signal y
Symbol
Period
BIT MAPPING CONSTELLATIONS
FOR A TWO-DIMENSIONAL LINE CODE
AND 64-CAP
Bit Mapping
Constellation for a
Tw o-Dimensional,
Binary Line Code
Bit Mapping
Constellation for 64-CAP
(a Tw o-Dimensional,
Multilevel Line Code)
y
y
x
1
x
UPSTREAM AND DOWNSTREAM
CHANNELS IN CAP SYSTEMS
CAP
Existing
Telephone
Service
Duplex
Transmission
and Control Channel
Simplex
Transmission
Control Channel
Tw o Wire
Tw isted Pair
Amplitude
Spectra
4 kHz f1
(low)
f1
(high)
f2
(low)
frequency (kHz)
f2
(high)
CAP TRANSCEIVER
ARCHITECTURE
Digital Signal Processing
CAP Transmitter Architecture
Downstream
Data
Reed-Solomon
Encoder w/
Interleaving
Scrambler
Trellis
Encoder
Analog Front End
Channel
Precoder
CAP
Transmitter
D/A
Tx Filter
Line
Driver
Hybrid
CAP Receiver Architecture
Upstream
Data
DeScrambler
Symbolto-Bit
Map
Viterbi
Decoder
Equalizer
A/D
AGC
Rx Filter
Line
DMT’S UTILIZATION
FREQUENCY SPECTRUM AVAILABLE ON A
SINGLE TWISTED PAIR WIRE
(I.E., TELEPHONE CABLE)
256 Sub-Channels

DMT
Existing
Telephone
Service
Amplitude
Spectra of
Single Pair
UTP Cable
4 kHz
1 MHz
4 kHz Interv als
f requency (kHz)
DMT TRANSMITTER AND
RECEIVER
ARCHITECTURES
DMT Transmitter Architecture
2n/T
Input Data
Inv erse
Bit
Fast
2n time
Buf f er n QAM
Fourier domain P/S
and Sy mbols
Transf orm samples
Encoder
(IFFT)
DAC
LPF
Transmit
Signal
DMT Receiver Architecture
2n/T
Receive
Signal
LPF
ADC
Fast
Bit
2n time
Fourier
n QAM Buf f er
S/P domain
Transf orm Sy mbols and
samples
(FFT)
Encoder
Receive
Data
MYTH VS. REALITY
Myth
1. DMT has higher consumption than CAP.
2. DMT was intended for Video on Demand (VoD) and has
been made obsolete by Internet access.
3. CAP invented rate adaptation.
Reality
1. For equivalent rates CAP requires more power than DMT.
2. DMT is very well suited for Internet support. The ANSI
standard explicitly addressed data access. The upstream
rate was chosen to reflect the 10:1 ratio that is optimum for
Internet traffic.
3. DMT has always been rate adaptive, takes it for granted, and
implements it in a highly flexible and elegant way. Indeed, the
coarse granularity of CAP (steps of ~300 kbps and no
downstream rates of less than 640 kbps) renders its rate
adaptation essentially useless for rural low-rate/long-reach
applications. In contrast DMT steadily adapts in 32 kbps
steps to support optimum rates on all loops.
MYTH VS. REALITY
1. Performance is equivalent.
2. DMT is heavily patented or inaccessible.
3. DMT is less available than CAP.
1. DMT is demonstrably more robust and has much better
performance—delivering higher rates, much longer reach, or
both.
2. DMT is defined in an open international standard mandating
fair access. Many manufacturers have independently
developed their own solutions. It is the CAP technology has
remained proprietary and with only one supplier.
3. Solutions designed to the ANSI standard are available now
from 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 VLSI
Algorithm versus algorithm implementation
2. The story of HFM
Getting repeated once every decade! IEEE Trans/ASSP
3. The story of GaAs
Ranking changes when technology changes! IEEE Trans/Computers
Acknowledgments:
Dejan Rašković, …