Introduction to ADSL Technology

by: Wenmei Zhao

May 3, 1999

Outline

ADSL — What is it? Why ADSL? ADSL Family Features ADSL vs. cable

modem System Model Channel Modeling

Noise Echo Canceller Line Codes

– DMT– CAP

Frame Structure Bell Atlantic ADSL References

ADSL — What is it?

ADSL — Asymmetric Digital Subscriber Line – High speed communications over twisted pair.– Concurrent with POTS (plain old telephone

service).– Secure way of Internet access.– Originally standardized in ANSI (American

National Standards Institute) T1.231-1993.– Currently standardized in ANSI T1.413-1998.– Growing really fast.

Why ADSL?

Over the past 15 years, a thousand-fold transmission rate is realized. But it still does not meet today’s need.– Viewing a full-motion movie requires about 5Mbps.– Downloading Netscape requires 10 minutes.

ADSL:– 20 fold faster

DSL Family

ADSL: Asymmetric DSL– It allows 6Mbit/s downstream and 640kbit/s

upstream to a distance of 18kft. It uses a modulated analog carrier.

HDSL: High-bit-rate DSL– It uses two twisted pairs of standard subscriber

copper telephone lines. It supports 1.544Mbit/s up to 12kft. It uses 2B1Q line code.

VDSL: Very High-bit-rate DSL– It is similar to ADSL, but supports about 26 Mbit/s

to 3kft and 51Mbit/s to 1.2kft.

DSL Family (cont.)

RADSL: Rate Adaptive DSL– Usually refer to a proprietary modulation

standard designed by Globespan Semiconductor. It uses CAP.

SDSL: Single line DSL– It’s a single-pair version of HDSL. Supports

T1/E1 on a single pair to a distance of 11,000ft. CDSL: Consumer DSL

– It’s a proprietary technology from Rockwell International.

DSL Family (cont.)

EtherLoop: Ethernet Local Loop– It’s a proprietary technology from Nortel.

ADSL Lite– It’s a lower data rate version of ADSL. It is known

as G.lite. Splitter is not required in the subscriber’s home. It offers up to 1.5Mbps downstream and up to 512Kbps upstream. ATM used as transport protocol.

Features

Appropriate for Internet requirement– downstream data rate of up to 6-8Mbps– upstream data rate of up to 640kbps to 1Mbps

Convenient– always on

Parallel with voice service– no need to unplug telephone line

Secure access to Internet– no need to worry about your noisy neighbors

ADSL vs. cable modem

Pro:– Secure. “Point to point

connectivity” of ADSL ensures the security of the service. Cable, by contrast, is shared media and is not secure at all.

– Bigger coverage area.– Cheap. ADSL uses

existing twisted pair, hence is cheap in installation and also cheap in monthly payment.

Cons:– Bandwidth. ADSL has

about 1.1MHz BW due to loop limitations, while cable modem has about 745MHz BW.

– Bridge taps, DLCs, load coils can lead to problems.

– Mutual noise among different DSL lines, T1 lines.

System Model

Channel Modeling(characteristic impedance, propagation constant, channel attenuation)

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)()()(

fsCfG

fsLfRsZ

)(686.8)(10ln

20),(log20),( 10 fdfdfdHfdLdB

)()()(),( fdjfdsd eeesdH

Noise

There are three main types of noise that affect

DSL system performance:

NEXT (Near End Crosstalk) FEXT (Far End Crosstalk) Impulse Noise

NEXT

When a transceiver sends a signal and a nearby transceiver at the same end “hears” the signal, it’s NEXT.

A simplified NEXT model for N disturbers:

2

3

13

6.0

10134.1

1)

49( fN

NEXTN

FEXT

When a transceiver sends a signal and a transceiver at the far end “hears” the signal, FEXT occurs.

A simplified FEXT model for N disturbers:

226.0 )()49

( fHdfkN

FEXTN

Impulse Noise

Impulse noises are large surges of noise with short duration. The sources of impulse noises are not well understood yet. It is a very devastating noise if not handled well.

A concatenated code, using a 2-dimensional 8-state trellis code and a 4-error-correcting Reed-Solomon code with an interleaving depth of 18 symbols, was found to be suitable for eliminating impulse noise.

FDM (Frequency Division Multiplexing) ECH (Echo Canceller with Hybrid)

Multiple Access

Two main contenders: DMT — Discrete MultiTone

– A multi-carrier system using Discrete Fourier Transforms to create and demodulate individual carriers.

CAP — Carrierless Amplitude and Phase– A version of suppressed carrier QAM.

Line Code

DMT

Existing ANSI and ETSI standards Consists of up to 256 sub-channels, (also

called tones or bins), of 4.3125KHz– upstream use 25-163KHz (bins 6 to 38)– downstream use 142KHz-1.1MHz (bins 33 to 255)– bins 16 (69KHz) and 64 (276KHz) are pilot tones.

Outperforms CAP in field trials More expensive and complex

DMT Line Code

Observations

Three Channels: POTS channel

– POTS channel is split off from the digital modem by filters, thus guaranteeing uninterrupted POTS.

High speed downstream channel– Its data rate depends on length of the copper line,

its wire gauge, presence of bridged taps, cross talk, etc.

Medium speed upstream channel

DMT Features

Discretely divides the available frequencies into 256 sub-channels or tones.

Incoming data is broken down into a variety of bits and distributed to a specific combination of sub-channels.

To rise above noise, more data resides in the lower frequencies and less in the upper frequencies.

DMT Transmission Parameters

Downstream– symbol rate: 4KHz– FFT size: 512– Cyclic prefix: 32– Sampling rate:

2.208MHz– Transmit power:

20dBm– Highpass filter:

62.5kHz

Upstream– Symbol rate: 4kHz– FFT size: 64– Cyclic prefix: 4– Sampling rate:

276kHz– Transmit

Power:7dBm– Lowpass filter:

43.875kHz

DMT Block Diagram

PSD of DMTPSD is useful for finding received signal power, thus useful for analyzing NEXT and FEXT noises.Upstream and downstream PSD models are:

838

8

8

6

2

, )1020()10104.1

(1

1)sin(2

f

fff

fT

ZT

VPSD DSADSL

2

22

, )()sin(2

fHf

fT

ZT

VPSD USUSADSL

Frame Structure

Frame Structure (cont.)

A super frame is defined for every 68 IFFT/FFT

operations.The super frame has a time duration

of 68/4k=17ms for baud rate of 4kHz.

CAP

Initial ADSL implementations were done using CAP

1996 - 90% of world-wide ADSL implementation based on CAP

Variant of QAM - widely understood Not yet incorporated in ANSI standards

T1.413 or ETSI Supported by GlobeSpan Technologies

CAP Transmission Parameters

Downstream– Constellation size: 64– Baud rate: 266.67KHz– Throughput: 1.6 Mbps– Sampling

rate:1.0667MHz– Transmit power: 12dBm– Signal spectrum:

170 ~ 410KHz

Upstream– Constellation size: 16– Baud rate: 6KHz– Throughput: 24Kbps– Transmit power:-

4.8dBm– Signal spectrum:

96 ~ 102KHz

Bell Atlantic ADSL Packets

Product Speeds Price Loop Qual.

Infospeed640k d:640 kbps $39.95 12,000ft.

u:90 kbps

Infospeed1.6M d:1.6 Mbps $59.95 8,000-12,000ft.

u:90 kbps

Infospeed7.1M d:7.1 Mbps $109.95 8,000ft.

u:680 kbps

References

ADSL Tutorial: "ADSL Application Notes", by ADSL Forum at www.adsl.com.

ADSL Tutorial: "Twisted Pair Access to the Information Highway", by ADSL Forum at www.adsl.com.

"ADSL and DSL Technologies", Walter Goralski, 1998, ISBN: 0-07-024679-3.

"ADSL Forum System Reference Model", by ADSL Forum at www.adsl.com.

References (cont.)

"Discrete Multitone vs. Carrierless Amplitude/Phase Line Codes", Aware white paper.

"ADSL: A New Twisted-Pair Access to the Information Highway", by Philip J.Kyees, etc., IEEE Communications Magazine, pp52-59, 1995.

"Evaluation of Near-End Crosstalk Noise Affecting ADSL Systems", by Marco Carbonelli,etc., TELECOM ITALIA.

References (cont.) "Comparison of Single-Carrier ad Multitone

Digital Modulation for ADSL Applications, by Burton R. Saltzberg, IEEE Communications Magazine, Nov., 1998.

"Coded 64-CAP ADSL in an Impulse-Noise Environment --- Modeling of Impulse Noise and First Simulatin Results", by Werner Henkel,etc., IEEE Selected Areas in Comm., December, 1995.

References (cont.)

"Forward Error Correction for Asymmetric Digital Subscriber Lines (ADSL)", by Kenneth J. Kerpez, Bellcore, GLOBECOM'91.

"Frequency Domain Echo Cancellation for Discrete Multitone Asymmetric Digital Subscriber Line Transceivers", by David Jones, IEEE Trans. On Communications, April, 1995.

etc.