Transcript ADSL and Line Codes

Introduction to ADSL Technology
by: Wenmei Zhao
May 3, 1999
Outline

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
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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)
R( f )  sL( f )
Z ( s) 
G( f )  sC ( f )
20
LdB (d , f )  20 log10 H (d , f ) 
d ( f )  8.686 d ( f )
ln 10
H (d , s )  e
 d  ( s )
e
 d  ( f )  j d  ( f )
e
Noise
There are three main types of noise that affect
DSL system performance:
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NEXT (Near End Crosstalk)
FEXT (Far End Crosstalk)
Impulse Noise
NEXT
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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:
N 0.6
1
NEXTN  ( )
f
13
49 1.134  10
3
2
FEXT
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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:
N 0.6
2
2
FEXTN  ( ) k  f  d  H ( f )
49
Impulse Noise
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
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.
Multiple Access
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FDM (Frequency Division Multiplexing)
ECH (Echo Canceller with Hybrid)
Line Code
Two main contenders:
 DMT — Discrete MultiTone
– A multi-carrier system using Discrete Fourier
Transforms to create and demodulate individual
carriers.
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CAP — Carrierless Amplitude and Phase
– A version of suppressed carrier QAM.
DMT
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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.
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
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
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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 DMT
PSD is useful for finding received signal power,
thus useful for analyzing NEXT and FEXT
noises.
Upstream and downstream PSD models are:





2V  sin(fT ) 
1
f8
 8




3 8 
f
ZT  f
 (20  10 ) 
8  f
 1  (
)

1.104  106 

2
PSDADSL, DS
PSDADSL,US 
2
2V
ZT
 sin(fT ) 
2

 HUS ( f )
f


2
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
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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
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Downstream
–
–
–
–
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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
Infospeed640k
Infospeed1.6M
Infospeed7.1M
Speeds
Price
Loop Qual.
d:640 kbps
u:90 kbps
d:1.6 Mbps
u:90 kbps
d:7.1 Mbps
u:680 kbps
$39.95
12,000ft.
$59.95 8,000-12,000ft.
$109.95
8,000ft.
References
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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.)
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"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, pp5259, 1995.
"Evaluation of Near-End Crosstalk Noise
Affecting ADSL Systems", by Marco
Carbonelli,etc., TELECOM ITALIA.
References (cont.)
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
"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.)
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"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.