Introduction to ADSL Technology

1
Introduction to ADSL Technology

• by Wenmei Zhao
• May 3, 1999

2
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

3
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.

4
Why ADSL?

• Over the past 15 years, a thousand-fold
  transmission rate is realized. But it still does
  not meet todays need.
• Viewing a full-motion movie requires about 5Mbps.
• Downloading Netscape requires 10 minutes.
• ADSL
• 20 fold faster

5
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.

6
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
• Its a single-pair version of HDSL. Supports
  T1/E1 on a single pair to a distance of 11,000ft.
• CDSL Consumer DSL
• Its a proprietary technology from Rockwell
  International.

7
DSL Family (cont.)

• EtherLoop Ethernet Local Loop
• Its a proprietary technology from Nortel.
• ADSL Lite
• Its a lower data rate version of ADSL. It is
  known as G.lite. Splitter is not required in the
  subscribers home. It offers up to 1.5Mbps
  downstream and up to 512Kbps upstream. ATM used
  as transport protocol.

8
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

9
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.

10
System Model
11
Channel Modeling(characteristic impedance,
propagation constant, channel attenuation)
12
Noise

• There are three main types of noise that affect
• DSL system performance
• NEXT (Near End Crosstalk)
• FEXT (Far End Crosstalk)
• Impulse Noise

13
NEXT

• When a transceiver sends a signal and a nearby
  transceiver at the same end hears the signal,
  its NEXT.
• A simplified NEXT model for N disturbers

14
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

15
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.

16
Multiple Access

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

17
Line Code

• 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.

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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

19
DMT Line Code
20
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

21
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.

22
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 Power7dBm
• Lowpass filter 43.875kHz

23
DMT Block Diagram
24
PSD of DMTPSD is useful for finding received
signal power, thus useful for analyzing NEXT and
FEXT noises.Upstream and downstream PSD models
are
25
Frame Structure
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Frame Structure (cont.)

• A super frame is defined for every 68 IFFT/FFT
• operations.The super frame has a time duration
• of 68/4k17ms for baud rate of 4kHz.

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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

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CAP Transmission Parameters

• Downstream
• Constellation size 64
• Baud rate 266.67KHz
• Throughput 1.6 Mbps
• Sampling rate1.0667MHz
• Transmit power 12dBm
• Signal spectrum
• 170 410KHz

• Upstream
• Constellation size 16
• Baud rate 6KHz
• Throughput 24Kbps
• Transmit power-4.8dBm
• Signal spectrum
• 96 102KHz

29
Bell Atlantic ADSL Packets

• Product Speeds Price
  Loop Qual.
• Infospeed640k d640 kbps 39.95
  12,000ft.
• u90 kbps
• Infospeed1.6M d1.6 Mbps 59.95
  8,000-12,000ft.
• u90 kbps
• Infospeed7.1M d7.1 Mbps 109.95
  8,000ft.
• u680 kbps

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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.

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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.

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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.

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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.