Jacek Ilow

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Residential Broadband

• Jacek Ilow
• Dalhousie University

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Why High Speed Access?

• Broadband Services
• large data files
• real time applications
• multimedia
• WWW
• Quality of Service
• bandwidth, end-to-end delay, loss of infromation
• Broadband Services to home

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The Last Mile

• High bandwidth demand
• Best use of existing coper
• Traditional telephone services without
  interruption
• Minimal line engineering and testing installation
• Off-load network switches

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Copper Cable

• Collective assets of millions of dollars
• Central Office- home interconnection by copper
  twisted pair local loop telephone lines
• T1/E1 lines from the backbone between Cos
• Early 1980s T1/E1 migrate to local loop
• expensive to install, reconfigure and maintain

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The x in DSL

• Copper Access Technologies
• voice band modems (V.22, V.32, V.34), xDSL
• DSL Digital Subscriber Line
• refers to a modem not the line (Belcore)
• transmission scheme designed for high speed data
  networking over twisted pair
• xDSL wide variation of DSL technologies
• ADSL, VDSL, HDSL, SDSL, BDSL

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Digital Subscriber Line

• Modem used for basic rate ISDN (128kbps)
• Characteristics
• duplex transmission, 160kbps, 18,000 feet, 24
  gauge wire multiplexing
• demultiplexing into two B channels (64kbps each),
  one D channel (16 kbps) plus overhead
• Twisted-pair bandwidth 0-80kHz
• Simultaneously provisioning of analog POTS -
  Plain Old Telephone System

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High Speed DSL

• Introduced in late 1980s (Bellcore)
• Reliable cost effective, provision of repear free
  T1/E1(1.544 / 2.048 Mbps) service
• HDSL creates mathematical model of copper wire,
  allowing the transmission device to compensate
  for copper-base distortions
• Immune to cross-talk, quality comparable to fibre
  optics, BER10E-10

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Wavelet DSL

• Symmetric DSL
• Supports one or two pair lines
• Supports 1-4 Mbps, 12,000-18,000 feet
• Uses Discrete Wavelet Multi-tone Technology
• dynamic bandwidth access
• immunity to narrow-band impulse noise
• simple instalation of line cards
• no interference with other equipment

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Single Line DSL

• Symmetric data rates over a single twisted pair
• Reliable, cost effective, provision of repeatless
  T1/E1 (1.544/2.048 Mbps) services
• Duplex, no line conditioning
• Advantages of HDSL, plus lower equipment and
  installation cost
• one transceiver instead of two

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VDSL/VASDL

• Introduced in mid-1995
• Designed to work in conjuction with fiber to the
  curb (FTC)
• Provides higher rates
• 51-55 Mbps upstream, 1000 feet
• 26.6-27.6 Mbps, 3000-4000 feet
• 12.96-13.8 Mbps, 4500 feet
• 13 Mbps upstream
• Lower cost than ADSL

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VDSL

• Still in definition state
• preliminary products exist
• little known about line characteristics, RFI
  emission for the used frequency
• Forward error control and interleaving
• Multiplexing of data stream
• Services environment ATM, other traffic format
• Integration with existing modem technologies

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Rate Adaptive DSL

• Rate adaptive based on
• line length
• transmission quality
• Selects the higher operational speed
  automatically
• Downstream 640 kbps-7.168Mbps, Upstream
  90.6kbps-1.008Mbps
• Reliable, less complex than ADSL

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Cable Modem

• What is cable modem?
• Device that provides high speed data transfer
  via cable TV networks
• External device connected to the PC via a
  10BASE-T Ethernet
• How fast is cable modem?
• Speeds vary widely based on cable modem system,
  cable network architecture, traffic load
• downstream up to 27 Mbps aggregated (1-5Mbps)
• upstream up to 10Mbps (500kbps-2.5Mbps)

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How Does it Work?

• Modulates/Demodulates digital signals
• More complicated than dial-in modem
• Part modem, tuner, encryption/decryption device,
  router, network interface, SNMP agent, Ethernet
  hub
• Downstream modulated digital data on a 6MHz TV
  channel
• 50-450 KHz, coaxial cable
• up to 750 MHz, hybrid fiber coax
• Upstream 5-42 MHz

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Modulation Schems

• Upstream
• QPSK (up to 10Mbps)
• 64 QAM (up to 30Mbps)
• no interference with TV signal
• Downstream, reverse channel
• 5-42 MHz
• QPSK or similar to reduce interference effects
• QPSK more robust but slower than QAM

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ADSL Technology Description

• It uses state of the art technology in more then
  one area.
• Challenges
• Full use of the copper line frequency spectrum
  (1.1Mhz) .
• An advanced coding/decoding method.
• Ability to work simultaneously with POTS on the
  same copper line.

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Discrete MultiTone

• The lower 4Khz are being used by the POTS.
• The amplification isn't the same in all
  frequencies.
• DMT divides the frequency rang to 256
  sub-frequencies from 64Khz to 1.1Mhz Each
  subferqency is an independent channel and has it
  own stream of signals . The ADSL protocol defines
  a basic stream of data which is known to both
  endpoints in advanced and enables them to find
  the specific SNR for each sub-frequency , and
  uses this information to split the data over the
  sub-frequencies

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Error Correction Coding

• Constellation encoding and decoding the
  information on the line can be damaged and yet
  the decoder rebuild the information in a very
  high reliability.
• To improve the performance of ADSL system some
  companies use 16 state 4 dimensional trellis
  code on top of the constellation encoding.
• Another useful method to increase the ADSL
  systems reliability is Forward Error Correction
  (FEC) , which is based on Reed Solomon coding
  method .

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Framing and Scrambling

• DSL uses a specific framing method .
• The main frame is called Superframe and it is
  composed of 68 ADSL data frames ,
• the ATU-C sends a superframe every 17 msec .
• Each data frame gets his information from two
  data buffers (interleaved buffer and fast buffer)
  which are scrambled at a specific sequence , this
  scrambling method makes the error correction and
  coding more efficient.

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Summary

• The end-user does not really care which
  technology is used he is looking at the bottom
  line, that is how much he will have to pay and
  whether what he pays is worth paying for.
• Solution
• offer the subscriber connectivity at varying
  costs and QoS