Chapter 3 Line Coding and the Subscriber Line

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Chapter 3 Line Coding and the Subscriber Line
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Motivation

• The extension of digital links to network
  subscribers is essential part of digital network
  evolution.
• The link between the network subscriber and
  network switch, known as the subscriber line,
  subscriber loop, or local loop, must be digital.

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Outline

• Basic subscriber line technology using twisted
  pair and optical fiber.
• Digital line coding
• Standard approach to providing high-speed
  subscriber line access for ISDN.
• xDSL

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Subscriber Line Technology

• Most of subscriber lines are twisted-pair cable.
• With the increasing demand for servers that
  require high data rates, there is increasing
  interest in the use of the optical fiber.

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Twisted Pair in the subscriber Line

• subscriber lt-gt local office or switch
• Full-duplex
• Change the twisted pair will require the
  installation of tremendous amount of new cable.
  (impossible)
• How to reach full-duplex?
• How to increase the speed?

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Approaches to Full-Duplex

• Analog -gtDigital
• Use modems to convert digital data into analog
  signals and to use a different frequency band in
  each direction.
• Example 300-bps Bell 108 Modem specification
• FSK (frequency-shift keying)
• Note that there is little overlap and thus little
  interference.

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FSK
1170
2125
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FSK

• Only half of the bandwidth of the lines is
  available for transmission in either direction.
• To satisfy ISDN requirements, the minimum data
  rate in each direction is 144 kbps.
• It is difficult to achieve these data rates with
  existing modem technology.

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Echo

• Alternative way dispense with modem and to
  transmit directly.
• some portion of the originators signal returns
  in the form of an echo.

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Echo

• Echo is a reflection of the transmitted signal
  back to the sender
• Near end echo from the senders hybrid
• Far end echo from the receivers echo
• Approaches
• Time-compression multiplexing (TCM)
• echo cancellation (superior system) (ANSI T1.601)

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(TCM) Time-compression multiplexing

• ping-pong method
• Data are transmitted in one direction at a time,
  with transmission alternating between the two
  directions.
• To achieve the desired data rate, the
  subscribers bit stream is divided into equal
  segment, compressed in time to a higher
  transmission rate, and transmitted in bursts,
  which are expanded at the other end to the
  original rate.
• Short quiescent period is used to allow the line
  to settle down.

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TCM
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TCM
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TCM
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TCM

• Tb Burst transmission time
• Tp Propagation delay
• Tg Guard time
• The time send one block is (TpTbTg).
• The rate at which blocks can be transmitted is
  only ½(TpTbTg).
• R be the desired data rate in bits per second
• B be the size of a block in bits
• The effective number of bits transmitted per
  second is RB/2(TpTbTg).

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TCM

• The actual data rate A on the medium AB/Tb.
• A2R(1(Tp Tq)/Tb)
• B16 to 24 bits.

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Echo cancellation

• With the echo cancellation method, digital
  transmission is allowed to proceed in both
  directions within the same bandwidth
  simultaneously.
• An estimate of the echo signal is generated at
  the transmitting end and is subtracted from the
  incoming signal.
• The exact behavior is hard to measure.
• To enable more accurate approximation, a feedback
  circuit is included.

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Echo cancellation
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Echo cancellation
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Comparison

• The echo cancellation avoids the necessity of
  transmitting at more than double the subscriber
  rate.
• TCM 2km 144kbps
• EC 4km 144kbps.
• TCM for 4km require the extensive use of
  equipment such as concentrators and repeaters to
  overcome the poor range of the technology
• Disadvantage requiring complex digital signal
  processing circuitry.
• used by ADSL (achieving digital subscriber lines)

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Optical Fiber in the subscriber line

• In Broadband ISDN, there has been significant
  effort devoted to design alternatives for
  bringing fiber to business and residential
  subscribers.
• Approaches
• the subscriber interface appears as a simple
  direct link
• the subscriber interface must implement multiple
  access logic

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Twisted Pair arrangement

• There is a direct, point-to-point, twisted pair
  link between each subscriber and the central
  office (3km).
• Star topology.

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Fiber-based arrangement

• The central office is connected to a set of nodes
  by feeder cables.
• TDM technology
• Subscriber may be connected to a node.
• Node multiplex and de-multiplex
• Active star topology

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Fiber-based arrangement
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Full-duplex transmission

• Two fiber
• Wavelength-division multiplexing (WDM)

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Cascade multiplex
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Active star

• The subscriber is not aware of the details of the
  implementation of the feeder and distribution
  network.
• TDM structure of the feeder cable is of no
  concern to the subscriber equipment.

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Passive star

• Simply the remote nodes at the cost of additional
  logic at the subscriber equipment.
• The feeder cable carries multiple channels as
  before.
• At the remote node, the signal is optically split
  onto a umber of fibers going to the individual
  subscribers. Thus, all subscribers recessive the
  same signal.
• Remote nodes do not require power.
• Disadvantage require more complex equipment at
  the subscriber end.

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Passive Star

• Two approaches to multiplexing are possible
• Dense WDM
• 40 to 50 wavelengths per fiber.
• 20 to 25 subscriber per feeder cable.
• TDM
• good because less expensive.

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Line Coding Technology

• In ISDN, both analogy and digital data are
  transmitted using digital signals.
• Digital signal
• 1-gt a constant positive voltage level
• 0-gt a constant negative voltage level.
• More complex encoding schemes may be used to
  improve performance or quality.

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Evaluation Criteria

• Interpreting Task
• The receiver must know the timing of each bit.
• The receiver must determine whether the signal
  level for each voltage pulse is high or low.
• Successful factor
• Signal to-noise ratio (S/N)
• Data rate
• Bandwidth
• Data rate ? ? bit error rate ?
• S/N ? ? bit error rate ?
• bandwidth ? ? data rate ?

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Encoding scheme

• Encoding Scheme mapping from data bits to
  signal elements, can be used to improve
  performance.
• Evaluate and compare criteria
• Signal spectrum
• Signal synchronization capacity
• Error-detection capacity
• Cost and complexity

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Signal spectrum

• A lack of high-frequency components means that
  less bandwidth is required for transmission.
• lack of DC component is also desirable. AC
  coupling can provide excellent electrical
  isolation and reduce interference.

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Signal synchronization capacity

• The receiver must know the timing of each bit.
• There must some signal synchronization capability
  between transmitter and receiver.
• Approaches
• provide a separate clock at each end (expensive).
• provide some synchronization mechanism based on
  the transmitted signal.

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Encoding scheme criteria

• Error-detection capacity
• error detection is the responsibility of a data
  link protocol.
• It is useful to have some error-detection
  capability built into the physical signaling
  scheme.
• Cost and complexity

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Encoding Method

• Non-return to zero
• Multilevel Binary
• Bipolar-AMI
• pseoduternary
• Code Substitution Techniques
• B8ZS
• HDB3

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Non-return to zero

• A negative voltage is used to represent one bit
  value and a positive voltage is used to represent
  the other.
• Easiest to engineer and make efficient use of
  bandwidth
• presence of DC component
• lack of synchronization
• used for digital magnetic recording.

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Multilevel Binary

• These codes use more than two signal levels.
• Bipolar-AMI (Alternate mark inversion)
• pseudoternary

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Bipolar-AMI

• a binary 0 is represented by no line signal,
• a binary 1 is represented by a positive or
  negative pulse.
• The binary 1 pulses must alternate in polarity.
• No loss of synchronization if a long string of 1s
  occurs.
• No DC component
• Bandwidth is less than the bandwidth for NRZ
• Pulse alternation property provides a simple
  means of error detection.

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Bipolar -AMI
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pseudoternary

• The binary 1 is represented by the absence of a
  line signal
• Binary 0 by alternating positive and negative
  pulse.

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NRZ vs.. Multilevel Binary

• Multilevel binary the line signal may take on
  one of three levels.
• each signal bit element could represent
  log231.58 bit of information.
• Multilevel binary is not as efficient as NRZ
  coding.
• receiver must distinguish between (A, -A, 0).
• Multilevel requires more 3dB power than NRZ for
  the same probability of bit error.

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Power requirement
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Code Substitution Techniques

• Sequence that would result in a constant voltage
  level on the line are replaced by filling
  sequence that will provide sufficient transition
  for receivers clock to maintain synchronization.
• The filling sequence is the same length as the
  original sequence, no data rate increase
• Design Goal
• No DC component
• No long sequences of zero-level line signals.
• No reduction in data rate.
• Error-detection capability.

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ISDN techniques

• B8ZS bipolar with 8 zeros substitution.
• If an octet of all zeros occurs and the last
  voltage pulse preceding this octet was positive,
  then the eight zeros of the octet are enclosed as
  0 0 0 - 0 - .
• If an octet of all zeros occurs and the last
  voltage pulse preceding this octet was negative,
  then the eight zeros of the octet are enclosed as
  0 0 0 - 0 -.

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B8ZS
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HDB3

• HDB3 (high-density bipolar 3 zero) the scheme
  replaces strings of four zeros with sequences
  containing one or two pulses.
• The fourth zero is replaced with a code
  violation.
• A rule is needed to ensure that successive
  violations are of alternate polarity so that no
  DC component is introduced.
• If the last violation was positive, the violation
  must be negative, and vice versa.
• suited for to high-data-rate transmission.

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HDB3
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3.3 U interface

• ITU-T ISDN recommendations for ISDN do not
  include a complete specification for the ISDN
  subscriber line.
• G. 961 address the interface between subscriber
  equipment and the subscriber line.
• G.961 is only a partial specification. It
  specifies the use of either echo cancellation or
  time-compression multiplexing over a single
  twisted pair.
• American use ANSI.601 coding

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2B1Q

• 2B1Q (two binary, one quaternary) coding each
  signaling element represent two bits instead of
  one.
• Four different voltage levels are used.

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2B1Q coding
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Performance

• data rate expressed in bits per second (bps) is
  the rate at which bit values are transmitted.
• Modulation rate, expressed in bauds, is the rate
  at which signal elements are generated.

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Analog Signaling Techniques

• ASK (Amplitude-shift keying)
• FSK (Frequency-shift keying)
• PSK (Phase-shift keying)

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ASK (Amplitude-shift keying)
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FSK (Frequency-shift keying)
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PSK (Phase-shift keying)
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3.4 Quadrature Amplitude Modulation

• QAM (Quadrature Amplitude Modulation) is a
  popular analog signaling technique used in ADSL.
• It is possible to send two different signals
  simultaneously on the same carrier frequency, by
  using two copies of the carrier frequency, one
  shifted by 90? with respect to the other.
• ASK is used.

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QAM
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QAM

• two level -gt 256 level.
• The greater the number of stages, the higher the
  potential error due to noise and attenuation.

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ADSL

• ADSL (Asymmetric Digital Subscriber Line)
• Asymmetric ADSL provides more capacity
  downstream than upstream.
• For video on demand services.
• ADSL provides a perfect fit for the INTERNET
  requirement.
• ADSL use FDM in a novel way to exploit the 1-MHz
  capacity of twisted pair.
• 5.5 km (95 of all U.S.A.)

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ADSL element

• Reserve lowest 25 MHz for voice (POTS plain old
  telephone service) 0-4hHz band.
• Use either echo cancellation or FDM to allocate
  two bands (upstream and down stream).
• Use FDM within the upstream and downstream bands.
  A single bit stream is split into multiple
  parallel bit streams and each portion is carried
  in s separate frequency band.

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ADSL (FDM)
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ADSL (echo cancellation)
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ADSL (echo cancellation)

• Advantages of Echo cancellation
• The higher the frequency, the greater the
  attenuation. More of the downstream bandwidth is
  in the good part of the spectrum.
• More flexible for changing upstream capacity
• Disadvantage
• echo cancellation logic circuit on both end.

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Discrete Multitone (DMT)

• DMT uses multitone carrier signals at different
  frequencies, sending some of the bits on each
  channel.
• 4-kHz per channel
• Process
• (1) DMT modem send out test signal on each
  sub-channel to determine the S/N ratio.
• Modem assigns more bits to channels according
  (1), 060kbps per channel.

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DMT channel allocation
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DMT transmitter
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ADSL/DMT

• Present ADSL employ 256 downstream channels.
• 4kbps-gt60 kbps
• 60kbps 256 15.36 Mbps.
• Current implementation operate at from 1.5 to 9
  Mbps depending on line distance and quality.

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xDSL

• HDSL (High-Data-Rate) DSL
• SDSL (Single) DSL
• VDSL (Very High Data Rate) DSL

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