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Chapter 3 Line Coding and the Subscriber Line

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Title: Chapter 3 Line Coding and the Subscriber Line


1
Chapter 3 Line Coding and the Subscriber Line
2
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.

3
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

4
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.

5
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?

6
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.

7
FSK
1170
2125
8
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.

9
Echo
  • Alternative way dispense with modem and to
    transmit directly.
  • some portion of the originators signal returns
    in the form of an echo.

10
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)

11
(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.

12
TCM
13
TCM
14
TCM
15
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).

16
TCM
  • The actual data rate A on the medium AB/Tb.
  • A2R(1(Tp Tq)/Tb)
  • B16 to 24 bits.

17
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.

18
Echo cancellation
19
Echo cancellation
20
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)

21
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

22
Twisted Pair arrangement
  • There is a direct, point-to-point, twisted pair
    link between each subscriber and the central
    office (3km).
  • Star topology.

23
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

24
Fiber-based arrangement
25
Full-duplex transmission
  • Two fiber
  • Wavelength-division multiplexing (WDM)

26
Cascade multiplex
27
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.

28
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.

29
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30
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.

31
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.

32
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 ?

33
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

34
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.

35
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.

36
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

37
Encoding Method
  • Non-return to zero
  • Multilevel Binary
  • Bipolar-AMI
  • pseoduternary
  • Code Substitution Techniques
  • B8ZS
  • HDB3

38
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.

39
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40
Multilevel Binary
  • These codes use more than two signal levels.
  • Bipolar-AMI (Alternate mark inversion)
  • pseudoternary

41
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.

42
Bipolar -AMI
43
pseudoternary
  • The binary 1 is represented by the absence of a
    line signal
  • Binary 0 by alternating positive and negative
    pulse.

44
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.

45
Power requirement
46
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.

47
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 -.

48
B8ZS
49
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.

50
HDB3
51
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

52
2B1Q
  • 2B1Q (two binary, one quaternary) coding each
    signaling element represent two bits instead of
    one.
  • Four different voltage levels are used.

53
2B1Q coding
54
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.

55
Analog Signaling Techniques
  • ASK (Amplitude-shift keying)
  • FSK (Frequency-shift keying)
  • PSK (Phase-shift keying)

56
ASK (Amplitude-shift keying)
57
FSK (Frequency-shift keying)
58
PSK (Phase-shift keying)
59
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.

60
QAM
61
QAM
  • two level -gt 256 level.
  • The greater the number of stages, the higher the
    potential error due to noise and attenuation.

62
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.)

63
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.

64
ADSL (FDM)
65
ADSL (echo cancellation)
66
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.

67
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.

68
DMT channel allocation
69
DMT transmitter
70
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.

71
xDSL
  • HDSL (High-Data-Rate) DSL
  • SDSL (Single) DSL
  • VDSL (Very High Data Rate) DSL

72
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