ICSA 411: Week 3b Data Communication, Contd

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ICSA 411 Week 3bData Communication, Contd

• Elizabeth Lane Lawley, Instructor

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Data Link Control

• Specified flow and error control for synchronous
  communication
• Data link module arranges data into frames,
  supplemented by control bits
• Receiver checks control bits, if data is intact,
  it strips them

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Bit Stuffing

• Allows data link layer to frame data with unique
  bit patterns
• Typically, the unique pattern is 6 consecutive 1s
  framed by 0s
• Whenever five 1s are found in the data, sending
  station inserts a 0 after the fifth 1
• Whenever receiver detects five 1s followed by a
  0, the 0 is removed

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Bit Stuffing Example

• Suppose network layer gives 16 1s to the data
  link layer for transmission
• Data link layer sends the following start/stop
  flags and data to physical layer01111110 111110
  111110 111110 1 01111110
• Data link layer on receiver removes start/stop
  sequences and the stuffed bits
• Note Example shows only start/stop flags and
  user data

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High-Level Data Link Control

• On transmitting side, HDLC receives data from an
  application, and delivers it to the receiver on
  the other side of the link
• On the receiving side, HDLC accepts the data and
  delivers it to the higher level application layer
• Both modules exchange control information,
  encoded into a frame

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HDLC Frame Structure

• Flag 01111110, at start and end
• Address secondary station (for multidrop
  configurations)
• Information the data to be transmitted
• Frame check sequence 16- or 32-bit CRC

• Control purpose or function of frame
• Information frames contain user data
• Supervisory frames flow/error control (ACK/ARQ)
• Unnumbered frames variety of control functions
  (see p.131)

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HDLC Operation

• Initialization S-frames specify mode and
  sequence numbers, U-frames acknowledge
• Data Transfer I-frames exchange user data,
  S-frames acknowledge and provide flow/error
  control
• Disconnect U-frames initiate and acknowledge

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Transmission Efficiency Multiplexing

• Several data sources share a common transmission
  medium simultaneously
• Line sharing saves transmission costs
• Higher data rates mean more cost-effective
  transmissions
• Takes advantage of the fact that most individual
  data sources require relatively low data rates

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Transmission Efficiency Data compression

• Reduces the size of data files to move more
  information with fewer bits
• Used for transmission and for storage
• Often combined with multiplexing to increase
  efficiency

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Alternate Approaches to Terminal Support

• Direct point-to-point links
• Multidrop line
• Multiplexer
• Integrated MUX function in host

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Direct Point-to-Point
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Multidrop Line
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Multiplexer
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Integrated MUX in Host
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Frequency Division Multiplexing

• Requires analog signaling transmission
• Total bandwidth sum of input bandwidths
  guardbands
• Modulates signals so that each occupies a
  different frequency band
• Standard for radio broadcasting, analog telephone
  network, and television (broadcast, cable,
  satellite)

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Synchronous Time-Division Multiplexing (TDM)

• Used in digital transmission
• Requires data rate of the medium to exceed data
  rate of signals to be transmitted
• Signals take turns over medium
• Slices of data are organized into frames
• Used in the modern digital telephone system
• US, Canada, Japan DS-0, DS-1 (T-1), DS-3 (T-3),
  ...
• Europe, elsewhere E-1, E3, ...

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Statistical Time Division Multiplexing

• requires digital signaling transmission
• data rate capacity required is well below the sum
  of connected capacity
• same concepts as synchronous TDM
• uses memory buffers to avoid loss of data
• widely used for remote communications with
  multiple terminals
• similar to medium-sharing done by LANs

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Data Compression

• Works on the principle of eliminating redundancy
• Codes are substituted for compressed portions of
  data
• Lossless reconstituted data is identical to
  original (GIF, ZIP)
• Lossy reconsituted data is only perceptually
  equivalent (JPEG, MPEG)

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Run Length Encoding

• Replace string of anything with flag, character,
  and count

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Huffman Encoding

• Length of each character code based on
  statistical frequency in text
• Modified Group III Fax
• Encodes runs of black or white
• 4 million pixels to
• full page

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Lempel-Ziv Encoding

• Used in V.42 bis, ZIP
• buffer strings at transmitter and receiver
• replace strings with pointer to location of
  previous occurrence
• algorithm creates a tree-based dictionary of
  character strings

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Lossy Algorithms (JPEG/MPEG)

• Scaling and color conversion (to YUV)
• Color subsampling (reduces hue info)
• Discrete cosine transformation
• Quantization
• Run-length encoding
• Huffman coding (lossless compression)
• Interframe compressions (MPEG only)