Data Link Control Protocols

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Data Link Control Protocols

• Interface standards provide means for which a
  strem of data can be transmitted but do not
  include function of error control and flow
  control
• A data link control protocol handles error
  control and flow control
• Examples of data link protocols
• HDLC High Level Data Link Control
• LAPB
• Connect devices to packet switched networks
• LAPF
• Frame relay

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HDLC

• Accepts user data from higher level software and
  delivers across link to another device
• On other side HDLC accepts user data and send it
  to application
• Both HDLC modules exchange control information to
  provide
• Flow control
• Error control

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HDLC

• Flag
• Used for synchronization
• Beg and end of frame
• Address
• Address of secondary stations
• Point to point or multidrop
• Control
• Identifies purpose and function of frame
• Information
• User Data
• Frame Check Sequence
• 16 or 32 bit CRC

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HDLC

• Information Frames
• I frame
• Carries user data
• Can also carry flow control and error control
• Supervisory Frames
• S Frames
• Flow control and error control
• Unnumbered frames
• Link set up and disconnection
• No acknowledgement information

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HDLC in Action

• Initialization
• Specifies which of 3 modes to operate in
• Specifies 3 or 7 bit sequence
• If B accepts B sends unnumbered acknowledgement
• If rejected B sends disconnected mode (DM)
• A can send a disconnect (DISC) and B acknowledges
  with unnumbered acknowledgement
• Because of fault or instruction from application
  software

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HDLC in Action
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Multiplexing

• Multiple sources but only one link

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Multiplexing

• Costs to a business
• Transmission service is very expensive
• Leased lines, packet switched networks
• Multiplexing and compression techniques save
  businesses money
• As data capacity of lines increase it becomes
  more cost effective for the company
• Most data devices require modest data rate support

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Multiplexing

• Alternative Approaches
• Direct Point to Point
• Need I/O port for each device
• Need line for each device
• Need large amount of wiring if on different floors

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Multiplexing

• Alternative Approaches
• Multidrop Line
• Host polls machines to see who wants to send
• Saves I/O ports
• Total communications load is not greater than
  data rate of line

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Multiplexing

• Using a multiplexer approach
• All devices to MUX and one line to host
• Number of lines in the number of lines out
• Link carries multiple channels of information

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Multiplexing

• FDM Frequency Division Multiplexing
• Sends signals in several distinct frequency
  ranges
• Used in cable TV
• Carry multiple video channels on a single cable
• Imagine cost of stringing wire for each channel
  to peoples homes
• Each signal is modulated onto a different carrier
  frequency and carrier frequencies are separated
  by guard bands
• Bandwidth of the transmission medium exceeds
  required bandwidth of all the signals

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Multiplexing
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Time Division Multiplexing

• Used for digital signals or analog signals
  carrying digital data
• Data rate of transmission media exceeds data rate
  of signals
• Uses a frame
• One slot for each slice of time
• One or more slots for each device channel
• Time slots are transmitted whether source has
  data or not

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Multiplexing
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