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Example Point-to-Point Data Link Layer protocol: HDLC

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• High-level Data Link Control (HDLC) protocol
  • Is a very popular data link protocol (used even today)

  • Original Frame format:

      FLAG       Address    Control            X            FCS(CRC)
    +----------+----------+----------+--------------------+---------------+----------+
    | 01111110 |  8 bits  |  8 bits  | Information (data) | 16 or 32 bits | 01111110 |
    +----------+----------+----------+--------------------+---------------+----------+
    

  • The address field is used to identify the recepient (like the SSN of the receiver)

  • The control field contains the Send Sequence number and a piggybacked ACK sequence number:

           Control  
         +----------+
         | 0SSS0AAA |        SSS = Send Sequence Number
         +----------+        AAA = ACK number
    

  • The address and control field was expanded to make the protocol useful for higher bandwidth network.