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Intro to bit-oriented (framing) protocols

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• Bit-oriented framing methods
  • Characteristics of bit-oriented framing methods:

    A bit-oriented protocol does not impart special meaning to byte patterns One exception: There is one special pattern to signal the start of a frame It's called the flag The smallest transmitted unit is: One bit !!!

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  • Note:

    Bit-oriented framing is used transmit large amount of data Consequently: To avoid clock drift, a bit-oriented protocol use Manchester encoding

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• Highlevel Datalink Link Control protocol (HDLC)
  • HDLC:

    HDLC is the most popular bit-oriented framing protocol HDLC is a commonly used Data link layer protocol on long haul (wide area) links HDLC is an ISO standard

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• The flag pattern of HDLC
  • Recall:

    Flag = a bit pattern that signals: The start of a frame (in a bit-oriented framing protocol) The end of a frame (in a bit-oriented framing protocol)

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  • The flag pattern of HDLC (happens to be a byte):

    01111110

    Example:

    Find the HDLC frame in the following transmission: Answer:

    • Remember: Frames are delimited by the flag sequence which is 01111110

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  • Problem:

    What do you do if the message contains the FLAG (01111110) ???

    Will be handled in the next webpage.....

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• The idle HDLC transmitter

  • If a sender has no frames to send, it must transmit the flag pattern continuously:

    ....011111100111111001111110.... (idle HDLC connection)