network framing

Framing - how portray sequence of bits into a complete message

• on packet-switched networks frames are exchanged between nodes via adapters
• adaptor needs to recognize a frame as a set of bits, and there are several protocols:
  • bit-oriented protocol
  • byte-oriented protocol - also called character-oriented protocol
  • clock-based protocol

Bit-Oriented Protocols (HDLC)

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• views frame as collection of bits
• types of bit-oriented protocols
  • SDLC/HDLC (Synchronous/High Level Data Link Control)
• SDLC/HDLC (Synchronous/High Level Data Link Control)
  • sentinel bits
    • 01111110 - denotes start and end of frame
  • bit stuffing
    • anytime sender transmits 5 consecutive 1s, transmit a 0 afterwards
    • anytime receiver reads 5 consecutive 1s, check next bit. If 0 remove it and continue reading, otherwise end-of-frame marker is read

Byte-Oriented Protocol

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• views each frame as a set of bytes rather than bits
• types of byte-oriented protocols
  • BISYNC (Binary Synchronous Communication)
  • PPP (Point-to-Point Protocol)
  • DDCMP (Digital Data Communication Message Protocol)
• sentinel-based approach
  • uses special characters to indicate where frames start and end
  • character stuffing - escapes data that may be misinterpreted as sentinel characters so that data may not be misinterpreted as sentinel characters
• BISYNC (Binary Synchronous Communication)
  • look at figure 2.7
  • uses sentinel characters and character stuffing (DLE (data-link-escape) character)
  • includes CRC (cyclic redundancy check) at end of frame which is used to detect errors
• PPP (Point-to-Point Protocol)
  • commonly used to carry Internet Protocol packets
  • look at figure 2.8
  • uses sentinel characters and character stuffing
• DDCMP (Digital Data Communication Message Protocol)
  • look at figure 2.9
  • COUNT field specifies how many bytes are contained in the frame’s body

Clock-Based Framing

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• dominant standard in long-distance optical networks
• Synchronous Optical Network (SONET)
  • addresses both framing problem and encoding problem
  • full specifications is bigger than the book
  • go to page 121