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Data Link Protocols

Data Link Protocols. Specifications to implement data link layer Asynchronous Protocols: Primarily used in modems Feature start and stop bits and variable length gaps between characters Synchronous Protocols: Developed for higher speed networks

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Data Link Protocols

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  1. Data Link Protocols • Specifications to implement data link layer • Asynchronous Protocols: • Primarily used in modems • Feature start and stop bits and variable length gaps between characters • Synchronous Protocols: • Developed for higher speed networks • No start and stop bits, much lower overhead, faster transmission ECE 766 Computer Interfacing and Protocols

  2. Synchronous Protocols • Character-oriented protocols: • Frames are interpreted as a sequence of characters • Example: Binary Synchronous Communication (BSC) • Character count-oriented protocols: • Frame length specified within the header • Example: Digital Data Communication Message Protocol, Count Type (DDCMP) • Bit-oriented protocols: • Frames are interpreted as a sequence of bits • Example: High-Level Data Link Control (HDLC) ECE 766 Computer Interfacing and Protocols

  3. BSC • Half duplex, can be used with ASCII, EBCDIC, and Six Bit Transcode • Control information is in the form of code words taken from the character set • Control information is carried in separate frames as well as within data frames • Line control, flow control, error control ECE 766 Computer Interfacing and Protocols

  4. SYN SYN SOH STX ETX BCC BCC Header …Data… BSC • Data Frames: • SYN = (0)0010110 (ASCII 268) • Header: • Address, sequence number for stop-and-wait ARQ • Non-standardized • BCC: Block Check Character • 1-character LRC • 2-character CRC ECE 766 Computer Interfacing and Protocols

  5. Block Block SYN SYN SOH STX ITB BCC BCC STX ETX BCC BCC Header …Data… …Data… SYN SYN SYN SYN SOH SOH STX STX ITB ITB BCC BCC BCC BCC STX STX ETX ETB BCC BCC BCC BCC Header Header …Data… …Data… …Data… …Data… BCC BCC BCC BCC AC1K AC0K SYN SYN SYN SYN BSC • Multiblock frame: • Multiframe transmission: ECE 766 Computer Interfacing and Protocols

  6. BSC • Control Frames: • Frames used only for signaling • SYN SYN {Control Characters} BCC • Connection establishment (bid, poll, select, positive and negative responses) • Connection termination (end of transmission) • Flow and error control(ACK0, ACK1, NAK, WACK (wait and ACK), reverse interrupt, temporary delay) ECE 766 Computer Interfacing and Protocols

  7. BSC • Data Transparency • When sending binary data, certain sequences may be identical to control characters • Solution: Define transparent blocks and use character stuffing • Start transparent text block with DLE STX • End transparent text block with DLE ETX, DLE ITB, DLE ETB • To send DLE character, send DLE DLE ECE 766 Computer Interfacing and Protocols

  8. SYN SYN HEADER SourceDest. MSGByteCount CRC CRC …Data… DDCMP • Character-count-oriented protocol • Instead of using control characters to mark the end of the frame, send the length of the frame • Can also be used asynchronously • What happens when header is damaged? ECE 766 Computer Interfacing and Protocols

  9. Flag Address Control Variable length data Check Flag HDLC • Bit-oriented protocol • Special bit pattern to signal beginning and end of frame (01111110) • This pattern cannot be repeated in the frame • Solution: Bit stuffing • Transmitter automatically stuffs dummy 0 after five 1s • If after five 1s the sequence 10 is received, it is the reserved pattern • If less than 15 1s are received, it signals abort, otherwise idle channel ECE 766 Computer Interfacing and Protocols

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