Computer Interfacing and 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 No start and stop bits, much lower overhead, faster transmission Winter 2006 ECE ECE 766 Computer Interfacing and Protocols

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) Winter 2006 ECE ECE 766 Computer Interfacing and Protocols

Computer Interfacing and Protocols 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 Winter 2006 ECE ECE 766 Computer Interfacing and Protocols

Computer Interfacing and Protocols 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 S Y N S O H S T X E T X B C C Header …Data… Winter 2006 ECE ECE 766 Computer Interfacing and Protocols

Computer Interfacing and Protocols BSC Multiblock frame: Multiframe transmission: S Y N S O H S T X I T B B C C Header …Data… E T X Block S Y N S O H S T X I T B B C C Header …Data… E T B A C1 K S Y N S O H S T X I T B B C C Header …Data… E T X A C0 K Winter 2006 ECE ECE 766 Computer Interfacing and Protocols

Computer Interfacing and Protocols 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) Winter 2006 ECE ECE 766 Computer Interfacing and Protocols

Computer Interfacing and Protocols 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 Winter 2006 ECE ECE 766 Computer Interfacing and Protocols

Computer Interfacing and Protocols 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? S Y N HEADER Source Dest. MSG Byte Count C R C …Data… Winter 2006 ECE ECE 766 Computer Interfacing and Protocols

Computer Interfacing and Protocols 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 Flag Address Control Variable length data Check Winter 2006 ECE ECE 766 Computer Interfacing and Protocols