Data Link Protocols Specifications to implement data link

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 766 Computer Interfacing and Protocols 16 - 1

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 766 Computer Interfacing and Protocols 16 - 2

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 766 Computer Interfacing and Protocols 16 - 3

BSC • Data Frames: S Y N S O H Header S T X …Data… E T X B C C – 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 Winter 2006 ECE 766 Computer Interfacing and Protocols 16 - 4

BSC • Multiblock frame: S Y N S S Y O N H Header S T X Block I T B B C C S T X …Data… I T B B C C S T X …Data… E T X B C C …Data… E T B B C C …Data… E T X B C C • Multiframe transmission: S Y N S O H B C C A S C 1 Y K N S Y N S O H B C C A S C 0 Y K N Winter 2006 ECE Header S T X S Y N ECE 766 Computer Interfacing and Protocols 16 - 5

BSC • Control Frames: – Frames used only for signaling • SYN {Control Characters} BCC – Connection establishment (bid, poll, select, positive and negative responses) – Connection termination (end of transmission) – Flow and error control (ACK 0, ACK 1, NAK, WACK (wait and ACK), reverse interrupt, temporary delay) Winter 2006 ECE 766 Computer Interfacing and Protocols 16 - 6

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 Winter 2006 ECE 766 Computer Interfacing and Protocols 16 - 7

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 Winter 2006 ECE S Source Y HEADER Dest. N MSG Byte Count C R C …Data… ECE 766 Computer Interfacing and Protocols C R C 16 - 8

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 1 s – If after five 1 s the sequence 10 is received, it is the reserved pattern – If less than 15 1 s are received, it signals abort, otherwise idle channel Flag Address Control Winter 2006 ECE Variable length data ECE 766 Computer Interfacing and Protocols Check Flag 16 - 9