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Chapter 11 Data Link Protocols

Chapter 11 Data Link Protocols. A data link protocol is a set of specifications used to implement the data link layer [A protocol is the set of rules or specifications used to implement one or more layers of the OSI model]. Asynchronous Protocols.

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Chapter 11 Data Link Protocols

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  1. Chapter 11Data Link Protocols • A data link protocol is a set of specifications used to implement the data link layer • [A protocol is the set of rules or specifications used to implement one or more layers of the OSI model]

  2. Asynchronous Protocols • in asynchronous transmission a data unit is transmitted with no timing coordination between sender and receiver • employed mainly in modems, asynchronous protocol feature start and stop bits and variable length between gaps

  3. Asynchronous Protocols • XMODEM • designed by Ward Christiansen in 1979 for file transfer over telephone line • half-duplex, stop and wait protocol field 1 – one byte SOHstart of header field 2 – two byte header seq. no. (frame no.) validity check for seq. no. field 3 – 128 bytes data field 4 – CRC for data only

  4. Asynchronous Protocols • YMODEM – similar to XMODEM except data unit is 1024 bytes, two CANs to abort transmission, use ITU-T CRC-16 and support multiple files • ZMODEM – combining features of XMODEM & YMODEM • BLAST – Blocked asynchronous transmission, full-duplex, sliding windows, support text and binary files • Kermit – the most widely used async. protocol, designed by Columbia Univ, used for terminal emulation and file tranfer

  5. Synchronous Protocols • character-oriented (byte-oriented) protocols – the frame or packet is interpreted as a series of characters • bit-oriented protocols – the frame or packet is interpreted as a series of bits

  6. Synchronous Protocols • Character-oriented Protocol - Binary Synchronous Communications (BSC) • developed by IBM in 1964 • usable in point-to-point and multipoint, supports half duplex, stop-and –wait ARQ flow control and error correction • BSC Control characters (represented by one or more char) • ACK 0 – good even frame received or ready to receive • ACK 1 – good odd frame received • ENQ – request for a response • NAK – bad frame received or nothing to send • SYN – alert receiver to incoming frame • …

  7. Synchronous Protocols • Character-oriented Protocol – BSC (cont) • BSC Frames

  8. Synchronous Protocols • Character-oriented Protocol – BSC (cont) • Data Frames • SYN character alert the receiver to the arrival of a new frame and provide a bit pattern for time synchronization • STX (start text), ETX (end text), BCC (block check count)

  9. Synchronous Protocols • Character-oriented Protocol – BSC (cont) • Data Frames with header field • SOH (start of header) • header may include address of receiving device, address of sending device, frame number for stop-and-wait ARQ

  10. Synchronous Protocols • Character-oriented Protocol – BSC (cont) • Multiblock Data Frames • message may be divided into several blocks • ITB (intermediate text block) • error checking done for each block, if any block contains error, the entire must be retransmitted • only single acknowledgment required

  11. Synchronous Protocols • Character-oriented Protocol – BSC (cont) • Multiframe Data Transmission • some messages may betoo long to fit into oneframe – split message • ETB (end transmissionblock) is used insteadof ETX to signify that theend of frame is not theend of transmission • receiver mustacknowledge each frame

  12. Synchronous Protocols • Character-oriented Protocol – BSC (cont) • Control Frames • carries information specific to the functioning of the data link layer itself – establishing connections, flow and error control, terminating connections

  13. Synchronous Protocols • Bit-oriented Protocols

  14. Synchronous Protocols • Bit-oriented Protocol • Synchronous Data Link Control (SDLC) • developed by IBM in 1975 • forwarded to the ISO for it to become standard • ISO answered with HDLC • High-level Data Link Control (HDLC) • ISO and ITU-T standard • all bit-oriented protocols are related to HDLC • support half-duplex, full-duplex modes and point-to-point, multipoint configurations

  15. Synchronous Protocols • Bit-oriented Protocol – HDLC • Station Types • Primary – has complete control of the link; sends commands to secondary stations • Secondary – sends responses on command received • Combined – can behave either as primary or secondary; send commands and response

  16. Synchronous Protocols • Bit-oriented Protocol – HDLC • Configuration – Unbalanced (master/slave) • one device is primary and others are secondary • point-to-point and multipoint

  17. Synchronous Protocols • Bit-oriented Protocol – HDLC • Configuration – Symmetrical • each physical station consists of two logical stations – a primary and a secondary • separate link links fortwo logical stations • control of link can shift between thetwo stations

  18. Synchronous Protocols • Bit-oriented Protocol – HDLC • Configuration – Balanced • both stations in point-to-point topology are of the combined type • a single link is used and can be controlled by either stations

  19. Synchronous Protocols • Bit-oriented Protocol – HDLC • Modes of Communication • Normal response mode (NRM) • standard primary-secondary relationship; a secondary device must have permission from the primary before transmitting • Asynchronous response mode (ARM) • a secondary may initiate a transmission without permission from the primary whenever the channel is idle; all transmission from a secondary (even to another secondary) must be through the primary

  20. Synchronous Protocols • Bit-oriented Protocol – HDLC • Modes of Communication • Asynchronous balanced mode (ABM) • in balanced configuration; all stations are equal; any one can initiate transmission without permission from the other

  21. Synchronous Protocols • Bit-oriented Protocol – HDLC • Frames • I-frames (information frames) – used to transport user data and control information relating to user data • S-Frames (supervisory frames) – used only to transport control information, e.g. flow control, error control • U-Frames (unnumbered frames) – reserved for system management, i.e. managing the link itself

  22. Synchronous Protocols • Bit-oriented Protocol – HDLC • I-frame • Flag – a bytewith bit pattern01111110that identifiesthe beginningand end of frame and for sync. • Address – one byte or multiple byte; a from address (if secondary) or a to address (if primary) • control – for acknowledgment of the number of frame received and the number of frame expected • FCS – error detection; two or four bytes CRC

  23. Synchronous Protocols • Bit-oriented Protocol – HDLC • S-frame • Flag, address, FCS - similar • Control – acknowledgment but no data to send

  24. Synchronous Protocols • Bit-oriented Protocol – HDLC • U-frame • Flag, address, FCS – similar • Control – identifies type if U-frame • Info – used for managing the network; may or may not be present.

  25. Synchronous Protocols • Bit-oriented – Link Access Procedures (LAP) • LAPB – LAP Balanced • a simplified subset of HDLC; uses ABM; used in ISDN on B channel • LAPD – LAP for D channel • uses ABM; used in ISDN on D channel • LAPM – LAP for Modem • a simplified subset of HDLC for modems; designed to do asynchronous-synchronous conversion, error detection and retransmission

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