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 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

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 SOH start 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

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

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 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 …

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

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)

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

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

Synchronous Protocols Character-oriented Protocol – BSC (cont) Multiframe Data Transmission some messages may be too long to fit into one frame – split message ETB (end transmission block) is used instead of ETX to signify that the end of frame is not the end of transmission receiver must acknowledge each frame

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

Synchronous Protocols Bit-oriented Protocols

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

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

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

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

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

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

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

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

Synchronous Protocols Bit-oriented Protocol – HDLC I-frame Flag – a byte with bit pattern that identifies the beginning and 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

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

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.

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