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

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]

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


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 headerseq. no. (frame no.)

validity check for seq. no.field 3 – 128 bytes datafield 4 – CRC for data only


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 …


Character-oriented Protocol – BSC (cont) BSC Frames


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)


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


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


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


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


Bit-oriented 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


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


Bit-oriented Protocol – HDLC Configuration – Unbalanced (master/slave)

one device is primary and others are secondary point-to-point and multipoint


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


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


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


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


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


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


Bit-oriented Protocol – HDLC S-frame

Flag, address, FCS - similar Control – acknowledgment but no data to send


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.


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

Documents

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