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DATA LINK CONTROL PROTOCOLS

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Page 1: DATA LINK CONTROL PROTOCOLS. 2 Introduction Data link control layer – often abbreviated simply to data link layer – is concerned with the transfer of

DATA LINK CONTROL PROTOCOLS

Page 2: DATA LINK CONTROL PROTOCOLS. 2 Introduction Data link control layer – often abbreviated simply to data link layer – is concerned with the transfer of

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Introduction Data link control layer – often abbreviated

simply to data link layer – is concerned with the transfer of data over a serial data link

The transmission mode may be either asynchronous or synchronous and based on either a character-oriented or a bit-oriented transmission control protocol

The data link layer is fundamental to the operation of all data communication applications

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Introduction

Depending on the application, the user service provided by the data link layer may be either a simple best-try (connectionless) service or a reliable (connection-oriented) service

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Introduction The connectionless service

means that although error check bits are used to detect errors, any frames that are found to contain transmission errors are simply discarded by the link layer protocol entity.

It is also referred to as an unacknowledged service

Retransmission becomes a function of a higher layer protocol

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Introduction The connection-oriented service

Data link protocol employs error and flow control procedures to provide a reliable service

To achieve this, prior to sending any data (information frames), a logical connection between the two data link layer protocol entities is established (using L_Connect service)

All data is transfer using a suitable retransmission and flow control protocol

When all data has been exchanged, the logical connection is clear (using L_Disconnect service)

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Introduction

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Application Environments In some instances the data link protocols

are located in the two communicating DTEs, and the protocol is said to operate on an end-to-end basis

In others, data link protocols operates over the local link connecting, for example, the DTE to a network, the protocol is said to have only local significance

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Application Environments

The data link topology may be Point-to-point circuit

Direct physical connection Operates on an end-to-end basis

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Application Envirionments

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Application Environments The data link topology may be

Multipoint or multidrop topology Single transmission line – known as a bus

or data highway – is used to connect all the computer together.

Normally used in applications that involve a single master (supervisory) computer communicating with a distributed community of slave computers

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Applications Environments

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Application Environments

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Character-oriented Protocols Character-oriented protocols

are in use in both point-to-point and multipoint applications

Are characterized by the selected transmission control characters used to perform the various transmission control function associated with link management, start of frame and end of frame – frame delimiting, error control and data transparency

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Character-oriented Protocols

Simplex protocols Half-duplex protocols Full-duplex protocols

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Character-oriented Protocols – Simplex protocols

Simplest since it allows a simplex – one direction only – transfer of data from one DTE to another

Point-to-point data link topology The most widely used protocols for

this function is Kermit

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Character-oriented Protocols – Simplex protocols

Kermit: Synchronous transmission (normally) Point-to-point data link Stop and Wait ARQ

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Character-oriented Protocols – Simplex protocols

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Character-oriented Protocols – Simplex protocols

Kermit: Frame format

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Character-oriented Protocols – Simplex protocols

Kermit: Frame format

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Character-oriented Protocols – Simplex protocols

Kermit Protocol operation

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Character-oriented Protocols – Half-duplex Protocols

Most character-oriented protocols operate in the half-duplex, stop and wait mode

The best known is BSC

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Character-oriented Protocols – Half-duplex Protocols

BSC: Synchronous transmission Connection-oriented protocol Multipoint network/ multidrop bus

network

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Character-oriented Protocols – Half-duplex Protocols BSC

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Character-oriented Protocols – Half-duplex Protocols

BSC: Frame format

Data supervisory

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Character-oriented Protocols – Half-duplex Protocols

BSC

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Character-oriented Protocols – Half-duplex Protocols

BSC

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Character-oriented Protocols – Half-duplex Protocols

BSC

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Character-oriented Protocols – Half-duplex Protocols

BSC Protocol operation

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Character-oriented Protocols – Half-duplex Protocols

BSC

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Character-oriented Protocols – Half-duplex Protocols BSC

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Character-oriented Protocols – Half-duplex Protocols

BSC Protocol

performance

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Character-oriented Protocols – Duplex Protocols

A few character-oriented protocols operate in full-duplex mode

Character-oriented, full duplex protocols ARPANET

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Character-oriented Protocols – Duplex Protocols

ARPANET

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Bit-oriented Protocols

All new data link protocols are bit-oriented protocols

Such protocols use defined bit patterns rather than transmission control characters to signal the start and end of frame – known as frame delimiting

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Bit-oriented Protocols – HDLC (High-level Data Link Control)

Network configuration Point-to-point with single primary and

secondary Multipoint with single primary and

multiple secondary Point-to-point with two primaries and

two secondaries

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Bit-oriented Protocols – HDLC (High-level Data Link Control)

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Bit-oriented Protocols – HDLC (High-level Data Link Control)

Operational modes Normal response mode (NRM)

used in balanced configurations Slave stations (or secondaries) can

transmit only when specially instructed by the master (primary) station

The link may be point-to-point or multipoint (in this case only one primary station is allowed)

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Bit-oriented Protocols – HDLC (High-level Data Link Control)

Operational modes Asynchronous response mode (ARM)

Used in unbalanced configurations Allows a secondary to initiate a

transmission without receiving permission from the primary.

Normally used with point-to-point configurations and duplex links

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Bit-oriented Protocols – HDLC (High-level Data Link Control)

Operational modes Asynchronous balanced mode (ABM)

Used mainly on duplex point-to-point links

Each station has an equal status and performs both primary and secondary functions

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Bit-oriented Protocols – HDLC (High-level Data Link Control)

Frame format

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Bit-oriented Protocols – HDLC (High-level Data Link Control)

Frame format Flag field:

a string of bits 01111110 Receiver hunts for flag sequence to synchronize Bit stuffing used to avoid confusion with data

containing 01111110 0 inserted after every sequence of five 1s If receiver detects five 1s it checks next bit If 0, it is deleted If 1 and seventh bit is 0, accept as flag If sixth and seventh bits 1, sender is indicating abort

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Bit-oriented Protocols – HDLC (High-level Data Link Control)

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Bit-oriented Protocols – HDLC (High-level Data Link Control)

Frame format Address field:

Depend on the mode operation In NRM, on multidrop line, every secondary

station is assigned a unique address. Whenever the primary station communicates with a secondary, the address field contains the address of the secondary

Not used in this way in ABM because only direct point-to-point links are involved. Instead, it is used to indicate the direction of commands and their associated responses

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Bit-oriented Protocols – HDLC (High-level Data Link Control)

Frame format FCS field

16-bit CRC with G(x) = x16 + x12 + x5 + 1

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Bit-oriented Protocols – HDLC (High-level Data Link Control)

Frame format Control field

Unnumbered frames used for such functions as link setup and

disconnect Information frames

carry the actual information or data and are normally referred to simply as I-frame

Supervisory frames used for error and flow control and hence

contain send and receive sequence numbers

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Bit-oriented Protocols – HDLC (High-level Data Link Control)

Frame format Control field

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Bit-oriented Protocols – HDLC (High-level Data Link Control)

Frame format

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Bit-oriented Protocols – HDLC (High-level Data Link Control)

Frame format

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Bit-oriented Protocols – HDLC (High-level Data Link Control)

Protocol operation Link management

Before any information (data) may be transmitted, a logical connection between the two communicating parties must be established

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Bit-oriented Protocols – HDLC (High-level Data Link Control)

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Bit-oriented Protocols – HDLC (High-level Data Link Control)

Protocol operation Data transfer

Error control uses a continuous RQ with either selective repeat or go-back-N retransmission strategy

Flow control is based on a window mechanism

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Bit-oriented Protocols – HDLC (High-level Data Link Control)

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Bit-oriented Protocols – HDLC (High-level Data Link Control)

Protocol operation Data transfer

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Bit-oriented Protocols – HDLC (High-level Data Link Control)

Protocol operation Data transfer

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Bit-oriented Protocols – HDLC (High-level Data Link Control)

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Bit-oriented Protocols – HDLC (High-level Data Link Control)

User interface

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Bit-oriented Protocols – Link Access Procedure version B (LAPB)

LAPB Subset of HDLC that is used to control the transfer of I-frame across a point-to-point duplex data link that connects a computer to a public or private packet-switching network (X.25 networks)

LAPB is an extended version of an earlier subset LAPA (Link Access Procedure version A)

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Bit-oriented Protocols – Link Access Procedure version B (LAPB)

The computer is the DTE and the packet switching exchange is the data circuit-terminating equipment (DCE)

LAPB is used to control the transfer of information frames across the local DTE-DCE interface (local significance)

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Bit-oriented Protocols – Link Access Procedure version B (LAPB)

LAPB used asynchronous balanced mode with DTE and DCE and

All I-frame are treated as command frames

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Bit-oriented Protocols – Link Access Procedure version B (LAPB)

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Bit-oriented Protocols – Link Access Procedure version B (LAPB)

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Bit-oriented Protocols – Link Access Procedure version B (LAPB)

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Bit-oriented Protocols – Link Access Procedure version B (LAPB)

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Bit-oriented Protocols – Link Access Procedure – D channel (LAPD)

ISDN (ITU-D) ABM Always 7-bit sequence numbers (no 3-

bit) 16 bit address field contains two sub-

addresses One for device and one for user (next

layer up

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Bit-oriented Protocols – Logical Link Control (LLC)

IEEE 802 Different frame format Link control split between medium access layer

(MAC) and LLC (on top of MAC) No primary and secondary - all stations are peers Two addresses needed

Sender and receiver Error detection at MAC layer

32 bit CRC Destination and source access points (DSAP,

SSAP)


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