Chapter7 - DATA LINK CONTROL PROTOCOLS docx

| 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 e

DATA LINK CONTROL

| 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

4 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

4 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

| 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)

L_DATA.indication

L_DATA request

- s ®

= 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

= Direct physical connection

= Operates on an end-to-end basis

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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Application Environments

PS = Packet switching PSE = Packet switching exchange

management, start of frame and end of frame —

frame delimiting, error control and data

transparency

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Character-oriented Protocols —

4 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

SOH = Marks start of a frame

LEN = The number of characters/bytes in the frame following this character

up to and including BCC The number is encoded in excess-32 notation using a single character in the range from ASCII # (decimal 35) to

ASCII ~ (decimal 126) where # indicates a length of 3 (no data) and ~

indicates the maximum length of 91 SEQ = The send sequence number of the frame The sequence number is

incremented modulo-64 and is again encoded as a single character in the

range ASCII SP (decimal 32) for zero to ASCII _ (decimal 95) for 63

TYPE = The type of frame encoded using a single character:

S = Send initiation (parameters)

BCC = Block check character

CR = End of block marker (ASCII carriage return)

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DATA BLOCK [3] SENT 4

DATA BLOCK [4] SENT

I[O, S, Init Params]

ACK(O, Y, Init Params]

Duplicate detected

ACK{S Y]

H6, BỊ

KERMIT CONNECT RECEIVE

DATA BLOCK [1] RECVD DATA BLOCK [2] RECVD

= DATA BLOCK [3] RECVD

DATA BLOCK [4] RECVD

END OF FILE

END OF TRANSACTION

ENQ (TCS) ACK (TC6) DLE (TC7) NAK (TC8)

End of transmission block: used to indicate the end of a block of data when a message is divided into a number of such blocks (frames)

2/

Either: Slave not ready {

Or: Slave ready

Next data block (2)

Next data block (2)

Last data block (CV)

EOT

Select (+) Sequence First data block (0)

L_CONNECT.-confirm

L_DATA request

(Message block)

L_DISCONNECT request L_DISCONNECT.confirm

L_DISCONNECT indication

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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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= 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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(c)

Primary secondary

=" 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)

3/

= 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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= 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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= Receiver hunts for flag sequence to synchronize

= Bit stuffing used to avoid confusion with data containing

01111110

= Q 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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— inverted

(b) An inverted bit splits a frame in two

= 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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= used for error and flow control and hence contain

send and receive sequence numbers

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Set Normal Response Mode Extended (SNRME) Set Asynchronous Balanced Mode (SABM)

Set Asynchronous Balanced Mode Extended (SABME)

Si ee A Bi So ee ek IE Adee 13, 14 2S) 3G Supervisory = | 1 | 0 | S — P/F N(R) |

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ty

90

S3

L_CONNECT indication

- c- aie w=

-

~_Ắ~

ca œ

L CONNECT.confrm L_DATA.request

= see

ae -~

L_DISCONNECT.confirm #4

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)

S/

Type Commands | Responses Commands _ Responses

Supervisory RR RR RR RR

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RR, REJ, RNR

DISC

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= 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

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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