Data Communication and Networks

Lecture 4

Data Link Control(Part 2)

September 29, 2005

Data Link Performance IssuesPerformance is computed as a measure of

the how efficiently a transmitter and receiver make use of the communications capacity of a give line (medium).

We want to know how much of the potential capacity of the line a protocol can actually use.

This is called utilization, and it varies based on the flow control and error control mechanisms used.

First, let’s review these mechanisms.

Stop and WaitSource transmits frameAfter reception, destination indicates

willingness to accept another frame in acknowledgement

Source must wait for acknowledgement before sending another frame

2 kinds of errors: Damaged frame at destination Damaged acknowledgement at source

Figure 11.4

Error-Free Stop and WaitT = Tframe + Tprop + Tproc + Tack + Tprop + Tproc

Tframe = time to transmit frame

Tprop = propagation time

Tproc = processing time at station

Tack = time to transmit ack

Assume Tproc and Tack relatively small

T ≈ Tframe + 2Tprop

Throughput = 1/T = 1/(Tframe + 2Tprop) frames/sec

Normalize by link data rate: 1/ Tframe frames/sec

U = 1/(Tframe + 2Tprop) = Tframe = 1

1/ Tframe Tframe + 2Tprop 1 + 2a

where a = Tprop / Tframe

Error-Free Stop and Wait (2)

The Parameter aa = propagation time = d/V = Rd transmission time L/R VLwhere

d = distance between stationsV = velocity of signal propagationL = length of frame in bitsR = data rate on link in bits per sec

Rd/V ::= bit length of the linka ::= ratio of link bit length to the length of

frame

Stop-and-Wait Link UtilizationIf Tprop large relative to Tframe then

throughput reducedIf propagation delay is long relative to

transmission time, line is mostly idleProblem is only one frame in transit at a

timeStop-and-Wait rarely used because of

inefficiency

Error-Free Sliding Window ARQCase 1: W ≥ 2a + 1

Ack for frame 1 reaches A before A has exhausted its window

Case 2: W < 2a +1A exhausts its window at t = W and cannot send

additional frames until t = 2a + 1

Figure 11.10

Normalized Throughput

1 for W ≥ 2a + 1 U = W for W < 2a +1 2a + 1

Stop-and-Wait ARQ with ErrorsP = probability a single frame is in error

Nx = 1

1 - P = average number of times each frame

must be transmitted due to errors

U = 1 = 1 - P Nx (1 + 2a) (1 + 2a)

Selective Reject ARQ

1 - P for W ≥ 2a + 1 U = W(1 - P) for W < 2a +1 2a + 1

Go-Back-N ARQ

1 - P for W ≥ 2a + 1 U = 1 + 2aP

W(1 - P) for W < 2a +1 (2a + 1)(1 – P + WP)

High-Level Data Link ControlHDLC is the most important data link

control protocolWidely used which forms basis of other

data link control protocols

Frame StructureSynchronous transmissionAll transmissions in framesSingle frame format for all data and

control exchanges

Frame Structure Diagram

Flag Fields Delimit frame at both ends 01111110 May close one frame and open another 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

Bit Stuffing Example with

possible errors

Other DLC Protocols (LLC)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 layer32 bit CRC

Destination and source access points (DSAP, SSAP)

Other DLC Protocols (Frame Relay) (1)Streamlined capability over high speed

packet witched networksUsed in place of X.25Uses Link Access Procedure for Frame-

Mode Bearer Services (LAPF)Two protocols

Control - similar to HDLC Core - subset of control

Other DLC Protocols (Frame Relay) (2)ABM7-bit sequence numbers16 bit CRC2, 3 or 4 octet address field

Data link connection identifier (DLCI) Identifies logical connection

More on frame relay later

Other DLC Protocols (ATM)Asynchronous Transfer ModeStreamlined capability across high speed

networksNot HDLC basedFrame format called “cell”Fixed 53 octet (424 bit)Details later