Data Communication and Networks Lecture 4 Data Link Control (Part 2) September 29, 2005

Data Link Performance Issues zPerformance is computed as a measure of the how efficiently a transmitter and receiver make use of the communications capacity of a give line (medium). zWe want to know how much of the potential capacity of the line a protocol can actually use. zThis is called utilization, and it varies based on the flow control and error control mechanisms used. zFirst, let’s review these mechanisms.

Stop and Wait zSource transmits frame zAfter reception, destination indicates willingness to accept another frame in acknowledgement zSource must wait for acknowledgement before sending another frame z2 kinds of errors: yDamaged frame at destination yDamaged acknowledgement at source

Figure 11.4

Error-Free Stop and Wait T = T frame + T prop + T proc + T ack + T prop + T proc T frame = time to transmit frame T prop = propagation time T proc = processing time at station T ack = time to transmit ack Assume T proc and T ack relatively small

T ≈ T frame + 2T prop Throughput = 1/T = 1/(T frame + 2T prop ) frames/sec Normalize by link data rate: 1/ T frame frames/sec U = 1/(T frame + 2T prop ) = T frame = 1 1/ T frame T frame + 2T prop 1 + 2a where a = T prop / T frame Error-Free Stop and Wait (2)

The Parameter a a = propagation time = d/V = Rd transmission time L/R VL where d = distance between stations V = velocity of signal propagation L = length of frame in bits R = data rate on link in bits per sec Rd/V ::= bit length of the link a ::= ratio of link bit length to the length of frame

Stop-and-Wait Link Utilization zIf T prop large relative to T frame then throughput reduced zIf propagation delay is long relative to transmission time, line is mostly idle zProblem is only one frame in transit at a time zStop-and-Wait rarely used because of inefficiency

Error-Free Sliding Window ARQ zCase 1: W ≥ 2a + 1 Ack for frame 1 reaches A before A has exhausted its window z Case 2: W < 2a +1 A 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 Errors P = probability a single frame is in error N x = P = average number of times each frame must be transmitted due to errors U = 1 = 1 - P N x (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 Control zHDLC is the most important data link control protocol zWidely used which forms basis of other data link control protocols

Frame Structure zSynchronous transmission zAll transmissions in frames zSingle frame format for all data and control exchanges

Frame Structure Diagram

Flag Fields zDelimit frame at both ends z zMay close one frame and open another zReceiver hunts for flag sequence to synchronize zBit stuffing used to avoid confusion with data containing y0 inserted after every sequence of five 1s yIf receiver detects five 1s it checks next bit yIf 0, it is deleted yIf 1 and seventh bit is 0, accept as flag yIf sixth and seventh bits 1, sender is indicating abort

Bit Stuffing zExample with possible errors

Other DLC Protocols (LLC) zLogical Link Control (LLC) yIEEE 802 yDifferent frame format yLink control split between medium access layer (MAC) and LLC (on top of MAC) yNo primary and secondary - all stations are peers yTwo addresses needed xSender and receiver yError detection at MAC layer x32 bit CRC yDestination and source access points (DSAP, SSAP)

Other DLC Protocols (Frame Relay) (1) zStreamlined capability over high speed packet witched networks zUsed in place of X.25 zUses Link Access Procedure for Frame-Mode Bearer Services (LAPF) zTwo protocols yControl - similar to HDLC yCore - subset of control

Other DLC Protocols (Frame Relay) (2) zABM z7-bit sequence numbers z16 bit CRC z2, 3 or 4 octet address field yData link connection identifier (DLCI) yIdentifies logical connection zMore on frame relay later

Other DLC Protocols (ATM) zAsynchronous Transfer Mode zStreamlined capability across high speed networks zNot HDLC based zFrame format called “cell” zFixed 53 octet (424 bit) zDetails later