1. Data Communication and Networks Lecture 5 Link Protocol Performance, Bit Stuffing, Multiplexing October 6, 2005

2. 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.

3. 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

4. Figure 11.4

5. 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

6. T ≈ T frame + 2T prop Throughput = 1/T = 1/(T frame + 2T prop ) frames/sec Utilization U is ratio of time to transmit data T frame and the total time to send the data and get the response T frame + 2T prop U = T frame = 1 T frame + 2T prop 1 + 2a where a = T prop / T frame Error-Free Stop and Wait (2)

7. 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

8. 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

9. 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

10. Figure 11.10

11. Normalized Throughput 1 for W ≥ 2a + 1 U = W for W < 2a +1 2a + 1

12. Stop-and-Wait ARQ with Errors P = probability a single frame is in error N x = 1 1 - P = average number of times each frame must be transmitted due to errors U = 1 = 1 - P N x (1 + 2a) (1 + 2a)

13. Selective Reject ARQ 1 - P for W ≥ 2a + 1 U = W(1 - P) for W < 2a +1 2a + 1

14. 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)

15. 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

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

17. Frame Structure Diagram

18. Flag Fields zDelimit frame at both ends z01111110 zMay close one frame and open another zReceiver hunts for flag sequence to synchronize zBit stuffing used to avoid confusion with data containing 01111110 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

19. Bit Stuffing zExample with possible errors

20. Multiplexing

21. Frequency Division Multiplexing zFDM zUseful bandwidth of medium exceeds required bandwidth of channel zEach signal is modulated to a different carrier frequency zCarrier frequencies separated so signals do not overlap (guard bands) ze.g. broadcast radio zChannel allocated even if no data

22. Frequency Division Multiplexing Diagram

23. Synchronous Time Division Multiplexing zData rate of medium exceeds data rate of digital signal to be transmitted zMultiple digital signals interleaved in time zMay be at bit level of blocks zTime slots preassigned to sources and fixed zTime slots allocated even if no data zTime slots do not have to be evenly distributed amongst sources

24. Time Division Multiplexing

25. Statistical TDM zIn Synchronous TDM many slots are wasted zStatistical TDM allocates time slots dynamically based on demand zMultiplexer scans input lines and collects data until frame full zData rate on line lower than aggregate rates of input lines