1. Data Communication and Networks Lecture 4 Data Link Control (Part 2) September 25, 2003 Joseph Conron Computer Science Department New York University conron@cs.nyu.edu

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 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)

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. HDLC Station Types zPrimary station yControls operation of link yFrames issued are called commands yMaintains separate logical link to each secondary station zSecondary station yUnder control of primary station yFrames issued called responses zCombined station yMay issue commands and responses

17. HDLC Link Configurations zUnbalanced yOne primary and one or more secondary stations ySupports full duplex and half duplex zBalanced yTwo combined stations ySupports full duplex and half duplex

18. HDLC Transfer Modes (1) zNormal Response Mode (NRM) yUnbalanced configuration yPrimary initiates transfer to secondary ySecondary may only transmit data in response to command from primary yUsed on multi-drop lines yHost computer as primary yTerminals as secondary

19. HDLC Transfer Modes (2) zAsynchronous Balanced Mode (ABM) yBalanced configuration yEither station may initiate transmission without receiving permission yMost widely used yNo polling overhead

20. HDLC Transfer Modes (3) zAsynchronous Response Mode (ARM) yUnbalanced configuration ySecondary may initiate transmission without permission form primary yPrimary responsible for line yrarely used

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

22. Frame Structure Diagram

23. 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

24. Bit Stuffing zExample with possible errors

25. Address Field zIdentifies secondary station that sent or will receive frame zUsually 8 bits long zMay be extended to multiples of 7 bits yLSB of each octet indicates that it is the last octet (1) or not (0) zAll ones (11111111) is broadcast

26. Control Field zDifferent for different frame type yInformation - data to be transmitted to user (next layer up) xFlow and error control piggybacked on information frames ySupervisory - ARQ when piggyback not used yUnnumbered - supplementary link control zFirst one or two bits of control filed identify frame type zRemaining bits explained later

27. Control Field Diagram

28. Poll/Final Bit zUse depends on context zCommand frame yP bit y1 to solicit (poll) response from peer zResponse frame yF bit y1 indicates response to soliciting command

29. Information Field zOnly in information and some unnumbered frames zMust contain integral number of octets zVariable length

30. Frame Check Sequence Field zFCS zError detection z16 bit CRC zOptional 32 bit CRC

31. HDLC Operation zExchange of information, supervisory and unnumbered frames zThree phases yInitialization yData transfer yDisconnect

32. Examples of Operation (1)

33. Examples of Operation (2)

34. Other DLC Protocols (LAPB,LAPD) zLink Access Procedure, Balanced (LAPB) yPart of X.25 (ITU-T) ySubset of HDLC - ABM yPoint to point link between system and packet switching network node zLink Access Procedure, D-Channel yISDN (ITU-D) yABM yAlways 7-bit sequence numbers (no 3-bit) y16 bit address field contains two sub-addresses xOne for device and one for user (next layer up)

35. 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)

36. 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

37. 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

38. 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