Understanding Data Link Control Mechanisms in Networks

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 • Performance 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.

3. Stop and Wait • Source transmits frame • After 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

4. Figure 11.4

5. Error-Free Stop and Wait T = 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 andTack relatively small

6. Error-Free Stop and Wait (2) 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

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 • If Tprop large relative to Tframe then throughput reduced • If propagation delay is long relative to transmission time, line is mostly idle • Problem is only one frame in transit at a time • Stop-and-Wait rarely used because of inefficiency

9. Error-Free Sliding Window ARQ • Case 1: W ≥ 2a + 1 Ack for frame 1 reaches A before A has exhausted its window • 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 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)

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 • HDLC is the most important data link control protocol • Widely used which forms basis of other data link control protocols

16. HDLC Station Types • Primary station • Controls operation of link • Frames issued are called commands • Maintains separate logical link to each secondary station • Secondary station • Under control of primary station • Frames issued called responses • Combined station • May issue commands and responses

17. HDLC Link Configurations • Unbalanced • One primary and one or more secondary stations • Supports full duplex and half duplex • Balanced • Two combined stations • Supports full duplex and half duplex

18. HDLC Transfer Modes (1) • Normal Response Mode (NRM) • Unbalanced configuration • Primary initiates transfer to secondary • Secondary may only transmit data in response to command from primary • Used on multi-drop lines • Host computer as primary • Terminals as secondary

19. HDLC Transfer Modes (2) • Asynchronous Balanced Mode (ABM) • Balanced configuration • Either station may initiate transmission without receiving permission • Most widely used • No polling overhead

20. HDLC Transfer Modes (3) • Asynchronous Response Mode (ARM) • Unbalanced configuration • Secondary may initiate transmission without permission form primary • Primary responsible for line • rarely used

21. Frame Structure • Synchronous transmission • All transmissions in frames • Single frame format for all data and control exchanges

22. Frame Structure Diagram

23. 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

24. Bit Stuffing • Example with possible errors

25. Address Field • Identifies secondary station that sent or will receive frame • Usually 8 bits long • May be extended to multiples of 7 bits • LSB of each octet indicates that it is the last octet (1) or not (0) • All ones (11111111) is broadcast

26. Control Field • Different for different frame type • Information - data to be transmitted to user (next layer up) • Flow and error control piggybacked on information frames • Supervisory - ARQ when piggyback not used • Unnumbered - supplementary link control • First one or two bits of control filed identify frame type • Remaining bits explained later

27. Control Field Diagram

28. Poll/Final Bit • Use depends on context • Command frame • P bit • 1 to solicit (poll) response from peer • Response frame • F bit • 1 indicates response to soliciting command

29. Information Field • Only in information and some unnumbered frames • Must contain integral number of octets • Variable length

30. Frame Check Sequence Field • FCS • Error detection • 16 bit CRC • Optional 32 bit CRC

31. HDLC Operation • Exchange of information, supervisory and unnumbered frames • Three phases • Initialization • Data transfer • Disconnect

32. Examples of Operation (1)

33. Examples of Operation (2)

34. Other DLC Protocols (LAPB,LAPD) • Link Access Procedure, Balanced (LAPB) • Part of X.25 (ITU-T) • Subset of HDLC - ABM • Point to point link between system and packet switching network node • Link Access Procedure, D-Channel • ISDN (ITU-D) • ABM • 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)

35. 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 layer • 32 bit CRC • Destination and source access points (DSAP, SSAP)

36. Other DLC Protocols (Frame Relay) (1) • Streamlined capability over high speed packet witched networks • Used in place of X.25 • Uses Link Access Procedure for Frame-Mode Bearer Services (LAPF) • Two protocols • Control - similar to HDLC • Core - subset of control

37. Other DLC Protocols (Frame Relay) (2) • ABM • 7-bit sequence numbers • 16 bit CRC • 2, 3 or 4 octet address field • Data link connection identifier (DLCI) • Identifies logical connection • More on frame relay later

38. Other DLC Protocols (ATM) • Asynchronous Transfer Mode • Streamlined capability across high speed networks • Not HDLC based • Frame format called “cell” • Fixed 53 octet (424 bit) • Details later