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Chapter 11

Chapter 11. Link-Level Flow and Error Control. Introduction. The need for flow and error control Link control mechanisms Performance of ARQ (Automatic Repeat Request). Flow Control and Error Control. Fundamental mechanisms that determine performance Can be implemented at different levels:

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Chapter 11

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  1. Chapter 11 Link-Level Flow and Error Control Chapter 11 Link-Level Flow and Error Control

  2. Introduction • The need for flow and error control • Link control mechanisms • Performance of ARQ (Automatic Repeat Request) Chapter 11 Link-Level Flow and Error Control

  3. Flow Control and Error Control • Fundamental mechanisms that determine performance • Can be implemented at different levels: link, network, or application • Difficult to model performance • Simplest case: point-to-point link • Constant propagation • Constant data rate • Probabilistic error rate • Traffic characteristics Chapter 11 Link-Level Flow and Error Control

  4. Flow Control • Limits the amount or rate of data that is sent • Reasons: • Source may send PDUs faster than destination can process headers • Higher-level protocol user at destination may be slow in retrieving data • Destination may need to limit incoming flow to match outgoing flow for retransmission Chapter 11 Link-Level Flow and Error Control

  5. Flow Control at Multiple Protocol Layers • X.25 virtual circuits (level 3) multiplexed over a data link using LAPB (X.25 level 2) • Multiple TCP connections over HDLC link • Flow control at higher level applied to each logical connection independently • Flow control at lower level applied to total traffic Chapter 11 Link-Level Flow and Error Control

  6. Figure 11.1 Chapter 11 Link-Level Flow and Error Control

  7. Flow Control Scope • Hop Scope • Between intermediate systems that are directly connected • Network interface • Between end system and network • Entry-to-exit • Between entry to network and exit from network • End-to-end • Between end user systems Chapter 11 Link-Level Flow and Error Control

  8. Figure 11.2 Chapter 11 Link-Level Flow and Error Control

  9. Error Control • Used to recover lost or damaged PDUs • Involves error detection and PDU retransmission • Implemented together with flow control in a single mechanism • Performed at various protocol levels Chapter 11 Link-Level Flow and Error Control

  10. Link Control Mechanisms 3 techniques at link level: • Stop-and-wait • Go-back-N • Selective-reject Latter 2 are special cases of sliding-window Assume 2 end systems connected by direct link Chapter 11 Link-Level Flow and Error Control

  11. Sequence of Frames Source breaks up message into sequence of frames • Buffer size of receiver may be limited • Longer transmission are more likely to have an error • On a shared medium, avoids one station monopolizing medium Chapter 11 Link-Level Flow and Error Control

  12. 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 Chapter 11 Link-Level Flow and Error Control

  13. ARQ • Automatic Repeat Request • Uses: • Error detection • Timers • Acknowledgements • Retransmissions Chapter 11 Link-Level Flow and Error Control

  14. Figure 11.3 Chapter 11 Link-Level Flow and Error Control

  15. Figure 11.4 Chapter 11 Link-Level Flow and Error Control

  16. 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 Chapter 11 Link-Level Flow and Error Control

  17. Sliding Window Techniques • Allow multiple frames to be in transit at the same time • Source can send n frames without waiting for acknowledgements • Destination can accept n frames • Destination acknowledges a frame by sending acknowledgement with sequence number of next frame expected (and implicitly ready for next n frames) Chapter 11 Link-Level Flow and Error Control

  18. Figure 11.5 Chapter 11 Link-Level Flow and Error Control

  19. Figure 11.6 Chapter 11 Link-Level Flow and Error Control

  20. Go-back-N ARQ • Most common form of error control based on sliding window • Number of un-acknowledged frames determined by window size • Upon receiving a frame in error, destination discards that frame and all subsequent frames until damaged frame received correctly • Sender resends frame (and all subsequent frames) either when it receives a Reject message or timer expires Chapter 11 Link-Level Flow and Error Control

  21. Figure 11.7 Chapter 11 Link-Level Flow and Error Control

  22. Figure 11.8 Chapter 11 Link-Level Flow and Error Control

  23. 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 Chapter 11 Link-Level Flow and Error Control

  24. T ≈ Tframe + 2Tprop Throughput = 1/T = 1/(Tframe + 2Tprop) frames/sec Normalize by link data rate: 1/ Tframe frames/sec S = 1/(Tframe + 2Tprop) = Tframe = 1 1/ Tframe Tframe + 2Tprop 1 + 2a where a = Tprop / Tframe Chapter 11 Link-Level Flow and Error Control

  25. 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 S = 1 = 1 - P Nx (1 + 2a) Nx (1 + 2a) Chapter 11 Link-Level Flow and Error Control

  26. 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 Chapter 11 Link-Level Flow and Error Control

  27. Table 11.1 Chapter 11 Link-Level Flow and Error Control

  28. Figure 11.9 Chapter 11 Link-Level Flow and Error Control

  29. 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 Chapter 11 Link-Level Flow and Error Control

  30. Figure 11.10 Chapter 11 Link-Level Flow and Error Control

  31. Normalized Throughput 1 W ≥ 2a + 1 S = W W < 2a +1 2a + 1 Chapter 11 Link-Level Flow and Error Control

  32. Selective Reject ARQ 1 - P W ≥ 2a + 1 S = W(1 - P) W < 2a +1 2a + 1 Chapter 11 Link-Level Flow and Error Control

  33. Go-Back-N ARQ 1 - P W ≥ 2a + 1 S = 1 + 2aP W(1 - P) W < 2a +1 (2a + 1)(1 – P + WP) Chapter 11 Link-Level Flow and Error Control

  34. Figure 11.11 Chapter 11 Link-Level Flow and Error Control

  35. Figure 11.12 Chapter 11 Link-Level Flow and Error Control

  36. Figure 11.13 Chapter 11 Link-Level Flow and Error Control

  37. 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 Chapter 11 Link-Level Flow and Error Control

  38. Figure 11.15 Chapter 11 Link-Level Flow and Error Control

  39. HDLC Operation • Initialization • Data transfer • Disconnect Chapter 11 Link-Level Flow and Error Control

  40. Figure 11.16 Chapter 11 Link-Level Flow and Error Control

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