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Understanding Link-Level Flow and Error Control in Network Architecture

This lecture provides an in-depth examination of flow and error control mechanisms in network architecture, addressing their significance in ensuring efficient data transmission. It covers the performance of Automatic Repeat Request (ARQ), techniques including Stop-and-Wait, Go-Back-N, and Selective Rejection, and the challenges of modeling flow control performance across multiple network layers. Key concepts such as hop scope, error recovery, and the implications of traffic characteristics are discussed, along with the importance of controlling data flow to prevent bottlenecks and optimize throughput.

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Understanding Link-Level Flow and Error Control in Network Architecture

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  1. CDA 6505 Network Architecture and Client/Server Computing Lecture 11 Link-Level Flow and Error Control by Zornitza Genova Prodanoff Lect11.ppt - 03/15/05

  2. Introduction • The need for flow and error control • Link control mechanisms • Performance of ARQ (Automatic Repeat Request) ZGP002

  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 ZGP003

  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 ZGP004

  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 ZGP005

  6. Figure 11.1

  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 ZGP007

  8. Figure 11.2

  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 ZGP009

  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 ZGP0010

  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 ZGP0011

  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 ZGP0012

  13. ARQ • Automatic Repeat Request • Uses: • Error detection • Timers • Acknowledgements • Retransmissions ZGP0013

  14. ZGP0014

  15. Figure 11.4 ZGP0015

  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 ZGP0016

  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) ZGP0017

  18. Figure 11.5 ZGP0018

  19. Figure 11.6 ZGP0019

  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 ZGP0020

  21. Figure 11.7 ZGP0021

  22. Figure 11.8 ZGP0022

  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 ZGP0023

  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 ZGP0024

  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) (1 + 2a) ZGP0025

  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 ZGP0026

  27. ZGP0027

  28. Figure 11.9 ZGP0028

  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 ZGP0029

  30. Figure 11.10 ZGP0030

  31. Normalized Throughput 1 W ≥ 2a + 1 S = W W < 2a +1 2a + 1 ZGP0031

  32. Selective Reject ARQ 1 - P W ≥ 2a + 1 S = W(1 - P) W < 2a +1 2a + 1 ZGP0032

  33. Go-Back-N ARQ 1 - P W ≥ 2a + 1 S = 1 + 2aP W(1 - P) W < 2a +1 (2a + 1)(1 – P + WP) ZGP0033

  34. Figure 11.11

  35. Figure 11.12 ZGP0035

  36. Figure 11.13

  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 ZGP0037

  38. ZGP0038

  39. HDLC Operation • Initialization • Data transfer • Disconnect ZGP0039

  40. ZGP0040

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