1 / 35

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. Motivation: these are fundamental mechanisms that determine performance

ranee
Télécharger la présentation

Chapter 11

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. 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 Controls

  3. Flow Control and Error Control • Motivation: these are fundamental mechanisms that determine performance • Can be implemented at different levels: link, network, transport, or application • Difficult to model flow/error control end-to-end • Simplest case: point-to-point link • Constant propagation • Constant data rate • Probabilistic error rate • Traffic characteristics Chapter 11: Link Level Flow and Error Controls

  4. Flow Control • A mechanism whereby the destination node imposes limits on the amount or rate of data that is sent, or is en route to the receiver • Reasons destination may need to limit flow: • Source may send PDUs faster than destination can process information • Higher-level protocol at destination may be slow in retrieving data • Destination may need to limit incoming flow to match outgoing flow for retransmission (bridging/forwarding) Chapter 11: Link Level Flow and Error Controls

  5. Flow Control Layering/Scope Chapter 11: Link Level Flow and Error Controls

  6. Hop Scope: • directly connected intermediate systems • managed at link-layer • e.g., X.25 LAPB flow control, or HDLC • simplest to model • End-to-End Scope: • exercised on logical connections between end systems • managed by end systems • e.g. TCP, FR LAPF Control • Network Interface Scope: • between end user and network interface (UNI) • limit total flow of packets from an end-system into the network • Network Entry to Exit Scope: • internetwork flows, such as virtual circuits • managed by network • prevents overflow of buffers at network exit (egress) nodes Flow Control Scope Chapter 11: Link Level Flow and Error Controls

  7. Error/loss Control • Used to recover lost or damaged PDUs • Involves error detection (via some type of FCS), and PDU retransmission • Typically implemented together with flow control in a single mechanism • Performed at various protocol levels Chapter 11: Link Level Flow and Error Controls

  8. Link Control Mechanisms • Three techniques for flow and error control at the link level: • Stop-and-wait • Go-back-N • Selective-reject (a.k.a. selective repeat) • Latter two are special cases of sliding-window ARQ (Automatic Repeat reQuest) • Simplifying assumption: end systems are connected by direct link Chapter 11: Link Level Flow and Error Controls

  9. ARQ • Automatic Repeat reQuest • A flow and error control mechanism which uses: • Error detection • Timers • Acknowledgements • Retransmissions Chapter 11: Link Level Flow and Error Controls

  10. Stop and Wait • Source transmits a frame • After reception, destination indicates willingness to accept another frame in ACKnowledgement • Source must wait for ACK before sending another frame • 2 kinds of errors: • Damaged frame received at destination • Damaged ACK received at source Chapter 11: Link Level Flow and Error Controls

  11. Stop-and-Wait ARQ Chapter 11: Link Level Flow and Error Controls

  12. Stop-and-Wait Link Utilization Chapter 11: Link Level Flow and Error Controls

  13. Stop-and-Wait Link Utilization • If Tprop is large relative to Tframe (a.k.a. Ttrans) then throughput is reduced • i.e., if propagation delay is long relative to transmission time, link 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 Controls

  14. Sliding Window (Pipelined) Techniques • Allow multiple frames to be in transit at the same time • Source can send n frames (a “window”) without waiting for acknowledgements • Destination can accept n frames (credit) • 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 Controls

  15. Sliding Window Example RR = Receive Ready window size = 7 segments Chapter 11: Link Level Flow and Error Controls

  16. 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 Controls

  17. Selective Reject ARQ • Error control based on sliding window • Number of un-acknowledged frames determined by window size • Upon receiving a good frame, destination stores that frame and sends reject (SREJ) for lowest sequence number frame that it still expects • Sender resends each frame for which it receives a SREJ message or timer expires Chapter 11: Link Level Flow and Error Controls

  18. Go-back-N Selective Reject/ Repeat Sliding Window Protocols Chapter 11: Link Level Flow and Error Controls

  19. Link Utilization - Efficiency The ratio between the use of a link for sending user data to the available bandwidth on the link… or User data that is sent Maximum capacity of link for sending user data Chapter 11: Link Level Flow and Error Controls

  20. Stop-and-Wait Link Utilization Chapter 11: Link Level Flow and Error Controls

  21. ARQ Performance:Error-Free Stop-and-Wait T = Tframe + Tprop +Tproc +Tack +Tprop + Tproc Tframe = time to transmit frame (Ttrans) 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 Controls

  22. Error-Free Stop and Wait T ≈ Tframe + 2Tprop (time required to send a frame) Throughput = 1/T = 1/(Tframe + 2Tprop) frames/sec Normalize to 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 Controls

  23. The Parameter a a = propagation time = d/V = Rd transmission time L/R VL where d = distance between stations V = velocity of signal propagation (“s”) L = length of frame in bits R = data rate on link in bits per sec Chapter 11: Link Level Flow and Error Controls

  24. Stop-and-Wait Timing tframe = 1, tprop = a Chapter 11: Link Level Flow and Error Controls

  25. Stop-and-Wait Performance a = Rd/VL Chapter 11: Link Level Flow and Error Controls

  26. Example Values of a: e.g., ISDN BRI, 1250 bytes e.g., FR FR via satellite, 1250 bytes e.g., LAN 100BaseT, 1250 bytes Chapter 11: Link Level Flow and Error Controls

  27. 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 Chapter 11: Link Level Flow and Error Controls

  28. 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 Controls

  29. Sliding Window Timing Case 1 Case 2 Chapter 11: Link Level Flow and Error Controls

  30. or, W times stop-and-wait throughput Normalized Throughput Error-Free Sliding Window ARQ 1 W ≥ 2a + 1 S = W W  2a +1 1 + 2a Chapter 11: Link Level Flow and Error Controls

  31. or, W times stop-and-wait throughput Selective Reject ARQ 1 - P W ≥ 2a + 1 S = W(1 - P) W 2a +1 2a + 1 Chapter 11: Link Level Flow and Error Controls

  32. Go-Back-N ARQ 1 - P W ≥ 2a + 1 1 + 2aP S = W(1 - P) W  2a +1 (2a + 1)(1 – P + WP) Chapter 11: Link Level Flow and Error Controls

  33. Sliding Window Utilization Chapter 11: Link Level Flow and Error Controls

  34. ARQ Utilization (P = 10-3) Chapter 11: Link Level Flow and Error Controls

  35. ARQ Throughput (P = 10-3) Chapter 11: Link Level Flow and Error Controls

More Related