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

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

ZGP003 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

ZGP004 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

ZGP005 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

Figure 11.1

ZGP007 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

Figure 11.2

ZGP009 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

ZGP0010 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

ZGP0011 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

ZGP0012 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

ZGP0013 ARQ Automatic Repeat Request Uses: –Error detection –Timers –Acknowledgements –Retransmissions

ZGP0014

ZGP0015 Figure 11.4

ZGP0016 Stop-and-Wait Link Utilization If T prop large relative to T frame 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

ZGP0017 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)

ZGP0018 Figure 11.5

ZGP0019 Figure 11.6

ZGP0020 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

ZGP0021 Figure 11.7

ZGP0022 Figure 11.8

ZGP0023 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

ZGP0024 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 S = 1/(T frame + 2T prop ) = T frame = 1 1/ T frame T frame + 2T prop 1 + 2a where a = T prop / T frame

ZGP0025 Stop-and-Wait ARQ with Errors P = probability a single frame is in error N x = P = average number of times each frame must be transmitted due to errors S = 1 = 1 - P N x (1 + 2a) (1 + 2a)

ZGP0026 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

ZGP0027

ZGP0028 Figure 11.9

ZGP0029 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

ZGP0030 Figure 11.10

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

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

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

Figure 11.11

ZGP0035 Figure 11.12

Figure 11.13

ZGP0037 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

ZGP0038

ZGP0039 HDLC Operation Initialization Data transfer Disconnect

ZGP0040