Lect11.ppt - 03/15/05

CDA 6505 Network Architecture

and Client/Server Computing

Lecture 11

Link-Level Flow and Error Control

byZornitza Genova Prodanoff

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Introduction

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

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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

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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

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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

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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

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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

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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

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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

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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

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ARQ

• Automatic Repeat Request• Uses:

– Error detection– Timers– Acknowledgements– Retransmissions

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Figure 11.4

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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

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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)

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Figure 11.5

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Figure 11.6

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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

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Figure 11.7

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Figure 11.8

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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 and Tack relatively small

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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

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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)

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The Parameter a

a = propagation time = d/V = Rd transmission time L/R VL

whered = distance between stationsV = velocity of signal propagationL = length of frame in bitsR = data rate on link in bits per sec

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Figure 11.9

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Error-Free Sliding Window ARQ

• Case 1: W ≥ 2a + 1Ack for frame 1 reaches A before A has

exhausted its window

• Case 2: W < 2a +1A exhausts its window at t = W and cannot send

additional frames until t = 2a + 1

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Figure 11.10

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Normalized Throughput

1 W ≥ 2a + 1

S =

W W < 2a +1

2a + 1

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Selective Reject ARQ

1 - P W ≥ 2a + 1

S =

W(1 - P) W < 2a +1

2a + 1

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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

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Figure 11.12

Figure 11.13

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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

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HDLC Operation

• Initialization• Data transfer• Disconnect

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