Link-Level%20Flow%20and%20Error%20Control

1
Chapter 11

• Link-Level Flow and Error Control

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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

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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

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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 11.3
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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) Nx (1 2a)

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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

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Table 11.1
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Figure 11.9
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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 lt 2a 1
• A 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 lt 2a
  1
• 2a 1

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Selective Reject ARQ

• 1 - P W 2a
  1
• S
• W(1 - P) W lt 2a 1
• 2a 1

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Go-Back-N ARQ

• 1 - P W 2a
  1
• S 1 2aP
• W(1 - P) W lt 2a 1
• (2a 1)(1 P WP)

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Figure 11.11
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Figure 11.12
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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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Figure 11.15
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HDLC Operation

• Initialization
• Data transfer
• Disconnect

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Figure 11.16