Highspeed Networks Chapter 11 Linklevel Flow and Error Control 2003' 04' 24

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High-speed NetworksChapter 11Link-level Flow
and Error Control2003. 04. 24???
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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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1.1 The need for flow and error control

• Flow control
• A protocol mechanism that enables a destination
  entity to regulate the flow of PDUs sent by a
  source entity.
• Flow control limit the amount or rate of data.
• Source may send PDUS faster than destination can
  process headers.
• Higher-level protocol user at destination may be
  slow in retrieving data.
• Destination may limit the incoming flow to match
  the outgoing flow.

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• Flow control scope

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• Error Control
• Used to recover from the loss or damage of PDUs
  in transit between source and destination.
• Involves error detection based on FCS, and PDU
  retransmission.
• Error control flow control
• Implemented together in a single mechanism.

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9.2 Link control mechanisms

• Flow and error control at the link level
• Stop and wait
• Go-back-N
• Selective reject

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Stop and Wait

• Source transmits frame
• After rectption, destination indicates
• willingness to accept another frame
• in acknowledgement
• Source must wait for acknowledgement
• before sending another frame

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Stop and Wait

• 2 kinds of errors.
• Dameged frame at destination at destination.
• Receiver detects an error with FCS.
• If an error is detected, discards the frame.
• Source waits for an ACK.
• If no ACK is received by the time that the timer
  expires,
• then, the same packet is sent again.
• Damaged acknowledgment at source.
• ACK is damaged, and is not recognizable by A
  (sender).
• Therefore time out and resend the same frame.
• Discards duplicate frame

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Stop and Wait

• Rarely used because of its inefficiency.

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Stop and Wait

• The line is underutilized.
• A (sender) would be capable of sending frames at
  a rate of 1/Tframe.
• The maximum transmission rate is reduced to
• If , substantial
  reduction in potential throughput.

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Sliding-window techniques

• Allowing 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
  acknowledges with sequence number of next frame
  expected (and implicitly ready for next n frames)

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Sliding-window (Go-back-N ARQ)

• Go-back-N ARQ
• Most commonly used protocol.
• If destination detects an error, sends a negative
  ack. (REJreject) for that frame.
• Destination discards that frame and all future
  incoming frames.
• Until the frame in error is correctly received.
• The source, when it receives a REJ, retransmits
  the frame in error plus all succeeding frames.

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Sliding-window (selective-reject ARQ)

• The only frames retransmitted are those that
  receive a negative ack.
• More efficient than go-back-N.
• Receiver maintains a large buffer.
• To save post-SREJ frames.
• Reinserting that frame in the proper sequence.
• Transmitter also requires more complex logic.

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9.3 ARQ Performance
Stop-and-wait ARQ

• Error-free Stop-and-wait
• The maximum rate frames can be transmitted over a
  line.
• Total time to send the data nT

A sends f1. B send an acknowledgment. A sends
f2. B send an acknowledgment. A
sends fn. B send an acknowledgment.
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Error-free Stop-and-wait ARQ

• Assume
• Processing time is negligible.
• Acknowledgment frame is very small.
• Fixed frame size (L)
• Link capacity R

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Stop-and-wait ARQ with Errors

• A frame is lost or its ACK is lost.

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• Nx can be derived.
• P probability that a single frame is in error.
• Assume ACKs are never in error.

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• The parameter a
• d distance of the link between two station
• V velocity of propagation of the signal along
  the link.
• L length of a link control frame in bits.
• R data rate on the link.

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Sliding-window ARQ (Error-free)

• Throughput depends on window size W and the value
  of a.
• Transmission time is normalized to 1.

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Selective-reject (with Error)

• Use the result of stop-and-wait ARQ.
• Throughput of selective-reject protocol.

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Go-back-N ARQ (with Error)

• Use the result of stop-and-wait ARQ.
• Nx calculation

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