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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

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chapter 11

Chapter 11

Link-Level Flow and Error Control

introduction
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

flow control and error control
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

flow control
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

flow control layering scope
Flow Control Layering/Scope

Chapter 11: Link Level Flow and Error Controls

flow control scope

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

error loss control
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

link control mechanisms
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

slide9
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

stop and wait
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

stop and wait arq
Stop-and-Wait ARQ

Chapter 11: Link Level Flow and Error Controls

stop and wait link utilization
Stop-and-Wait Link Utilization

Chapter 11: Link Level Flow and Error Controls

stop and wait link utilization1
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

sliding window pipelined techniques
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

sliding window example
Sliding Window Example

RR = Receive Ready

window size = 7 segments

Chapter 11: Link Level Flow and Error Controls

go back n arq
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

selective reject arq
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

sliding window protocols

Go-back-N

Selective

Reject/

Repeat

Sliding Window Protocols

Chapter 11: Link Level Flow and Error Controls

link utilization efficiency
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

stop and wait link utilization2
Stop-and-Wait Link Utilization

Chapter 11: Link Level Flow and Error Controls

arq performance error free stop and wait
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

error free stop and wait
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

the parameter a
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

stop and wait timing
Stop-and-Wait Timing

tframe = 1, tprop = a

Chapter 11: Link Level Flow and Error Controls

stop and wait performance
Stop-and-Wait Performance

a = Rd/VL

Chapter 11: Link Level Flow and Error Controls

example values of a
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

stop and wait arq with errors
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

error free sliding window arq
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

sliding window timing

Sliding Window Timing

Case 1

Case 2

Chapter 11: Link Level Flow and Error Controls

normalized throughput error free sliding window arq

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

selective reject arq1

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

go back n arq1
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

sliding window utilization
Sliding Window Utilization

Chapter 11: Link Level Flow and Error Controls

arq utilization p 10 3
ARQ Utilization (P = 10-3)

Chapter 11: Link Level Flow and Error Controls

slide35

ARQ Throughput (P = 10-3)

Chapter 11: Link Level Flow and Error Controls