Chapter 11

1

Chapter 11Chapter 11

Link-Level Flow and Link-Level Flow and Error ControlError Control

Chapter 11: Link Level Flow and Error Controls2

Introduction Introduction

The need for flow and error The need for flow and error controlcontrol

Link control mechanismsLink control mechanismsPerformance of ARQ (Automatic Performance of ARQ (Automatic

Repeat Request)Repeat Request)


Flow Control and Error Flow Control and Error ControlControl Motivation: these are fundamental Motivation: these are fundamental

mechanisms that determine mechanisms that determine performanceperformance Can be implemented at different levels: Can be implemented at different levels:

link, network, transport, or applicationlink, network, transport, or application Difficult to model flow/error control end-Difficult to model flow/error control end-

to-endto-end Simplest case: point-to-point linkSimplest case: point-to-point link

– Constant propagationConstant propagation– Constant data rateConstant data rate– Probabilistic error rateProbabilistic error rate– Traffic characteristicsTraffic characteristics


Flow ControlFlow Control A mechanism whereby the destination A mechanism whereby the destination

node imposes limits on the amount or node imposes limits on the amount or rate of data that is rate of data that is sentsent, or is en route to , or is en route to the receiverthe receiver

Reasons destination may need to limit Reasons destination may need to limit flow:flow:– Source may send PDUs faster than Source may send PDUs faster than

destination can process informationdestination can process information– Higher-level protocol at destination may be Higher-level protocol at destination may be

slow in retrieving dataslow in retrieving data– Destination may need to limit incoming flow Destination may need to limit incoming flow

to match outgoing flow for retransmission to match outgoing flow for retransmission (bridging/forwarding)(bridging/forwarding)


Flow Control Flow Control Layering/ScopeLayering/Scope


Flow Control ScopeFlow Control Scope Hop Scope:• directly connected

intermediate systems• managed at link-layer• e.g., X.25 LAPB flow

control, or HDLC • simplest to model

Network Entry to Exit Scope:• internetwork flows, such as

virtual circuits• managed by network• prevents overflow of buffers

at network exit (egress) nodes

Network Interface Scope:• between end user and

network interface (UNI)• limit total flow of packets

from an end-system into the network

End-to-End Scope:• exercised on logical

connections between end systems

• managed by end systems• e.g. TCP, FR LAPF Control


Error/loss ControlError/loss Control

Used to recover Used to recover lost or damagedlost or damaged PDUsPDUs

Involves Involves error detectionerror detection (via some (via some type of FCS), and PDU type of FCS), and PDU retransmissionretransmission

Typically implemented together Typically implemented together with flow control in a single with flow control in a single mechanismmechanism

Performed at various protocol levelsPerformed at various protocol levels


Link Control MechanismsLink Control Mechanisms Three techniques for flow and error Three techniques for flow and error

control at the link level:control at the link level:– Stop-and-waitStop-and-wait– Go-back-NGo-back-N– Selective-reject (a.k.a. selective repeat)Selective-reject (a.k.a. selective repeat)

Latter two are special cases of Latter two are special cases of sliding-sliding-window ARQwindow ARQ (Automatic Repeat (Automatic Repeat reQuest)reQuest)

Simplifying assumption: end systems Simplifying assumption: end systems are connected by direct linkare connected by direct link


ARQARQ

AAutomatic utomatic RRepeat reepeat reQQuestuestA flow and error control A flow and error control

mechanism which uses:mechanism which uses:– Error detectionError detection– TimersTimers– AcknowledgementsAcknowledgements– RetransmissionsRetransmissions


Stop and WaitStop and WaitSource Source transmitstransmits a frame a frameAfter reception, destination indicates After reception, destination indicates

willingness to accept another frame willingness to accept another frame in in ACKACKnowledgementnowledgement

Source must Source must waitwait for ACK before for ACK before sending another framesending another frame

2 kinds of errors:2 kinds of errors:– Damaged frameDamaged frame received at destination received at destination– Damaged ACKDamaged ACK received at source received at source


Stop-and-Wait ARQStop-and-Wait ARQ


Stop-and-Wait Link Stop-and-Wait Link UtilizationUtilization


Stop-and-Wait Link Stop-and-Wait Link UtilizationUtilization If If TTprop prop is largeis large relative to T relative to Tframe frame (a.k.a. (a.k.a.

TTtranstrans) then throughput is reduced) then throughput is reduced– i.e., if propagation delay is long i.e., if propagation delay is long

relative to transmission time, link is relative to transmission time, link is mostly idlemostly idle

Problem is only one frame in transit Problem is only one frame in transit at a timeat a time

Stop-and-Wait rarely used because Stop-and-Wait rarely used because of inefficiencyof inefficiency


Sliding Window (Pipelined) Sliding Window (Pipelined) TechniquesTechniquesAllow Allow multiple framesmultiple frames to be to be in transitin transit

at the same timeat the same timeSource can send Source can send n n frames (a frames (a

““windowwindow”) without waiting for ”) without waiting for acknowledgementsacknowledgements

Destination can accept Destination can accept nn frames frames (credit)(credit)

Destination acknowledges a frame Destination acknowledges a frame by sending acknowledgement with by sending acknowledgement with sequence number of next frame sequence number of next frame expectedexpected (and implicitly ready for (and implicitly ready for next next nn frames) frames)


Sliding Window ExampleSliding Window Example

RR = Receive Ready

window size = 7 segments


Go-back-N ARQGo-back-N ARQ

Most common formMost common form of error control of error control based on sliding windowbased on sliding window

Number of un-acknowledged frames Number of un-acknowledged frames determined by window sizedetermined by window size

Upon receiving a frame in error, Upon receiving a frame in error, destination discards that frame and all destination discards that frame and all subsequent framessubsequent frames until damaged frame until damaged frame received correctlyreceived correctly

Sender Sender resends frame (and all resends frame (and all subsequent frames)subsequent frames) either when it either when it receives a Reject message or timer receives a Reject message or timer expiresexpires


Selective Reject ARQSelective Reject ARQ

Error control based on sliding windowError control based on sliding window Number of un-acknowledged frames Number of un-acknowledged frames

determined by window sizedetermined by window size Upon receiving a good frame, Upon receiving a good frame,

destination stores that frame and destination stores that frame and sends reject (SREJ) for lowest sequence sends reject (SREJ) for lowest sequence number frame that it still expects number frame that it still expects

Sender Sender resends each frame for which it resends each frame for which it receives a receives a SREJ message or timer SREJ message or timer expiresexpires


Sliding Window ProtocolsSliding Window Protocols

Go-back-NGo-back-N

SelectiveSelectiveReject/Reject/RepeatRepeat


Link Utilization - EfficiencyLink Utilization - Efficiency

The ratio between the use of The ratio between the use of a link for sending user data to a link for sending user data to the available bandwidth on the available bandwidth on the link… orthe link… or

User data that is sentUser data that is sentMaximum capacity of link for sending user Maximum capacity of link for sending user

datadata


Stop-and-Wait Link Stop-and-Wait Link UtilizationUtilization


ARQ Performance:ARQ Performance:Error-Free Stop-and-WaitError-Free Stop-and-Wait

T = T = TTframeframe + + TTprop prop ++ TTproc proc ++ TTack ack ++ TTprop prop + T+ Tprocproc

TTframe frame = time to transmit frame (T= time to transmit frame (Ttranstrans))

TTprop prop = propagation time= propagation time

TTproc proc = processing time at station= processing time at station

TTack ack = time to transmit ack= time to transmit ack

Assume TAssume Tproc proc andand TTackack relatively small relatively small


T T ≈ ≈ TTframeframe + 2T + 2Tprop prop (time required to send a (time required to send a frame)frame)

ThroughputThroughput = 1/T = 1/(T = 1/T = 1/(Tframeframe + 2T + 2Tpropprop) frames/sec) frames/sec

Normalize to link data rate: 1/ TNormalize to link data rate: 1/ Tframeframe frames/sec frames/sec

SS = 1/(T = 1/(Tframeframe + 2T + 2Tpropprop) = T) = Tframe frame = = 11

1/ T1/ Tframe frame T Tframeframe + 2T + 2Tprop prop 1 + 2a1 + 2a

where where a = a = TTprop prop / T/ Tframeframe

Error-Free Stop and WaitError-Free Stop and Wait


The Parameter aThe Parameter a

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

wherewhered = distance between stationsd = distance between stationsV = velocity of signal propagation V = velocity of signal propagation

(“s”)(“s”)L = length of frame in bitsL = length of frame in bitsR = data rate on link in bits per secR = data rate on link in bits per sec


Stop-and-Wait TimingStop-and-Wait Timing

tframe = 1, tprop = a


Stop-and-Wait Stop-and-Wait PerformancePerformance

a = Rd/VL


Example Values of a:Example Values of a:

e.g., ISDNe.g., ISDN

e.g., FRe.g., FR

e.g., LANe.g., LAN

100BaseT, 1250 bytes

BRI, 1250 bytes

FR via satellite, 1250 bytes


Stop-and-Wait ARQ with Stop-and-Wait ARQ with ErrorsErrorsP = probability a single frame is in errorP = probability a single frame is in error

NNxx = 1 = 1 1 - P1 - P = average number of times each = average number of times each

frame must be transmitted due to frame must be transmitted due to errorserrors

SS = 1 = = 1 = 1 – P 1 – P

NNxx (1 + 2a) (1 + 2a) 1 + 2a1 + 2a


Error-Free Sliding Window ARQError-Free Sliding Window ARQ

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

has exhausted its windowhas exhausted its windowCase 2: W Case 2: W 2a +1 2a +1

A exhausts its window at t = W and A exhausts its window at t = W and cannot send additional frames until t cannot send additional frames until t = 2a + 1= 2a + 1


Sliding Window Sliding Window TimingTiming

Case 1

Case 2


Normalized Throughput Normalized Throughput Error-Free Sliding Window ARQError-Free Sliding Window ARQ 1 W 1 W ≥≥ 2a 2a

+ 1+ 1 S =S = W W W W 2a 2a

+1+1 1 + 2a1 + 2a

or, W times stop-and-or, W times stop-and-wait throughputwait throughput


Selective Reject ARQSelective Reject ARQ

1 - P W 1 - P W ≥≥ 2a 2a + 1+ 1

S =S = W(1 - P) W(1 - P) WW 2a 2a

+1+1 2a + 12a + 1

or, W times stop-and-or, W times stop-and-wait throughputwait throughput


Go-Back-N ARQGo-Back-N ARQ

1 - P W 1 - P W ≥≥ 2a 2a + 1+ 1

1 + 2aP1 + 2aPS =S = W(1 - P) W(1 - P) W W 2a 2a

+1+1 (2a + 1)(1 – P + WP)(2a + 1)(1 – P + WP)


Sliding Window UtilizationSliding Window Utilization


ARQ Utilization (P = 10ARQ Utilization (P = 10-3-3))


ARQ Throughput (P = 10ARQ Throughput (P = 10-3-3))

Chapter 11

Documents

linklevel flow

link level

error control mechanism

error controlstop

outgoing flow

error controlintroduction

incoming flow

error controlchapter