This document summarizes flow and error control mechanisms at the link layer. It discusses stop-and-wait, go-back-N, and selective reject protocols for automatic repeat request (ARQ). These protocols use sliding windows, acknowledgments, and retransmissions to provide reliable data transfer over unreliable links. Performance analysis is presented showing how throughput is affected by window size, propagation delay, and error rates. High-level data link control (HDLC) is also introduced as an important link layer protocol.

1. Chapter 11 Link-Level Flow and Error Control
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Chapter 11Chapter 11
Link-Level Flow and Error
Control

2. Chapter 11 Link-Level Flow and Error Control
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IntroductionIntroduction
The need for flow and error control
Link control mechanisms
Performance of ARQ (Automatic Repeat
Request)

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Flow Control and Error ControlFlow 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 ControlFlow 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 ProtocolFlow Control at Multiple Protocol
LayersLayers
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 11.1Figure 11.1

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Flow Control ScopeFlow 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 11.2Figure 11.2

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Error ControlError 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 MechanismsLink 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 FramesSequence 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 WaitStop 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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ARQARQ
Automatic Repeat Request
Uses:
– Error detection
– Timers
– Acknowledgements
– Retransmissions

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Figure 11.3Figure 11.3

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Figure 11.4Figure 11.4

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Stop-and-Wait Link UtilizationStop-and-Wait Link Utilization
If Tproplarge 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 TechniquesSliding 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 11.5Figure 11.5

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Figure 11.6Figure 11.6

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Go-back-N ARQGo-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 11.7Figure 11.7

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Figure 11.8Figure 11.8

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Error-Free Stop and WaitError-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 TprocandTack 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 ErrorsStop-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 aThe 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 11.1Table 11.1

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Figure 11.9Figure 11.9

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Error-Free Sliding Window ARQError-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

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Figure 11.10Figure 11.10

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Normalized ThroughputNormalized Throughput
1 W ≥ 2a + 1
S =
W W < 2a +1
2a + 1

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Selective Reject ARQSelective Reject ARQ
1 - P W ≥ 2a + 1
S =
W(1 - P) W < 2a +1
2a + 1

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Go-Back-N ARQGo-Back-N ARQ
1 - P W ≥ 2a + 1
S = 1 + 2aP
W(1 - P) W < 2a +1
(2a + 1)(1 – P + WP)

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Figure 11.11Figure 11.11

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Figure 11.12Figure 11.12

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Figure 11.13Figure 11.13

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High-Level Data Link ControlHigh-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 11.15Figure 11.15

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HDLC OperationHDLC Operation
Initialization
Data transfer
Disconnect

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Figure 11.16Figure 11.16