high speed network

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