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

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

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

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

5. Flow Control at Multiple Protocol Layers
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
Chapter 11 Link-Level Flow and Error Control

6. Figure 11.1
Chapter 11 Link-Level Flow and Error Control

7. Flow Control Scope Hop Scope Network interface Entry-to-exit
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
Chapter 11 Link-Level Flow and Error Control

8. Figure 11.2
Chapter 11 Link-Level Flow and Error Control

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

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

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

12. Stop 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
Chapter 11 Link-Level Flow and Error Control

13. ARQ Automatic Repeat Request Uses: Error detection Timers
Acknowledgements
Retransmissions
Chapter 11 Link-Level Flow and Error Control

14. Figure 11.3
Chapter 11 Link-Level Flow and Error Control

15. Figure 11.4
Chapter 11 Link-Level Flow and Error Control

16. Stop-and-Wait Link Utilization
If Tprop large 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
Chapter 11 Link-Level Flow and Error Control

17. Sliding 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)
Chapter 11 Link-Level Flow and Error Control

18. Figure 11.5
Chapter 11 Link-Level Flow and Error Control

19. Figure 11.6
Chapter 11 Link-Level Flow and Error Control

20. 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 Control

21. Figure 11.7
Chapter 11 Link-Level Flow and Error Control

22. Figure 11.8
Chapter 11 Link-Level Flow and Error Control

23. Error-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 Tproc and Tack relatively small
Chapter 11 Link-Level Flow and Error Control

24. where a = Tprop / Tframe 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 a
where a = Tprop / Tframe
Chapter 11 Link-Level Flow and Error Control

25. Stop-and-Wait ARQ with Errors
P = probability a single frame is in error
Nx =
1 - P
= average number of times each frame must be transmitted due to errors
S = = P
Nx (1 + 2a) Nx (1 + 2a)
Chapter 11 Link-Level Flow and Error Control

26. The 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
Chapter 11 Link-Level Flow and Error Control

27. Table 11.1
Chapter 11 Link-Level Flow and Error Control

28. Figure 11.9
Chapter 11 Link-Level Flow and Error Control

29. 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 Control

30. Figure 11.10
Chapter 11 Link-Level Flow and Error Control

31. Normalized Throughput
W ≥ 2a + 1
S =
W W < 2a +1
2a + 1
Chapter 11 Link-Level Flow and Error Control

32. Selective Reject ARQ 1 - P W ≥ 2a + 1 S = W(1 - P) W < 2a +1 2a + 1
Chapter 11 Link-Level Flow and Error Control

33. Go-Back-N ARQ 1 - P W ≥ 2a + 1 S = 1 + 2aP W(1 - P) W < 2a +1
Chapter 11 Link-Level Flow and Error Control

34. Figure 11.11
Chapter 11 Link-Level Flow and Error Control

35. Figure 11.12
Chapter 11 Link-Level Flow and Error Control

36. Figure 11.13
Chapter 11 Link-Level Flow and Error Control

37. High-Level Data Link Control
HDLC is the most important data link control protocol
Widely used which forms basis of other data link control protocols
Chapter 11 Link-Level Flow and Error Control

38. Figure 11.15
Chapter 11 Link-Level Flow and Error Control

39. HDLC Operation Initialization Data transfer Disconnect
Chapter 11 Link-Level Flow and Error Control

40. Figure 11.16
Chapter 11 Link-Level Flow and Error Control