Flow Control & Error control in Data Link Layer .ppt
- 1. 1 It coordinates the amount of data that can be sent before receiving acknowledgment. It provides the receiver’s acknowledgment of frames received corrupted. Flow control
- 3. 3 Normal operation In Stop-and-Wait ARQ, numbering frames prevents the retaining of duplicate frames.
- 4. 4 Stop-and-Wait ARQ, lost frame
- 5. 5 Stop-and-Wait ARQ, lost ACK frame
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- 8. 8 Sliding Window Sender Sliding Window
- 9. 9 Sliding Window Receiver Sliding Window
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- 13. 13 Error Control
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- 15. Go Back N ARQ What is Go Back N ARQ? • This protocol is based on the method of using the Sliding Window Protocol as the basis of data exchange, where the ’N’ in the protocol represents the window size. • The term ARQ represents the Automatic Repeat Request, which refers to sending multiple frames simultaneously from the sender to the receiver end. 15
- 16. Go Back N ARQ Important Feature of the Protocol • The frames shared in the protocol are sequenced for better efficiency, to avoid any retransmission of shared data, and differentiate between the frames. 16 0 1 2 3 4 5 6 7 Sequence Number
- 17. Go Back N ARQ Important Feature of the Protocol 17 The protocol is designed to share multiple frames at a time, with the receiver end, before expecting any acknowledgment from it. This simultaneous exchange of data is termed protocol pipelining.
- 18. Go Back N ARQ Important Feature of the Protocol 18 If the acknowledgment is not shared to the sender side within a certain time frame, all the frames after the non-acknowledged frame are to be retransmitted to the receiver side.
- 19. Go Back N ARQ Working of the Protocol The working of the Go-Back-N ARQ protocol involves applying the sliding window method for the basis of sharing data, and the number of frames to be shared is decided by the window size. 19
- 20. Go Back N ARQ Working of the Protocol 1. To begin with, the sender side will share the data frames simultaneously according to the window size assigned, over to the receiver side, and wait for the acknowledgment. 20
- 21. Go Back N ARQ Working of the Protocol 2. After the receiver side receives the frames, it will use the first frame and send the acknowledgment to the sender side. 21
- 22. Go Back N ARQ Working of the Protocol 3. After the sender receives the acknowledgment for the first frame, the sender will share the next frame with the receiver. 22
- 23. Go Back N ARQ Working of the Protocol 4.This exchange continues until, due to some external or internal interruption in the network, the acknowledgment is not received by the sender side 23
- 24. Go Back N ARQ Working of the Protocol 5.Then, the sender side will go back to the unacknowledged frame and retransmit that frame, along with all the frames shared after that frame with the receiver. This represents the Go-Back-N ARQ protocol method. 24
- 25. Go Back N ARQ Advantages • Multiple frames can be simultaneous to the receiver side. • Increase the efficiency of the data transfer and has more control over the flow of frames. • Time delay is less for sharing data frames. 25
- 26. Go Back N ARQ Disadvantages • The storage of data frames at the receiver side. • Retransmission of frames, when the acknowledgement is not received by the sender end. 26
- 27. Go Back N ARQ Problem Statement In a network channel, the number of frames given is 10 with the window’s size assigned as 3 and every 4th packet is lost. Find the total number of messages sent from the sender to the receiver side. a) 27 b) 25 c) 22 d) 29 27
- 28. Go Back N ARQ Problem Statement In a network channel, the number of frames given is 10 with the window’s size assigned as 3 and every 4th packet is lost. Find the total number of messages sent from the sender to the receiver side. 1 2 3 4 5 6 4 5 6 7 5 6 7 8 6 7 8 9 7 8 9 10 8 9 10 9 10 28
- 29. Go Back N ARQ Problem Statement In a network channel, the number of frames given is 10 with the window’s size assigned as 3 and every 4th packet is lost. Find the total number of messages sent from the sender to the receiver side. a) 27 b) 25 c) 22 d) 29 29
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- 35. Comparison Stop and Wait ARQ Go back N Selective Repeat Remarks Efficiency 1 / (1+2a) N / (1+2a) N / (1+2a) Go back N and Selective Repeat gives better efficiency than Stop and Wait ARQ. 35
- 36. Comparison Stop and Wait ARQ Go back N Selective Repeat Remarks Window Size Sender Window Size = 1 Receiver Window Size = 1 Sender Window Size = N Receiver Window Size = 1 Sender Window Size = N Receiver Window Size = N Buffer requirement in Selective Repeat is very large. If the system does not have lots of memory, then it is better to choose Go back N. 36
- 37. Comparison Stop and Wait ARQ Go back N Selective Repeat Remarks Minimum number of sequence numbers required 2 N+1 2 x N Selective Repeat requires large number of bits in sequence number field. 37
- 38. Comparison Stop and Wait ARQ Go back N Selective Repeat Remarks Retransmissions required if a packet is lost Only the lost packet is retransmitted The entire window is retransmitted Only the lost packet is retransmitted Selective Repeat is far better than Go back N in terms of retransmissions required. 38
- 39. Comparison Stop and Wait ARQ Go back N Selective Repeat Remarks Bandwidth Requirement Bandwidth requirement is Low Bandwidth requirement is high because even if a single packet is lost, entire window has to be retransmitted. Thus, if error rate is high, it wastes a lot of bandwidth. Bandwidth requirement is moderate Selective Repeat is better than Go back N in terms of bandwidth requirement. 39
- 40. Comparison Stop and Wait ARQ Go back N Selective Repeat Remarks CPU usage Low Moderate High due to searching and sorting required at sender and receiver side Go back N is better than Selective Repeat in terms of CPU usage. 40
- 41. Comparison Stop and Wait ARQ Go back N Selective Repeat Remarks Level of difficulty in Implementation Low Moderate Complex as it requires extra logic and sorting and searching Go back N is better than Selective Repeat in terms of implementation difficulty. 41
- 42. Comparison Stop and Wait ARQ Go back N Selective Repeat Remarks Acknowledgeme nts Uses independent acknowledgemen t for each packet Uses cumulative acknowledgemen ts (but may use independent acknowledgemen ts as well) Uses independent acknowledgemen t for each packet Sending cumulative acknowledgemen ts reduces the traffic in the network but if it is lost, then the ACKs for all the corresponding packets are lost. 42
- 43. Comparison Stop and Wait ARQ Go back N Selective Repeat Remarks Type of Transmission Half duplex Full duplex Full duplex Go back N and Selective Repeat are better in terms of channel usage. 43
- 44. Conclusions 44 • Go back N is more often used than other protocols. • SR protocol is less used because of its complexity. • Stop and Wait ARQ is less used because of its low efficiency. • Depending on the context and resources availability, Go back N or Selective Repeat is employed. • Selective Repeat and Stop and Wait ARQ are similar in terms of retransmissions. • Go back N and Selective Repeat are similar in terms of efficiency if sender window sizes are same. • SR protocol may be considered as a combination of advantages of Stop and Wait ARQ and Go back N. • SR protocol is superior to other protocols but because of its complexity, it is less used.
- 45. Thanks!! 45