Stop and Wait ARQ | Go back N | SR Protocol

Flow Control Protocols-

 

In computer networking, there are various flow control protocols-

 

 

Sliding Window Protocol-

 

• Sliding window protocol allows the sender to send multiple frames before needing the acknowledgements.
• It is more efficient.

 

Read More- Sliding Window Protocol

 

Implementations-

 

Various implementations of sliding window protocol are-

1. Stop and Wait ARQ
2. Go back N
3. Selective Repeat

 

In this article, we will compare these sliding window protocols.

 

Comparison Table-

 

	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.
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.
Minimum number of sequence numbers required	2	N+1	2 x N	Selective Repeat requires large number of bits in sequence number field.
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.
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.
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.
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.
Acknowledgements	Uses independent acknowledgement for each packet	Uses cumulative acknowledgements (but may use independent acknowledgements as well)	Uses independent acknowledgement for each packet	Sending cumulative acknowledgements reduces the traffic in the network but if it is lost, then the ACKs for all the corresponding packets are lost.
Type of Transmission	Half duplex	Full duplex	Full duplex	Go back N and Selective Repeat are better in terms of channel usage.

 

Conclusions-

 

• 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.

 

Important Notes-

 

Note-01:

 

Protocols at data link layer like HDLC (Low level protocols) use Go back N.

This is because-

1. Bandwidth is high
2. CPU is very busy doing routing job
3. Error rate is low since out of order packets are not possible in wired medium

 

Note-02:

 

Protocols at transport layer like TCP (High level protocols) use selective repeat.

 

PRACTICE PROBLEMS BASED ON SLIDING WINDOW PROTOCOLS-

 

Problem-01:

 

If the bandwidth between the sender and receiver is sufficient, CPU and buffers are moderate, then which flow control protocol would you suggest to use?

 

Solution-

 

The suggested protocol would be Go back N.

 

Problem-02:

 

If the bandwidth between the sender and receiver is moderate, CPU and buffers are sufficient, then which flow control protocol would you suggest to use?

 

Solution-

 

The suggested protocol would be Selective Repeat.

 

To gain better understanding about sliding window protocols comparison,

Watch this Video Lecture

 

Next Article- Miscellaneous Problems On Flow Control Protocols

 

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