Sliding Window Protocol-
Before you go through this article, make sure that you have gone through the previous article on Sliding Window Protocol.
The two well known implementations of sliding window protocol are-
- Go back N Protocol
- Selective Repeat Protocol
In this article, we will discuss about Go back N protocol.
Go back N Protocol-
Go back N protocol is an implementation of a sliding window protocol.
The features and working of this protocol are explained in the following points-
Point-01:
| In Go back N, sender window size is N and receiver window size is always 1. |
In Go back N,
- Sender window size = N. Example in Go back 10, sender window size will be 10.
- Receiver window size is always 1 for any value of N.
Point-02:
| Go back N uses cumulative acknowledgements. |
In Go back N,
- Receiver maintains an acknowledgement timer.
- Each time the receiver receives a new frame, it starts a new acknowledgement timer.
- After the timer expires, receiver sends the cumulative acknowledgement for all the frames that are unacknowledged at that moment.
NOTE-
- A new acknowledgement timer does not start after the expiry of old acknowledgement timer.
- It starts after a new frame is received.
Point-03:
| Go back N may use independent acknowledgements too. |
- The above point does not mean that Go back N can not use independent acknowledgements.
- Go back N may use independent acknowledgements too if required.
- The kind of acknowledgement used depends on the expiry of acknowledgement timer.
Example-
- Consider after the expiry of acknowledgement timer, there is only one frame left to be acknowledged.
- Then, Go back N sends the independent acknowledgement for that frame.
Point-04:
| Go back N does not accept the corrupted frames and silently discards them. |
In Go back N,
- If receiver receives a frame that is corrupted, then it silently discards that frame.
- The correct frame is retransmitted by the sender after the time out timer expires.
- Silently discarding a frame means-
“Simply rejecting the frame and not taking any action”
(like not sending a NACK to the sender to send the correct frame)
Point-05:
| Go back N does not accept out of order frames and silently discards them. |
In Go back N,
- If receiver receives a frame whose sequence number is not what the receiver expects, then it silently discards that frame.
- All the following frames are also discarded.
- This is because receiver window size is 1 and therefore receiver can not accept out of order frames.
Point-06:
| Go back N leads to retransmission of entire window if for any frame, no ACK is received by the sender. |
In Go back N,
- Receiver silently discards the frame if it founds the frame to be either corrupted or out of order.
- It does not send any acknowledgement for such frame.
- It silently discards the following frames too.
Thus,
- If for any particular frame, sender does not receive any acknowledgement, then it understands that along with that frame, all the following frames must also have been discarded by the receiver.
- So, sender has to retransmit all the following frames too along with that particular frame.
- Thus, it leads to the retransmission of entire window.
- That is why, the protocol has been named as “Go back N“.
Point-07:
| Go back N leads to retransmission of lost frames after expiry of time out timer. |
In Go back N,
- Consider a frame being sent to the receiver is lost on the way.
- Then, it is retransmitted only after time out timer expires for that frame at sender’s side.
Efficiency of Go back N-
Efficiency of any flow control protocol is given by-
| Efficiency = Sender Window Size in Protocol / (1 + 2a) |
In Go back N protocol, sender window size = N.
Thus,
| Efficiency of Go back N = N / (1 + 2a) |
To gain better understanding about Go back N ARQ,
Next Article- Practice Problems On Go Back N Protocol
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