Tag: Go back N ARQ Protocol

Go back N | Sliding Window Protocol

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-

 

 

  1. Go back N Protocol
  2. 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┬ácan 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,

Watch this Video Lecture

 

Next Article- Practice Problems On Go Back N Protocol

 

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