CITS3002 Computer Networks  
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The go-back-N Protocol

The first solution, termed go-back-N, requires the receiver to simply discard all frames after a bad one.

  • The sender's window size corresponds to the number of frames transmitted by not yet received - it varies, grows and shrinks, over time.
  • The receiver's window size corresponds to the number of frames that the receiver is willing to receive - it is always fixed, 1.

In the following diagram, from [Tan 5/e], the sender's transmitted frames appear in the top row, and received frames appear in the bottom row. The frames are 'offset' because they take time to be encoded onto the media and to then travel through the media. The frames are not necessarily all the same size, nor necessarily transmitted at regular intervals.

 

 

Notice the waste of bandwidth - because the receiver only buffers a single frame, all frames transmitted after a lost (or corrupted frame) require later re-transmission - we go back N frames and then restart transmitting.

CITS3002 Computer Networks, Lecture 3, Data Link Layer protocols, p26, 13th March 2024.