Computer Network & Network Security System — Data Link Layer, NEC licence examination syllabus (Nepal Engineering Council).
The second numerical goldmine of this subject.
Imagine trying to pour water from a fire hose into a small cup — most of it just spills. Flow control exists so a fast sender doesn't overwhelm a receiver that can't process data quite as fast.
How it worksSender sends ONE frame, then waits for an acknowledgment before sending the next.
ProblemExtremely simple, but wastes huge amounts of time waiting — especially over long distances where the round-trip time is large.
Leave the window at 1 and raise the link speed. Utilisation falls — a faster link transmits the frame sooner and then waits just as long for the acknowledgement, so it idles a greater fraction of the time. The window has to grow with bandwidth×delay, which is why long fast links need large windows.
Instead of waiting after every single frame, the sender is allowed to send several frames ahead ("the window") before needing an acknowledgment — dramatically improving efficiency for anything beyond very short links.
Go-Back-NIf one frame is lost/damaged, the receiver discards THAT frame and every frame after it — sender must resend the entire batch from that point. Simple, but wasteful on errors.
Selective RepeatOnly the specific lost/damaged frame is resent — everything else already received is kept. More efficient, but needs more buffer memory at the receiver to hold out-of-order frames.
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