Aspect	Go-Back-N (GBN)	Selective Repeat (SR)
Receiver behavior	Accepts only frames arriving strictly in order; discards any out-of-order frame, even if correctly received	Accepts and buffers out-of-order frames, filling gaps later as missing frames arrive
Acknowledgement type	Cumulative ACK — ACK(n) means "all frames up to n-1 received correctly"	Individual/selective ACK — each correctly received frame is acknowledged separately, regardless of order
Timers	Single timer for the oldest unacknowledged frame	Multiple timers — one per outstanding frame
Retransmission on loss	Resends the lost frame and every frame sent after it (even ones already successfully received)	Resends only the specific lost/corrupted frame
Maximum window size (n-bit sequence numbers)	2^n - 1	2^(n-1) (must be half the sequence space to avoid ambiguity between old and new frames)
Receiver buffering needed	None (only tracks "next expected" sequence number)	Requires buffer space to hold out-of-order frames until gaps are filled
Bandwidth efficiency	Lower — wastes bandwidth resending already-correct frames after a loss	Higher — minimal wasted retransmission, only the actually lost frame is resent
Implementation complexity	Simpler (minimal receiver logic/state)	More complex (per-frame timers, receiver-side buffering and reordering logic)
Best suited for	Low-error-rate links, or where simplicity matters more than bandwidth efficiency	High-bandwidth or higher-error-rate links, where the retransmission overhead of GBN would be too costly

Summary: Go-Back-N trades bandwidth efficiency for simplicity (a "dumb" receiver, minimal state), while Selective Repeat trades implementation complexity for much better bandwidth efficiency (a "smart" receiver that buffers and reorders) — making SR the better choice on links where retransmitting entire windows would be too costly, at the cost of more sender/receiver bookkeeping.