Solution

Go-Back-N (GBN) and Selective Repeat (SR) are sliding-window ARQ protocols used for reliable data transmission. Both use acknowledgements, sequence numbers, timers, and retransmission to recover from lost or damaged frames, but they differ in how they handle errors.

Feature	Go-Back-N ARQ	Selective Repeat ARQ
Receiver window	Usually 1	Greater than 1; accepts multiple frames
Out-of-order frames	Discarded	Buffered
Retransmission	Retransmits the erroneous/lost frame and all subsequent outstanding frames	Retransmits only the specific lost/damaged frame
ACKs	Typically cumulative	Typically selective/individual
Receiver complexity	Lower	Higher
Buffer requirement	Low	Higher
Bandwidth efficiency	Lower when errors are frequent	Higher, especially on noisy links
Implementation	Simpler	More complex

Example

Suppose frames 0, 1, 2, 3 are sent and frame 2 is lost.

Go-Back-N: The receiver detects the missing frame and does not accept frame 3 as the next in-order frame. The sender eventually retransmits frames 2 and 3 (and any later outstanding frames).

Selective Repeat: The receiver can accept and buffer frame 3 while waiting for frame 2. The sender retransmits only frame 2. After frame 2 arrives, the buffered frame 3 can be delivered in order.

Conclusion

Go-Back-N is simpler and requires less receiver buffering, but it can waste bandwidth because correctly received frames may be retransmitted. Selective Repeat is more efficient on error-prone or long-delay links because only lost or damaged frames are retransmitted, but it requires more receiver memory and more complex control logic.