# Q1(b) · Selective Repeat versus Go-Back-N

Subject: Computer Networks

Exam: TEE

Reader: https://deep-learning-study-room.vercel.app/cn/#tee/papers/paper-2024-final/q1b

Canonical: https://deep-learning-study-room.vercel.app/read/cn/tee/papers/paper-2024-final--q1b/

Actual paper; 100 marks attempted. Current-syllabus alignment is stated per question.

### Q1(b). Compare and contrast Selective Repeat and Go Back N protocol. (Any 5 points) [5 marks]

### Answer

| Property | Stop-and-Wait | Go-Back-N | Selective Repeat |
| --- | --- | --- | --- |
| Outstanding sender frames | 1 | Up to 2^m−1 | Up to 2^(m−1), equal windows |
| Receiver window | 1 | 1 | Same maximum as sender in standard equal-window SR |
| ACK convention used here | Next expected | Cumulative, next expected | Individual frame accepted |
| Out-of-order data | Not delivered | Discarded | Buffered within valid window |
| After timeout | Repeat the one frame | Repeat outstanding frames from base | Repeat only timed-out frame |
| Buffer/tracking cost | Low | Sender window; simple receiver | Sender and receiver windows; per-frame tracking |
| Tradeoff | Simple but idle during RTT | Good pipelining, waste after loss | Efficient loss recovery, greater complexity |

Every comparison assumes finite sequence numbers, correct duplicate handling and the textbook protocol models. The same text “ACK 3” can mean different things under different ACK conventions; state its meaning before explaining a trace.

#### Practical example

If one frame out of a seven-frame burst is lost, GBN may retransmit several frames, whereas SR can retain later frames and resend the missing one.