# Q2(b) · CSMA/CD

Subject: Computer Networks

Exam: TEE

Reader: https://deep-learning-study-room.vercel.app/cn/#tee/papers/paper-2024-final/q2b

Canonical: https://deep-learning-study-room.vercel.app/read/cn/tee/papers/paper-2024-final--q2b/

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

### Q2(b). Explain the working of CSMA/CD with flow diagram. [10 marks]

### Answer

**Definition.** CSMA with Collision Detection monitors the medium during transmission, aborts a detected collision and retries after a randomized backoff.

**Simple meaning.** CD = takkar detect hui toh transmission rok do.

Swipe across the diagram to read its labels ↔

![CSMA/CD: sender must still be sending after a round trip](https://deep-learning-study-room.vercel.app/cn/fig/collision-roundtrip.42358ec7bf.svg)

Diagram: CSMA/CD: sender must still be sending after a round trip. Labels: CSMA/CD: sender must still be sending after a round trip | CSMA/CD: sender must still be sending after a round trip | Station A | Station B | Signal travels: Tp | Collision indication returns: Tp | Worst-case timing bound: Tframe ≥ 2Tp, so Lmin ≥ 2RTp

CSMA/CD: detect a collision while transmitting

## Understand it step by step

1. Sense the channel. In traditional shared half-duplex Ethernet, begin transmission when permitted by the access rule.
2. Continue monitoring while transmitting. If a collision is detected, send the collision/jam indication and abort the current frame.
3. Choose a random delay using binary exponential backoff; then attempt again subject to the access rules.
4. The sender must still be transmitting when a worst-case collision indication returns. Hence the ideal timing requirement is Tframe ≥ 2Tp.
5. Modern switched full-duplex Ethernet links do not use collision-based CSMA/CD access. Keep the historical/shared-medium assumptions explicit.



$$
T_{\mathrm{frame}}\geq 2T_p
$$





$$
L_{\min}\geq 2RT_p
$$



#### Practical example

On a shared half-duplex Ethernet segment, a station that detects a collision stops sending the remaining frame bits, saving time compared with completing a corrupted long frame.

CSMA/CD senses before transmission and detects collisions during transmission. A collision causes abort/jam and a random backoff before retry. To detect the most distant collision before the frame finishes, the transmission duration must be at least the worst-case round-trip propagation delay. Thus Lmin/R ≥ 2Tp in the idealized model.

**Yaad rakho:** Detect karna hai toh sender tab tak bhej raha hona chahiye.

### CSMA/CD flow diagram

```
Frame ready → sense channel
                 │
          Busy? yes → keep sensing (1-persistent case)
                 no
                 ↓
          Transmit and monitor
                 │
          Collision? no → frame finished? → success
                 yes
                 ↓
          Jam indication → abort frame → increment attempt count
                 │
          Retry limit reached? yes → report failure
                 no
                 ↓
          Random binary-exponential backoff → sense again
```

For Ethernet, the backoff range grows with collisions, capped after the specified stage; transmission is abandoned at the retry limit. Carrier sensing must still be obeyed after the random delay.