---
title: "Short note: CSMA/CD"
subject: "Computer Networks and Security"
chapter: 3
key: "semester_7/computer_networks_and_security:csma-cd-note"
source: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/csma-cd-note
site: BCT Engineering
---

# Short note: CSMA/CD

Computer Networks and Security · Chapter 3: Data Link Layer

- Asked in: 2074 Asoj, 4 marks
- Video: [CSMA/CD protocol in computer networks | Carrier Sense Multiple Access With Collision Detection  | CN](https://www.youtube.com/watch?v=xGzDbJz-AJ8) — Sudhakar Atchala
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/csma-cd-note
- Whole chapter: https://www.bctengineer.com/md/semester_7/computer_networks_and_security/chapter-3

## Question

Write a short note on CSMA/CD.

## Short answer

CSMA/CD is the random-access protocol of classic Ethernet. Stations listen before sending; if idle, transmit. While transmitting, they detect collisions by comparing signals. On collision, they send a jam signal and perform binary exponential backoff. It requires a minimum frame size (e.g., 64 bytes) to detect collisions within transmission time.

## Answer

**CSMA/CD (Carrier Sense Multiple Access with Collision Detection)** is the random-access MAC protocol of classic Ethernet (IEEE 802.3) over a shared bus. Nodes listen before and during transmission.

#### Working
1. **Listen** to the channel (carrier sense). If it is busy, wait until it is idle.
2. When idle, **transmit** the frame.
3. While sending, **detect collision** by comparing the transmitted and received signal.
4. On collision, **stop**, send a 32-bit **jam signal**, and wait for a random time by **binary exponential backoff** (0 to 2ⁿ − 1 slots after the nth collision).
5. Retry up to 16 times, then report failure.

```text
 Sense -> idle? -> Send -> collision? --no--> success
   ^        |no               |yes
   +--wait--+         Jam -> backoff -> retry
```

#### Features
- Minimum frame length must satisfy: transmission time ≥ 2 × propagation delay (64 bytes in Ethernet at 10 Mbps).
- Efficient at low load; efficiency falls with heavy load or long cable.
- Not suitable for wireless, where collisions cannot be detected (CSMA/CA is used).
- Not needed in modern full-duplex switched Ethernet.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does a station do first in CSMA/CD?
   A. Transmits immediately
   B. Listens to the channel and waits if busy
   C. Sends a jam signal
   D. Waits for a token
   Answer: B. Carrier sense comes first: if the channel is busy, wait until it is idle, then transmit. (Key point: CSMA/CD step order)

2. What does a station do on detecting a collision?
   A. Continues sending the whole frame
   B. Stops, sends a jam signal and backs off randomly
   C. Sends an ACK
   D. Switches to token passing
   Answer: B. It stops, sends a 32-bit jam so all stations know, then waits by binary exponential backoff. (Key point: Jam signal)

3. Why is CSMA/CD not used in wireless networks?
   A. Wireless is too fast
   B. Collisions cannot be detected, so CSMA/CA is used
   C. Wireless has no carrier
   D. Frames are too long
   Answer: B. Wireless stations cannot listen while transmitting, so collision avoidance is used instead. (Key point: Limitations)

## Related in Computer Networks and Security

- [Static channel assignment and CSMA/CD](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Astatic-channel-csmacd) — Computer Networks and Security, chapter 3

## Related in other subjects

- [Ethernet with CSMA and CSMA/CD](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aethernet-csma-cd) — Telecommunications, chapter 8