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Short note: 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.

Write a short note on CSMA/CD.

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.
 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.

✓ Answer checked by Claude Opus 5.5 (high effort)

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