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Q2(b) · CSMA/CD

2024-12-06 · Final 2024-25 / Re-exam 2023-24

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
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_pLaTeX: T_{\mathrm{frame}}\geq 2T_p
L_{\min}\geq 2RT_pLaTeX: L_{\min}\geq 2RT_p

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.