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Q4(a) · CSMA persistence and CSMA/CD

2025-12-05 · 2022-23

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

Q4(a). Explain the persistence method defined in CSMA. In which way these methods are used in CSMA/CD. [10 marks]

Answer

Definition. Persistence specifies how a station behaves after observing that the medium is busy or becomes idle.

Simple meaning. 1 = turant; non = random wait; p = probability ke basis pe start.

CSMA persistence: what happens after carrier sensing? — 1-persistent, non-persistent and p-persistent CSMA

Understand it step by step

In 1-persistent CSMA, a ready station transmits immediately when idle; when busy it continuously senses. Several waiting stations may all start at the same idle transition.
In non-persistent CSMA, a busy observation causes a random wait before the next sensing attempt. This reduces simultaneous retries but can waste some idle time.
In p-persistent CSMA, when an eligible idle slot begins, transmit with probability p. With probability 1−p, defer to a later slot and sense again. The probabilistic decision is repeated, not a one-time discard.

Rule	When idle	When busy	Trade-off
1-persistent	Transmit immediately	Keep listening; transmit as soon as idle	Low idle delay, but waiting stations may start together
Non-persistent	Transmit	Wait a random time, then sense again	Less synchronized contention; may leave avoidable idle time
p-persistent (slotted)	Transmit with probability p; defer a slot with probability 1−p	Wait until available, then resume the probabilistic slot rule	Controls contention; needs slots and a choice of p

Yaad rakho: 1 = start now; non = wait then listen; p = chance per idle slot.

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.

CSMA/CD: sender must still be sending after a round trip — CSMA/CD: detect a collision while transmitting

Understand it step by step

Sense the channel. In traditional shared half-duplex Ethernet, begin transmission when permitted by the access rule.
Continue monitoring while transmitting. If a collision is detected, send the collision/jam indication and abort the current frame.
Choose a random delay using binary exponential backoff; then attempt again subject to the access rules.
The sender must still be transmitting when a worst-case collision indication returns. Hence the ideal timing requirement is Tframe ≥ 2Tp.
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

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.

Persistence decision flows

1-persistent:
Ready → sense → busy? yes → sense again
                      no → transmit immediately

p-persistent (slotted medium):
Ready → sense → busy? yes → wait and sense
                      no → draw random decision
                    probability p → transmit
                  probability 1−p → wait one slot → sense again

Non-persistent CSMA instead waits a random time after a busy observation before sensing again. A collision leads to the protocol's retry/backoff procedure; the persistence rule alone does not guarantee success.