# Q1(c) · CSMA persistence

Subject: Computer Networks

Exam: TEE

Reader: https://deep-learning-study-room.vercel.app/cn/#tee/papers/paper-2024-feb-reexam/q1c

Canonical: https://deep-learning-study-room.vercel.app/read/cn/tee/papers/paper-2024-feb-reexam--q1c/

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

### Q1(c). Compare and contrast 1 persistent and p persistent schemes in multiple access with flow diagrams. [5 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.

Swipe across the diagram to read its labels ↔

![CSMA persistence: what happens after carrier sensing?](https://deep-learning-study-room.vercel.app/cn/fig/persistence.528dafcd8a.svg)

Diagram: CSMA persistence: what happens after carrier sensing?. Labels: CSMA persistence: what happens after carrier sensing? | CSMA persistence: what happens after carrier sensing? | 1-persistent | Busy: keep listening. Idle: transmit immediately. | Non-persistent | Busy: random wait, then sense again. Idle: transmit. | p-persistent | Eligible idle slot: transmit with probability p; defer with probability 1−p.

1-persistent, non-persistent and p-persistent CSMA

## Understand it step by step

1. 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.
2. 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.
3. 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 |

#### Practical example

With p=0.25, the station has a 25% chance of transmitting at that idle-slot decision and a 75% chance of deferring. It does not mean it transmits exactly once every four slots.

**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.

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.

### 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.