Difference between 1-persistent, p-persistent and Non-persistent CSMA

Last Updated : 16 Apr, 2025

Carrier Sense Multiple Access (CSMA) is a network protocol used to control access to a shared communication channel. Before a device (or station) sends data, it first listens to the channel (also called carrier sensing) to check if it is busy or idle. If the channel is idle, the device starts transmitting data. If the channel is busy, the device waits for it to become free before trying again. Imagine you're in a room where several people want to talk, CSMA works like:

1-persistent CSMA is like someone who keeps checking if the room is quiet, and the second it is, they start talking with no hesitation. This often causes people to talk over each other because everyone’s jumping in at the same time.
p-persistent CSMA is a bit more polite. The conversation is broken into small time slots, and people only speak based on a certain chance. So even if the room is quiet, they might wait a bit before talking, which helps avoid overlap.
Non-persistent CSMA is like waiting a while after noticing the room is busy, then randomly checking again. It reduces the chance of people talking at the same time, but it can slow things down a bit.

Let us discuss them in detail below:

1. 1-Persistent CSMA

Working:

A station continuously checks the channel to see if it's idle or busy.
If the channel is busy, the station waits until it becomes idle.
Once the channel is idle, the station immediately transmits its data with no delay.
The station transmits with 100% probability (hence the name 1-persistent CSMA).

Key Points:

High risk of collision: If multiple stations detect the channel is idle at the same time, they all transmit at once, causing a collision.
After a collision: Each station waits for a random backoff time, then tries again once the channel is idle.
Additional risk: If two stations are already communicating, other stations (called intermediate stations) will wait and not transmit.

2. P-Persistent CSMA

Working:

Used when the channel has time slots, and each slot is equal to or longer than the maximum propagation delay.
When a station is ready to send data, it first checks the channel.
If the channel is busy: The station waits for the next time slot.
If the channel is idle :The station will Transmit the frame with probability p and wait for the next time slot with probability q = 1 - p.
Repeats until: The frame is successfully transmitted or another station starts transmitting (causing the station to stop and try again later)

Key Points :

Balanced approach between speed and collision avoidance.
Reduces the chance of collisions compared to 1-persistent CSMA.
Useful when time-slotted communication is in place.

3. Non-Persistent CSMA

Working:

Only the station that has data to send will sense the channel.
If the channel is idle: The station immediately transmits the data.
If the channel is busy: The station waits for a random amount of time, then senses the channel again.
The station does not keep checking continuously just to grab the channel as soon as it’s free.

Key Points:

Focuses on collision avoidance over speed.
Suitable for systems where minimizing collisions is more important than transmission speed.