Differentiate between 1-Persistent and p-Persistent CSMA

The three types of Carrier Sense Multiple Access (CSMA) Protocols are as follows −

  • 1-persistent CSMA
  • Non- Persistent CSMA
  • p-persistent CSMA

1-persistent CSMA

In this method, to transmit data the station wants to continuously sense the channel to check whether the channel is busy or idle.

Suppose, if the channel is busy, then the station will wait until it becomes idle. Whenever the station detects an idle-channel, then it transmits the frame with probability. Therefore this persistence is called 1-persistent CSMA.

The 1-persistent CSMA has the highest chance of collision because two or more stations identify a channel to be idle at the same time and transmit their frames.

Whenever the collision occurs, the stations will wait a random amount of time and start again. The structure of 1-persistent CSMA is as follows −

Drawback of 1-persistent

In this 1-persistence the propagation delay time is more.

For example − If station 1 begins its transmission, at the same time station 2 is also ready to send its data and senses the channel. If the station 1 signal has not still reached station 2, station 2 thought that the channel is idle and will begin its transmission. This will result in a collision.

Even though the propagation delay time is zero, the collision will occur. If two stations are ready in the middle of the third station's transmission, then both stations have to wait till the transmission of the first station ends and then both will begin their transmission exactly at the same time. This will also result in collision.

p-persistent CSMA

The p-persistent CSMA is used whenever the channel has time slots so that the time slot duration is equal to or greater than the maximum propagation delay time. The p-persistence senses the channel whenever a station becomes ready to send.

Suppose if the channel is busy, then the station has to wait till the next slot is available.

The p-persistence transmits with a probability p when the channel is idle. With a probability q=1-p, the station has to wait for the beginning of the next time slot. If the next slot is idle, it either transmits or waits again with probabilities p and q.

This process will continue until the frame has been transmitted or another station has begun transmitting. If another station is transmitting, the station acts as though a collision has occurred and it waits a random amount of time and starts again.


The advantages of p-persistent CSMA are as follows −

  • The p-persistence reduces the chance of collision.

  • Improves the efficiency of the network.


The major differences between 1-persistent CSMA and p-persistent CSMA are as follows −

1-persistent CSMAp-persistent CSMA
Whenever the carrier sense channel is idle the 1-persistence send with probability 1Whenever the carrier sense channel is idle the p-persistent send with probability p
The 1-persistent will continuously sense the channel for the frames transmission, it will not wait.The p-persistence will wait for the next slot for the frames transmission.
In 1-persistent CSMA, the chance of collisions is more.In p-persistence the chance of collisions is less when compared to 1-persistent.
Delay low load is small in 1-persistent.Delay low load is large when probability p is small.
The 1-persistent utilization is above ALOHA.The p-persistent utilization is always dependent on probability p.