What are the differences between Non-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

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.

Non-Persistent CSMA

Non-persistent is called as a non-aggressive version of CSMA protocols which comes under the MAC (Medium Access Control) layer. Here the station which has the frames to send, that station only senses the channel.

Suppose if it is an idle channel, then non-persistent frames will send frames immediately to that channel. So, here the channel seems to be busy, then it will wait for the random time and again sense the state of the station whether idle or busy.

In a non-persistent method, the station will not immediately sense the channel. The main advantage is it reduces the chances of collision. But the problem in on-persistence is that it reduces the efficiency of the network.

The advantage of non-persistent CSMA is that it reduces the chance of collision because the stations wait a random amount of time. The disadvantage is that it reduces the efficiency of the network because the channel remains idle when there are stations with frames to send.


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

p-persistent CSMANon-persistent CSMA
Whenever the carrier sense channel is idle the p-persistent send with probability pThe Non-persistence sends when the channel is idle.
The p-persistence will wait for the next slot for the frames transmission.The Non-persistent waits for the random amount of time to check the carrier.
In p-persistence the chance of collisions is less when compared to non-persistent.In non-persistent there is more chances of collision when compared to p-persistence
Delay low load is large when probability p is small.Delay low load is small whenever the channel is found idle.
The p-persistent utilization is always dependent on probability p.The non-persistent utilization is above 1-persistent because all stations are not constantly checking the channel at the same time.