Difference Between Non-Persistent and p-Persistent CSMA

Last Updated : 10 Sep, 2024

Carrier Sense Multiple Access (CSMA) is a network protocol that determines on how data should be transmitted in a shared medium communication channel so that there will be no collisions. It provides for the possibility for several equipment or devices to send out data while at the same time not clashing with one another. CSMA also signifies some variations as to when and how a device would try to actually send out data over cable if the channel is occupied. Some of them are Non-persistent CSMA and p-persistent CSMA. It is therefore important to understand how each of these two methods works so that the most appropriate be used in a given path or scenario.

Non-Persistent CSMA

In Non-persistent CSMA, the station has frames to send only senses for the channel. In the case of an idle channel, it will send the frames immediately to that channel. In case when the channel is found busy, it will wait for a fixed amount of time and again sense for the state of the station to be idle or busy. In this method, the station does not immediately sense the channel for only the purpose of capturing it when it detects the end of the previous transmission. This method reduces the chances of collision but reduces the efficiency of the network.

Advantages of Non-persistent CSMA

Reduced Collision Probability: As the name suggests, Non-persistent CSMA does not allow devices to continuously try to acquire the medium as is the case with other types of CSMA; instead of repeating the process at a fixed time interval, Non-persistent CSMA waits for a random amount of time before it attempts to do so which drastically decreases the likelihood of collisions.
Lower Network Congestion: The random wait time also comes in handy in dispersing the transmission attempts in the network thus lowering overall congestion.

Disadvantages of Non-persistent CSMA

Increased Delay: The random waiting period results in enhanced delay because the network devices take long time in the channel before they can gain access due to high traffic congestion in the network.
Lower Throughput: Because of the extra delays, the performance of the network as a whole can be less than in other methods that indicate that devices want to transmit actively.

p-Persistent CSMA

p-Persistent CSMA is used when a channel has time-slots and that time-slot duration is equal to or greater than the maximum propagation delay time for that channel. When the station is ready to send the frames, it will sense the channel. If the channel found to be busy, the station will wait for the next time-slot. But if the channel is found to be idle, the station transmits the frame immediately with a probability 'p'. The station thus waits for the left probability i.e. q which is equal to 1-p, for the beginning of the next time-slot. If the next time-slot is also found idle, the station transmits or waits again with the probabilities p and q. This process repeats until either the frame gets transmitted or another station starts transmitting.

Advantages of p-Persistent CSMA

Better Throughput: p-Persistent CSMA therefore performs better than Non-persistent CSMA for these reasons: it allows devices to transmit when the channel is free than in case of Non-persistent CSMA.
Controlled Collision Risk: It’s possible to reciprocate between the throughput and collision probability by changing the value of p.

Disadvantages of p-Persistent CSMA

Complexity in Probability Management: Finding the right value to use for ‘p’ can also be quite challenging given that it depends with the current condition of the network, the traffic levels and the number of devices connected to the network at any one time.
Potential for Collisions: However, if the probability p is set at a higher value then there could be a lot of interference as many devices may attempt to broadcast at the same time.