Difference between Non-persistent and p-Persistent CSMA

Prerequisite – Carrier Sense Multiple Access (CSMA)

1. Non-persistent CSMA :
In Non-persistent CSMA, the station that 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 for 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.

Figure – Non-persistent CSMA

2. 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.

Figure – p-persistent CSMA

Difference between Non-persistent and p-persistent CSMA :

Parameter Non-persistent CSMA p-persistent CSMA
Carrier sense When channel is idle, it will send frame. When channel is idle, it will send with probability p.
Waiting It will wait for a random amount of time to check the carrier. It will wait for the next time-slot for the transmission of frames.
Chance of collision In this method, chance of collisions are more than in p-persistent. In this method, there are less chances of collision than in non-persistent.
Utilization Its utilization is above 1-persistent because in this all the stations constantly check for the channel at the same time. Its utilization depends upon the probability p.
Delay low load It is longer than 1-persistent as it only checks randomly when the channel is busy. It is large when the probability p is small because the station will not send always in the idle state of the channel.
Delay high load It is high due to collision. It is large when the probability p of sending is small when the channel is found in the idle state.

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