Difference between 1-Persistent and p-Persistent CSMA

Prerequisite – Carrier Sense Multiple Access (CSMA)
1. 1-persistent CSMA :
In 1-persistent CSMA, station continuously senses channel to check its state i.e. idle or busy so that it can transfer data. In case when channel is busy, station will wait for channel to become idle. When station finds an idle channel, it transmits frame to channel without any delay with probability 1. Due to probability 1, it is called 1-persistent CSMA. The problem with this method is that there is a huge chance of collision, as two or more stations can find channel in idle state and transmit frames at the same time. At the time when a collision occurs station has to wait for random time for channel to be idle and to start all again.


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


Figure – p-persistent CSMA





Difference between 1-persistent and p-persistent CSMA :

Basis 1-persistent CSMA p-persistent CSMA
Carrier Sense When channel is idle it will send with probability 1. When channel is idle it will send with probability p.
Waiting It will continuously sense channel for transmission of frames. It will wait for next time slot for transmission of frames.
Chance of Collision In this method, there are highest number of collisions observed. In this method, there are less chances of collision than in 1-persistent.
Utilization It’s utilization is above ALOHA because frames are sent only when channel is found in idle state. It’s utilization is dependent on probability p.
Delay low load It is small because frames are sent only in idle state. It is large when probability p is small because station will not send always in idle state of channel.
Delay high load It is high due to collision. It is large when probability p of sending is small when channel is found in idle state.

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