P-persistent CSMA is an approach of Carrier Sense Multiple Access (CMSA) protocol that combines the advantages of 1-persistent CMSA and non-persistent CMSA. Using CMSA protocols, more than one users or nodes send and receive data through a shared medium that may be a single cable or optical fiber connecting multiple nodes, or a portion of the wireless spectrum.
In p-persistent CSMA, when a transmitting station has a frame to send and it senses a busy channel, it waits for the end of the transmission, and then transmits with a probability p. Since, it sends with a probability p, the name p – persistent CSMA is given.
The algorithm of p-persistent CMSA is:
When a frame is ready, the transmitting station checks whether the channel is idle or busy.
If the channel is idle then it transmits the frame immediately.
If the channel is busy, the station waits and continually checks until the channel becomes idle.
When the channel becomes idle, the station transmits the frame with a probability p.
With a probability ( 1 – p ), the channel waits for next time slot. If the next time slot is idle, it again transmits with a probability p and waits with a probability ( 1 – p ).
The station repeats this process until either frame has been transmitted or another station has begun transmitting.
If another station begins transmitting, the station waits for a random amount of time and restarts the algorithm.
It is the most efficient among 1-persistent CSMA, non-persistent CSMA and p-persistent CSMA. It reduces the number of collisions considerably as compared to 1-persistent CSMA. The channel utilization is much better than non-persistent CSMA.
The throughput of a network system is defined as the number of successful transmissions per frame time. The through put of p-persistent CMSA depends upon the value of p. Generally speaking, lower the value of p, greater the throughput. However, with lower values of p, channel utilization also reduces.
The following graph compares the throughput of p-persistent CMSA with that of other MAC protocols. Here, the throughputs of three values of p: 0.5, 0.1 and 0.01 are plotted.