Non-persistent CSMA protocol

Non-persistent CSMA is a non-aggressive version of Carrier Sense Multiple Access (CSMA) protocol that operates in the Medium Access Control (MAC) layer. Using CSMA protocols, multiple 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 non-persistent CSMA, when a transmitting station has a frame to send and it senses a busy channel, it waits for a random period of time without sensing the channel in the interim, and repeats the algorithm again.

Algorithm

The algorithm of non-persistent CSMA 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 for a random time period during which it does not check whether the channel is idle or busy.

• At the end of the waiting time period, it again checks the status of the channel and restarts the algorithm.

Advantages

The rate of collisions is much reduced compared to 1-persistent CSMA. This is because each station waits for a random amount of time before attempting retransmission. The probability that multiple stations will wait for the same amount of time is extremely low, so collision between contending stations is greatly reduced.

Disadvantages

Non-persistent CSMA reduces the bandwidth usage of the network. This occurs because the channel remains idle even if there are stations that have frames to transmit. Since each station waits for a random time before attempting retransmission, there may be multiple stations waiting while the channel is idle, leading to underutilization of the available bandwidth.

Comparison of CSMA Variants

Conclusion

Non-persistent CSMA reduces collision rates by having stations wait for random periods without sensing the channel, but this comes at the cost of reduced bandwidth utilization. It provides a balance between collision avoidance and network efficiency in shared medium environments.