To minimize the chance of collision and, therefore, increase the performance, the CSMA method was developed. The chance of collision can be reduced if a station senses the medium before trying to use it. CSMA requires that each station first listen to the medium (or check the state of the medium) before sending. it is based on the principle "sense before transmit“. CSMA can reduce the possibility of collision, but it cannot eliminate it. The possibility of collision still exists because of propagation delay.
What should a station do if the channel is busy or idle? Three methods have been devised are:
1. I-Persistent: The I-persistent method is simple and straightforward. In this method, after the station finds the line idle, it sends its frame immediately (with probability I). This method has the highest chance of collision because two or more stations may find the line idle and send their frames immediately, Ethernet uses this method.
2. Nonpersistent: In the nonpersistent method, a station that has a frame to send senses the line. If the line is idle, it sends immediately. If the line is not idle, it waits a random amount of time and then senses the line again. The nonpersistent approach reduces the chance of collision because it is unlikely that two or more stations will wait the same amount of time and retry to send simultaneously. However, this method reduces the efficiency of the network because the medium remains idle when there may be stations with frames to send.
3. P-Persistent: The p-persistent method is used if the channel has time slots with a slot duration equal to or greater than the maximum propagation time. n The p-persistent approach combines the advantages of the other two strategies. It reduces the chance of collision and improves efficiency.
