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 the station has to wait for random time for channel to be idle and to start all again.
2. Non-persistent CSMA :
In Non-persistent CSMA, station that has frames to send only senses for channel. In the case of an idle channel, it will send frames immediately to that channel. In case when channel is found busy, it will wait for a fixed amount of time and again sense for state of station to be idle or busy. In this method, station does not immediately sense for channel for only purpose of capturing it when it detects end of previous transmission. This method reduces the chances of a collision but reduces efficiency of network.
Difference between 1-persistent and Non-persistent CSMA :
|Basis||1-persistent CSMA||Non-persistent CSMA|
|Carrier Sense||When channel is idle it will send with probability 1.||When channel is idle it will send frame.|
|Waiting||It will continuously sense channel for transmission of frames.||It will wait for random amount of time to check carrier.|
|Chance of Collision||In this method, there are highest number of collisions observed.||In this method, chance of collision are less 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 above 1-persistent because in this all stations constantly check for channel at same time.|
|Delay Low Load||It is small because frames are sent only in idle state.||It is longer than 1-persistent as it only checks randomly when channel is busy.|
|Delay High Load||It is high due to collision.||It is longer than 1-persistent because stations check randomly when channel is busy.|
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