Carrier Sense Multiple Access Protocol

The Carrier Sense Multiple Access Control (CSMA) Protocol is a Media Access Control (MAC) system. It operates when a sender wishes to transmit data on shared media.

Function

The transmitter listens on the line for a carrier signal. If it detects one, it knows that the transmission medium is occupied and waits. This is the "Carrier Sense" part of the protocol's name. "Multiple Access" means that the transport medium is shared among nodes.

Features

Several nodes are connected to the same wire. Each may send signals down that wire. Only one signal can be sent at a time. All nodes receive the transmitted signal.

Variations

The CSMA protocol is the basis for two other protocols. The most commonly known of these is CSMA/CD (Carrier Sense Multiple Access/Collision Detection). This terminates transmission if a collision occurs to prevent another collision. CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) is the second variation. With CSMA, if the line is busy, the testing node waits a set period before trying again. All nodes wait the same period. With CSMA/CA, that wait period is a random number reducing the possibility that two nodes will try to access the line again at the same moment.

References

• Search Mobile Computing: What Kind of Systems Use the CSMA Protocol