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Carrier Sense Multiple Access

Carrier Sense Multiple Access is a Media Access Control (MAC) protocol in which a node verifies the absence of other traffic before transmitting on a shared physical medium, such as an electrical bus, or a band of electromagnetic spectrum.

"Carrier Sense" describes the fact that a transmitter listens for carrier before trying to send. That is, it tries to detect the presence of an encoded signal from another station before attempting to transmit.

"Multiple Access" describes the fact that multiple nodes may concurrently send and receive on the medium.

Concurrent transmission by multiple nodes may result in frame collisions, in which a receiver is unable to extract frames from overlapping received signals.

In pure CSMA a transmitter does not detect collisions, and a receiver cannot distinguish between collisions and other sources of frame errors. Collision recovery relies on the ability of the receiver to detect frame error and invoke an error recovery procedure. For example, it may not send a required ACK, causing transmitters to time out and retry.

Since collisions are not seen by the transmitter, a frame is always entirely transmitted, even if it is garbled at the receiver by a collision. Also, retries may result in further collisions. Two common modifications to CSMA, Collision Detection and Collision Avoidance, attempt to improve performance by addressing these problems.