In computer networking, Carrier Sense Multiple Access With Collision Detection (CSMA/CD) is a network control protocol in which (a) a carrier sensing scheme is used and (b) a transmitting data station that detects another signal while transmitting a frame, stops transmitting that frame, transmits a jam signal, and then waits for a random time interval before trying to send that frame again.

CSMA/CD is a modification of pure Carrier Sense Multiple Access (CSMA). Please visit this article for a complete description of the basic protocol.

Collision detection is used to improve CSMA performance by terminating transmission as soon as a collision is detected, and reducing the probability of a second collision on retry.

Methods for collision detection are media dependent, but on an electrical bus such as Ethernet, collisions can be detected by comparing transmitted data with received data. If they differ, another transmitter is overlaying the first transmitter's signal (a collision), and transmission terminates immediately. A jam signal is sent which will cause all transmitters to back off by different random intervals, reducing the probability of a collision when the first retry is attempted.

Ethernet is the classic CSMA/CD protocol.

See also the similar Carrier sense multiple access with collision avoidance (CSMA/CA) protocol.

Very partly derived from Federal Standard 1037C