Carrier-Sense Multiple Access with Collision Detection

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Carrier-Sense Multiple Access with Collision Detection (often given as the acronym, CSMA-CD) is a way of controlling access to a LAN using a broadcast bus (i.e. a shared broadcast channel on which all stations can hear any station which is transmitting).

A station wanting to transmit waits until it hears silence ('carrier sense'), and then starts transmitting. If it 'hears' another station start to transmit while it is so doing (a 'collision' - hence 'collision detection'), it stops, 'backs off' a random delay, and re-tries. (This is essentially exactly identical to what a group of people chatting at a party will do.)

With both stations doing the 'random backoff after collision', the chances are that one will start before the other, and succeed in acquiring the channel: i.e. its new transmission will arrive at the other station, so it can suppress the other station from transmitting, before the second station's random time-out will have run out.

A version of CSMA-CD was first used on the ALOHA network, before being adopted for Experimental Ethernet.