CSMA/CD

networking

Stands for: Carrier Sense Multiple Access with Collision Detection

Classic Ethernet's channel etiquette: listen before you talk, talk only when the wire is quiet, and if two stations collide anyway, stop, jam, and retry after a random backoff.

Metcalfe and Boggs described the scheme in their July 1976 CACM paper on Ethernet, and IEEE 802.3 standardized it: a station senses the carrier, transmits when idle, keeps listening while sending, and on detecting a collision emits a jam signal so everyone notices, then waits a random interval chosen by binary exponential backoff before trying again. The 64-byte minimum frame exists for it, sized so a transmission lasts at least one round trip across the maximum segment and collisions are always detected in time. It is also the losing side of the great 1980s argument with Token Ring's deterministic turn-taking. Switched, full-duplex Ethernet retired the contention entirely, so CSMA/CD survives in the framing, the minimum frame size, and the exams.

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