CSMA/CD
acronymStands 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.
In full duplex the whole scheme is switched off. Cisco's troubleshooting note puts the two worlds side by side: on a shared, half-duplex segment 'Collisions are not bad; they are essential to correct Ethernet operation', while 'In full-duplex Ethernet, collision detection is disabled'.
What survives in operations is the late collision. A collision 'detected by a station after it has sent the 512th bit of its frame' counts as late, because by then every station should already have heard the frame; Cisco lists the usual causes as incorrect cabling, too many hubs and faulty network cards. On a modern switched network, a late-collision counter that moves almost always means one thing instead: one end of the link is running half duplex and the other full, the duplex mismatch.