CSMA/CD
/ see-ess-em-AY-see-DEE /
Plain CSMA tells a node to listen before talking, but once it commits to a frame it keeps blasting away even if a collision has already ruined that frame — wasting the whole transmission. CSMA/CD adds a clever refinement: keep listening WHILE you talk. If a transmitting node detects that the signal on the wire no longer matches what it is sending, it knows a collision is happening, so it stops immediately instead of finishing a frame that is already garbage. This is how classic wired Ethernet shared a single cable.
The full dance goes: sense the channel and wait until idle; begin transmitting while monitoring the line; if a collision is detected, abort at once and send a brief jam signal so every station clearly registers the collision; then wait a random backoff time before trying again. The backoff uses binary exponential backoff — after each successive collision a node doubles the range of random wait times it picks from, so repeated collisions spread the contenders further apart and the channel settles down. Aborting early means a collision wastes only a tiny fragment of airtime, not a whole frame, which is what makes CSMA/CD efficient on a wire.
Two honest caveats. First, collision detection requires a minimum frame size: a frame must take at least as long to send as a signal takes to travel to the far end and back, or a sender could finish before a distant collision's evidence returns and never notice — this is why Ethernet has a minimum frame length. Second, and importantly, CSMA/CD is a WIRED idea. On wireless, a station usually cannot hear collisions while transmitting (its own signal drowns out everything), so Wi-Fi uses collision AVOIDANCE (CSMA/CA) instead. Modern switched, full-duplex Ethernet has no shared medium and no collisions at all, so CSMA/CD is effectively retired there too — it lives on mainly as the protocol that defined how Ethernet grew up.
Two stations on old shared-cable Ethernet both sense idle and start at nearly the same time. Each soon detects that the cable's signal differs from its own, recognizes the collision, aborts, sends a jam signal, and backs off a random time using binary exponential backoff — wasting only a sliver of airtime, not two full frames.
Listen while you talk: abort the instant a collision is detected to save the rest of the airtime.
CSMA/CD is for wired links. Wi-Fi cannot reliably detect collisions while transmitting, so it uses collision AVOIDANCE (CSMA/CA), not /CD. And on today's switched full-duplex Ethernet there is no shared medium, so CSMA/CD no longer runs at all.