CSMA/CD — Carrier Sense Multiple Access with Collision Detection — is the access method of classic half-duplex Ethernet (IEEE 802.3). It improves on ALOHA with one piece of common sense: listen before you talk. A station senses the channel and transmits only if it sounds idle. But sensing alone isn't enough, because signals take time to travel — so it also keeps listening while transmitting to catch a clash the instant it happens.
The collision window: why propagation delay matters
Two stations at opposite ends of the cable can both sense 'idle' and start, because neither has heard the other's signal yet. The danger lasts until a signal can cross the wire — the propagation delayτ. A collision can therefore happen up to 2τ after you start: your signal reaches the far end at τ, and that station's frame (begun an instant before yours arrives) takes another τ to travel back.
Frame time must be ≥ 2τ
For a station to *detect* a collision, it must still be transmitting when the collision signal gets back — so the frame transmission time must be at least 2× the propagation delay. This is exactly why Ethernet mandates a minimum frame size of 64 bytes and limits cable length: a frame too short could finish before its own collision returned, leaving the clash undetected.
What a station actually does
1Sense the carrier. If busy, wait until it goes idle.
2Begin transmitting, and keep listening to its own signal.
3If the signal it hears differs from what it sent → a collision is detected.
4Stop sending the frame and emit a short JAM signal (32 bits in Ethernet) so every station reliably notices and discards the garbled frame.
5Compute a random backoff and wait that long before retrying from step 1.
Binary exponential backoff
After the n-th collision a station waits a random number of slot times chosen uniformly from 0 to 2^n − 1. The range doubles with each successive collision, spreading contending stations further apart so retries seldom clash again. The exponent is capped at n = 10 (range 0–1023), and after 16 failed attempts the frame is discarded and the failure reported upward.
Why modern Ethernet rarely collides
Switched, full-duplex Ethernet gives each device a dedicated link, so CSMA/CD no longer fires — there's no shared medium to contend for. The protocol still defines the half-duplex legacy mode, and its ideas live on in CSMA/CA, the collision-*avoidance* cousin used by Wi-Fi where stations can't reliably hear collisions.
OperationTimeSpace
Collision detect · min 64-byte frame in 802.3≤ 2τ after startframe ≥ 2τ
Backoff after n-th clash · give up after 16 triesrandom 0..2^n−1 slotsn capped at 10
