Carrier Sense Multiple Access (CSMA/CD and CSMA/CA)

Carrier Sense Multiple Access is the family of rules that lets many devices share one transmission medium. Each device checks whether the medium is idle (carrier sense) before it sends. CSMA with Collision Detection (CSMA/CD), specified for half-duplex Ethernet in IEEE 802.3, keeps listening while it transmits; if two stations collide, both stop, signal the collision, and retry after a random backoff. CSMA with Collision Avoidance (CSMA/CA), used by IEEE 802.11 Wi-Fi, cannot reliably hear a collision while transmitting, so it waits a random interval before sending, relies on acknowledgements, and can use a request-to-send and clear-to-send exchange to reserve the channel.

Both operate at the media access part of the data link layer of the OSI model. The collision problem belongs to shared segments such as those built on a hub; a network switch running full duplex gives each port its own collision domain, so CSMA/CD has little work left to do there. The security angle is availability: a shared medium can be saturated or, on wireless, jammed.

Exam relevance: questions in this area tend to turn on matching the variant to the medium. Detection pairs with wired, half-duplex Ethernet; avoidance pairs with wireless. Candidates are expected to contrast both with token passing, where a device transmits only while it holds the token.