CSMA/CA

Carrier Sense Multiple Access (with Collision Avoidance)

CSMA checks wether the medium is free before accessing it.

Collisions resulting from simultaneous access are avoided by deferring access to the medium.

The Distributed Coordination Function (DCF) builds the main method of the CSMA/CA mechanism.
Stations choose random backoff time after each transmission, the one with the lowest number occupies the channel first.

The Point Coordination Function(PCF) provides contention-free transmission.
Chosen stations called Point Coordinators (situated in access points) are used to define which station is allowed to transmit at a given time.
So PCF is only possible in infrastructured networks.