The fundamental access method of the MAC used by 802.11 devices is a distributed coordination function (DCF) known as carrier sense multiple access with collision avoidance (CSMA/CA). The DCF MUST be implemented in all STAs.
For a STA to transmit, it MUST sense the medium to determine if another STA is transmitting. If the medium is
not determined to be busy the transmission may proceed.
There must be a wait time before stations can transmit… this will be covered in a later section
A transmitting STA must verify that the medium is idle for this required duration before attempting to transmit. If the medium is determined to be busy, a station must defer until the end of the current transmission.
After deferral, or prior to attempting to transmit again immediately after a successful transmission, the STA must select a random backoff interval and shall decrement (count down) the backoff interval counter while the medium is idle.
Hybrid coordination function (HCF)
The QoS facility includes an additional coordination function called HCF that is usable only in QoS network
configurations. The HCF shall be implemented in all QoS STAs except mesh STAs.
Instead, mesh STAs implement the MCF. The HCF combines functions from the DCF and PCF with some enhanced, QoS-specific mechanisms and frame subtypes to allow a uniform set of frame exchange sequences to be used for QoS data transfers during both the CP and CFP.
The HCF uses both a contention based channel access method, called the enhanced distributed channel access (EDCA) mechanism for contention based transfer and a controlled channel access, referred to as the HCF controlled channel access (HCCA) mechanism, for contention free transfer.