hybrid coordination function (WLAN) (HCF)
The Hybrid Coordination Function (HCF) is used to ensure the quality of service in WLANs according to 802.11e. The HCF coordination function is functionally based on the radio channels and provides defined channel capacities at any time via the base station.
HCF works in conjunction with an Enhanced Distributed Coordination Function( DCF) of the Enhanced Distributed Coordination Function( EDCF). It includes two access methods, distributed and competitive access, Enhanced Distributed Channel Access( EDCA), and centrally controlled and non-competitive access, HCF Controlled Channel Access( HCCA). To delineate these access methods, the time period between 802.11 frames is divided into two: the non-competitive, Contention Free Period( CFP), and the competitive, Contention Period( CP). In the CFP period, access to the transmission medium is by means of HCCA, and in the CP period by means of both HCCA and EDCA.