Only a brief description of HCF is provided here. The interested reader can refer the 802.11-2012 or the 802.11-2016 standard.
In the EDCF mechanism seen previously – it was seen that all stations contend for the medium and the station that wins contention normally is allowed to transmit. The Hybrid co-ordination function combined features of Point Coordination Function (PCF) and Distributed Coordination Function (DCF).
The HCF mechanism uses the contention based Channel access mechanism (EDCF) for contention based data transfer and an Access Point controlled channel access termed HCF Controlled Channel Access (HCCA) where-in the stations do not contend for the medium and the Access point allocates time slots for each individual station to transmit packets. The HCCA mechanism is an extension of the PCF (Point Coordination Function) mechanism.
The Contention based channel access time is termed as Contention period (CP) and the Access Point allocated Time Period is termed as Contention Free Period (CFP).
In addition to the Contention Free Period, the HCCA mechanism also incorporates a Controlled Access Phase (CAP) which can be initiated in the Contention Period (CP) itself. In the Controlled Access Phase, the Access Point controls the medium. In this phase, the Access Point starts the CAP and exchanges frames with a WLAN station without contending for the medium
The HCF mechanism was not majorly implemented due to the difficulty in the development of implementing the HCCA mechanism with the Access point maintaining all of the scheduling responsibilities. The 802.11ax standard might re-look this mechanism for its operation.