The Point Coordination Function was another media access mechanism that is described in the 802.11 WLAN standard. In the PCF mechanism, the Access Point would function as a Point Coordinator. The time for which the Access Point would control the medium was termed as the Contention Free Period (CFP). The Access Point can place the stations intending to participate in the CFP in a CF-POLL list.

The basic mechanism of PCF is provided below:

• A WLAN Station’s setting of CF-Pollable, CF-Poll Request and QoS bits in the capability Information field determines whether the station is CF-Pollable and the station intends the Access Point to place the station in a CF-Poll list

• The Access Point sends a Contention Free Parameter Set in the beacon frame which provides CFP parameters for the station to set

1. CFP Count — indicates how many delivery traffic indication maps (DTIMs) (including the current frame) appear before the next CFP start
2. CFPPeriod — indicates the number of DTIM intervals between the start of CFPs
3. CFPMaxDuration — indicates the maximum duration, in TU, of the CFP that may be generated by this PCF
4. CFPMaxDuration — indicates the maximum duration, in TU, of the CFP that may be generated by this PCF

• The Access Point would set the Network Allocation vector of all the stations associated to it to the duration of the CFP via the CFPMaxDuration field
• The Length of the CFP is controlled by the Access Point
• During the CFP, the Access Point polls the station via CF-Poll frame, data frame or data frame+CF-Poll frame, or a management frame.
• A station can respond to a poll by the AP and partake in a Contention free frame transmission.
• An Access point ends the CFP via CF-End frame

The below diagrams indicate CFP and CP time period allocations

The below diagram indicates a CFP frame transfer

Drawback of the DCF Mechanism and development of EDCF and HCF