IEEE 802.11 - PCF

To support time-bounded services, the IEEE 802.11 standard defines the Point Coordination Function (PCF). The PCF lets stations to have priority access to the wireless medium, coordinated by a station called Point Coordinator (PC). The PCF has higher priority than the DCF, because it may start transmissions after a shorter duration than DIFS; this time is called PIFS. Time is divided into repeated periods, called superframes. A Contention Free Period (CFP) with PCF used for accessing the medium and Contention Period (CP) with DCF used for accessing the medium alternate over time. A superframe starts with so-called beacon frame and contains CFP and CP. The beacon frame is a management frame that maintains the synchronization of the timers in the stations and deliveres protocol related parameters. The PC, which is typically collocated with the AP, generates beacon frames at regular beacon frame intervals, thus every station knows when the next beacon frame will arrive. See Fig. 8.1 for an illustration of a superframe.



Fig. 8.1. The superframe of IEEE 802.11.