Point coordination function

Article summary

Point Coordination Function (PCF) is a media access control (MAC) technique used in IEEE 802.11 based WLANs, including Wi-Fi. It resides in a point coordinator also known as access point (AP), to coordinate the communication within the network. The AP waits for PIFS duration rather than DIFS duration to grasp the channel. PIFS is less than DIFS duration and hence the point coordinator always has the priority to access the channel. The PCF is located directly above the distributed coordination function (DCF), in the IEEE 802.11 MAC Architecture. Channel access in PCF mode is centralized and hence the point coordinator sends CF-Poll frame to the PCF capable station to permit it to transmit a frame. In case the polled station does not have any frames to send, then it must transmit null frame. Due to the priority of PCF over DCF, stations that only use DCF might not gain access to the medium. To prevent this, a repetition interval has been designed to cover both (Contention free) PCF & (Contention Based) DCF traffic. The repetition interval which is repeated continuously, starts with a special control frame called Beacon Frame. When stations hear the beacon frame, they start their network allocation vector for the duration of the contention free period of the repetition period.…

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