Point coordination function

Point coordination function (PCF) is a Media Access Control (MAC) technique used in IEEE 802.11 based WLANs. It resides in a point coordinator also known as Access Point (AP), to coordinate the communication within the network. The AP wait 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 stations does not have any frames to send, then it must transmit null frame.

Since most APs have logical bus topologies (they are shared circuits) only one message can be processed at one time (it is a contention based system), and thus a media access control technique is required.

Wireless networks may suffer from a hidden node problem where some regular nodes (which communicate only with the AP) cannot see other nodes on the extreme edge of the geographical radius of the network because the wireless signal attenuates before it can reach that far. Thus having an AP in the middle allows the distance to be halved, allowing all nodes to see the AP, and consequentially, halving the maximum distance between two nodes on the extreme edges of a circle-star topology.

PCF seems to be implemented only in very few hardware devices as it is not part of the Wi-Fi Alliance's interoperability standard.

See also

• Contention free pollable
• Distributed Coordination Function
• Hybrid Coordination Function

External links

• 802.11 Medium Access Methods on wi-fiplanet.com