802.11 Wireless Networks: The Definitive Guide, 2nd Edition

Chapter 9. Contention-Free Service with the PCF

To support applications that require near real-time service, the 802.11 standard includes a second coordination function to provide a different way of accessing the wireless medium. The point coordination function (PCF) allows an 802.11 network to provide an enforced “fair” access to the medium. In some ways, access to the medium under the PCF resembles token-based medium access control schemes, with the access point holding the token. This chapter describes medium access under the PCF, detailed frame diagrams for the PCF frames, and how power management operations interact with the PCF.

The PCF has not been widely implemented. A media server product for the home implemented the PCF, though it was not commercially successful. Some enterprise-class products have implemented the PCF because it gives the access points more control over access to the wireless medium, and helps the network to to wrest control away from the anarchy of a herd of individual stations. For many readers, this chapter may not be necessary. If you are not using a product that implements the PCF, there is no need to read this chapter unless you have an interest in the standard itself.

Contention-Free Access Using the PCF

If contention-free delivery is required, the PCF may be used. The PCF is an optional part of the 802.11 specification; products are not required to implement it. However, the IEEE designed the PCF so stations that implement only the distributed coordination ...