A Field Guide to Wireless LANs for Administrators and Power Users

The topics of medium access, MAC timing, power saving, fragmentation, and retransmission, are all inter-related. This can cause considerable confusion, since it is often difficult to remember which rule trumps another in a given situation. In this chapter, we have seen all the important bits and perhaps also exposed the reader to the complexity, in that the pieces are all inter-dependent and it is often necessary to mention special cases while trying to describe the "main mode" of operation.

DCF is that main mode, and it embodies the CSMA/CA access control protocol and a slew of timing-related rules. PCF is optional, and allows for polling-oriented access managed by an entity in the AP known as the Point Coordinator. PCF is built on DCF, and leverages mechanisms in the Beacon to control when CFPs begin. Only STAs that are capable of CF operation can participate in the CFP, and the rest of the STAs' NAVs are updated so they do not attempt to contend for the medium while the AP is in polling mode.