802.11 Protocol Architecture
FIGURE 2.16
Protection Announced in Beacons
The duration value will be understood by NonERP stations so they will
update their NAV, causing them to refrain from transmitting during the
reservation period specified by the ERP station.¹
Point Coordination Function (PCF)
The optional priority-based point coordination function provides
contention-free frame transfer for processing time-critical information
transfers via a station polling mechanism. There are very few vendors
that support this access method, so this book only covers the basic
operation. Using PCF, a point coordinator (a software function) resides
in the access point to control the transmission of frames to and from
stations.
Only Infrastructure Basic Service Sets may use PCF. Additionally,
when PCF is in use, it is not actually PCF only, but rather a
combination of PCF and DCF. PCF operates on top of DCF. Polling
happens during the Contention-Free Period (CFP), and regular
CSMA/CA access happens during the Contention Period (CP).
1
For more information on Protection Mechanism use in ERP wireless LANs,
see Protection Ripple in 802.11 WLANs (Devin Akin, June 2004) at
