802.11 Protocol Architecture
FIGURE 2.17
CFP/CP Alternation (a.k.a. Contention-Free Repetition Interval)
Contention-Free Period
At the beginning of the CFP, the point coordinator has an opportunity
to gain control of the medium. The point coordinator senses the
medium at the beginning of each CFP. If the medium is idle after a
PIFS interval, the point coordinator sends a beacon frame that includes
the CF Parameter Set element.
Before receiving a CFP beacon, stations set their NAVs to the
CFPMaxDuration value found in the previous beacon’s CF Parameter
Set. Stations set their NAV at the expected Target Beacon
Transmission Time (TBTT), which is the same time the beacon is
expected to be transmitted. Since stations anticipate their backoff in
this manner, there is a less likely chance of colliding with the beacon
when it is sent by the point coordinator. Stations know at which TBTT
the CFP beacon will be transmitted based on the CFPPeriod value
found in the CF Parameter Set.
When stations receive the beacon, they see that the beacon contains a
duration field value of 32,768 which is reserved for use in all frames
sent during a CFP. This special value causes receiving stations to skip
the normal NAV update procedure based on Duration field found in
every received frame and continue counting down from the NAV value
set by the CFPDurRemaining value in the CF Parameter Set found only
in beacons.
