The contention-free transfer protocol is based on a polling scheme controlled
by a PC operating at the AP of the BSS. The PC gains control of the medium at the beginning of the CFP and attempts to
maintain control for the entire CFP by waiting a shorter time between transmissions than the stations (STAs) using the
DCF access procedure.
At the beginning of each CFP, the PC senses the medium. When the medium is determined to be idle for one PIFS period,
the PC transmits a Beacon (B) frame containing the CF Parameter Set element. Each station presets
its NAV to the CFPMaxDuration value (obtained from the CF Parameter Set element). This prevents stations from
taking control of the medium during the CFP.
After the initial beacon frame, the PC waits for at least one SIFS period, and then transmits one of the following:
a Data frame, a CF-Poll frame, a Data+CF-Poll frame, or a CF-End frame. A station that
receives a directed data frame of any subtype that includes CF-Poll may transmit one data frame a SIFS period after
receiving the CF-Poll. For frames that require MAC-level acknowledgment, stations that received a CF-Poll (of any type)
performs this acknowledgment using the Data+CF-ACK subtype in the response to the CF-Poll.
The lengths of the data frames may be variable.
If a station does not respond to a CF-Poll (of any type) within the SIFS period following a transmission from the PC,
then the PC resumes control and transmits its next frame after a PIFS period from the end of the PC’s last transmission.
A station shall always respond to a CF-Poll directed to it and received without error. If the station has no frame to send
when polled, the response shall be a Null frame.
If the station has no frame to send when polled, but an acknowledgment is required for the frame that conveyed the CF-Poll,
the response shall be a CF-ACK (no data) frame.
The CFP ends when the CFPDurRemaining time (obtained from the CF Parameter Set element) has elapsed since
the Beacon frame originating the CFP or when the PC has no further frames to transmit nor stations to poll.
In either case, the end of the CFP is signaled by the transmission of a CF-End by the PC. If there is a received
frame that requires acknowledgment at the time the CF-End is to be transmitted, the PC transmits a CF-End+ACK
frame instead. All stations of the BSS receiving a CF-End or CF-End+ACK resets their NAVs so they may attempt
to transmit during the CP.
See Fig. 8.2 for an illustration of PCF scheme.
Fig. 8.2. An illustration of Point Coordination Function.