PCF Operation
The PC begins a period of operation called the CFP (Contention-Free Period), during which PCF is operating.
CFP begins when the PC gains access to the medium, using the normal DCF procedures, and transmits a Beacon frame (Containing Max. expected length of CFP).
Once PC has control of the medium, it begins delivering traffic to stations in the BSS.
In addition, PC sends CF-Poll frame to those stations that have requested contention-free service.
PCF uses PIFS to seize & maintain control of the medium.
A wireless station can transmit only after being polled by PC. May send one frame for each CF-Poll received.If PC doesn’t get response ( Either Ack or Data frame) within SIFS, it will transmit next frame before PIFS interval. This is secondary mechanism to prevent access to medium during CFP.
It is possible to piggyback both the ACK & CF-Poll onto data frames.
PC announces the end of CFP by transmitting a CF-End frame.
