Power Management and the PCF

Power conservation during the contention-free period is similar to power conservation during the contention-based period, with a few minor exceptions. The basic distinction between the two is that frame delivery must obey the PCF rules, so buffered frames can be delivered only to CF-Pollable stations. Stations that do not support PCF operations must wait until contention-based service resumes before retrieving buffered frames.

Stations on the polling list are not allowed to sleep during the contention-free period. When the access point is performing its point coordination functions, it may poll any station on the polling list at any time. Frames destined for stations on the polling list do not need to be buffered during the contention-free period because those stations do not sleep.

Frame buffering is identical under contention-free and contention-based service. By maintaining power-saving status for each station, the access point can buffer frames for any station in a low-power mode. Broadcast and multicast frames are buffered whenever an associated station is in a low-power mode.

In addition to the buffer status associated with contention-free service, the access point also sets bits in the TIM for any station it intends to poll. The reason for setting these bits is related to how buffered frames are delivered. Like contention-based service, DTIM frames trigger the transmission of broadcast and multicast frames. If the total time required to transmit multicast and broadcast frames exceeds the Beacon interval, the access point will transmit one Beacon interval's worth of buffered frames and stop. Remaining frames will, however, cause the access point to keep the bit corresponding to AID 0 set.

After transmitting the buffered broadcast and multicast frames, the access point goes through the list of AIDs whose TIM bits are set in increasing order and transmits any pending data. Transmissions are conducted according to the rules of the PCF, so it is not necessary to include a delay before beginning transmission. Stations on the polling list are added to the TIM, so they will be included in this process. Multiple buffered frames can be transmitted, but this is entirely up to the access point implementationin contention-free service, mobile stations can transmit only when given permission by the access point. A station is not allowed to resume sleeping until all frames have been delivered to it, as indicated by a 0 More Data bit. When a station is cleared to resume sleeping, it sleeps until the next DTIM transmission. DTIM frames signal the beginning of the contention-free period, so all stations that implement the PCF are required to wake up for every DTIM.

If a station switches from a low-power mode to the active mode, any frames buffered for it are transferred to the point coordination function for delivery during the contention-free period. The transfer does not result in immediate delivery, but the access point can place the frames into a queue for transmission as soon as the point coordination function permits.