Basics of WiFi Power Saving Mechanisms

·         Power savings can be achieved by the STA when it indicates to the AP that it is entering power save mode and shutdown its receive path.

·         The AP stores frames destined to an STA in power save mode and sends them to the STA when requested to do so.

·         During association, an STA uses the Listen Interval parameter to indicate to the AP how many beacon intervals it shall sleep before it retrieves the queued frames from the AP( The AP shall not drop any queued frames until the STA's Listen Interval elapses).

Legacy Power Save Polling

·         An STA enters power-save mode by sending a Null frame to the AP with the Power Management bit set.

·         From then on, the AP stores all packets destined to the STA in a per-STA queue and sets the TIM field in the beacon frame to indicate that packets destined for the STA have been queued at the AP.

·         An STA wakes up from sleep every Listen Interval to receive the beacon frame and when it detects that the TIM field for it has been set, it sends a PS-Poll frame to the AP.

·         In response, the AP sends the first queued frame to the STA. The STA receives the queued data frame and if the More Data field in this frame is set, it sends another PS-Poll frame to the AP.

·         The STA continues to send PS-Poll frames to receive all the queued frames and when none are left, it goes back to sleep until the next Listen Interval.

·         In legacy power-save mode, the frames are exchanged with DIFS separation so that other STAs can take over the medium.http://www.wifi-insider.com/wlan/psd.htm