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To preserve battery life on small footprint wireless devices, the 802.11 standard defines a Power Save mode. Stations that enter Power Save mode announce their intention to sleep before temporarily shutting down all radio functions, with the exception of a timing function. The timing function wakes the station briefly at regular intervals to listen for AP beacons.
The AP does indeed buffer frames received for a sleeping station. The AP lets all stations know whether unicast frames are pending transmission by sending beacons containing a Traffic Indication Map (TIM). When a sleeping station hears one of those beacons, it leaves Power Save mode, turns its radio back on, and becomes ready to receive buffered frames. A slightly different mechanism is used to deliver buffered broadcast/multicast frames to all stations at specified intervals, known as the DTIM (Delivery Traffic Indication Message) interval.
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