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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