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As in legacy power-save mode, an STA sets the Power Management bit in a Null frame to indicate to the AP that it is in power-save mode. An STA in power-save mode sends a QoS Null or QoS Data frame to trigger the AP to send queued frames. The AP acknowledges the trigger and then follows it up with up to a negotiated maximum number of frames. If there are no queued frames on the AP when the trigger is received, the AP will respond with a QoS NULL frame.