In Trigger enabled U-APSD mechanism – the 802.11 station needs to send a trigger frame to de-queue a single frame and send more trigger frames to obtain the remaining data packets that are buffered at the access point. The trigger frame that is sent out is normally a PS-POLL frame and the mechanism is the PS-POLL mechanism to obtain the buffered packets from the Access Point
The Access point will buffer the Trigger based traffic in a separate queue termed the Legacy PS buffer queue. The TIM element sent by the Access Point in the beacon will indicate buffered data for the Trigger Access Categories.
FIG: Example of a Trigger based U-APSD SP with PS-POLL Mechanism