UAPSD
Unscheduled Automatic Power Save Delivery
The term U-APSD is used in wireless networks supporting the IEEE 802.11e Quality of Service enhancement. It was introduced in order to support power management in QBSS. In essence, it involves a QAP delivering downlink frames which belong to a specified access class to power saving Stations automatically. The Unscheduled variant is triggered by a Station receiving either a Delivery or Trigger enabled frame from a particular access class.