APSD
Automatic Power Save Delivery
Automatic Power Save Delivery is a term used in WiFi to describe a power saving technique originally introduced in the IEEE 802.11e specification. It involves a QAP (Quality Access Point) delivering downlink frames (which belong to a specified Access Class) to Power Saving stations automatically, thereby preventing the need to poll for each frame as is usually performed. APSD defines two delivery mechanism – U-APSD (Unscheduled – Automatic Power Save Delivery) and S-APSD (Scheduled – Automatic Power Save Delivery).