Automatic Power Save Delivery (APSD) is an extension to the IEEE 802.11e standard. APSD is available in two versions:
- Unscheduled APSD (U-APSD)
- Scheduled APSD (S-APSD)
These two methods differ in the way that they use the transmission channels, among others. LANCOM APs and wireless routers support U-APSD, which forms the basis for the Wi-Fi-certified WMM Power Save (WMMPS).
U-APSD allows WLAN devices to save considerable amounts of energy. This function has come into demand due to the increasing use of WLAN-capable telephones (Voice over WLAN – VoWLAN).
Activating U-APSD for a wireless LAN enables WLAN devices making calls to switch into "doze mode" while they wait for the next data packet. Transmission of VoIP data takes place in a fixed time pattern—WLAN devices synchronize their phases of activity with this cycle, so that they are ready in good time to receive the next packet. This significantly reduces power consumption and the batteries provide a considerably longer call time.
The precise behavior of the power-saving mode is negotiated between the AP and WLAN client under consideration of the actual application at hand. This makes APSD much more flexible than former power saving methods, referred to in this context as "legacy power save".
