Spatial Multiplexing (SM) Power Save is an 802.11n improvement to the original 802.11 power save feature. All 802.11 clients can inform access points (APs) that they plan to sleep in order to conserve power. With SM Power Save, 802.11n clients can keep just one antenna (actually, one transmit/receive chain) turned on while sleeping, turning other transmit/receive chains off to conserve more power.
Power Save Multi-Poll (PSMP) is an 802.11n improvement based on 802.11e Quality of Service scheduled and unscheduled power save features. With unscheduled PSMP, sleeping clients can inform APs when they're ready to wake up and send traffic instead of waiting until the AP broadcasts a Delivery Traffic Indication Map (DTIM) frame. With scheduled PSMP, clients can actually "book a reservation" with the AP by sending a Traffic Specification (T-Spec), eliminating the power otherwise wasted when contending with other clients using the same channel.
To learn more about how 802.11e (WMM) Power Save conserves battery life, see this Wi-Fi Alliance white paper: WMM™ Power Save for Mobile and Portable Wi-Fi® CERTIFIED Devices.
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