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Understanding WMM Power Save and WLAN Client Battery Life


Wi-Fi mobile devices need as much battery power as possible because the power demands of voice, audio, and video applications are ever-increasing. To maintain laptop battery life in these situations, WMM Power Save was certified by the Wi-Fi Alliance. WMM Power Save is an addition to WMM, the technology that enables Quality of Service (QoS) functionality in Wi-Fi networks by prioritizing traffic from different applications. WMM is a required feature for 802.11n capable devices—almost all modern Wi-Fi devices support it.

WMM Power Save is optimized for latency-sensitive applications such as voice, audio, or video, but benefits any Wi-Fi device. With WMM Power Save, the same amount of data can be transmitted in fewer frames in a shorter time, while enabling the Wi-Fi device to preserve power in a low-power, dozing state in between transmissions.

This topic describes:

How Does WMM Power Save Extend Battery Life?

Power save uses mechanisms from 802.11e and legacy 802.11 to save power for battery powered equipment and fine-tune power consumption. Products targeted for power-critical applications such as mobile phones, smart phones, and other portable power devices typically use WMM Power Save.

The underlying concept of WMM Power Save is that the client triggers the release of buffered data from the access point by sending an uplink data frame. Upon receipt of that data trigger frame, the access point releases previously buffered data stored in each of its four queues. Queues can be configured to be either trigger enabled (receipt of a data frame corresponding to each queue acts as a trigger), or delivery enabled (data stored in all queues is released upon receipt of a frame).

WMM operates by dividing traffic into four access categories: background, best effort, video, and voice. QoS policy determines the different handling of each access category. The result is that different packets are handled differently.

Where is WMM Defined?

WMM was a precursor to the 802.11e standard. Before the 802.11e standard could be ratified, some organizations agreed on and published a draft standard called WMM. Once the 802.11e standard was finalized, WMM became an enhancement to the 802.11e standard, and was referred to as 802.11e quality- of- service (QoS) enhancements. Both the original WMM standard and 802.11e are now deprecated, but the industry continues to use that terminology.

How is WMM Power Save Implemented on Juniper Networks WLANs?

To take advantage of WMM Power Save functionality, both the Wi-Fi client and the access point must be Wi-Fi CERTIFIED for WMM Power Save. In addition, the applications used also need to support WMM Power Save to inform the client of the requirements of the traffic they generate.

WMM is supported by Radio profiles, along with the corresponding QoS policies that describe access classes. When a wireless client using WMM Power Save associates with an access point using a Radio profile that includes WMM, the client selects the access classes (voice, video, best effort, background) in WMM Power Save.

WMM Power Save is Disabled by Default

WMM Power Save is disabled by default on access points, even though it saves client battery life, because clients using Power Save must send a separate PSpoll to retrieve each unicast packet buffered by the access point radio. This can sometimes slow performance, depending on the network configuration. Also, your applications need to support WMM Power Save to inform the client of the requirements of the traffic they generate.

Why Should I Enable WMM Power Save in a Radio Profile?

WMM Power Save preserves client battery life, especially for applications such as voice and video. Wi-Fi CERTIFIED for WMM Power Save devices can operate in any Wi-Fi network and coexist with 802.11 legacy power save mechanisms.