Power management and battery-life conservation have been part of the IEEE 802.11 standard from its first release in 1997. Over time, the techniques and basic technologies involved have become more diverse.
Power Save Mode is just one of a range of options, however. Here's a list of today's WLAN power-conservation technologies, but keep in mind that not all adapters will implement all of these choices, and specific implementations will vary:
Constantly Awake Mode (CAM). This is how most WLANs are operated today, with power-saving features disabled. This is partly because of fears about reduced performance in terms of throughput when power saving measures are enabled (as we saw in our testing), but such is also the default for most products shipped today, and users typically change few, if any, options of any form upon installation and initial configuration, and very seldom thereafter.
Power Save Mode (PSM). This is the original power-conservation technique defined in 802.11, and was tested in this article. The methodology is for the mobile device to suspend radio activity after a variable but pre-determined (by the vendor) period of inactivity, and then wake up periodically (usually about three beacon frames, which are normally 100 ms each) to see if the infrastructure has queued any traffic for it.
Unscheduled Automatic Power Save Delivery (U-APSD). This is an asynchronous approach to power conservation – defined in 802.11, and serves as the basis of WMM Power Save (below), allowing the client to request queued traffic at any time rather than waiting for the next beacon frame. This technique may thus be more efficient with lighter traffic loads, like voice.
WMM Power Save (WMM-PS). This technique is a product of the Wi-Fi Alliance and was introduced with the development of 802.11e and the corresponding Wireless Multimedia (WMM) specification. It is based on U-APSD, and is often implemented in Wi-Fi handsets. A scheduled (synchronous) version (S-APSD) is also defined.
Power Save Multi-Poll (PSMP). This approach is specified as part of 802.11n, and was developed because of concerns that MIMO-based products, using multiple radios and more circuitry regardless, would become power hogs with a significant adverse impact on battery life. An extension to U-APSD and S-APSD, the scheduled version reserves a time slot for a given client station and thus temporarily silences others associated. This technique may be better with relatively heavy traffic loads.
Dynamic MIMO Power Save. This technique allows MIMO-based (802.11n) radios to downshift to less-aggressive radio configurations (for example, from 2x2 to 1x1) when traffic loads are light.
Wake on Wireless. Atheros implements a technique in some of its WLAN chips that is not unlike the wake-on-LAN frequently seen in Ethernet adapters, allowing the infrastructure to initiate the waking of a dozing radio.
Wi-Fi protocol-related power-saving techniques are, of course, only one piece of the overall mobile power solution. Other elements include the use of low-power semiconductor process technologies and resulting components wherever possible, design engineering (boards and systems) with power consumption in mind, and improvements in batteries – although progress in this area has historically been slow. And, of course, other power-conservation variables like processor speed also must be taken into account.