Windows Vista has a napping problem: I've been reading about an issue regarding low-power and brief sleep modes in "802.11" and "Wi-Fi" (used sometimes inaccurately as interchangeable commodities) for days now to understand what Microsoft did right or wrong in how they configured Vista to save battery power when using Wi-Fi. Ars Technica, as usual, has the right combination of technical detail and comprehensibility.
The story started as if Vista would "drain" batteries, which made little sense. Reading the original coverage in TechWeb and a Microsoft blog post on the matter didn't enlighten me. Why would any Vista setting use more power than XP SP2? Surely, XP SP2 has an optimized, but inferior set of options for Wi-Fi, because Vista is reported to offer better control over networking and wireless usage. (The Microsoft blog post has been deleted, by the way, with no placeholder. The TechWeb story has quotes from the blog.)
The discussion of "802.11 power save" made things even murkier. The Wi-Fi Alliance approved a test to certify part of 802.11e known as WMM (Wireless Multimedia). WMM as a whole deals with prioritizing packets in different queues so that voice packets can be given priority over ordinary data, and streaming data packets likewise. These queues are only part of the answer--ask Ruckus Wireless and others about that--but within 802.11e, there's an option for reducing power usage through cleverer brief naps while a transceiver isn't active. The alliance calls this WMM Power Save, and just a few devices currently carry that certification standard. (T-Mobile's HotSpot@Home service offers a D-Link router with WMM Power Save for this reason to preserve battery life on its UMA [unlicensed mobile access] handsets that work over Wi-Fi or cellular networks for calls.)
These tiny naps can add up. By catching a few milliseconds here and there, the Wi-Fi Alliance has estimated a 15 to 40 percent improvement in battery life over regular Wi-Fi. This will be significant in phones, games, and cameras where every electron counts.
So what's "802.11 power save"? Ars Technica notes that a power save mode appears in 802.11 specifications, and that different vendors have implemented this in different ways. Vista's default setting for its last version before the product was actually installed on hard drives heading to corporate customers was "Medium Power," which made adapters use this older, uncertified, non-interoperable mode. Because adapters and access points from different manufacturers--perhaps just different models--handle this power save feature differently, "Medium Power" would find APs sending packets when adapters were sleeping.
In the release to manufacture (RTM) version of Vista actually pushed out the door, the setting was change to "Maximum Performance," which disables sleep, and provides the most compatibility. It's unclear whether that setting would disable WMM Power Save--that might be implemented at a lower layer of the stack and only work with compatible devices.