Less than satisfying results
What jumps out almost immediately from this data is that PSM in any form delivered very little in terms of additional run time, and occasionally had a major detrimental impact on throughput.
The best improvement in runtime that we saw was a little over 8% in the case of the Linksys AP/Linksys Adapter running on our HP notebook with PSM enabled. That said, this combination also simply decimated throughput to less than half that of the CAM case. Interesting, this same combination of gear with "Fast" PSM enabled still resulted in 4% better run time and yielded a .5% gain in throughput.
Overall, though, it was clear that PSM was not contributing to significantly longer runtimes, and thus appears to have a negligible impact on notebook battery life. Moreover, in most cases, throughput was adversely affected and, where it was not, no real benefit was noted.
And the reason for this is the relatively large amount of power consumed in modern notebooks in comparison with the energy used by today's Wi-Fi adapters.
The 802.11 standard was initially developed during a time when processor clocks were in the 100MHz to 200MHz. range, and initial WLAN designs involved a significant number of power-hungry components. Today, however, Wi-Fi adapters are highly integrated — meaning fewer chips are required to implement a Wi-Fi solution — and designs are more power-efficient. While the notebooks' other components — most notably the processor (because of higher clock rates) and display and backlighting (due to much higher resolutions) — often consume more energy than in the past.
Notebook designers have compensated with larger batteries and a continual emphasis on power-conservative designs and provisions for a high degree of end-user control over power conservation settings in many cases, but the proportion of energy consumed between the computer and the WLAN adapter has clearly flipped.
As a consequence, it would be hard to encourage users to enable PSM in their daily operations. PSM is mostly harmless, but can also have very negative performance impacts. We also noted in the testing of some of the power-save modes on the Intel adapter that test runs would not complete, timing out with an error message, indicating that the notebook was simply not responding fast enough to meet application demands. Users thus need to be cautioned about setting PSM options without some knowledge of the possible consequences.
Saving energy in any form is, as Martha Stewart might say, a good thing. But, more importantly, anyone who is mobile knows that, after dropping one's mobile computer or communicator on a concrete floor, the most likely failure mode for these devices is a battery going dead. While I still recommend carrying a fully charged spare battery for all critical mobile devices essentially everywhere (understanding that is problematic with notebook batteries that tend to be large, heavy, and expensive), anything we can do to optimise battery life without creating a significant impact on network throughput needs to be considered, if not implemented as a matter of policy. Our tests show, however, that a slam-dunk case for Wi-Fi Power Save Mode cannot be made.
As a final note, it's also important to point out that we've only been considering the client-side elements of power conservation. While infrastructure plays a critical role in the implementation of the protocol-related elements of WLAN power management, it also makes sense to examine the power, and thus the cooling and cost, impacts of all WLAN infrastructure-side equipment, most importantly access points. While not all 802.11n access points, for example, can run on 802.3af power over Ethernet, it is wise to consider access point power consumption when evaluating new equipment. While this may not be the deciding factor in a purchasing decision, it makes sense that such at least be an item in the RFP.
Mathias is a principal with Farpoint Group, an advisory firm specialising in wireless networking and mobile computing. He can be reached at firstname.lastname@example.org.