Mobility and Wireless

One of the challenges in mobile computing is battery life. It's hard to be productive with a dead battery, so IT personnel and users alike need to think about maximising run time between charges.

Optimising the power conservation settings of a mobile computer or communicator, including dimming the display when on battery, turning off the display and hard drive after a pre-set period of time, suspending (keeping memory alive but the computer otherwise powered down) and hibernating (writing the image of main memory to disk for later resumption) help in getting the most out of any given charge.

And there are also power conservation settings in most Wi-Fi adapters that (at first glance, anyway) are intended to allow a high degree of control over the power consumed by the wireless network interface card (NIC) found in almost all notebooks and many handhelds as well. In gross terms, wireless power conservation involves turning off the radio, synchronously or asynchronously with the fixed infrastructure, for a portion of time — a technique used in various forms on essentially all production wireless systems today, including WANs. But this technique motivates an interesting and fundamental question: do Wi-Fi power-conservation techniques, when enabled, actually save a meaningful amount of energy or have any negative impact on throughput?

We set out to define a simple test to answer these questions as they pertain to 802.11's Power Save Mode (PSM), the most common form of Wi-Fi power saving implemented today. We do note that there are several new power saving mechanisms defined for 802.11n gear, but we have not found those to be widely implemented, so we could not assess those at this juncture.

Vendors have delivered a number of PSM variants, with the primary difference being how quickly and how often the adapter wakes up. Having a NIC wake up faster could negatively affect power consumption, the fundamental tradeoff in this strategy, although this could theoretically improve throughput. The opposite of PSM is Constantly Awake Mode (CAM), in which PSM is disabled. Our test compared various forms and implementations of PSM against CAM and, for good measure, a wired Gigabit Ethernet baseline test.

Using PSM in our tests produced only a marginal benefit in terms of battery life (and was even slightly worse than CAM in one test). In terms of throughput, the results ranged from marginally positive to having a very negative impact on throughput in two cases tested

Bottom line: PSM isn't likely to be of any value in contemporary implementations, and may even hurt performance.

We contacted all vendors whose products were included in this test regarding the results. Only Broadcom's PR department would comment, saying that its internal testing showed that battery life gains from PSM implementations in notebooks varies between brands, sometimes showing that PSM can maximize battery life with no impact on throughput.