Wireless computing power saving measures may not be worth the effort

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 maximizing run time between charges.

Optimizing 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. (Read a related story on how to get the most out of your battery.)

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 (see related story on standards) 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.

Test configuration and procedures

The basic test strategy was to copy a file consisting of roughly 1MB (1,095,680 bytes, to be precise) from a source computer to a destination computer as many times as possible, beginning with a fully charged battery and ending each test run when the notebook computer went into hibernation as a result of near exhaustion of the battery, defined in this case as 5% battery charge remaining.

The test was driven by a simple DOS .bat file, the logic for which was to print the time of day, copy the file from source to destination, pause for three seconds, increment and display a counter indicating the loop iteration, and then run continuously until the battery gave out. The purpose of the pause was to allow more than enough time for the notebook to go into PSM and to simulate a fairly low Wi-Fi usage duty cycle, so as to maximize the time the radio was asleep. The test script was run on the destination computer so that transactions would be initiated and recorded by the mains-powered computer.

The destination in all test cases was a Dell 4500 desktop upgraded with a PCI gigabit Ethernet adapter and running Windows XP Pro with all current updates applied as of the date that testing began. Power conservation features on this machine were disabled for all test runs as it was operating on AC power.

We used two different source computers, both notebooks: an Acer Aspire 5920 notebook equipped with Vista Ultimate (including all updates available as of the date of the test run, but not including SP1), and featuring both gigabit Ethernet ports and an integral Intel 4965 a/g/n wireless adapter; and an HP Compaq nx6125 with gigabit Ethernet and a Broadcom 802.11 b/g radio.