Testing Wi-Fi's Power Save Mode
PSM is supposed to save power, but does it?
By Craig Mathias, Farpoint Group | Techworld | Published: 10:00, 13 May 2008
Wi-Fi has long been regarded as a big drain on mobile battery life. Several techniques have been developed to reduced the power demands of Wi-Fi, by using the radio sparingly.
Farpoint Group tested whether 802.11's Power Save Mode (PSM) actually delivers any significant benefit, and found that by and large, it does not.
Here are the details of the test.
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 percent of 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 maximise 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.
Both machines were directly connected to each other either via a gigabit Ethernet link (for baseline testing), and via a wireless connection (using an access point). At no time were any elements connected to any other network or the Internet.
The Acer's built-in wireless adapter was used in 802.11g mode only, with a Netgear WNR854T router (used only as an access point in this test) forced to 802.11g mode as well. The Acer's internal adapter was also used in 802.11n mode with a Linksys WRT350N router (again, as an access point). Similar 802.11g testing was performed with the HP notebook, using the Linksys AP, and also with two external 802.11n adapters (a Linksys PC card and an SMC USB adapter) connected to the Linksys AP. All of this provided a good variety of test cases.
Our test procedure involved first establishing a baseline for performance in terms of throughput. We then repeated the test with each Wi-Fi client adapter/access point pair, in each case the only variable being the changing of the level of client Wi-Fi power conservation. Both notebooks kept the hard drive on all the time, and the Acer was set to 50 percent display brightness while the HP's display was kept all the way up. We used a spectrum analyzer to monitor for any high-amplitude interference that might affect results throughout all test runs, and none was observed.