May 12, 2008, 2:00 PM — 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.
