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Green computing: do Wi-Fi power saving adaptors work?

We put their power-saving consumption rates to the test

Many Wi-Fi adaptors claim to have power-conservation capability. We set out to test just how useful they are.

When it comes to mobile computing, there's nothing worse than a dead battery, especially when you urgently need to send an email or finish a document. Maximising battery life is one of the biggest issues for IT personnel and users alike.

We all know that optimising the power conservation settings including dimming the display, 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.

There are also power conservation settings in most Wi-Fi adaptors 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 laptops 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 did not assess those.

Vendors have delivered a number of PSM variants, with the primary difference being how quickly and how often the adaptor 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 laptops varies between brands, sometimes showing that PSM can maximise battery life with no impact on throughput.

NEXT PAGE: Test configuration and procedures

  1. We put power-saving consumption rates to the test
  2. Test configuration and procedures
  3. How power-saving adaptors fared in our test
  4. Our verdict on power saving Wi-Fi adaptors

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