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Want to significantly boost your smartphone's battery life? US Duke University researcher Justin Manweiler reckons he's worked out how.

Manweiler's work centres on Wi-Fi, specifically when the phone - or tablet or laptop, for that matter - is operating within range of more than one wireless access point.

If access points are able to co-operatively manage their shared radio environment, client gadgets can keep their Wi-Fi adaptors in sleep mode for longer, extending battery life.

Currently, clients periodically wake up their radios to check in with their associated access point and see if data is available to be transfered. Existing Wi-Fi energy saving technology, called Wi-Fi Power Save Mode (PSM), ensures data is buffered in the access point to ensure the client radio doesn't need to be woken up for dribs and drabs.

Instead, data packets are bundled and sent as bursts big enough to maximise radio sleep time without introducing so much latency that the user perceives that the data isn't being streamed - video playback that isn't smooth, for example.

Now throw multiple access points into the mix and the upshot is that a given client's radio needs to stay awake for greater periods - up to five times longer, Manweiler found - than it would when in range of a single access point, thanks to contention between access points.

As so many of we city dwellers and workers know, these days there are lots of access points around us, and the longer the device's radio is on, the more power it's burning.

Manweiler's solution, dubbed SleepWell, has a compatible access point dynamically reschedule those bursts of traffic to its clients so they don't occur when other access points are sending data to their clients. Clients sleep when other clients are active, and can be sure they can download data without interruption when it's their turn.

"By carefully modifying the timestamps - as a part of the Wi-Fi clock synchronisation process - the SleepWell AP regulates the client’s sleep and wake-up schedules," writes Manweiler in his paper Avoiding the Rush Hours: Wi-Fi Energy Management via Traffic Isolation.

"Results show a median gain of up to 2x when Wi-Fi links are strong," he says. "When links are weak and the network density is high, the gains can be even more."

SleepWell was implemented by tweaking the open source ath9k driver for Atheros 802.11n PCIe interfaces.

Since bursts take place periodically, SleepWell can work out the best time to send its data even if some or all of the surrounding APs are not SleepWell-enabled themselves.

Manweiler said that SleepWell only needs to be implemented at the access point, so there's no need to change the radio code running on the client - it's compatible with all existing client devices.

Testing SleepWell with a variety of laptops and smartphones, Manweiler said he recorded "energy reductions vary between 38 per cent to 51 per cent, across a variety of real online applications, including YouTube, Pandora, Last.fm and... FTP".

The gotcha is VoIP and other "interactive, highly latency sensitive traffic", which requires lower levels of latency than even PSM can ordinarily introduce without being noticed.

You can read Manweiler's paper on the Duke University website (PDF). ®

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