Controlling Wi-Fi radio 'nap-time' reaps big power savings

Tests show up to doubling of battery life for mobile devices

A Duke University grad student has come up with a way to double, or more, battery life in Wi-Fi devices, without any changes needed on the device itself. Essentially, the technique regulates how long and when client radios sleep, so that data transfers can be scheduled more efficiently.

In a test using eight laptops and nine Nexus One Android-based smartphones on an 802.11n network, the researchers found that the scheduling technique, dubbed SleepWell, resulted in energy reductions of 38% to 51% across a variety of online applications, including YouTube, Pandora and Last.fm Internet radio, and TCP bulk data transfers. What’s more, they found that as the quality of radio links degrades (with each packet transmitted at lower bit rates, resulting in a longer transmit interval), the relative energy gains are even higher.

BACKGROUND: Wi-Fi client surge forcing fresh wireless LAN thinking

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A Duke University grad student has come up with a way to double, or more, battery life in Wi-Fi devices, without any changes needed on the device itself. Essentially, the technique regulates how long and when client radios sleep, so that data transfers can be scheduled more efficiently.

In a test using eight laptops and nine Nexus One Android-based smartphones on an 802.11n network, the researchers found that the scheduling technique, dubbed SleepWell, resulted in energy reductions of 38% to 51% across a variety of online applications, including YouTube, Pandora and Last.fm Internet radio, and TCP bulk data transfers. What’s more, they found that as the quality of radio links degrades (with each packet transmitted at lower bit rates, resulting in a longer transmit interval), the relative energy gains are even higher.

BACKGROUND: Wi-Fi client surge forcing fresh wireless LAN thinking

"In light of these results, we believe that SleepWell may be an effective solution for the future, not only to sustain a demanding suite of [mobile] applications, but also to improve ‘immunity’ to increasingly dense WiFi environments," the researchers conclude, in a paper "Avoiding the Rush Hours: WiFi Energy Management via Traffic Isolation," presented this week at the annual MobiSys Conference in Washington, D.C. It’s authored by the principal researcher, grad student Justin Manweiler, and Romit Roy Choudhury, Assistant Professor, Dept. of Electrical & Computer Engineering and. Dept. of Computer Science, at Duke, Durham, N.C. The complete paper is available online.

In effect, as the two researchers note, SleepWell introduces a kind of distributed time division multiple access (TDMA) technique to a Wi-Fi network. TDMA is a channel access method for "shared medium" networks, for example, cellular networks. Several users can share the same frequency channel by dividing the signal into different time slots that are used by different users.

Wi-Fi radios are a major drain on a device’s battery. It’s being made worse by a change in the way mobile devices are used. Instead of intermittent, bursty data traffic, users today often are streaming media and interacting with always-on social networking applications. 

Increasingly, Wi-Fi networking takes place in locations with numerous access points and wireless devices, creating a demanding radio environment that adds still more to the power drain. [See "Wi-Fi client surge forcing fresh wireless LAN thinking"] The SleepWell paper notes that earlier Wi-Fi energy optimization efforts have been designed as if the network consisted of a single access point.

"However, network contention among different APs can dramatically increase a client’s energy consumption. Each client may have to keep awake [and using power] for long durations before its own AP gets a chance to send packets to it. As the AP density increases in the vicinity, the waiting time inflates, resulting in a proportional decrease in battery life."

SleepWell is designed to sidestep this contention. It gives the AP the ability to regulate the "sleeping window" of their associated clients, and in effect coordinate these windows with those of other nearby APs. The result is that different APs are active or inactive during non-overlapping time windows. "The solution is analogous to the common wisdom of [commuters] going late to the office and coming back late, thereby avoiding the rush hours," the authors write.