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

Wi-Fi radios are a major drain on a device’s battery

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 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.

The IEEE 802.11 WLAN standard, more popularly known as Wi-Fi, actually has an optional protocol for energy efficiency, Power Save Mode or PSM. It can turn off the radio when it doesn’t have to be used. The AP queues packets for the client, which partially wakes up periodically to listen for alerts from the AP, which is collecting and queuing that client’s packets. If it detects an alert, the entire client radio wakes up, an action that draws a lot of power, and collects the packets as a group rather than one at a time, a more efficient use of the radio channel.

The Google Nexus One uses this effectively, the researchers note: "In current Nexus One phones running Android, the WiFi PSM mode wakes up in the order of 300ms to download bursts of packets. This is a judicious design decision, with proven energy benefits."

But there are drawbacks, noted by other researchers."[D]epending on the PSM implementation strategies used by the clients/Access Points (APs), [then] the presence of competing background traffic results in one or more of the following negative consequences: a significant increase, up to 300%, in a client’s energy consumption, a decrease in wireless network capacity due to unnecessary retransmissions, and unfairness.

This has led to development of improved power management via Network-Assisted Power Management or NAPman. "NAPman leverages AP virtualization and a new energy-aware fair scheduling algorithm to minimize client energy consumption and unnecessary retransmissions, while ensuring fairness among competing traffic." [You can find the 2009 paper "NAPman: Network-Assisted Power Management for WiFi Devices" online]

But, say the SleepWell authors, NAPman is most effective "where an isolated AP is connected to multiple clients." But that’s rarely the case in real-world networking. "In reality, multiple APs are within the wireless vicinity, and this strongly impacts the energy consumption of individual clients," the authors write. "Specifically, when a PSM client wakes up to download its own burst of packets, it has to share the channel with all other clients of all other APs in the vicinity. In homes or dense office areas, it is not unusual to overhear 5 to 10 other APs. Since the APs are likely to share the channel fairly between them, it is possible that a client remains awake almost 5 times longer than it would if there was no contention with other APs. Thus, the energy wastage during network activity can be 5 times, and even more if other APs have multiple clients associated to them."

The SleepWell protocol is designed to sidestep this contention.

Always-on APs monitor the ongoing wireless traffic of nearby APs. In a SleepWell network, each APs track the periodic bursts of traffic from surrounding access points. With that information, each one can dynamically re-schedule its own traffic burst to coincide with an open period, creating minimal overlap with other bursts.

"Reduced overlap reduces competition, allowing each client to download its own packets uninterrupted, and [then] sleep when the channel is occupied by other transmissions. This bears resemblance to a distributed TDMA scheme, but executed with energy-efficiency in mind," according to the paper.

It does so neatly, without requiring additional intelligence on the client, or by breaking 802.11 compatibility: "By carefully modifying the timestamps (as a part of the WiFi clock synchronization process), the SleepWell AP regulates the client’s sleep and wake-up schedules. The client remains unaware of the changes in its own duty cycle; neither does it get disassociated. 802.11a/g/n standard-compatibility remains intact."

John Cox covers wireless networking and mobile computing for "Network World."



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Tags Wi-FiDuke Wi-Fi researchNetworkingPandorawirelessDuke UniversityWLANs / Wi-Fiyoutube

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