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 ] 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."