Some access points sidestep those problems by bumping Power Saving Mode traffic to the head of the queue, but that can degrade the quality of wireless signal for everyone else. “We witnessed reduced network capacity due to unnecessary retransmissions and unfairness to network traffic,” says Rozner. He and his colleagues developed an alternative way of handling Power Saving Mode traffic that slashes the energy use of mobile devices and maintains a fair playing field for all traffic.
Their system, dubbed NAPman, carefully enforces a first-come, first-served approach to all data, whether it’s from a device using Power Saving Mode or not. It also only wakes a phone to retrieve its data when its data it is at the front of the queue, preventing the phone from waiting around and burning energy. The system also tracks devices that go to sleep after a fixed time, to ensure they aren’t sent data while asleep.
NAPman also uses the ability of Wi-Fi access points to pose as virtual access points to assign different virtual connections to different clients. The result is that devices do not compete for traffic so directly, and the access point can carefully time the sending of its beacons to ensure that devices only wake when necessary.
“Not only could we provide 70 percent energy savings compared to the conventional implementation, but NAPman is fair to background traffic,” says Rozner. In a test that involved streaming a 128 kilobit-per-second radio stream to an HP iPAQ smart phone using a crowded hot spot, NAPman doubled the device’s battery life from 4.7 to 10 hours, although if the backlight was turned up high, the effect was slightly reduced.
“It seems that though systems today may be deploying power saving, they’re doing it wrong,” says Philip Levis, who works on networking at Stanford University, speaking after Rozner presented his work at the MobiSys conference in San Francisco last week. “But I wonder how specific this is to just devices today,” he adds.
Rozner acknowledges that as new devices are released, his fixes may become unnecessary. “But as new schemes are implemented, I think they will need to take into account some of our ideas,” he says. The team hasn’t entered discussions with any wireless router vendors. “But the potential for adaptation is there,” says Rozner.