Constant Connection

For most of the 20th century, the paradigm of wireless communication was a radio station with a single high-power transmitter. As long as you were within 20 miles or so of the transmitter, you could pick up the station.

With the advent of cell phones, however, and even more so with Wi-Fi, the paradigm became a large number of scattered transmitters with limited range. When a user moves out of one transmitter’s range and into another’s, the network has to perform a “handoff.” And as anyone who’s lost a cell-phone call in a moving car or lost a Wi-Fi connection while walking to the bus stop can attest, handoffs don’t always happen as they should.

Most new phones, however, have built-in motion sensors — GPS receivers, accelerometers and, increasingly, gyros. At the Eighth Usenix Symposium on Networked Systems Design and Implementation, which took place in Boston in March, MIT researchers presented a set of new communications protocols that use information about a portable device’s movement to improve handoffs. In experiments on MIT’s campus-wide Wi-Fi network, the researchers discovered that their protocols could often, for users moving around, improve network throughput (the amount of information that devices could send and receive in a given period) by about 50 percent.

The MIT researchers — graduate student Lenin Ravindranath, Professor Hari Balakrishnan, Associate Professor Sam Madden, and postdoctoral associate Calvin Newport, all of the Computer Science and Artificial Intelligence Laboratory — used motion detection to improve four distinct communications protocols. One governs the smart phone’s selection of the nearest transmitter. “Let’s say you get off at the train station and start walking toward your office,” Balakrishnan says. “What happens today is that your phone immediately connects to the Wi-Fi access point with the strongest signal. But by the time it’s finished doing that, you’ve walked on, so the best access point has changed. And that keeps happening.”

By contrast, Balakrishnan explains, the new protocol selects an access point on the basis of the user’s inferred trajectory. “We connect you off the bat to an access point that has this trade-off between how long you’re likely to be connected to it and the throughput you’re going to get,” he says. In their experiments, the MIT researchers found that, with one version of their protocol, a moving cell phone would have to switch transmitters 40 percent less frequently than it would with existing protocols. A variation of the protocol improved throughput by about 30 percent.