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MIT Researchers Tackle Challenge of Dropped Calls

Researchers at MIT have developed a new set of protocols to define how a handoff should happen during wireless communication. The findings could eventually help improve cell phone and WiFi performance for users on the move.

In the research paper, "Improving Wireless network Performance Using Sensor Hints," members of the Computer Science and Artificial Intelligence Laboratory described the challenge of mobile devices: They rely on network protocols that have to deal with both static and mobile usage in a short period, which degrades performance in both scenarios. The team put forth the idea that the protocols used in wireless networks can adapt to a given context--stationary or on the move--by taking advantage of the "external sensor hints" provided by a device's GPS receiver, accelerometer, and gyroscope. The outcome: fewer calls dropped and fewer Internet connections disappearing.

Graduate student Lenin Ravindranath, professor Hari Balakrishnan, associate professor Sam Madden, and postdoctoral associate Calvin Newport presented their findings at a Usenix Symposium on Networked Systems Design and Implementation in Boston last month. The team discovered that their protocols could often improve device network throughput by about half as much.

The paper offered improvements on four specific communication 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 said. "What happens today is that your phone immediately connects to the WiFi 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 said, the new protocol selects an access point on the basis of the user's inferred direction. "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."

In experiments a moving cell phone switched transmitters 40 percent less frequently than it would with existing protocols. A variation of the protocol improved throughput by about 30 percent.

Another protocol addressed the phone's selection of bit rate--the rate at which it optimally sends and receives information. When a device is in motion, the available bandwidth is constantly fluctuating, which makes the selection of the optimal rate more challenging. Because a device using the MIT protocol knows when it's in motion, it also knows when to be more careful in choosing a bit rate. In the researchers' experiments, the gains in throughput from bit rate selection varied between 20 percent and 70 percent but consistently hovered around 50 percent.

A third protocol governs the functionality of the wireless base stations delivering the connectivity. Ordinarily, a base station knows that a device has broken contact only after a period of silence. During that period, the base station will continue sending the same data, waiting for an acknowledgement. But with information about the device's trajectory, the base station can make an "educated guess" about when it will lose contact and therefore will cease its communication with that device at a more appropriate moment.

The fourth protocol uses motion data to determine routing procedures for networks of wirelessly connected smart cars, whose relative positions are constantly changing.

The team said they've identified additional protocols that could stand for some improvement, but apparently they'll leave those for others to uncover. "What we are really hoping is that this opens up a really exciting direction for work in the community," he explained. "Other people will come up with more creative ideas, now that you know that you can get these sensor hints in a fairly robust way."

About the Author

Dian Schaffhauser is a writer who covers technology and business for a number of publications. Contact her at dian@dischaffhauser.com.

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