UCSD Jacobs School of Engineering University of California San Diego
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Roaming with Wi-Fi

Road warriors, take note: you may no longer have to stay in one place while surfing the Web over a broadband 802.11 Wi-Fi network.Wi-Fi's narrow range - under 100 meters - and the length of time it takes to hand off a signal from one access point to the next have deterred its use by consumers on the run.

Computer science professor Stefan Savage (left) and grad student Ishwar Ramani test their Wi-Fi handoff algorithm.

Now computer science and engineering professor Stefan Savage and graduate student Ishwar Ramani have filed initial patent papers on key elements of a process called SyncScan that achieves practical, fast handoff for 802.11 networks.

"The SyncScan algorithm can cut the handoff time by a factor of a hundred over existing solutions," says Savage."It also allows mobile devices to make better handoff decisions and therefore improve signal quality overall."

At present,Wi-Fi devices do not begin scanning for a stronger signal nearby until the link to the current access point weakens substantially and packets of data are lost.When that happens, the device broadcasts an S.O.S. on all channels to find nearby access points. The resulting delay of up to one second can be unacceptable if the user is trying to listen to Internet radio,watch streaming video clips, or talk on the phone."With voice over Wi-Fi," notes Savage,"the interruptions can disrupt the illusion of continuous connectivity."

The solution proposed by Savage and Ramani is simple and effective. Instead of scanning for surrounding access points just when the current signal is running low, SyncScan continuously monitors the proximity of nearby 802.11 access points but only for very brief periods, timed to coincide precisely with regularly scheduled beacon messages transmitted by the Wi-Fi access points.

The process replaces the long scanning delay with many small delays that are imperceptible to the user. Notes Ramani:"Increasingly, users want to be able to make voice over IP [VoIP] calls wirelessly, and a one-second disruption can seem like an eternity."

Ramani tested the SyncScan algorithm on a laptop running a popular VoIP application called Skype. He carried the laptop between areas of the Applied Physics & Mathematics building served by neighboring Wi-Fi access points.

Using SyncScan the handoff delay was virtually imperceptible - roughly five milliseconds. Repeating the tests without SyncScan, the average handoff time was 450 milliseconds, but ranging up to a full second in some cases.

The researchers also observed a big difference in the number of lost data packets. Zero packets were dropped using the SyncScan algorithm in the UCSD tests, compared to substantial packet losses using current technology."We expect that the same improvements can be achieved on most Wi-Fi devices and using most applications, not just voice," adds Ramani.

SyncScan is also economical because it can be deployed incrementally and implemented in software without requiring any hardware upgrades or changes to the 802.11 standard.