US researchers turbocharge Wi-Fi roaming

The time it takes roam between Wi-Fi wireless networks can be dramatically shortened thanks to software developed by two computer scientists at the University of California, San Diego.

Professor Stefan Savage, of the Jacobs School of Engineering, and graduate student Ishwar Ramani have a patent pending on the basic invention behind a technology known as SyncScan.

They believe this can overcome a major obstacle in Wi-Fi roaming by allowing fast handoff for 802.11 infrastructure networks. Their study will be published in the Proceedings of the IEEE InfoCom 2005.

"Wi-Fi offers tremendous speeds if you stay in one place or at least within 100 metres of the same access point," said Savage.

"SyncScan is a handoff algorithm which can cut the time it takes to switch from one Wi-Fi access point to another by a factor of 100 over existing solutions.

"This is a requirement for demanding applications like voice over Wi-Fi, where even short interruptions can disrupt the illusion of continuous connectivity."

Savage claimed that SyncScan also allows mobile devices to make better handoff decisions and therefore improve overall signal quality.

According to the researchers, Wi-Fi handoffs are cumbersome and time-consuming. Not until the access point signal weakens substantially and begins losing packets of data does a Wi-Fi enabled laptop, PDA or mobile phone begin scanning for a stronger signal.

At this point, it broadcasts requests on all channels to find nearby access points. The result is a delay of up to one second, during which packets may be lost.

Although Savage noted that such a delay may not seem inordinate when downloading data, it can be unacceptable if the user is trying to listen to internet radio, watch a streaming movie trailer or talk on a Wi-Fi phone.

"Today most Wi-Fi users accept being tethered to a single location in exchange for the broadband speeds that Wi-Fi offers," said Ramani.

"But increasingly they want to be able to make voice over IP phone calls or stream multimedia while on the move, and a one-second disruption can seem like an eternity."

The SyncScan technology is a way to continuously monitor the proximity of nearby 802.11 access points.

Instead of looking for surrounding access points just when the current signal is running low, a Wi-Fi device with SyncScan regularly checks signal strengths nearby, but only for very short periods of time.

These times are picked to coincide precisely with regularly scheduled 'beacon' messages sent by all standard Wi-Fi access points.

The process eliminates the current need to start from scratch when looking for a stronger signal, and replaces the long scanning delay with many small delays that are imperceptible to the user.

To test the SyncScan algorithm, the researchers used a laptop running a voice application while walking between two areas of the UCSD campus served by neighbouring Wi-Fi access points.

"We used a popular VoIP service called Skype which uses user datagram protocol packets exchanged between two clients for voice communication," explained Savage.

"Using SyncScan with a measurement interval of 500 milliseconds, handoff delay was virtually imperceptible at 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."

Just over 110,000 voice over Wi-Fi handsets were sold in 2004, mostly in Japan, and sales of dual-use phones incorporating mobile and voice over Wi-Fi could reach $3bn by 2009, according to a recent study by Infonetics Research.

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