Scientists from the University of California, San Diego have developed a way to speed up roaming from one wireless access point to another - by synchronising the clocks in Wi-Fi equipment.
At present, handoff between one access point and another can take up to 250ms, which is death to real-time applications such as voice. The delay is caused because a Wi-Fi device waits till the connection it has degrades, then scans for other access points, picks the best one and makes a connection.
SyncScan, from professor Stefan Savage and graduate student Ishwar Ramani, requires software installed on access points and clients. All the access points in an area synchronise their clocks, and synchronise the beacon signals they use to advertise their existence, which makes scanning for alternative signals more efficient. The client can then scan in the background, keep track of the signals available and change directly to a new access point at the drop of a hat.
Access points use defined time slots to advertise their presence, according to which channel they are using - so the client can look for an access point that is on a different channel to the one it is currently linked to.
"The scanning phase completely dominates the cost of handoff," say the authors in an IEEE paper [pdf], "usually contributing more than 90 percent of the overhead." By eliminating that cost, SyncScan could make handoffs much faster.
"By synchronising the announcement of beacon packets, a client can arrange to listen to other clients with very low overhead," say the authors. "As a result, handoff using this SyncScan approach is an order of magnitude faster than using the conventional approach."
The researchers tested the algorithm with a laptop running Skype, while walking between two access points on the UCSD campus. “Using SyncScan with a measurement interval of 500 millseconds, handoff delay was virtually imperceptible - roughly five milliseconds," said Savage. "Repeating the tests without SyncScan, the average handoff time was 450 milliseconds, but ranging up to a full second in some cases." SyncScan also cut the number of lost packets to zero.
The software can be depoyed incrementally - someone running a SyncScan-enabled client will roam as normal to non-SyncScan access points, but roam faster where SyncScan has been installed. No hardware upgrades are needed. The software - for which the pair have a patent pending - can be downloaded here.
Just over 110,000 voice-over-Wi-Fi handsets were sold in 2004, mostly in Japan. Vonage is set to roll it out commercially in the US later this spring as an add-on to its popular VoIP service, and sales of dual-use phones incorporating both cellular and Wi-Fi connectivity could reach $3 billion by 2009, according to a study by Infonetics Research.
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