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Wireless Lookout
Fast handoff for Wi-Fi networks
Context: People routinely access the Internet via the
tens of thousands of Wi-Fi access points dotting
airports, university campuses, cafés, and other public
places. But a Wi-Fi device can connect to an access
point only if it is close by—usually within 100 meters.
When a device moves beyond the signal range of one
access point, it is “handed off” to a nearer one, a
process that disrupts data flow. For someone making a
phone call over a Wi-Fi phone or watching live streaming
multimedia, a one-second delay during handoff can be
highly irritating. Ishwar Ramani and Stefan Savage of
the University of California, San Diego, have developed
a new approach, called SyncScan, that allows faster
handoffs.
Methods and Results: Right now, a Wi-Fi device
searches for a new access point only after the signal
quality from the one it’s using degrades markedly. Then,
the device scans all available wireless channels for
beacons broadcast by access points, leaving little
bandwidth for other incoming data.
With SyncScan, a Wi-Fi device regularly records the
signal strengths of other channels, but it checks them
only at the precise times that they are scheduled to
transmit beacons. Such timing avoids needless channel
switching, so the device receives more of the data being
sent to it. It also makes better-timed and better-placed
handoffs. In a prototype—a laptop computer running the
popular Internet-telephony program Skype—the delay
during handoff was reduced a hundredfold to only a few
milliseconds. The algorithm is expected to work for all
Wi-Fi devices.
Why it Matters: Internet telephony and streaming
multimedia are emerging as hot applications in Wi-Fi
networks. Wi-Fi phones already exist in Japan and are
expected in the United States by this spring, but long
handoff delays will discourage their adoption. SyncScan
shrinks the handoff delay without the need for hardware
upgrades or changes to IEEE 802.11, the most widely
deployed standard for wireless networks. Though SyncScan
is still not perfectly synchronized, it promises to
greatly improve the quality, convenience, and value of
communication in Wi-Fi networks.
Source: Ramani, I.,
and S. Savage. 2005. SyncScan: practical fast handoff
for 802.11 infrastructure networks. Proceedings of IEEE
Infocom 2005 (in press). |