Ismail's Corner

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 fl ow. 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 handoff s. 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).