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Plan rolling to improve vehicle-based Wi-Fi nets

By , Network World
September 04, 2008 04:38 PM ET

Network World - You could think of this as the Tarzan protocol for Wi-Fi. The goal is to improve interactive Wi-Fi connections dramatically for moving vehicles.

Dubbed "Vi-Fi," the protocol lets Wi-Fi clients keep in touch with several access points at once. In a sense, Vi-Fi lets overlapping access points coordinate with the moving client, minimizing the disruptions that can zap interactive applications. The tests, published in a recent technical paper, showed that Vi-Fi doubles the number of successful short TCP transfers and doubles the length of disruption-free VoIP sessions compared to an existing, more fragile Wi-Fi handoff protocol.

One member of the investigating team, Ratul Mahajan of Microsoft Research, uses the analogy of Tarzan swinging through the jungle on a vine attached to one branch. If either breaks, Tarzan's forward motion is abruptly interrupted. If multiple vines and branches are readily at hand, however, he easily can keep moving forward. Vi-Fi in effect provides those additional resources.

The results of Vi-Fi tests in two location are the subject of "Interactive WiFi Connectivity for Moving Vehicles," a paper presented August 21 at the Association for Computing Machinery's Special Interest Group on Data Communications(SIGCOMM) in Seattle. (Todd Bishop of the Seattle Post-Intelligencer wrote about the project in his "Software Notebook" column.) The co-authors are Aruna Balasubramanian, Arun Venkataramani and Brian Neil Levine, all of the University of Massachusetts at Amherst; Microsoft's Mahajan; and John Zahorjan, from the University of Washington. Balasubramanian and Mahajan talked with Network World about the research.

Today, linking people and computers in moving vehicles with a network requires relatively slow and expensive satellite or cellular data links, Mahajan says. The demand is growing, however, for instant, continuous, interactive access for Web browsing, voice, multimedia and similar applications. The researchers wanted to explore the feasibility of using the ever-growing number of Wi-Fi access points as a low-cost alternative.

The original vision for 802.11 wireless LANs, and the basis for its protocol design, was for stationary nodes relatively close together. "But it's evolved: It's very flexible and we've been pushing it further ever since," Mahajan says

The big problem: gray periods

Sustaining a vehicle-based Wi-Fi network for interactive applications runs into a big problem. "What happens today is that any Wi-Fi client talks to only one base station [access point] at a time," Mahajan says. If that interaction falters or fails, the vehicle-based client either suffers poor performance or is marooned completely until it can connect to a new base station.

"The connection can degrade or drop unexpectedly, and unpredictably. Users see a drop in application performance or a complete loss of connection," Mahajan says. The researchers dubbed these interruptions "gray periods."

The researchers found frequent disruptions even when moving vehicles' radios were close to Wi-Fi base stations. There are numerous sources for these disruptions, Balasubramanian notes: tall buildings, topography, weather and many more factors can "all impact the signal," she says.

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