Wi-Fi Nets Come to Vehicles
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.
What the team also noticed is that in many cases, a Wi-Fi client is actually in range of two or more base stations at any given moment, so the packets are "overheard" by all of them, even though only one is chosen to associate with the client. "What if the client could talk with multiple base stations?" Balasubramanian asks. "Then you're not relying on just one. We want to leverage that for more robust connectivity."
Coordinated Packet Forwarding
Here's how they did it. A cluster of base stations overhear a client packet, and listen for an acknowledgement (ACK) from one member of the cluster (both packets and ACKs are sent as broadcasts). "If they hear the ACK [from one base station], they don't have to act," Balasubramanian says. If they don't hear the ACK, all these other base stations make use of our Vi-Fi algorithm to determine which one of them should forward the packet."
The algorithm use a "probabilistic" approach, essentially figuring out at any given moment what is the probability for each of the overhearing base stations to be able to forward the packet. The base station with the best probability gets the forwarding assignment, whether upstream to a media server or downstream to the vehicle-based client. Vi-Fi isn't duplicating packets: The packet is forwarded only once, by the base station with the best chance of doing so.
Wireless mesh protocols in multi-hop networks have something like this already. To minimize the overhead associated with this kind of opportunistic packet forwarding, however, they group or batch packets together. "We wanted a system that avoids this grouping because it introduces delay," Mahajan says. Delay would not be a problem for file transfers, but could be damaging or fatal for video feeds, interactive camera movements and the like, Balasubramanian says.
Putting Vi-Fi to the Test
The research team tested the Vi-Fi protocol at two locations: VanLAN, at Microsoft's Redmond, Wash., headquarters complex; and DieselNet, at the UMass-Amherst campus. In both cases, computers with, or linked to, Wi-Fi radios were mounted in vans or buses making periodic circuits past Wi-Fi base stations. The team compared the Vi-Fi results with those from a conventional Wi-Fi handoff mechanism.
"Performance improved by a factor of roughly 2 [with Vi-Fi]," Mahajan says. "Voice calls were clearer on average, and the number of disruptions dropped [also] by a factor of 2." Another way to look at the results is that disruption-free time doubled.
Web browsing showed similar, noticeable improvements in terms of faster download times and a greater number of successful download completions. "Pages loaded faster, and there were fewer aborted transfers," Mahajan says.
Vi-Fi is software code that sits above the 802.11 interface. No hardware or firmware had to be changed. "Any [Wi-Fi] vendor today could pick this up and implement it just as we did," Mahajan says.
The tests also suggest that Vi-Fi may be applicable outside vehicle-based wireless networks. "We found it can be used for static or nomadic [Wi-Fi] settings as well," Balasubramanian says. "Gray periods are part of these networks as well," she says. Testing that discovery more rigorously is part of the team's future work, Mahajan says.
"The vision is that Wi-Fi is everywhere," Mahajan says. "If it's already here, and a completely novel class of applications can be supported on it [using Vi-Fi], that's striking. If Wi-Fi is widespread [enough], you wouldn't need cellular plans."