Image: Paul Torrens

WIRELESS GEOGRAPHY: All of downtown Salt Lake City is covered by a cloud of Wi-Fi, but the strength of the signal varies.

28 February 2008—U.S. cities such as Tempe, Ariz., and Philadelphia have struggled to set up municipal Wi-Fi systems, but some researchers believe a city could make do by piggybacking on the many residential and business hotspots already dotting most cities. Among the many difficulties in making such a scheme work, one of the main ones is determining just how large and dense the existing Wi-Fi clouds are.

Paul Torrens, an Arizona State University geographer recently completed the first maps of Wi-Fi coverage on a citywide scale. “I was using Wi-Fi myself and noticing the phenomenon unfolding before my eyes,” he says. “I thought there must be a geography to it.”

His maps, which will appear in the Annals of the Association of American Geographers, show that private businesses and homes already envelop Salt Lake City in a dense Wi-Fi cloud. Such unplanned wireless networks could provide the foundation for integrated citywide wireless coverage without huge investments in new infrastructure. More than 175 U.S. cities have already tried to build citywide or partial systems, but few have actually succeeded in providing the coverage they promised.

Torrens convinced a small team of graduate students and family members to walk, bike, and drive around Salt Lake City while carrying wireless antennas, to collect signals from the airwaves. They looked for “beacon frames,” a type of digital handshake that computers and other wireless access points broadcast to advertise their presence on a network.

With the aid of GPS, Torrens mapped 1739 unique access points among 500 000 data samples. His analysis shows access points clustering around dense areas of houses and offices, as well as around the dorms on the University of Utah campus.

The range of each access point was largely determined by urban geography; Torrens found that one well-located antenna provided the equivalent of a Wi-Fi hotspot over most of downtown Salt Lake City’s 5 square kilometers. Despite congestion caused by overlapping coverage, Torrens found that the majority of access points supported data rates between 11 and 54 megabits per second.

While turning a sprawling, unplanned network into a municipal system might be cheaper than building a dedicated network, it’s not without its challenges. Such a system would require home and business networks to grant access to guest users while preventing such users from stealing private data or downloading illegal content under a borrowed Internet Protocol (IP) address.

One possible solution, developed by computer scientists at the University of Cambridge and MIT, are “tunnels.” Guest users entering a public Internet gateway would get routed through the host’s network to their own home IP addresses and then back. The extra signal runaround would cause some slowdown for guest users, but it would ensure that they sign onto the network as themselves.

“You want traceability, so if things do go badly, there’s accountability,” says Jon Crowcroft, a Cambridge computer scientist. He developed the idea of tunnels with graduate students at Cambridge and MIT, setting up his own shared network with neighbors.

Torrens and Crowcroft both remain skeptical about cities tapping their unplanned wireless anytime soon. Cities would need to hash out a deal with each private home or business, not to mention install the necessary hardware to ensure secured sharing. Internet service providers (ISPs) also frown on open Wi-Fi networks that allow people to get “free” access and often prohibit such sharing in their customer contracts.

“In the UK, we’ve had stories of ISPs blocking open Wi-Fi access,” Crowcroft says. He also points to Verizon, an American broadband and telecommunications company that lobbied against the city of Philadelphia’s plans to deploy its own low-cost wireless broadband network that would have allowed customers to buy access from a number of Verizon competitors.