Philadelphia is well on its way to becoming one of the world's biggest Wi-Fi hot spots

In May, after a 15-square-mile test zone passed muster, the city gave EarthLink the green light to cover the entire 135-square-mile city with a wireless mesh network by year-end.

EarthLink is moving full-speed ahead, adding Tropos Networks access points to light poles around the city, testing and optimizing the network, and building out coverage at a pace of 5,000 potential households per workday. Today, coverage has expanded to 80% of the city.

From a technology perspective, creating a full-blown mesh network across an entire city -- from parks to tourist attractions to downtown skyscrapers to residential neighborhoods with brownstones jammed together -- presents quite a challenge.

"It was important for people to be mobile and treat the entire city network as one large, unified network, so if they attached in one place they could start surfing the Web and keep the connection even if they went to another point across town," says Jeb Linton, director and chief architect for EarthLink.

"No system in the world had ever been able to scale to that level of mobility in a Wi-Fi network," Linton adds. "So we had a unique architectural challenge to enable this."

EarthLink is building out the network using Tropos 802.11b/g access points, which connect to a complex backhaul system that uses Motorola Canopy line-of-sight radios and Alvarion BreezeAccess VL non-line-of-sight radios.

As with any project of this scale, problems have emerged and original planning assumptions have had to be adjusted. For example, EarthLink has had to double the number of access points per square mile, from an original estimate of 20 to the current figure of 42, to provide the requisite level of coverage. In addition, after the access points are installed, there's a four- to six-week optimization process designed to maximize coverage.

"We are using our tools to drive testing, and we use customer feedback to improve coverage. We don't think we'll ever get to 100% coverage, but we are very happy with 90% until technology, both on our side and on the user's side improves," says Donald Berryman, president of EarthLink Municipal Networks.