Once Philadelphia gave EarthLink permission to move ahead with the citywide rollout, the ISP wasted no time. EarthLink had leased and built out towers so it could deploy nodes quickly once the proof of concept was approved.

Crews were dispatched to accelerate the installation of access points on light poles. The Philadelphia streets department inspects light poles for the access points' use and approves the selection of poles, and then EarthLink deploys the Tropos Networks units and goes through an optimization process, and the city does a final inspection for safety and mounting.

Within 15 days of getting the green light EarthLink had doubled network coverage to 30 square miles. Today, 80% of the city is covered, and EarthLink plans to have all 135 square miles blanketed with wireless signal by the end of September. Additional network optimization has to be completed, and EarthLink expects to get the city's official acceptance by year-end.

EarthLink is building a layered network that starts with the Wi-Fi mesh at the street level that the company says will deliver 1Mbps to 3Mbps data rates to customers. The Tropos 802.11b/g access points contain built-in routers and use point-to-multipoint radios. That creates the mesh, says Jeb Linton, director and chief architect for EarthLink. The Tropos units dynamically select the best wireless channel to avoid congestion and interference.

At the backhaul or capacity injection layer, data is fed into the mesh at rates of 20Mbps to 50Mbps by Motorola Canopy radios and Alvarion VL radios on the tops of buildings and towers. In some areas of this first layer of the backhaul system, EarthLink is running fiber.

The tower tops are connected by either fiber optics or line-of-sight microwave communication at 200Mbps-to-1Gbps data transfer rates.

Click to see: Philadelphia Wi-Fi map

EarthLink already has a major point of presence (POP) in Philadelphia, which connects to the ISP's national backbone network. The local POP contains the service gateway, which manages every user session and all details of the user experience.