The city of Philadelphia has awarded EarthLink a high-profile contract to build a Wi-Fi network stretching over 135 square miles, marking the formal start of the largest municipal effort in the United States to offer wireless Net access. Dianah Neff, Philadelphia’s chief information officer, said the Atlanta-based Internet provider has won the contract to place Wi-Fi access points on telephone poles throughout the city, beating out a competing proposal from Hewlett-Packard. Most city residents will pay $20 a month for access.