21.   The company is providing various software, as well as transmission equipment and some bigger optic lines that are increasingly connected to cell sites.

22.   The frequencies in a given cell site become crowded, increasing interference and cross-talk as cellphones inadvertently intercept signals on nearby frequencies.

23.   The phone companies will keep their cell sites on the towers, but will lease space back from the tower company.

24.   The switches and cell sites are linked by Sprint phone lines to the network center.

25.   The success sparked interest among other groups interested in controlling the number of cell sites in residential areas.

26.   They set up a cell site there, then another on a higher building, then another even higher.

27.   Those new cell sites are badly needed.

28.   To compensate, carriers were forced to reuse the same limited radio spectrum more often by adding more cell sites covering smaller areas.

29.   To be sure, the wireless carriers have heard the complaints of cell phone users and escalated the construction of cell sites in response to demand.

30.   Today, the number of cell sites in the United States is one of the most visible signs of that growth.