11.   Digital phones, in particular, are likely to fade as the distance from a cell site increases because they operate at higher frequencies.

12.   In addition, some calls reach only one cell site and thus cannot be located.

13.   Nokia, Ericsson and the other equipment companies are preparing their factories to produce millions of phones and tens of thousands of cell sites.

14.   One method is to locate calls by measuring the time it takes the signal to reach three nearby cell sites.

15.   Others are striking roof-top management deals, luring owners with the promise that every multistory building will become a revenue-producing cell site.

16.   Several have asked cell phone companies to install cell sites on top of light poles or on buildings where they are less noticeable.

17.   SnapTrack is actually a hybrid system, requiring special circuits at the cell sites and inside the phones.

18.   Sometimes, however, it is not signal strength that is the problem, but the lack of digital cell sites.

19.   Such a system would work well in rural areas where callers might reach only a single cell site.

20.   That meant companies considering geographic expansion could get the same coverage with far fewer cell sites, Reudink said.