-=> Quoting Pascal Schmidt to Leonard Erickson <=-

 LE>> Alas, one reason for the different networks is the fact to to
 LE>> support the density of phones found in major cities without
 LE>> needeing a cell site every block, you need to use different
 LE>> frequency bands and different protocols.

 PS> Or, as in GSM, the network can be designed to allow sharing.

*Any* cell phone network "allows sharing" or it wouldn't work. But each
phone needs channel to the chell tower. And there are only so many
channels *possible* in a given band.
 
 JB>> how does multiple incompatible networks lead to fewer cell sites?
 LE> Fewer sites *per network*.
 PS> In GSM networks, there are always two cells using different
 PS> frequencies between two that use the same. This is by design. GSM
 PS> operates on two distinct frequency bands, each subdivided to allow two
 PS> different providers. That means four providers can use a single cell
 PS> site, and this often happens in Germany. 

Doesn't change the problem I was talking about. Namely, that for any
"type" of network, there is a *small*, *finite* number of channels
available. 

So as the number of phones *simultaneously* in use in an area goes up,
you have to go to smaller "service areas" for each cell tower so as to
have enough channels available for the traffic. 

*Or* you have to have more than one "type" (frequency band or
modulation) of cellular service hosted at the sites, with (hopefully) a
decent mix of phones able to use one or the other or both.


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