Internetworkers: http://www.ibiblio.org/internetworkers/

[Mark Turner <markt AT siteseers.net>]

Alan MacHett wrote:

> This is purely a mental exercise for me.  I'm curious to know at what
> distance (measured by infrastructure, not lateral increments) a
> line-originated call is jumped to microwave or other "air" transmission.

Nowadays few, if any, calls go through the air. It's all fiber now. I 
think AT&T operates the last microwave towers. Or did, anyway. Now 
they're out of the long-distance business.

> Conversely, is there any point at which a cell-originated call switches to
> land lines (when not actually calling a land line number)?

Check this page out for a description:
http://www.telecomwriting.com/Cellbasics/Cellbasics.html

> And finally,
> depending on the distance of the call and its originating demographics
> (number of carriers in the area), how many different carriers'
> infrastructures are being traversed?

It's pretty much:

originating telecom -> ld-service (or VoIP) -> terminating telecom.

Every time a call is delivered to another telecom, money changes hands 
due to the way the tariffs work. This encourages efficiency, as you 
might imagine. :)

There are lots of good telecom resource pages on the Net. Here are a few 
of my favorites:

http://www.telecomwriting.com/index.html
http://www.dmine.com/phworld/
http://www.bellsystemmemorial.com/

--
Mark Turner
www.markturner.net