AT&T/Cingular; These are pictures of a powerpoint that explains their cellular network and how it is setup. Everything is covered from the first GSM phones with Internet access (GPRS) clear up to EDGE, 3G, 4G, DoCoMo, UMTS, etc... 

Also explains how call are routed internally between 2 att/cingular customers and outside callers as well. Very informative. 

This includes Cell One / Cingular / AT&T. Which are all owned by AT&T. This covers all the technical aspects of their network, all the nodes and devices and how calls are approved through their system.