First, there was the Integrated Services Digital Network (ISDN), and then came portable 800 numbers. Soon, cellular providers were talking of tying their networks together so that subscribers could roam from one cellular network to another without special roaming numbers. And now we are talking over the Internet using our computers as telephones.

During all these discussions, however, nothing (or at least very little) has been mentioned about how all this is possible. How is it possible that many networks can communicate with one another, and which control mechanism is used to maintain the connections and data communications?

Behind the scenes is a quiet, highly reliable, fault-tolerant data communications network that links the world s telephone networks together and allows them to share vital signaling and control information. This one network, which controls telecommunications networks around the world, is being prepared to control the broadband networks that will form the framework for tomorrow s communications infrastructure.

This data communications network is so robust and so sophisticated that few really know much about it. Only recently has anything been written about this network, and what information does exist has been sparse and ambiguous. The very standards that define the network are written to answer the needs of so many different audiences that they remain vague and reliant on reader interpretation.

This network, often dubbed the world s largest data communications network, is known as Signaling System 7 (SS7). It started as a way to access 800 databases here in the United States,...