PSTN: What Is It, and How Does It Work?

Today, the PSTN is the most broadly interconnected communications system in the world, and is likely to remain so for at least another decade or more. For voice, it has no equal. VoIP services like Skype have banked on this fact; their business model depends on a steady flow of PSTN interconnect charges. But the PSTN provides FAX, data, telex, video, and hundreds of other multimedia services as well. And for many decades, the PSTN has enjoyed a universal numbering scheme called E.164. When you see a number that begins with “+” and a country code, you are seeing an E.164 number. In most of the world, connectivity to the PSTN is considered as essential as electricity or running water. Even the Internet itself depends on the PSTN to deliver dedicated access circuits as well as dial-up.

In the early days following Bell’s invention, wired communications at its most advanced meant two (or more) devices sharing a single iron wire, whether you were using a telegraph or telephone. A grounded wire to earth completed the circuit running between phones, each with its own battery to generate the current necessary to transmit. It was noisy and lines couldn’t run very far, and it would be many decades before it could truly be called a global network, much less a national one.

To fully define today’s PSTN, we’ll need to focus on several areas in turn. First, the physical “cable plant” required for signal distribution, from twisted-pair copper and coaxial electric to the latest fiber-optic cabling. Second, its signal transmission models, combining analog and digital signal processing and transmission over electrical, optical, and radio interfaces. This directly affects the kinds of content it can carry. Third, the increasing sophistication of associated signaling (control) protocols and “intelligent network” design introduced with the Integrated Services Digital Network (ISDN). And finally, its associated operational and regulatory infrastructure on international, national, state, and local levels