Section 1.1. The PSTN

1.1. The PSTN

In 2004, there were close to 200 million traditional, landline telephones in use in the United States. ^[*] The network that connects all these phones together wraps around the globe. The job of the PSTN (Public Switched Telephone Network) is to reliably facilitate telephone conversations at any time of day, year-round. The PSTN combines analog, digital, and electromechanical data links that strive to make sure that every time you pick up your phone receiver, you hear a dial-tone, and every time you're hungry, you can reliably dial up the local pizza parlor.

^[*] International Telecommunications Union (ITU) statistics provided by the World Bank.

1.1.1. Mesh Versus Switched

When designing a network that connects many phones (called endpoints in the world of telephony), there are two approaches: mesh networks and switched networks. In a mesh network design, every endpoint has a permanent connection to every other endpoint, so all can communicate with one another. The first experimental networks developed by telephone pioneers worked this way.

Meshes are not very practical, because once you add more than a few endpoints, the number of permanent links between endpoints becomes absurd. For example, in a mesh with 10 endpoints, 100 separate links have to be maintained. In a mesh with 100 endpoints, 10,000 links have to be maintained . It doesn't make much sense to maintain so many network links, because there's a better way: the switched network.

In switched networks, links between endpoints don't need to be permanent because they aren't needed constantly. The only time a link between two endpoints is needed is when a call is in progress between them; the rest of the time, the link is idle, unused. Switching is a method whereby links are established and removed as needed, eliminating the need for a mesh. The PSTN is a switched network.

The PSTN carries each phone call by setting up and tearing down a temporary link, usually an electrical circuit, between a caller and a callee. The links that carry the calls may be comprised of copper wires, fiber optics, or radio systems, depending on the network infrastructure that exists between the caller and callee.

While the phone connections used in most homes are traditionally analog, electromechanical ones, connections between large telephone switchesfor instance, calls between subscribers of competing phone companiestend to be digital.

1.1.2. Signaling System 7

SS7 is the PSTN's signaling component: a second network that runs alongside the PSTN, whose purpose is coordinating communication between switches, telephone company databases and billing systems, and other parts of the public voice network. Toll-free calling and long distance call routing are both functions of SS7. While SS7 is secondary to the main function of the PSTN (setting up and tearing down links on the switched network), the modern-day PSTN couldn't function correctly without it.

1.1.3. Plain Old Telephone Service

When you pick up a traditional telephone, like the one in your house, the dial-tone you hear is a signal transmitted over the simplest kind of voice circuit: an analog phone line, also called a POTS (plain old telephone service) line. The line is a simple electrical loop. On the other end of the line is the telephone company's switch, or exchange. It is this switch that gives your phone its electrical power, sends the sound signals emitted by its audio transducer, and knows how to handle the numbers you dial on its keypad.

The switch also knows how to provide POTS calling features like call-waiting, a service that allows your call in progress to be placed on hold while you answer another caller who is trying to call your linewhen you tap the receiver's hook, the switch recognizes the electromagnetic signal you created (called a flash ), connects your phone line to the call that's waiting on another line on the switch, and places your first caller on hold. Later, when you send the flash signal again, the first caller is re-connected with your line and you can resume the original conversation. On the PSTN, calling features are always provided by the telephone company switchnot by the phone itself.

Despite historically increasing demand for features and gains in efficiency, POTS has been the most popular type of service provided by phone companies for the last several decades. Its core of a copper loop with an electromagnetic telephone receiver on one end and a central office switch on the other end hasn't changed much during that time. POTS' stalwart reputation and simplicity have been key reasons for its longevity.

So why not keep that copper telephone loop forever? POTS works great in low-density scenarios; that is, places with a small number of simultaneous callers , such as a one-family home. POTS can also excel where extensive calling features aren't important or where the telephone is rarely used. But when density and features become more critical, POTS isn't always the best solution.

Many phone companies offer an enhanced type of analog phone service known as Centrex to permit more calling features, such as call transfers and call-forwarding . Since Centrex and POTS lines are owned (and billed) by the phone company, they can create high reliance upon the phone company. This can be cost-prohibitive and inconvenient. To solve this problem, organizations on the grow usually choose to bring their telephony facilities in-house. By facilitating its own calling features, the enterprise can make telephony less costly and even leverage it as a productivity enhancer .