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Let’s take an example from the U.S. telephone system. The actual numbering sequence varies in other countries, though the concept is identical. The phone company runs a UTP (unshielded twisted-pair) cable (called the local loop) from your location (called the demarcation point, or demarc for short) to a phone company building called the Central Office. All the pairs from all the local loop cables that are distributed throughout a small regional area come together at a central point, similar to a patch panel in a UTP-based LAN.


This centralized point has a piece of equipment attached, which is called a switch. This switch functions almost exactly like the switches mentioned in Chapter 2, “The OSI Model,” in that a communications session, once initiated by dialing the phone number of the receiver, exists until the “conversation” is closed. The switch can then close the connection. On one side of the switch is the neighborhood wiring. On the other side are lines that may connect to another switch or to a local set of wiring. The number of lines on the other side of the switch depends on the usage of that particular exchange. Figure 7.1 shows a PSTN system that utilizes these components.


FIGURE 7.1 A local PSTN (or POTS) network

When you want to make a call, you pick up the phone. This completes a circuit, which in most cases gives you a dial tone. The tone is the switch’s way of saying, “I’m ready to accept your commands.” Failure to get a dial tone indicates either a break in the equipment chain or that the switch is too busy at the moment processing other commands. In many areas of the world, you may hear a fast on-and-off tone after giving a command string (phone number) to the local switch. This means that other switches with which the local switch is attempting to communicate are too busy right now. Recently, this has been replaced with a localized voice, which typically says, “We’re sorry. All circuits are busy. Hang up and try your call later.” This happens frequently on holidays or during natural disasters. The phone company in a local area uses only a few wires (called trunk lines) for normal capacity and some auxiliary lines for unexpected usage. This is because wiring and switches are very expensive. It is a trade-off between 100-percent uptime and keeping the costs of leasing the connection from the phone company affordable.

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