4.1 Introduction to telephony

In this chapter, we assume you're familiar with how to use a telephone, but know little about telecommunications technology.

There are many different types of phone line, but we'll illustrate as many concepts as possible with Plain Old Telephone Service (POTS), the kind of service you probably have at home.

A POTS line is a loop of wire which starts at a local telephone company Central Office (CO) switching station. The CO switch applies a small amount of power to the line, this is called battery voltage, or just battery. Your telephone has a small on/off switch (not to be confused with the CO switching equipment) called a hook switch. When the telephone is in its resting position, the hook switch is off and there is no current flowing; the phone is said to be on the hook, or simply on-hook. When you pick up the telephone, this closes the hook switch (turns it on), completing the circuit and causing current to start flowing. Since this current is flowing along the local loop between your phone and the CO switch, this current is called loop current.

The local loop is sometimes referred to as the last mile because the distance is typically within a mile or two. It is the most troublesome link in the telecommunications network because even though most central offices are now based on sophisticated digital equipment, the last mile and the customer's phone are generally based on this antiquated analog technology.

If...