7.1. Analog Telephony

The purpose of the Public Switched Telephone Network (PSTN) is to establish and maintain audio connections between two endpoints.

Although humans can perceive sound vibrations in the range of 20-20,000 Hz,^[] most of the sounds we make when speaking tend to be in the range of 250-3,000 Hz. Since the purpose of the telephone network is to transmit the sounds of people speaking, it was designed with a bandwidth of somewhere in the range of 300-3,500 Hz. This limited bandwidth means that some sound quality will be lost (as anyone who's had to listen to music on hold can attest to), especially in the higher frequencies.

^[] If you want to play around with what different frequencies look like on an oscilloscope, grab a copy of Sound Frequency Analyzer, from Reliable Software. It's a really simple and fun way to visualize what sounds "look" like. The spectrograph gives a good picture of the complex harmonics our voices can generate, as well as an appreciation for the background sounds that always surround us. You should also try the delightfully annoying NCH Tone Generator, from NCH Swift Sound.

7.1.1. Parts of an Analog Telephone

An analog phone is composed of five parts: the ringer, the dial pad, the hybrid (or network), and the hook switch and handset (both of which are considered parts of the hybrid). The ringer, the dial pad, and the hybrid can operate completely independently from one another.

7.1.1.1. Ringer

When the central office (CO) wants to signal an incoming call, it will connect an alternating current (AC) signal of roughly 90 volts to your circuit. This will cause the bell in your telephone to produce a ringing sound. (In electronic telephones, this ringer may be a small electronic warbler rather than a bell. Ultimately, a ringer can be anything that is capable of reacting to the ringing voltagefor example, strobe lights are often employed in noisy environments such as factories.)

Ringing voltage can be hazardous. Be very careful to take precautions when working with an in-service telephone line.

Many people confuse the AC voltage that triggers the ringer with the direct current (DC) voltage that powers the phone. Remember that the ringer will not respond to DC voltage, and you've got it.

In North America, the number of ringers you can connect to your line is dependent on the Ringer Equivalence Number (REN) of your various devices. (The REN must be listed on each device.) The total REN for all devices connected to your line cannot exceed 5.0. An REN of 1.0 is equivalent to an old-fashioned analog set with an electromechanical ringer. Some electronic phones have an REN of 0.3 or even less.

7.1.1.2. Dial pad

When you place a telephone call, you need some way of letting the network know the address of the party you wish to reach. The dial pad is the portion of the phone that provides this functionality. In the early days of the PSTN, dial pads were rotary devices that used pulses to indicate digits. This was a rather slow process, so the telephone companies eventually introduced touch-tone dialing . With touch-tonealso known as Dual-Tone Multi Frequency (DTMF)dialing, the dial pad consists of 12 buttons. Each button has two frequencies assigned to it (see Table 7-1).

Table 7-1. DTMF digits

When you press a button on your dial pad, the two corresponding frequencies are transmitted down the line.

We assume that you've used a telephone, so we won't spend any more time on DTMF.

7.1.1.3. Hybrid (or network)

The hybrid is a type of transformer that handles the need to combine the signals transmitted and received across a single pair of wires in the PSTN and two pairs of wires in the handset. One of the functions the hybrid performs is regulating sidetone , which is the amount of your transmitted signal that is returned to your earpieceits purpose is to provide a more natural-sounding conversation. Too much sidetone, and your voice will sound too loud; too little, and you'll think the line has gone dead.

7.1.1.3.1. Hook switch (or switch hook)

This device signals the state of the telephone circuit to the CO. When you pick up your telephone, the hook switch closes the loop between you and the CO, which is seen as a request for a dial tone. When you hang up, the hook switch opens the circuit, which indicates that the call has ended.^[*]

^[*] When referring to the state of an analog circuit, people often speak in terms of "off-hook" and "on-hook." When your line is "off-hook," your telephone is "on" a call. If your phone is "on-hook," the telephone is essentially "off."

The hook switch can also be used for signaling purposes. Some electronic analog phones have a button labeled "Link" that causes an event called a flash. You can perform a flash manually by depressing the hook switch for a duration of between 200 and 1,200 milliseconds. If you leave it down for longer than that, the carrier will assume you've hung up. The purpose of the Link button is to handle this timing for you. If you've ever used call waiting or three-way calling on an analog line, you have performed a hook switch flash for the purpose of signaling the network.

7.1.1.3.2. Handset

The handset is composed of the transmitter and receiver. It performs the conversion between the sound energy humans use and the electrical energy the telephone network uses.

7.1.2. Tip and Ring

In an analog telephone circuit, there are two wires. In North America, these wires are referred to as Tip and Ring.^[*] This terminology comes from the days when telephone calls were connected by live operators sitting at cord boards. The plugs they used had two contacts, one located at the tip of the plug and the other connected to the ring around the middle (Figure 7-1).

^[*] They may have other names elsewhere in the world (such as "A" and "B").

Figure 7-1. Tip and Ring

The Tip lead is the positive polarity wire. In North America, this wire is typically green and provides the return path. The Ring wire is the negative polarity wire. In North America, this wire is normally red. When your telephone is on-hook, this wire will have a potential of -48V DC with respect to Tip. Off-hook, this voltage drops to roughly -7V DC.