The purpose of the Public Switched Telephone Network (PSTN) is to establish and maintain audio connections between two endpoints in order to carry speech.
Although humans can perceive sound vibrations in the range of 20–20,000 Hz,[188] 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.
An analog phone is composed of five parts: the ringer, the dialpad, the hybrid (or network), and the hook switch and handset (both of which are considered parts of the hybrid). The ringer, the dialpad, and the hybrid can operate completely independently of one another.
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 voltage; for example, strobe lights are often employed in noisy environments such as factories.)
Warning
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 a ringer needs an alternating current in order to oscillate (just as a church bell won’t ring if you don’t supply the movement), 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 RENs of 0.3 or even less. If you connect too many devices that require too much current, you will find that none of them will be able to ring.
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 dialpad is the portion of the phone that provides this functionality. In the early days of the PSTN, dialpads were in fact 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-tone—also known as Dual-Tone Multi Frequency (DTMF)—dialing, the dialpad consists of 12 buttons. Each button has two frequencies assigned to it (see Table A.1, “DTMF digits”).
Table A.1. DTMF digits
|
1633 Hz [a] | ||||
|---|---|---|---|---|
[a] Notice that this column contains letters that are not typically present as keys on a telephone dialpad. They are part of the DTMF standard nonetheless, and any proper telephone contains the electronics required to create them, even if it doesn’t contain the buttons themselves. (These buttons actually do exist on some telephones, which are mostly used in military and government applications.) | ||||
When you press a button on your dialpad, the two corresponding frequencies are transmitted down the line. The far end can interpret these frequencies and note which digit was pressed.
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 earpiece; its 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.
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 dialtone. When you hang up, the hook switch opens the circuit, which indicates that the call has ended.[189]
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 may 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.
In an analog telephone circuit, there are two wires. In North America, these wires are referred to as Tip and Ring.[190] This terminology comes from the days when telephone calls were connected by live operators sitting at cord boards. The plugs that they used had two contacts—one located at the tip of the plug and the other connected to the ring around the middle (Figure A.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. For modern Cat 5 and 6 cables, the Tip is usually the white wire, and Ring is the colored wire. 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.
[188] 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.
[189] 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,” or idle.
[190] They may have other names elsewhere in the world (such as “A” and “B”).