Page 196 - Asterisk™: The Future of Telephony

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.

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
of one another.

Ringer
When the central office (CO) wants to signal an incoming call, it will connect an al-
ternating 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.)

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 cur-
rent 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 elec-
tromechanical ringer. Some electronic phones have an REN 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.

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 in fact rotary
devices that used pulses to indicate digits. This was a rather slow process, so the tele-
phone companies eventually introduced touch-tone dialing. With touch-tone—also

168 | Chapter 7: Understanding Telephony