Page 58 - Asterisk™: The Future of Telephony

Physical Telephones
Any physical device whose primary purpose is terminating an on-demand audio com-
munications circuit between two points can be classified as a physical telephone. At a
minimum, such a device has a handset and a dial pad; it may also have feature keys, a
display screen, and various audio interfaces.
This section takes a brief look at the various user (or endpoint) devices you might want
to connect to your Asterisk system. We’ll delve more deeply into the mechanics of
analog and digital telephony in Chapter 7.

Analog telephones
Analog phones have been around since the invention of the telephone. Up until about
20 years ago, all telephones were analog. Although analog phones have some technical
differences in different countries, they all operate on similar principles.

This contiguous connection is referred to as a circuit, which the
telephone network used to use electromechanical switches to create—
hence the term circuit-switched network.

When a human being speaks, the vocal cords, tongue, teeth, and lips create a complex
variety of sounds. The purpose of the telephone is to capture these sounds and convert
them into a format suitable for transmission over wires. In an analog telephone, the
transmitted signal is analogous to the sound waves produced by the person speaking.
If you could see the sound waves passing from the mouth to the microphone, they
would be proportional to the electrical signal you could measure on the wire.
Analog telephones are the only kind of phone that are commonly available in any retail
electronics store. In the next few years, that can be expected to change dramatically.

Proprietary digital telephones
As digital switching systems developed in the 1980s and 1990s, telecommunications
companies developed digital Private Branch eXchanges (PBXes) and Key Telephone
Systems (KTSes). The proprietary telephones developed for these systems were com-
pletely dependent on the systems to which they were connected and could not be used
on any other systems. Even phones produced by the same manufacturer were not cross-
compatible (for example, a Nortel Norstar set will not work on a Nortel Meridian 1
PBX). The proprietary nature of digital telephones limits their future. In this emerging
era of standards-based communications, they will quickly be relegated to the dustbin
of history.
The handset in a digital telephone is generally identical in function to the handset in
an analog telephone, and they are often compatible with each other. Where the digital

30 | Chapter 2: Preparing a System for Asterisk