System information

This section takes a brief look at the various user (or endpoint) devices you might want
to connect to your Asterisk system. We delve more deeply into the mechanics of analog
and digital telephony in Appendix A.
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.
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 phones 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 (PBXs) and key telephone sys-
tems (KTSs). The proprietary telephones developed for these systems were completely
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-com-
patible (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
phone is different is that inside the telephone, the analog signal is sampled and con-
verted into a digital signal—that is, a numerical representation of the analog waveform.
We discuss digital signals in more detail in Appendix A; for now, suffice it to say that
the primary advantage of a digital signal is that it can be transmitted over limitless
distances with no loss of signal quality.
658 | Appendix C: Preparing a System for Asterisk