Asterisk: The Definitive Guide, 4th Edition

Appendix A. Understanding Telephony

Utility is when you have one telephone, luxury is when you have two, opulence is when you have three—and paradise is when you have none.

—Doug Larson

In this appendix, we are going to talk about some of the technologies of the traditional telephone network—especially those that people most commonly want to connect to Asterisk. (We’ll discuss Voice over IP in Appendix B.)

While tomes could be written about the technologies in use in telecom networks, the material included here was chosen based on our experiences in the community, which helped us to define the specific items that might be most useful. Although this knowledge may not be strictly required in order to configure your Asterisk system, it will be of great benefit when interconnecting to systems (and talking with people) from the world of traditional telecommunications.

Analog Telephony

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,^[217] 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 ...