Page 195 - Asterisk™: The Future of Telephony

CHAPTER 7

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

We’re now going to take a break from Asterisk for a chapter or two, because we want
to spend some time discussing the technologies with which your Asterisk system will
need to interface. In this chapter, 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 the next chapter.)
While tomes could be written about the technologies in use in telecom networks, the
material in this chapter was chosen based on our experiences in the community, which
helped us to define the specific items that might be most useful. Although this knowl-
edge 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, 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

* 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.

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