Chapter 7. Understanding Telephony

Table of Contents

Analog Telephony
Parts of an Analog Telephone
Dial pad
Hybrid (or network)
Tip and Ring
Digital Telephony
Pulse-Code Modulation
Digitally encoding an analog waveform
Increasing the sampling resolution and rate
Nyquist’s Theorem
Logarithmic companding
The Digital Circuit-Switched Telephone Network
Circuit Types
The humble DS-0―the foundation of it all
T-carrier circuits
SONET and OC circuits
Digital Signaling Protocols
Channel Associated Signaling (CAS)
Signaling System 7
Packet-Switched Networks

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 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.