Packet Guide to Voice over IP

Chapter 2. Traditional Telephony

Telecommunication: Any transmission and/or emission and reception of signals representing signs, writing, images and sounds or intelligence of any nature by wire, radio, optical or other electromagnetic systems.

—ITU-T G.701 Vocabulary of digital transmission and multiplexing and pulse code modulation terms (PCM)

Introduction

Whether you are converting to Voice over IP or already have, network administrators will be required to handle a collection of issues of a more traditional nature and connect to the outside world. So even though we may have embraced the future, it is difficult to divorce ourselves entirely from the past. For the foreseeable future, an understanding of analog and digital circuit-switched technology, telephony wiring, and the tried-and-true methods for connecting to ISPs is necessary. For example, one of the most popular circuits sold is still a T-1. In addition, data from the International Telecommunication Union indicates that while the growth in Internet use and devices accessing the Internet is significant, it does not signal the death knell of more traditional fixed services. This data is shown in Figure 2-1.

It is quite common for graduates entering the workforce armed with network degrees and certifications to be told upon being hired that their responsibilities include the care and feeding of a legacy PBX or a bundle of blue-and-white wires.

Figure 2-1. Data from the ITU

Since the Public Switched Telephone Network (PSTN) is ...