Chapter 1 - Synchronous Hierarchical Networks

1.1 INTRODUCTION

Historically, the first method of telecommunications after the industrial revolution
depended on telegraphy. Telegraphy, however, was a point-to-point, one-way wired
connection, and it used a simplistic on–off modulation method by which an electric
current was manually interrupted to provide Morse-like electric pulses that were decoded
by a skilled operator into characters at the receiving end. The telecommunications
revolution began with the ability to modulate DC electric current with a microphone.
When analog voice impinges on its diaphragm, the modulated signal is
transmitted over copper wires and converted into an acoustic signal, reproducing
voice (Figure 1.1). The ability to simultaneously talk and listen at any time, created
such a demand that the first communication network was created with switching
nodes in between (albeit operated manually) and telephony was created. The evolving
network, however, reached a point of growth that was becoming uneconomical
due to labor involved in the manual switching from node to node, becoming particularly
noticeable in geographically large countries with city centers located apart
from each other, such as the United States. As a result, the old analog network was
not profitable and new methods were needed to automate connectivity, increase networking
capability, and transport more traffic over the same copper cables. The result
was digital telephony over a synchronous network.

However, voice still remains an analog signal, and the analog transducer, the microphone,
still generates an analog electrical signal, as it did more than 100 years
ago. In fact, the analog signal generated by a telephone is transmitted over a twisted
pair of wires (TP 2W), and separated by electrical signals flows over the pair in
both directions simultaneously, circuits known as “hybrids.” An alternate method
uses two pairs of wires (TP 4W), one pair per direction, without hybrids.

In a 2W system (popular in narrowband), where the transmitted and received
signals are separated by a “hybrid,” part of the signal from the microphone, a
sidetoneorecho, leaks from the hybrid and finds its way to the earpiece (receiver);
this is eliminated with an echo-canceller.

At the receiving line unit (or channel unit), the analog signal is converted to a
pulse-code modulated (PCM) digital signal by means of a coder/decoder (CODEC)
(Figure 1.2).

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