ications works from a technical perspective. For an introduction to electricity, the user should consult Appendix A. After completion of this chapter, the reader of this text should have a grasp of electrical communications and its units of measure. Specifically, we will introduce an electrical signal and how it can carry intelligence. We will differentiate analog and digital transmission with a very first approximation. Binary digital transmission will then be introduced starting with binary numbers and how they can be represented electrically in a simple fashion. We then delve into conducted transmission. That is the transport of an electrical signal on a copper-wire pair, on coaxial cable, and then by light in a fiber-optic strand of glass. Radio transmission and the concept of modulation will then be introduced. 2.2 SIGNALS IN EVERYDAY LIFE Prior to the advent of practical electrical communication, human beings had been signaling over a distance in all kinds of ways. The bell in the church tower called people to religious services or "for whom the bell tolls"-the announcement of a death. We knew a priori several things about church bells. We knew approximately when services were to begin, and we knew that a long, slow tolling of the bells announced death. Thus we could distinguish one from the other, namely a call to religious services or the announcement of death. Let's call lesson 1 a priori knowledge. The Greeks used a relay of signal fires to announce the fall of Troy. They knew a priori that, a signal fire in the distance announced victory at Troy. We can assume that "no fire" meant defeat. The fires were built in a form of relay, where a distant fire was Fundamentals of Telecommunications, Second Edition, by Roger L. Freeman ISBN 0-471-71045-8 Copyright © 2005 by Roger L. Freeman SIGNALS CONVEY INTELLIGENCE just visible with the naked eye, the sight of which caused the lighting of a second fire, and then a third, fourth, and SO on, in a line of fires on nine hills terminating in Queen Clytemnestra's pal