he signal to a standard. The information content of a digital signal is concerned with discrete states of the signal, such as the presence or absence of a voltage, a contact is in an open or closed position, a voltage is either positive- or negative-going, or a light is on or off. The signal is given meaning by assigning numerical values or other information to the various possible combinations of the discrete states of the signal (Ref. 1). The examples of digital signals given above are all binary. Of course, with a binary signal (a bit), the signal can only take on one of two states. This is very provident for several reasons. First, of course, with a binary system, we can utilize the number base 2 and apply binary arithmetic, if need be. The other good reason is that we can use a decision circuit where there can be only two possible conditions. We'll call those conditions a 1 and a 0. Key to the principal advantage of digital transmission is the employment of such simple decision circuits. We call them regenerators. A corrupted digital signal enters on one side, and a good, clean, nearly perfect square-wave digital signal comes out the other side. Accumulated noise on the corrupted signal stops at the regenerator. This is the principal disadvantage of analog transmission: noise accumulates. Not SO with digital transmission. Let's list some other advantages of binary digital transmission. It is compatible with the integrated circuits (ICs) such as LSI, VLSI, and VHSIC. PCs are digital. A digital signal is more tolerant of noise than its analog counterpart. It remains intelligible under very poor error performance, with bit error rates typically as low as 1 X 10-2 for voice operation. The PSTN in the industrialized world is 100% digital. Most of the world's evolv- ing economies are also entirely digital. However, there are segments of a few that remain analog. Fundamentals of Telecommunications, Second Edition, by Roger L. Freeman ISBN 0-471-71045-8 Copyright © 2005 by Roger L. Freeman DIGITAL NETWORKS The digit