tunately, the amplifiers used in the network are not particularly dis- criminating: They have no way of separating the voice noise; the result is that they convert a weak, noisy signal into a loud, noisy signal-better, but far from ideal. A better solution was needed. The better solution came about with the development of time-division multiplexing (TDM), which became possible because of the transistor and integrated circuit electronics that arrived in the late 1950s and early 1960s. TDM is a digital transmission scheme, which employs a small number of discrete signal states, rather than the essentially infinite range of values employed in analog systems. While digital systems are as susceptible to noise impairment as their analog counterparts, the dis- crete nature of their binary signaling makes it relatively easy to separate the noise from the transmitted signal. In digital carrier systems, there are only three valid signal values, one positive, one negative, and zero; anything else is construed to be noise. It is, therefore, a trivial exercise Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Telephony Chapter 3 for digital repeaters to discern what is desirable and what is not, thus, eliminating the problem of cumulative noise. The role of the regenerator, as shown is Figure 3-64, is to receive a weak, noisy digital signal; remove the noise; reconstruct the original signal; and amplify it before transmit- ting the signal on to the next segment of the transmission facility. For this reason, repeaters are also called regenerators, because that is pre- cisely the function they perform. One observation: It is estimated that as much as 60 percent of the cost of building a transmission facility lies in the regenerator sections of the span. For this reason, optical networking, discussed a bit later, has vari- ous benefits, not the least of which 