iminated, thus giving each user access to the full breadth of available spectrum across the shared physical medium. In frequency-division systems, we observed that we give users "some of the frequency all of the time;" in time-division systems, we turn that around and give users "all of the frequency some of the time." As a result, high- frequency digital signals can be transmitted without restriction. Digitization brings with it a cadre of advantages including improved voice and data transmission quality; better maintenance and trou- bleshooting capability, and therefore reliability; and dramatic improve- ments in configuration flexibility. In digital carrier systems, the time-division multiplexer is known as a channel bank; under normal cir- cumstances, it allows either 24 or 30 circuits to share a single, 4-wire facility. The 24-channel system is called T-Carrier; the 30-channel sys- tem, used in most of the world, is called E-Carrier. Originally designed in 1962 as a way to transport multiple channels of voice over expensive transmission facilities, they soon became useful as data transmission networks as well. That, however, came later. For now, we focus on voice. Voice Digitization The process of converting analog voice to a digital representation in the modern network is a logical and straight-forward process. It has four dis- tinct steps: Pulse Amplitude Modulation (PAM) sampling, in which the amplitude of the incoming analog wave is sampled every 125 microsec- onds; companding, during which the values are weighted toward those most receptive to the human ear; quantization, in which the weighted samples are given values on a nonlinear scale; and finally encoding, dur- ing which each value is assigned a distinct binary value. Each of these stages of Pulse Code Modulation (PCM) will now be discussed in detail. 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