rew out of work performed at Bell Laboratories shortly after the turn of the century. That work led to a discrete understanding of not only the biological nature and spectral makeup of the human voice, but also to a better understanding of language, sound patterns, and the sounded emphases that make up spoken language. The Nature of Voice A typical voice signal is made up of frequencies that range from approx- imately 30 Hz to 10 KHz. Most of the speech energy, however, lies between 300 Hz and 3,300 Hz, the so-called voice band. Experiments 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 Telephony have shown that the frequencies below 1 KHz provide the bulk of recog- nizability and intelligibility, while the higher frequencies provide rich- ness, articulation, and natural sound to the transmitted signal. The human voice has a remarkably rich mix of frequencies, and this richness comes at a considerable price. In order for telephone networks to transmit the voice's entire spectrum of frequencies, significant net- work bandwidth must be made available to every ongoing conversation. There is a substantial price tag attached to bandwidth; it is a finite com- modity within the network, and the more of it that is consumed, the more it costs. The Network Thankfully, work performed at Bell Laboratories at the beginning of the twentieth century helped network designers confront this challenge head-on. To understand it, let's take a tour of the telephone network. The typical network, as shown in Figure 3-62, is divided into several regions: the access plant; the switching, multiplexing, and circuit con- nectivity equipment (the central office); and the long-distance transport plant. The access and transport domains are often referred to as the out- side plant; the central office is, conversely, the inside plant. The outside plant has the res