r 4: 12,000-16,000 Hz Subscriber 5: 16,000-20,000 Hz 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 more efficient systems that will be discussed later, is still used in analog cellular telephone and microwave systems, among others. This model worked well in early telephone systems. Because the lower regions of the 300 to 3,300 Hz voiceband carry the frequency components that allow for recognizability and intelligibility, telephony engineers concluded that while the higher frequencies enrich the transmitted voice, they are not necessary for calling parties to recog- nize and understand each other. This understanding of the makeup of the human voice helped them create a network that was capable of faithfully reproducing the sounds of a conversation while keeping the cost of consumed bandwidth to a minimum. Instead of assigning the full complement of 10 KHz to each end of a conversation, they employed filters to bandwidth-limit each user to approximately 4,000 Hz, a resource savings of some 60 percent. Within the network, sub- scribers were frequency-division multiplexed across shared physical facilities, thus allowing the telephone company to efficiently conserve network bandwidth. Time, of course, changes everything. As with any technology, there were downsides to frequency-division multiplexing. It is an analog tech- nology and, therefore, suffers from the shortcomings that have histori- cally plagued all transmission systems. The wire over which information is transmitted behaves like a longwire antenna, picking up noise along the length of the transmission path and very effectively homogenizing it with the voice signal. Additionally, the power of the transmitted signal diminishes over distance, and if the distance is far enough, the signal will have to be amplified to make it intelligible at the receiving end. Unfor- 