ponsibility to aggregate inbound traffic for switching and transport across the long haul, as well as to terminate traffic at a Figure 3-62 Network regions Access Plant Transport Plant Access 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 particular destination. The inside plant, on the other hand, has the responsibility to multiplex incoming traffic streams, switch the streams, and select an outbound path for ultimate delivery to the next central office in the chain or the final destination. Switching, therefore, was cen- trally important to the development of the modern network. Multiplexing Equally important as the development of the central office switch was the concept of multiplexing, which allowed multiple conversations to be carried simultaneously across a single shared physical circuit. The first such systems used frequency-division multiplexing (FDM), a tech- nique made possible by the development of the vacuum tube, in which the range of available frequencies is divided into "chunks" which are then parceled out to subscribers. For example, Figure 3-63 illustrates that subscriber #1 might be assigned the range of frequencies between 0 and 4,000 Hz, while subscriber #2 is assigned 4,000 to 8,000 Hz, #3 8,000 to 12,000 Hz, and so on, up to the maximum range of frequencies available in the channelized system. In FDM, we often observe that users are given "some of the frequency all of the time," meaning that they are free to use their assigned frequency allocation at any time, but may not step outside the bounds given to them. Early FDM sys- tems were capable of transporting 24, 4 KHz channels, for an overall system bandwidth of 96 KHz. FDM, while largely replaced today by Figure 3-63 Subscriber 1: 0-4,000 Hz Frequency- division multiplexing Subscriber 2: 4,000-8,000 Hz (FDM) Subscriber 3: 8,000-12,000 Hz Subscribe