 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 The best way to gain an understanding of how the telephone network operates is by studying a modern railroad system. Consider Figure 3-8, which is a route map for the mythical Midland, Amarillo, and Roswell Railroad. Rail yards in each of the three cities are interconnected by high-volume trunk lines. The train yards, also known as switch yards, are used as aggregation, storage, and switching facilities for the cargo that arrives on trains entering each yard. A 90-car train from El Paso, for example, may arrive in Midland as planned. Half the cars are destined for Midland, while the other half are destined for Amarillo. At the Mid- land yard, the train is stored, disassembled, reassembled for transport, and switched as required to move the Amarillo traffic on to Amarillo. Switches in the yards create temporary paths from one side of the yard to the other. Meanwhile, route bosses in the yard towers analyze traffic patterns and route trains across a variety of alternative paths to ensure the most efficient delivery. For example, traffic from Amarillo to Roswell could be routed through Midland, but it would obviously be more effi- cient to send it on the direct line that runs between Amarillo and Roswell. Assuming that the direct route is available and is not congested, the route boss might very well choose that alternative. Notice also that there are short local lines, called feeder lines, which pump local traffic into the switchyards. These lines typically run shorter trains, destined most likely for local delivery. Some of them, however, carry cargo destined for distant cities. In those cases, the cargo would be combined with that of other trains to create a large, efficient train with all cargo bound for the same destination. Figure 3-8 Amarillo Midland Midland, To Dallas Amarillo, and Roswell To Paris Railr