ty to select one's 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 long-distance provider from a pool of available service providers-in this case AT&T, MCI, or Sprint. This, of course, was the realization of a truly competitive marketplace in the long-distance market segment. To understand this evolution, it is helpful to have a high-level under- standing of the overall architecture of the network. In the predivestiture world, AT&T was the provider for local service, long-distance service and communications equipment. An AT&T central office, therefore, was awash in AT&T hardware-switches, cross-connect devices, multiplex- ers, amplifiers, repeaters, and myriad other devices. Figure 3-6 shows a typical network layout in the predivestiture world. A customer's telephone is connected to the service provider's network by a local loop connection (so-called twisted pair wire). The local loop, in turn, connects to the local switch in the central office. This switch is the point at which customers first touch the telephone network, and it has the responsibility to perform the initial call setup, maintain the call while it is in progress, and tear it down when the call is complete. This switch is called a local switch because its primary responsibility is to set up local calls that originate and terminate within the same switch. It has one other responsibility, though, and that is to provide the necessary interface between the local switch and the long-distance switch, so that calls between adjacent local switches (or between far-flung local switches) can be established. The process, then, goes something like this: When a customer lifts the handset and goes off-hook, a switch in the telephone closes, completing a circuit that allows current flow, which, in turn, brings dial tone to the customer's ear. Upon hearing the dial tone, th