he cor- rect number of trunks involves a discipline known as traffic engineering, discussed at length in Chap. 23. Most PBXs support DID, the ability for callers to use a PSTN telephone number to dial the telephones directly behind the PBX rather than to call a main number and then to give an extension number. DID requires that the business purchase from the PSTN a range of telephone numbers, which the business then assigns to the telephones behind the PBX. When a call comes to the local exchange office for a telephone in that range of numbers, it will seize an idle trunk from the trunk group going to the PBX and pass the call to the PBX, much as it would to another local exchange office. The PBX is responsible for associating the telephone number with the line leading to the correct telephone and completing the call. A recent development in the PBX marketplace is the Internet Protocol (IP) PBX. In its most basic form, it is a PBX that places calls through an IP network and internally routes calls to the correct telephone, which itself is an IP telephone. Advanced IP PBXs are capable of routing calls from IP or traditional telephones either via an IP network or the PSTN. 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. The PSTN and Telephone Switching The PSTN and Telephone Switching 11.1.4 The organization of the PSTN In 1985, the North American PSTN was organized as a clear hierarchy of levels from the class 5 local exchange office to the most central class 1 tandem switches of the long- distance network. Many of the concepts associated with telephony today draw their heritage from the Bell System, which was broken up in the 1980s. The Bell System of the early 1980s provides a comparatively straightforward example of the architecture of a telephone network and for this reason will be used as a starting point for our ex- ploration of the org