hone to the switch is a line, since it is static at the telephone end and only switched at the local exchange office end. If the twisted pair were connecting a pair of switches such that the twisted pair could be switched at both ends in order to complete a circuit, then the twisted pair would be called a trunk, a transmission path that can be switched at both ends. If the trunk virtual, that is, digitally multiplexed with other trunks onto a digital facility, then it is still referred to as a trunk. The physical facility on which the multiplexed digital sig- nals are being carried is often referred to as a line, but the trunks that are being car- ried remain trunks. A collection of several trunks between two switches is referred to as a trunk group. 11.1.2 PSTN equipment The term local exchange office is often used to describe the telephone switch used to connect customer equipment to the PSTN (via the local loop). Two other common terms for this switch are central office and class 5 office. The latter term has its origins in the early architecture of the PSTN, which used a hierarchy of switches, class 1 through class 5, to perform the function of routing calls across long distances. Unless the calling and called parties are connected to the same local exchange office, the call will be routed through additional switches to get from the calling to the called party. The calls will be routed between the switches over trunks. The trunks may, in rare instances, still be a pair of analog wires. A more common form of analog trunk uses Twisting of the wires reduces the susceptibility of the wires to interference from magnetic fields. 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 a pair of wires in each direction so that the two voice paths remain separate. The vast majority of trun