hat another caller might have used to place a call were not available to them, because they were tied up transporting call setup data-which produces no revenue. SS7 changes all that. With SS7, the signaling data travels across a dedicated packet network from the calling party to the called party. SS7 verifies the availability of the called party's line, reserves it, and then- and only then-seizes - a talk path. Once the talk path has been created it rings the called party's phone and places ringing tone in the caller's ear. As soon as the called party answers, SS7 silently bails out until one end or the other hangs up. At that point it returns the path to the pool of available network resources. Interoffice Trunking We have not discussed the manner in which offices are connected to one another. As Figure 3-36 illustrates, an optical fiber ring with add-drop multiplexers interconnects the central offices SO that interoffice traffic can be safely and efficiently transported. The switches have trunk units Figure 3-36 Typical network showing fiber ring intercon- necting offices Switch Switch Optical LU TU TU LU Williston CO Charlotte CO Tandem Switch 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 Telephony (TUs) that connect the back side, called the trunk side, of the switch to the wide area network; in this case the optical ring. Trunks that interconnect offices have historically been four-wire cop- per facilities (a pair in each direction). Today they are largely optical, but are still referred to as "four-wire" because of their two-way nature. An interesting point about trunks: In the 1960s and early 1970s, most interoffice trunks were analog rather than digital. To signal, they used single frequency tones. Because these trunks did not "talk" directly to customers, there was no reason to worry about a human voice inadver- tently e