oper disposition of the call and any special service 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 information about Cristina that it should invoke. The local switch routes the call over intraoffice facilities to a tandem switch, which in turn con- nects the call to the POP of whichever long-distance provider Cristina is subscribed. The long-distance carrier invokes the capabilities of SS7 to determine whether Adam's phone is available, and if it is, hands the call off to the remote local service provider. When Adam answers, the call is cut through and it progresses normally. There are, of course, other ways that calls can be routed. The local loop could be wireless, and if it is, the call from the cell phone is received by a cell tower (see Figure 3-37) and is transported to a special dedicated cel- lular switch in a central office called a Mobile Telephone Switching Office (MTSO). The MTSO processes the call and hands it off to the wireline network via interoffice facilities. From that point on the call is handled like any other. It is either terminated locally on the local switch or handed to a tandem switch for long-distance processing. The fact of the Figure 3-37 Cell tower 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 matter is that the only part of a cellular network that is truly wireless is the local loop-everything else is wired. Before we wrap this chapter, we should take a few minutes to discuss carrier systems, voice digitization, and multiplexed transport, all of which take place (for the most part) in the central office. In spite of all the hype out there about the Internet and IP magic, plain old telephone serv