 the same switch. It does this by looking at the area code (number- ing plan area, or NPA) and prefix (NXX) of the dialed number. A telephone number is made up of three sections, shown here. (802) 555-7837 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 The first three digits are the NPA, which identifies the called region. For example, I live in Vermont where the entire state is served by a sin- gle NPA-802. Other states, such as California, have a dozen or more NPAs to serve their much denser population bases. Each NPA identifies a calling area. The second three digits are the NXX, which identify the exchange, or office, that the number is served from, and by extension the switch to which the number is connected. Each NXX can serve 10,000 numbers (555-0000 through 555-9999). Remember the New York fire? Twelve exchanges were lost, under which 104,000 subscribers were operating (out of a possible 120,000). A modern central office switch can typically handle as many as 15 to 20 exchanges of 10,000 lines each. In our example, Adam is served out of a different switch than Cristina, so the call must be routed to a different central office. Before that rout- ing can take place, however, Cristina's local switch routes a query to the signaling network, known as Signaling System 7 (SS7). SS7 provides the network with intelligence. It is responsible for setting up, maintaining, and tearing down a call, while at the same time providing access to enhanced services such as Custom Local Area Signaling Services (CLASS), 800 Number Portability, Local Number Portability (LNP), Line Information Database (LIDB) lookups for credit card verification, and other enhanced features. In a sense, it makes the local switch's job eas- ier by centralizing many of the functions that formerly had to be per- formed locally. The original concept behind SS