designers to make sure nothing unexpected is going to happen, for example, to ensure that there are no glare conditions, as described in Sec. 23.6. In call processing, we do not like surprises. 2Call waiting and three-way calling are discussed later. 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. Call States Call States 16.2 PSTN Call States The public switched telephone network (PSTN) provides many services besides basic call setup and teardown and handles these extra capabilities by cleverly using the call states we discussed earlier and extending them to provide these services. Many of these extra capabilities were added in support of the idea that the telephone service should switch from being location-based to being personal, or subscriber-based. We call these extra capabilities vertical services that started with custom calling (see Sec. 12.1) and have expanded into voice-mail messaging services and beyond. Let us examine the ba- sic PSTN call first and then delve into call waiting and three-way calling. The other two custom calling features, speed dialing and call forwarding, are less in- teresting to us here. Speed dialing determines how a call is routed from its abbreviated dialed digits, and call forwarding affects how the network routes an incoming call, a landline function similar to roaming. 16.2.1 Basic PSTN call states Consider regular landline telephone calls being made on a regular landline telephone, with no call waiting and no three-way calling. This model will be more detailed than the two-state model examined earlier. Let's examine the four states QUIET, RINGING, DIALING, and TALKING shown in Fig. 16.2. The QUIET state has the telephone just sitting there. Restricting our attention to PSTN call processing, we ask ourselves, "What can happen next?" One stimulus is that somebody calls, and the phone starts ringing. A ring