n connected in their own call. This feature is often called call transfer because it is used to transfer incom- ing calls.3 Adding three-way calling to a diagram with call waiting makes the call-state picture more complicated. The interactions between the two features may differ on different systems. In most, if a single call is in progress and the originating caller initiates 3The typical scenario is I get a call from somebody who really should be talking to you, SO I flash and three-way call you; I tell you who is calling; I flash again to make the call three-way; and then I hang up. 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 Key Telephony Concepts Call Waiting Hang Up TALK 1 Hang Up RINGING Hang Up QUIET TALKING TALK 2 One of Them Hangs Up They Hang Up Connect DIALING DIAL 2 TALK 3 Connect Flash Hang Up Hang Up Three-Way Calling Figure 16.4 PSTN states with call waiting and three-way calling. another call with a flash, the call will become three-way when the originating caller nals the system with a second flash. However, if the call is interrupted by a third party, then the call-waiting mode will be active, and the flash key will alternate between the two calls. A fairly complete state diagram with call waiting and three-way calling is shown in Fig. 16.4. 4 Call state diagrams can be used to describe the state of a single piece of equipment, the states of an entire network, or the states of multiple components interacting on a network. When designing a messaging system using a protocol such as Integrated Ser- vices Digital Network (ISDN), it is important that every state of every node have some 4We say fairly complete for two reasons. First, not all telephone systems do the exact same thing when call waiting and three-way calling are both involved. We have seen systems that do not al- low both call waiting and three-way 