li that can move the component or system from state to state. Making state diagrams and enumerating the various stimuli are major components of almost any system design. 1 A state diagram is only a model and is not required to have every level of detail. Just as a map can be high level or close up, state diagrams can show varying levels of detail depending on what we are using them for. In creating the state diagram, we decide which states are important to model. Once the states are determined, we need to find each relevant stimulus that changes the state of the telephone. The notion of a stimu- lus being relevant gives us latitude in describing the call process. Depending on what we want to accomplish, we can choose a more general diagram to give a simple big pic- ture, or we can work through all the possible stimuli and get the details right. Allow us to illustrate the process with a basic description of the states of a landline telephone. At the highest level, a normal residential telephone has two basic states, sitting idle or being used. When the telephone is sitting idle with the receiver in the 1A call-state diagram is not quite the same as the flowchart that our teachers told us to make before writing a computer program, but the two are similar. Making call-state diagrams forces system designers to think about all the relevant states and stimuli in an organized way. For peo- ple with an information technology background, call-state diagrams are deceptively similar to data-flow diagrams. However, the circles in a call state represent states, not processes, and the ar- rows represent transitions between states, not data items that move from one process to another. 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 Hang-Up On-Hook Off-Hook Pick-Up Figure 16.1 Call-state diagram of the two basic states of 