a landline telephone. cradle, we say that it is in the ON-HOOK state. And any time the receiver is not in the cradle, the telephone is in the OFF-HOOK state. These states are depicted in Fig. 16.1. We use circles for the states and arrows for the state-to-state transitions. The labels on the arrows name the stimulus that causes the transition from one state to another. In the case of these two states, there are only two stimuli that can cause the tele- phone to change from one state to another. When the telephone is idle, in the ON-HOOK state, somebody can come along and pick up the receiver and put the telephone into the OFF-HOOK state. This is the PICK-UP stimulus. The telephone will act differently depending on both what event occurs and what state it is in when this event occurs. In the ON-HOOK state, the telephone rings when a 90-V, 20-Hz signal is applied. In the OFF-HOOK state, the telephone sends digits and en- gages in two-way voice communication when someone makes a call. So long as the tele- phone does these things, the telephone is working properly, at least at this level of mod- eling. It is important to understand that the telephone behaves differently in each of the two states of this model. Another important behavioral difference between the two states is the response to an incoming call. This two-state model may not be sophisticated enough to represent the difference between incoming and outgoing calls, but the rest of the telephone network still cares. A telephone in the ON-HOOK state is receptive to an incoming call. When telephone is in the OFF-HOOK state, the switch returns a busy signal to the caller. 2 There is a level of abstraction in analyzing states. Terms such as PICK-UP are eral terms that can represent a large number of actual events. For example, the PICK- UP stimulus that sends the telephone from ON-HOOK to OFF-HOOK can be picking up a telephone receiver, pressing the SPEAKER button on a speakerphone, pressing the TALK button on a cordless telephone, or having a fax machine automat