ing telephone is in a different state than an idle telephone; it responds differently when somebody picks up the receiver and goes off-hook. An INCOMING-CALL stimulus sends the telephone from the QUIET state to the RINGING state. The other stimulus is somebody picking up the receiver to RINGING Hang Up QUIET TALKING They Hang Up DIALING Figure 16.2 Four-state diagram of a basic PSTN telephone. 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 make an outgoing telephone call. A PICK-UP stimulus sends the telephone from the QUIET state to the DIALING state. From the RINGING state, two things can happen. One stimulus is that the caller gives up, and the telephone stops ringing. The THEY-HANG-UP stimulus sends the tele- phone from the RINGING state to the QUIET state. The other stimulus is that the tele- phone is answered, in which case the PICK-UP stimulus sends the telephone from the RINGING state to the TALKING state, the stable call we described in Sec. 12.1. Is the PICK-UP stimulus that sends the telephone from QUIET to DIALING the same stimulus as the PICK-UP stimulus that sends the same telephone from RINGING to TALK- ING? This model has two stimuli from two different states with the same name. From the state-diagram point of view, it does not matter: It is the state and stimulus together that determine the flow. We could be more precise and call them PICK-UP-WHILE- QUIET and PICK-UP-WHILE-RINGING, but this does not change the state model. From the DIALING state, the HANG-UP stimulus sends the telephone back into the QUIET state. And the CONNECT event sends the DIALING telephone into the TALKING state. Finally, the HANG-UP stimulus or the THEY-HANG-UP stimulus sends the tele- phone from the TALKING state back into its QUIET state. This four-state picture shows the progression of a telephone in a simple cal