calling to be used at the same time, and we have seen other systems that do. Second, as discussed earlier, call state diagrams operate at a level of abstraction that may not include all details. 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 response to every message in the protocol. This response may be not to do anything, not to change state, or not to send any messages anywhere, but freezing up and not know- ing what to do is not a healthy design option. The entity sending the message has its own state space and is responding to its own stimuli, including other protocol mes- sages. The design challenge of a multinode messaging system is in resolving the con- fusing combinations of states without creating glare conditions. One example of this is in the ANSI-41 protocol (see Chap. 15). A serving mobile switching center (MSC) looking for handoff opportunities asks candidate MSCs to re- spond. If the candidate MSC is unable to take the call, it simply does not respond within a specified time period. This is a valid action. The serving MSCs call state dia- gram would include an appropriate action to take when not receiving a response. That action would be to drop the switch that did not respond from the candidate list. 16.3 Wireless Call States With its preorigination dialing, the wireless telephone has no DIALING state. Instead, dialed digits are kept in the user terminal until the SEND key is pressed. The digits are dialed and kept in the cellular telephone until the subscriber presses SEND, and then the system sends a message requesting dialed digits. The simple four-state wireless diagram is in Fig. 16.5. Dialing digits, the DIGITS stimulus, puts digits into the internal telephone registers but does not change the state of the telephone. While dialing digits does not change the state on this diagram, we feel that i