aling networks just as the figure shows. Another implementation of the IWF is a dual-mode HLR, a single HLR machine that signals in both GSM and ANSI- 41 protocols. In any case, neither GSM nor ANSI-41 is going away any time soon, so in- ternetwork communication is going to be a part of the wireless signaling system for many years to come. 15.7 Conclusion The ANSI-41 standard governs inter-MSC communications in support of advanced cel- lular services. This standardization of the cellular network specification simplifies management of cellular networks and reduces the cost of equipment design. ANSI-41 continues to grow, meeting new demands of cellular subscribers and service providers. 6There is a separate interworking function that translates between circuit-switched and packet- switched (IP) transport networks described at the beginning of this chapter. 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. ANSI-41 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. Source: CDMA CAPACITY AND QUALITY OPTIMIZATION Chapter Call States The call state diagramming method is a useful tool for describing the operations of a telephone network. Modeling with call states allows system designers to make sure that the system is conceptually robust. If the physical model and physical implemen- tation deliver the conceptual model successfully, and the conceptual model is robust, then the system should work as specified. State diagrams are also useful to engineers who are tracing network problems. 16.1 Defining Call States A telephone call is a sequential process with a beginning, an end, and a logical pro- gression of call states along the way. We model sequential processes as states and stim- u