 voice mail or waits a considerable time before trying the call again. 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 the Federal Communications Commission's (FCC's) enhanced 911 (E911) standard. Thus the call processing has to have a state where the telephone is able to call 911 and not much else. 8 The second is an accounting issue for three-way calls. Wireless has a separate billing concept called air time. We pay not only for how much telephone time we use but also for how much radio time we use. Some wireless providers waive the air-time charges for incomplete, busy or no-answer calls, and it is tempting to use the call-processing billing records. After all, each complete call appears, from beginning to end, in the au- tomatic message accounting (AMA) records. The rub is that three-way calls are counted twice in the AMA records. Consider a 10- minute call using 10 minutes of air time. Suppose that the subscriber spent 6 of those minutes on a three-way call. The total call time is 16 minutes, but the total air time is still only 10 minutes. Just using AMA records will double-bill the customer for those 6 minutes. This billing issue means that the system has to have some record of radio-link starts and stops as well as telephone-call starts and stops. For those wireless service providers not billing customers for busy and no-answer calls, the accounting system has to have some combined understanding of radio-link and call-processing states. 16.5 Conclusion Call-state diagrams are valuable tools for component design, network planning, and network troubleshooting. Any cellular engineer will benefit from being able to read interpret state diagrams. In addition, it is valuable to practice creating them and ex- plaining them to others. Walking through plans and problems together with diagrams allows cellular