confusion because inter- national country codes look a lot like U.S. area codes. 5 New numbering plans are in the works as the international standards committees work on the mobile numbering problem. cdmaOne, cdma2000, and wideband CDMA (W-CDMA) all use ANSI-41 networks, but there are 700,000,000 GSM cell phones in service that use the GSM signaling pro- tocols. Since W-CDMA is an outgrowth of GSM, we expect W-CDMA user terminals to be dual-mode mobile telephones supporting both GSM and W-CDMA. This means that the same user terminal may find itself in a GSM system one day and in an ANSI-41 system the next, so we need some form of signaling communication between GSM and ANSI-41. While the two signaling languages are not the same, they have much vocabulary in common. Since telephone signaling systems do not have the universal translators used on the "Star Trek" television show, we have to employ a more down-to-earth solution. 5 For example, Mexico's country code is 52 and Arizona has a 520 area code, so an Arizona sys- tem could refuse service to a Mexican roamer with a confusing number. 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 ANSI-41 ANSI-41 Network Network Figure 15.4 Interworking function (IWF). The term interworking is used for the communication between GSM and ANSI-41 sig- naling systems. There are enough differences that there is not going to be full compat- ibility between the two systems, but there is enough in common that a subscriber can have a single subscription that operates in both environments. The interworking func- tion (IWF) is the functional entity that does the translations. The IWF could be physi- cally located in either system or somewhere between the two systems, but the basic pic- ture is shown in Fig. 15.4.6 6 The most basic form of the IWF is a separate machine that connects to the GSM and ANSI-41 sign