andard is used globally. The development and formal acceptance of standards run parallel to the deployment of equipment according to those standards, but the two are not synchronized. When a standard is useful, it is often deployed before it is formally approved. [This was certainly the case with Qualcomm's IS-95, which was used widely on code division multiple access (CDMA) systems before it was accepted as cdmaOne.] ANSI-41 is a valuable and useful standard that allows the mobile switching center (MSC) and other signaling-related equipment to interact on and across cellular networks. As a result, it is being built and deployed worldwide, even though, at the time of publication, it is officially only an American standard. The Ameri- can National Standards Institute is a member of the International Telecommunications Union (ITU), and it is quite likely that the ITU-T (the Telephony Division of the ITU) is considering or will consider adopting ANSI-41 as a formal international standard. ANSI-41 is a moving target as it evolves to meet increasing customer demand for fea- tures and mobility management and increasing system operator demand for opera- tions, administration, and maintenance (OA&M) support. The major third-generation (3G) systems, cdma2000, wideband CDMA (W-CDMA), and UWC-136 (time division multiple access, or TDMA) all use ANSI-41 signaling. ANSI-41 can use one of two signaling environments, X.25 packet protocol or Signal- ing System 7 (SS7). Today X.25-based systems are legacy systems because SS7 is the signaling service of choice, particularly for large carriers that already have SS7 net- works in place. Such ANSI-41 entities as MSCs, home location registers (HLRs), visi- tor location registers (VLRs), and so on connect using SS7 A links to signal transfer points (STPs). Figure 15.1 shows the first version of the ANSI-41 logical network reference model. We know most of the cast of characters already, but there are a couple of new faces. 1 Reproduced under written permission of the Telecommunications 