MC-to-MC M2 interface and the SME-to-SME M3 interface. SMS is designed into ANSI-41 as an integral part of today's wireless service. The Internet has been added in the form of the public packet data network (PPDN) with an unspecified P interface. The interworking function (IWF) provides protocol conversions between packet-switched and circuit-switched entities. An example is con- verting circuit-based voice data streams from subscriber calls to packet-based voice over Internet Protocol (VoIP) to be sent over the Internet. 3 Portable telephone numbers have been added in the unspecified Z interface to the number portability database (NPDB), which is standardized in TIA/EIA/TS-756. This is for telephone numbers formerly owned and administered by a wireless service provider and now belonging to subscribers served by another wireless or a landline provider. The OTASP function uses ANSI-41 SMS operations to transfer customer information between the serving VLR and the HLR to support a new subscription. This allows cus- tomers to buy a cellular telephone and have it automatically activated when it is first 2Reproduced under written permission of the Telecommunications Industry Association (TIA). Superscript(3)There is a separate interworking function, discussed in Sec. 15.6, that translates between ANSI-41 networks and GSM networks. 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 Key Telephony Concepts OTASP authentication center base station customer service station equipment identity register home location register integrated services digital network interworking function message center mobile station mobile switching center number portability database OTASP over-the-air service provisioning function public packet data network public switched telephone network short message entity visitor location register Figure 15.2 The most recent ANSI