Industry Association. 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 authentication center base station cellular subscriber station equipment identity register home location register integrated services digital network mobile switching center public switched telephone network visitor location register Figure 15.1 Original ANSI-41 network reference model. The MSC is the mobile switching center, connected to a collection of base stations (BS) by an A interface not specified in ANSI-41. The base stations are connected to cel- lular subscriber stations (CSSs) by an air interface Um, also not specified in ANSI-41. The MSC is also connected to the public switched telephone network (PSTN) and the Integrated Services Digital Network (ISDN) by A and Di interfaces, also not specified in ANSI-41. (A and D originally stood for analog and digital.) Thus a typical mobile telephone call goes from cell phone to base station to MSC to PSTN to landline tele- phone without using an ANSI-41 link. (ANSI-41 signaling is still used for subscriber account verification and establishing call processing features, however.) It would seem that the entire wireless telephone system is not specified by ANSI- 41-that its designers have ignored every essential part of the system. However, ANSI- 41 is not there to support the essential cellular concept; rather, ANSI-41 is there to 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 enhance wireless telephony by allowing its users to roam freely supported by seamless handoffs from one MSC to another. ANSI-41 is also there to support short message ser- vice (SMS) and over-the-air service provisioning (OTASP) for 