r. OTASP and OTAPA require good radio signal because we do not want any mistakes to happen here. We also do not want anybody to overhear these transactions, so a very sophisticated encryption scheme is used for OTASP and OTAPA, the Diffie-Hoffman Key Agreement Standard. Diffie-Hoffman has no public keys to publish and no private 4Although we avoid the term trunks when referring to the wireless network, in the case of inter-MSC pipes, these are dedicated transport channels. 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 Loop- Switch Switch Figure 15.3 Loop-back trunk testing. keys to store securely, but it is computationally complex and typically takes a user ter- minal several minutes to calculate. It is worth this kind of wait to ensure ultrasecure transactions for service provisioning and parameter adjustment. CDMA-based ANSI- 41 networks use the SMS capability to send their OTASP data. Proprietary services can extend the functionality of OTASP and OTAPA. For exam- ple, some cellular services, including Sprint PCS, are able to reconfigure telephones to correct technical problems or update user features across the air interface. The service provider may maintain a database of cellular telephone equipment and its internal tem control codes, making it possible to modify specific equipment in specific ways. 15.6 Interaction with Other Networks Most of the internetwork issues for ANSI-41 networks are solved by SS7. We do not have to worry about network addressing for packet messages, internetwork communi- cation issues, or even international roaming because the worldwide SS7 network has already dealt with most of these issues. There are issues with international numbering plans because the mobile identification number (MIN) is usually the mobile directory number in the United States and Canada. This creates some 