 con- sidered, a corporation should evaluate several factors unique to the wireless world. Security aspects may be the foremost concern, especially for what might appear to be the most vulnerable portion of the mobile-corporation path: the air link. Time Division Multiple Access (TDMA) and the other GPRS access technologies have designed the mobile network SO that the packet data traffic is protected by encryption over the air. This improves performance, as many end-to-end encryption methods add extra bytes to each packet sent over the air. They also interfere with the data compression techniques implemented between the mobile system and the GPRS opera- tor's network. The availability of VPN service for roaming users should be discussed. Some corporations will only want their networks accessed using selected wireless operators or from selected geographical locations. Multinational corporations may decide that roaming users should connect with local VPN access points. The performance of the air link, especially the throughput seen by data users, varies. Although the GPRS air link has multiple meth- ods to ensure reliable air-link data transmission, factors such as fading and multipath may reduce performance. Enhancements to the air link and net- work infrastructure to meet enhanced quality of service (QoS) requirements are underway in standards organizations. VPN-Service Provider Independent (SPI) End-to-end, voluntary tunneling technology is used to support corporate- based VPNs. The best example of this is IPSec, an Internet Engineering Task Force (IETF) standard. The IPSec tunnel is from a gateway server that matches security parameters with client software on the user's PC, which is 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. SGSN-to-GGSN (Gn) and GGSN-to-PDN (Gi) Interface SGSN-to-GGSN (G) and GGSN-to-PDN (G) Interface distributed by