unnels can exist between the SGSN and the GGSN. A TID that is unique for that pair of nodes identifies each tunnel. Different network applications on the same mobile could use different tunnels between the GPRS support nodes. The tables in the SGSN and the E-Mail Figure 10-4 The GTP tunnel GPRS PLMN identities. Web Browsing 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 GGSN have identifiers that map a particular mobile address with its NSAPI, Temporary Logical Link Identity (TLLI), and PDP contexts. During handover, when a mobile attaches itself to a different SGSN, the queued packets are tunneled to the new SGSN from the old SGSN. Virtual Private Networks (VPNs) GPRS must support access to private corporate networks. Corporations expect convenient, but secure, access from wireless data networks. Roaming mobile corporate users should have secure, trusted access to the company's data vaults. The term Wireless Virtual Private Network (W-VPN) is used to describe such an environment. We will review key tunneling, authentica- tion, and encryption techniques and ways that GPRS can use them to pro- vide secure corporate network access. Figure 10-5 shows the VPN. A VPN is an extension of an organization's private intranet across a public network (the Internet), creating a secure connection essentially through a tunnel. VPNs securely convey information across the Internet connecting remote users, branch offices, and business partners into the cor- porate network. VPNs are owned by the carriers, but are used by corporate customers as if the customers owned them. A VPN is a secure connection that offers the privacy and management controls of a dedicated point-to-point leased line, but actually operates over a shared routed network. A VPN provides a corpora