The SIM Application Toolkit (SAT) is a technology that lets the SIM card issue commands to the telephone. These commands range from displaying menus and getting user input Security to sending and receiving SMS messages. The SIM toolkit is excellent for implementing security critical applications, since it allows for custom encryption. One potential draw- back is that the operator owns the SIM card, and to place applications on it you need the operator's permission. What is USIM? USIM (universal subscriber identity module) is the smart card for third generation (3G/UMTS) mobile terminals. The role of SIM cards in 3G concerns the traditional benefits of portability, security and individuality. Mobile commerce is a big driver for security functions in the terminals for authentica- tion and cryptography. Tunnelling In GPRS and WCDMA/UMTS one of the main protection methods is to send end user data in tunnels. GPRS and WCDMA/UMTS use tunnels for signalling and user data traffic. The pro- tocols are called GPRS tunnelling protocol - user (GTP-U) and GPRS tunnelling proto- col - control (GTP-C). GPRS tunnelling protocol (GTP) is used in the internal backbone. If required, IPSec can be used internally as well. See Figure 6.28. Tunnelling towards external networks will only be performed if the interworking case SO requires. The media gateway and the GGSN are the tunnel end points for the GTP-U for packet mode communication. The GGSN tunnel end point is situated on the border between the basic WCDMA/UMTS core network and the ISP's point-of-presence (POP). Destination/source private IP addresses Application server Corp. network GTP tunnel IPSec tunnel Backbone GGSN/FW public IP addresses Figure 6.28 Packet routing and tunnelling Understanding Changing Telecommunications - Building a Successful Telecom Business The SGSN (serving GPRS support node) supports the GTP-C protocol for control sig- nalling between SGSN servers and GGSN. GTP-C is transported by UDP/IP and contains functionality for SGSN media gateway function - GGSN 