 inter- working function at the PCUSN to SGSN that is converted to ATM in other stages of transition across the network. 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. CHAPTER SGSN-to- GGSN (Gn) n and GGSN-to-PDN (Gi) Interface SGSN-to-GGSN (Gn) and GGSN-to-PDN (Gi) Interface Chapter 10 Objectives Upon completion of this chapter, you should be able to Describe the SGSN-to-GGSN interface. Discuss the GPRS Tunneling Protocols. Understand the way GPRS provides data security across the PLMN. Describe the components that can assist in securing the data. Understand why ETSI chose the use of IPSec and other Layer 2 protocols. GPRS Tunneling Protocol (GTP) The GPRS Tunneling Protocol (GTP) is the protocol between GPRS Support Nodes (GSNs) in the Universal Mobile Telephone Systems/General Packet Radio Systems (UMTS/GPRS) backbone network. It includes both the GTP signaling and control (GTP-C) and user data transfer (GTP-U) procedures. The two different types of tunnels deal with either network signaling and control for control purposes and for actual user data. GTP is defined for the Gn interface, the interface between GSNs within a Public Land Mobile Network (PLMN), and for the Gp interface, the inter- face between GSNs in different PLMNs. Only GTP-U is defined for the Iu interface between the Serving GPRS Support Node (SGSN) and the UMTS Terrestrial Radio Access Network (UTRAN). On the Iu interface, the Radio Access Network Application Part (RANAP) protocol is performing the con- trol function for GTP-U. Figure 10-1 shows the reference model for the GTP. GTP enables multiprotocol packets to be tunneled through the UMTS/GPRS backbone between GSNs and between SGSN and UTRAN. In the signaling plane, GTP specifies a tunnel control and management pro- tocol (GTP-C), which enables the SGSN to provide packet data network (PDN) access for a mobile system.