P over UDP/IP. The tunnelling protocol corresponds to an evolution of the user plane part of the GTP protocol used in GPRS stacked on top of UDP/IP. When The UMTS Network and Radio Access Technology transport data uses ATM PVCs, the Iu IP layer provides Iu network layer services such as routing, addressing, load sharing and redundancy. This leads to an IP network con- figured to transfer Iu data units between RNSs and 3G-SGSNs. We can access common layer 2 resources between UTRAN and the IP domain of a CN through one or several AAL5/ATM permanent VCs. More than one permanent AAL5/ ATM VCs, for example allows load sharing and redundancy. The UMTS user data plane in the network consists of two tunnels, i.e. a first IP/UDP/GTP tunnel between RNC and 3G SGSN on the Iu interface and a second IP/UDP/GTP tunnel between GGSN and 3G SGSN on the Gn interface. The double tunnel architecture provides hierarchical mobility, allows direct RNC con- nection to the IP domain backbone, ensures traffic routing through 3G-SGSN to per- form appropriate charging and legal interface functions, and it also makes room for fu- ture exploitation of Iu and Gn interfaces. The protocol stack is shown in Figure 3.6. Iu-PS GTP-U GTP-U GTP-U GTP-U UDP/IP UDP/IP UDP/IP UDP/IP 3G-SGSN 3G-GGSN Figure 3.6 Protocol architecture for the IP domain user plane. Specifications in [3] outline user data retrieval principles in UMTS and at GSM-UTMS handover for the PS domain. In the following we cover the Radio Access Domain, which in part will cover UTMS Mobility Management (UMM) and UMTS call control to complete the context of interoperability between 2G and 3G systems, i.e. GSM and UMTS. THE ACCESS NETWORK DOMAIN While the access network domain incorporates elements starting the physical layer and radio protocols, here we concentrate on the UTRAN as part of the radio access network. 3.3.1 UTRAN Architecture The UTRAN as illustrated in Figure 3.7, contains Radio Network Subsystems (RNSs) communicating with the Core Network (CN) through the Iu interface. In