lding blocks of the radio network in forthcoming chapters. 3.2.6.1 lu Interface Inter-Working Characteristics The Iu principles presented in [5] apply to PS and CS networks. In this context, UTRAN supports two logically independent signalling flows via the Iu interface to combined or separated network nodes of different types like MSC and SGSN [3]. Thus, UTRAN contains domain distribution function routing application independent UE control signalling to a corresponding CN domain. The UE indicates the addressed application type through a protocol discriminator for example. Then UTRAN maps this onto a correct Iu instance to forward signalling. UTRAN services, including radio ac- cess bearers, are CN domain independent, e.g. we can get speech bearer either through the PS or CS core network. The Iu includes control and user planes. Because only a RNC can identify the actual packet volume successfully transferred to a UE, it indicates the volume of all not transferred downlink data to the 3G-SGSN SO this latter can correct its counter. 3.2.6.1.1 Iu Control Plane Both PS and circuit CS domains use the Signalling Connection Control Part (SCCP) protocol to transport Radio Access Network Application Part (RANAP) messages over the Iu interface. Likewise, both SCCP and RANAP protocols comply with ITU-T rec- ommendations. In R99, SCCP messages in CS domain use a broadband SS7 stack com- prising MTP3b on top of SAAL-NNI. In the PS domain UMTS specs allow operators to chose one out of two standardized protocol suites, i.e. broadband SS7 stack comprising MTP3b on top of SAAL-NNI or IETF/Sigtran CTP protocol suite for MTP3 users with adaptation to SCCP. Figure 3.5 illustrates the different RANAP stack options. RANAP MTP-3b IETF/Sigtran CTP Protocol SCCP/MTP3 users module SAAL-NNI Figure 3.5 Stack options in the RANAP protocol. 3.2.6.1.2 Iu User Plane The user plane towards the IP domain works based on an evolved Gn interface, where we achieve tunnelling of user data packets over the Iu interface through the user plane part of GT