 elements mainly involved in actual switching and handling of user data. The control plane network elements perform call control, subscriber mobility management, charging and MGW control functions. The 3G release 4 architecture introduces several new network elements, i.e. the application server (APSE), the MSC server (MSS), the gateway control server (GCS), the media gateway (MGW) and the circuit switched data server (CDS). Core Network Planning and Optimisation Application Plane CAP/INAP CAP/INAP Control Plane MSS/GCS MSC Server BICC-CS-2 / SIP-T Sigtran/ Sigtran/ Megaco, H248 Megaco, H248 User Plane IP/ATM/TDM GSM/EDGE BSS Backbone lu-CS 3G RAN Figure 4.8 3G release 4 network architecture MSC Server (MSS) As mentioned earlier, in the MSC server concept, the user plane and the control plane are separated, as introduced in the 3GPP specification TS 23.205. Separation of the two planes in the core network means that 2G/3G MSC functionalities evolve SO that user plane traffic switching functionality is devolved to a new user data switching element called the media gateway (MGW). The MSS and GCS elements maintain call control, mobility management and MGW control functionalities of the network. The MSS/GCS also handle the connection of media channels in the MGWs. The MSS has several main functionalities: Gateway MSC (GMSC) functionality for call control Visited MSC (VMSC) functionality for mobility management Media gateway control function (MGCF) functionality for media gateway control Switching functionality available in an integrated MSC server solution Interworking function (IWF) functionality is also available for integrated MSC server solution The GCS is a new network element in the MSS solution and provides for interconnections with PSTN and external networks. The GCS is intended for operators with high transit traffic in the network and thus require a transit layer or dedicated PSTN access for IP telephony. The main functionalities of the GCS GMSC functionality MGCF functionality The standard also introduces 