 The 3G MSC is a central network element for release 99 mobile networks. The release 99 3G MSC consists of two network elements, the MSC and MGW. The MSC contains the visitor location register (VLR) and service switching point (SSP) functionalities. The 3G MSC is able to serve the 3G radio access network (RAN) by supporting the Iu interface through a new network node, the ATM module (otherwise known as the MGW), and acting as a multiplexing point for the Iu interface, SO that one Iu interface connection is needed from the RNC. The A interface towards the GSM BSS is supported by the 3G MSC, as required. The MSC function- alities can be categorised into three main areas: Call control and switching functionality Visitor location register (VLR) Service switching point (SSP) Network Evolution Internet GPRS + 3G (TCP/IP) mobile Co-sited GSM + 3G Network Subsystem Base Station Subsystem (GSM+3G) GSM / 3G GSM BTS mobile Modul PSTN/ISDN mobile 3G (WCDMA) IN SCP Figure 4.7 3GPP (Third Generation Partnership Project) release 99 network architecture Release 99 MGW The main function of the release 99 MGW (ATM module) is to provide UMTS terrestrial radio access network (UTRAN) interworking with the mobile switching centre (MSC). The release 99 MGW also provides transcoding (TC) services for calls originating in the UMTS network. The new interfaces introduced to the network are: the Iu interface connecting the RNC to the MGW; the A interface connecting the MGW to the MSC. The MGW's main functionalities in 3G release 99 environments can be categorised as: the IWU function for the ATM to TDM transcoding; signalling Gateway functionality for the RANAP to BSSAP conversion; the ATM interface to RAN connectivity. 4.3.3 3GPP Release 4 Network Release 4 Network Architecture The next step of network evolution contains the 3G release 4 networks. The network architecture of a 3G release 4 network comprises three functional planes: the user plane, the control plane and the application plane (Figure 4.8). The user plane contains all network