gy could be IP instead of ATM. Besides UTRAN the evolved GSM BSS named GERAN can be connected to the core network with the Iu interface. Thus the traffic coming from GERAN gets the same treatment as the traffic coming from UTRAN in the sense of interfaces. If the operator has the IMS in use, the CN CS domain is not basically needed any more; the main step (besides the new radio access alternatives) between the 3GPP release 4 and release 5 networks is whether to quit the CN CS domain or not. This will depend significantly on the direction and maturation of the VoIP development. Release 6 is an enhancement of earlier 3GPP releases, aiming to bring mobile users a complete 3G experience. The 3GPP release 6 network includes numerous new features, among them being high speed uplink packet access (HSUPA), the second phase of the IP multimedia subsystem (IMS), interworking with wireless local area networks (WLAN), the multimedia broadcast multicast service (MBMS) and enablers for push-to-talk (PoC). Release 6 brings many notable new services to mobile device users. Symmetrical data services enabling two-way high speed data communications for services such as video conferencing and mobile email are specified. High speed uplink packet access (HSUPA) will complement high speed downlink packet access (HSDPA) for a rich, two-way, interactive wireless broadband user experience. HSUPA and HSDPA together will enable symmetrical data communications at a high speed, supporting multimedia, VoIP, etc. The second phase of the IMS comprises all of the core network elements offering multimedia services. The IMS makes it possible for operators to offer mobile users multimedia services using the Session Initiation Protocol (SIP). The SIP enables mobile users to use services based on Internet applications. In release 6, the IMS is developed to support interworking with circuit switched networks, non-IMS networks and 3GPP2 based CDMA systems. Release 6 also defines interworking with wireless local area networks (WLANs). The interworking is