ies, and core network to circuit switched inter-working to IMS and IMS charging. IMS is a broad and evolving standard and it will continue to evolve over the next few years. 3GPP Release 7 and beyond, supported by ETSI TISPANs activities (Telecoms and Internet converged Services and Protocols for Advanced Networks), will improve and even extend the IMS standard towards the principle of fixed-mobile convergence. 9.3.1 IMS Requirements The 3GPP TS 22.228 [1] defines the requirements from both users' and operators' perspectives for the support of IP multimedia applications. Some key requirements are: End-to-end QoS for voice at least as good as the circuit switched voice. IP multimedia sessions may support a variety of media types. Support for roaming. Implementation of IP Policy Control. IP multimedia applications should not be allowed to be standardized, allowing operator specific variations in a vendor independent manner. An architectural framework that enables an operator to efficiently deploy IP multimedia applications in a network-agnostic manner. IP multimedia sessions provide the ability for users to send and receive IP multimedia voice and data applications, even when roaming. This includes inter-working with existing voice and data networks for both fixed (PSTN, ISDN, etc.) and mobile users. The inter-working with existing fixed and mobile voice is supported by IP multimedia CN subsystem. The IP multimedia CN subsystem does not necessarily have to support all services offered by the circuit switched telephony network. The following support is expected: Verification that the user is entitled to use the resources of the IM CN subsystem. A user can register and de-register on the IMS CN subsystem. A user or the network can identify an alternative destination for an IP multimedia session or individual media of an IP multimedia session. A user can either accept or reject an incoming IP multimedia session. A user can suspend an IP multimedia session, and resume that IP multimedia session at a later time. The use