hone services, it will be possible to create new value-added services and conve- nient solutions. In NGN, various services are provided by IMS, which is the service control function located in the service stratum. Introducing IMS enables providing IP-based multimedia services without depending on the access network to mobile phones, Wireless LAN (WLAN) terminals, and SO on. Therefore, in NGN, IMS is expected to be used as the service control architecture common to a fixed and mobile network. 4.2.1.1 NGN Service Stratum The standardization of IMS had been carried on by 3rd Generation Partnership Project (3GPP/3GPP2) taking the leadership, on assumption that the IMS should be used mainly on the mobile network. Then later, ESTI TISPAN and ITU-T reviewed and extended the specification of service platform common to fixed and mobile network, to be used as the service control function for NGN. Today, 3PPP reviews the specification of functions common to fixed and mobile networks, as common IMS. IMS consists of components such as Call Session Control Function (CSCF) controlling session and services, Multimedia Resource Function (MRF) con- trolling multimedia resources, Home Subscriber Server/Subscription Locator Function (HSS/SLF) executing user profile management, Media Gateway Function/Media Gateway Control Function (MGF/MGCF) performing inter- work with existing networks, and Application Server (AS) providing applications used in IMS. Out of these components, the CSCF is the server controlling ses- sions and services via Session Initiation Protocol (SIP). The CSCF performs access control of user terminals, roaming control, and activation of services provided by AS, playing the central role in the IMS architecture. Figure 4.1 shows the NGN architecture overview. Detailed architectural descriptions can be found, for example, at the International Telecommunication Union (ITU) [1] as well as 3GPP [2]. 4.2.1.1.1 S-CSCF Using the user profile information and user location information managed by HSS, Serving-CSCF (S-CSCF) pe