rforms routing based on terminating address specified by originating user (SIP-URI, telephone number, etc.) to establish, manage, and release the sessions between originating and terminating users. Also, via IMS Service Control (ISC), the SIP-based standard interface, the S-CSCF is connected to common enablers shared by different services (general-purpose functions used to realize each service such as presence and messaging), and the AS performing control for each service. Therefore, S-CSCF interworking with multiple ASs enables the Telecom operators to provide various services in a flexible Service Delivery Platforms Session control Media and interworking Application server Service stratum Internet Core network Transport control functions Other mobile Transport stratum networks Figure 4.1 System view of the NGN and IMS architecture. manner. Moreover, as the solution to easily realize cooperation among various ASs, the ISC function provided by the S-CSCF can be converted via a gateway into logical API, which is more accessible from applications. Through such API, various ASs can use the functions provided by the IMS such as setting and releas- ing of sessions, and acquisition of user location information, development, and deployment of services are simplified and accelerated. In NGN, the application service support function located in the service stratum realizes this function, enabling connection to external applications provided by a third party via Application-Network Interface (ANI). 4.2.1.1.2 P-CSCF Proxy-CSCF (P-CSCF) is connected to a user terminal via the access network, and it is allocated upon the connection setup with the user terminal. In W-CDMA networks, the connection-to-user terminal is established via a packet switch, which is called Gateway GPRS Support Node (GGSN). In fixed broadband access and WLAN access, the connection is established via a relay router in the access IP network. The user terminal connected to the access network is authenticated at connection setup. An IPSec tunnel is set up wi