he media streams between subscribers and between sub- scribers and IMS media (announcements) generating and processing functions. The Session or Control Layer controls access to services and provides operator control over subscrip- tions and services. The Application Layer consists application servers (presence servers, messaging severs, etc). It also needs to be noted that all the IMS entities are located in the 3GPP core network architecture as shown in Figure 9.2. The service requirements of the 3GPP IMS (IP Multimedia Core Network Subsystem) are specified in 3GPP TS 22.228 [1] and the technical details can be found in 3GPP TS 23.228 [2]. This section describes the key functions of these IMS entities which can be roughly classified into following categories [3-5]: A signaling routing entity called Common Session Control Function (CSCF). Databases namely Home Subscriber Server (HSS) and Subscription Locator Function (SLF). A number of supporting and inter-working functions like application servers, Media Resource Func- tion, and SO on. 9.3.2.1 The CSCF is used to process SIP signaling packets in IMS. It consists of three controlling functions namely Proxy CSCF (P-CSCF), Serving CSCF (S-CSCF) and Interrogating CSCF (I-CSCF). All the three CSCFs play important roles during registration and session establishment as discussed next: P-CSCF: The P-CSCF is the first point of contact within the IMS for a user. It means that the entire SIP signaling traffic from the user will be sent to the P-CSCF and the entire terminating SIP signaling from the network will be sent via P-CSCF to the user. The P-CSCF may be located in the home or visited network. The P-CSCF ensures that registration of the user is passed to the correct home network and that SIP session messages are passed to the correct S-CSCF once registration has occurred. The P-CSCF has been assigned the following four main tasks: IMS, Services and Applications SIP AS IM-SSF OSA-SCS Application Servers Application Plane IPv4/IPv6 S-CSCF I-CSCF P-CSCF Control/Session