 Plane IM-MGW Transport Plane 3GPP IP Multimedia Core Network Subsystem CS Networks (PSTN) Figure 9.2 IMS layered model. Note: The architecture is similar to the 3GPP Release 5 IMS architecture SIP Compression: The P-CSCF compresses messages if the user has indicated that he/she wants to receive compressed signaling messages. IPSec Security Association: P-CSCF is responsible for maintaining Security Associations (SAs) and applying integrity and confidential protection for SIP signaling. This is achieved during SIP registration as the user and P-CSCF negotiates IPSec SAs. After the initial registration, the P-CSCF is able to apply integrity and confidential protection to SIP signaling. Interaction with PDF: The P-CSCF is also tasked to pass session and media related information to the PDF (Policy Decision Function). Emergency Session Detection: IMS emergency sessions over GPRS and EPS are expected to be fully specified in Release 9. S-CSCF: The Serving-CSCF (S-CSCF) is the central brain of the CSCF. The operation of the S-CSCF is controlled by the policy stored in the HSS (Home Subscriber Server). It is responsible for users' authentications, registrations and authorizations. It is also responsible for call processing including retrievals of service triggering information and user profiles from HSS, and finally for routing of calls Next Generation Mobile Communications Ecosystem to applications. The S-CSCF also sends accounting related information for online charging purposes (i.e., supporting prepaid subscribers). I-CSCF: I-CSCF is a contact point within an operator's network for all connections destined to a subscriber of that network operator. There are four unique tasks assigned for the I-CSCF: Obtaining the name of the next hop (either S-CSCF or application server) from the HSS. Assigning an S-CSCF based on received capabilities from the HSS. The assignment of the S-CSCF will take place when a user is registering with the network or a user receives a SIP request while one is unregistered from the network but 