em PSTN - Public switched telephone network ISDN - Integrated services digital network S-CSCF - Serving call session control function MGCF - Media gateway control function SLF - Subscriber location function MGW - Media gateway Figure 3.4 IMS architecture. many implementations P-CSCF also behaves as a user agent (UA), with the aim of hiding IMS functions from end-user terminals. Serving-call session control function (S-CSCF). This is the central node in the IMS architecture which has SIP server functionalities. The main function of this node is session/call control. S-CSCF behaves also as a SIP registrar because it accepts regis- tration requests and then makes the registration information available via the location server (that is, the home subscriber server (HSS)). Also, it binds the public IP address (allocated to the network interface of the end-user equipment) with the SIP address. Unlike P-CSCF, S-CSCF is always located in the home network of the subscriber and it uses Diameter protocol to access the HSS database with the aim of obtaining 3 QoS in NGN and Future Networks user profiles. A given telecom operator may implement several S-CSCF nodes in the network for the purposes of load balancing and smaller round-trip time for signaling. One should note that S-CSCF may also behave as a SIP proxy server or a SIP user agent to independently initiate or terminate SIP sessions, and to support interaction with service platforms. Interrogating-call session control function (I-CSCF). This is a SIP location function located at the edge of an IMS network. I-CSCF is the first contact for all incoming messages from other IMS-based networks, SO the IP address of the I-CSCF node is the one that is stored in the carrier-grade DNS, SO terminating calls/sessions from IMS in another network can find the destination IMS (where the called party is located). However, all SIP requests that are received by an I-CSCF from other networks are routed toward S-CSCF as the central node of the IMS. Besides the three CSCFs, the "core" part