media gateway transport control function signalling GW multimedia signalling protocols IM-MGW PSTN/ Media gateway Figure 14.15 IMS functions and other capabilities The BGCF is a signalling entity for call/session control and acts as a router (at the SIP level) to another SIP entity. When multiple MGCFs exist within the network operator's network, BGCF acts as a 'SIP in, SIP out' entity trying to find the best MGCF for interworking with the circuit-switched network for a call from a UE to a PSTN address. A CSCF in a network may perform one or more of the following roles: Interrogating CSCF (I-CSCF) - the I-CSCF function interrogates an external location-service function to determine which CSCF is serving at a given time and acts as a SIP proxy firewall to fulfil network security and privacy functions. Serving CSCF (S-CSCF) - the S-CSCF is the serving network element with which subscribers register in order to be reached when roaming. It temporarily stores user profile-related data, which is downloaded from the HSS as registration takes place. The S-CSCF also triggers the call- and session-related services to which the user has subscribed. Control Network IP/SIP services with: Security Charging Combinations Mobile convergence connectivity network router Internet Figure 14.16 The IMS domain position. IP networks used to be stupid in the central network part, but with IMS we are heading towards a central intelligence again Proxy CSCF (P-CSCF) - the P-CSCF, which contains a very limited CSCF function, is the only Internet multimedia function placed in the network being visited; that is, when the subscriber's unit is roaming outside of its home network. The P-CSCF con- tains address translation functions in order to proxy the session request directed at the I-CSCF in the home network. Figure 14.17 shows possible CSCF locations. The upper half of the diagram shows the UE served at the home network and the lower half of the diagram shows the UE roaming to a visited network. Home network P-CSCF S-CSCF I-CSCF Visited Home 