LTIMEDIA CORE NETWORK FUNCTIONS See Figures 14.15 and 14.16. The IM core network functions include: Call state control function (CSCF) - Basic functions includes call routing, SIP session set up, interface to media gateway, HSS and MRF for call connection and control. Home subscriber server (HSS) - HSS contains authorization and authentication infor- mation for a user, terminal-based information, and information about the SGSN/VLR that a user is currently being served by. Other functions such as border gateway control function (BGCF), media resource func- tion (MRF) and media gateway control function (MGCF). Understanding Changing Telecommunications - Building a Successful Telecom Business Legacy R-SGW mobile signalling network roaming signalling Breakout gateway control function Service control Call state control function Multimedia Applications IP network services Subscriber Server multimedia resources function T-SGW UTRAN 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-rela