network network P-CSCF S-CSCF I-CSCF Figure 14.17 CSCF locations for mobile-terminated sessions Understanding Changing Telecommunications - Building a Successful Telecom Business The MRF performs multiparty call, multimedia conferencing and transcoding functions. It is responsible for bearer control (with, for example, the GGSN and MG) in the case of mul- tiparty/conference sessions. The MRF is also responsible for performing announcement functions. The MRF communicates with the CSCF for call control of announcements and conferences. The MRF is separated in two logical functions: MRC (media resource con- trol) and MRG (media resource gateway). The MRG is the media-related part, co-located with the MG. The MRC is the control part, which can be located with the server nodes. MGC controls call-related state information that pertains to the call connection in the media gateway. The MGCF communicates with the CSCF for call information. It requests the media gateway to handle the bearer connection. See Figure 14.15. 'A call between a mobile roaming subscriber and for example, an ISDN subscriber, may involve many control networks. An example with a home control network, a visited control network, a gateway ISDN network and their common connectivity layer is presented in Chapter 15, Section 15.10. 14.8 HLR/HSS FOR ALL PREVIOUS DOMAINS 14.8.1 THE HOME LOCATION REGISTER (HLR) The HLR is a network database for mobile telecommunications. The HLR holds all mobile specific subscriber data and contains a number of functions for managing this data, controlling services and enabling subscribers to access and receive their services when roaming within and outside their home PLMN. The HLR communicates with the SGSN, MSC and other network elements via the MAP protocol. 14.8.2 THE HOME SUBSCRIBER SERVICE (HSS) The HSS is responsible for keeping a master list of features and services (either directly or via servers) associated with each user, and for tracking location and means of access for the subscribers. It holds user-specific inf