the VCC UE and the DTF. E2E Network Architecture and Mobility Management VCC UE P-CSCF Multimedia Networks Unspecified interface Domain CAMEL Selection Service Function VCC Application Domain Adaptation Transfer (ISDN Function Function Part) I-CSCF S-CSCF PLMN/ Figure 11.16 VCC reference architecture. Reproduced with permission from 2007 3GPP [18] are Camel Service, Domain Selection Function, CS Adaptation Function and Domain Transfer Function. An application server supporting the VCC application will implement one or more of these functional roles. The functional entities of the overall VCC architecture are as follows: User Equipment: A VCC capable UE will have an active VCC subscription. The VCC enabled UE will support voice both over IMS and CS domains. Domain Transfer Function (DTF): The DTF uses the ISC reference point towards the S-CSCF for execution of the domain transfer functions. The functions include maintaining information describing the current domain, maintaining domain transfer policies, preserving information for delivering calling line identity, and SO on. Domain Selection Function (DSF): The DSF provides selection of the domain to be used for delivery of a VCC UEs incoming call, as specified by operator policy or user preferences. The DSF performs functions like determining IMS and CS registration statuses, retrieving current domain used for VCC UE, and SO on. CS Adaptation Function (CSAF): The CSAF acts as the VCC UEs proxy into IMS for: CS originated calls and CS legs established for domain transfers to CS. It also communicates call related data from the CS domain to IMS. CAMEL Service: CAMEL (Customized Applications for Mobile networks Enhanced Logic) service function operates either independently or in collaboration with the CSAF. It performs functions like Next Generation Mobile Communications Ecosystem Applications Media (Operations Packet Core Gateway Support System) CS Core PacketCable Mobile Network PSTN/ISDN (DSL) (3G/3.5G) packet switched 2G (CS) Figure 11.17 IMS-based FMC (long term)