 interface is the reference point between the PCEF and OFCS entities. This interface supports the DIAMETER signaling. The S9 interface is the reference point located between the H-PCRF entity located in the home network and the V-PCRF entity located in the visited network. This interface supports the DIAMETER signaling. The Sp interface is the reference point between the PCRF and SPR entities. The protocol used in this interface is not defined. 1.6. DIAMETER routers DIAMETER agent is a DIAMETER router that can reduce the meshing of DIAMETER sessions between different nodes located, on the one hand, in the entities of EPS or IMS networks, and, on the other hand, in the PCC, OCS and OFCS functions and in the HSS and SLF database (Figure 1.11). Databases Charging Roaming interface Figure 1.11. DIAMETER routers VoLTE and ViLTE DIAMETER routing is done on the basis of the operator's domain name to obtain the IP address of the next hop and on the basis of the identity of the destination to obtain the IP address of the DIAMETER node. DIAMETER routing agent (DRA) only performs routing DIAMETER messages and does not analyze the content of DIAMETER messages. The DRA router is deployed in the home network of the operator. DIAMETER edge agent (DEA) performs the routing of DIAMETER messages and control of their content according to rules established by the operator. The DEA router is deployed at roaming interfaces between the visited network and the home network. 1.7. ENUM system The ENUM mechanism allows for using the phone number (E.164 or TEL URI) to determine the network of the called. The ENUM mechanism is based on domain name system (DNS) resolution which converts the E.164 identity or TEL URI to a SIP URI identity containing the domain name of the destination network. The ENUM system is structured in three levels of servers: - level 1: these servers contain pointers to the level-2 servers. The DNS level-1 server manages the e164enum.net domain. The response to the request relating to a phone number contains the identit