urce and admission control function MMCF - Mobility management control function S-CSCF - Serving call session control function NACF - Network access control function SCF - Service control function Figure 6.1 Voice service in the next generation network (NGN) environment. For voice call establishment in the NGN, the originating user terminal (i.e. terminal A) sends a request for establishing a voice call to the P-CSCF, which then forwards it to the S-CSCF, and it finally reaches the I-CSCF of network operator A (which is needed in cases when the called user, B party, belongs to another operator). To obtain the IP address of the I-CSCF in the target operator B, the I-CSCF node of operator A performs DNS resolving of the domain name of operator B. Let's say the called party has telephone number +38912345678@operatorB.example.com (which is a typical addressing scheme for SIP signaling, used in IMS and in NGN). The I-CSCF node of operator B receives the call request from operator A and forwards it to the S-CSCF entity in its IMS, which further obtains the P-CSCF to which the called user B is registered in the operator B network. Finally, the service request from user A is forwarded to called user B (to termi- nating user terminal) and its response is carried back to the operator A network by using the reverse path back. Each P-CSCF in both networks (operator A and operator B) com- municates with the RACF for QoS provision, including admission control (for the given voice call) and transfer of QoS setup information (via QoS intra-network signaling) in 6 Services in Fixed and Mobile Ultra-Broadband NGN operator A NGN operator B Access Transit Access network A network network B (e.g. 5G) network A network B (e.g. FTTH) equipment equipment QoS signaling for VolP Voice user data RACF - Resource and admission control function Figure 6.2 VolP transfer over NGN. the transport IP networks (which is done independently in each of the two operator networks, i.e. on both ends). After the VoIP call is established by using SIP signa