le access networks the terminal can change its point of attach- ment during an ongoing voice call (that is called a handover), which requires additional mobility management support regarding QoS (e.g. to have the same QoS in the new cell in the mobile network, after the handover, as was in the old cell, before the handover). QoS-enabled mobile VoIP in the NGN uses service control functions, mobility man- agement and control functions, network attachment control functions, and resource and admission control functions, as shown in Figure 6.1. The SCFs perform mobile terminal registration in the network, service establishment/ service release, by using the CSCFs of the IMS [1]. The VoIP service is always established by using signaling messages, unlike the legacy Internet client-server communication. Why? Because in the case of voice, a caller user (the originating user terminal, i.e. user A) requires to establish a connection with the called party (the terminating user termi- nal, regarding the given call, typically denoted as user B) by using a certain addressing scheme (e.g. ITU-T E.164 numbering schemes are used since the PSTN era, recently updated with ITU-T E.212, and continue to be used for operator's VoIP, while OTT VoIP service providers typically use different proprietary addressing schemes). 6.1 QoS-enabled VolP Services NGN operator A NGN operator B 2) Service request processing 5) Service request processing 3) URL query/ response; S-CSCF S-CSCF 4) Service request P-CSCF I-CSCF I-CSCF P-CSCF 10) Service response 11) QoS setup 8) QoS setup 13) Service 7) Service response 12) QoS setup 9) QoS setup response to network 1 to network 2 Access Access Transit network B network B network A network network B (e.g., 5G) (e.g., FTTH) 1) Service Originating Terminating 6) Service request mobile terminal mobile terminal request HSS - Home subscriber server NGN - Next generation network FTTH - Fiber to the home P-CSCF - Proxy call session control function I-CSCF - Interrogating call session control function RACF - Reso