e Data plane Service stratum Transport stratum 3.3 Service Architecture in NGN Applications Service stratum Application/service support functions Service control and content delivery functions Service profiles Transport control functions Transport functions Transport stratum Media CDF - Content delivery functions Control MMCF - Mobility management control functions NACF - Network attachment and control functions Management RACF - Resource and admission control functions SCF - Service control functions Figure 3.3 NGN functional architecture. service stratum (service control functions (SCFs)), and with customer equipment. A single NACF can be used for multiple access networks connected to the same NGN core, but it is not required in the NGN architecture (it is a network design choice). Service Architecture in NGN The main part of the service architecture in NGN is targeted at standardized signaling systems as a replacement for SS7 (Signaling System 7), which was the legacy in digital telephone networks (PSTNs). Although SS7 was a packet-based system, it was not an IP-based one. With the transition to all-IP networks, the SS7 traffic started to be tun- neled over IP-based core networks (e.g. in many mobile operators' core networks in the 2010s). The standardized signaling in the all-IP environment for legacy as well as for innovative services is based on the Internet Protocol multimedia subsystem (IMS). IMS was initially standardized with 3GPP Release 5 in a form of an application development environment. 3GPP Release 7 has further directed IMS usage focus toward telephony replacement in IP-based networks, regardless of access type (i.e. fixed or mobile). The common IMS version was standardized in 3GPP Release 8, which implemented different requirements from all other bodies for standardization of mobile 3 QoS in NGN and Future Networks and fixed networks (ITU, 3GPP2, etc.). Although IMS is standardized by the 3GPP, it is included as an integral part of the NGN service stratum [4]. Also, it has become a legacy appro