Here we explain how voice over long-term evolution (VoLTE) user equipment (UE) obtains required Internet protocol (IP) connectivity [evolved packet core (EPC) authentication, IP address allocation, retrieval of packet data network (PDN) subscription context, default bearer activation] and discovers both that Internet protocol multimedia subsystem (IMS) based voice over Internet protocol (VoIP) is possible and what is the address of the IMS entry point. We will further explain the required radio and EPC bearers for VoLTE and associated quality of service (QoS) properties. In addition from a long term evolution (LTE) point of view, we cover packet scheduling, power saving features mobility concepts, circuit switched fallback (CSFB) options (CSFB call cases will be covered separately in Section 5.8), expected UE capabilities and UE positioning. Furthermore here we introduce two different identity modules [universal subscriber identity module (USIM) and IP multimedia services identity module (ISIM)], IMS identities, IMS service execution and IMS telephony service which are discussed further in the next chapter.
4.2 Radio Functionalities
This section describes the main radio functionalities that are needed to carry high quality voice traffic efficiently in LTE radio. First, packet scheduling and QoS differentiation are discussed, which are relevant functionalities from the voice capacity and quality perspective. Second, the mobility solutions ...