chmarking purposes each application was tested individually on two different smart- phones with different OSs. All active tests consisted on mobile-to-mobile calls in lab envi- ronment in good radio conditions and without additional load. External tools were connected to the UEs and to different network interfaces in order to collect key performance indicators (KPIs) such as battery consumption, voice quality, delay, throughput and network signalling. 16.7.1 VoLTE System Architecture The VoLTE system enables Voice over IP in a LTE network with QoS. LTE network consists of evolved Node B (eNodeB), a mobility management entity (MME) as well as Serving and Packet Data Network Gateways (SGW and PGW, respectively). The MME and S/PGW form the so-called Evolved Packet Core (EPC). Policy and Charging Resource Function (PCRF) enables QoS differentiation for VoLTE calls. Core IMS functionality is found in the Call Session Control Function (CSCF). Actual IMS services such as VoLTE and Rich Call Services (RCS) are provided by specific Application Servers (AS). Due to the interworking requirements, there are interfaces from the IMS system and EPC to the CS core and to other relevant control functions such as Media Gateway Control Function (MGCF). Important interworking scenarios are voice calls between VoLTE and CS-only devices as well as call continuity between LTE and 2G/3G coverage areas. Finally, it is worth noting that subscriptions for both LTE and IMS services are managed in the home subscriber server (HSS). VoLTE system architecture is illustrated in Figure 16.21. More details about VoLTE architecture can be found from References [3] and [4]. VoLTE optimization aspects are considered also in Reference [5]. Communication core HLR/HSS VoLTE AS MSC-S RCS AS 2G/3G radio BTS/NB BSC/RNC LTE radio LTE packet core Internet S/PGW Figure 16.21 Overview of VoLTE system architecture LTE Small Cell Optimization Invite 100 Trying 183 Session progress Ringing Figure 16.22 SIP call establishment signalling flow 16.7.2 VoLTE Performan