d core network nodes (3G-SGSN and 3G-GGSN, together with subscribers database, the Home Subscriber Server (HSS)). This was referred to as system architecture evolution (SAE). Why did such 3GPP evolution result in a flat architecture? The main reason was the QoS in the mobile networks based on IP, including UTRAN (in 3G) and the mobile core network, i.e. for provision of lower delay in the mobile network which is needed for real-time services over all-IP mobile networks (e.g. carrier-grade VoIP as the pri- mary service in this regard) as well as emerging critical services over a mobile network (e.g. control of driverless vehicles or drones, remote surgery in real-time, and similar examples in the future). 5.1.2 4G Network Architecture The 4G mobile network from 3GPP is LTE/LTE-Advanced. LTE was standardized ini- tially with 3GPP Release 8, together with the common IMS (Internet Protocol multi- media subsystem) for signaling based on SIP and Diameter (for communication with the users' database HSS) as well as with evolved packet core (EPC). Evolution of 3GPP mobile standards is shown in Figure 5.2. Overall, there are three segments that are stan- dardized by 3GPP: High-speed access, which refers to radio access technology. IP core network, which includes all controllers/gateways and databases in the mobile networks as well as their mutual interfaces. Services, which include service overlay network implemented over a given mobile network, with IMS as the central node in the services part. LTE-Advanced was initially standardized as 4G with 3GPP Release 10 and further continued with the following Releases 11-14 (Figure 5.2). In fact, the requirements R99 R4 R12 R13 R14 R15 3GPP radio access networks HSDPA - High speed downlink packet access LTE-A - LTE-advanced HSPA - High speed packet access UMTS - Universal mobile telecommunication system LTE - Long term evolution Figure 5.2 Timeline of 3GPP mobile networks. 5.1 Mobile Ultra-Broadband Network Architectures for 4G were specified by the ITU-R in International Mobile T