elecommunications (IMT)-Advanced umbrella, and in 2010 the assessment of all candidates for 4G mobile broadband, i.e. IMT-Advanced, resulted in the acceptance of two technologies as 4G: LTE-Advanced (from the 3GPP); Mobile WiMAX 2.0 (i.e. Wireless MAN-Advanced), with IEEE 802.16m standard in the radio access interface. The development in the 2010s showed market dominance by the LTE/LTE-Advanced mobile standards, driven by the evolution of the predecessor 3G/3.5G technolo- gies (UMTS/HSPA). Although LTE was initially referred to as 3.9G because it did not satisfy the required 1 Gbit/s downlink speeds (initially required for 4G mobile networks), in practice LTE is marketed as 4G technology because it is almost the same as LTE-Advanced, which provides higher bitrates with carrier aggregation (i.e. simultaneous use of single IP layer over multiple underlying LTE frequency carriers in adjacent and non-adjacent spectrum bands). 3GPP Release 8 also introduced evolved packet system (EPS), which consists of a flat-IP core network EPC and Evolved Universal Mobile Telecommunication System Radio Access Network (E-UTRAN) as LTE-based RAN. However, the transition from hierarchical to a flat architecture started with 3GPP Release 7 with direct tunnel, and it was completed in 3GPP Release 8, then continued in further 3GPP releases (Figure 5.3). Base stations in LTE/LTE-Advanced mobile networks, eNodeBs (i.e. eNBs), which are the main nodes in E-UTRAN, are connected directly to the EPC core network gateway via the "S1 interface" (Figure 5.4). Additionally, adjacent eNodeBs are directly connected through an "X2 interface," which provides fast handovers by rerouting the IP packets from the old eNodeB toward the new eNodeB after the handover execution (which is ini- tially managed by the adjacent eNodeBs over the X2 interface). This approach provides lower handover delay because otherwise it would take longer to establish communi- cation with centralized nodes in EPC and then to instigate rerouting (also, certain IP packet losses ma