 the GPRS (General Packet Radio Service) tunnel control protocol (GTPv2-C). The S5 interface is the reference point between the SGW and PGW entities for signaling via the GTPv2-C protocol and traffic data (IP packets) via the GPRS tunnel protocol user (GTP-U). The S10 interface is the reference point between the MME entities for signaling, via the GTPv2-C protocol. The S1-U interface is the reference point between the eNB and SGW entities for traffic data, via the GTP-U protocol. The SGi interface is the reference point between the PDW entity and the PDN network (Internet). The X2 interface is the reference point between two eNB entities for signaling, via the X2-AP protocol (Figure 1.4) and for incoming traffic data via the GTP-U protocol when mobile changes cells (Figure 1.5). X2-AP X2-AP eNB cible eNB source Figure 1.4. Protocol architecture of the X2 interface: control plane The bearer established between the two eNB entities is unidirectional (eNB source to eNB target). It allows for the transfer of traffic data received VoLTE and ViLTE from the SGW entity to the target eNB entity. It is established temporarily, for the time of the handover of the mobile. GTP-U GTP-U GTP-U GTP-U GTP-U Bearer Bearer Bearer LTE-L1 LTE L1 eNB cible eNB source Figure 1.5. Protocol architecture traffic plane during handover based on the X2 interface 1.1.3. Bearers 1.1.3.1. Bearer structure The EPS network carries traffic data (IP packets) transparently to the PGW entity that performs packet routing. The IP packet is carried in bearers constructed between EPS network entities (Figure 1.6). EPS Bearer Radio Access Bearer Radio Bearer S1 Bearer S5 Bearer GTP-C Figure 1.6. Construction of the bearers Network Architecture The data radio bearer (DRB) is constructed between the user equipment (UE) and the eNB entity. The RRC signaling, which is exchanged between the mobile and the eNB entity, is responsible for constructing this bearer. The S1 bearer is constructed between the eNB and SGW entities. The S1-AP signaling, exchanged between