es 3) The source eNB entity forwards the information element Handover Command to the mobile in the RRC ConnectionReconfiguration message, which also triggers the handover. 4) 5) The random access procedure comprises the preamble transmission transmitted by the mobile and the MAC RAR frame response of the eNB entity. 6) The mobile connection is finalized when the mobile transmits the RRC ConnectionReconfigurationComplete message. source Target X2-AP HANDOVER REQUEST X2-AP HANDOVER REQUEST ACK RRC ConnectionReconfiguration (Preamble) (Preamble) PRACH (Preamble) PRACH (Preamble) PDCCH (RA-RNTI) PDSCH / MAC RAR (TA, UL Grant, TC-RNTI) PUSCH / MAC (C-RNTI) / RRC ConnectionReconfigurationComplete Figure 3.2. Random access without contention, in case of handover 3.1.3. Data transfer During the attachment procedure, the 4G core network is aware of either of the mobile's support dual 4G and 5G connectivities (radio capabilities). The Mobility and Management Entity (MME) performs the attachment procedure in order to authenticate the mobile. It checks with the HSS (Home Subscriber Server) database whether the customer has subscribed to the 5G-NSA offer. If applicable, when the mobile wishes to establish an end-to-end bearer (E2E), the MME entity will inform the eNB entity that the mobile is authorized to connect to the 5G-NR radio access. The selection of the eNB base station is performed by the mobile according to the PLMN priorities stored on the USIM module. A 5G-NSA mobile will therefore search for an eNB base station that can implement dual connectivity, as a priority. This information is contained in the SIB1 channel broadcast by the base station NG-RAN and 5G-NR 3.1.3.1. Establishment session procedure on the 4G radio access network 1) 2) The mobile performs the random access procedure with the selected serving eNB base station (i.e. the selected eNB when the mobile is in standby mode). The access procedure aims to create a C-RNTI identity, control radio resources on UpLink and allow us to adjust its Timing Advanced 