aling radio bearer to the eNB. A signaling radio bearer (SRB) is used for the transmission of RRC and NAS messages. The establishment of a RRC connection starts via the random access procedure. The procedure is initiated by the mobile and the radio link is controlled by the base station. During the UE attach, the eNB transmits a radio configuration to the UE. The radio configuration is used to define the type of measurements to be performed on the LTE and NR (New Radio) radio accesses for dual connectivity. When the UE requests dedicated SRB and DRB (Data Radio Bearer) to exchange message/data with the network, the eNB applies a RRC Connection reconfiguration procedure. This procedure aims to establish a UE context to define a radio identification in the connected mode (C-RNTI: Connected Radio Network Temporary Identifier), and an E-RAB (E-UTRAN Radio Access Bearer) for one-to-one mapping between the E-RAB and EPS bearer. For dual connectivity, two radio bearers are set up, one between the mobile and the master MN (Master Node) and the other between the mobile and the secondary SN (Secondary Node). NG-RAN and 5G-NR: 5G Radio Access Network and Radio Interface, First Edition. Frédéric Launay. C ISTE Ltd 2021. Published by ISTE Ltd and John Wiley & Sons, Inc. NG-RAN and 5G-NR In the case of the 5G-NSA option 3x, the MN node is the eNB (evolved Node Base Station) entity and the SN node is the en-gNB (E-UTRAN/NR new Generation Base Station) entity. A traffic tunnel is set up between the en-gNB entity and the 4G EPC (Evolved Packet Core) network. 3.1.1. LTE search procedure The selection of the 4G radio access network is initiated by the mobile. When the mobile switches on, it searches for the synchronization signal. The synchronization signal is periodically transmitted by each eNB. If the mobile was previously attached to a PLMN (Public Land Mobile Network), it listens to the frequency that was used (and stored on the SIM card) before detachment. If the mobile is switched on for the first time, or in the case of roa