nectionSetupComplete RRC Security Mode Command RRC Security Mode Complete RRC Connection Reconfiguration RRC Connection Reconfiguration Complete Traffic Figure 3.3. Procedure for establishing a DRB 3.1.3.2. Establishment of dual connectivity option 3X The procedure for adding a secondary node (en-gNB entity) is initialized by the eNB entity (master node). It is used to establish context at the en-gNB entity to provide radio resources to the mobile. For SCG (Secondary Cell Group), a new radio resource Pcell is required. The SCG is a group of serving cells assigned by the secondary cell, like the MCG (Master Cell Group) assigned by the master cell. en-gNB Traffic SSB signal Measurement reports X2-AP SgNB Addition Request X2-AP SgNB Addition Request ACK RRC Connection Reconfiguration RRC Connection Reconfiguration Complete X2-AP SgNB Reconfiguration Complete X2-AP SN Transfer Status Data Transfer Synchronization and RACH Procedure Data transfer Figure 3.4. Procedure to add a secondary node. For a color version of this figure, see www.iste.co.uk/launay/5g.zip NG-RAN and 5G-NR 1) The eNB entity sends the X2-AP SgNB Addition Request message to the en-gNB entity to allocate radio resources for the establishment of an S1-U bearer with the serving gateway (SGW). The X2-AP message allows the optional establishment of SRB3 signaling between the mobile and the en-gNB entity. The eNB entity sends the TNL transport layer information for the establishment of a tunnel, depending on the dual connectivity option (option 3, 3a or 3x) and the radio capabilities of the mobile. At the secondary node, the RRM (Radio Resource Management) function, which manages the admission request, can accept or refuse the request of the master node. 2) If the request is accepted, the SN node (en-gNB entity) determines the primary cell and possibly the secondary cells (SCell). The radio resource configuration is sent to the eNB entity in a NR RRC configuration encapsulated in the X2-AP SgNB Addition Request Acknowledge message. 3) The eNB entity forwa