 gNB. Secondary node in multi-RAT dual connectivity architecture is either a secondary eNB or a secondary gNB. 5G Network Architecture Master node Secondary NR RRC RRC (MeNB RRC (master state) node state) EN-DC MR-DC Figure 1.75 Control-plane architecture for EN-DC and MR-DC with 5GC [13]. provided by the SN. In EN-DC and NG-RAN E-UTRA-NR dual connectivity (NGEN- DC75) scenarios, during initial connection establishment SRB1 uses LTE PDCP; however, after initial connection establishment MCG SRB (SRB1 and SRB2) can be configured by the network to use either LTE PDCP or NR PDCP. The PDCP version change (release of old PDCP and establishment of new PDCP) of SRBs can be supported via a handover pro- cedure (reconfiguration with mobility) or through a reconfiguration without mobility, when the network is aware that there is no uplink data in the UE buffer. For EN-DC capable UEs, NR PDCP can be configured for DRBs and SRBs before EN-DC is configured. If the SN is a gNB (i.e., for EN-DC and NGEN-DC), the UE can be configured to establish an SRB with the SN (e.g., SRB376 to enable RRC PDUs for the SN to be sent directly between the UE and the SN. The RRC PDUs for the SN can only be sent directly to the UE for SN RRC reconfiguration without any coordination with the MN. Measurement reporting for mobility within the SN can be conducted directly from the UE to the SN, if SRB3 is config- ured. The MCG split SRB is supported for all MR-DC cases, allowing duplication of RRC PDUs generated by the MN, via the direct path and through the SN. The MCG split SRB uses NR PDCP. The SCG split SRB is not currently supported in 3GPP specifications [13]. NG-RAN supports NGEN-DC, in which a UE is connected to one ng-eNB that acts as a master node and one gNB that acts as a secondary node. The ng-eNB is connected to the 5GC and the gNB is connected to the ng-eNB via the Xn interface. SRB3 in EN-DC and NGEN-DC represents a direct signaling radio bearer between the secondary node and the user equipment. 156 Chapter 1 bearer split split bearer 