ion if the node is a gNB) which can generate RRC PDUs to be sent to the UE. The RRC PDUs generated by the SN can be transported via the MN to the UE. The MN always sends the initial SN RRC configuration via MCG SRB (SRB1); however, subsequent reconfigurations may be transported via MN or SN. When en-gNB is a node providing NR user-plane and control-plane protocol terminations toward the UE and acts as a secondary node in EN-DC. A secondary node in MR-DC is a radio access node, with no control-plane connection to the core network, providing only additional radio resources to the UE. It may be an en-gNB (in EN-DC), a secondary ng-eNB (in NE-DC), or a secondary gNB (in NR-DC and NGEN-DC) [4]. 278 Chapter 2 transporting RRC PDU from the SN, the MN does not modify the UE configuration pro- vided by the SN [4]. When an LTE node is connected to the EPC, upon initial connection establishment, SRB1 uses LTE PDCP. If the UE supports EN-DC, regardless of whether EN-DC is configured, after initial connection establishment, the MCG SRBs (SRB1 and SRB2) can be configured by the network to use either LTE PDCP or NR PDCP (either SRB1 and SRB2 are both con- figured with LTE PDCP, or they are both configured with NR PDCP). A change from LTE PDCP to NR PDCP (or vice versa) is supported via a handover procedure (reconfiguration with mobility) or, for the initial change of SRB1 from LTE PDCP to NR PDCP, with a reconfiguration without mobility before the initial security activation. If the SN is a gNB (i.e., the case for EN-DC, NGEN-DC, and NR-DC scenario), the UE can be configured to establish an SRB with the SN (SRB3) to enable RRC PDUs for the SN to be directly transferred between the UE and the SN. The RRC PDUs for the SN can only be transported directly to the UE for SN RRC reconfiguration without any coordination with the MN. Measurement reporting for mobility within the SN can be sent directly from the UE to the SN, if SRB3 is configured. The split SRB is supported for all MR-DC options, allowing duplication of RRC PDUs generat