Split bearer in multi-RAT dual connectivity is a bearer whose radio protocols are split either at the master node or at the secondary node and belongs to both secondary cell group and master cell group. 154 Chapter 1 split split bearer bearer bearer bearer Figure 1.74 MgNB and SgNB bearers for dual connectivity [16]. links (or both) that it transmits uplink PDCP PDUs. The RLC sublayer only transmits corre- sponding feedback for the downlink data over the link which is not used for transmitting PDCP PDUs. Multi-RAT DC (MR-DC) is a generalization of the intra-LTE dual connectivity, where a UE with multiple RF transceivers may be configured to utilize radio resources provided by two distinct schedulers located in two different nodes and connected via non-ideal backhaul, one providing LTE access and the other one providing NR connectivity. One scheduler is located in the master node (MN) 73 and the other in the secondary node (SN). 74 The MN and SN entities are connected via a network interface and the MN is typically connected to the core network. LTE supports MR-DC via E-UTRA-NR dual connectivity (EN-DC), in which a UE is connected to one eNB that acts as the MN and one gNB that acts as the SN. The eNB is connected to the EPC and the gNB is connected to the eNB via the X2 interface (i.e., the logical interface between LTE eNBs). In MR-DC scenarios, the UE has a single RRC state, based on the MN RRC state and a sin- gle control-plane connection toward the core network. Fig. 1.75 shows the control-plane architecture for MR-DC. Each radio node has its own RRC entity, 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, for example, SRB1, but subsequent reconfigurations may be sent via the MN or the SN entities. When transporting RRC PDU from the SN, the MN does not modify the UE configuration Master node in a multi-RAT dual connectivity architecture is either a master eNB or a master