d S1-U bearers may further split and carried over the X2/Xn interface to the eNB and over the LTE air interface. The deployments based on architecture option 3 do not require interface with the 5GC, and allow service over the NR air interface with Uu (i.e., the interface between the UE and the serving gNB) and X2/Xn interfaces defined. As such, this is seen as the most likely architectural scenario for early NR deployments. From control-plane perspec- tive, there is only one RRC state in the UE, which is based on the LTE RRC protocols and there is only one control-plane connection toward the core network. 164 Chapter 1 The deployments based on architecture option 4 are essentially the opposite of option 3, with the gNB and the eNB representing the MCG and the SCG, respectively. The control- plane connection is established between the gNB and the 5GC over the NG-C, and the eNB exchanges control-plane information with gNB over Xn. In option 4A, direct user-plane bearers with the 5GC are terminated at the eNB. In architecture option 5, the ng-eNB (i.e., an LTE eNB compliant with LTE Rel-15 onward) is connected to the 5GC. The deployments based on architecture option 7 use the same topology as option 3, with the eNB acting as MCG and the gNB acting as SCG. The difference between the two options is that the 5GC serves as the core network instead of the EPC, requiring the eNB upgrade to ng-eNB interfaces with the 5GC. In this scenario, each radio node has its own RRC entity which can generate RRC PDUs to be sent to the UE. Note that RRC PDUs generated by the gNB (SN) can be transported via the LTE Uu interface or NR Uu interface to the UE, if configured. The eNB (MN) always sends the initial SN RRC configuration via MCG SRB (SRB1); however, subsequent reconfigurations may be transported via the MN or SN. Furthermore, the UE can be configured to establish an SRB with the SN (i.e., SRB3) to enable RRC PDUs for the SN to be sent directly between the UE and the SN. 1.5 Network Aspects of Mobility and Power Management Mobil