Many 5G deployment options have been defined yet. Majorly one is known as non-standalone while the other one is known as standalone deployment. In case of non-standalone deployment, both eLTE RAN and NG RAN nodes would be available to serve 5G handsets either through LTE EPC or NGCN. In case of standalone deployment, either eLTE RAN or NG RAN node will be deployed with NGCN.
In non-standalone deployment, SRB3 is a dedicated signalling radio bearer which is used to transport NR RRC PDUs directly between UE and NR node. If SRB3 is not established between UE and NG RAN, NG RAN RRC PDUs are sent to UE by using the SRB1 (established between eLTE and UE).