5G: What is the use of signalling radio bearer 3 (SRB3) and when it is established ?

Question

In the LTE network, as per my knowledge only SRB1 and SRB2 two signalling radio bearers were supported for an UE. SRB1 mainly used to RRC messages and optionally NAS messages if SRB2 (optional) is unavailable. If SRB2 is available, NAS messages are transported between UE and MME using SRB2 signalling bearer.
Now I want to know what is the use of another SRB3 ? Is it established along with SRB1 and SRB2 every time or it is optional and established only on specific cases/scenarios ?

Answer 1

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).

Answer 2

some add on -
SRB3 shall carry only following messages
1- rrcConnectionReconfiguration
2- rrcConnectionReconfigurationComplete
3- measurementReport