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 ?