4G/LTE - LTE NB
SRB (Signaling Radio Bearer) mapping for LTE Signaling Message
One of the most common mistake we make and spend a lot of time for troubleshooting would be 'Network send some message but does not get any response from UE'. One of the most common reason in this situation (especially when you see the message has gone through L1) would be mismatchs between Signalling message and SRB.
3GPP 36.331 4.2.2 Signalling radio bearers says :