4G/LTE - SRB
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 :
Default SRB Configuration
In many cases (especially with testing environment), default setting is used for SRBs. In most cases, you wouldn't need to check on the detailed parameter of these default settings but you would need to check on the details in case that you need to do some troubleshooting. The detailed parameter setting for the default SRB are specified in 36.331 - 9 Specified and default radio configurations as shown below.
< SRB1 >
< 36.331-220.127.116.11 SRB1 >
< 36.331-18.104.22.168 SRB1 >
< SRB1bis >
< 36.331-22.214.171.124a SRB1bis >
< SRB2 >
< 36.331-126.96.36.199 SRB2>
< 36.331-188.8.131.52 SRB2 >
SRB Mapping to Signalling Message
Also 36.331 defines the SRB mapping for each message as shown below. I recommend you to pay attention to these mapping examples. If SRB number does not match between the sender and the reciever, the message would not reach the destination even though the message is properly sent and reach the lower layer of the destination.