SRB are used to transmission of RRC and NAS messages.
There are 4 different types of SRB defied in TS 38.331.
1. SRB0.
2. SRB1.
3. SRB2.
4. SRB3.
SRB0: It is for RRC messages using the CCCH logical channel;
SRB1: It is for RRC messages including piggybacked NAS message as we as NAS messages prior to the establishment of SRB2 using DCCH channel.
SRB2: It is used for NAS and RRC messages using DCCH channel after the AS security activation. They have lower priority than SRB1.
SRB3: It is used for specific RRC messages when the UR is in (NG)EN-DC or NR-DC, all using DCCH logical channel.
-> Once the security mode is activated, all RRC messages in SRB1, SRB2 and SRB3, including those containing NAS
messages, are integrity protected and ciphered by PDCP.
SRBs Default Configuration:
NAS and RRC Message Mapping with SRBs:
3GPP TS 38.331