5G NR SRB configuration and NAS and RRC message mapping
SRB are used to carry RRC and NAS messages.
There are 4 different types of SRB in 5G NR.
It is used to transmit RRC messages on the logical channel CCCH.
It is used to transmit RRC messages with NAS messages piggybacked on the logical channel DCCH.
They have higher priority than SRB2.
It is used to transmit NAS messages that has lower priority than SRB1 and SRB2 is used/configured after security mode is activated.
It is used to initiate connection establishment, DL NAS messages in attachment process.
SRB configuration table
NAS and RRC message mapping in SRBs
RRC Signalling is between UE and BS. SRB are used to transfer RRC messages. They can encapsulate NAS messages for transportation.
NAS signalling is between UE and AMF/SMF. NGAP is used to transfer NAS messages between BS and AMF.
There are 4 different types of SRB in NR.
SRB0: It transfers RRC messages using CCCH logical channel.
SRB1 SRB2, SRB3 transfers RRC messages using DCCH logical channel. IT uses AM RLC mode.
SRB 1 is is used prior to establishment of SRB2 and can also encapsulate NAS messages. It uses TM RLC.
SRB2 will be established after security activation.
SRB3 is used when EN-DC configuration.
RRCResumeRequest1 message is uses CCCH1 logical channel and is used to transfer larger messages.