LTE Signalling Radio Bearer types-SRB0,SRB1,SRB2

This page describes LTE Signalling Radio Bearer types which include LTE SRB0, SRB1 and SRB2. LTE Signalling radio bearers(SRB) are used for the transfer of RRC and NAS signalling messages.
•  RRC messages are used as signalling between UE and eNodeB.
•  NAS(Non Access Stratum) messages are used as signalling between UE and MME.

RRC messages can be used to encapsulate NAS messages for their transfer between UE and eNodeB. The S1 application protocol is later used to transfer NAS messages between eNode and MME.

LTE SRB Type Direction RRC message RLC Mode
LTE SRB0 (CCCH) Downlink RRC Connection Setup
RRC Connection Reject
RRC Connection Re-establishment
RRC Connection Re-establishment reject
Uplink RRC Connection Request
RRC Connection Re-esblishment Request
LTE SRB1(DCCH) Downlink RRC Connection Reconfiguration
RRC Connection Release
Security Mode Command
UE Capability Enquiry
DL information transfer(if no SRB-2)
Mobility from EUTRA Command
Handover from EUTRA preparation request
CS Fallback parameter response CDMA2000
Counter Check
Uplink RRC Connection Setup Complete
Security Mode Complete
Security Mode Failure
RRC Connection Reconfiguration Complete
RRC Connection Re-establishment Complete
Measurement report
UE Capability information
UL Information Transfer(if no SRB2)
UL handover preparation transfer
CS fallback parameters request CDMA2000
Counter Check response
LTE SRB2 (DCCH) Downlink DL Information Transfer
Uplink UL Information Transfer

As mentioned in the LTE SRB table, there are three types of SRB in the LTE technology.
•  SRB0 used to transfer RRC messages which use CCCH channel.
•  SRB1 used to transfer RRC messages which use DCCH channel.
•  SRB2 used to transfer RRC messages which use DCCH channel and encapsulates a NAS message.

SRB1 is also used to encapsulate NAS message if SRB2 has not been configured.
SRB2 has lower priority then SRB1 and it is always configured after security activation.
SRB0 uses transparent mode RLC while SRB1 and SRB2 use acknowledged mode RLC.

LTE Signalling Radio Bearers(LTE SRB) : 3GPP reference TS 36.331