Signalling Radio Bearers (SRB) are used to transmit RRC and NAS messages.
We know that there are total of 11 radio bearers.
The first 3 types are reserved for signalling.
SRB0: It is used to transfer RRC messages which use the CCCH channel;
SRB1: It is used to transfer RRC messages which use DCCH channel, prior to establishment of SRB2.
SRB2: It is used to transfer RRC messages which use DCCH channel and encapsulates a NAS message.
RRC messages are used as signalling between UE and eNB.
NAS(Non Access Stratum) messages are used as signalling between UE and MME.
RRC messages are used to encapsulate NAS messages for their transfer between UE and eNB to MME. S1AP protocol is used to transfer message between eNB and MME.
Below we shall see the mapping of SRB.
MasterInformationBlock
Signalling radio bearer: N/A
RLC-SAP: TM
Logical channel: BCCH
Direction: E-UTRAN to UE
SystemInformationBlockType1
Signalling radio bearer: N/A
RLC-SAP: TM
Logical channel: BCCH
Direction: E-UTRAN to UE
SRB0:
RRC Connection Request
Signalling radio bearer: SRB0
RLC-SAP: TM
Logical channel: CCCH
Direction: UE to E-UTRAN
RRC Connection Re-esblishment Request
Signalling radio bearer: SRB0
RLC-SAP: TM
Logical channel: CCCH
Direction: UE to E-UTRAN
RRC Connection Setup
Signalling radio bearer: SRB0
RLC-SAP: TM
Logical channel: CCCH
Direction: E-UTRAN to UE
RRC Connection Reject/RRC Connection Re-establishment reject
Signalling radio bearer: SRB0
RLC-SAP: TM
Direction: E-UTRAN to UE
SRB1:
RRC Connection Setup Complete
Signalling radio bearer: SRB1
RLC-SAP: AM
Logical channel: DCCH
Direction: UE to E-UTRAN
Measurement Report
Signalling radio bearer: SRB1
RLC-SAP: AM
Logical channel: DCCH
Direction: UE to E-UTRAN
Measurement Report/ Security Mode Complete/ Security Mode Failure
RRC Connection Reconfiguration Complete/ RRC Connection Re-establishment Complete
UE Capability information/ UL Information Transfer(if no SRB2)
UL handover preparation transfer/ CS fallback parameters request CDMA2000
Signalling radio bearer: SRB1
RLC-SAP: AM
Direction: UE to E-UTRAN
RRC Connection Reconfiguration
Signalling radio bearer: SRB1
RLC-SAP: AM
Logical channel: DCCH
Direction: E-UTRAN to UE
MobilityFromEUTRACommand
Signalling radio bearer: SRB1
RLC-SAP: AM
Logical channel: DCCH
Direction: E-UTRAN to UE
UECapabilityEnquiry
Signalling radio bearer: SRB1
RLC-SAP: AM
Logical channel: DCCH
Direction: E-UTRAN to UE
UEInformationRequest
Signalling radio bearer: SRB1
RLC-SAP: AM
Logical channel: DCCH
Direction: E-UTRAN to UE
SRB 2:
DLInformationTransfer
Signalling radio bearer: SRB2 or SRB1 (only if SRB2 not established yet. If SRB2 is suspended, E-UTRAN does not send this message until SRB2 is resumed.)
RLC-SAP: AM
Logical channel: DCCH
Direction: E-UTRAN to UE
UL Information Transfer
Signalling radio bearer: SRB2
Logical channel: DCCH
Direction: UE to E-UTRAN
Paging
Signalling radio bearer: N/A
RLC-SAP: TM
Logical channel: PCCH
Direction: E-UTRAN to UE