Skip to main content

SRB stands for Signaling Radio Bearers. They are defined as radio bearers that are used only for the transmission of RRC (Radio Resource Control) and NAS (Non-Access Stratum) messages.

Four different types of SRB in 5G NR

  • SRB 0: it is for RRC messages using the CCCH(Common Control Channel) logical channel.
  • SRB 1: it is for RRC messages as well as for NAS messages prior to the establishment of SRB 2, all using DCCH(dedicated control channel) logical channel.
  • SRB 2: it is for NAS messages, all using the DCCH logical channel. It has a lower priority than SRB 1.
  • SRB 3: it is for specific RRC messages when UE is in EN-DC, all using DCCH logical channel.

The UE in the RRC_IDLE state does not have any allocated  SRB or DRB. The UE requires dedicated SRBs or DRBs to exchange data or messages with the network. For the transfer of NAS messages, SRBs were established.

Default SRB Configuration

RRC information elementsSRB1/1SSRB2/2SSRB3
PDCP-Config   
    t-ReorderingInfinityInfinityInfinity
RLC-Config CHOICEAMAMAM
ul-RLC-Config   
    sn-FieldLengthsize12size12size12
    t-PollRetransmitms45ms45ms45
    pollPDUInfinityInfinityInfinity
    pollByteInfinityInfinityInfinity
    maxRetxThresholdt8t8t8
dl-RLC-Config   
    sn-FieldLengthsize12size12size12
    t-Reassemblyms35ms35ms35
    t-StatusProhibitms0ms0ms0
logicalChannelIdentity123
LogicalChannelConfig   
    priority131
    prioritisedBitRateInfinityInfinityInfinity
    logicalChannelGroup000

SRB Mapping to Signalling Message

MessageDirectionLogical ChannelRLCSRB
MasterInformationBlockUE <– NWBCCHTMN/A
MeasurementReportUE –> NWDCCHAMSRB1,SRB3
MobilityFromNRCommandUE <– NWDCCHAMSRB1
PagingUE <– NWPCCHTMN/A
RRCReestablishmentUE <– NWDCCHAMSRB1
RRCReestablishmentCompleteUE –> NWDCCHAMSRB1
RRCReestablishmentRequestUE –> NWCCCHTMSRB0
RRCReconfigurationUE <– NWDCCHAMSRB1,SRB3
RRCReconfigurationCompleteUE –> NWDCCHAMSRB1,SRB3
RRCRejectUE <– NWCCCHTMSRB0
RRCReleaseUE <– NWDCCHAMSRB1
RRCResumeUE <– NWDCCHAMSRB1
RRCResumeCompleteUE –> NWDCCHAMSRB1
RRCResumeRequestUE –> NWCCCHTMSRB0
RRCResumeRequest1UE –> NWCCCH1TMSRB0
RRCSetupUE <– NWCCCHTMSRB0
RRCSetupCompleteUE –> NWDCCHAMSRB1
RRCSetupRequestUE –> NWCCCHTMSRB0
RRCSystemInfoRequestUE –> NWCCCHTMSRB0
SecurityModeCommandUE <– NWDCCHAMSRB1
SecurityModeCompleteUE –> NWDCCHAMSRB1
SecurityModeFailureUE –> NWDCCHAMSRB1
SIB1UE <– NWBCCHTMN/A
SystemInformationUE <– NWBCCHTMN/A
UEAssistanceInformationUE –> NWDCCHAMSRB1
UECapabilityEnquiryUE <– NWDCCHAMSRB1
UECapabilityInformationUE –> NWDCCHAMSRB1
ULInformationTransferUE –> NWDCCHAMSRB1,SRB2

Leave a Reply

%d bloggers like this: