SRB(

SignalingRadioBearer)就是無線信令承載;SRBs(SignallingRadioBearers）是傳輸RRC和NAS消息的無線承載；根據(jù)3GPPTS38.331，5G(NR)的SRB有四種：1.SRB的類型SRB#0

RRC消息承載于CCCH信道SRB#1

RRC和NAS消息,用在SRB2(含NAS消息)建立之前，承載于DCCH信道；SRB#2

NAS消息，承載于DCCH信道；空口加密完成后由網(wǎng)絡(luò)側(cè)配置，SRB2較SRB1優(yōu)先級低，；SRB#3

UE雙連接（EN-DC）中RRC消息，承載于DCCH信道；

SRB的功能在5G(NR)和LTE網(wǎng)絡(luò)中相同。5G(NR)網(wǎng)絡(luò)中只是增加了SRB3，它用在雙連接（EN-DC）中，通過5G(NR)無線網(wǎng)絡(luò)傳遞UE與SgNB之間SRB3消息；2.SRB配置表InformationElementSRB#1SRB#2SRB#3PDCP-Configt-ReorderingInfiniteInfiniteInfiniteRLC-ConfigCHOICEAMAMAMul-RLC-Configsn-FieldLengthsize12size12size12t-PollRetransmitms45ms45ms45pollPDUInfinityInfinityInfinitypollByteInfinityInfinityInfinitymaxRetxThresholdt8t8t8dl-RLC-Configsn-FieldLengthsize12size12size12t-Reassemblyms35ms35ms35t-StatusProhibitms0ms0ms0logicalChannelIdentity123LogicalChannelConfigpriority131prioritisedBitRateInfinityInfinityInfinitylogicalChannelGroup0003.SRBs中NAS和RRC消息映射Message（消息）Direction（方向）Logical

Channel

(邏輯信道）RLC

Mode(模式)SRB

Mapping（映射）MasterInformationBlockUE<–NWBCCHTMN/ASIB1UE<–NWBCCHTMN/ASystemInformationUE<–NWBCCHTMN/ARRCSetupRequestUE–>NWCCCHTMSRB0RRCSetupUE<–NWCCCHTMSRB0RRCResumeRequest1UE–>NWCCCH1TMSRB0RRCSystemInfoRequestUE–>NWCCCHTMSRB0RRCRejectUE<–NWCCCHTMSRB0RRCResumeRequestUE–>NWCCCHTMSRB0RRCReestablishmentRequestUE–>NWCCCHTMSRB0RRCSetupCompleteUE–>NWDCCHAMSRB1RRCReleaseUE<–NWDCCHAMSRB1RRCReconfigurationUE<–NWDCCHAMSRB1,SRB3RRCReconfigurationCompleteUE–>NWDCCHAMSRB1,SRB3MeasurementReportUE–>NWDCCHAMSRB1,SRB3MobilityFromNRCommandUE<–NWDCCHAMSRB1PagingUE<–NWPCCHTMN/ARRCReestablishmentUE<–NWDCCHAMSRB1RRCReestablishmentCompleteUE–>NWDCCHAMSRB1RRCResumeUE<–NWDCCHAMSRB1RRCResumeCompleteUE–>NWDCCHAMSRB1SecurityModeCommandUE<–NWDCCHAMSRB1SecurityModeCompleteUE–>NWDCCHAMSRB1SecurityModeFailureUE–>NWDCCHAMSRB1UEAssistanceInformationUE–>NWDCCHAMSRB1UECapabilityEnquiryUE<–