SRB( Signaling Radio Bearer)就是无线信令承载;SRBs(Signalling Radio Bearers）是传输RRC和NAS消息的无线承载；根据3GPP TS38.331，5G (NR)的SRB有四种： 1.SRB的类型 • SRB#0 RRC消息承载于CCCH信道 • SRB#1 RRC和NAS消息,用在SRB2(含NAS消息)建立之前，承载于DCCH信道； • SRB#2 NAS消息，承载于DCCH信道；空口加密完成后由网络侧配置，SRB2较SRB1优先级低，； • SRB#3 UE双连接（EN-DC）中RRC消息，承载于DCCH信道； SRB的功能在5G(NR)和LTE网络中相同。5G(NR)网络中只是增加了SRB3，它用在双连接（EN-DC）中，通过5G(NR)无线网络传递UE与SgNB之间SRB3消息； 2.SRB配置表 Information Element SRB#1 SRB#2 SRB#3 PDCP-Config t-Reordering RLC-Config CHOICE ul-RLC-Config sn-FieldLength t-PollRetransmit pollPDU pollByte maxRetxThreshold dl-RLC-Config sn-FieldLength t-Reassembly t-StatusProhibit logicalChannelIdentity LogicalChannelConfig priority prioritisedBitRate Infinite AM Infinite AM Infinite AM size12 ms45 Infinity Infinity t8 size12 ms45 Infinity Infinity t8 size12 ms45 Infinity Infinity t8 size12 ms35 ms0 1 size12 ms35 ms0 2 size12 ms35 ms0 3 1 Infinity 3 Infinity 1 Infinity logicalChannelGroup 0 0 0
3.SRBs中NAS和 RRC消息映射
Message（消息）
Direction（方向） UE <– NW UE <– NW UE <– NW UE –> NW UE <– NW UE –> NW UE –> NW UE <– NW UE –> NW UE –> NW UE –> NW UE <– NW UE <– NW UE –> NW UE –> NW UE <–
Logical Channel (逻辑信道）
RLC Mode(模式)
SRB Mapping（映射）
N/A
MasterInformationBloc
k
SIB1
BCCH TM
BCCH TM N/A
SystemInformation BCCH TM N/A
RRCSetupRequest CCCH TM SRB0
RRCSetup CCCH TM SRB0
RRCResumeRequest1 CCCH1 TM SRB0
RRCSystemInfoReques
t
RRCReject
CCCH TM SRB0
CCCH TM SRB0
RRCResumeRequest CCCH TM SRB0
RRCReestablishmentR
equest RRCSetupComplete
CCCH TM SRB0
DCCH AM SRB1
RRCRelease DCCH AM SRB1
RRCReconfiguration DCCH AM SRB1,SRB3
RRCReconfigurationCo
mplete MeasurementReport
DCCH AM SRB1,SRB3
DCCH AM SRB1,SRB3
MobilityFromNRCommDCCH AM SRB1
and Paging
NW UE <– NW UE <– NW UE –> NW UE <– NW UE –> NW UE <– NW UE –> NW UE –> NW UE –> NW UE <– NW UE –> NW UE –> NW
PCCH
TM
N/A
RRCReestablishment DCCH AM SRB1
RRCReestablishmentC
omplete
RRCResume
DCCH AM SRB1
DCCH AM SRB1
RRCResumeComplete DCCH AM SRB1
SecurityModeComman
d SecurityModeComplete
DCCH AM SRB1
DCCH AM SRB1
SecurityModeFailure DCCH AM SRB1
UEAssistanceInformati
on UECapabilityEnquiry
DCCH AM SRB1
DCCH AM SRB1
UECapabilityInformatio
n ULInformationTransfer
DCCH AM SRB1
DCCH AM SRB1,SRB2
