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
Infinite
Infinite
Infinite
RLC-Config CHOICE
AM
AM
AM
ul-RLC-Config
sn-FieldLength
size12
size12
size12
t-PollRetransmit
ms45
ms45
ms45
pollPDU
Infinity
Infinity
Infinity
pollByte
Infinity
Infinity
Infinity
maxRetxThreshold
t8
t8
t8
dl-RLC-Config
sn-FieldLength
size12
size12
size12
t-Reassembly
ms35
ms35
ms35
t-StatusProhibit
ms0
ms0
ms0
logicalChannelIdentity
1
2
3
LogicalChannelConfig
priority
1
3
1
prioritisedBitRate
Infinity
Infinity
Infinity
logicalChannelGroup
0
0
0
3.SRBs中NAS和 RRC消息映射
Message（消息）
Direction（方向）
Logical Channel (逻辑信道）
RLC Mode(模式)
SRB Mapping（映射）
MasterInformationBlock
UE <– NW
BCCH
TM
N/A
SIB1
UE <– NW
BCCH
TM
N/A
SystemInformation
UE <– NW
BCCH
TM
N/A
RRCSetupRequest
UE –> NW
CCCH
TM
SRB0
RRCSetup
UE <– NW
CCCH
TM
SRB0
RRCResumeRequest1
UE –> NW
CCCH1
TM
SRB0
RRCSystemInfoRequest
UE –> NW
CCCH
TM
SRB0
RRCReject
UE <– NW
CCCH
TM
SRB0
RRCResumeRequest
UE –> NW
CCCH
TM
SRB0
RRCReestablishmentRequest
UE –> NW
CCCH
TM
SRB0
RRCSetupComplete
UE –> NW
DCCH
AM
SRB1
RRCRelease
UE <– NW
DCCH
AM
SRB1
RRCReconfiguration
UE <– NW
DCCH
AM
SRB1,SRB3
RRCReconfigurationComplete
UE –> NW
DCCH
AM
SRB1,SRB3
MeasurementReport
UE –> NW
DCCH
AM
SRB1,SRB3
MobilityFromNRCommand
UE <– NW
DCCH
AM
SRB1
Paging
UE <– NW
PCCH
TM
N/A
RRCReestablishment
UE <– NW
DCCH
AM
SRB1
RRCReestablishmentComplete
UE –> NW
DCCH
AM
SRB1
RRCResume
UE <– NW
DCCH
AM
SRB1
RRCResumeComplete
UE –> NW
DCCH
AM
SRB1
SecurityModeCommand
UE <– NW
DCCH
AM
SRB1
SecurityModeComplete
UE –> NW
DCCH
AM
SRB1
SecurityModeFailure
UE –> NW
DCCH
AM
SRB1
UEAssistanceInformation
UE –> NW
DCCH
AM
SRB1
UECapabilityEnquiry
UE <– NW
DCCH
AM
SRB1
UECapabilityInformation
UE –> NW
DCCH
AM
SRB1
ULInformationTransfer
UE –> NW
DCCH
AM
SRB1,SRB2