5G NR Signaling Radio Bearers (SRBs)

5G, LTE, New Radio, NR, Tech Fundas

SRB stands for Signaling Radio Bearer.”Signalling Radio Bearers” (SRBs) are defined as Radio Bearers (RBs) that are used only for the transmission of RRC and NAS messages.

As per 3GPP specification TS38.331,   Signalling radio bearers, there are four different types of SRB in 5G New Radio (NR) defined as follows.

  • SRB#0 is for RRC messages using the Common Control Channel (CCCH) logical channel
  • SRB#1 is for RRC messages as well as for NAS messages prior to the establishment of SRB2 (which may include a piggybacked NAS message), all using Dedicated Control Channel (DCCH) logical channel
  • SRB#2 is for NAS messages, all using DCCH logical channel. SRB2 has a lower-priority than SRB1 and is always configured by the network after security activation
  • SRB#3 is for specific RRC messages when UE is in EN-DC, all using DCCH logical channel

Here you may notice, the type and functions of each SRB in New Radio is same as the SRBs in LTE except that it has one more SRB named as SRB3. In EN-DC, SRB3 configured for the transfer of some NR RRC messages between UE and SgNB via the NR radio interface.

SRBs Default Configuration

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

NAS and RRC Message Mapping with SRBs

Message Direction Logical Channel  RLC Mode SRB Mapping
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

Reference:

3GPP 38.331; NR; Radio Resource Control (RRC); Protocol specification