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 elements | SRB1/1S | SRB2/2S | SRB3 |
| PDCP-Config | |||
| t-Reordering | Infinity | Infinity | Infinity |
| 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 |
SRB Mapping to Signalling Message
| Message | Direction | Logical Channel | RLC | SRB |
| MasterInformationBlock | UE <– NW | BCCH | TM | N/A |
| 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 |
| RRCReestablishmentRequest | UE –> NW | CCCH | TM | SRB0 |
| RRCReconfiguration | UE <– NW | DCCH | AM | SRB1,SRB3 |
| RRCReconfigurationComplete | UE –> NW | DCCH | AM | SRB1,SRB3 |
| RRCReject | UE <– NW | CCCH | TM | SRB0 |
| RRCRelease | UE <– NW | DCCH | AM | SRB1 |
| RRCResume | UE <– NW | DCCH | AM | SRB1 |
| RRCResumeComplete | UE –> NW | DCCH | AM | SRB1 |
| RRCResumeRequest | UE –> NW | CCCH | TM | SRB0 |
| RRCResumeRequest1 | UE –> NW | CCCH1 | TM | SRB0 |
| RRCSetup | UE <– NW | CCCH | TM | SRB0 |
| RRCSetupComplete | UE –> NW | DCCH | AM | SRB1 |
| RRCSetupRequest | UE –> NW | CCCH | TM | SRB0 |
| RRCSystemInfoRequest | UE –> NW | CCCH | TM | SRB0 |
| SecurityModeCommand | UE <– NW | DCCH | AM | SRB1 |
| SecurityModeComplete | UE –> NW | DCCH | AM | SRB1 |
| SecurityModeFailure | UE –> NW | DCCH | AM | SRB1 |
| SIB1 | UE <– NW | BCCH | TM | N/A |
| SystemInformation | UE <– NW | BCCH | TM | N/A |
| 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 |
