5G NR Signaling Radio Bearers (SRBs)
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
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