Radio Bearers
In the protocol stack of the RAN, at each layer, we use different terms to indicate how the data is treated. The table below shows the terms.
| Layer | Term |
|---|---|
| PDCP | Radio Bearer (RB) |
| RLC | Logical Channel (LC) |
| MAC | Transport Channel |
| PHY | Physical Channel |
We have two types of radio bearers:
- Signaling Radio Bearer (SRB): SRBs are used for the transmission of RRC messages.
- Data Radio Bearer (DRB): DRBs are used for the transmission of user data.
There are four types of SRBs in 5G-NR:
- SRB0: Maps to CCCH LC and is used for the transmission of RRC messages.
- SRB1: Maps to DCCH LC (ID 1) and is used for the transmission of RRC messages with NAS piggybacked data perhaps.
- SRB2: Maps to DCCH LC (ID 2) and is used for the transmission NAS messages, it has lower priority than SRB1 and maybe configured after the AS security is established.
- SRB3: Maps to DCCH LC (ID 3) and is used when the UE is in NSA mode.
The table below shows the mapping of the signaling messages to the SRBs.
| Message | Direction | Logical Channel | RLC | SRB |
|---|---|---|---|---|
| MasterInformationBlock | UE <-- BS | BCCH | TM | N/A |
| MeasurementReport | UE --> BS | DCCH | AM | SRB1,SRB3 |
| RRCReestablishment | UE <-- BS | DCCH | AM | SRB1 |
| RRCReestablishmentComplete | UE --> BS | DCCH | AM | SRB1 |
| RRCReestablishmentRequest | UE --> BS | CCCH | TM | SRB0 |
| RRCReconfiguration | UE <-- BS | DCCH | AM | SRB1, SRB3 |
| RRCReconfigurationComplete | UE --> BS | DCCH | AM | SRB1, SRB3 |
| RRCSetup | UE <-- BS | CCCH | TM | SRB0 |
| RRCSetupComplete | UE --> BS | DCCH | AM | SRB1 |
| RRCSetupRequest | UE --> BS | CCCH | TM | SRB0 |
| SecurityModeCommand | UE <-- BS | DCCH | AM | SRB1 |
| SecurityModeComplete | UE --> BS | DCCH | AM | SRB1 |
| SIB1 | UE <-- BS | BCCH | TM | N/A |
| SystemInformation | UE <-- BS | BCCH | TM | N/A |
| UEAssistanceInformation | UE --> BS | DCCH | AM | SRB1 |
| UECapabilityEnquiry | UE <-- BS | DCCH | AM | SRB1 |
| UECapabilityInformation | UE --> BS | DCCH | AM | SRB1 |
| ULInformationTransfer | UE --> BS | DCCH | AM | SRB1, SRB2 |
At the RRC layer, we have the following logical channels:
- Common Control Channel (CCCH) and Dedicated Control Channel (DCCH) are used to transfer RRC signaling messages, used both in uplink and downlink.
- Dedicated Traffic Channel (DTCH) is used to transfer application data, used both in uplink and downlink.
- Broadcast Control Channel (BCCH) is used to transfer both the Master Information Block (MIB) and System Information Blocks (SIB). MIB is mapped to BCH and PBCH; SIBs are mapped to DL-SCH and PDSCH (downlink only).
- Paging Control Channel (PCCH) is used to transfer paging messages from the network to the UE (downlink only).
Figures below taken from this website show these channels and how they are mapped to the transport channels and physical channels in the uplink and downlink directions.
- CCCH, DCCCH, DTCH all map to UL-SCH and PUSCH.
- Physical Uplink Control Channel (PUCCH) transfers Uplink Control Information (UCI), including Scheduling Requests, HARQ Acknowledgements and Channel State Information (CSI). It is not used to transfer higher layer information.
- Random Access Channel (RACH) is used to transfer the index of the preamble sequence.
- Demodulation Reference Signals (DM-RS) are sequences which are known to the base station. They are needed for base station to estimate the channel properties for PUCCH and PUSCH.
- Sounding Reference Signal (SRS) is used by the base station for channel-aware packet scheduling and link adaptation.
- Phase Tracking Reference Signal (PT-RS) is used to compensate for phase noise generated by the local oscillators at both the transmitter and receiver.
- CCCH, DCCCH, DTCH all map to DL-SCH and PDSCH.
- Physical Downlink Control Channel (PDCCH) transfers Downlink Control Information (DCI), which is used by the base station to allocate uplink and downlink resources. DCI can also be used to provide uplink power control commands, config slot format, and indicate that pre-emption has occured. It is not used to transfer higher layer information.
- Demodulation Reference Signals (DM-RS) are sequences which are known to the UE. They are needed for UE to estimate the channel properties for PBCH, PDCCH, and PDSCH.
- Phase Tracking Reference Signal (PT-RS) is used to compensate for phase noise generated by the local oscillators at both the transmitter and receiver.
- Channel State Information Reference Signal (CSI-RS) is used by the UE to measure and report channel quality. This information can be used by the link adaptation algorithm at the Base Station. It can also be used for beam management and connected mode mobility procedures.
- Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS) are used during the cell search procedure and beam management procedure. SSS is used for Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ) measurements.