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This post describes the LTE Signalling Radio Bearer types. Please read further down in order to find more details about the topic.
Topic description
The LTE Signalling Radio Bearers (SRB) include LTE SRB0, SRB1 and SRB2.
The LTE Signalling Radio Bearers are used for the transfer of the RRC and NAS signaling messages.
• RRC messages are used as signaling between UE and eNodeB.
• NAS (Non-Access Stratum) messages are used as signaling between UE and MME.
RRC messages can be used to encapsulate the NAS messages for their transfer between UE and eNodeB.
The S1 application protocol is later used to transfer NAS the messages between eNode and MME.
| LTE SRB Type | Direction | RRC message | RLC Mode |
| LTE SRB0 (CCCH) | Downlink | RRC Connection Setup | Transparent |
| RRC Connection Reject | |||
| RRC Connection Re-establishment | |||
| RRC Connection Re-establishment reject | |||
| Uplink | RRC Connection Request | ||
| RRC Connection Re-esblishment Request | |||
| LTE SRB1(DCCH) | Downlink | RRC Connection Reconfiguration | Acknowledged |
| RRC Connection Release | |||
| Security Mode Command | |||
| UE Capability Enquiry | |||
| DL information transfer(if no SRB-2) | |||
| Mobility from EUTRA Command | |||
| Handover from EUTRA preparation request | |||
| CS Fallback parameter response CDMA2000 | |||
| Counter Check | |||
| Uplink | RRC Connection Setup Complete | ||
| Security Mode Complete | |||
| Security Mode Failure | |||
| RRC Connection Reconfiguration Complete | |||
| RRC Connection Re-establishment Complete | |||
| Measurement report | |||
| UE Capability information | |||
| UL Information Transfer(if no SRB2) | |||
| UL handover preparation transfer | |||
| CS fallback parameters request CDMA2000 | |||
| Counter Check response | |||
| LTE SRB2 (DCCH) | Downlink | DL Information Transfer | |
| Uplink | UL Information Transfer |
As mentioned in the LTE SRB table, there are three types of SRB in the LTE technology:
• SRB0 used to transfer RRC messages which use the CCCH channel;
• SRB1 used to transfer RRC messages which use DCCH channel;
• SRB2 used to transfer RRC messages which use DCCH channel and encapsulates a NAS message.
SRB1 is also used to encapsulate the NAS message if SRB2 has not been configured.
SRB2 has a lower priority than SRB1 and it is always configured after the security activation.
SRB0 uses the transparent mode RLC, while SRB1 and SRB2 use the acknowledged mode RLC.
LTE Signalling Radio Bearers (LTE SRB): 3GPP reference TS 36.331
