RFLTE Signalling Radio Bearers: SRB0, SRB1, SRB2 Explained
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This document describes the different types of LTE Signalling Radio Bearers (SRBs), specifically SRB0, SRB1, and SRB2.
LTE Signalling Radio Bearers (SRB) are used to transfer RRC (Radio Resource Control) and NAS (Non-Access Stratum) signalling messages.
- RRC messages facilitate signalling between the User Equipment (UE) and the eNodeB (Evolved Node B, i.e., the base station).
- NAS messages facilitate signalling between the UE and the MME (Mobility Management Entity).
RRC messages can encapsulate NAS messages for transfer between the UE and the eNodeB. The S1 application protocol is subsequently used to transfer NAS messages between the eNodeB and the MME.
LTE SRB Types and Characteristics
The following table summarizes the different LTE SRB types and their key characteristics:
| LTE SRB Type | Direction | RRC Message | RLC Mode |
|---|---|---|---|
| LTE SRB0 (CCCH) | Downlink | RRC Connection Setup, RRC Connection Reject, RRC Connection Re-establishment, RRC Connection Re-establishment Reject | Transparent |
| Uplink | RRC Connection Request, RRC Connection Re-establishment Request | ||
| LTE SRB1 (DCCH) | Downlink | RRC Connection Reconfiguration, 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 | Acknowledged |
| 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 | Acknowledged |
| Uplink | UL Information Transfer |
Detailed Explanation of SRB Types
As outlined in the table above, LTE technology employs three distinct SRB types:
-
SRB0: This SRB is used for transferring RRC messages using the CCCH (Common Control Channel).
-
SRB1: This SRB is used for transferring RRC messages using the DCCH (Dedicated Control Channel).
-
SRB2: This SRB is also used for transferring RRC messages using the DCCH channel and encapsulates a NAS message. SRB1 is also used to encapsulate NAS messages if SRB2 has not yet been configured.
SRB2 has a lower priority than SRB1 and is always configured after security activation. SRB0 utilizes Transparent Mode RLC, while SRB1 and SRB2 employ Acknowledged Mode RLC.
Reference: 3GPP TS 36.331 (LTE Signalling Radio Bearers)
