Signalling Radio Bearers (SRBs) are the Radio Bearers (RBs) that are used by UE for transmitting and receiving RRC messages with the eNodeB. For NB-IoT UE, only the following radio bearers are defined.
- SRB 0: It is used for RRC messages using the CCCH logical channel. It is used for the exchanging of the RRCConenctionRequest, RRCConnectionSetup, RRCConnectionResumeRequest, RRCConnectionReject, RRCConnectionReestablishmentRequest, and RRCConnectionReestablishment messages with eNodeB.
- SRB1bis: It is for RRC messages (which may include a piggybacked NAS message) as well as for NAS messages prior to the activation of security, all using DCCH logical channel. It is established implicitly when establishing SRB1 after the UE receives the RRCConnectionSetup. SRB1bis is the same as SRB1 except that it bypasses the PDCP layer. SRB1bis is used as long as security is not activated. If security is activated, SRB1bis is not used but only SRB1 is used.
- SRB1: Used for RRC signalling message transfer after the security is activated. SRB0 is used before Access Stratum (AS) security is activated and only SRB1 is supported after AS security is activated.
- DRB0 and DRB1: A maximum of two data radio bearers are used for exchanging data messages with the eNodeB. For a UE to support two DRBs, it must have its multi-DRB capability enabled; otherwise, UE supports only a single DRB
SRB0 and SRB1bis both use Transparent Mode (TM) at RLC sublayer
RRC Default Configuration Parameters for SRB1
RRC Messages Mapping to Signaling Radio Bearers (SRBs) and Channels