SRB (Signalling Radio Bearer) is used for transmission of signalling messages likes RRC message, NAS messages. Three SRBs have been defined in LTE system as described below:
SRB 0 : This signalling radio bearer is used for transmission of common rrc messages such as rrc connection request, rrc connection setup, rrc connection re-establishment request, rrc connection connection re-establishment. Since this radio bearer is used for all the user that's why it is not security and integrity protected.
SRB 1 : This signalling radio bearer is dedicated to a single UE and it carries dedicated rrc signalling messages such has rrc connection setup complete, rrc connection re-establishment complete, rrc connection re-configuration complete etc. Once the AS security is activated, all the signalling messages over this bearer will be security and integrity protected.
SRB 2: This is also a dedicated signalling radio bearer for an UE. This bearer is established for the transmission of dedicated NAS signalling messages such as dedicated bearer creation message.
DRB is established for the transmission of data plane packets. Multiple DRBs can be established for an UE due to different types of services used by UE such as internet browsing, voip call etc. A UE can have maximum of eight data radio bearers at a particular moment.
