SRBs

List of Signalling Radio Bearers and their intentions

SRB0
Initial RRC connection access, RRC connection request, Cell Update in TM mode

Logical Channel - CCCH

RLC PDU TYPE: UL - TM, DL - UM

No Encryption

SRB1
Logical Channel: DCCH

RRC messages in UM mode

SRB2
Logical Channel: DCCH

All RRC messages in dedicated channel

RLC PDU TYPE: AM

SRB3
Logical Channel: DCCH

RLC PDU Type: AM

Reserved for NAS Signalling - MM, GMM, SM, CC (SAPI 0, SAPI 3)

SRB4 (Optional)
Logical Channel - DCCH

RLC PDU Type: AM (SAPI 3)

Reserved for NAS Signalling - SS, SMS (SAPI 3)