RRC Connection Establishment in 5G

RRC Connection establishment is the initial step for a UE to be able to communicate with the cellular network.

RRC Connection Establishment, Successful (Fig. 5.3.3.1–1 in TS 38.331)

The UE initiates the procedure to establish an RRC connection by sending RRCSetupRequest while it is in RRC_IDLE mode and it has acquired essential System Information through SIBs messages.

The UE shall set the contents of the RRCSetupRequest message contents as presented in Section 5.3.3.3 in TS 38.331. Upon Reception of RRCSetupRequest from UE, the gNB (specifically CU-CP) sends either an RRCSetup or RRCReject message.

RRCSetupRequest message

This message is sent over SRB0 over CCCH logical channel. The main message is as follows:

The above message contains Information Elements (IEs) that carry relevant information about the UE, along with Establishment Cause for UE to initiate the connection request.

Information Elements for RRCSetupRequest-IEs

Side Note: The EstablishmentCause is pretty important in establishing the setup with the gNB. For example, if the gNB is running at a capacity and does not have room for more UEs to grant access, then reading the EstablishmentCause gNB can decide if it is gonna ignore…