RRC connection

The purpose of establishing an RRC connection is to make the UE known to the network so that it can exchange both user and signalling data with the network.To establish a connection, the RRC submits an RRC Connection Setup Request message on the CCCH channel to the cell it is camped on. The UTRAN will then respond with an RRC Connection Setup message, returned on a FACH. The connection setup will assign the UE Radio Network Temporary Identifiers for use within the cell (C-RNTI) and across UTRAN (U-RNTI), and it will set up dedicated radio bearers to carry RRC signalling. Four or five radio bearers (depending on the network) are used for RRC signalling during a connection:

RB0 is the CCCH logical channel. It uses TM RLC and RACH in the uplink and UM RLC and FACH in the downlink, and is used, e.g. to establish a connection.

RB1 is a DCCH logical channel using UM RLC in both directions. It is used for peer RRC messages.

RB2 is a DCCH using the AM RLC. It is used for peer RRC messages which require Acknowledged Mode (AM)

RB3 is a DCCH using AM RLC. It is used for NAS messages carried through the RRC.

RB4 is an optional DCCH using AM RLC. It is used for low priority NAS messages. If this RB is not configured then RB3 is used.