RRC Connection Establishment Procedure

RRC Connection Establishment Procedure

RRC connection establishment is shown in the figure

The RRC layer in the UE leaves the idle mode and initiates an RRC connection establishment by sending a RRC Connection Request message using transparent mode  RLC on CCCH logical channel. This logical channel will be mapped onto RACH transport channel by MAC. The Physical channel maps this RACH onto PRACH and transmits this message to network. Remember that the UE cannot send such a request message spontaneously through PRACH, because this PRACH will be used by all the UEs in the cell. So the UE will listen to AICH channel to know the time slot assigned for this UE to use PRACH. (To know more about this click here).
BTS is completely transparent to this message, because it doesn’t have any RRC layer .
On the network side, upon the reception of RRC Connection Request, the RRC layer performs admission control, assigns an s-RNTI for the RRC connection and selects radio resource parameters (such as transport channel type, transport format sets etc). If a DCH is to be established, CPHY-RL-Setup and CPHY-TrCH-Config request primitives (transmitted as one RADIO LINK SETUP PDU) are sent to all Node Bs that would be involved in the channel establishment. The physical layer operation is started and confirmation primitives are returned from each Node B. RRC configures parameters on layer 2 to establish the DCCH logical channel locally. The selected parameters including the RNTI, are transmitted to the UE in an RRC Connection Setup message using unacknowledged mode on the CCCH logical channel.
Upon reception of the RRC Connection Setup message, the RRC layer in the UE configures the L1 and L2 using these parameters to locally establish the DCCH logical channel. In case of DCH, layer 1 indicates to RRC when it has reached synchronisation.
The RLC signalling link is locally established on both sides. The establishment can be mapped on either RACH / FACH or DCH by MAC. When the UE has established the RLC signalling link, it transmits an RRC Connection Setup Complete message to the network using acknowledged mode on the DCCH.