6.7 The RRC state machine

23.2213GPPArchitectural requirementsRelease 17TS

The UTRAN RRC state machine is a description model of how the UE and the RAN co-operate regarding RRC functionality. The RRC state describes the state of the UE in the RAN. Here follows a brief description of the UTRAN RRC state machine, for more information see TS 25.301 [6] and TS 25.303 [7]. The E-UTRAN RRC state machine is described in TS 36.331 [29].

NOTE: RRC idle mode and RRC connected mode refer to the UE idle mode and UE connected mode respectively in TS 25.301 [6] and TS 25.303 [7].

The RRC state machine exists as peer entities, one in the UE and one in RAN. Apart from transient situations and error cases they are synchronised. Figure 6.4 illustrates the main modes/states of the RRC state machine.

Figure 6.4: RRC modes, main RRC states and main mode/state transitions

RRC-Idle_mode:

In the Idle mode there is no connection established between UE and RAN. There is no signalling between RAN and the UE except for system information that is sent from RAN down link on a Broadcast channel to the UE. The UE can also receive paging messages with a CN identity on the PCH. There is no information on the UE stored in RAN in this state.

RRC-Connected_mode:

In the Connected mode the main states are Cell Connected state and URA connected state. In this mode there is one RNC that is acting as Serving RNC (SRNC), and an RRC connection is established between the UE and this SRNC.

– When the UE position is known on cell level, the UE is in the cell connected state. When in cell connected state, the RRC connection mobility is handled by handover procedures.

– When the UE position is known on URA level, the UE is in the URA connected state. The URA contains a set of cells. URA updating procedures provides the mobility functionality in this state. In URA connected state no dedicated radio resources are used.