There are two states:
Idle Sate: There is no uplink connection. Utran don't have any information about this UE.
UE has to monitor regularly to perfom cell-reselection when necessary
-> For the above UE has to monitor the reception of Broadcast info
-> and also reception of Pagin message for this UE
UE will be allways identified by NAS identities (IMSI,IMEI,TMSI/P-TMSI)
Monitors downlink PCH/PICH but not DCH/DPCH
Connected Mode: There are 4 states CELL_DCH, CELL_PCH, URA_PCH, CELL_FACH
UE will be in Connected Mode when RRC Connection is established and gets RNTI which is used in Common Transport Channels
CELL_DCH: When dedicated connection exists in both directions.
Entered into this state when RRC Connection Established with Dedicated Channels.
and comes out when the connection is released.
CELL_FACH: Here there is no dedicated connection but stilldata can be transfered. This will be done through common channels. This will be helpful to transfer less amount data or bursy transfer using RACH and FACH for d/l and u/l.
Cell Fach state requires the mobile to monitor FACH which takes battery. So, if there is no data transmission UE can move to CELL_PCH state.
*In RRC Conn.setup message the file RRC StateIndicator tell to the n/w for which state UE wants to move i.e CELL DCH
--> The same filed will be present in Cell Update it can mention URA_PCH also
--> Radio Bearer Setup /release also has this RRCStateIndicator field.
In this tate, UE will monitor FACH continuosly and no soft or hard HO might be initiated.
At this state UE uses C-RNTI as UE Identity.
Initiates Cell Update on cell change of another URA
CELL_PCH: Much like idle Mode
But will be listening to PICH , but here no UPLINK activity
Broacast also received but no soft or hard Handovers
*The diff.between Idle mode and CeLL_PCH state is, RRC Connection still exists . A simple paging or any uplink access will move to CELL_FACH
To move to idle state, it should move to CELL_FACH state and then move to idle. This inturn triggers RRC release Message.
For cell change also UE should move to CELL_FACH. (Reason being is CELL_PCH state, there will not be any uplink information. To send cell_update uplink message UE will move to CELL_FACH and sends the message)
Similar to CELL_PCH but every cell change does not trigger CELL_UPDATE.
Here Update procedure hapens only when UEs UTRAN registered area changes.
UE will be moved into this state when n/w observes the UE Activity is very low.
*Cons: When UTRAN is paging for this UE, N/w should expand the paging area from single cell to several cells as UEs location is not acurately known
Actually NAS is not bothered about the internal states of RRC as this is internal info of RRC. For NAS what matters is whether UE is idle or connected state. If is in connected NAS always sends data and RRC will manaage these state accodingly.