UE RRC CELL STATES (UMTS- 3G)

UE RRC CEL STATES (UMTS- 3G)

RRC ( Radio resource control) is also called as packet data  transfer state.

The RRC handles the Control plane  signalling  at layer 3 between the UE and the RAN(Radio access network)

RRC allows the a dialog between the RAN and the UE and also between  UE and the core network.



RRC connection is a logical connection between the UE and the RAN.

There can only be one RRC connection per UE.

When there is a signalling connection between the UE and the RAN , there is an RRC Connection  and the UE is in UTRAN connected Mode. In the UTRAN connected mode the the position of the UE is known on cell level.

When the UE connection is known in the cell level it is either in
CELL_FACH
CELL_DCH
CELL_PCH state.

The RRC Connection mobility is handled by handover procedures and cell updates.

If the UE leaves the UTRAN connected mode and returns to idle mode, the RRC connection is released .

 1. CELL_DCH State :
 ---------------------------

 This state is characterized by dedicated transport channel to the UE.

 The UE is transferred from an IDLE mode to CELL_DCH mode through a setup of RRC connection
 or
 By establishing a dedicated channel from CELL_FACH state.

 Transition from CELL_DCH state to IDLE state is realised through the relaese of RRC Connection.

 When mobile is moving from idle mode to dedicated mode and data tranmission is of large amount of data then CELL_DCH
state comes in to picture

  Transition from CELL_DCH substate to CELL_FACH substate is performed when the last active NRT DCH is released and the RT RABs do not exist. ( Do not know what is NRT DCH , Research).
 
  2. CELL_FACH State:
  ----------------------------
 
  In this state the UE monitors a forward access channel (FACH). 
 
  In this state the UE is able to transmit uplink control signals and may be able to transmit small data packets on the Random access chanel (RACH).
 
  If data transmission is of small amout of data then CELL_FACH State will come in to picture

  A transition from CELL_FACH to CELL_DCH  occurs when a dedicated transport channel is established through explicit signalling.
 
  In this cell state the UE constiniously monitors the FACH state , when the data services are inactive for a while it  is moved from CELL_FACH to CELL_PCH substate as defined by an elaspsed activity timer(T313). When the timer expires the UE is moved to CELL_PCH  to reduce UE power consumption.
 
  The UE is moved from CELL_PCH to CELL_FACH  state in oder to perform cell update procedure(handover). After the cell update procedure is complete  the UE state is changed back to CELL_PCH  if there is no data activity to be performed by the UE or the network.
 
  When the RRC connection is released the UE back in the IDLE mode.
 
  3. CELL_PCH state:
  --------------------------
 
  In this susbtstate the UE listens to the PCH Transport channel.
 
  If the network wants to make any activity it has to make a paging request on the PCCH logical channel in the known cell to intiate any downlink activity.
 
  The UE initiates a cell update only when its selects a new cell. The Only function in keeping the UE in CELL_PCH state is the possibillity of any Cell update procedures that might occur when the UE moves to another cells. to reduce this function of UE in this state it is moved to URA_PCH when low activity is observed.
 
WHEN MOBILE IS MOVING FROM ONE CELL TO ANOTHER CELL IT SHOULD PERFORM HANDOVER THEN MOBILE RRC STATE WILL BE IN  CELL_PCH STATE.
 
  The UE state changes from CELL_PCH to CELL_FACH state when there is an uplink activity from the UE or any Paging command from the RNC.
 
  URA  and URA_PCH State:
  -----------------------------------
 
  URA MEANS UTRAN REGISTRATION AREA

THE AREA COVERED BY ONE RNC IS KNOWN AS URA . ( RNC in 3G , BSC in 2G)

SO WHEN MOBILE IS MOVING FROM  one URA AREA TO ANOTHER URA AREA
THEN LOCATION UPDATE SHOULD  BE PERFORMED THEN MOBLIE RRC STATE
COMES TO URA_PCH STATE .
 
  When the UE is in this cell state it performs cell updates, but if the UE is in continious mobility and is updating Cells frequently and after a number of cell updates the UE cell state is moved to URA_PCH.  Becoz in cell_pch the position of the UE is known in the cell level and everytime it moves  from one cell to another cell  it has to update the UTRAN about the  handover, so this defeats the pupose of keeping the UE in the cell-pch state. In the CELL-PCH state the UE is in connected mode and the position is known in the cell level where as in the UTRAN mode it is in IDLE.
  When the UE is in URA_PCH it only updates the UTRAN when the UE changes the URA .
  Also the service providers can define one CELL with Multiple URA's. For example a slow moving UE can be defined to a smaller URA in the URA_PCH state, where as the faster moving UE can be defined in larger URA's.
 
 One cell may belong to several diffrent URAs  and the UEs in that cell may have been registered to diffrent URAs.  SIB 2 contains a list of URA identities indicating which URAs this cell belongs to