UMTS RCC States

Picture below shows the RRC states in UTRA RRC Connected Mode, including transitions between UTRA RRC connected mode and GSM connected mode for CS domain services, and between UTRA RRC connected mode and GSM/GPRS packet modes for PS domain services. It also shows the transitions between Idle Mode and UTRA RRC Connected Mode and furthermore the transitions within UTRA RRC connected mode. 

RRC States and State Transitions including GSM

CELL_DCH state is characterised by:

A dedicated physical channel is allocated to the UE in uplink and downlink.

The UE is known on cell level according to its current active set.

Dedicated transport channels, downlink and uplink (TDD) shared transport channels, and a combination of these transport channels can be used by the UE.

CELL_FACH state is characterised by:

No dedicated physical channel is allocated to the UE.

The UE continuously monitors a FACH in the downlink.

The UE is assigned a default common or shared transport channel in the uplink (e.g. RACH) that it can use anytime according to the access procedure for that transport channel. The position of the UE is known by UTRAN on cell level according to the cell where the UE last made a cell update.

In TDD mode, one or several USCH or DSCH transport channels may have been established.

CELL_PCH state is characterised by:

No dedicated physical channel is allocated to the UE.

The UE selects a PCH with the algorithm, and uses DRX for monitoring the selected PCH via an associated PICH.

No uplink activity is possible.

The position of the UE is known by UTRAN on cell level according to the cell where the UE last made a cell update in CELL_FACH state.

URA_PCH State is characterised by:

No dedicated channel is allocated to the UE.

The UE selects a PCH with the algorithm, and uses DRX for monitoring the selected PCH via an associated PICH.

No uplink activity is possible.

The location of the UE is known on UTRAN Registration area level according to the URA assigned to the UE during the last URA update in CELL_FACH state.

Call reselection procedures:

States and procedures in the cell reselection process in connected mode

When a cell reselection is triggered, the UE evaluates the cell reselection criteria based on radio measurements, and if a better cell is found that cell is selected, procedure Cell reselection. If the change of cell implies a change of radio access technology, the RRC connection is released, and the UE enters idle mode of the other RAT. If no suitable cell is found in the cell reselection procedure, the UE eventually enters idle mode.

When an Initial cell reselection is triggered, the UE shall use the Initial cell reselection procedure to find a suitable cell. One example where this procedure is triggered is at radio link failure, where the UE may trigger an initial cell reselection in order to request re-establishment of the RRC connection. If the UE is unable to find a suitable cell, the UE eventually enters idle mode.

Difference b/w Cell PCH and URA-PCH:In Cell-PCH , the UE‘s current cell is known to the RNC. If the RNC wants to exchange data with the UE, it only needs to page the UE there. If the UE changes the cell, it must perform a cell update. Also periodical cell updates can be requested by UTRAN. To perform updates, the UE must change to the CELL_FACH sub-state.

Now, If the UE is in the CELL_PCH and moving fast, a lot of cell updates have to be performed. URAs are a combination of one or several cells under one C-RNC. URAs may overlap, i.e. a cell may belong to several URAs. If UTRAN wants to transmit something to the UE, it must page the UE within the URA. The UE is responsible for URA updates – when it changes the URA – and periodic URA updates – when required by UTRAN. 

So in this way UTRAN doesn't need to keep cell level information on an UE.

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