RRC states and RRC State transitions

We would try to find the answer for below questions:

What are various RRC States and why?

How and when to move a UE from one state to another state?

Is it worth putting extra burden of signaling on UE and Network?

What is Paging and Cell update?

RRC States: 

UE can be either in connected mode or in idle mode in UMTS network. While being in connected mode a UE can stay in one of these 4 RRC States/Modes which are:

• CELL_DCH  
• CELL_FACH
• CELL_PCH
• URA_PCH

When a UE sends a request to a Network for connection establishment then the Network has the prerogative to select appropriate RRC state for the UE based upon the certain parameters like Cause, Resources,  Qos, etc.   
A UE can only be admitted into states where UL and DL data transfer can be possible like CELL_DCH or CELL_FACH, However a UE can also be released from CELL_PCH state(supported only in R5 and above) other than CELL_DCH and CELL_FACH. 

An UMTS Network always takes below parameters into account to ensure best results:

• Limited Power, (Most impotently UE power)
• Limited Channel Resource( Code in case of UMTS)
• Qos (Service demanded by UE)

Different RRC States have been introduced for better resource utilization, for better connectivity, for best Qos with less possible power uses.  So to achieve all this, Network implements algorithms which take all the above points into account and directs a connected UE to move from one state to another state.

In this post I would like to talk about CELL_DCH, CELL_FACH and CELL_PCH states only and will take 3GPP Rel 6 into consideration.

RRC States:

CELL_DCH:

As the name suggests CELL_DCH  is the dedicated state where UE gets the access to the costly dedicated and data intensive resources in the network. The Transport and Physical channels which can be allocated by the network to the UE in CELL DCH state are:

Transport Channels:

• DCH (UL/DL)
• HS-DSCH (DL)
• EDCH (UL)

Physical Channels:

• DPDCH/DPCCH
• HS-PDSCH/HS-SCCH/HS-DPCCH
• E-DPDCH/E-DPCCH/E-RGCH/E-HICH/E-AGCH

All above DCH channels are used for high data rate services(CS AMR is a real time service and can only be in DCH) so consumes more Network and UE resources like Battery and processing resources in UE and Power, Code, Processing resources in Network. If we talk specifically about the PS connection the traffic behavior is bursty in nature like if we access an internet page and go idle, in this scenario having DCH resource allocated for a long time is not a good idea. To deal with this scenario network can either have a low resources intensive state or may put UE in Idle, But putting UE in idle can cause more problem as network has to bring the UE back to the network as soon as the data requirements comes for the UE.

And that is why UMTS supports a low data rate state with shared resources called CELL_FACH

An example scenario:

if((UE_Data_Rate< 3kbps) && (for Time T))
{ Put UE in FACH } 

CELL_FACH:

UE having only low data rate PS connection can be moved from DCH to FACH (obviously as CS calls cannot be established in FACH STATE) and also A UE can be directly admitted into FACH STATE based on the UE admission cause(eg Registration), However it also depends on admission control algorithms sometimes.  Suppose a UE just wanted to do location update/Routing area update then allocating and de-allocating DCH resource is pointless, such UEs can be admitted in FACH, can setup a PS RAB in FACH and also can be moved to DCH if required to transfer high volume of PS Data or wanted to make CS Call.  The transport and physical channels are as follows in CELL_FACH State.

Transport Channels:

• RACH (UL)
• FACH (DL)

Physical Channels:

• PRACH, AICH
• SCCPCH

The capacity of data transfer of FACH channel is generally 32kbps and can also be increased of decreased slightly. FACH and RACH are shared channels with very low capacity. All the UEs in the CELL_FACH state have only these channels available (till Rel 6, In later releases 3GPP has also added HS-DSCH and EDCH in CELL-FACH state).

• The UE is known on cell level according to the cell where the UE last made a cell update.
• Use C-RNTI assigned in the current cell as the UE identity on common transport channels unless when a new cell is selected
• UE performs cell reselection and upon selecting a new UTRA cell, initiate a cell update procedure

If an UE is being idle for a longer period of time with PS connection then it's a point of concern for network because there is a certain limit on the number of UE supported in this state and also congestion. To deal with this scenario again network has two option either it can put the UE into IDLE or put it in a power/resource saving connected mode call CELL_PCH. Putting UE back to idle is tough task as network has to do fresh setup for the UE if the data requirement comes and may result into the flooding of signaling messages and high latency, If supported network will put this UE in CELL-PCH state.

CELL_PCH:

Network does not provide any resource to the UE to do any UL or DL data transfer in CELL_PCH. In this state UE can only  listen to paging requests sent by network in DL or if it has to send data in UL then it has to come to CELL_FACH rather transient CELL_FACH state and send CELL_UPDATE_IND to the network with cause UL_DATA_TRANSFER. We can say that while in CELL_PCH state UE uses CELL_FACH as stepping state momentarily without the consents of network. 

• UE monitors the paging occasions according to the DRX cycle and receive paging information on the PCH
• The UE is known on cell level according to the cell where the UE last made a cell update in CELL_FACH state
• UE mobility is performed through cell reselection procedures. The UE performs cell reselection and upon selecting a new UTRA cell, it moves to CELL_FACH state and initiates a cell update procedure in the new cell.
• Network uses URNTI to identify a UE is CELL_PCH state wich is SRNTI + SRNC-ID a unique identifier of RRC Connection in a PLMN.
• Paging Type 1 sent (on PCCH) is used to page a UE in CELL_PCH state.

Transport Channels:

• PCH (DL)

Physical Channels:

• PICH, SCCPCH

Paging a UE in cell PCH_STATE is more efficient than paging a UE is Idle state. IU-PS Network connection is maintained with CN and network also preserves the UE context at various back-end nodes but UE does not have any radio resources at hand .

State Transitions:

TBD..