UMTS RRC STATES

INTRODUCTION

User Equipment (UE) that is powered on, but does not have a connection to the Radio Network is defined as being in Idle Mode. A UE in Idle Mode can both access and be reached by the system.
A UE that has an RRC connection is defined as being in Connected Mode. A UE in Connected Mode using the common channels RACH (or E-DCH if Enhanced Uplink for FACH feature (FAJ 121 1652) is activated) in the UL and FACH (or HS-DSCH if Enhanced Uplink for FACH is activated) in the DL is defined as being in CELL_FACH state. A UE in Connected Mode monitoring the paging channel is defined as being in URA_PCH state. The location of a UE in URA_PCH is known by the WCDMA RAN on a UTRAN Registration area (URA) level.

Before we understand different RRC States it i important to know various RNTI used in UMTS Network

What are the different RNTI used in UTRAN …

RNTI -(Radio Network Temporary Identities) are used as UE identities within the UTRAN and in signalling messages between UE and UTRAN.
RNTI is used , so that UE original identity can be hide , so that no one can detect the UE , but as a identity UE can also send IMSI or IMEI , in which we try to avoid sending the MEI number to the network, except in the case , when UE does not have the SIM or it is not properly camped to a cell and user wants to make the “Emergency call”.

These RNTI are,
SRNC RNTI (S-RNTI)
Drift RNC RNTI(D-RNTI)
Cell RNTI(C-RNTI)
UTRAN RNTI(U-RNTI)

S-RNTI is allocated in association with RRC connection setup by the SRNC to which the UE has the RRC connection, so that both can identify each other.

D-RNTI is allocated by a DRNC in association with context establishment and is used to handle the UE connection and context over the Iur interface.

C-RNTI is allocated when the UE accesses a new cell , so that we can say that it identifies the UE when UE is in common channels ( when UE is in cell FACH), i.e. with in a cell. By this CRNC and UE communicate each other.(in most cases CRNC and SRNC are incorporated in a single RNC )

But, now the most important RNTI comes into the picture, and its the URNTI, it identifies the UE , uniquely within UTRAN. URNTI is used as a UE identifier for the first cell access , and it generated by the combination of SRNC identity and SRNTI. It is an unique one in UTRAN.

Apart from them , we also have E-RNTI and H-RNTI , used in HSUPA and HSDPA respectively, which also have the same use as the above ones.
we will explore all these more in a later post.

The U-RNTI is assigned by the SRNC and uniquely identifies the UE within the UTRAN.

It is used when a UE cannot be identified in the UTRAN, e.g.,

first access after a UE did cell change (cell update or URA update) or for UTRAN originated paging.

U-RNTI: RNC id + S-RNTI

32 bits

Why is it required to have CHANNEL SWITCHING (DIFFERENT RRC STATES)

Traffic on the Interactive Radio Access Bearer (RAB) causes large variations in offered traffic over time for a particular user. The packet-switched (PS) Interactive RAB traffic transferred through the WCDMA Radio Access Network (RAN) is generated mainly by web browsing, e-mail and file transfer. Especially web browsing causes large variations in the traffic stream. After a web page is downloaded and the user is reading the page, there is very little data to transfer. This changes when the user requests a new web page. Consequently, reserving resources for a dedicated channel continuously is not efficient.

The purpose of the Channel Switching feature is to dynamically change the physical resources allocated to a User Equipment (UE) based on the amount of data that has to be transmitted in the Uplink (UL) and in the Downlink (DL). This is achieved by switching the Interactive RAB of the UE between transport channels of different rates. When an Interactive RAB has only a small amount of data to send and receive, it is switched to common transport channels mapped onto the Random Access Channel (RACH) or E-DCH (if Enhanced Uplink for FACH is activated) in UL and Forward Access Channel (FACH) or High Speed Downlink Shared Channel (HS-DSCH) in DL. This allows more users to share the radio resources than in a circuit-switched scenario. When the traffic increases, the UE is switched back to one of the following transport channels: Dedicated Channel (DCH), Enhanced Dedicated Channel (E-DCH) or HS-DSCH , if there are resources available.

The Channel Switching feature applies only to packet traffic on the Interactive RAB, which has little or no Quality of Service (QoS) attributes that apply. It belongs to the Interactive and Background QoS classes, which have no guaranteed bit rates and no packet delay requirements. When sufficient resources are available, the Interactive RAB receives high bit rates, but when the system is heavily loaded and not many resources are available, the offered bit rates can be low. In a situation of heavy load, it can be refused any bandwidth, since there are no reserved resources for the Interactive RAB.

The Channel Switching feature switches only between transport channels. The logical channels are not affected. When large amounts of data are being sent or received, the DCH transport channel with various rates is available on both the UL and the DL. In addition, the HS-DSCH is available on the DL and E-DCH is available on the UL, both depending on UE and cell capability.

For small amounts of data, the RACH and FACH common transport channels are used. In this case, for DL, a maximum of 32 kbps is shared between all users in a cell. Alternatively, HS-DSCH without associated Uplink HS-DPCCH can be activated instead of FACH and enable bit rates higher than 32 kbps, and E-DCH can be activated instead of RACH.
For no data activity, the Interactive RAB is switched to URA_PCH state, where no transport channels are allocated and no data transmission is possible.
The Channel Switching feature works both on a Single RAB and on a Multi-RAB combination.

PROCESSES IN UE MODES/STATES

UE Idle mode -The UE shall perform a periodic search for higher priority PLMNs

UTRA RRC Connected mode :

1)URA_PCH or CELL_PCH state

In the URA_PCH or CELL_PCH state the UE shall perform the following actions:
NOTE: Neither DCCH nor DTCH are available in these states.

1> if the UE is "in service area": 

•  maintain up-to-date system information as broadcast by the serving cell
• cell reselection
• Periodic search for high priority PLMN's
• Monitoring the Paging occasions and PICH and receive paging information on PCH mapped on the S-CCPCH selected by the UE
• act on RRC Messages received on PCCH and BCCH
• perform measurment process
• maintain up to date BMC Data if it supports CBS
• act in RRC Messages received on MCCH
• run timer T305 for periodical URA update if the UE is in URA_PCH state and periodic cell update if the UE is in Cell_PCH state

2) If the UE is Out of Service area

• Perform Cell selection
• Run timer T316
• Run time T305
• if the cell selection process fails to find a suitable cell after a complete scan of all RATs and all frequency
  bands supported by the UE, the UE shall after a minimum of TimerOutOfService time (default value 30 s) of being "out of service area"
• indicate all available PLMNs to NAS to enable the selection of a new PLMN
• if an acceptable cell is found then the UE shall camp on that cell to obtain limited service
•  else if no acceptable cell is found, the UE shall continue looking for an acceptable cell

CELL_FACH State

In the CELL_FACH state the UE shall perform the following actions:
NOTE: DCCH and, if configured, DTCH are available in this state.

1> if the UE is "in service area":

• maintain up-to-date system information as broadcast by the serving cell
• Cell Reselection
• Perform Meausrement process
• Run Timer T305
• Select and Configure the RB Multiplexing option applicable for the transport channels to be used in RRC states
• listen to all FACH transport channels mapped on the S-CCPCH selected by the UE
•  act on RRC messages received on BCCH, CCCH and DCCH
•  act on RRC messages received on MCCH if it supports MBMS and has activated

2> if the UE is "out of service area":

• perform cell selection process
• run timers T305 (periodical cell update), and T317 (cell update when re-entering "in service") or T307 (transition to Idle mode), if started
• run timers T314 and/or T315, if started
• if the cell selection process fails to find a suitable cell after a complete scan of all RATs and all frequency
  bands supported by the UE, the UE shall after a minimum of TimerOutOfService time (default value 30
  seconds) of being "out of service area":
  3> indicate all available PLMNs to NAS to enable the selection of a new PLMN;
  3> if an acceptable cell is found then the UE shall camp on that cell to obtain limited service

CELL_DCH state
In the CELL_DCH state the UE shall perform the following actions:
NOTE: DCCH and, if configured, DTCH are available in this state.
1> read the system information as specified in subclause 8.1.1 (for UEs in TDD mode);
1> perform measurements process
1> select and configure the RB multiplexing options applicable for the transport channels to be used in this RRC
state;
1> act on RRC messages received on DCCH;
1> act on RRC messages received on BCCH (applicable only to UEs with certain capabilities and in FDD mode);
1> act on RRC messages received on MCCH if it supports MBMS and has activated an MBMS service as 1> act on RRC messages received on BCCH (TDD only) and, if available, SHCCH (TDD only)

What is URA and URA_PCH state?

URA or UTRAN Registration Area is a colection of cells that are used for fast moving UE's in Connected mode when they are not transferring any data. In this case the UE is in CELL_PCH state. Everytime a fast moving UE in CELL_PCH state changes the cell, a CELL UPDATE needs to be performed to let the UTRAN know of the new position of the UE. This is done because in the connected mode (CELL_PCH), UE is known at cell level rather than UTRAN level as in IDLE state. If too many CELL UPDATES are performed, it defeats the purpose of UE being in CELL_PCH. Hence in this case the UE is put in URA_PCH state. Now the UE will perform CELL UPDATE only when the URA is changed for a UE. The drawback is that when UE needs to be paged the paging area is now extended to many cells belonging to the URA. 

Also Note that the CELL_PCH state is actually a subset of the URA_PCH state. It is possible to define overlapping URAs to be used in the URA_PCH state. Thus, the UTRAN operator could define that each cell is a separate URA in addition to other larger URAs. Then the operator could assign small one-cell URAs for slow-moving mobiles, and larger URAs for mobiles with greater mobility. The small URAs could nicely perform the task of the CELL_PCH state. However, it has been decided to keep these states separate. 



The URAs can be overlapping or even hierarchical. The same cell may belong to several different URAs, and the UEs in that cell may have been registered to different URAs. SIB 2 contains a list of URA identities indicating which URAs this cell belongs to. This arrangement is done to further reduce the amount of location update signaling because now the UEs moving back and forth in the boundary area of two URAs do not have to update their URA location information if the boundary cells do belong to both URAs.