Download - WPO-14 Signaling Flow of WCDMA Radio Interfaces

Signaling Flow of WCDMA Advanced Radio Interfaces

ZTE University

Objectives

• To understand the main service flows and classification of WCDMA radio interfaces.

• To understand the handover flow in the system.

• To understand the call service flow.

Content

Overview of the Basic Signaling Flow

Handover Flow in the System

Call Service Flow

Service Release Flow

Classification of Flows

In terms of protocol stack, signaling flows can be access layer signaling flows or non-access layer signaling flows.

In terms of network composition, signaling flows can be categorized as circuit-switched or packet-switched.

Access layer and non-access layer signaling flows are actually so named from the perspective of protocol stack.

Classification of Flows

The flows at the access layer include PLMN selection, cell selection, and radio resource management flows.

The radio resource management flows are the flows at the RRC layer, including the RRC connection setup flow, flow of the signaling setup between UEs and CNs, RAB setup flow, call release flow, handover flow, and SRNS redirection flow.

Called service flow

Startup

Cell SearchingStartup

Cell Searching

Location

UpdateLocation

Update

StandbyStandby

RRC

Connection SetupRRC

Connection Setup

NAS

Connection SetupNAS

Connection Setup

HandoverHandoverCell/URA

UpdateCell/URA

Update

RL

ReconfigurationRL

Reconfiguration

RB/RAB

ReconfigurationRB/RAB

Reconfiguration

RAB

AssignmentRAB

Assignment

RRC

Connection ReleaseRRC

Connection Release

Call/Called

initiateCall/Called

initiate

①

②

③

④ ⑤ ⑥

⑦

⑧

⑨

UE Working Modes

There are two basic working modes: Idle

UE stands by without any service. There’s no connection between UE and UTRAN.

Connected UE transferred to the connected mode after

accomplishing RRC Connection Setup. UE has 4 states under the connected mode:

– Cell-DCH, Cell-FACH, Cell-PCH, URA-PCH

UE State

UE States Under the Connected Mode

Cell-DCH UE is activated, and in service using its dedicated

channels. There are dedicated channels in both uplink and

downlink. UTRAN knows exactly which cell the UE is in.

Cell-FACH UE is activated, but without too much date to transmit,

so there’s no need to assign dedicated channels to it. Downlink data transmitted in FACH, while uplink in

RACH. UTRAN knows exactly which cell the UE is in.

UE States Under the Connected Mode

Cell-PCH There is no data transmitted in both uplink and downlink. UE needs to listen to PICH for paging information. UE works in discrete receiving, which saves the power

effectively. UTRAN knows exactly which cell the UE is in. UE needs to do cell update procedure when it passes cells.

URA-PCH There is no data transmitted in both uplink and downlink. UE needs to listen to PICH for paging information and

receive discretely. UTRAN only knows which URA (UTRAN Registration Area)

UE exists. UE updates its location information only when its URA

changes, which further reduces the signaling saves resource.

RRC State Transition (I)

After RRC Connection Setup, UE will

transit to CELL_FACH or CELL_DCH.

CN indicates UE using dedicated channel

(CELL_DCH) or common channel

(CELL_FACH) according to the service

request.

If there is no service right now, UE will be

indicated into PCH mode or release RRC

connection back to IDLE mode.

URA_PCH

CELL_DCH CELL_FACH

CELL_PCH

IDLE

DEAD

IDLE

DEAD

CELL_DCH CELL_FACH

IDLE

Sketch of RRC State Transition

—— Transit from IDLE Mode

URA_PCH

CELL_DCH

CELL_PCH

IDLE

DEAD

IDLE

DEAD

CELL_FACH

IDLE

RRC State Transition (II)

UE do not occupy dedicated channels In the mode of CELL_FACH.

UE receives downlink data through FACH(S-CCPCH).

UE transmits uplink data through RACH(PRACH).

——Transit from CELL_FACH Mode


URA_PCH

CELL_DCH CELL_FACH

CELL_PCH

IDLE

DEAD

IDLE

DEAD

CELL_DCH

IDLE

URA_PCH CELL_PCH

CELL_DCH

RRC State Transition (III)

UE transceives uplink and downlink data through dedicated channels (DPDCHs and DPCCHs).

During the time when there is no data transmitted, CN may indicate UE’s transiting to URA_PCH or CELL_PCH.

——Transit from CELL_DCH Mode


URA_PCH

CELL_DCH CELL_FACH

CELL_PCH

IDLE

DEAD

IDLE

DEAD

CELL_DCH

IDLE

CELL_DCH

URA_PCH CELL_PCH

CELL_FACH

——CELL_PCH和 URA_PCH

RRC State Transition (IV)

UE has no activated uplink.

Paging Information makes UE back to

CELL_FACH or CELL_DCH mode.

In CELL_PCH, UE should transit to

CELL_FACH first to finish cell updating, and

go back if no service request.

In URA_PCH, if URA changes, UE also

transits to CELL_FACH to finish cell updating

first before it goes back to URA_PCH.

System resource can be saved in these two

modes.Sketch of RRC State Transition

Content



Call Service Flow


Category of Handover

Intra-RNC, inter-Node B Inter-RNC

Soft handover (SHO)

Same Node B, Inter-sector

Softer handover

Intra-frequency Inter-frequency Inter-system (3G&2G) Inter-mode (FDD&TDD)

Hard handover (HHO)

WCDMA system support

multiple handover technology

Handover Demonstration

Soft

Handover

Hard

Handover

A

B

C

A

B

C

A

B

C

A

B

C

A

B

C

A

B

C

Soft Handover/Softer Handover

Soft Handover

Soft-Softer Handover

Softer Handover

Between Cells in NodeB

In this case, the radio uplink can be merged in NodeB or the SRNC. If the radio uplink is merged in NodeB, it is known as a softer handover.

Between NodeBs in the Same RNC

The softer handover flow is basically the same as the soft handover flow between NodeBs. The only difference is that a softer handover is a handover in NodeB, with Iub interface message as RADIO LINK ADDITION REQUEST, while the switching Iub interface message between NodeBs is RADIO LINK SETUP REQUEST.

Between RNCs

Inter-Frequency Hard Handover under the Same RNC

Inter-RNC Hard Handover under the same MSC

Cell Update with SRNS Relocation

Cell Update via Iur without SRNS Relocation

Content



Call Service Flow


Establishing an RRC Connection on a Common Channel

Establishing an RRC Connection on a Dedicated Channel

RRC Connection Rejection

If the RNC determines that the RRC connection cannot be established (for example, due to insufficient resources), the RNC directly sends the UE an RRC CONNECTION REJECT message which contains the reason for the rejection of the RRC connection.

Initial Direct Transfer

The initial direct transfer process is used to establish a signaling connection between the RNC and CN, carrying an initialized NAS message. The contents of the NAS message is not explained in the RNC but forwarded to the CN.

Description When the UE is in the CELL_PCH or URA_PCH state, for initial direct

transfer, a cell update is performed first so that the state of the UE transitions to CELL_FACH, with a update reason of "uplink data transmission". When the cell update is completed successfully, the UE continues to perform initial direct transfer.

Uplink Direct Transfer

When the UE needs to send a NAS message to the CN on an existing signaling connection, the UE initiates the uplink direct transfer procedure.

Description of the Signaling Flow: The UE sends an UPLINK DIRECT TRANSFER message to the RNC

to initiate the uplink direct transfer process. The message contains such information as NAS message and CN identification.

The RNC routes the message according to the CN identifier in the message and sends the NAS information carried in the message to the CN through a DIRECT TRANSFER message on Iu interface. The uplink direct transfer process is complete.

Signaling Flow Description

The CN sends a DIRECT TRANSFER message to the RNC to initiate the downlink direct transfer process. The message contains the NAS message.

The UTRAN sends a DOWNLINK DIRECT TRANSFER message through the DCCH channel in AM RLC mode. The message carries the NAS information that the CN sends to the UE and CN identifier.

The UE receives and reads the DOWNLINK DIRECT TRANSFER message carrying the NAS information. If the received message contains a protocol error, the UE sends an RRC STATUS message on the uplink DCCH in AM RLC mode.

UE Capability Information Flow

Through a UE CAPABILITY ENQUIRY message, the UTRAN requests the UE to initiate the UE capability query process. The UTRAN sends a UE CAPABILITY ENQUIRY message through the downlink DCCH logical channel in AM RLC mode to complete the UE capability enquiry process.

UE Capability Information Update

If the UTRAN initiates the UE capability information enquiry process or the UE capability information changes during the RRC connection, the UE initiates the UE capability information update process.

The UE capability information update process is used to transfer the radio network related capabilities supported by the UE to the UTRAN.

The UE sends a UE CAPABILITY INFORMATION message in AM or UM RLC mode on uplink DCCH. The message carries the UE capability information.

The UTRAN reads the UE capability information and sends a UE CAPABILITY INFORMA CONFIRM message in AM or UM RLC mode on the downlink DCCH channel. The UE capability information update process is complete.

Content



Call Service Flow


Signaling Connection Release Request

The Iu connection release flow is generally initiated directly the CN and can also be initiated by the CN at the request of the UTRAN. The Iu connection release request flow is used by the UTRAN to request the CN to initiate the Iu connection release process.

The SRNC sends an IU RELEASE REQUEST message to the CN domain to initiate the Iu connection release request process. The message indicates the reason for releasing the Iu connection. The CN determines how to react to the Iu connection release request. For example, if the CN decides to release the Iu connection, the CN initiates the Iu connection release process.

Signaling Connection Release The Iu connection release process is used by the CN to release an Iu

connection, releasing all the UTRAN resources related to a specific Iu connection. The following shows the signaling flow.

The CN sends an IU RELEASE COMMAND message to the UTRAN to initiate the signaling connection release process. The message contains the reason for releasing the signaling connection, for example, "Successful Relocation", "Normal Release", "Release due to UTRAN Generated Reason", "Relocation Cancelled", and "No Remaining RAB". After sending the message, the CN no longer sends any connection-oriented RNAP message on this connection.

Upon receiving the message, the RNC clears the related resources in the UTRAN. The RNC sends an IU RELEASE COMPLETE message to the CN. The Iu connection release process is complete.

RAB Release Flow

Joint Release Flow of CS-Domain lu Signaling Connection and RAB

If only one service is created in the CS domain, when the service is released, the MSC first sends an IU RELEASE COMMAND message to the RNC. Upon receiving the message, the RNC releases the Iu signaling connection and RAB on Iu-CS interface at the same time.

Signaling Flow

Releasing an RRC Connection Established on a Dedicated Channel