Flow Control PD

RAN

Flow Control Parameter Description

Issue 01

Date 2009-03-30

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RANFlow Control Parameter Description

About This Document

About This Document

Author

Prepared by Qian Yan Date 2008-10-16

Edited by Sun Jingshu Date 2008-12-09

Reviewed by Date

Translated by Fang Qin Date 2008-12-09

Tested by Lu Feng Date 2009-01-10

Approved by Duan Zhongyi Date 2009-03-30

Issue 01 (2009-03-30) Huawei Proprietary and Confidential Copyright © Huawei Technologies

Co., Ltd

iii


Contents

Contents

1 Change History...........................................................................1-3

2 Introduction...............................................................................2-3

3 Flow Control Principles...............................................................3-33.1 Flow Control System Model..........................................................................................................................3-3

3.1.1 System Resources..................................................................................................................................3-3

3.1.2 Flow Control Items...............................................................................................................................3-3

3.2 Flow Control Thresholds (CPU Usage)..........................................................................................................3-3

3.3 Flow Control Thresholds (Message Block Usage).........................................................................................3-3

3.4 Flow Control Algorithms................................................................................................................................3-3

3.4.1 Switch Algorithm..................................................................................................................................3-3

3.4.2 Linear Algorithm...................................................................................................................................3-3

3.4.3 Hierarchical Algorithm..........................................................................................................................3-3

3.5 RNC Load Balancing.....................................................................................................................................3-3

3.5.1 Load Balancing on the Control Plane...................................................................................................3-3

3.5.2 Load Balancing on the User Plane........................................................................................................3-3

3.6 Access Class Restriction.................................................................................................................................3-3

3.7 Signaling Flow Control of the Iu Interface.....................................................................................................3-3

3.7.1 Congestion in Signaling Links on the Iu Interface................................................................................3-3

3.7.2 Congestion in the Peer SCCP Subsystem..............................................................................................3-3

3.8 Signaling Flow Control of the Iub Interface...................................................................................................3-3

4 Flow Control Parameters.............................................................4-34.1 Description.....................................................................................................................................................4-3

4.2 Values and Ranges..........................................................................................................................................4-3

5 Reference Documents.................................................................5-3

Issue 01 (2009-03-30) Huawei Proprietary and Confidential Copyright © Huawei

Technologies Co., Ltd

v


1 Change History

1 Change History

The change history provides information on the changes in different document versions.

Document and Product Versions

Table 1-1 Document and product versions

Document Version RAN Version

01 (2009-03-30) 11.0

Draft (2009-03-10) 11.0

Draft (2009-01-15) 11.0

This document is based on the BSC6810 and 3900 series NodeBs.

The available time of each feature is subject to the RAN product roadmap.

There are two types of changes, which are defined as follows:

Feature change: refers to the change in the flow control.

Editorial change: refers to the change in the information that was inappropriately described or the addition of the information that was not described in the earlier version.

01 (2009-03-30)

This is the document for the first commercial release of RAN11.0.

Compared with draft (2009-03-10), this issue optimizes the description.

Draft (2009-03-10)

This is the second draft of the document for RAN11.0.

Compared with draft (2009-01-15), draft (2009-03-10) optimizes the description.



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1 Change History

Draft (2009-01-15)

This is the initial draft of the document for RAN11.0.



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2 Introduction

2 Introduction

Flow control refers to the internal flow control of the Radio Network Controller (RNC) by regulating the input transmission rate, which is called feedback flow control mechanism. Flow control is a set of mechanisms applied to the RNC to prevent the system from being overloaded.

Flow control suppresses and prevents overload in the RNC. Therefore, flow control ensures the stable performance of the RNC.

Flow control is performed to:

Ensure system stability and robustness.

Ensure that the high-priority services work normally when the traffic is heavy.

Intended Audience

This document is intended for:

System operators who need a general understanding of flow control.

Personnel working on Huawei products or systems.

Impact Impact on system performance

Frequent flow control decisions affect the system performance. Thus, the interval between flow control decisions cannot be too short.

Impact on other features

None.

Network Elements Involved

Table 2-1 lists the Network Elements (NEs) involved in flow control.

Table 2-1 NEs involved in flow control

UE NodeB RNC MSC Server

MGW SGSN

GGSN

HLR

- - √ √ √ √ - -



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2 Introduction

UE NodeB RNC MSC Server

MGW SGSN

GGSN

HLR

NOTE

-: not involved √: involved

UE = User Equipment, RNC = Radio Network Controller, MSC Server = Mobile Service Switching Center Server, MGW = Media Gateway, SGSN = Serving GPRS Support Node, GGSN = Gateway GPRS Support Node, HLR = Home Location Register



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3 Flow Control Principles


3.1 Flow Control System ModelRNC board software performs real-time monitoring of system resources. Flow control decisions are based on the usage of the key system resources such as the CPU and message blocks.

Figure 3-1 shows the flow control system model of the RNC.

Figure 3-1 Flow control system model

3.1.1 System ResourcesThe system determines whether to enable flow control based on the current system flow. It obtains the internal flow conditions by monitoring the following RNC system resources:



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CPU usage

The CPU is the primary resource for the system processing capabilities. When the CPU usage reaches a specified threshold, the flow control for the corresponding function is enabled. When the CPU usage is lower than a specified threshold, the flow control for the corresponding function is disabled. This ensures proper functioning of the system.

Message block usage

The message block is the primary resource for communication in the RNC. High message block usage indicates a risk of insufficient processing capability in the system. In this case, the flow control for corresponding functions is enabled to ensure proper functioning of the system. When the message block usage is lower than a specified threshold, the flow control for corresponding functions is disabled.

3.1.2 Flow Control ItemsEach flow control item corresponds to a function in the system. The system enables, disables, or partially disables the functions according to the current CPU usage and message block usage to ensure system stability and robustness.

If a flow control item is controlled, the corresponding function is disabled or partially disabled.

If a flow control item is restored, the corresponding function is enabled again.

Table 3-1 lists the flow control items for the RNC boards and subsystems.

Table 3-1 Flow control items for RNC boards

Board Type on the LMT Flow Control Item

SCUa Printing Debugging Performance monitoring Log

SPUa Printing Debugging Performance monitoring Resource auditing Paging Handover Iur uplink transfer Iur downlink transfer CBS Log Access control Cell URA update

DPUb Printing Debugging Performance monitoring



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Board Type on the LMT Flow Control Item

Log

AEUa/PEUa/AOUa/UOIa/FG2a/GOUa/POUa Printing Debugging PVC Log

GCUa/GCGa Printing Debugging Performance monitoring Log

MPU Printing Debugging Performance monitoring Resource auditing Paging Handover Iur uplink transfer Iur downlink transfer CBS Log Access control Cell URA update

3.2 Flow Control Thresholds (CPU Usage)The system checks the CPU usage once a second. When the CPU usage reaches a specified threshold and the flow control switch is on, flow control is performed.

To minimize the impact of instant fluctuation of the CPU usage on the flow control decision, the average value of CPU usage measured during the previous seconds is used. The data in the previous seconds forms a filter window, as shown in Figure 3-1.

Figure 3-1 Filter window



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When the average CPU usage in the filter window reaches or exceeds the corresponding control threshold, the associated flow control mechanism is started. When the average CPU usage is lower than the corresponding restore threshold, the associated flow control mechanism is stopped.

To control the flow in the case that the CPU usage becomes high within a short period, the system provides the fast judgment window. That is, the system compares the average CPU usage measured in the preceding period with the critical threshold. If all the CPU usage values in the fast judgment window reach or exceed the critical threshold, all the flow control items are put under control.

The filter window, fast judgment window, critical threshold, and flow control item control threshold and restore threshold of each type of board are different, and they are defined by the system software.

The critical threshold takes precedence over the thresholds of other flow control items. That is, the control mechanism of the critical threshold takes effect when the CPU usage reaches the critical threshold. The decisions of other flow control thresholds are effective only when the CPU usage is lower than the critical threshold.

3.3 Flow Control Thresholds (Message Block Usage)

A decision on the message block usage is made by the system when the system allocates the message for the 10th time. When the message block usage reaches a specified threshold, flow control is started.

Message blocks are essential to internal communication of the system. The system collapses once the message blocks are exhausted. Therefore, the system provides a critical threshold for the message block usage. When the message block usage reaches or exceeds the critical threshold, all the flow control items are put under control.

To prevent state transition of flow control due to instant increase of the message block usage, the system provides the filter window. That is, the system compares the average message block usage measured in the preceding period with the threshold.

When the average message block usage in the filter window reaches or exceeds the corresponding control threshold, the associated flow control mechanism is started.

When the average message block usage is lower than the restore threshold, the associated flow control mechanism is stopped.

Each flow control item for the message block flow control has two thresholds: the flow control item control threshold and the flow control item restore threshold.

The filter window, critical threshold, and flow control item control threshold and restore threshold of each type of board are different, and they are defined by the system software.

The critical threshold takes precedence over the thresholds of other flow control items. That is, the control mechanism of the critical threshold takes effect when the message block usage reaches the critical threshold. The decisions of other flow control thresholds are effective only when the message block usage is lower than the critical threshold. The critical threshold decision does not use the filter mechanism. That is, the size of the filter window is 1.



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3.4 Flow Control AlgorithmsThree types of flow control algorithms are adopted in the RNC: switch algorithm, linear algorithm, and hierarchical algorithm. Different services usually use different algorithms. These algorithms, however, cannot be set on the RNC LMT.

3.4.1 Switch AlgorithmThe principles of the switch algorithm are as follows:

When the resource usage, such as the CPU usage or message block usage, is higher than the control threshold of a flow control item, the flow control function for the item is disabled.

When the resource usage is lower than the restore threshold, the flow control function for the item is enabled.

Figure 3-1 shows the switch algorithm.

Figure 3-1 Switch algorithm

3.4.2 Linear AlgorithmThe principles of the linear algorithm are as follows:

When the resource usage is higher than the control threshold of a flow control item, flow control is performed.

When the resource usage is lower than the restore threshold of a flow control item, flow control is not performed.

When the resource usage is between the restore threshold and the control threshold, the flow control level changes linearly with the resource usage.

Figure 3-1 shows the linear algorithm.



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Figure 3-1 Linear algorithm

The flow control level of the linear algorithm, that is, the probability (P) of the flow control item being disabled, is calculated as follows:

P = (resource usage – restore threshold) x 100%/(control threshold – restore threshold)

3.4.3 Hierarchical AlgorithmThe principles of the hierarchical algorithm are as follows:

When the resource usage is higher than the control threshold of a flow control item, flow control is performed.

When the resource usage is lower than the restore threshold of a flow control item, flow control is not performed.

When the resource usage is between the restore threshold and the control threshold, the flow control level changes hierarchically with the resource usage.

Figure 3-1 shows the hierarchical algorithm.

Figure 3-1 Hierarchical algorithm

The flow control level of the hierarchical algorithm is calculated as follows:



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Flow control level = [(resource usage – restore threshold) x total number of flow control grades for the flow control item/(control threshold – restore threshold)]

[ ] means that an integer value is taken.

The total flow control grades for each flow control item are specified in the system software, and they cannot be set on the LMT. The flow control grades may differ for different flow control items.

3.5 RNC Load BalancingLoad balancing on the control plane and user plane can be implemented on the basis of resource sharing. Each SPUa board has four subsystems. By default, subsystem 0 of the main control SPUa board in a subrack is the main control Signaling Process Unit (SPU) subsystem, called Main Processing Unit (MPU). The MPU maintains control plane and user plane resources in the subrack and performs Destination Signaling Point (DSP) status management. Any SPU subsystem, other than the MPU, can be responsible for signaling processing. Hereinafter, the term SPU refers to an SPU subsystem on an SPUa board.

In the RNC, the SPU that controls a NodeB refers to the host SPU of the NodeB, and the subrack that is connected to a NodeB over the Iub interface refers to the host subrack of the NodeB.

3.5.1 Load Balancing on the Control PlaneWhen a service request arrives, the SPU determines whether to process the service request directly or to forward it to the MPU according to the current load and Call Attempts per Second (CAPS). The SPU can be in one of the three states shown in Figure 3-1.

Figure 3-1 Load states of the control plane

State I: The SPU has a light load. State I requires the following conditions to be met:

The message block load is lower than the message block overload threshold.

The CPU load is lower than the value of CtrlPlnSharingOutThd, and the CAPS is lower than the value of MaxCAPSLowLoad. Alternatively, the CPU load is lower than



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the CPU overload threshold, and the CAPS is lower than the value of SharingOutCAPSMidLoad.

In state I, the SPU processes all received service requests and accepts the service requests forwarded by the MPU.

State II: The SPU shares out its load. State II requires the following conditions to be met:

The message block load is lower than the message block overload threshold.

The CPU load is between the CPU overload threshold and the value of CtrlPlnSharingOutThd, and the CAPS is between the value of SharingOutCAPSMidLoad and the value of MaxCAPSMidLoad.

In state II, the SPU forwards all received service requests to the MPU and accepts the service requests forwarded by the MPU.

State III: The SPU is overloaded.

State III refers to any state other than states I and II. In state III, the SPU forwards all received service requests to the MPU, but the MPU does not forward any service request to this SPU.

When a service request arrives, the host SPU processes the service request if the SPU is in state I, or the host SPU forwards the service request to the MPU for further processing if the SPU is in state II or III. In the case of state II or III, the MPU selects a proper SPU in either the host subrack or a different subrack to process the service request.

If the control plane load of the host subrack minus the value of CtrlPlnSharingOutOffset is higher than the control plane load of any other subrack, the MPU in the host subrack forwards the service request to the MPU in the RNC with the minimum control plane load. The target MPU then forwards the service request to the SPU with the minimum load in the target subrack.

Otherwise, the MPU selects an SPU from the same subrack, that is, the host subrack. If the load of the host SPU is equal to or higher than the CPU overload threshold, or if the load of the host SPU minus the value of CtrlPlnSharingOutOffset is higher than the load of any other SPU in the host subrack, the MPU forwards the service request to the SPU with the minimum load in the host subrack.

If the MPU fails to find a proper SPU, the service request is rejected.

3.5.2 Load Balancing on the User PlaneThe MPU in a subrack manages and allocates the user plane resources in the subrack. When the load of the subrack is heavy, the MPU forwards the resource requests to other subracks.

The MPU maintains the user plane resources in the subrack and performs DSP status management. In addition, the MPU in the subrack exchanges user plane load information with other MPUs. Figure 3-1 shows how the MPUs manage and allocate user plane resources.



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Figure 3-1 User plane resource sharing

When a service request arrives, the service applies for user plane resources. In this case, the SPU requests the MPU in the host subrack for the resources. The MPU proceeds as follows:

If the user plane load of the host subrack is lower than the value of UserPlnSharingOutThd, or if the user plane load of the host subrack plus the value of UserPlnSharingOutOffset is lower than the user plane load of any other subrack, then the MPU allocates the user plane resources of the DSP with the minimum load in the host subrack to the service.

If the user plane load of the host subrack is equal to or higher than the value of UserPlnSharingOutThd, or if the user plane load of the host subrack minus the value of UserPlnSharingOutOffset is higher than the user plane load of any other subrack, then the MPU selects the subrack with the minimum load in the RNC and forwards the user plane resource request to the MPU in the target subrack. The target MPU then allocates the resources of the DSP with the minimum load in the target subrack to the service.

If the subrack with the minimum load in the RNC is overloaded, the service request is rejected.

3.6 Access Class RestrictionWhen the resource usage of the SPU subsystem is high, the access class restriction function provides access flow control. Only specific UEs are permitted to access the network, thus reducing the signaling load (call attempts).

The RNC monitors the CPU usage and message block usage of the SPU subsystem in real time. To enable the access class restriction function, ensure that AcRstrctSwitch is set to ON.

If the CPU usage or message block usage of an SPU subsystem is higher than the value of AC control threshold, the access class restriction function is enabled. The RNC performs the access class restriction for all cells under the SPU subsystem at intervals specified by AcIntervalOfCell.



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If the CPU usage or message block usage of an SPU subsystem is lower than the value of AC restore threshold, the access class restriction function is disabled and all restricted access classes of the cells are not barred.

The AC control threshold and the AC restore threshold are defined by the system software.

The UE services are categorized into 15 access classes: AC0 to AC14. The access class restriction applies sequentially to access classes AC0 to AC9 only. At any time during a specified period defined by AcRstrctIntervalLen, only a certain percentage (defined by AcRstrctPercent) of the 10 access classes are restricted. In the next period, the barred access classes are unrestricted and another set of access classes are restricted. This process goes on until the CPU usage or message block usage becomes normal.

You can run the MML command SET ACALGO to set the value of AcRstrctPercent. If AcRstrctPercent is set to 1, one AC service is restricted during access polling; if AcRstrctPercent is set to 2, two AC services are restricted during access polling. The value of AcRstrctPercent determines the number of AC services to be restricted.

The value of AcRstrctPercent for AC0 to AC9 ranges from 1 to 10.

Figure 3-1 shows an example of 10% access class restriction.

Figure 3-1 Example of 10% access class restriction

The monitoring mechanism of the CPU usage and that of the message block usage are independent from each other. The access class restriction is triggered when either of the resource usage reaches its associated AC control threshold.

3.7 Signaling Flow Control of the Iu InterfaceWhen a signaling link on the Iu interface or the peer SCCP subsystem is congested, the RNC rejects services selectively based on the congestion level. Emergency calls are not rejected. When a signaling link on the Iu interface is congested, the signaling flow control helps the link recover soon by reducing the flow on the link. When the peer SCCP subsystem is



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congested, the signaling flow control helps the subsystem recover soon by reducing the signaling exchange between the RNC and the peer SCCP subsystem.

3.7.1 Congestion in Signaling Links on the Iu Interface

The RNC checks the load of signaling links on the Iu interface and takes relevant measures if the load is heavy. With the increase of the load, the RNC rejects the access of services with lower priority and then those with higher priority. In this way, the flow control mechanism ensures that the admitted services are effectively processed in the system and that the services beyond the load capacity of the RNC are temporarily barred from the network to avoid continuous overloading of the system.

The SCCP over the Iu interface determines the congestion level, based on the congestion status reported from the lower layer. The congestion levels range from 0 through 8, where 0 denotes no congestion and 8 denotes the severest congestion.

3.7.2 Congestion in the Peer SCCP SubsystemIf the peer SCCP subsystem is congested, the RNC receives an SCCP Subsystem-Congested (SCCP-SSC) message from the peer equipment. This message indicates the congestion level.

When a Radio Resource Control (RRC) connection is set up, the RNC determines whether to start flow control according to the congestion level and the service type of the RRC connection. If the flow control conditions are met, this RRC connection setup request is rejected. The RRC connection services are of three types: short message service, call, and location registration. According to the characteristics of services, the RNC rejects setup of services upon congestion in the following order: short message service > call > location registration service. All the three types of services use the linear algorithm for flow control.

3.8 Signaling Flow Control of the Iub InterfaceThe RNC checks the load of signaling links on the Iub interface. When the load is heavy, the RNC rejects the RRC connection requests of services other than emergency calls and Attach and Detach services. The signaling flow control of the Iub interface helps to ease the congestion caused by the increase of the NCP link load and enables the NCP link to recover more rapidly.



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4 Flow Control Parameters


4.1 Description

Table 4-1 Flow control parameter description

Parameter ID Description

CtrlPlnSharingOutOffset The sharing offset should be added to the target subrack or subsystem. This parameter is used for preferable selection of the homing subrack and homing subsystem during call forwarding.

UserPlnSharingOutThd Percentage of User Plane Sharing Out threshold.

UserPlnSharingOutOffset Percentage of User Plane Sharing Out Offset.

AcRstrctSwitch OFF indicates that the AC algorithm is automatically disabled. ON indicates that the AC algorithm is automatically enabled.

AcRstrctPercent Access restriction ratio. When a cell performs AC restriction, you can select some ACs from AC0 to AC9 based on the ratio specified in this parameter and perform AC restriction on the selected ACs. After AC restriction goes on for AcRstrctIntervalLen, the original AC restriction is released and other ACs of the local cell are selected for AC restriction based on the ratio specified in this parameter.

AcIntervalOfCell Interval of automatic AC restriction between cells. When a subsystem of an RNC performs AC restriction on cells managed by this subsystem, it selects the first cell at random. After waiting for the time specified in this parameter, the subsystem selects the second cell and the first cell is still with AC restriction. The process lasts until all the cells are going through AC restriction. For detailed information of this parameter, refer to 3GPP TS 22.011.

AcRstrctIntervalLen Specifies the interval delay between consecutive pollings according to the flexible polling restriction mode.

CtrlPlnSharingOutThd Forwarding threshold of control plane load sharing. When the CPU usage is between the sharing threshold and overload threshold, and call number in each second reaches [SharingOutCAPSMidLoad], new arrival call attempts will be shared out to other CPU.

SharingOutCAPSMidLoa Numbers of incoming calls to be shared when the load exceeds the forwarding



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Parameter ID Description

d threshold. When the CPU usage is between the sharing out threshold and overload threshold, and number of incoming calls in each second reaches the threshold, new arrival call attempts will be shared out to other SPU.

MaxCAPSMidLoad Maximum numbers of incoming calls in one second when the load exceeds the forwarding threshold. When the CPU usage is between the sharing out threshold and overload threshold, and call number in one second reaches the threshold, new arrival call attempts will be shared out to other SPU and none will be shared in this SPU.

MaxCAPSLowLoad Maximum numbers of incoming calls in one second when the load is lower than the forwarding threshold. When the CPU usage is lower than the sharing out threshold and overload threshold, and call numbers in each second reach the threshold, new arrival call attempts will be shared out to other SPU and none will be shared in this SPU.

4.2 Values and Ranges

Table 4-1 Flow control parameter values and parameter ranges

Parameter ID

Default Value

GUI Value Range

Actual Value Range

Unit MML Command NE

CtrlPlnSharingOutOffset

- 1~10 0.01~0.1, step:0.01

per cent

SET CTRLPLNSHAREPARA(Optional)

RNC

UserPlnSharingOutThd

- 50~100 50~100 per cent

SET USERPLNSHAREPARA(Optional)

RNC

UserPlnSharingOutOffset

- 5~20 5~20 per cent

SET USERPLNSHAREPARA(Optional)

RNC

AcRstrctSwitch

- OFF, ON OFF, ON None SET ACALGO RNC

AcRstrctPercent

- 1~10 0.1~1, step:0.1 per cent

SET ACALGO(Mandatory)

RNC

AcIntervalOfCell

- 1~36000 10~360000, step:10

ms SET ACALGO(Mandatory)

RNC

AcRstrctIntervalLen

- 6~3600 6~3600 None SET DSACAUTOALGO(Mandatory)SET ACALGO(Mandatory)

RNC



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Parameter ID

Default Value

GUI Value Range

Actual Value Range

Unit MML Command NE

CtrlPlnSharingOutThd

- 0~100 0~1, step:0.01 per cent

SET CTRLPLNSHAREPARA(Optional)

RNC

SharingOutCAPSMidLoad

- 0~255 0~255 None SET CTRLPLNSHAREPARA(Optional)

RNC

MaxCAPSMidLoad


RNC

MaxCAPSLowLoad


RNC

The Default Value column is valid for only the optional parameters.

The "-" symbol indicates no default value.



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5 Reference Documents

The following lists the reference documents related to the feature:

1. Basic Feature Description of Huawei UMTS RAN11.0 V1.5

2. Optional Feature Description of Huawei UMTS RAN11.0 V1.5



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