flow control(ran13.0 03)

Flow Control RAN13.0

Feature Parameter Description

Issue 03

Date 2012-05-30

HUAWEI TECHNOLOGIES CO., LTD.

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WCDMA RAN

Flow Control Contents

Issue 03 (2012-05-30) Huawei Proprietary and Confidential

Copyright Huawei Technologies Co., Ltd

i

Contents

1 Introduction ................................................................................................................................ 1-1

1.1 Scope ............................................................................................................................................ 1-1

1.2 Intended Audience ........................................................................................................................ 1-1

1.3 Change History .............................................................................................................................. 1-1

2 Overview ..................................................................................................................................... 2-1

2.1 Definition ....................................................................................................................................... 2-1

2.2 Overall Picture of Flow Control ..................................................................................................... 2-1

3 Flow Control for Overloaded RNC Units............................................................................. 3-1

3.1 Principle ......................................................................................................................................... 3-1

3.1.1 Overview ............................................................................................................................... 3-1

3.1.2 CPU Usage Monitoring ......................................................................................................... 3-2

3.1.3 Message Block Occupancy Rate Monitoring ........................................................................ 3-2

3.2 Whole Picture of Flow Control for Overloaded RNC Units ............................................................ 3-3

3.3 Flow Control Triggered by CPUS Overload .................................................................................. 3-6

3.3.1 Overview ............................................................................................................................... 3-6

3.3.2 CPUS Basic Flow Control..................................................................................................... 3-6

3.3.3 Access Control ...................................................................................................................... 3-7

3.3.4 Paging Control ...................................................................................................................... 3-8

3.3.5 RRC Flow Control ................................................................................................................. 3-9

3.3.6 Flow Control on Signaling Messages over the Iur Interface................................................3-11

3.3.7 CBS Flow Control ............................................................................................................... 3-12

3.3.8 Cell/URA Update Flow Control ........................................................................................... 3-12

3.3.9 Flow Control over the Iur-g Interface .................................................................................. 3-13

3.3.10 Measurement Report Flow Control .................................................................................. 3-14

3.3.11 Queue-based RRC Shaping ............................................................................................. 3-14

3.4 Flow Control Triggered by MPU Overload .................................................................................. 3-16

3.4.1 Basic Flow Control for the MPU ......................................................................................... 3-16

3.4.2 MPU Overload Backpressure ............................................................................................. 3-17

3.5 Flow Control Triggered by INT Overload ..................................................................................... 3-18

3.5.1 INT Basic Flow Control ....................................................................................................... 3-18

3.5.2 Flow Control Triggered by INT Overload on the Control Plane .......................................... 3-19

3.5.3 Flow Control Triggered by Iub Interface Board Overload on the User Plane ..................... 3-20

3.6 Flow Control Triggered by DPU Overload ................................................................................... 3-20

3.6.1 DPU Basic Flow Control ..................................................................................................... 3-20

3.6.2 Flow Control Triggered by DSP CPU Overload .................................................................. 3-20

3.7 Flow Control Triggered by SCU Overload ................................................................................... 3-21

3.7.1 Principle .............................................................................................................................. 3-21

3.7.2 Overload Indication ............................................................................................................. 3-21

3.8 Flow Control Triggered by GCU Overload .................................................................................. 3-21

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3.8.1 Principle .............................................................................................................................. 3-21

3.8.2 Overload Indication ............................................................................................................. 3-22

4 Flow Control Triggered by NodeB/Cell Overload ............................................................. 4-1

4.1 CAPS Control ................................................................................................................................ 4-1

4.1.1 Principle ................................................................................................................................ 4-1

4.1.2 Overload Indication ............................................................................................................... 4-3

4.2 PCH Congestion Control ............................................................................................................... 4-4

4.2.1 Principle ................................................................................................................................ 4-4

4.2.2 Overload Indication ............................................................................................................... 4-4

4.3 FACH Congestion Control ............................................................................................................. 4-4

4.3.1 Overview ............................................................................................................................... 4-4

4.3.2 Flow Control Based on Limited Number of UEs in the CELL_FACH State .......................... 4-5

4.3.3 CCCH Flow Control .............................................................................................................. 4-7

4.3.4 DCCH Flow Control .............................................................................................................. 4-9

4.3.5 DTCH Flow Control .............................................................................................................4-11

4.3.6 FACH Efficiency Boost ....................................................................................................... 4-12

5 Flow Control over the Iu Interface ........................................................................................ 5-1

5.1 SCCP Flow Control ....................................................................................................................... 5-1

5.1.1 Overview ............................................................................................................................... 5-1

5.1.2 Flow Control Based on Iu Signaling Load ............................................................................ 5-2

5.1.3 Flow Control Based on SCCP Setup Success Rate ............................................................ 5-2

5.1.4 CN SCCP Congestion Control ............................................................................................. 5-3

5.2 Flow Control Triggered by CN RANAP Overload .......................................................................... 5-3

6 Service Flow Control ............................................................................................................... 6-1

7 Load Sharing .............................................................................................................................. 7-1

7.1 Overview ....................................................................................................................................... 7-1

7.2 Load Sharing on the Control Plane ............................................................................................... 7-2

7.2.1 Procedure for Load Sharing on the Control Plane ............................................................... 7-2

7.2.2 Service Request Processing by a CPUS ............................................................................. 7-4

7.3 Load Sharing on the User Plane ................................................................................................... 7-6

7.3.1 Overview ............................................................................................................................... 7-6

7.3.2 Procedure for Load Sharing on the User Plane ................................................................... 7-6

8 Engineering Guidelines ........................................................................................................... 8-1

8.1 Access Control and Domain-Specific Access Control ................................................................... 8-1

8.1.1 Factors That Affect Access control and Domain-Specific Access Control ............................ 8-1

8.1.2 Configuration Principles and Suggestions............................................................................ 8-1

8.1.3 Performance Optimization .................................................................................................... 8-2

8.1.4 Key Parameter Settings........................................................................................................ 8-2

8.2 Queue-based RRC Shaping ......................................................................................................... 8-2

8.2.1 Factors That Affect Queue-based RRC Shaping ................................................................. 8-2

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8.3 CAPS Control ................................................................................................................................ 8-2

8.3.1 Factors That Affect CAPS Control ........................................................................................ 8-2


8.3.3 Performance Optimization .................................................................................................... 8-3

9 Parameters ................................................................................................................................. 9-3

10 Counters ................................................................................................................................. 10-1

11 Glossary .................................................................................................................................. 11-1

12 References ............................................................................................................................. 12-1

13 Appendix - Flow Control Algorithms ............................................................................... 13-1

13.1 Switch Algorithm ........................................................................................................................ 13-1

13.2 Linear Algorithm ........................................................................................................................ 13-1

13.3 Hierarchical Algorithm ............................................................................................................... 13-2

WCDMA RAN

Flow Control 1 Introduction



1-1

1 Introduction

1.1 Scope

This document concerns the feature WRFD-040100 Flow Control. It describes the functions and principles of RNC flow control, as well as the overload indications.

Load control includes admission control and overload control. The goal is to ensure service quality and maximize system capacity. Flow control is part of overload control. Overload control works for the air interface, equipment, and the Iub/Iu interface. In addition, end-to-end (E2E) flow control can be implemented for the radio access network (RAN).

This document describes flow control for overloaded RNC units, flow control triggered by NodeB/cell overload, flow control over the Iu interface, and flow control on user services. For details about E2E flow control for the RAN, see the E2E Flow Control Feature Parameter Description. For details about other types of overload control, see the Load Control Feature Parameter Description.

This document describes the principles of flow control. If you need specific overload control measures for mass gathering events, contact Huaweis professional service teams, who can provide tailored solutions.

To learn more about admission control, see the Call Admission Control Feature Parameter Description.

1.2 Intended Audience

This document is intended for:

Personnel who have a good understanding of WCDMA principles

Personnel who need to learn about flow control

Personnel who work on Huawei products

1.3 Change History

This section describes the change history of this document. There are two types of changes:

Feature change: refers to a change in the flow control feature of a specific product version.

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

Document Issues

The issues of this document are as follows:

03 (2012-05-30)

02 (2011-10-30)

01 (2011-04-30)

Draft B (2011-03-30)

Draft A (2010-12-30)

03 (2012-05-30)

This is the third commercial release of the document for RAN13.0.

Compared with 02 (2011-10-30) of RAN13.0, this issue incorporates the changes described in the following table.

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Change Type Change Description Parameter Change

Feature change The switch for changing the maximum number of UEs in the FACH state from 30 to 60 is now RESERVED_SWITCH_0_BIT1 under ReservedSwitch0, instead of PERFENH_FACH_USER_NUM_SWITCH under PerfEnhanceSwitch. PERFENH_FACH_USER_NUM_SWITCH is no loger used. For details, see section 4.3.2 "Flow Control Based on Limited Number of UEs in the CELL_FACH State."

None

The wait time for RRC connection attempts can be configured based on service types. For details, see 4.1 "CAPS Control."

Added the RsvdPara3 parameter, which is used for configuring the wait time for RRC connection attempts.

Corrected the function of the DSPRestrainCpuThd parameter in user-plane load sharing. For details, see 7.3.2 "Procedure for Load Sharing on the User Plane."

None

Editorial change Optimized wording for the following functions: flow control triggered by CN RANAP overload. For details, see section 5.2 "Flow Control Triggered by CN RANAP Overload."

None

02 (2011-10-30)

This is the second commercial release of RAN13.0.

Compared with issue 01 (2011-04-30) of RAN13.0, this issue incorporates the changes described in the following table:


Feature change None None

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1-3


Editorial change Optimized the entire document and added the following sections:

3.3.3 "Access Control"

3.3.4 "Paging Control"

3.3.5 "RRC Flow Control"

3.3.6 "Flow Control on Signaling Messages over the Iur Interface"

3.3.7 "CBS Flow Control"

3.3.8 "Cell/URA Update Flow Control"

3.3.9 "Flow Control over the Iur-g Interface"

3.3.10 "Measurement Report Flow Control"

3.3.11 "Queue-based RRC Shaping"

3.5.2 "Flow Control Triggered by INT Overload on the Control Plane"

3.6.2 "Flow Control Triggered by DSP CPU Overload"

4.2 "PCH Congestion Control"

4.3 "FACH Congestion Control

For changes in parameters, see related descriptions in the chapters.

01 (2011-04-30)

This is the first commercial release for RAN13.0.

Compared with issue Draft B (2011-03-30) of RAN13.0, this issue optimizes the description.

Draft B (2011-03-30)

This is an updated draft for RAN13.0.

Compared with issue Draft A (2010-12-30) of RAN13.0, this issue optimizes the description.

Draft A (2010-12-30)

This is the first draft for RAN13.0.

Compared with issue 01 (2010-03-30) of RAN12.0, this issue incorporates the changes described in the following table:


Feature change None None

Editorial change Added "MPU Overload Backpressure." None

WCDMA RAN

Flow Control 2 Overview



2-1

2 Overview

2.1 Definition

Flow control is a protective measure for communications between the RNC and its peer equipment. Flow control provides protection in the following ways:

It restricts incoming traffic to:

Protect equipment from overload, thereby maintaining system stability.

Ensure that equipment can properly process services even during heavy traffic.

It restricts outgoing traffic to reduce the load on the peer equipment.

2.2 Overall Picture of Flow Control

During mass gathering events, the amount of services surges, generating a significantly increased traffic volume that exceeds the processing capabilities of the system. As a result, the system becomes overloaded, which may lead to messages being randomly discarded and NE resetting, as well as response failures, call drops, service access failures, and other unexpected events.

Resources in a WCDMA system are limited, so how they are used affects system performance. The resources concerned here are:

Equipment system resources, including CPU resources and memory

Air interface resources, including channels, codes, and power

Transmission resources

Core network processing capabilities

To keep system stability and capabilities at the maximum possible level, Huawei RNCs perform flow control at five points in the system, which are numbered in Figure 2-1.

Figure 2-1 Five points in flow control

Flow control involves discarding originating messages (such as RRC connection requests) that overload the system when system resources are insufficient, refusing to process low-priority services, and rejecting access requests for low-priority services.

WCDMA RAN

Flow Control 2 Overview



2-2

To address problems caused by limited RNC resources (labeled in Figure 2-1), the RNC performs flow control for RNC units. The software of each RNC board monitors the system resource usage. When necessary, the RNC starts basic flow control functions that suspend non-critical functions, such as recording logs and printing to reduce the system load. Then, based on the system load and the switch status of flow control functions, the RNC may perform other flow control functions to ensure system stability. For details, see chapter 3 "Flow Control for Overloaded RNC Units."

To address problems caused by limited air interface resources (labeled in Figure 2-1), the RNC performs CAPS (Call Attempts Per Second) control, PCH congestion control and FACH congestion control.

When the NodeB or cell is overloaded with services, RNC limits the number of RRC connection requests admitted to a cell or NodeB each second. For details, see 4.1 "CAPS Control."

When the paging channel is congested, the RNC allows CS-domain paging messages to preempt PS-domain paging messages in order to raise the paging success rate in the CS domain. For details, see section 4.2 "PCH Congestion Control."

When the FACH (Forward Access Channel) is congested, the RNC restricts message retransmissions on the logical channels, rejects certain PS service requests, and triggers state transitions such as CELL_PCH to CELL_DCH (P2D) and CELL_DCH to idle (D2Idle). This gives priority to access requests for high-priority services such as CS services, keeps the cell update success rate high, and reduces call drops. For details, see section 4.3 "FACH Congestion Control."

The RNC performs admission control, load reshuffling, and overload control on code and power resources. For details about admission control, see the Call Admission Control Feature Parameter Description. For details about load reshuffling and overload control, see the Load Control Feature Parameter Description.

To address problems caused by limited signaling bandwidth over the Iu interface (labeled in Figure 2-1), the RNC works with the core network to perform flow control over the Iu interface. Based on link congestion conditions detected at the local end and congestion indications reported from the peer end, the RNC performs flow control on initial UE messages to reduce the signaling traffic over the Iu interface. This prevents severe congestion on the signaling link between the RNC and the core network and reduces the load on the core network when it is overloaded. For details, see chapter 5 "Flow Control over the Iu Interface."

The RNC supports user-plane congestion control over the Iub interface to restrict transmission rates when there is transmission congestion over the Iub interface. This prevents packet loss and makes more efficient use of the bandwidth. For details, see chapter 6 "Service Flow Control."

For access requests, the RNC supports load sharing within one subrack or between subracks on the user plane and control plane. This achieves dynamic sharing of resources, balancing the load among subracks and boards and improving service processing efficiency. For details, see chapter 7 "Load Sharing."

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Flow Control 3 Flow Control for Overloaded RNC Units



3-1

3 Flow Control for Overloaded RNC Units

3.1 Principle

3.1.1 Overview

Each RNC board monitors the following in real time to keep track of resource consumption:

CPU usage: The CPU resources of a board determine the processing capabilities of the board. All functions running on the board use CPU resources.

Message block occupancy rate: Message blocks are resources used to send and receive messages within the RNC.

When the CPU usage or message block occupancy rate of a board is high, the board processing capabilities may become insufficient. When this occurs, the board triggers flow control to ensure that basic functions can continue to run properly. Flow control based on message block occupancy rate is independent of flow control based on CPU usage. Related flow control functions will be triggered when either the message block occupancy rate or the CPU usage is excessively high. Generally, it is rare to run out of message blocks.

Figure 3-1 shows the flow control model that each board follows based on CPU usage and message block occupancy rate.

Figure 3-1 Flow control model

The XPUs, interface boards (collectively known as INTs), DPUs, SCUs and GCUs mentioned in this document are board types displayed on the LMT. An XPU comprises MPUs and CPUSs, which have the following functions:

An MPU manages resources on the user plane, control plane, and transport plane, informs MPUs in other subracks about the load on the current subrack, and makes decisions regarding load sharing.

A CPUS processes services on the control plane.

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The XPU, INT, DPU, SCU, and GCU boards correspond to the following physical boards:

XPU: SPUa or SPUb

INT: AEUa, AOUa, AOUc, FG2a, FG2c, GOUa, or GOUc

DPU: DPUb or DPUe

SCU: SCUa or SCUb

GCU: GCUa or GCGa

For the detailed functions of each board, see the BSC6900 UMTS Hardware Description.

3.1.2 CPU Usage Monitoring

The system checks CPU usage in real time. If the CPU usage has reached the threshold for starting a flow control function that is based on the CPU usage and currently enabled, this function is started.

For details about flow control switches and CPU usage thresholds, see sections 3.3 "Flow Control Triggered by CPUS Overload," 3.4 "Flow Control Triggered by MPU Overload," 3.5 "Flow Control Triggered by INT Overload," 3.6 "Flow Control Triggered by DPU Overload," 3.7 "Flow Control Triggered by SCU Overload,", 3.8 "Flow Control Triggered by GCU Overload."

To prevent frequent flow control triggered by CPU usage fluctuations, the system also calculates the average CPU usage during a period of time that has just elapsed, and determines whether to perform flow control based on this CPU usage. The CPU usage values used to calculate the average CPU constitute a filter window, as shown in Figure 3-2.

Figure 3-2 Filter window for calculating the average CPU usage

The filter window of a flow control function is configurable only if this function is controlled by using the SET FCSW command. For details, see section 3.2 "Whole Picture of Flow Control for Overloaded RNC Units."

3.1.3 Message Block Occupancy Rate Monitoring

Once it has allocated message blocks ten times, the system checks the message block occupancy rate. If the message block occupancy rate has reached the threshold for starting a flow control function that is based on the message block occupancy rate and currently enabled, this function is started.

For details about flow control switches and CPU usage thresholds, see sections 3.3 "Flow Control Triggered by CPUS Overload," 3.4 "Flow Control Triggered by MPU Overload," 3.5 "Flow Control Triggered by INT Overload," 3.6 "Flow Control Triggered by DPU Overload," 3.7 "Flow Control Triggered by SCU Overload,", 3.8 "Flow Control Triggered by GCU Overload."

To prevent frequent flow control triggered by message block occupancy rate fluctuations, the system also calculates the average message block occupancy rate. The message block occupancy rate values used to calculate the average message block occupancy rate constitutes a filter window, as shown in Figure 3-3.

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Figure 3-3 Filter window for calculate the average message block occupancy rate

The filter window of a flow control function is configurable only if this function is controlled by using the SET FCSW command. For details, see section 3.2 "Whole Picture of Flow Control for Overloaded RNC Units."

3.2 Whole Picture of Flow Control for Overloaded RNC Units

Table 3-1 provides a whole picture of flow control for overloaded RNC units.

Table 3-1 Whole picture of flow control for overloaded RNC units

Overload Source Flow Control Function

Flow Control Object Impact on Services

Controlling Command

CPUS High CPU usage or message block occupancy rate

Printing flow control

Printing No SET FCSW

Debugging flow control

Debugging

Performance monitoring flow control

Performance monitoring

Logging flow control

Logging

Resource audit flow control

Resource audit

MR flow control MR function

Paging control Paging messages Yes SET FCSW

Access control Users in AC0 to AC9 SET FCSW

RRC flow control RRC connection requests

None

High CPU usage

Queue-based RRC shaping

RRC connection requests

SET UCACALGOSWITCH

High CPU usage or message block

Flow control on signaling messages over the Iur interface

Some signaling messages over the Iur interface

SET FCSW

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Controlling Command

occupancy rate

Flow control over the Iur-g interface

All messages over the Iur-g interface

CBS flow control All broadcast messages delivered by the CBC

Cell/URA update flow control

Some cell/URA update messages

MPU High CPU usage or message block occupancy rate


Printing None SET FCSW


Debugging


Logging

High CPU usage

MPU overload backpressure


Yes SET URRCTRLSWITCH

INT High CPU usage or message block occupancy rate




Debugging


Logging

High CPU usage

Flow control triggered by INT overload on the control plane


Yes SET TNSOFTPARA

Congestion in queues at the ports

Flow control triggered by Iub interface board overload on the user plane

BE service rates See related descriptions in 3.5.3 "Flow Control Triggered by Iub Interface Board Overload on the User Plane."

DPU High CPU usage or message block occupancy rate




Debugging


Logging

High DSP Flow control BE service rates Yes None

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Controlling Command

CPU usage

triggered by DSP CPU overload

SCU High CPU usage or message block occupancy rate




Debugging

Performance monitoring flow control

Performance monitoring


Logging

GCU High CPU usage or message block occupancy rate




Debugging


Logging

The filter windows for flow control functions configured by the SET FCSW command are configurable. The details are as follows:

For flow control decisions based on CPU usage, the SMWINDOW parameter of the SET FCCPUTHD command is used to configure the filter window.

For flow control decisions based on message block occupancy rate, the SMWINDOW parameter of the SET FCMSGQTHD command is used to configure the filter window.

For flow control functions configured by the SET FCSW command, the system also uses a fast judgment window to prevent the CPU usage and message block occupancy rate from rapidly rising to a high level. The details are as follows:

If all CPU usage values during this fast judgment window are greater than or equal to a critical threshold, all currently enabled flow control functions based on CPU usage are started. The FDWINDOW and CTHD parameters of the SET FCCPUTHD command are used to configure the fast judgment window and critical threshold, respectively. The value of SMWINDOW should be at least twice the value of FDWINDOW.

If the current message block occupancy rate value is greater than or equal to a critical threshold, all currently enabled flow control functions based on message block occupancy rate are started. The size of the fast judgment window for flow control based on the message block occupancy rate is 1. That is, the critical threshold decision does not use the filter mechanism. The CTHD parameter of the SET FCMSGQTHD command is used to configure the critical threshold.

When the FCSW parameter is set to OFF for a board, all flow control functions configured by the SET FCSW command are disabled for this board.

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3.3 Flow Control Triggered by CPUS Overload

3.3.1 Overview

The CPUS software monitors the CPU usage and message block occupancy rate of the CPUS in real time. Upon detecting a high CPU usage or message block occupancy rate, the CPUS software starts basic flow control, which is performed on non-critical functions, such as printing and logging. When the CPU usage or message block occupancy rate reaches or exceeds their respective thresholds, the CPUS software starts the following flow control functions if they are enabled:

Access control

RRC flow control

Flow control over the Iur interface

CBS flow control

Cell or URA update flow control

Flow control over the Iur-g interface

MR flow control

In addition, the CPUS software supports queue-based RRC shaping, which helps stabilize the CPU usage.

3.3.2 CPUS Basic Flow Control

Principle

Basic flow control for a CPUS is performed on printing, debugging, performance monitoring, logging, and resource auditing. The CPUS software monitors the CPU usage and message block occupancy rate of the CPUS in real time. Based on the monitored data, the CPUS software starts or stops all or some of the basic flow control functions.

When the CPU usage or message block occupancy rate reaches the threshold, the CPUS software starts flow control.

When the CPU usage and message block occupancy rate fall below their respective thresholds, the CPUS software stops flow control.

The SET FCSW command is used to enable the basic flow control functions. By default, all basic flow control functions are enabled.

The SET FCCPUTHD command is used to configure the thresholds for flow control based on CPU usage, and the SET FCMSGQTHD command is used to configure the thresholds for flow control based on message block occupancy rate.

Basic flow control for the CPUS has no impact on services.

Overload Indication

When the CPU usage reaches the preset threshold (configured by the SET CPUTHD command), ALM-20256 CPU Overload is reported. To find out whether the alarm was reported for an XPU, check the subrack number and slot number in the alarm.

EVT-22835 Flow Control is reported when a basic flow control function is started. To find out which basic flow control function was started, check the flow control type in the event.

The following counters indicate the CPU usage and message block occupancy rate.

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Counter Description

VS.XPU.CPULOAD.MEAN Average CPU usage of the XPU

VS.XPU.MSGLOAD.MEAN Average message block occupancy rate of the XPU

3.3.3 Access Control

Principle

When the network is heavily loaded, an access class (AC) identifies the access priority of specific UEs. ACs are numbered from 0 to 15. If the CPU usage of a CPUS is higher than the preset threshold, the CPUS software restricts the access of UEs of AC0 through AC9. This reduces the impact of traffic on the network.

The RNC starts access control when the CPU usage of the CPUS exceeds the value of ACCTHD configured with the SET FCCPUTHD command or the message block occupancy rate exceeds the value of ACCTHD configured with the SET FCMSGQTHD command.

After the RNC starts access control for the CPUS, all cells under the CPUS are affected. The RNC first starts access control for a random cell. After a period of time defined by AcIntervalOfCell, the RNC starts access control for another random cell. This pattern continues until access control has been started for all cells under the CPUS.

Users of certain ACs cannot access the access-controlled cell for each period of time defined by AcRstrctIntervalLen. The number of ACs affected by access control in each period is 10 times the value of AcRstrctPercent, and the ACs are chosen in turn. Assuming that the value of AcRstrctPercent is 20%, AC0 and AC1 users cannot initiate RRC connections under the cell during the first period of time defined by AcRstrctIntervalLen, and AC2 and AC3 users cannot initiate RRC connections under the cell during the second such period. This pattern continues under a cell this way until access control is stopped for this cell, as shown in Figure 3-4.

Figure 3-4 Access control on users under a cell when the value of AcRstrctPercent is 20%

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The RNC stops access control when the CPU usage of the CPUS falls below the value of ACRTHD configured with the SET FCCPUTHD command and the message block occupancy rate of the CPUS falls below the value of ACRTHD configured with the SET FCMSGQTHD command.

Whether access control yields noticeable effects depends on the following factors:

How the operator defines users

If SIM cards are evenly distributed among ACs before being sold, access control or DSAC can yield noticeable effects. If SIM cards are unevenly or incorrectly distributed among ACs, do not enable access control or DSAC because it may fail to yield noticeable effects.

UE compliance

DSAC is applicable only to UEs that comply with 3GPP Release 6 or later. Access control is applicable to all UEs.

To determine whether AC is yielding notable effects, run the DSP UCELLACR command or check the value of the counter VS.RRC.AttConnEstab.Msg. Assuming AcRstrctPercent is set to 20%, access control is considered yielding noticeable effects if the value of this counter is 20% less than its value before access control was enabled.

By default, access control is disabled. To enable it, set the values of ACSW and AcRstrctSwitch to ON. Users making emergency calls are all put into AC10 and are not subject to access control.

The RNC can perform domain-specific access control (DSAC) to differentiate between the CS domain and the PS domain. When one domain is overloaded or unavailable, DSAC keeps the other domain from being negatively affected. This makes the network more resilient in the event of service interruption. For more details about DSAC, see the DSAC Feature Parameter Description.

Overload Indication


Access control uses system information to prevent users in certain ACs from accessing the network. To determine access control effects on the RNC side, compare the values of the counter VS.RRC.AttConnEstab.Msg before and after access control is enabled. This counter indicates the total number of RRC connection requests that the RNC has received from UEs.

3.3.4 Paging Control

Principle

Upon detecting that the CPU usage or message block occupancy rate of a CPUS is higher than the preset threshold, the RNC starts paging control to reduce paging traffic and ensure high paging success rates for high-priority services. The PAGESW parameter in the SET FCSW command is used to enable paging control. By default, it is enabled.

When the CPU usage or message block occupancy rate exceeds the threshold, the RNC starts paging control and discards paging messages.

When the CPU usage and message block occupancy rate fall below their respective thresholds, the RNC stops paging control.

Paging control based on CPU usage varies by service. The SET FCCPUTHD command is used to configure paging control thresholds for different services, as described in Table 3-2. The higher the threshold for starting a flow control function, the more difficult it is for the flow control function to start.

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Table 3-2 Thresholds for paging control based on CPU usage

Service Types Threshold for Starting Paging Control

Threshold for Stopping Paging Control

Real-time services CPAGECTHD CPAGERTHD

BE services (background and interactive services), supplementary services, and location services

SLPAGECTHD SLPAGERTHD

SMS SMPAGECTHD SMPAGERTHD

To ensure a high paging success rate for high-priority services, such as CS services, the thresholds for starting paging control should be ranked as follows:

CPAGECTHD > SLPAGECTHD > SMPAGECTHD

This way, when paging control is in progress, SMS paging messages are the first to be discarded.

Paging control applies to terminating UEs, and load sharing is not used for paging messages. As a result, paging control for one CPUS affects all paging processes within the same RNC. The thresholds for starting paging control should be higher than the thresholds for starting other flow control functions triggered by CPUS overload.

Paging control based on message block occupancy rate does not vary by service. The threshold for starting this flow control function is configured by using the PAGECTHD parameter, and the threshold for stopping this flow control function is configured by using the PAGERTHD parameter.

Overload Indication


The following counters are related to paging control.

Counter Description

VS.Paging.FC.Disc.Num.CPUS Number of paging messages discarded because of paging control

VS.Paging.FC.Disc.Time.CPUS Duration of paging control in a measurement period

3.3.5 RRC Flow Control

Principle

Upon detecting that the CPU usage or message block occupancy rate of a CPUS is higher than the preset threshold, the RNC starts rejecting or discarding RRC connection requests to avoid raising the CPU load further. RRC Flow Control is enabled by default. It is started after load sharing fails for RRC connection requests. For more details on load sharing for RRC connection requests, see chapter 7 "Load Sharing."

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When the CPU usage or message block occupancy rate of the CPUS exceeds the threshold, the RNC starts RRC flow control and rejects RRC connection requests. When the number of rejected RRC connection requests per second exceeds the value of SysRrcRejNum configured with the SET UCALLSHOCKCTRL command, the CPUS starts discarding subsequent RRC connection requests messages, without responding with RRC CONNECTION REJECT messages. When the CPU usage and message block occupancy rate fall below their respective thresholds, the RNC stops RRC flow control.

RRC flow control varies by service. The SET SHARETHD command is used to configure the necessary thresholds for the different services, as shown in Table 3-3.

Table 3-3 Thresholds for RRC flow control

Service Type Threshold for Starting RRC Flow Control Based on CPU Usage

Threshold for Stopping RRC Flow Control Based on CPU Usage

Threshold for Starting RRC Flow Control Based on Message Block Occupancy Rate

Threshold for Stopping RRC Flow Control Based on Message Block Occupancy Rate

Inter-RAT cell reselection, IMSI detach procedure, registration, and incoming voice calls

CRRCCONNCCPUTHD

CRRCCONNRCPUTHD

CRRCCONNCMSGTHD

CRRCCONNRMSGTHD

BE services and UE-originated voice calls

LRRCCONNCCPUTHD

LRRCCONNRCPUTHD

LRRCCONNCMSGTHD

LRRCCONNRMSGTHD

SMS SMRRCCONNCCPUTHD

SMRRCCONNRCPUTHD

SMRRCCONNCMSGTHD

SMRRCCONNRMSGTHD

To ensure high-priority services such as CS services are processed first, the thresholds for starting RRC flow control should be ranked as follows:

CRRCCONNCCPUTHD > LRRCCONNCCPUTHD > SMRRCCONNCCPUTHD

This way, when RRC flow control is in progress, RRC connection requests for SMS are the first to be discarded.

When the CPU usage of the CPUS exceeds 90%, the RNC starts discarding all RRC connection requests except those for emergency calls.

Overload Indication


The following counters indicate the number of RRC connection requests discarded because of RRC flow control.

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Counter Description

VS.LowPriRRC.FC.Disc.Num.CPUS Number of discarded RRC connection requests for SMS because of RRC flow control based on CPU usage

VS.NormPriRRC.FC.Disc.Num.CPUS Number of discarded RRC connection requests for BE services and outgoing voice services because of RRC flow control based on CPU usage

VS.HighPriRRC.FC.Disc.Num.CPUS Number of discarded RRC connection requests for registration and incoming voice services because of RRC flow control based on CPU usage

3.3.6 Flow Control on Signaling Messages over the Iur Interface

Principle

Upon detecting that the CPU usage or message block occupancy rate of a CPUS is higher than the preset threshold, the RNC starts flow control to reduce signaling traffic over the Iur interface so that the CPU load does not rise further.

When the CPU usage or message block occupancy rate exceeds the threshold, the RNC starts flow control over the Iur interface and discards signaling messages over the Iur interface.

When the CPU usage and message block occupancy rate fall below their respective thresholds, the RNC stops flow control over the Iur interface.

Flow control on signaling messages over the Iur interface consists of uplink transmission flow control over the Iur interface and downlink transmission flow control over the Iur interface, as described in Table 3-4.

Table 3-4 Flow control over the Iur interface

Flow Control Function Flow Control Objects Switch

Uplink transmission flow control over the Iur interface

UPLINK SIGNALLING TRANSFER INDICATION messages

IURULSW

Downlink transmission flow control over the Iur interface

RADIO LINK SETUP REQUEST messages

PAGING REQUEST messages

COMMON TRANSPORT CHANNEL RESOURCES REQUEST messages

IURDLSW


Flow control over the Iur interface affects cell updates, handovers, and paging over the Iur interface. In addition, It affects ongoing service procedures because signaling messages are discarded. This may increase call drop rates.

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Overload Indication


3.3.7 CBS Flow Control

Principle

In cases where the UTRAN uses an external cell broadcast center (CBC) to provide the cell broadcast service (CBS), the RNC starts CBS flow control upon detecting that the CPU usage or message block occupancy rate of a CPUS is higher than the preset threshold. This reduces signaling traffic over the Iu-BC interface and thereby prevents the CPU load from rising further. The CBSSW parameter in the SET FCSW command is used to enable CBS flow control. By default, it is enabled.

When the CPU usage or message block occupancy rate exceeds the threshold, the RNC starts CBS flow control and discards all CBC broadcast messages.

When the CPU usage and message block occupancy rate fall below their respective thresholds, the RNC stops CBS flow control.


CBS flow control affects cell broadcast services.

Overload Indication


The following counters are related to CBS flow control.

Counter Description

VS.CBS.FC.Disc.Num.CPUS Number of broadcast messages discarded because of CBS flow control

VS.CBS.FC.Disc.Time.CPUS Duration of CBS flow control in a measurement period

3.3.8 Cell/URA Update Flow Control

Principle

Upon detecting that the CPU usage or message block occupancy rate of a CPUS is higher than the preset threshold, the RNC starts cell/URA update flow control to reduce the number of cell/URA update messages so that the CPU load does not rise further. The CELLURASW parameter in the SET FCSW command is used to enable cell/URA update flow control. By default, it is enabled.

When the CPU usage or message block occupancy rate exceeds the threshold, the RNC starts cell/URA update flow control. During cell/URA update flow control, the RNC discards cell or URA update requests originated by a UE in the CELL_PCH or URA_PCH state that involves a P2P

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transition (CELL_PCH to URA_PCH or URA_PCH to CELL_PCH) or a P2F transition (CELL_PCH or URA_PCH to CELL_FACH).

When the CPU usage and message block occupancy rate fall below the threshold, the RNC stops cell/URA update flow control.


Cell/URA update flow control lowers the cell update success rate and affects uplink data transmission. In addition, UE locations recorded by the RNC may not be accurate because cell update messages are discarded. This may affect paging.

For more details about UE state transitions, see the State Transition Feature Parameter Description.

Overload Indication


The following counters are related to cell/URA update flow control.

Counter Description

VS.CU.FC.Disc.Num.CPUS Number of cell update requests discarded because of cell/URA update flow control

VS.CU.FC.Disc.Time.CPUS Duration of cell/URA update flow control in a measurement period

3.3.9 Flow Control over the Iur-g Interface

Principle

Upon detecting that the CPU usage or message block occupancy rate of a CPUS is higher than the preset threshold, the RNC starts Iur-g flow control to reduce signaling traffic over the Iur-g interface so that the CPU load does not rise further. The IURGSW parameter in the SET FCSW command is used to enable flow control over the Iur-g interface. By default, it is disabled.

When the CPU usage or message block occupancy rate exceeds the threshold, the RNC starts flow control and discards all messages sent over the Iur-g interface.

When the CPU usage and message block occupancy rate fall below their respective thresholds, the RNC stops flow control over the Iur-g interface.


When flow control over the Iur-g interface is started, the RNC is not informed of real-time information about the GSM network load. This may cause the following problems:

When the GSM network load is heavy, inter-RAT handovers initiated by the RNC fail.

When the GSM network load is light, the RNC does not initiate inter-RAT handovers, service distribution cannot be performed for UMTS services, and load sharing cannot be achieved between the UMTS and GSM networks.

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For more details about load-based handovers, service distribution, and load balancing over the Iur-g interface, see the Common Radio Resource Management Feature Parameter Description.

Overload Indication


3.3.10 Measurement Report Flow Control

Principle

Upon detecting that the CPU usage or message block occupancy rate of a CPUS is higher than the preset threshold, the RNC starts measurement report (MR) flow control to reduce the number of MR messages so that the CPU load does not rise further. The MRFCSW parameter in the SET FCSW command is used to enable MR flow control. By default, it is enabled.

When the CPU usage or message block occupancy rate exceeds the threshold, the RNC starts MR flow control. After MR flow control is started, the RNC stops sending MR measurement control messages to newly admitted UEs. Consequently, NodeBs and these UEs stop submitting MR measurement reports. MR flow control does not apply to UEs admitted before MR flow control is started.

When the CPU usage and message block occupancy rate fall below their respective thresholds, the RNC stops MR flow control.


The MR function collects the following measurement reports:

Intra-frequency cell measurement reports

Inter-frequency cell measurement reports

Inter-RAT cell measurement reports

DL BLER (downlink block error ratio) measurement reports

Iub SIR (signal-to-interference ratio) measurement reports

UE transmit power measurement reports

LCS (location services) measurement reports, including UE location reports, Iub RTT (round trip time) measurement reports, and UE RX/TX (reception-transmission) measurement reports

RACH (random access channel) measurements reports

Overload Indication


3.3.11 Queue-based RRC Shaping

Principle

When new service attempts generate a traffic volume that exceeds the maximum processing capability of the CPU in a CPUS, the CPU usage rises to a high level. When a large number of service setup attempts are made in a short period of time, the CPU usage fluctuates sharply. To address these

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problems, the RNC adopts a token- and queue-based shaping solution, which performs flow control on RRC connection requests. This solution stabilizes the CPU usage and increase RRC and RAB setup success rates when traffic is heavy.

Tokens are permits to use the CPU resources of the CPUS. When an RRC connection request arrives, it applies for a token. RRC connection processing can proceed only after being granted a token. If the RRC connection request fails to obtain a token, it attempts to enter a specific queue and remains there until a token is available. If the queue is full, the RRC connection request is discarded. Figure 3-5 shows how queue-based RRC shaping works.

Figure 3-5 Queue-based RRC shaping

By default, queue-based RRC shaping is disabled. To enable it, run the SET UCACALGOSWITCH command with the RsvdPara1 parameter set to RSVDBIT14-1.

When an RRC connection request arrives, if the CPU usage of a CPUS is higher than 90%, the CPUS discards all RRC connection requests that are not from emergency calls. If the CPU usage is not higher than 90%, the CPUS checks whether the CPU load meets the conditions for load sharing. If so, the CPUS forwards the RRC connection request to the MPU for load sharing. If not, RNC performs queue-based RRC shaping. For details about load sharing, see chapter 7 "Load Sharing." Queue-based RRC shaping is as follows:

1. The RRC connection request applies for a token.

If the request manages to obtain a token, the request is processed and this procedure is done.

If the request fails to obtain a token and the queue is not full, the request enters the queue. Step 2 starts.

If the request fails to obtain a token and the queue is full, the request is discarded and this procedure ends.

2. The RRC connection request enters the queue.

3. The RRC connection request leaves the queue.

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The CPUS periodically scans the queues. If the RRC connection request has remained in a queue for longer than half of the value of T300, the CPUS discards the message.

If a token is available for the request, the request leaves the queue and is processed. Step 4 starts.

The CPUS first processes RRC connection requests from emergency calls and terminated voice calls.

4. The CPUS processes the RRC connection request.

The RNC does not perform flow control on emergency calls, and emergency calls do not enter queues.

Overload Indication


The counter VS.RRC.FC.Disc.Num.RRCQueue.CPUS indicates the number of RRC connection requests discarded because of queue-based RRC shaping.

3.4 Flow Control Triggered by MPU Overload

Flow control triggered by MPU overload is twofold: basic flow control for the MPU and MPU overload backpressure.

3.4.1 Basic Flow Control for the MPU

Principle

Basic flow control for an MPU is performed on printing, debugging, and logging. The MPU software monitors the CPU usage and message block occupancy rate of the MPU in real time. Based on the monitored data, the MPU software starts or stops all or some of the basic flow control functions.

When the CPU usage or message block occupancy rate reaches the threshold, the MPU software starts flow control.

When the CPU usage and message block occupancy rate fall below the threshold, the MPU software stops flow control.


The SET FCSW command is used to enable the basic flow control functions. By default, all basic flow control functions are enabled. Basic flow control for the MPU has no impact on services.

Overload Indication



The following counters indicate the CPU usage and message block occupancy rate.

Counter Description

VS.XPU.CPULOAD.MEAN Average CPU usage of the XPU

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Counter Description

VS.XPU.MSGLOAD.MEAN Average message block occupancy rate of the XPU

3.4.2 MPU Overload Backpressure

Principle

Under heavy traffic, the CPU of the MPU may be overloaded and fail to process services properly as a result. The RNC adopts an overload backpressure function. With this function, CPUSs work with MPUs to perform flow control on RRC CONNECTION REQUEST messages to alleviate the impact of heavy traffic on MPUs.

Congestion detection is performed based on the instantaneous CPU usage of the MPU. When the CPU usage of the MPU reaches 80% (this percentage is unconfigurable) or higher, the MPU sends a congestion message to the CPUS bound to it, as shown in Figure 3-6.

Figure 3-6 Flow control based on MPU overload

Upon receipt of the congestion message from the MPU, the CPUS adjusts the flow control level. The RNC adjusts the number of RRC connection requests that can be admitted on the CPUS each second according to the flow control level change. Flow control for the CPUS is performed on a scale of 30 levels. A higher flow control level means fewer RRC connection requests admitted each second.

The CPUS adjusts the flow control level by using two timers, one with a value of 2.2 seconds, the other with a value of 0.8 seconds.

Upon receiving a congestion message from the MPU, the CPUS increases the flow control level by one and starts the two timers.

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If MPU congestion messages are received before the 0.8-second timer expires, the CPUS does not take any actions.

If MPU congestion messages are received after the 0.8-second timer expires but before the 2.2-second timer expires, the CPUS increases the flow control level by one and restarts the two timers. After the 2.2-second timer expires, the CPUS decreases the flow control level by one.

When the RsvdPara1 parameter in the SET URRCTRLSWITCH command is set to RSVDBIT1_BIT19-1, MPU overload backpressure is enabled. By default, it is enabled.

Overload Indication

When the CPU usage reaches the preset threshold (configured by the SET CPUTHD command), ALM-20256 CPU Overload is reported. To find out whether the alarm was reported for an XPU, check

the subrack number and slot number in the alarm.

The counter VS.RRC.FC.Disc.Num.MPU.CPUS indicates the number of RRC connection requests discarded because of MPU overload backpressure.

3.5 Flow Control Triggered by INT Overload

Flow control triggered by INT overload is threefold: basic flow control for the INT, flow control triggered by INT overload on the control plane, and flow control triggered by Iub interface board overload on the user plane.

3.5.1 INT Basic Flow Control

Principle

When an interface board (INT) is heavily loaded, it starts basic flow control. Basic flow control for an INT is performed on printing, debugging, and logging. The INT software monitors the CPU usage and message block occupancy rate of the INT in real time. Based on the monitored data, the INT software starts or stops all or some of the basic flow control functions.

When the CPU usage or message block occupancy rate reaches the threshold, the INT software starts flow control.

When the CPU usage and message block occupancy rate fall below the threshold, the INT software stops flow control.

The SET FCCPUTHD command is used to configure the thresholds for flow control based on CPU usage, and the SET FCMSGQTHD command is used to configure the thresholds for flow control based on message block occupancy rate. Basic flow control for the INT has no impact on services.


Overload Indication

When the CPU usage reaches the preset threshold (configured by the SET CPUTHD command), ALM-20256 CPU Overload is reported. To find out whether the alarm was reported for an INT, check the subrack number and slot number in the alarm.


The counter VS.INT.CPULOAD.MEAN indicates the CPU usage.

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3.5.2 Flow Control Triggered by INT Overload on the Control Plane

Principle

After a UE initiates an RRC connection request and obtains transmission resources on the MPU, the CPUS sends a session setup request to the interface board. When a large number of service setup requests are made in a short period of time, the interface board needs to process a large number of session setup requests and may be overloaded. The MPU adopts a flow control process based on service priorities and the instantaneous CPU usage of the interface board. This type of flow control improves the RAB setup success rate when the interface board is heavily loaded.

The interface board reports its CPU usage to the MPU each second, as shown in Figure 3-7. Based on the CPU usage of the interface board, the MPU adjusts the maximum number of session setup requests admitted by the interface board. If the number of RRC connection requests already admitted is larger than the maximum number allowed, the RNC only processes RRC connection requests from emergency calls and high-priority services. The FcOnItfBrd parameter in the SET TNSOFTPARA command is used to enable this type of flow control. It is enabled by default and applies to Iub, Iu, and Iur interface boards. The maximum number of session setup requests allowed determines the signaling processing capability of the interface board. High-priority services involved in this type of flow control refer to incoming and outgoing voice calls, inter-RAT cell reselection, and registration.

Figure 3-7 Flow control triggered by INT overload

When the CPU usage of the interface board exceeds 90%, the MPU starts discarding RRC connection requests from all UEs.

Overload Indication

When the CPU usage reaches the preset threshold (configured by the SET CPUTHD command), ALM-20256 CPU Overload is reported. To find out whether the alarm was reported for an INT, check the subrack number and slot number in the alarm.

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3.5.3 Flow Control Triggered by Iub Interface Board Overload on the User Plane

When the amount of user-plane data sent from the DPU to the interface board exceeds the processing capability of the interface board, the interface board throughput decreases and the packet loss rate increases. To address this problem, the RNC adopts backpressure-based downlink congestion control. For more details, see the Transmission Resource Management Feature Parameter Description.

3.6 Flow Control Triggered by DPU Overload

3.6.1 DPU Basic Flow Control

Principle

When a DPU is heavily loaded, it starts basic flow control. Basic flow control for a DPU is performed on printing, debugging, and logging. The DPU software monitors the CPU usage and message block occupancy rate of the DPU in real time. Based on the monitored data, the DPU software starts or stops all or some of the basic flow control functions.

When the CPU usage or message block occupancy rate reaches the threshold, the DPU software starts flow control.

When the CPU usage and message block occupancy rate fall below the threshold, the DPU software stops flow control.

The SET FCCPUTHD command is used to configure the thresholds for flow control based on CPU usage, and the SET FCMSGQTHD command is used to configure the thresholds for flow control based on message block occupancy rate. Basic flow control for the DPU has no impact on services.


Overload Indication

When the CPU usage reaches the preset threshold (configured by the SET CPUTHD command), ALM-20256 CPU Overload is reported. To find out whether the alarm was reported for a DPU, check the subrack number and slot number in the alarm.


When all DSPs under the RNC have been heavily loaded for an extended period of time, the RNC reports ALM-22305 Resource overload on the user plane.

3.6.2 Flow Control Triggered by DSP CPU Overload

Principle

To ensure admission of CS services and quality of ongoing CS services, the RNC lowers the rates of BE services when the CPU of a DSP is heavily loaded. By default, this type of flow control is enabled.

Each DSP of the DPU periodically monitors its own CPU usage.

When the CPU usage is between SSDSPAVEUSAGEALMTHD and SSDSPMAXUSAGEALMTHD, the RNC lowers the rates of BE services.

When the CPU usage is lower than the threshold SSDSPAVEUSAGEALMTHD, the RNC raises the rates of BE services.

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To prevent the DSP from crashing, if the CPU usage is higher than 90% during a monitoring period, the RNC further lowers BE service rates and discards some packets of BE services and AMR services.

When the RNC raises or lowers service rates, the current monitoring period is ended. To prevent frequent changes in service rates, the RNC waits a period of time before starting the next monitoring period. During this period, the RNC does not increase or decrease rates of BE services.

Overload Indication

There are no indications when the CPU of a DSP is overloaded.

3.7 Flow Control Triggered by SCU Overload

3.7.1 Principle

When an SCU is heavily loaded, it starts basic flow control. Basic flow control for an SCU is performed on printing, debugging, performance monitoring, and logging. The SCU software monitors the CPU usage and message block occupancy rate of the SCU in real time. Based on the monitored data, the SCU software starts or stops all or some of the basic flow control functions.

When the CPU usage or message block occupancy rate reaches the threshold, the SCU software starts flow control.

When the CPU usage and message block occupancy rate fall below the threshold, the SCU software stops flow control.

The SET FCCPUTHD command is used to configure the thresholds for flow control based on CPU usage, and the SET FCMSGQTHD command is used to configure the thresholds for flow control based on message block occupancy rate. Basic flow control for the SCU has no impact on services.


3.7.2 Overload Indication

When the CPU usage reaches the preset threshold (configured by the SET CPUTHD command), ALM-20256 CPU Overload is reported. To find out whether the alarm was reported for an SCU, check the subrack number and slot number in the alarm.


The counter VS.SCU.CPULOAD.MEAN indicates the CPU usage.

3.8 Flow Control Triggered by GCU Overload

3.8.1 Principle

When a GCU is heavily loaded, it starts basic flow control. Basic flow control for a GCU is performed on printing, debugging, and logging. The GCU software monitors the CPU usage and message block occupancy rate of the GCU in real time. Based on the monitored data, the GCU software starts or stops all or some of the basic flow control functions.

When the CPU usage or message block occupancy rate reaches the threshold, the GCU software starts flow control.

When the CPU usage and message block occupancy rate fall below the threshold, the GCU software stops flow control.

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The SET FCCPUTHD command is used to configure the thresholds for flow control based on CPU usage, and the SET FCMSGQTHD command is used to configure the thresholds for flow control based on message block occupancy rate. Basic flow control for the GCU has no impact on services.



When the CPU usage reaches the preset threshold (configured by the SET CPUTHD command), ALM-20256 CPU Overload is reported. To find out whether the alarm was reported for a GCU, check the subrack number and slot number in the alarm.


The counter VS. GCU.CPULOAD.MEAN indicates the CPU usage.

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4-1

4 Flow Control Triggered by NodeB/Cell Overload

4.1 CAPS Control

4.1.1 Principle

When the number of calls in a cell or NodeB sharply increases, most system resources (mainly radio resources) are consumed processing the enormous amount of RRC connection setup requests. Therefore, the remaining resources are insufficient for processing subsequent RAB assignment requests, resulting in call failures.

To solve this problem, the RNC implements the CAPS control function. This function limits the number of RRC connection requests admitted to a cell or NodeB each second. By preventing the traffic of a single cell or NodeB from surging, CAPS control helps maintain a stable traffic volume on the network. Figure 4-1 shows the procedure for CAPS control.

Figure 4-1 Procedure for CAPS control

By default, CAPS control is disabled.

To enable cell-level CAPS control:

Set the CallShockCtrlSwitch parameter to SYS_LEVEL-0&NODEB_LEVEL-0&CELL_LEVEL-1; and

Set the RsvdPara1 parameter (by running the ADD UCELLALGOSWITCH command) to RSVDBIT4-1.

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To enable NodeB-level CAPS control:


Set the RsvdPara1 parameter (by running the ADD UNODEBALGOPARA command) to RSVDBIT1-1.

To enable both cell-level and NodeB-level CAPS control:


Set the RsvdPara1 parameter (by running the ADD UCELLALGOSWITCH command) to RSVDBIT4-1. Set the RsvdPara1 parameter (by running the ADD UNODEBALGOPARA command) to RSVDBIT1-1.

The CallShockCtrlSwitch parameter is for the RNC. To enable cell- or NodeB-level CAPS control, you need to set the associated parameter for the cell or NodeB. The SYS_LEVEL field of the CallShockCtrlSwitch parameter is used to enable CPUS-level CAPS control, which is no longer applicable.

After CAPS control is enabled, the RNC periodically checks the total number of RRC connection requests received by a cell or NodeB. When this number exceeds the preset threshold, the RNC triggers the cell- or NodeB-level flow control. The check period is set with the CallShockJudgePeriod parameter. Table 4-1 describes the conditions for triggering the cell- and NodeB-level flow control.

Table 4-1 Conditions for triggering the cell- and NodeB-level flow control

Flow Control Level Triggering Condition

Cell The total number of RRC connection requests received by a cell during a specified period exceeds the value of CellTotalRrcNumThd.

NodeB The total number of RRC connection requests received by a NodeB during a specified period exceeds the value of NBTotalRrcNumThd; or

NCP link congestion over the Iub interface is detected.

Table 4-2 describes the flow control policy for different services.

Table 4-2 Flow control policy

Service Flow Control Policy

PS BE services (interactive service and background service), streaming service, short message service (SMS), and inter-RAT cell change

The RNC rejects the access requests of these services.

AMR service Cell-level flow control:

The number of RRC connection requests admitted for AMR services in a cell each second must not exceed the value of the CellAmrRrcNum parameter. Once the limit is reached, the RNC rejects all subsequent requests.

NodeB-level flow control:

The number of RRC connection requests admitted for AMR services in a

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Service Flow Control Policy

NodeB each second must not exceed the value of the NBAmrRrcNum parameter. Once the limit is reached, the RNC rejects all subsequent requests.

Registration and inter-RAT cell reselection

When the RegByFachSwitch parameter is set to ON, the RNC forcibly sets up the RRC connection of registrations on the FACH.

Cell-level flow control:

The number of RRC connection requests for registrations and inter-RAT cell reselections in a cell each second must not exceed the value of the CellHighPriRrcNum parameter. Once the limit is reached, the RNC rejects all subsequent requests.

NodeB-level flow control:

The number of RRC connection requests for registrations and inter-RAT cell reselections in a NodeB each second must not exceed the value of the NBHighPriRrcNum parameter. Once the limit is reached, the RNC rejects all subsequent requests.

Emergency call Flow control is not applied to emergency calls.

To prevent a UE from frequently retransmitting the RRC connection requests, the RNC adds an IE "wait time" to the RRC connection reject message sent to the UE. The UE waits for the length of time specified by RrcConnRejWaitTmr and then retransmits the RRC connection request. In this way, network congestion will not be aggravated. The UE needs to support the processing associated with "wait time."

To prevent the UE from frequently triggering RRC connection attempts and aggravating network congestion, the RNC carries the wait time in the RRC CONNECTION REJECT message sent to the UE, requesting the UE to wait the specified period of time before making the next attempt. This requires the UE to support the wait time function.

The wait time can be configured by using a parameter. Different parameters are used for different service types, as shown in Table 4-3.

Table 4-3 Parameters for the wait time

Service Type Parameter

Conversational services and emergency calls RrcConnRejWaitTmr

BE services and streaming services RsvdPara3

If the value of the RsvdPara3 parameter is set to 0, this parameter has no impact on BE services and streaming services. The wait time for BE services and streaming services is configured by using the RrcConnRejWaitTmr parameter.

When the number of RRC connection requests rejected per second exceeds the value of the SysRrcRejNum parameter, the RNC discards subsequent RRC connection requests.


The counter VS.RRC.FC.Disc.Num.CallShock.CPUS indicates the number of RRC connection requests discarded on the CPUS because of CAPS control.

WCDMA RAN




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4.2 PCH Congestion Control

4.2.1 Principle

Because PS services are growing so rapidly, the number of paging messages consuming a large amount of paging resources is also increasing rapidly. As a result, the paging success rate of CS services may be affected. To address this issue, the RNC implements PCH congestion control. With PCH congestion control, CS services are allowed to preempt the paging resources of PS services in the event of PCH congestion, increasing the paging success rate of CS services.

When the n

flow control(ran13.0 03)

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