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4 Flow ControlAbout This Chapter

4.1 Overview

This topic describes flow control. The flow control function enables the BSS to decrease the

access traffic flow. This ensures the smooth processing of services.

4.2 Availability

This topic describes the availability of flow control. The realization of flow control depends on

the cooperation of related hardware, software, and configuration parameters.

4.3 ImpactThis topic describes the impact of flow control on system performance and on other features.

4.4 Technical Description

The technical description of flow control consists of the causes, measures, and algorithms of

flow control.

4.5 Implementation

The implementation of flow control refers to configuring flow control.

4.6 Maintenance Information

This topic describes the alarms and counters related to flow control.

4.7 References

HUAWEI BSC6000 Base Station Subsystem

BSS Feature Description 4 Flow Control

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4.1 Overview

This topic describes flow control. The flow control function enables the BSS to decrease the

access traffic flow. This ensures the smooth processing of services.

Definition

When the system traffic flow or load exceeds the design specifications, the BSS system performs

flow control to reduce the service connections or forbid some OM based on priorities. This

ensures the smooth processing of primary services and enables the system to restore to the normal

state within a short period of time.

Flow control can be classified into two types:

l Traffic flow control

Traffic flow control is an overload precaution. The traffic flow control function enables the

system to monitor the message arrival rate and take measures to ensure the stable running

of the system.

l System congestion control

System congestion control is an overload protection measure. The system congestion

control function enables the system to control the message arrival rate when the system is

overloaded. Thus, the system load is reduced.

Purposes

Under the specific system capacity and load, the BSC can normally provide services. When thetraffic flow exceeds the design specifications of the BSC, flow control should be performed to

ensure the following functions:

l The BSC runs normally and provide services.

l The BSC provides optimal services.

l The BSC provides differentiated services based on the QoS policies.

Terms

Terms Definition

Message arrival rate Message arrival rate indicates the number of specified messages

received by the BSC in a unit time.

Abbreviations

None.

4.2 Availability

This topic describes the availability of flow control. The realization of flow control depends onthe cooperation of related hardware, software, and configuration parameters.

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BSS Feature Description

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Network Elements Involved

Table 4-1 lists the NEs involved in flow control.

Table 4-1 NEs involved in flow control

MS BTS BSC MSC MGW SGSN GGSN HLR

NOTE

l : not involved

l : involved

Software Releases

Table 4-2 describes the versions of GBSS products that support flow control.

Table 4-2 GBSS products and related versions

Product Version

BSC BSC6000 V900R001C01 and later releases

BTS BTS All the releases

MiscellaneousThe realization of flow control requires the cooperation of the MSC and the BTS.

l The MSC can process Overload messages and Load Indication messages on the A interface.

The MSC can process Overload messages and Load Indication messages sent by the BSC

according to the 3GPP 48008 protocol. When the BSC is overloaded, the MSC should

reduce the number of messages sent to the BSC.

l The BTS can process the Overload, CCCH Load Indication, and CBCH Load Indication

messages on the Abis interface.

When the BTS is overloaded, it can send flow control messages (such as Overload messages

and CCCH Load Indication messages) on the Abis interface to the BSC to inform the BSCof the BTS load conditions.

4.3 Impact

This topic describes the impact of flow control on system performance and on other features.

Impact on System Performance

Flow control requires the system to measure the system load and message arrival rate in real

time. The measurement has impacts on the system performance.




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Impact on Other Features

Flow control has the following impacts on other features:

l When the flow control takes effect, some system OM functions may be forbidden and some

service flow may be controlled. For example, debug logs cannot be outputted, or sometracing messages may be limited.

l When the flow control takes effect, some functions cannot be used. For example, the

SDCCH dynamic adjustment function is forbidden.

l When the flow control takes effect, the BSC may discard some access request messages,

for example, access request messages sent by MSs, paging messages, incoming BSC

handover request messages.

4.4 Technical Description

The technical description of flow control consists of the causes, measures, and algorithms offlow control.

4.4.1 Flow Control Measures

This topic describes why the BSS performs flow control and the flow control measures taken

by the BSC.

4.4.2 Flow Control Algorithms

This topic describes the flow control algorithms involved in flow control.

4.4.1 Flow Control Measures

This topic describes why the BSS performs flow control and the flow control measures takenby the BSC.

The causes that lead to system overload are as follows:

l Internal causes

Internal causes such as OM in batches or internal exceptions may increase system

processing loads or exhaust key system resources.

l External causes

Excessive traffic flow from other NEs may lead to the BSS overload. For example, the

MSC sends a large number of paging messages within a short period of time, or a large

numberof MSs send or resend access request messages simultaneously. All this increases

the traffic flow and processor load of the BSC and exhausts key system resources. Thus,the BSS is overloaded.

The BSC uses various flow control measures to avoid network overload. When the network is

overloaded, the traffic flow can be reduced significantly. This ensures the smooth processing of

primary services and enables the system to restore the normal state within a short period of time.

The BSC has the following flow control measures:

l Flow control of message arrival rates

l Flow control on LAPD links

l Traffic flow control on the Um interface

l Cell traffic flow control

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l Internal flow control

l OM flow control

4.4.2 Flow Control Algorithms

This topic describes the flow control algorithms involved in flow control.

4.4.2.1 Flow Control of Message Arrival Rates

This topic describes the flow control of message arrival rates. The flow control of message arrival

rates enables the BSC to receive only a certain number of messages in a statistical period

4.4.2.2 Flow Control on LAPD Links

This topic describes the flow control on LAPD links. The flow control on LAPD links is

classified into DL flow control and UL flow control.

4.4.2.3 Traffic Flow Control on the Um Interface

This topic describes the traffic flow control on the Um interface. The traffic flow control on theUm interface involves the BTS processor overload, paging channel overload, and cell broadcast

channel overload.

4.4.2.4 Cell Flow Control

This topic describes cell flow control. To enable the cell flow control, each cell has a state

machine for cell flow control. The flow control of one cell is independent of the other cells.

4.4.2.5 Internal Flow Control

This topic describes the internal flow control. The BSC performs flow control on key system

resources such as messages and CPU usage. This ensures that the primary services run normally

and the BSS gradually restores the normal capacity and load when the traffic flow exceeds the

capacity and flow specifications.

4.4.2.6 OM Flow Control

This topic describes the OM flow control. The GBAM and service boards determine whether to

perform OM or whether to send OM messages to other boards based on relevant information.

Flow Control of Message Arrival Rates

This topic describes the flow control of message arrival rates. The flow control of message arrival

rates enables the BSC to receive only a certain number of messages in a statistical period

The MSC sends a large number of paging messages within a short period, or a large number of

MSs send or resend random access request messages. These paging messages or access messages

increase the BSC load, exhaust key system resources, and thus overload the BSC.

In this case, the BSC controls the message arrival rates of the paging messages on the A interface

and of the random access request messages on the Abis interface. When the message arrival rate

control function is enabled, the BSC monitors the paging messages on the A interface and the

random access request messages on the Abis interface, and calculates in real time the message

arrival rates.

l If the calculated message arrival rate is lower than the configured message arrival rate, the

system does not enable the message arrival rate control function.

l If the calculated message arrival rate in a specified unit time is higher than the configured

maximum message arrival rate, the system directly discards the messages the number ofwhich exceeds the maximum allowed number.




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In the message arrival rate control function, the parameters such as whether to start message

arrival rate control, the statistical period, and the number of messages in a statistical period are

configurable. The parameters are as follows:

l Start Pg Arrival Ctrl Pg State Period

l Pg Max Message Number In Period

l Start Channel Request Arrival Ctrl

l Channel Request Stat Period

l Channel Request Max Message Number In Period

l CPU usage threshold of XPUT channel required flow control in critical congestion

Flow Control on LAPD Links

This topic describes the flow control on LAPD links. The flow control on LAPD links is

classified into DL flow control and UL flow control.

The flow control on LAPD links over the Abis interface enables the system to control the number

of messages (including paging messages and access request messages) sent on the Abis interface

based on the message arrival rates and system load. Thus, the requirements for signaling load

and system load on LAPD links are met. This ensures the smooth operations of call services.

The DL flow control on LAPD links refers to controlling the number of paging messages. The

UL flow control on LAPD links refers to controlling the number of random access request

messages.

DL Flow ControlIf the rate that the BSC sends messages to the LAPD links is greater than the rate that the messages

on the Abis interface are sent to the BTS, DL messages are buffered in the I frame queue or are

even discarded. Thus, the length of the I frame queue should be calculated and flow control

should be performed.

The parameters of DL flow control on LAPD links are as follows:

l Flow Control End Threshold

l Flow Control Start Threshold

UL Flow Control

If a large number of random access request messages are sent on the LAPD links on the Abis

interface, the traffic flow of the system is greatly increased. Thus, the LAPD links are overloaded.

To avoid this case, the BSC controls the message arrival rate of random access request messages

sent by the BTS. If the actual message arrival rate exceeds a configured rate, the BSC discards

some of the random access request messages based on the configured flow control data.

The parameters ofthe UL flow control on LAPD links are as follows:

l Ra Max Count Percent Period

l Ra Check Period (50 ms)

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Traffic Flow Control on the Um Interface

This topic describes the traffic flow control on the Um interface. The traffic flow control on the

Um interface involves the BTS processor overload, paging channel overload, and cell broadcast

channel overload.

The traffic flow control on the Um interface is controlled by the BSC based on the load reports

sent by the BTS.

BTS Processor Overload

When a TRX processor in the BTS is overloaded, the BTS sends an Overload message to the

BSC with the cause value Processor Overload and the overloaded TRX.

On receiving the Overload message, the BSC starts the TRX overload timer and sets the state

of the overloaded TRX to the overload state. In this case, the BSC stops assigning the SDCCHs

and TCHs of the TRX. It also stops the SDCCH dynamic adjustment and reconversion.

If the TRX runs normally after the TRX overload timer expires, the BSC can perform channelassignment and SDCCH dynamic adjustment.

If the TRX overload timer does not expire (the TRX is in the overload state) and the BSC again

receives an Overload (processor overload) message related to the TRX, the BSC restarts the

TRX overload timer and sets the TRX to the overload state.

Paging Channel Overload

The network can locate a called MS by paging the MS in its location area. Then, the MS requests

access to the network.

If many paging messages are sent on the Um interface, PCHs may be overloaded. In this case,the BTS sends the BSC a CCCH Overload Indication message, indicating that the PCHs of the

current cell are overloaded.

On receiving the CCCH Overload Indication message, the BSC sends an Overload message to

the BSC with the overload cause CCCH Overload. The Overload message also indicates the

overloaded cell.

After receiving the Overload message, the MSC reduces paging messages sent to the BSC.

The flow control related to paging channel overload is a basic function of the BSS and does not

require data configurations.

Cell Broadcast Channel OverloadThe BSC supports the cell broadcast short message function. Cell broadcast short messages are

sent on the CBCHs on the Um interface.

Based on the loads of the CBCHs on the Um interface, the BTS sends the CBCH Load Indication

messages to the BSC. The CBCH Load Indication messages indicate whether the CBCH is

overloaded or underloaded.

If the CBCH Load Indication message indicates that the CBCHs of the current cell are

overloaded, the BSC does not send cell broadcast message to the cell in a period of N*1.883

seconds. Assume that N is the delay value that is indicated in the CBCH Load Indication message.

If the CBCH Load Indication message indicates that the CBCHs of the current cell areunderloaded and indicates the number N, the BSC determines how many cell broadcast messages




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can be sent. Assume that N is the number of cell broadcast messages that can be sent to the

current cell, M is the number of cell broadcast messages that need to be sent, and S = Min {M,

N}. The number of cell broadcast messages that can be sent by the BSC to the BTS is S.

The flow control related to cell broadcast messages does not require data configurations, but

requires the system to support cell broadcast short messages.

NOTE

The minimum interval of two consecutive cell broadcast messages is 1.883 seconds.

Cell Flow Control

This topic describes cell flow control. To enable the cell flow control, each cell has a state

machine for cell flow control. The flow control of one cell is independent of the other cells.

Cell flow control is performed to avoid the following conditions:

l When a large number of MSs simultaneously send location update requests (channel

requests), the BSC sends a large number of Immediate Assignment messages or Immediate

Assignment Reject messages to the BTS. Thus, the BTS is overloaded and the transmission

queue of AGCHs overflows.

l After sending a channel request message, the MS retransmits the channel request message

if it does not receive a real-time response from the network. When an Immediate

Assignment message is discarded, the MS cannot use the activated signaling channel, which

is not released until the relevant timer expires. Thus, the signaling channel is wasted.

After the BSC receives a channel request message, there is no available channel to be assigned.

In this case, cell flow control is triggered.

The cell flow control function uses the double timer mechanism to adjust the flow control level.

The double timer mechanism is described in the 3GPP 48058 protocol. When a cell is initialized,

the flow control level of the cell is zero and the flow control state is Idle state. Timers T1 and

T2 are not running. Table 4-3 describes the transfer of the flow control states.

Table 4-3 Transfer of the flow control states

Current State Event Action Next State

Idle state The current cell has

no available channel.

The flow control

level is increased by

one. Start timers T1and T2.

Timers T1 and T2 are

running.


running.

The current cell has


Take statistics of

traffic.


running.

Timer T1 expires. No action is

performed.

Timer T2 is running.

Timer T2 is running. The current cell has


The flow control

level is increased by

one. Start timers T1

and T2.


running.

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Current State Event Action Next State

Timer T2 expires. The flow control

level is decreased by

one. If the flow

control level after the

decrease is greater

than zero, restart

timer T2.

The flow control

level is decreased by

one. The flow control

level after the

decrease is zero.

Timer T2 is running.

Idle state

Cell flow control has six levels: 0 to 5.

l Level 0 indicates the normal state.

l Level 5 indicates the most serious flow control state.

When the flow control level changes, the BSC modifies the parameters MS MAX Retrans and

Tx-integer in the system message and retransmits the system message. The new system message

requests the MS to reduce channel request messages and to increase the interval of sending

channel request messages. Table 4-4 describes the flow control measures that are taken when

the cell flow control level changes.

Table 4-4 Cell flow control level

Adjustment ofCell FlowControl

MS MAX Retrans Tx-integer

01 Min(4, initial configuration) If the flow control level is smaller

than 4, set the flow control level

to 4.

12 Min(2, current value) If the flow control level is smaller


to 7.

23 Min(2, current value) If the flow control level is smaller than 10, set the flow control level

to 10.

34 1 If the flow control level is smaller


to 13.

45 1 The flow control level is 15.

54 1 The flow control level is Max(13,

initial configuration)



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Adjustment ofCell FlowControl

MS MAX Retrans Tx-integer

43 Min(2, initial configuration) The flow control level is Max(10,






10 Initial configuration Reset to initial configuration.

The Immediate Assignment Reject message sent by the BSC to an MS indicates the time that

the MS should wait before retransmitting a channel request message. The time depends on theflow control level. The higher the flow control level is, the longer the MS should wait. Table

4-5 lists the mapping between the flow control level and the length of timer T3122.

Table 4-5 Mapping between the flow control level and the length of timer T3122

Cell Flow Control Level Length of Timer T3122 (Second)

0 10

1 30

2 90

3 130

4 170

When configuring the cell flow control, you should set the Abis-interface flow control timers

T1 and T2. Timer T1 and T2 work together to determine the cell flow control level. When the

cell flow control level changes, the parameters MS MAX Retrans and Tx-integer in the system

message are modified. The parameters of timers T1 and T2 are as follows:

l Abis Flow Control Timer 1 (s)

l

Abis Flow Control Timer 2 (s)NOTE

When setting timers T1 and T2, ensure that the length of timer T1 should be smaller than that of timer T2.

Internal Flow Control

This topic describes the internal flow control. The BSC performs flow control on key system

resources such as messages and CPU usage. This ensures that the primary services run normally

and the BSS gradually restores the normal capacity and load when the traffic flow exceeds the

capacity and flow specifications.

The BSC monitors in real time the usage of key system resources, performs calculation, andcompares the calculation results with thresholds. In this way, the BSC determines the current

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flow control level and the discard proportion of random access request messages and paging

messages.

The internal flow control function enables the system to sample the number of resources used

by all messages. Then the number serves as an input of a low pass filter, the output of the low

pass filter is the average number (avg) of resources used by each system message. Through thefiltering, burst flow under normal conditions can be filtered and congestion can be detected.

NOTE

At present, the low pass filter in use is a WIN_SIZE-point rectangular window filter. The filter calculates

the average value of recent WIN_SIZE points input to obtain the current output.

The average message usage (avg) is used to determine the system load. The system message

usage changes rapidly, as shown in Figure 4-1. If the system message usage is directly used to

determine the system load, a tiny traffic burst may trigger the internal flow control, that is, the

internal flow control is triggered under normal conditions. If the flow control function is enabled

frequently, normal calls are rejected and call loss occurs. Thus, the system processing capability

is underused. Smoothing (low pass filter) filters tiny burst flow. When congestion occurs, themessage queue is long and average message usage (avg) continually increases. At last, the traffic

flow exceeds the flow control begin threshold.

Figure 4-1 BSS internal flow control

Th2

Th1

t

Assume that avg is the average system message usage, Th1 is the internal flow control discard

begin threshold, Th2 is the internal flow control discard all threshold, and P is the message

discard probability. Figure 4-2 shows the calculation of P.

l If avg < Th1, then P = 0.

l If Th1 avg

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Figure 4-2 Calculation of P

P

avgTh1 Th2

When the BSC receives paging messages on the A interface or random access request messages

on the Abis interface, it discards some of these messages based on P.

The internal flow control levels are calculated based on P and are classified into 12 levels: level0 through level 11.

l Level 0 indicates that internal flow control is not required.

l Level 11 indicates the most serious flow control state.

The BSS limits some auxiliary functions based on the internal flow control levels. Table 4-6

lists the auxiliary functions related to the internal flow control levels.

Table 4-6 Auxiliary functions related to the internal flow control levels

Control Item Flow Control Level

Level 01

Level 23

Level 45

Level67

Level 89

Level 1011

Cell frequency scan - Inhibit Inhibit Inhibit Inhibit Inhibi

t

Normal information

print

- Inhibit Inhibit Inhibit Inhibit Inhibi

t

Normal log - Inhibit Inhibit Inhibit Inhibit Inhibi

t

Call resource check - Inhibit Inhibit Inhibit Inhibit Inhibit

Forced handover - - Inhibit Inhibit Inhibit Inhibi

t

Switching candidate

query

- - - Inhibit Inhibit Inhibi

t

Channel allocation

queue function

- - - - Inhibit Inhibi

t

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Control Item Flow Control Level

Level 01

Level 23

Level 45

Level67

Level 89

Level 1011

Radio resource

indication processing

(interference band)

- - - - Inhibit Inhibi

t

Call preemption - - - - Inhibit Inhibi

t

Channel conversion - - - - - Inhibi

t

Conversion between

traffic channel and

signaling channel

- - - - - Inhibi

t

The parameters of the internal flow control are as follows:

l Internal Flow Control Allowed

l Internal Flow Control Discard Begin Threshold

l Internal Flow Control Window Size

l Internal Flow Control Discard All Threshold

OM Flow ControlThis topic describes the OM flow control. The GBAM and service boards determine whether to

perform OM or whether to send OM messages to other boards based on relevant information.

The system monitoring processes in the GBAM and service boards monitor in real time key

system resources such as the CPU usage and system message resources, and then calculate the

usage of these resources and the flow control level of each board. Then, these flow control levels

are used to control the flow of log messages and signaling tracing messages.

The flow control level of each service board is broadcast to other boards through periodical

message broadcasts among the GBAM and serviceboards. A board determines whether to

perform OM or send OM messages to other boards based on its flow control level and the flow

control levels of other boards received from periodical message broadcasts.

By default, the OM flow control of the BSC uses hierarchical flow control and has multiple flow

control thresholds (ThSt). For each flow control level, the BSC performs relevant operations.

l If the value of a flow control source is smaller than ThSt1, the flow of the flow control

source is not controlled.

l If the value of a flow control source is greater than ThStn, all the flow control sources are

discarded.

l If the value of a flow control source is between ThSt1 and ThStn, the flow of the flow

control source is controlled based on its flow control level.

Figure 4-3 shows the relations between the OM flow control levels and the flow controlthresholds.



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Figure 4-3 Relations between the OM flow control levels and the flow control thresholds

Not overload state

Serious overload

stateBegin overload state

1 2

3

4

When the flow control level reaches level 2, the BSS controls the flow of the signaling tracing

messages.

4.5 Implementation

The implementation of flow control refers to configuring flow control.

4.5.1 Configuring Flow Control

This topic describes how to configure flow control on the BSC6000 Local Maintenance

Terminal.

4.5.1 Configuring Flow Control

This topic describes how to configure flow control on the BSC6000 Local Maintenance

Terminal.

Procedure

Step 1 On the Management Tree tab page of the BSC6000 Local Maintenance Terminal, right-clickBSC6000, and then choose Configure BSC Attributes on the shortcut menu. A dialog box is

displayed, as shown in Figure 4-4.

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Figure 4-4 Configure BSC Attributes dialog box

Step 2 Click the Flow Control Data tab. Set the relevant parameters, as shown in Figure 4-5 andFigure 4-6.

Figure 4-5 Configuring flow control data (1)




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Figure 4-6 Configuring flow control data (2)

Step 3 Click the BSC Timer tab. Set the parameters, as shown in Figure 4-7.

Figure 4-7 Configuring BSC timer

Step 4 Click the Software Parameters tab. Set flow control data of LAPD links, as shown in Figure4-8.

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Figure 4-8 Configuring software parameters

Step 5 Set Flow Control Start Threshold and Flow Control End Threshold.

1. On the Management Tree tab page of the BSC6000 Local Maintenance Terminal, right-

click the target TRX, and then choose Configure TRX Attributes on the shortcut menu.

A dialog box is displayed, as shown in Figure 4-9.

Figure 4-9 Configure TRX Attributes dialog box



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2. Select the target TRX, and then clickSet TRX Attributes. A dialog box is displayed, as

shown in Figure 4-10.

Figure 4-10 Configure TRX Attributes dialog box

3. Click the RSL Setting tab, and then set Flow Control Start Threshold and Flow Control

End Threshold.

----End

4.6 Maintenance Information

This topic describes the alarms and counters related to flow control.

Alarms

Table 4-7 lists the alarms related to flow control.

Table 4-7 Alarms related to flow control

Alarm ID Alarm Name

201 Internal Flow Control Level Changed

202 Internal Flow Control Released

101 MSC Overload

21001 LAPD Link Congestion

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Counters

Table 4-8 describes the counters related to flow control.

The cell flow control is performed through a double-timer mechanism:

l If a trigger event occurs when timer T1 is running, the trigger event is ignored.

l When a trigger event occurs, if timer T1 is not running, the flow control level is increased

by one unless it reaches the highest level. At the same time, timers T1 and T2 (T2 > T1)

are started.

If timer T2 expires, the flow control level is decreased by one. If the flow control level is not

zero after the decrease, timer T2 is restarted.

Table 4-8 Counters related to flow control

Counter Description

Increaes of Flow Control Levels This counter indicates the number of times the trigger

event occurs when T1 stops and the flow control does

not reach the highest level.

Decreases of Flow Control Levels The counter indicates the number of times timer T2

expires.

Ignored Trigger Events This counter indicates the number of times that trigger

events occur when timer T1 is running.

Highest Level Delays This counter indicates the number of times trigger events

occur when timer T2 is running and the flow control level

is the highest level.

SM Pagings Discarded on LAPD

Link

This counter indicates the number of discarded short

message (SM) pagings on the LAPD link.

CS Pagings Discarded on LAPD

Link

This counter indicates the number of discarded CS

pagings on the LAPD link.

PS Pagings Discarded on LAPD

Link

This counter indicates the number of discarded PS

pagings on the LAPD link.

MSG CCCH LOAD IND (RACH)

Messages Sent on Abis Interface

The counter indicates the number of times that the BSC

receives CCCH Load Indication messages with the cause

value UL CCCH (RACH) Overload from the BTS.

MSG CCCH LOAD IND (PCH)


The counter indicates the number of times the BSC

receives CCCH Load Indication messages with the cause

value DL Circuit Service CCCH (PCH) Overload from

the BTS.

PACKET CCCH LOAD IND


This counter indicates the number of times the BSC

receives Packet CCCH Load Indication messages in a

statistical period.

MSG ABIS OVERLOAD (CCCH

OVERLOAD) Messages Sent on

Abis Interface


receives Overload messages with the cause value CCCH

Overload in a statistical period.




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Counter Description

MSG ABIS OVERLOAD

(PROCESSOR OVERLOAD)



receives Overload messages with the cause value TRX

Processor Overload in a statistical period.

4.7 References

l 3GPP 48.058 Base Station Controller - Base Transceiver Station (BCS-BTS) Interface

Layer 3 Specification

l 3GPP 48.008 Mobile Switching Centre - Base Station system (MSC-BSS) Interface Layer

3 Specification

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