Downlink TBF Establishment Success Rate Optimization Manual

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GSM BSS Network Performance PS KPI (Downlink TBF Establishment Success Rate)

Optimization Manual

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GSM BSS Network Performance PS KPI(Downlink TBF Establishment Success Rate)

Optimization Manual

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Optimization Manual

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Optimization Manual

GSM BSS Network Performance PS KPI (Downlink TBF

Establishment Success Rate) Optimization Manual

Keywords

Downlink TBF, establishment success rate

Abstract

This manual mainly describes the method of collecting statistics about the downlinkTBF establishment success rate and the methods of optimizing the downlink TBFestablishment success rate.

List of abbreviations

Abbreviation Full Spelling

PDCH Packet Data Channel

PCU Packet Control Unit

MS Mobile Station

CQT Call Quality Test

KPI Key Performance Index

DT Drive Test

GPRS General Packet Radio Service

EDGE Enhanced Data rates for GSM Evolution

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Optimization Manual

Contents

1.1 Counter Definition....................................................................................................................................................6

1.1.1 Definition in the Case of No MS Response .......................................................................................................6

1.1.2 Definition in the Case of No Available Resources ............................................................................................6

1.1.3 Definition in the Case of No MS Response or No Available Resources ...........................................................7

1.2 Theory.......................................................................................................................................................................7

2.1 Number of Successful Downlink TBF Establishments............................................................................................8

2.1.1 Description..........................................................................................................................................................8

2.1.2 Measurement Point.............................................................................................................................................8

2.2 Number of Failed Downlink TBF Establishments...................................................................................................9

2.2.1 Description..........................................................................................................................................................9

2.2.2 Measurement Point...........................................................................................................................................10

2.3 Number of Downlink TBF Establishment Attempts..............................................................................................10

2.3.1 Description........................................................................................................................................................10

2.3.2 Measurement Point...........................................................................................................................................10

3.1 Checking the Abis Link..........................................................................................................................................15

3.2 Checking the Delivery of Assignment Messages...................................................................................................16

3.2.1 Failed Downlink TBF Establishments Due to Overloaded CCCH..................................................................16

3.2.2 Failed Downlink TBF Establishments Due to No Channel Resources............................................................16

3.3 Checking the Air Interface......................................................................................................................................19

3.4 Checking Failed Downlink TBF Establishments Due to No Response from the MS............................................20

3.4.1 Failed Downlink TBF Establishments Due to Improper Parameter Configurations........................................20

3.4.2 Failed Downlink TBF Establishments Due to Incorrect Information Elements in Assignment Messages......22

3.4.3 Failed Downlink TBF Establishments Due to Imbalance Between the Uplink and the Downlink..................233.4.4 Checking the Feed System...............................................................................................................................24

3.4.5 Checking the KPIs of the PS Field...................................................................................................................24

4.1 Case 1: Low Success Rate of Downlink TBF Establishment Due to Incorrect Frequency-Hopping Parameter

Settings in Czech Republic...........................................................................................................................................25

4.2 Case 2: Low Success Rate of Downlink TBF Establishment Due to No Responses from MSs After the PCU Is

Upgraded......................................................................................................................................................................28

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Optimization Manual

Figures

Successful establishment of downlink TBF on the CCCH.....................9

Successful downlink TBF establishment on the PACCH.......................9

Downlink TBF establishment attempts on the CCCH.........................11

Downlink TBF establishment attempts on the PACCH.......................11

Uplink and downlink TBF establishment procedure on the CCCH.......13

Overall analysis process.................................................................14

Analysis of the frame error rate of the G-Abis interface is normal.....26

Downlink packet assignment message............................................26

MA Numeber in the downlink packet assignment message...............27

Frequency-hopping information being null in the SI 13 message.......28

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Optimization Manual

GSM BSS Network Performance PS KPI (Downlink TBF

Establishment Success Rate) Optimization Manual

1Basic Principle

1.1 Counter Definition

The definition of the downlink TBF establishment success rate varies with theassessment item.

1.1.1 Definition in the Case of No MS ResponseIf the network side delivers an assignment message to a mobile station (MS) but failsto receive the Packet Control Acknowledgement message from the MS, thenumber of failed downlink TBF establishments due to MS no response is added byone.

The definition of the downlink TBF establishment success rate is as follows:

Downlink GPRS TBF Establishment Success Rate = 1 - Number of FailedDownlink GPRS TBF Establishments due to MS No Response/Number of Downlink GPRS TBF Establishment Attempts

Downlink EGPRS TBF Establishment Success Rate = 1 - Number of FailedDownlink EGPRS TBF Establishments due to MS No Response/Number of Downlink EGPRS TBF Establishment Attempts

1.1.2 Definition in the Case of No Available Resources

If the downlink TBF establishment fails because the resources such as channels andTFIs at the network side are unavailable, the number of failed downlink TBFestablishments due to no channel is added by one.


Downlink GPRS TBF Establishment Success Rate = 1 - Number of FailedDownlink GPRS TBF Establishments due to No Channel/Number of DownlinkGPRS TBF Establishment Attempts

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Downlink EGPRS TBF Establishment Success Rate = 1 - Number of FailedDownlink EGPRS TBF Establishments due to No Channel/Number of DownlinkEGPRS TBF Establishment Attempts

1.1.3 Definition in the Case of No MS Response or NoAvailable Resources

If the downlink TBF establishment fails because the network side fails to receive thePacket Control Acknowledgement message from an MS or the resources at thenetwork side are unavailable, the number of failed downlink TBF establishments isadd by one.


Downlink GPRS TBF Establishment Success Rate = Number of SuccessfulDownlink GPRS TBF Establishments/Number of Downlink GPRS TBFEstablishment Attempts

Downlink EGPRS TBF Establishment Success Rate = Number of SuccessfulDownlink EGPRS TBF Establishments/Number of Downlink EGPRS TBFEstablishment Attempts

1.2 Theory

The downlink TBF establishment success rate shows the downlink accessperformance and is an important counter for assessing the network. When thedownlink TBF fails to be established, the network side continues to trigger theestablishment of the downlink TBF in a short time because the network side has

some data blocks that are not delivered. Therefore, the downlink TBF establishmentsuccess rate is slightly low in this case, but customer experience is not affected.

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2 Signaling Procedure

2.1 Number of Successful Downlink TBFEstablishments

2.1.1 Description

This counter provides the number of successful downlink TBF establishments in a measurementperiod.

2.1.2 Measurement Point

Successful downlink TBF establishments involve the following aspects:

1. Successful downlink TBF establishment on the CCCH

The network side initiates the downlink TBF establishment procedure by sendingan IMMEDIATE ASSIGNMENT message with Starting Time on the CCCH to theMS. When the starting time is reached, the network side sends a POLLINGmessage to the MS to obtain a TA value. The BSC reserves block resources for the MS to respond with an assignment acknowledgement message. If thenetwork side receives a Packet Control Acknowledgement message from theMS on the reserved block resources in the assigned channel, it indicates that thedownlink TBF is established. In addition, the network side can calculate the TAvalue by using the Packet Control Acknowledgement message that isreceived.

The following figure shows the procedure for establishing downlink TBFs on theCCCH. Each time the network side receives a Packet ControlAcknowledgement message (see measurement point A), the value of thecounter Number of Successful Downlink TBF Establishments is added byone.

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Figure 1.1 Successful establishment of downlink TBF on the CCCH

M S Network

A

IMMEDIATE ASSIGNMENT (CCCH)

POLLING(RRBP)

PACKET Control Acknowledgement

2. Successful downlink TBF establishment on the PACCH

The network side can initiate the downlink TBF establishment procedure bysending a Packet downlink assignment message on the PACCH to the MS.The message contains the information about the block resources reserved bythe network side for the MS to respond with an assignment acknowledgementmessage. If the network side receives a Packet Control Acknowledgementmessage from the MS on the reserved block resources in the assigned channel,it indicates that the downlink TBF is established.

2.1 shows the procedure for establishing the downlink TBF on the PACCH. Eachtime the network side receives a Packet Control Acknowledgement messagecorresponding to a Packet downlink assignment message (see measurementpoint A), the value of the counter Number of Successful Downlink TBFEstablishments is added by one.

Figure 2.1 Successful downlink TBF establishment on the PACCH

M S Network

A

Packet downlink assignment

PACKET Control Acknowledgement

2.2 Number of Failed Downlink TBF Establishments

2.2.1 Description

This counter measures the number of failed downlink TBF establishments in ameasurement period.

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Failed downlink TBF establishments involve the following aspects:

1. Failed downlink TBF establishments due to no channel

If the BSC receives a new downlink PDU request from the SGSN but fails toestablish a download TBF because channels are unavailable, the value of Number of Failed Downlink TBF Establishments due to No Channel is added by one.

2. Failed downlink TBF establishments due to MS no response

In the procedure for establishing a downlink TBF, the BSC sends a POLLINGmessage on the CCCH or sends a Packet downlink assignment message on thePACCH, and reserves block resources for the MS to respond with an assignmentacknowledgement message. If the BSC does not receive a Packet ControlAcknowledgement message from the MS on the reserved block resources, the BSCsends the IMMEDIATE ASSIGNMENT messages repeatedly until the maximum

number of retry times is exceeded. Each time the maximum number of retrying timesis exceeded, the value of Number of Failed Downlink TBF Establishments due toMS No Response is added by one.

2.3 Number of Downlink TBF Establishment Attempts

2.3.1 Description

This counter measures the number of downlink TBF establishment attempts in ameasurement period.


Downlink TBF establishment attempts involve the following aspects:

1. Downlink TBF establishment attempts on the CCCH

The network attempts to establish the downlink TBF by sending the IMMEDIATEASSIGNMENT message at the sub-timeslot corresponding to the CCCH groupto which the MS belongs. Figure shows the procedure for the network side tosend the IMMEDIATE ASSIGNMENT message on the CCCH. Each time thenetwork side sends an IMMEDIATE ASSIGNMENT message (see measurementpoint A), the value of the counter Number of Downlink TBF EstablishmentAttempts is added by one.

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Figure 1.1 Downlink TBF establishment attempts on the CCCH.

M S Network

A

IMMEDIATE ASSIGNMENT

(On CCCH)

2. Downlink TBF establishment attempts on the PACCHThe network side can attempt to establish the downlink TBF by sending aPACKET DOWNLINK ASSIGNMENT message to an MS during thetransmission process of the previous uplink TBF or the release process of thecurrent downlink TBF. 2.1 shows the procedure for the network side to send thePACKET DOWNLINK ASSIGNMENT message on the PACCH. Each time thenetwork side sends a PACKET DOWNLINK ASSIGNMENT message (seemeasurement point A), the value of the counter Number of Downlink TBFEstablishment Attempts is added by one.

Figure 2.1 Downlink TBF establishment attempts on the PACCH

M S Network

A

PACKET DOWNLINK ASSIGNMENT

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Optimization Manual

3 Analysis and Optimization Methods

The procedure for establishing the downlink TBF on the CCCH is as follows:

1. The RR entity at the network side initiates the downlink TBF establishment byusing the downlink packet assignment procedure. The downlink packetassignment procedure is triggered by a request from upper layers to transfer aLLC PDU. Before transferring an LLC PDU, the network side determineswhether the MS is in the Ready state. If the MS is in the Ready state, thenetwork side transfers an LLC PDU to the BSS. When receiving the LLC PDU,the BSS delivers an IMMEDIATE ASSIGNMENT message. If the MS is in theStandby state, the network side sends a paging message to the BSS. Thenetwork side sends the LLC PDU only after receiving a paging response fromthe BSS. The request from upper layers contains the priority, including the RLCmode, DRX parameter (optional), QoS script file of the IMSI (optional), and MSradio access capability associated with the packet transfer (optional). For such arequest, the network side determines whether the MS is in packet idle mode or packet transfer mode. If the MS is in packet idle mode, the network side initiatesthe downlink packet assignment procedure on the CCCH. If the MS is in packettransfer mode, the network side initiates the downlink packet assignmentprocedure on the PACCH.

2. The network side selects an encoding scheme and applies for radio resourcesaccording to the resource occupation in the accessed cell for establishing thedownlink TBF. After the application is approved, the network side assigns radioresources to the downlink TBF and counts the times the downlink TBF is startedat the network side and at the MS side.

3. The network side delivers an IMMEDIATE ASSIGNMENT message. If the MS isin DRX mode, the network side delivers the message in the PCH channel. If theMS is in Non-DRX mode, the network side delivers the message in the AGCHchannel.

4. When the MS receives the IMMEDIATE ASSIGNMENT message, radioresources are assigned. After receiving the frame number indicated by the TBFStarting Time (optional), the MS accesses the assigned channel, starts to listenon the RLC radio block of the downlink TBF, and starts timer T3190.

5. If the network side has the TA value of the MS, the network side directly trasfersthe TA value to the MS by sending a Packet Power Control/Timing Advancemessage after the Starting Time of the downlink TBF is reached. If the networkside does not have the TA value, the network side obtains the TA value by

sending a PACKET POLLING REQUEST message after the downlink TBF is

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Optimization Manual

started.

6. Before timer T3190 times out, if the MS addresses the downlink RLC radio blockin the assigned channel according to the TFI field and the MS receives thePACKET POLLING REQUEST message from the network side, the MS sends a

Packet Control Acknowledgement message on the uplink radio blockcorresponding to the message and resets timer T3190. Otherwise, the MSnotifies the upper layers of the downlink TBF establishment failure.

7. The network side obtains the TA value of the MS by using the valid PacketControl Acknowledgement message received on the reserved uplink RLCradio block. In this case, the network side regards that the downlink TBFestablishment is successful. Otherwise, the network side initiates the downlinkimmediate assignment procedure again.

Figure 7.1 Uplink and downlink TBF establishment procedure on the CCCH

MS PCU

PDCH

IMMEDIATE ASSIGNMENTCCCH

LLC PDU

PACKET POLLING REQUEST

PACKET CONTROL ACKNOWLEDGMENT

PDCH

BTS

This section taske the uplink and downlink TBF establishment on the CCCH as anexample to describe the optimization ideas about identifying the signaling and cellswhere problems occur throughout the signaling procedure. In the downlink TBFestablishment procedure, you can identify problematic singaling and cells as follow:

Checking whether transmission problems occur in the Abis links.

Checking whether the IMMEDIATE ASSIGNMENT and PACKET POLLING

REQUEST messages are properly sent to the BTS.

Checking whether the IMMEDIATE ASSIGNMENT and PACKET POLLINGREQUEST messages are sent to MSs according to the air interface quality.

Checking whether MSs respond to POLLING messages by sending PacketControl Acknowledgement messages.

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Figure 7.2 Overall analysis process

Analyze the cause of downlink TBF

establishment failure

Start

Check whether the status of

the Abis link is normal

Yes

Check whether assignment

messages are sent normally

Check whether the downlinkair interface is normal

Yes

Check whether MSs

respond to ASSIGNMENT

and POLLING messages

No

No

Check whether the

problem is solved

Yes

Check the transmission

Check whether the

CCCH is overloaded

Check whether channels

are available

No

Check whether the

downlink air interface

quality is poor

Perform the CQT test

No

Check whether theparameter configurations

are correct

Check whether the

importance cells are

correct

Yes

Check whether the uplink

and downlink are

balanced

Check whether the feed

system is normal

Check whether the

parameters of the CS

fieldNo

Yes

End

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3.1 Checking the Abis Link

Transmission problems such as out-of-synchronization or intermittent Abis interfacelinks may cause the failure to establish the downlink TBF.

You can determine the trasmission status of the Abis interface by calculating theframe error rate of the G-Abis interface as follows:

Frame error rate of the G-Abis interface = (Number of Received Check Error TRAUFrames + Number of Received Out-of-Synchronization TRAU Frames)/(Number of Sent Valid TRAU Frames + Number of Sent Empty TRAU Frames)

1. Generally, the frame error rate is equal to or lower than 10e–5, that is, 1/10,000.This indicates one error frame in a channel every four minutes on average. Inthis case, you can infer that the link is of good quality, and the MS can transmitdata in a stable way.

2. If the frame error rate is lower than 10e-4, the quality of the transmission link is

poor. In this case, one to three error frames occur in a channel every minute onaverage. Error frames are unpredictable. Therefore, affected MSs are likely toexperience low transmission rate, long transmission delay, or evendisconnection.

3. If the frame error rate is higher than 10e-4, the link becomes unstable. Out-of-synchronization is likely to occur and the rate of out-of-synchronization framegreatly increases. In this case, MSs may be able to perform data services thatrequire only a small volume, such as the high-layer signaling and certain WAPservices. Mass data transmission, such as FTP services, becomes difficult.

In actual running, carriers are not able to directly control leased lines, such asmicrowave satelites. Therefore, a frame error rate lower than 5/1000 is acceptable. If you find that a transmission problem has occurred because the frame error rate of acell stays high for a long time, check the transmission line and optimize the network.

The following table lists the relavant KPIs.

KPI Cell-Level

Frame error rate of the G-Abisinterface

G-Abis Measurement -> Performance measurement of BSC packet assignment ->

Number of Received Normal TRAU Frames

Number of Received Out-of-Synchronization TRAUFrames

Number of Received Check Error TRAU Frames

Number of Sent Valid TRAU Frames

Number of Sent Empty TRAU Frames

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Number of received frames = Number of Received Normal TRAU Frames + Number of Received Out-of-Synchronization TRAU Frames + Number of Received Check Error TRAU Frames + Number of Sent Empty TRAU Frames. In versions earlier than V9R8C11,Number of Sent Empty TRAU Frames is not included in the statistics about the number of

received frames. Therefore, when the frame error rate of the G-Abis interface is calculated, thesum of Number of Sent Valid TRAU Frames and Number of Sent Empty TRAU Frames isused to indicate the number of received frames. In the version V9R8C12, this problem issolved by using the sum of Number of Received Normal TRAU Frames, Number of Received Out-of-Synchronization TRAU Frames, Number of Received Check Error TRAUFrames, and Number of Sent Empty TRAU Frames to indicate the number of receivedframes.

3.2 Checking the Delivery of Assignment Messages

3.2.1 Failed Downlink TBF Establishments Due to Overloaded

CCCHIf the CCCH is overloaded, the IMMEDIATE ASSIGNMENT message sent on theCCCH may be discarded. As a result, the downlink TBF fails to be established. Youcan check whether the CCCH is overloaded by viewing flow control traffics. If theCCCH is overloaded, you can increase the load threshold of the CCCH to preventdownlink TBF establishment failure due to flow control.

The following table lists the relevant counters.

Cause Cell-Level

Overloaded CCCH Call Measurement -> Flow control measurement ->

PACKET CCCH LOAD IND Messages Sent on Abis Interface

MSG ABIS OVERLOAD (CCCH OVERLOAD) MessagesSent on Abis Interface

MSG DEL IND Messages Sent on Abis Interface

3.2.2 Failed Downlink TBF Establishments Due to No ChannelResources

Hardware FaultFaults on hardware such as the TRX may affect the success rate of downlink TBFestablishment. Therefore, you should check hardware faults.

You can locate hardware faults by checking the traffic measurement related to thehardware faults.

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Cause BSC-Level Cell-Level

Equipment faults

BSC Measurement -> AccessMeasurement ->

TCH Availability per BSCConfigured TCHs per BSC

Available TCHs per BSC

KPI Measurement ->

TCH Availability

Available TCHs

Configured TCHs

TRX Measurement ->

Activated TRXs in cell

Available TRXs in cell

Insufficient Channel Resources

Insufficient channel resources, which may cause congestion, occur in the following

situations:

1. The number of channels configured in a cell is small, and the traffic of packetservices is heavy. As a result, MSs are multiplexed on the channels in the cell tothe maximum degree. In this case, you need to add more static and dynamicchannels. In addition, you need to check the PS-domain channel managementparameters and set PDCH Downlink Multiplex Threshold to 80, that is, themaximum number of TBFs multiplexed on the downlink is eight.

2. Check whether the preemption of dynamic PDCHs by CS services leads to theinsufficiency of PDCHs. You can check the number of times of reclaimingdynamic PDCHs by the BSC and the number of times of reclaiming dynamicPDCHs with load. If the numbers are great, you can infer that busy CS services

preempt channel resources of PS services. As a result, you need to add staticPDCHs. You can also set Level of Preempting Dynamic Channel to Controlchannels cannot be preempted.

The following table describes the relevant parameters.

Name Description Setting Principle Value Range

MaximumRatioThresholdof PDCHsin a Cell

The total number of TCHs and PDCHsavailable in a cell isfixed. This parameter determines the

proportion of PDCHs tothe total number of TCHs and PDCHs.

If this parameter is setto an excessive value,there are excessivePDCHs and insufficientTCHs. This affects CS

services.If this parameter is setto a modest value,there are insufficientPDCHs and excessiveTCHs. This affects PSservices.

Value range: 0-100.

Default value:50

PDCHDownlinkMultiplexThreshold

PDCH DownlinkMultiplex Threshold

If this parameter is setto a lower value, theTBFs established onthe PDCH and the

subscribers are fewer,and the downlink

Value range: 10-80

The value 10indicates that at

most one TBF

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bandwidth for eachsubscriber is higher.

If this parameter is setto a higher value, thenumber of TBFsestablished on thePDCH and the number of subscribers aregreater, and thedownlink bandwidth for each subscriber is less.

can beaccessed. Thevalue 80indicates that amaximum of 8TBFs can beaccessed.

Default value:80.

Level of Preempting

DynamicChannel

Level of dynamicchannel preempted byCS services and PS

services The TCH/Fs aredynamic channels thatcan be preempted. If thisparameter is set to Alldynamic channels canbe pre-empted, it meansthat the CS services canpreempt all dynamicchannels; if thisparameter is set toControl channels cannotbe pre-empted, it means

that the CS services canpreempt any dynamicchannels except thecontrol channels; if thisparameter is set toDynamic channelscarrying services cannotbe pre-empted, it meansthat the CS servicescannot preempt thedynamic channels thatcarry services.

Generally, CS serviceshave the highestpriority. This parameter

must be set to Alldynamic channels canbe pre-empted so thatCS services canpreempt all dynamicchannels.

To ensure dataservices, you can setthis parameter toControl channelscannot be pre-emptedor Dynamic channels

carrying servicescannot be pre-empted.

Value options:

All dynamicchannels can bepreempted,

Controlchannels cannotbe preempted

Dynamicchannelscarryingservices cannotbe preempted

Default value:

All dynamicchannels can bepreempted.

The following table lists the related traffic measurement counters.

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Cause Cell-Level

Insufficientresources

Packet Switch Channel Measurement -> PDCH ResourceCapability Measurement ->

Number of TCH to PDTCH Conversion AttemptsNumber of Successful TCH to PDTCH Conversions

Number of Reclaimed Dynamic PDCHs

Number of Reclaimed Busy Dynamic PDCHs

Packet Switch Call Measurement -> Downlink GPRS TBFEstablish and Release Capability Measurement ->

Number of Downlink GPRS TBF Establishment Attempts

Number of Successful Downlink GPRS TBF Establishments

Number of Failed Downlink GPRS TBF Establishments due to No

Channel

Average Number of Concurrent Downlink GPRS TBFs

Packet Switch Call Measurement ->Downlink EGPRS TBFEstablish and Release Capability Measurement ->Number of Downlink EGPRS TBF Establishment Attempts

Number of Successful Downlink EGPRS TBF Establishments

Number of Failed Downlink EGPRS TBF Establishments due toNo Channel

Average Number of Concurrent Downlink EGPRS TBFs

3.3 Checking the Air Interface

MSs may not receive downlink ASSIGNMENT messages or POLLING messages dueto poor air interface quality. You can check the air interface quality by checking theBEP distribution or performing the CQT test. To check the BEP distribution, you cancount the 8PSK_MEAN_BEP variants and the GMSK_MEAN_BEP variants. Toconduct the CQT, you can use a dedicated tool, such as the TEMS.

If the air interface is experiencing serious interruption, you can improve the air interface quality by adjusting the frequency point.

The following table lists the related traffic measurement counters.

KPI Cell-Level

Downlink air interface quality

Packet Switch Call Measurement ->Measurement of numbers of 8PSK_MEAN_BEP variants

Measurement of numbers of GMSK_MEAN_BEP variants

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3.4 Checking Failed Downlink TBF Establishments Dueto No Response from the MS

3.4.1 Failed Downlink TBF Establishments Due to Improper Parameter Configurations

If the TBF at the network side and the TBF at the MS are not started at the sametime, the downlink TBF establishment may fail. Therefore, the TBF at the networkside cannot be started before the TBF at the MS. Otherwise, the MS may miss thePOLLING message from the network side, and thus the downlink TBF establishmentfails.

The parameters are as follows:


Retry Times of Downlink TBFReassignment

This parameter specifies the maximumnumber of attempts toresend theIMMEDIATEASSIGNMENTmessage. Themessage is sent whenthe network side failsto receive a validPacket ControlAcknowledgement

message on thereserved uplink RLCblock in the procedurefor establishing thedownlink TBF. If thenumber is exceeded,the network sidereleases the downlinkTBF.

If the value of thisparameter is toosmall, the networkside will release thedownlink TBF withonly a few attempts toresend theIMMEDIATEASSIGNMENTmessage, whichleads to downlinkestablishment failure.

If the number of faileddownlink TBFestablishments isgreat, you can setthis value to a higher value.

Default value:2.

Retry Times of Downlink TBFPolling

This parameter specifies the maximumnumber of attempts toresend the POLLINGmessage in theprocedure for establishing thedownlink TBF.

If the value of thisparameter is toosmall, the downlinkTBF establishmentmay fail because thesending of thePOLLING messagefails in the procedurefor establishing thedownlink TBF on theCCCH. If the number of failed downlinkTBF establishmentsis great, you canchange this value to a

higher value.

Default value:5.

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Delay for DownlinkImmediateAssignmentDRX (block)

In DRX mode, thenetwork sidedetermines the time tostart downlink TBF onthe network side byusing this parameter.

In addition, the networkside calculates theTBF Starting Timeassigned to MSs tonotify the MSs of thetime to access theassigned channels.

If the value of thisparameter is great,the starting time of the downlink TBF onthe network side islate. After starting thedownlink TBF, thenetwork side sendsthe PACKETPOLLING REQUESTmessage or thedownlink data blocksimmediately. If thevalue of thisparameter is small,

the network sidestarts the downlinkTBF before an MS.The MS fails toreceive thesubsequent downlinkRLC radio blockssent from the networkside because the MShas not accessed theassigned channel atthe time. Therefore,

the accessperformance isdegraded.

Default value:12.

Delay for DownlinkImmediateAssignmentNon-DRX(block)

In Non-DRX mode, thenetwork sidedetermines the time tostart downlink TBF onthe network side byusing this parameter.

In addition, the networkside calculates theTBF Starting Timeassigned to MSs tonotify the MSs of thetime to access theassigned channels.

If the value of thisparameter is great,the starting time of the downlink TBF onthe network side islate. After starting thedownlink TBF, thenetwork side sendsthe PACKET

POLLING REQUESTmessage or thedownlink data blocksimmediately. If thevalue of thisparameter is small,the network sidestarts the downlinkTBF before an MS.Because the MS hasnot accessed theassigned channel at

the time, the MS failsto receive the

Default value:26.

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subsequent downlinkRLC radio blockssent from the networkside. Therefore, theaccess performanceis degraded.

3.4.2 Failed Downlink TBF Establishments Due to IncorrectInformation Elements in Assignment Messages

You need to check whether the important cells in assignment messages are correct,including the parameters such as the frequency-hopping parameter. Currently, power

control is not performed for downlink TBF establishment. Therefore, you do not needto check the power control parameters.

Check the frequency-hopping parameters as follows:

Check whether the values of the GPRS Mobile Allocation parameter in the SI 13message and the frequency parameters in the downlink assignment message areconsistent with data configurations.

The frequency parameters are described as follows:

A downlink assignment message contains the frequency parameters that indicate whether frequency hopping is applied for MSs and encoding scheme of the frequency points.

The parameters are as follows:

ARFCN: no frequency hopping

Indirect encoding: frequency hopping and indirect encoding

Direct encoding 1: frequency hopping and direct encoding 1

Direct encoding 2: frequency hopping and direct encoding 2

< Frequency Parameters IE > ::=

< TSC : bit (3) >

{ 00 < ARFCN : bit (10) >

| 01 < Indirect encoding : < Indirect encoding struct > >

| 10 < Direct encoding 1 : < Direct encoding 1 struct > >

| 11 < Direct encoding 2 : < Direct encoding 2 struct > > } ;

< Indirect encoding struct > ::=

< MAIO : bit (6) >

< MA_NUMBER : bit (4) >

{ 0 | 1 < CHANGE_MARK_1 : bit (2) >

{ 0 | 1 < CHANGE_MARK_2 : bit (2) > } } ;

< Direct encoding 1 struct > ::=

< MAIO : bit (6) >

< GPRS Mobile Allocation : < GPRS Mobile Allocation IE > > ;

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< Direct encoding 2 struct > ::=

< MAIO : bit (6) >

< HSN : bit (6) >

< Length of MA Frequency List contents : bit (4) >

< MA Frequency List contents : octet (val(Length of MA Frequency List contents) +3) > ;

Indirect encoding: Information used by MSs is obtained from the PSI 2 and PSI or from the SI13 and the previous assignment messages. Therefore, you need to check whether the valuesof the frequency-hopping parameters in the system messages or assignment messages areconsistnet with data configurations according to MA_NUMBER.

MA_NUMBER = 0–13 shall be used to reference a GPRS mobile allocation received in aPSI2 message;

MA_NUMBER = 14 shall be used to reference a GPRS mobile allocation received in a SI13or PSI13 message;

MA_NUMBER = 15 shall be used to reference a GPRS mobile allocation received in a

previous assignment message using the direct encoding.Direct encoding 1: MSs use the frequency-hopping index information specified by the GPRSMobile Allocation parameter in system messages.

Direct encoding 2: MSs use the frequency-hopping information such as MAIO, HSN, Length of MA Frequency List contents, and MA Frequency List contents specified in assignmentmessages.

3.4.3 Failed Downlink TBF Establishments Due to ImbalanceBetween the Uplink and the Downlink

When the imbalance between the uplink and the downlink occurs, no signals may bereceived on the uplink or downlink at the edge of a cell coverage area. As a result,

TBF fails to be established.To analyze the balance between the uplink and the downlink, check whether thetransmit power of the BTS is high. Then, check whether the components such as theTMA, BTS amplifier, and antenna port that affect uplink/downlink receive level haveproblems. For details, see the GSM BSS Network Performance KPI (Uplink and Downlink Balance) Optimization Manual.

If the uplink and downlink are imbalanced, the following conditions may occur: Thedifference between the mean uplink receive level and the mean downlink receivelevel is great; the uplink and downlink balance level is high; both the immediateassignment success rate and the assignment success rate are low. The followingtable lists the traffic measurement counters related to the balance between the uplink

and the downlink.

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Cause Cell-Level TRX-Level

Balancebetweenuplinkanddownlink

Call Measurement -> Assignment

Measurement ->Success Rate of TCH Assignment

Success Rate of CallEstablishmentCall Measurement -> ImmediateAssignment Measurement ->

Success Rate of ImmediateAssignment

MR Measurement ->

Uplink-and-Downlink BalanceMeasurement

MR Measurement ->

TCHF Receive LevelMeasurement

MR Measurement ->

TCHH Receive LevelMeasurement

3.4.4 Checking the Feed System

Hardware faults in the feed system or incorrect parameter configurations, such as theincorrect TMA factor, lead to low success rate of downlink TBF establishment. Inaddition, the faulty feed system may lead to the imbalance between the uplink andthe downlink. Therefore, you can check whether the feed system is faulty by usingthe traffic measurement counters related to the balance between the uplink and thedownlink.

3.4.5 Checking the KPIs of the PS Field

Low success rate of downlink TBF establishment may not be caused simply by theincorrect parameter configurations of the CS field. Incorrect parameter settings of theCS field may affect the KPIs of the PS field. Therefore, you must check whether theimportant KPIs of the CS field are normal. This helps you to locate faults. It isrecommended that you focus on the success rate of immediate assignment. If thesuccess rate is low, you can infer that this is caused by the air interface.

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4 Cases

4.1 Case 1: Low Success Rate of Downlink TBFEstablishment Due to Incorrect Frequency-HoppingParameter Settings in Czech Republic

Symptom

The success rate of downlink TBF establishment decreased suddenly sinceNovember 4, and the number of abnormal TBF releases increased. The statistics areas follows:

Troubleshooting

1. The frame error rate of the G-Abis interface is normal.According to the formula for calculating the frame error rate of the G-Abisinterface, the frame error rate was normal and stable on and near November 4.

Formula: Frame error rate of the G-Abis interface = (Number of ReceivedCheck Error TRAU Frames + Number of Received Out-of-SynchronizationTRAU Frames)/(Number of Sent Valid TRAU Frames + Number of SentEmpty TRAU Frames)

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Figure 1.1 Analysis of the frame error rate of the G-Abis interface is normal

0.00000000

0.00200000

0.00400000

0.00600000

0.00800000

0.01000000

0.01200000

0.01400000

0 : 0 0 : 0 0

1 : 3 0 : 0 0

3 : 0 0 : 0 0

4 : 3 0 : 0 0

6 : 0 0 : 0 0

7 : 3 0 : 0 0

9 : 0 0 : 0 0

0 : 3 0 : 0 0

2 : 0 0 : 0 0

3 : 3 0 : 0 0

5 : 0 0 : 0 0

6 : 3 0 : 0 0

8 : 0 0 : 0 0

9 : 3 0 : 0 0

1 : 0 0 : 0 0

2 : 3 0 : 0 0

03/ 11/ 2

04/ 11/ 2

05/ 11/ 2

06/ 11/ 2

07/ 11/ 2

08/ 11/ 2

09/ 11/ 2

2. MSs did not respond to assignment messages.

The analysis of the TEMS signaling indicates that the MSs received Downlinkpacket assignment messages but did not send Packet Control

Acknowledgement to respond. Therefore, the problem is cased by no responsefrom MSs.

Figure 2.1 Downlink packet assignment message

The MSs did not respond to the downlink assignment messages because of incorrectfrequency-hopping information.

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The frequency-hopping information in the assignment messages were MAnumber=14.

Figure 2.2 MA Numeber in the downlink packet assignment message

According to the relevant protocol, the frequency-hopping information is determined

based on the value of the GPRS Mobile Allocation parameter in the SI 13 message.In the SI 13 message, the frequency-hopping information is null, which is differentfrom the data configuration. Therefore, the fault occurs because of this productdefect. The frequency-hopping information in the system messages is incorrect dueto the product defect, and thus the downlink TBF establishment fails. When thecorresponding channel is moved to a frequency point that does not involve frequencyhopping, the success rate of downlink TBF establishment becomes normal.

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Figure 2.3 Frequency-hopping information being null in the SI 13 message

Solution

Frequency-hopping is disabled to prevent this fault. This defect will be rectified inlater versions.

4.2 Case 2: Low Success Rate of Downlink TBFEstablishment Due to No Responses from MSs After thePCU Is Upgraded

Symptom

The purpose of upgrading the PCU version to C05SP01 is to solve the problem thatthe success rate of uplink TBF assignment in the GPRS network is low. After thePCU is upgraded, the failure rate of uplink assignment drops from 80% to 20%. Thefailure rate of downlink assignment, however, rises from 2% to 20%.

Troubleshooting

Analysis indicates that the frame error rate of the G-Abis interface is normal, theCCCH is not overloaded, congestion does not occur due to no channel, and the air interface quality is fine. Therefore, the fault occurs due to no response from MSs.

The following figure shows that in the downlink assignment procedure the MS doesnot return the Packet Control Acknowledgement message after the PCU sends thePACKET POLLING REQUEST message to the MS. In C04, if the PCU fails toresend the IMMEDIATE ASSIGNMENT message for three times, the systemincreases the number of failed downlink TBF establishments due to MS no responseby one. In C05, if the MS does not respond with the Packet ControlAcknowledgement message, the PCU resends the IMMEDIATE ASSIGNMENT for one time. If the MS still does not respond, the system increases the number of failed

downlink TBF establishments due to MS no response by one.

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The difference between C04 and C05 is as follows: The numbers of times that thePCU resends the IMMEDIATE ASSIGNMENT message are different. As a result,during the establishment of the downlink TBF, traffic statistics about no responsefrom the MS are different.

MS PCU

2.Proces

PDCH

3.IMMEDIATE ASSIGNMENTCCCH

8.PACKET POWER CONTROL/TIMING ADVANCE

1.LLC PD

4.Process 5.PACKET POLLING REQUEST

7.PACKET CONTROL ACKNOWLEDGMENT6.Process

PDCH

9.DOWNLINK RLC DATA BLOCK

MS PCU

2.Proces

PDCH



1.LLC PD



PDCH


MS PCU

2.Proces

PDCH



1.LLC PD



PDCH


ResendIMMEDIA

ASSIGNMor PACK POLLIN

REQUEmessag

Solution

Increase the values of innner software parameters Retry Times of Downlink TBFEstablishment and Retry Times of Downlink TBF Polling.

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5 Problem Feedback

1. Traffic counters

Function Type Measurement Type

DSP Measurement DSP CPU Performance Measurement

Abis interfacemeasurement

TRAU link measurement

PTRAU Measurement

PS CallMeasurement

Measurement of packet assignment capability per BSC

Uplink GPRS TBF establishment and release capabilitymeasurement

Uplink EGPRS TBF establishment and release capabilitymeasurement

PDCH resource capability measurement

Performance measurement of PDCH extremes

Downlink GPRS TBF establishment and release capabilitymeasurement

Downlink EGPRS TBF establishment and release capabilitymeasurement

PS ChannelMeasurement

Cell radio channel capability measurement

PDCH resource capability measurement

2. Feedback on signaling tracing at the PCU side (Um and Gb interfaces)

3. Feedback on the versions of the BTS and BSC

4. Data configuration

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Documents

Downlink TBF Establishment Success Rate Optimization Manual