52 GSM BSS Network PS KPI (Downlink TBF Establishment Success Rate) Optimization Manual

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

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Product Version

Total 30 pages

INTERNAL

GSM BSS Network Performance PS KPI (Downlink TBF Establishment Success Rate) Optimization Manual For internal use only

Prepared by

GSM &UMTS Network Performance Research Department

Date

2008-12-26

Reviewed by

Date

yyyy-mm-dd

Reviewed by

Date

yyyy-mm-dd

Granted by

Date

yyyy-mm-dd

Huawei Technologies Technologies Co., Ltd. All rights reserved


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Revision Record Date

Revision version

Change Description

Author

2008-05-05

V0.8

Initial draft completed

Wang Guanghua

2008-08-26

V1.0

Revised according to review suggestions

S u S hi

2008-12-26

V1.1

Optimized the document according to the accumulated optimization experience

Wang Guanghua

2011-11-7 3:35:36 a11/p11

Huawei Technologies Proprietary

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GSM BSS Network Performance PS KPI (Downlink TBF Establishment Success Rate) Optimization Manual Keywords Downlink TBF, TBF, establishment success rate Abstract This manual mainly describes the method of collecting statistics about the downlink TBF establishment success rate and the methods of optimizing the downlink TBF establishment success rate. List of abbreviations Abbreviation

Full Spelling

PDCH

Packet Data Channel

PCU

Packet Control Unit

MS

Mobile Station

CQT

Call Quality Qualit y Test

K PI

Key Performance Index

DT

Drive Test

GPRS

General Packet Radio Service

EDGE

Enhanced Data rates for GSM Evolution

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Contents 1 Basic Principle..............................................................................7 1.1 Counter Definition....................................................................................................................................................7 1.1 Counter Definition....................................................................................................................................................7 1.1.1 Definition in the Case of No MS Response .......................................................................................................7 1.1.1 Definition in the Case of No MS Response .......................................................................................................7 1.1.2 Definition in the Case of No Available Resources ............................................................................................7 1.1.2 Definition in the Case of No Available Resources ............................................................................................7 1.1.3 Definition in the Case of No MS Response or No Available Resources ...........................................................8 1.1.3 Definition in the Case of No MS Response or No Available Resources ...........................................................8 1.2 Theory....................................................................................................................................................................... Theory.......................................................................................................................................................................8 8 1.2 Theory....................................................................................................................................................................... Theory.......................................................................................................................................................................8 8

2 Signaling Procedure......................................................................9 2.1 Number of Successful Downlink TBF Establishments............................................................................................9 2.1 Number of Successful Downlink TBF Establishments............................................................................................9 2.1.1 Description............................................................................. Description..........................................................................................................................................................9 .............................................................................9 2.1.1 Description............................................................................. Description..........................................................................................................................................................9 .............................................................................9 2.1.2 Measurement Point.............................................................................................................................................9 2.1.2 Measurement Point.............................................................................................................................................9 2.2 Number of Failed Downlink TBF Establishments.................................................................................................10 2.2 Number of Failed Downlink TBF Establishments.................................................................................................10 2.2.1 Description............................................................................. Description........................................................................................................................................................10 ...........................................................................10 2.2.1 Description............................................................................. Description........................................................................................................................................................10 ...........................................................................10 2.2.2 Measurement Point...........................................................................................................................................11 2.2.2 Measurement Point...........................................................................................................................................11 2.3 Number of Downlink TBF Establishment Attempts...............................................................................................11 Attempts...............................................................................................11 2.3 Number of Downlink TBF Establishment Attempts...............................................................................................11 Attempts...............................................................................................11 2.3.1 Description............................................................................. Description........................................................................................................................................................11 ...........................................................................11 2.3.1 Description............................................................................. Description........................................................................................................................................................11 ...........................................................................11 2.3.2 Measurement Point...........................................................................................................................................11 2.3.2 Measurement Point...........................................................................................................................................11

3 Analysis and Optimization Methods..............................................13 3.1 Checking the Abis Link..........................................................................................................................................16

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3.1 Checking the Abis Link..........................................................................................................................................16 3.2 Checking the Delivery of Assignment Messages...................................................................................................17 3.2 Checking the Delivery of Assignment Messages...................................................................................................17 3.2.1 Failed Downlink TBF Establishments Due to Overloaded CCCH..................................................................17 3.2.1 Failed Downlink TBF Establishments Due to Overloaded CCCH..................................................................17 3.2.2 Failed Downlink TBF Establishments Due to No Channel Resources............................................................17 3.2.2 Failed Downlink TBF Establishments Due to No Channel Resources............................................................17 3.3 Checking the Air Interface......................................................................... Interface......................................................................................................................................20 .............................................................20 3.3 Checking the Air Interface......................................................................... Interface......................................................................................................................................20 .............................................................20 3.4 Checking Failed Downlink TBF Establishments Due to No Response from the MS............................................21 3.4 Checking Failed Downlink TBF Establishments Due to No Response from the MS............................................21 3.4.1 Failed Downlink TBF Establishments Due to Improper Parameter Configurations........................................21 3.4.1 Failed Downlink TBF Establishments Due to Improper Parameter Configurations........................................21 3.4.2 Failed Downlink TBF Establishments Due to Incorrect Information Elements in Assignment Messages......23 3.4.2 Failed Downlink TBF Establishments Due to Incorrect Information Elements in Assignment Messages......23 3.4.3 Failed Downlink TBF Establishments Due to Imbalance Between the Uplink and the Downlink..................24 3.4.3 Failed Downlink TBF Establishments Due to Imbalance Between the Uplink and the Downlink..................24 3.4.4 Checking the Feed System...............................................................................................................................25 3.4.4 Checking the Feed System...............................................................................................................................25 3.4.5 Checking the KPIs of the PS Field...................................................................................................................25 3.4.5 Checking the KPIs of the PS Field...................................................................................................................25

4 Cases.........................................................................................26 4.1 Case 1: Low Success Rate of Downlink TBF Establishment Due to Incorrect Frequency-Hopping Parameter Settings in Czech Republic...........................................................................................................................................26 4.1 Case 1: Low Success Rate of Downlink TBF Establishment Due to Incorrect Frequency-Hopping Parameter Settings in Czech Republic...........................................................................................................................................26 4.2 Case 2: Low Success Rate of Downlink TBF Establishment Due to No Responses from MSs After the PCU Is Upgraded......................................................................................................................................................................29 4.2 Case 2: Low Success Rate of Downlink TBF Establishment Due to No Responses from MSs After the PCU Is Upgraded......................................................................................................................................................................29

5 Problem Feedback.. Feedback....................................... ..................................................................... ................................31 31

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Figures Successful establishment of downlink TBF on the CCCH...................10 Successful downlink TBF establishment on the PACCH................... ACCH......................10 ...10 Downlink TBF establishment attempts on the CCCH.........................12 Downlink TBF establishment attempts on the PACCH.......................12 Uplink and downlink TBF establishment procedure on the CCCH.......14 Overall analysis process.................................................................15 Analysis of the frame error rate of the G-Abis interface is normal.....27 Downlink packet assignment message............................................27 MA Numeber in the downlink packet assignment message...............28 Frequency-hopping information being null in the SI 13 message.......29

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1

Basic Principle

1.1 Counter Definition The definition of the downlink TBF establishment success rate varies with the assessment item.

1.1.1 Definition in the Case of No MS Response If the network side delivers an assignment message to a mobile station (MS) but fails to receive the Packet Control Acknowledgement message from the MS, the number of failed downlink TBF establishments due to MS no response is added by one. The definition of the downlink TBF establishment success rate is as follows: Downlink GPRS TBF Establishment Success Rate = 1 - Number of Failed Downlink GPRS TBF Establishments due to MS No Response/ Response /Number of Downlink GPRS TBF Establishment Attempts Downlink EGPRS TBF Establishment Success Rate = 1 - Number of Failed Downlink EGPRS TBF Establishments due to MS No Response/ 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 and TFIs at the network side are unavailable, the number of failed downlink TBF establishments due to no channel is added by one. The definition of the downlink TBF establishment success rate is as follows: Downlink GPRS TBF Establishment Success Rate = 1 - Number of Failed Downlink GPRS TBF Establishments due to No Channel/ Channel /Number of Downlink GPRS TBF Establishment Attempts

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

1.1.3 Definition in the Case of No MS Response or No Available Available Resources Res ources If the downlink TBF establishment fails because the network side fails to receive the Packet Control Acknowledgement message from an MS or the resources at the network side are unavailable, the number of failed downlink TBF establishments is add by one. The definition of the downlink TBF establishment success rate is as follows: Downlink GPRS TBF Establishment Success Rate = Number of Successful Downlink GPRS TBF Establishments/ Establishments /Number of Downlink GPRS TBF Establishment Attempts Downlink EGPRS TBF Establishment Success Rate = Number of Successful Downlink EGPRS TBF Establishments/ Establishments /Number of Downlink EGPRS TBF Establishment Attempts

1.2 Theory The downlink TBF establishment success rate shows th e downlink access performance and is an important counter for assessing the network. When the downlink TBF fails to be established, the network side continues to trigger the establishment of the downlink TBF in a short time because the network side has some data blocks that are not delivered. Therefore, the downlink TBF establishment success rate is slightly low in this c ase, but customer experience is not affected.

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Signaling Procedure

2.1 Number of Successful Downlink TBF Establishments 2.1.1 Description This counter provides the number nu mber of successful downlink TBF establishments in a measurement period.

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 sending an IMMEDIATE ASSIGNMENT message with Starting Time on the CCCH to the MS. When the starting time is reached, the network side sends a POLLING message to the MS to obtain a TA value. The BSC reserves block resources for the MS to respond with an assignment acknowledgement message. If the network side receives a Packet Control Acknowledgement message from the MS on the reserved block resources in the assigned channel, it indicates that the downlink TBF is established. In addition, th e network side can calculate the TA TA value by using the Packet Control Acknowledgement message that is received. The following figure shows the procedure for establishing downlink TBFs on the CCCH. Each time the network side receives a Packet Control Acknowledgement message (see measurement point A), the value of the counter Number counter Number of Successful Downlink TBF Establishments is added by one.

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Figure 1.1 Successful establishment of downlink TBF on the CCCH MS

Network IMMEDIATE ASSIGNMENT (CCCH) POLLING(RRBP) PACKET Control Acknowledgement A

2.

Successful downlink TBF establishment on the PACCH The network side can initiate the downlink TBF establishment procedure by sending a Packet downlink assignment message on the PACCH to the MS. The message contains the information about the block resources reserved by the network side for the MS to respond with an assignment acknowledgement message. If the network side receives a Packet Control Acknowledgement message 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 esta blishing the downlink TBF on the PACCH. Each time the network side receives a Packet Control Acknowledgement message corresponding to a Packet downlink assignment message (see measurement point A), the value of the counter Number counter Number of Successful Downlink TBF Establishments is added by one.

Figure 2.1 Successful downlink TBF establishment on the PACCH MS

Network Packet downlink assignment

PACKET Control Acknowledgement A

2.2 Number of Failed Downlink TBF Establishments 2.2.1 Description This counter measures the number of failed downlink TBF establishments in a measurement period.

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2.2.2 Measurement Point 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 to establish a download TBF because channels are unavailable, the value of Number 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, TBF, the BSC sends a POLLING message on the CCCH or sends a Packet downlink assignment message on the PACCH, and reserves block resources for the MS to respond with an assignment acknowledgement message. If the BSC does not receive a Packet Control Acknowledgement message from the MS on the reserved block resources, the BSC sends the IMMEDIATE ASSIGNMENT messages repeatedly until the maximum number of retry times is exceeded. Each time the maximum number of retrying times is exceeded, the value of Number of Number of Failed Downlink TBF Establishments due to MS 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 a measurement period.

2.3.2 Measurement Point 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 IMMEDIATE ASSIGNMENT message at the sub-timeslot corresponding to the CCCH group to which the MS belongs. Figure shows the procedure for the network side to send the IMMEDIATE ASSIGNMENT message on the CCCH. Each time the network side sends an IMMEDIATE ASSIGNMENT message (see measurement point A), the value of the counter Number counter Number of Downlink TBF Establishment Attempts is added by one.

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Figure 1.1 Downlink TBF establishment attempts on the CCCH. MS

Network

IMMEDIATE ASSIGNMENT (On CCCH) A

2.

Downlink TBF establishment attempts on the PACCH The network side can attempt to establish the downlink TBF by sending a PACKET DOWNLINK ASSIGNMENT message to an MS during the transmission process of the previous uplink TBF or the release process of the current downlink d ownlink TBF. 2.1 shows the procedure for the network side to send the PACKET DOWNLINK ASSIGNMENT message on the PACCH. Each time the network side sends a PACKET DOWNLINK ASSIGNMENT message (see measurement point A), the value of the cou nter Number nter Number of Downlink TBF Establishment Attempts is added by one.

Figure 2.1 Downlink TBF establishment attempts on the PACCH MS

Network

PACKET DOWNLINK ASSIGNMENT A

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3

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Analysis and Optimization Methods

The procedure for establishing the downlink TBF on the CCCH is as follows: 1.

The RR entity entity at the network network side initiates initiates the downlink downlink TBF establishme establishment nt by usin using g the the down downli link nk pack packet et assi assign gnme ment nt proc proced edur ure. e. The The down downli link nk pack packet et assignment procedure is triggered by a request from upper layers to transfer a LLC PDU. PDU. Before Before transf transferr erring ing an LLC PDU, PDU, the networ network k side side determ determine ines s whether the MS is in the Ready state. If the MS is in the Ready state, the network 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 the Standby state, the network side sends a paging message to the BSS. The network side sends the LLC PDU only after receiving a paging response from the BSS. The request from upper layers contains the priority, including the RLC mode, DRX parameter (optional), QoS script file of the IMSI (optional), and MS radio access capability associated with the packet transfer (optional). For such a request, 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 initiates the downlink packet assignment assignment procedure procedure on the CCCH. If the MS is in packet transf transfer er mode, mode, the networ network k side side initia initiates tes the downli downlink nk packet packet assign assignmen mentt procedure on the PACCH.

2.

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

3.

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

4.

When When the the MS rece receiv ives es the the IMMEDIAT IMMEDIATE E ASSIGNMENT ASSIGNMENT messag message, e, radio radio resources are assigned. After receiving the frame number indicated by the TBF Starting Time (optional), the MS accesses the assigned channel, starts to listen on 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 trasfers the TA value to the MS by sending a Packet Power Control/Timing Advance message message after the Starting Time of the downlink TBF is reached. reached. If the network side 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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started. 6.

Before timer T3190 times out, if the MS addresses the downlink RLC radio block in the assigned channel according to the TFI field and the MS receives the PACKET POLLING REQUEST message from the network side, the MS sends a Packet Control Control Acknowledg Acknowledgement ement mess messag age e on the the upli uplink nk radi radio o bloc block k corres correspon pondin ding g to the messag message e and resets resets timer timer T3190. T3190. Otherw Otherwise ise,, the MS notifies the upper layers of the downlink TBF establishment failure.

7.

The network side obtains the TA value of the MS by using the valid Packet Control Acknowledgement messag message e receiv received ed on the reserv reserved ed uplink uplink RLC radio radio block. block. In this this case, case, the network network side side regard regards s that that the downlink downlink TBF establishment is successful. Otherwise, the network side initiates the downlink immediate assignment procedure again.

Figure 7.1 Uplink and downlink TBF establishment procedure on the CCCH MS

BTS

PCU LLC PDU

CCCH

IMMEDIATE ASSIGNMENT PACKET PACKET POLLING R EQUEST

PDCH PACKET CONTROL ACKNOWLEDGMENT PDCH

This section taske the uplink and downlink TBF establishment on the CCCH as an example to describe the optimization ideas about identifying the signaling and cells where problems occur throughout the signaling procedure. In the downlink TBF establishment 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 POLLING REQUEST messages are sent to MSs according to the air interface quality.



Checking whether MSs respond to POLLING messages by sending Packet Control Acknowledgement messages.

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Figure 7.2 Overall analysis process Start

Analyze the cause of downlink TBF establishment failure

Check whether the status of the Abis link is normal

No

Check the transmission

Yes

Check whether assignment messages are sent normally

Check whether the CCCH is overloaded

No

Check whether channels are available

Yes

Check whether the downlink air interface is normal

No

Check whether the downlink air interface quality is poor Perform the CQT test

Yes Check whether the parameter configurations are correct Check whether the importance cells are correct Check whether MSs respond to ASSIGNMENT and POLLING messages Yes

No

Check whether the problem is solved

No

Check whether the uplink and downlink are balanced Check whether the feed system is normal Check whether the parameters of the CS field

Yes End

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3.1 Checking the Abis Link Transmission Transmission problems such as out-of-synchronization or intermittent Abis interface links may cause th e failure to establish the dow nlink TBF. TBF. You can determine the trasmission status of the Abis interface by calculating the frame error rate of the G-Abis interface as follows: Frame error rate of the G-Abis interface = (Number (Number of Received Check Error TRAU Frames + Number of Received Out-of-Synchronization TRAU Frames)/( Frames )/(Number Number of Sent Valid TRAU Frames + Number of Sent Empty TRAU Frames) 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. In this case, you can infer that the link is of good quality, quality, and the MS can transmit transmit data in a stable way.

2.

If the frame frame error rate is lower than 10e-4, the quality quality of the transmission transmission link is poor. In this case, one to three error frames occur in a channel channel every minute on average. Error frames are unpredictable. Therefore, affected MSs are likely to expe experi rien ence ce low low tran transm smis issi sion on rate rate,, long long tran transm smis issi sion on dela delay, y, or even even disconnection.

3.

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

In actual running, carriers are not able to directly control leased lines, such as microwave 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 a cell 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 G-Abis Measurement -> Performance measurement of the G-Abis BSC packet assignment -> interface Number of Received Normal TRAU Frames Number of Received Out-of-Synchronization TRAU Frames 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. Frames. In versions earlier than V9R8C11, 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, the sum of Number of Number of Sent Valid TRAU Frames and Number of Sent Empty TRAU Frames is used to indicate the number of received frames. In the version V9R8C12, this problem is solved by using the sum of Number of Number of Received Normal TRAU Frames, Frames, Number of Received Out-of-Synchronization TRAU Frames, Frames, Number of Received Check Error TRAU Frames, Frames, and Number of Sent Empty TRAU Frames to indicate the number of received frames.

3.2 Checking the Delivery of Assignment Assignment Messages 3.2.1 Failed Downlink TBF Establishments Due to Overloaded CCCH If the CCCH is overloaded, the IMMEDIATE ASSIGNMENT message sent on the CCCH may be discarded. As a result, the downlink TBF fails to be established. You can check whether the CCCH is overloade d by viewing flow control traffics. If the CCCH is overloaded, you can increase the load threshold of the CCCH to prevent downlink 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) Messages Sent on Abis Interface MSG DEL IND Messages Sent on Abis Interface

3.2.2 Failed Downlink TBF Establishments Due to No Channel Resources Hardware Fault Faults on hardware such as the TRX may affect the success rate of downlink TBF establishment. Therefore, you should check hardware faults. You can locate hardware faults fa ults by checking the traffic measurement related to the hardware faults.

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Cause

BSC-Level

Cell-Level

Equipmen t faults

BSC Measurement -> Access Measurement ->

KPI Measurement ->

TCH Availability per BSC

Available TCHs

Configured TCHs per BSC

Configured TCHs

Available TCHs per BSC

TRX Measurement ->

TCH Availability

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 packet services is heavy. As a result, MSs are multiplexed on the channels in the cell to the maximum degree. In this case, you need to add more static and dynamic channels. In addition, you need to check the PS-domain channel management parameters parameters and set PDCH Downlink Multiplex Threshold to 80, that is, the maximum number of TBFs multiplexed on the downlink is eight.

2.

Check whether the preemption of dynamic PDCHs by CS services leads to the insuff insuffici iciency ency of PDCHs. PDCHs. You can check check the number number of times times of reclai reclaimin ming g dynamic PDCHs by the BSC and the number of times of reclaiming dynamic PDCHs 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 static PDCHs. You can also set Level of Preempting Dynamic Channel to Control channels cannot be preempted. preempted.

The following table describes the relevant parameters. Name

Description

Setting Principle

Value Range

Maximum Ratio Threshold of PDCHs in a Cell

The total number of TCHs and PDCHs available in a cell is fixed. This parameter determines the proportion of PDCHs to the total number of TCHs and PDCHs.

If this parameter is set to an excessive value, there are excessive PDCHs and insufficient TCHs. This affects CS services.

Value range: 0100.

PDCH Downlink Multiplex Threshold

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PDCH Downlink Multiplex Threshold

Default value: 50

If this parameter is set to a modest value, there are insufficient PDCHs and excessive TCHs. This affects PS services. If this parameter is set to a lower value, the TBFs established on the PDCH and the subscribers are fewer, and the downlink


Value range: 101 080 The value 10 indicates that at most one TBF

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Name

Description

Setting Principle

Value Range

bandwidth for each subscriber is higher hig her..

can be accessed. The value 80 indicates that a maximum of 8 TBFs can be accessed.

If this parameter is set to a higher value, the number of TBFs established on the PDCH and the number of subscribers are greater, and the downlink bandwidth for each subscriber is less. Level of Preemptin g Dynamic Channel

Level of dynamic channel preempted by CS services and PS services The TCH/Fs are dynamic channels that can be preempted. If this parameter is set to All dynamic channels can be pre-empted, it means that the CS services can preempt all dynamic channels; if this parameter is set to Control channels cannot be pre-empted, it means that the CS services can preempt any dynamic channels except the control channels; if this parameter is set to Dynamic channels carrying services cannot be pre-empted, it means that the CS services cannot preempt the dynamic channels that carry services.

INTERNAL

Generally, Generally, CS services have the highest priority. priority. This parameter must be set to All dynamic channels can be pre-empted so that CS services can preempt all dynamic channels. To ensure data services, you can set this parameter to Control channels cannot be pre-empted or Dynamic channels carrying services cannot be pre-empted.

Default value: 80.

Value options: o ptions: All dynamic channels can be preempted, Control channels cannot be preempted Dynamic channels carrying services cannot be preempted Default value: All dynamic channels can be preempted.

The following table lists the related traffic measurement counters.

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Cause

Cell-Level

Insufficient resources

Packet Switch Channel Measurement -> PDCH Resource Capability Measurement -> Number of TCH to PDTCH Conversion Attempts Number of Successful TCH to PDTCH Conversions Number of Reclaimed Dynamic PDCHs Number of Reclaimed Busy Dynamic PDCHs Packet Switch Call Measurement -> Downlink GPRS TBF Establish 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 TBF Establish 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 to No Channel Average Number of Concurrent Downlink EGPRS TBFs

3.3 Checking the Air Interface MSs may not receive downlink ASSIGNMENT messages or POLLING messages due to poor air interface quality. You You can check the air interface quality by checking the BEP distribution or performing the CQT test. To check the BEP distribution, you can count the 8PSK_MEAN_BEP variants and the GMSK_MEAN_BEP variants. To conduct 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 Due to 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 same time, the downlink TBF establishment may fail. Therefore, the TBF at the network side cannot be started before the TBF at the MS. Otherwise, the MS may miss the POLLING message from the network side, and thus the downlink TBF establishment fails. The parameters are as follows: Name

Setting Principle

Value Range

Retry Times of This parameter Downlink TBF specifies the maximum Reassignment number of attempts to resend the IMMEDIATE ASSIGNMENT message. The message is sent when the network side fails to receive a valid Packet Control Acknowledgement message on the reserved uplink RLC block in the procedure for establishing the downlink TBF. If the number is exceeded, the network side releases the downlink TBF.

If the value of this parameter is too small, the network side will release the downlink TBF with only a few attempts to resend the IMMEDIATE ASSIGNMENT message, which leads to downlink establishment failure. If the number of failed downlink TBF establishments is great, you can set this value to a higher value.

Default value: 2.

Retry Times of This parameter Downlink TBF specifies the maximum Polling number of attempts to resend the POLLING message in the procedure for establishing the downlink TBF.

If the value of this parameter is too small, the downlink TBF establishment may fail because the sending of the POLLING message fails in the procedure for establishing the downlink TBF on the CCCH. If the number of failed downlink TBF establishments is great, you can change this value to a higher value.

Default value: 5.

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Description


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INTERNAL

Name

Description

Setting Principle

Value Range

Delay for Downlink Immediate Assignment DRX (block)

In DRX mode, the network side determines the time to start downlink TBF on the network side by using this parameter.

If the value of this parameter is great, the starting time of the downlink TBF on the network side is late. After starting the downlink TBF, the network side sends the PACKET POLLING REQUEST message or the downlink data blocks immediately. immediately. If the value of this parameter is small, the network side starts the downlink TBF before an MS. The MS fails to receive the subsequent downlink RLC radio blocks sent from the network side because the MS has not accessed the assigned channel at the time. Therefore, the access performance is degraded.

Default value: 12.

If the value of this parameter is great, the starting time of the downlink TBF on the network side is late. After starting the downlink TBF, the network side sends the PACKET POLLING REQUEST message or the downlink data blocks immediately. immediately. If the value of this parameter is small, the network side starts the downlink TBF before an MS. Because the MS has not accessed the assigned channel at the time, the MS fails to receive the

Default value: 26.

In addition, the network side calculates the TBF Starting Time assigned to MSs to notify the MSs of the time to access the assigned channels.

Delay for Downlink Immediate Assignment Non-DRX (block)

In Non-DRX mode, the network side determines the time to start downlink TBF on the network side by using this parameter. In addition, the network side calculates the TBF Starting Time assigned to MSs to notify the MSs of the time to access the assigned channels.

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Name

Description

Setting Principle

INTERNAL

Value Range

subsequent downlink RLC radio blocks sent from the network side. Therefore, the access performance is degraded.

3.4.2 Failed Downlink TBF Establishments Due to Incorrect Information 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, Currently, power control is not performed for downlink TBF establishment. Therefore, you do not need to 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 13 message and the frequency parameters in the downlink assignment message are consistent with data configurations. The frequency parameters are described as follows: A downlink assignment message 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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INTERNAL

< 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 SI 13 and the previous assignment messages. messages. Therefore, you need to check whether the values of the frequency-hopping parameters in the system messages messages or assignment messages are consistnet with data configurations according to MA_NUMBER. MA_NUMBER. MA_NU MA_NUMB MBER ER = 0–13 0–13 PSI2 message;

shall shall be used used to refer referenc ence e a GPRS GPRS mobile mobile alloca allocatio tion n receiv received ed in a

MA_NUMBE MA_NUMBER R = 14 shall be used used to reference reference a GPRS mobile mobile allocation allocation received received in a SI13 or PSI13 message; MA_NUMBE MA_NUMBER R = 15 shall be used used to reference reference a GPRS GPRS mobile mobile allocation allocation received received in a previous assignment message using the direct encoding. Direct encoding 1: MSs use the frequency-hopping index information specified by theGPRS theGPRS Mobile 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 assignment messages.

3.4.3 Failed Downlink TBF Establishments Due to Imbalance Between the Uplink and the Downlink When the imbalance between the uplink and the downlink occurs, no signals may be received 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 b alance between the uplink and the downlink, check whether wh ether the transmit power of the BTS is high. Then, check whether the components such as the TMA, BTS amplifier, and antenna port that affect uplink/downlink receive level have problems. 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: The difference between the mean uplink receive level and the mean downlink receive level is great; the uplink and downlink balance level is high; both the immediate assignment success rate and the assignment success rate are low. The following table lists the traffic measurement counters related to the balance between the uplink and the downlink.

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Cause

Cell-Level

TRX-Level

Balance between uplink and downlink

Call Measurement -> Assignment Measurement -> Success Rate of TCH Assignment

MR Measurement ->

Success Rate of Call Establishment Call Measurement -> Immediate Assignment Measurement ->

MR Measurement ->

Success Rate of Immediate Assignment

MR Measurement ->

INTERNAL

Uplink-and-Downlink Balance Measurement TCHF Receive Level Measurement TCHH Receive Level Measurement

3.4.4 Checking the Feed System Hardware faults in the feed system or incorrect parameter configurations, such as the incorrect TMA factor, factor, lead to low success rate of downlink TBF establishment. In addition, the faulty feed system may lead to the imbalance between the uplink and the downlink. Therefore, you can check whether the feed system is faulty by using the traffic measurement counters related to the balance between the uplink and the downlink.

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

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4

INTERNAL

Cases

4.1 Case 1: Low Success Rate of Downlink TBF Establishment Due to Incorrect Frequency-Hopping Parameter Settings in Czech Republic Symptom The success rate of downlink TBF establishment decreased suddenly since November 4, and the number nu mber of abnormal TBF releases increased. The statistics are as 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-Abis interface, the frame error rate was normal and stable on and near November 4. Formula: Frame error rate of the G-Abis int erface = (Number (Number of Received Check Error TRAU Frames + Number of Received Out-of-Synchronization TRAU Frames)/( Frames)/(Number Number of Sent Valid TRAU Frames + Number of Sent Empty TRAU Frames) Frames )

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INTERNAL

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

0. 01400000 0. 01200000 0. 01000000

03/ 11/ 2 04/ 11/ 2 05/ 11/ 2 06/ 11/ 2 07/ 11/ 2 08/ 11/ 2 09/ 11/ 2

0. 00800000 0. 00600000 0. 00400000 0. 00200000 0. 00000000

2.

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 : : : : : : : : : : : : : : : : 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3 0 3 0 3 0 3 0 3 0 3 0 3 0 3 : : : : : : : : : : : : : : : : 0 1 3 4 6 7 9 0 2 3 5 6 8 9 1 2

MSs did not respond to assignment messages. The analysis of the TEMS signaling indicates that the MSs received Downlink packet assignment messages but did not send Packet Control Acknowledgement to respond. Therefore, the problem is cased by no response from MSs.

Figure 2.1 Downlink packet assignment message

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

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INTERNAL

The frequency-hopping information in the assignment messages were MA number=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 different from the data configuration. Therefore, the fault occurs because of this product defect. The frequency-hopping information in the system messages is incorrect due to the product defect, and thus the downlink TBF establishment fails. When the corresponding channel is moved to a frequency point that does not involve frequency hopping, the success rate of downlink TBF establishment becomes normal.

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INTERNAL

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

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

4.2 Case 2: Low Success Rate of Downlink TBF Establishment Due to No Responses from MSs After the PCU Is Upgraded Symptom The purpose of upgrading the PCU version to C05SP01 is to solve the problem that the success rate of uplink TBF assignment in the GPRS network is low. After the PCU is upgraded, the failure rate of uplink assignment drops from 80% to 20%. The failure rate of downlink assignment, h owever, rises from 2% to 20%. Troubleshooting Analysis indicates that the frame error rate of the G-Abis interface is normal, the CCCH 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 does not return the Packet Control Acknowledgement message after the PCU sends the PACKET POLLING REQUEST message to the MS. In C04, if the PCU fails to resend the IMMEDIATE ASSIGNMENT message for three times, the system increases the number of failed downlink TBF establishments due to MS no response by one. In C05, if the MS does not respond with the Packet Control Acknowledgement 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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INTERNAL

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

PCU PCU 1.LLC PD

CCCH 4.Process

3.IMMEDIATE ASSIGNMENT 5.PACKET POLLING REQUEST

PDCH 6.Process

7.PACKET CONTROL ACKNOWLEDGMENT

PDCH 8.PACKET PACKET POWER CONTROL CONTROL/TIMING /TIMING ADVANCE

2.Proces Resend IMMEDIA ASSIGNM or PACK POLLIN REQUE messag

9.DOWNLINK NLINK RLC DATA BLOCK

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

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5 1.

INTERNAL

Problem Feedback

Traffic counters

Function Type

Measurement Type

DSP DSP Mea Measu sure reme ment nt

DSP DSP CPU CPU Perf Perfor orma manc nce e Meas Measur urem emen entt

Abis interface measurement

TRAU link measurement

PS Call Measurement

Measurement of packet assignment capability per BSC

PTRAU Measurement

Uplink GPRS TBF establishment and release capab ility measurement Uplink EGPRS TBF establishment and release capab ility measurement PDCH resource capability measurement Performance measurement of PDCH extremes Downlink GPRS TBF establishment and release capability measurement Downlink EGPRS TBF establishment and release capability measurement

PS Channel Measurement

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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52 GSM BSS Network PS KPI (Downlink TBF Establishment Success Rate) Optimization Manual

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