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GSM BSS Network KPI (TCH Assignment Success Rate)
Optimization Manual

INTERNAL

Product Name

Confidentiality Level

G3BSC

INTERNAL

Product Version

Total 25 pages

GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual
(For internal use only)

Prepared by

WCDMA & GSM Network

Date

Performance Research Dept.
2008-03-29
Su Shi
Reviewed by

Date

yyyy-mm-dd

Reviewed by

Date

yyyy-mm-dd

Granted by

Date

yyyy-mm-dd

Huawei Technologies Co., Ltd.

All rights reserved
GSM BSS Network KPI (TCH Assignment Success Rate)
Optimization Manual

INTERNAL

Contents
1

Definition of TCH Assignment Success Rate ........................................................................... 5
1.1
1.2

2

Recommended Formula ................................................................................................ 5
Signaling Procedure and Counters ................................................................................ 5

Factors in Assignment Success Rate ......................................................................................... 7
2.1
2.2

Transmission Fault ........................................................................................................ 8

2.3

Parameter Settings......................................................................................................... 8

2.4

TCH Congestion ........................................................................................................... 9

2.5

Intra-network and Inter-network Interference ............................................................... 9

2.6

Coverage Problems ..................................................................................................... 10

2.7

Antenna Problems ....................................................................................................... 10

2.8

Repeater Problems ...................................................................................................... 10

2.9
3

Hardware Fault .............................................................................................................. 8

Mobile Phone Problems .............................................................................................. 10

Analysis Procedure and Optimization Method of Assignment Failures ................................. 11
3.1

Analysis Procedure ..................................................................................................... 11

3.2

Analysis Procedure ..................................................................................................... 13

4

Test Methods ........................................................................................................................... 25

5

Description of Assignment Requests ...................................................................................... 25

6

Cases of Optimizing Assignment Success Rate ...................................................................... 27
6.1 High Assignment Failure Rate After BTS Capacity Expansion .................................... 27
6.2 High Assignment Failure Rate Because of Improper Parameter Settings ......................... 28
6.3 Decrease in Assignment Success Rate due to Insufficient Channel Resources ................ 30
6.4 Low Assignment Success Rate Caused by Insufficient Ater Resources ........................... 31
6.5 Low Assignment Success Rate Because of Configuration of Half-rate Channels ............ 32

7

Feedbacks on Assignment Success Rate ................................................................................. 33
GSM BSS Network KPI (TCH Assignment Success Rate)
Optimization Manual

INTERNAL

Revision Record
Date

Revision

Change Description

Author

Draft completed

Su Shi

Version
2008-03-31

1.0

This

1.0
modified

2008-06-06

according

document
to

the

is

Su Shi

review

comments.

References
Number
1

References
Guide to Tackle G-Interference

Author

Date

Chen Baolin

2005-03-11

Guide to Tackle Wireless

Coverage

2005-02-28

Coverage

Tackle Team

(20050311-A-1.0)
2

Problems-20050228-A-3.0
3

GSM BSS Network KPI (TCH
Assignment Success Rate)
Baseline

Wu Zhen

2007-05-30
GSM BSS Network KPI (TCH Assignment Success Rate)
Optimization Manual

INTERNAL

GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual
Keywords: TCH Assignment Success Rate, KPI
Abstract: This document introduces the definition, test method, and optimization
method of TCH assignment success rate.
Abbreviations
Abbreviation

Full Spelling

TCH
MS

Traffic Channel
Mobile Station

BSC
KPI

Base Station Controler
Key Performance Index
GSM BSS Network KPI (TCH Assignment Success Rate)
Optimization Manual

1

INTERNAL

Definition of TCH Assignment Success Rate
The TCH assignment success rate refers to the rate that the TCHs assigned by the

BSC are successfully seized. Successful TCH assignment indicates the procedure
from the BSC receiving the Channel Request message sent by the MSC to receiving
the Assignment Complete message. TCH Assignment Success Rate is one of
accessibility KPIs. It indicates the rate of the MSs successfully seizing the TCHs to
make calls. This KPI directly affects the user experience.
1.1

Recommended Formula

The TCH assignment success rate is obtained through the traffic statistics. The
recommended formula is as follows: TCH Assignment Success Rate = (Completed
TCH Assignments/TCH Assignment Requests) x 100%
For detailed formula, refer to the GSM BSS Network KPI (TCH Assignment Success
Rate) Baseline.
1.2 Signaling Procedure and Counters

Common assignment procedure
GSM BSS Network KPI (TCH Assignment Success Rate)
Optimization Manual

INTERNAL

Inter-BSC directed retry procedure
Note: The measurement points in the previous figure indicate the following.
A——Assignment Requests (Seizure Requests)
B——Completed Assignments (Successful Seizures)
C——Successful Inter-BSC Directed Retries
The measurement points in the preceding figures show that the assignment
success rate of the BSC32/BSC6000 includes the assignment requests (seizure
requests) and completed assignments (successful seizures).
TakeInter-BSC Directed Retry as an example. In the Inter-BSC Directed Retry
procedure, the BSC increases the value of the successful assignment in the originating
cell, on receiving the Clear CMD message from the MSC. In the Intra-BSC Directed
Retry procedure, the BSC increases the value of the successful assignment in the
originating cell, on receiving the Clear CMD message from the MSC. If the directed
retry fails, the BSC decreases the value of the successful assignment in the originating
cell. In the Directed Retry procedure, the counters related to handover are also
measured.
The following figure shows the processing workflow after the BSC receives the
channel request message and the measurement points of successful TCH seizures.

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Gsm bss-network-kpi-tch-assignment-success-rate-optimization-manual(asr)

  • 1. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual INTERNAL Product Name Confidentiality Level G3BSC INTERNAL Product Version Total 25 pages GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual (For internal use only) Prepared by WCDMA & GSM Network Date Performance Research Dept. 2008-03-29 Su Shi Reviewed by Date yyyy-mm-dd Reviewed by Date yyyy-mm-dd Granted by Date yyyy-mm-dd Huawei Technologies Co., Ltd. All rights reserved
  • 2. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual INTERNAL Contents 1 Definition of TCH Assignment Success Rate ........................................................................... 5 1.1 1.2 2 Recommended Formula ................................................................................................ 5 Signaling Procedure and Counters ................................................................................ 5 Factors in Assignment Success Rate ......................................................................................... 7 2.1 2.2 Transmission Fault ........................................................................................................ 8 2.3 Parameter Settings......................................................................................................... 8 2.4 TCH Congestion ........................................................................................................... 9 2.5 Intra-network and Inter-network Interference ............................................................... 9 2.6 Coverage Problems ..................................................................................................... 10 2.7 Antenna Problems ....................................................................................................... 10 2.8 Repeater Problems ...................................................................................................... 10 2.9 3 Hardware Fault .............................................................................................................. 8 Mobile Phone Problems .............................................................................................. 10 Analysis Procedure and Optimization Method of Assignment Failures ................................. 11 3.1 Analysis Procedure ..................................................................................................... 11 3.2 Analysis Procedure ..................................................................................................... 13 4 Test Methods ........................................................................................................................... 25 5 Description of Assignment Requests ...................................................................................... 25 6 Cases of Optimizing Assignment Success Rate ...................................................................... 27 6.1 High Assignment Failure Rate After BTS Capacity Expansion .................................... 27 6.2 High Assignment Failure Rate Because of Improper Parameter Settings ......................... 28 6.3 Decrease in Assignment Success Rate due to Insufficient Channel Resources ................ 30 6.4 Low Assignment Success Rate Caused by Insufficient Ater Resources ........................... 31 6.5 Low Assignment Success Rate Because of Configuration of Half-rate Channels ............ 32 7 Feedbacks on Assignment Success Rate ................................................................................. 33
  • 3. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual INTERNAL Revision Record Date Revision Change Description Author Draft completed Su Shi Version 2008-03-31 1.0 This 1.0 modified 2008-06-06 according document to the is Su Shi review comments. References Number 1 References Guide to Tackle G-Interference Author Date Chen Baolin 2005-03-11 Guide to Tackle Wireless Coverage 2005-02-28 Coverage Tackle Team (20050311-A-1.0) 2 Problems-20050228-A-3.0 3 GSM BSS Network KPI (TCH Assignment Success Rate) Baseline Wu Zhen 2007-05-30
  • 4. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual INTERNAL GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual Keywords: TCH Assignment Success Rate, KPI Abstract: This document introduces the definition, test method, and optimization method of TCH assignment success rate. Abbreviations Abbreviation Full Spelling TCH MS Traffic Channel Mobile Station BSC KPI Base Station Controler Key Performance Index
  • 5. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual 1 INTERNAL Definition of TCH Assignment Success Rate The TCH assignment success rate refers to the rate that the TCHs assigned by the BSC are successfully seized. Successful TCH assignment indicates the procedure from the BSC receiving the Channel Request message sent by the MSC to receiving the Assignment Complete message. TCH Assignment Success Rate is one of accessibility KPIs. It indicates the rate of the MSs successfully seizing the TCHs to make calls. This KPI directly affects the user experience. 1.1 Recommended Formula The TCH assignment success rate is obtained through the traffic statistics. The recommended formula is as follows: TCH Assignment Success Rate = (Completed TCH Assignments/TCH Assignment Requests) x 100% For detailed formula, refer to the GSM BSS Network KPI (TCH Assignment Success Rate) Baseline. 1.2 Signaling Procedure and Counters Common assignment procedure
  • 6. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual INTERNAL Inter-BSC directed retry procedure Note: The measurement points in the previous figure indicate the following. A——Assignment Requests (Seizure Requests) B——Completed Assignments (Successful Seizures) C——Successful Inter-BSC Directed Retries The measurement points in the preceding figures show that the assignment success rate of the BSC32/BSC6000 includes the assignment requests (seizure requests) and completed assignments (successful seizures). TakeInter-BSC Directed Retry as an example. In the Inter-BSC Directed Retry procedure, the BSC increases the value of the successful assignment in the originating cell, on receiving the Clear CMD message from the MSC. In the Intra-BSC Directed Retry procedure, the BSC increases the value of the successful assignment in the originating cell, on receiving the Clear CMD message from the MSC. If the directed retry fails, the BSC decreases the value of the successful assignment in the originating cell. In the Directed Retry procedure, the counters related to handover are also measured. The following figure shows the processing workflow after the BSC receives the channel request message and the measurement points of successful TCH seizures.
  • 7. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual INTERNAL Channel Request Idle Channel Available No Normal Assignment Procedure Queuing Allowed No Successful Assignment Yes Queuing No Yes Reassign Allowed Queuing Successfully No Yes No Idle Channels Available No No Yes Directed Retry Allowed Yes Yes Reassign Directed Retry Reassign Successfully Yes Measure TCH Seizure Failures Measure Successful TCH Seizures No Directed Retry Successfully Yes Processing procedure after the BSC receives the CHAN REQ message 2 Factors in Assignment Success Rate According to the user complaint cases and network optimization experience, the major factors that influence the TCH assignment success rate are summarized as follows: 1. Hardware fault; 2. Transmission fault; 3. Parameter settings; 4. TCH congestion; 5. Intra-network or inter-network interference;6. Coverage problems; 7. Antenna problems; 8. Repeater problems; 9. Mobile phone problems.
  • 8. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual 2.1 INTERNAL Hardware Fault When a TRX or a combiner is faulty, some of the TCHs become unavailable for use. Therefore, TCH assignment success rate decreases. 2.2 Transmission Fault Bad transmission quality on the A interface or Abis interface due to various reasons and unstable transmission links also cause TCH assignment failures. 2.3 Parameter Settings The settings of some parameters of BSC and MSC also affect TCH assignment success rate. The main parameters include: 1. Directed Retry Allowed and Allow Reassign are set to No. Therefore, the TCH reassignment and directed retry functions are not enabled. 2. Speech Version does not support half-rate and AMR half-rate. 3. When the BSC enables half-rate or AMR half-rate, the parameters on the MSC side are not configured consistently. 4. TCH Traffic Busy Threshold (percentage) is set too great. 5. Assignment Cell Load Judge Enable is set to No. 6. The functions such as queuing and preemption are not enabled on the MSC side. 7. The power level difference between the BCCH and the TCH is great. 8. SDCCH Dynamic Allocation Allowed is set to Yes, and immediate assignment requests are great. 9. T3107 (Timer for the ASS CMP message) is set too short. 10. T3103A (Timer for the HO CMP message) is set too long. 11. Level of Preempting Dynamic Channel is set to Dynamic channels carrying services cannot be preempted. Therefore, the priority of the PS services is higher.
  • 9. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual 2.4 INTERNAL TCH Congestion When the TCH congestion occurs, new access requests are rejected due to the unavailability of the TCH. Thus, the TCH assignment success rate is affected. The possible causes are: 1. Half-rate and AMR half-rate are not configured for better speech quality. 2. High traffic volume caused by dense subscriber distribution in the cell or cross-coverage Burst traffic increases because the neighboring BTS is out of service or other burst events occur. 3. The cell is configured with too much static PDCHs or dynamic PDCHs, and the PS services have higher priority over CS services. 4. Some TRXs in the cell are faulty or the TRXs are manually blocked. 5. very early assignment is enabled. 2.5 Intra-network and Inter-network Interference If inter-network interference or repeater interference occurs, or if severe intra-network interference occurs because of tight frequency reuse, the MS may fail to decode the assigned TCH properly. Thus, the TCH seizure failure occurs. This affects the TCH assignment success rate. The following types of interference may occur: 1. Interference from inter-network scramblers or privately installed antennas 2. Interference from Unicom CDMA network 3. Interference from the repeater 4. Interference from intermodulation 5. Co-channel interference or adjacent-channel interference
  • 10. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual 2.6 INTERNAL Coverage Problems Due to the low coverage level caused by the poor coverage, the MS may fail to decode the assigned TCH properly. Thus, the TCH seizure failure occurs. This affects the TCH assignment success rate. 2.7 Antenna Problems For the double transmit antennas, inconsistent azimuth and tilt caused by improper installation of the two antennas may cause different coverage of the BCCH TRX and the TCH. Thus, the assignment failure occurs. The inverse connection of the antennas also causes the improper signal coverage. Therefore, the MS cannot easily seize the TCH and the assignment failure occurs. 2.8 Repeater Problems The band selective repeater magnifies the signals carried on the BCCH frequency; therefore, the coverage ranges of the BCCH and the TCH are inconsistent. Thus, the MSs on the edge of the cell can receive BCCH signals but fail to seize the TCH. 2.9 Mobile Phone Problems Some MSs of early models do not support half-rate speech version 1. Therefore, when the BSC assigns the half-rate channel to the MS, the MS fails to seize the channel and returns the ASSIGNMETN FAILURE message. The cause of the failure is channel mode unacceptable. Therefore, assignment failure occurs.
  • 11. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual 3 INTERNAL Analysis Procedure and Optimization Method of Assignment Failures 3.1 Analysis Procedure
  • 12. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual Begin Choose the cell with high TCH assignment failure rate Transmission or hardware problems exist Yes Solve the transmission or hardware problem Yes Adjust relevant parameters No Data configuration problem No Check internetwork inteference No Interference Yes Intra-network inteference Yes No Yes Coverage Refer to the Coverage Optimization Manual No Yes Antenna No Adjust the antenna No Repeater or MS Low assignment rate does not esixt Yes End Yes Solve the repeater or MS problem Check frequency configuration INTERNAL
  • 13. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual 3.2 INTERNAL Analysis Procedure The procedure for solving the problems associated with low TCH assignment success rate is as follows: 1. Choose the cells with high assignment failure rate (higher than 1%) from the traffic statistics. 2. Check the hardware status of the cells with high TCH assignment failure rate. If the TRX or the combiner is faulty or if the RF cables are incorrectly connected, some TCHs become unavailable for use; therefore, TCH assignment failure occurs.<Relevant Cases> You can check whether the hardware is faulty by querying the BTS alarms or the hardware status on the Site Device Panel on the LMT. In addition, you can check the traffic statistics related to the hardware failures (as shown in the following table).
  • 14. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual Cause BSC Level BSC Measurement-> Cell-Level INTERNAL TRX-Level Call Measurement ->Channel Activation Assignments Measurement per Cell-> Measurement per CHAN ACTIV NACK BSC-> Messages Sent by BTS in Hardwa Failed Assignments Assignment Procedure re per BSC Channel Activation Failure (Equipment Failure) Failed Assignments Timeouts in Assignment Procedure KPI Measurement-> per BSC (Invalid State) 3. TRX Availability TCH Availability Check the transmission in the cells with high assignment failure rate Poor transmission quality, unstable transmission links, or insufficient resources on the Abis interface and A interface may lead to TCH assignment failure. You can check the transmission status by checking the alarms related to transmission. If a large number of transmission alarms are generated, you can infer that transmission failures occur. Then, you should check the transmission connections. <Relevant Cases> In addition, you can check the traffic statistics related to transmission failures, as shown in the following table. TRX-Lev Cause BSC Level Cell-Level el
  • 15. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual INTERNAL BSC Measurement->As signments Measurement per BSC-> Failed Assignments per BSC Call Measurement ->Assignment Measurement-> Transm (Terrestrial Circuit Number of Assignment Failures ission Already Allocated) (No Abis Resource Available) failure Failed Assignments BSC Measurement-> LAPD per BSC Non e. Measurement (Requested Terrestrial Resource Unavailable) Resource Measurement 4. Check the parameter settings of the cells with high TCH assignment failure rate Settings of the parameters on the BSC and MSC also affect the TCH assignment success rate. You should check the parameter settings of the faulty cell from the following aspects: 1) Check whether Directed Retry Allowed and Allow Reassign are set to Yes. The settings of Directed Retry and Allow Reassign have great impact on the assignment success rate. If the signaling load of the
  • 16. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual INTERNAL current network is not heavy, Huawei recommends that enable these two functions to increase the assignment success rate. 2) Check whether Speech Version is set to support half-rate CS-1 and half-rate CS-3. If half-rate and AMR half-rate are allowed at the site, Huawei recommends that enable them to increase the assignment success rate. 3) After the BSC enables half-rate or AMR half-rate, check whether the parameters on the MSC side are configured consistently. If they are not set on the BSC side, adjust the data configuration on the MSC to be consistent with that on the BSC. 4) Check whether TCH Traffic Busy Threshold (percentage) is set too high in the condition of supporting half-rate. If TCH Traffic Busy Threshold (%) is set over 80, the full-rate is hard to change into half-rate. If TCH Traffic Busy Threshold (%) is set too low, more half-rate channels can be used. 5) Check whether Assignment Cell Load Judge Enable is enabled. Enable the Directed Retry function. If the cell supports directed retry and the load of the cell is equal to that defined by the directed retry threshold, then proceed with the directed retry procedure. This avoids the MSC sending TCH requests when the cell is congested, and thus decreases assignment failures caused by congestion. 6) Check whether the queuing and preemption functions are enabled on the MSC side. These two functions should be enabled on the MSC side to improve TCH assignment success rate. Check whether the power level difference between the BCCH TRX and the non-BCCH TRX is great. If the difference is higher than two steps, the downlink receives level difference between the BCCH and
  • 17. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual INTERNAL the TCH is great. Thus, the TCH seizures of MSs on the edge of the cell are difficult. Therefore, the static power level difference between the BCCH TRX and non-BCCH TRX should not be over two steps. 7) Check whether SDCCH Dynamic Allocation Allowed is set to Yes. If it is set to Yes and there are many immediate assignment requests (such as location updating) in the cell, the congestion may occur and thus the assignment success rate decreases. If the SDCCH traffic volume is too high, the capacity of the cell should be expanded. 8) Check whether T3017 (ms) (Timer for the ASS CMP message) is set too small. If it is set too low, the timer may expire before the assignment is completed, thus leading to assignment failures. Setting the timer to a greater value can improve the assignment success rate. 9) Check whether T3103A (ms) (Timer for the HO CMP message) is set too great. If it is set too great, the TCH resources may be wasted, thus leading to assignment failures. Setting the timer to a small value can improve the assignment success rate. 10) Check whether Level of Preempting Dynamic Channel is set to Dynamic channels carrying services cannot be preempted. If the PS services have higher priority over the CS services and that the PS flow is high, the dynamic PDCHs are seized by PS services and the TCHs are insufficient. This parameter should be set to Control channels cannot be preempted. Query the related traffic statistics to see whether the data configuration is improper. The related traffic statistics are shown in the following table. Cause BSC- Cell-Level TRX-Level
  • 18. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual INTERNAL Level Channel Measurement ->Channel Conversion Measurement per Cell-> Channel Assignment Requests (TCHH) Number of PCU-Initiated Requests for Dynamic PDCH TRX Attributes-> Improper Number of Channel Parameter Channel Attributes None. Conversions (TCH-SDCCH) Channel Type Setting Successful Channel Conversions (TCHH-TCHF) Channel Measurement ->Channel Assignment Queue Measurement per Cell-> Queuing Requests 5. Analyze the congestion in the cells with high assignment failure rate .TCH Congestion Rate = [Failed TCH Seizures due to Busy TCH (Signaling Channel)] + [Failed TCH Seizures due to Busy TCH (Traffic Channel)] + [Failed TCH Seizures in TCH Handovers due to Busy TCH (Traffic Channel)]/TCH Seizure Requests (Signaling Channel) + TCH Seizure Requests (Traffic Channel) + TCH Seizure Requests in TCH Handovers (Traffic Channel) For details on the computing method, refer to the GSM BSS Network KPI (TCH Assignment Success Rate) Baseline. If the congestion rate of the faulty cell is high (over 10%), the failed
  • 19. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual INTERNAL assignment is mainly caused by congestion. Then, check the causes of TCH congestion from the following aspects. <Relevant Cases> 1) Check whether the cell supports half-rate CS-1 and half-rate CS-3 from the parameter configuration. If the speech quality meets the demand, you can enable half-rate and AMR half-rate. 2) Check whether cross coverage or unbalanced distribution of traffic exists in the cell by drive tests. If the faults exist, decrease the coverage range of the cell by reducing the power, lowering the antenna tilt, or increasing Min Access Level. 3) Query the alarm logs to see whether the surrounding cells are out of service, and whether an emergency such as an assembly occurs. The congestion caused by these reasons can be handled later. 4) Check whether the cell is configured with too much static PDCHs or dynamic PDCHs from the channel configuration (PS services first). If the PS service volume is relatively small, reduce the number of PDCHs properly. 5) Check whether the very early assignment function is enabled. If it is, disable the function or expand the capacity. 6) Check whether TCH Availability is 100% from the traffic statistics. If it is not 100%, the TRXs may be faulty or some channels may be manually blocked or suspended. In this case, check the status of TRX and channel and restore them in time. The relevant counters are described in the following table. Cause TRX-Lev BSC-Level Cell-Level s Conge stion el BSC Measurement->Assign -{}- Non Call Measurement->KPI e.
  • 20. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual INTERNAL ment Measurement per Measurement->TCH Congestion BSC->Failed Rate (overflow) Assignments per BSC TCH Availability (Channel Unavailable) Seizures due to Busy TCH (Traffic Channel) BSC TCH Seizure Requests (Traffic Channel) Measurement(BSC)-> Access Measurement per BSC-> TCH Congestion Rate per BSC 6. Failed TCH Call Measurement ->Assignment Measurement-> Failed Assignments (Channel Unavailable) Analyze the interference in the cells with high assignment failure rate When serious intra-network interference and inter-network interference occur, the MS may not seize the assigned TCH successfully. Therefore, assignment failures occur. You can obtain the information on UL interference by checking the distribution status of the interference bands. The higher percentage of bands 3-5 indicates that serious UL interference exists. You can obtain the information on DL by drive test or checking the frequency configuration on NASTAR. The interference is classified into inter-network interference and intra-network interference. For the detailed method of eliminating the interference, refer to the Guide to Tackle G-Interference (20050311-A-1.0). The relevant counters are listed in the following table. Cause BSC-Level Cell-Level TRX-Level
  • 21. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual INTERNAL s MR BSC Measurement->Analyzed Measurement->Hand Call over Attempt Measurement->Outgoing Measurement per Internal Inter-Cell Measurement of Interference Band per TRX-> BSC-> Attempted Handover Measurement Handovers per BSC per Cell->Outgoing Uplink Signal Internal Inter-Cell Interf Quality) erence Attempted Handovers Handover Requests per Cell (Signal per BSC Quality)->Outgoing Downlink Signal Internal Inter-Cell Quality) Handover Requests per Attempted Handovers Cell (Other Causes) Mean Number of TCHs in Interference Band 1 Mean Number of TCHs in Interference Band 2 Mean Number of TCHs in Interference Band 3 Mean Number of TCHs per BSC in Interference Band 4 (Other Causes) Mean Number of TCHs in Interference Band 5 7. Check the coverage in the cells with high assignment failure rate To check the coverage status, query the signal level distribution. If the DL levels of a large number of MRs are lower than –95 dBm, the coverage of the cell is relatively poor. If the coverage of the cell is poor but the congestion does not occur, solve the coverage problems by adjusting the down tilt of the antenna, increasing the transmit power, adding repeaters, or changing the combination mode. In addition, query the traffic statistics to locate the coverage problems.
  • 22. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual INTERNAL The relevant counters are listed in the following table. Cau BSC-Level Cell-Level TRX-Level ses BSC Call Measurement-> Outgoing Measurement->Handov Internal Inter-Cell Handover er Attempt Measurement per Cell->Outgoing Measurement per Internal Inter-Cell Handover BSC-> Attempted Requests (Signal Strength) MR Measurement->TCHF Receive Level Handovers per BSC Cov Measurement erag (Uplink Signal Call Measurement->Outgoing MR Strength) External Inter-Cell Handover Attempted Handovers Measurement per Cell->Outgoing per BSC e External Inter-Cell Handover Measurement->TC HH Receive Level Measurement (Downlink Signal Strength) Requests(Uplink Signal Strength) Outgoing External Inter-Cell Handover Requests(Downlink Signal Strength) For detailed method of solving coverage problems, refer to the GSM BSS Network Performance KPI (Coverage) Optimization Manual. 8. Check the antenna in the cells with high assignment failure rate For double transmit antennas, check whether the tilt and the azimuth of the antennas are consistent. If they are inconsistent, adjust them to be consistent. In addition, analyze the data of the drive test to see whether the antenna jumpers are reversely connected. If they are, connect the jumpers correctly. If the antenna system (antenna and feeder) is faulty, the coverage range
  • 23. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual INTERNAL of the cell becomes smaller, thus leading to failed TCH seizures. If the faults are found, change the antenna or feeder. The fault on antenna system is generally shown in the following ways: The drop rate and handover failure rate of the cell are relatively high; the difference between the UL quality and the DL quality is great; both the UL quality and the DL quality are poor. You can query the relevant counters as listed in the following table to see whether the fault on antenna system exists. BSC-L Causes Cell-Level TRX-Level evel Call Measurement-> Call Drop Measurement->Call Drop Rate on TCHF per cell MR Measurement->Receive Call Drop Rate on TCHH per cell TRX MR Call Drop Rate on TCH per cell Measurement->Uplink-an (Excluding Handover) d-Downlink Balance Traffic Call Drop Ratio per cell None. Call Measurement ->KPI Measurement-> Antenna Quality Measurement per Measurement per TRX Success Rate of Radio Handover 9. Check whether the cell with high assignment rate is configured with the band selective repeater. Check whether Directly Magnifier Site Flag on the LMT is set to Yes. If it is set to Yes, a repeater is configured in the cell. If it is set to No, check whether other telecom operators mount the repeaters around the cell. If there is a repeater mounted around, check whether the repeater is the band selective repeater corresponding to the BCCH frequency band. If it is,
  • 24. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual INTERNAL check whether the UL/DL magnifying coefficient is excessively high. If it is, reduce the coefficient. If the impact is serious, disable the band selective repeater. In addition, check whether the difference between the TA distribution of the BCCH TRX and non-BCCH TRX is great in the cell with high assignment failure rate. If the BCCH TRX with great TA reports certain numbers of MRs while the non-BCCH TRX does not, the BCCH frequency band may be configured with the band selective repeater. This may cause inconsistent coverage between the BCCH TRX and non-BCCH TRX, which leads to many assignment failures. Cell-L Causes BSC-Level TRX-Level evel BCCH Frequency MR Measurement-> Number of Band None. None. MRs Based on Configured TA per TRX with Band Selective Repeater 10. Check whether a large number of MSs do not support half-rate channel in the cell with high assignment failure rate. Check whether many subscribers use old-model mobile phones that do not support the half-rate channel. If there are many such mobile phones, half-rate and full-rate dynamic conversion should be enabled to expand the capacity.<Relevant Cases> In addition, check the related counters to see whether the problems exist.
  • 25. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual INTERNAL The relevant counters are listed in the following table. BSCCauses Cell-Level TRX-Level Level Channel Measurement->Channel Assignment Request Measurement per Cell->Channel Assignment Requests The MS does None (TCCH) not support None. half-rate . channel. Successful Channel Assignments (TCCH) 4Test Methods TCH assignment success rate is one of traffic measurement counters, which can be obtained through registration or reporting of the related traffic measurement counters. Generally, this counter is not measured through drive tests. Drive tests cannot indicate the actual assignment success rate because of the limited samples and incomplete test routes. Currently, manufacturers and operators use different formulas for calculating TCH assignment success rate, thus leading to different values of this counter. In actual measurement, you should register the specific counters and use an appropriate formula to calculate the value of the counter. 5 Description of Assignment Requests Trace the RSL messages on the Abis interface. You can query the signaling
  • 26. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual INTERNAL tracing file on the LMT or by using the Signal Analyze Tool., Filter the signaling to find Data Request including the CHAN REQ message, as shown in the following figure. The meaning of each message is as follows: Channel Type: Channel Type indicates the type of the requested channel. Generally, the TCH is requested. It is represented by the first five bits of byte 12. TN indicates the number of the timeslot of the requested channel. It is represented by the last three bits of byte 12. TSC stands for training sequence code. It is represented by the first three bits of
  • 27. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual INTERNAL byte 13. Hopping Flag indicates whether the cell is enabled with frequency hopping. It is represented by bit 4 of byte 13. MAIO stands for mobile allocation index offset. It is represented by the last four bits of byte 13 and the first two bits of byte 14. It includes six bits. HSN stands for hopping sequence number. It is represented by the last six bits of byte 13. Power Level indicates the level of MS power. It is represented by the last five bits of byte 15. CA ARFCN is the frequency of the cell. It is represented by the last four bytes of byte 17 to byte 32. Channel Mode indicates the mode of requesting channels such as FR, HR, EFR, AMR FR, and AMR HR. It is represented by byte 34. Multi–Rate Configuration indicates the parameters related to AMR such as rate conversion threshold and hysteresis if the requested channel is AMR channel. It is represented by byte 38 to byte 45. 6 Cases of Optimizing Assignment Success Rate 6.1 High Assignment Failure Rate After BTS Capacity Expansion [Problem Description] After the capacity expansion of a BTS, the BTS is reset. The initialization of the cell is normal. Then, conduct a dialing test to check the performance of the newly expanded TRX. The result shows that the MS cannot seize the TCH after the SDCCH assignment is completed. Continue the dialing test and find that the TCH seizure fails when TRXs 12 to 15 are seized. The TCH assignment is successful sometimes, but the receive level decreases to –102 dBm, which leads to poor speech quality. [Problem Analysis and Solution] Cause Analysis:
  • 28. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual INTERNAL 1. Fault on the newly expanded TRX board 2. Fault on the RF cables of the newly expanded board 3. Faults on the SCU port and CDU receive diversity port 4. Wrong connections of RF cables 1. Troubleshooting: Lock the frequency and conduct a dialing test. The result shows that the seizure of TRXs 0 to 4 is normal. Manually block TRXs 1 to 3 except the main BCCH. Conduct the dialing test again with the frequency locked and find that the receive level decreases to –102 dBm during TCH assignment. The assignment fails. 2. Exchange the newly expanded TRX with another TRX and perform the test. The fault persists. 3. Replace the RF cables and the fault persists. 4. Because faults occur on the TRXs 12 to 15, the SCU may be faulty. During the replacement of the SCU, the RF cables are found to be incorrectly connected to the TX_COM of CDU. The TRXs 12–15 do not transmit RF signals. Connect the RF cables to the TX_DUP port and then perform a dialing test. All timeslots are normally seized. Summary: Ensure correct cable connections when using SCU and CDU during capacity expansion of a BTS. Wrong connections of cables can cause assignment failure. 6.2 High Assignment Failure Rate Because of Improper Parameter Settings [Problem Description] An office uses the equipment of Huawei and manufacturer S. The statistics show that the TCH success rate cannot reach 98%, the optimal value for the office. The Huawei equipment does not outperform the equipment from
  • 29. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual INTERNAL manufacturer S. The TCH assignment success rate of Huawei equipment is lower than the average value of the whole network. [Problem Analysis and Solution] Cause Analysis: TCH Seizure Requests indicates the number of ASSIGNMENT REQUEST messages that the BSC receives from the MSC in the assignment procedure. Successful TCH Seizures is measured in the following cases: 1. When the BSC receives a CHANNEL ACTIVATION ACKNOWLEDGE message from the BTS in the procedure of very early assignment, this counter is incremented by one.2. In the assignment procedure, if the directed retry procedure with the target cell under another BSC is successful, the MSC sends a CLEAR COMMAND message to the originating cell to clear the original connection. When the originating BSC receives the CLEAR COMMAND message, this counter is incremented by one. 3. In the assignment procedure, if the directed retry procedure with the target cell under another BSC is successful, the target cell sends an Inter Clear Request message to the originating cell to inform the originating cell to clear the resources and connection. When the originating BSC receives the message, Successful TCH Seizures is measured. 4. The value of this counter excludes the number of ASSIGNMENT COMPLETE messages that the BSC sends to the MSC after the directed retry procedure is performed successfully in the target cell within the BSC. The analysis of the assignment procedure shows that TCH seizure failure is mainly caused by channel unavailability due to congestion and no receipt of the CHANNEL ACTIVATION ACKNOWLEDGE message from the BTS. In addition, the directed retry procedure has great impact on successful TCH seizure. The analysis of parameters indicates that the current network does not enable the functions of directed retry and reassignment. In this case, directed retry can be enabled to reassign a TCH to increase the TCH assignment success rate (excluding handover). Troubleshooting:
  • 30. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual INTERNAL Enable directed retry in the BSC with low TCH assignment success rate and compare the traffic statistics of the same time of two days. The result shows that the TCH seizure success rate increases by about 2%, from 96.1% to 98 %.Perform the same procedure in the other two BSCs in the network. The TCH seizure success rate of the entire network increases from 96.5% to 98.2%. Then, adjust Allow Reassign to Yes. The TCH seizure success rate of the network increases to 98.4%. This value exceeds that of the equipment from manufacturer S and reaches the demand of the office. Thus, the user satisfaction is improved. Summary: Adjusting Directed Retry and Allow Reassign to Yes can increase TCH seizure success rate. 6.3 Decrease in Assignment Success Rate due to Insufficient Channel Resources [Problem Description] The office in Pakistan reports that the assignment success rate on site is too low. [Problem Analysis and Solution] Cause Analysis: By analyzing the traffic statistics before the patch loading, the engineers in the headquarters think that the low assignment success rate is not caused by the slow release of the A interface circuits but the insufficient TCHF resources on the BSC. The main reasons are Other Causes, Channel Unavailable, and Terrestrial Circuit Already Allocated. The engineers in the headquarters think that Other Causes and Channel Unavailable indicate that the TCHF resources are insufficient. Terrestrial Circuit Already Allocated indicates that release delay of the circuit exists; however, this is not the main reason that causes assignment failure on site. Troubleshooting:
  • 31. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual INTERNAL Suggestions: 1. Register measurement job of assignment procedure on the BSC. Inquire the seizure conditions of TCHHs and TCHFs. 2. Add TCHFs properly. Decrease TCHHs according to the seizure conditions of TCHHs. 3. With the MS support on the half-rate scheme in the current network and with the consent of the customer, use only half-rate channels to decrease full-rate channel seizures based on ACPOOL or data configuration on the BSC. Seizing TCHHs affects the speech quality. Summary: Insufficient channel resources can cause congestion, which affects assignment success rate. 6.4 Low Assignment Success Rate Caused by Insufficient Ater Resources [Problem Description] The assignment success rate is low after the BSC6000 is cut over at a site. The TCH assignment success rate per BSC is only about 60%. The handover success rate is low. Making calls is difficult. [Problem Analysis and Solution] Troubleshooting: 1. Check the settings of the parameters related to assignment: Allow Reassign and T3107, and find that they are normal. 2. The BSC6000 is configured with adaptive circuit pool. The site is in circuit pool 27. Therefore, exclude the impact of the circuit pool and speech version. 3. Check the BSC alarms. The result shows that all equipment run normally. 4. Check the traffic statistics and find that the TCH congestion does not occur. Therefore, exclude the situation of channel unavailability. The assignment failure mainly occurs on the A interface. The difference between the assignment requests sent by the MSC and the assignment commands sent by the BSC is great, that is, many assignment commands are not sent. Check the status of the A interface and find that the trunk is normal and the
  • 32. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual INTERNAL resources are sufficient. In the current network, the TC is configured remotely and the transmission resources are insufficient. Check the resources on the Ater interface and find that the BSC is configured with only two E1s and 176 trunks. The traffic volume of fault occurrence time is 165 Erl. Therefore, we can infer that the resources on the Ater interface are insufficient, which is acknowledged by the tracing signaling. Increase the transmission resources on the Ater interface and find that the assignment success rate is up to 99 %. Summary: In the BSC32, the trunks on the A and Ater interfaces are activated correspondingly, that is, the resources on the A and Ater interfaces are the same. If the trunks are insufficient, the MSC does not send the Assignment Request message but the Disconnect message with the cause value resource unavailable after the MSC receives the SETUP message. Therefore, note the ratio of traffic volume to the Ater resources to avoid limited communication because of insufficient Ater resources. 6.5 Low Assignment Success Rate Because of Configuration of Half-rate Channels [Problem Description] In a newly deployed BTS in town A, the traffic volume is high after the BTS is started. When the half-rate scheme is enabled, the subscribers complain that the mobile phones cannot make calls frequently even though the signals are strong. The assignment success rate is low. [Problem Analysis and Solution] Troubleshooting: 1. Check the alarm console and find that no alarm is generated. The TRX availability is 100%, which indicates that the probability of TRX faults is small. 2. Query RACH Min. Access Level and find that it is set to –109 dBm. The subscriber reports that the mobile phone has full signal strength indicators. Therefore, the error in parameter setting is excluded.
  • 33. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual INTERNAL 3. Query the status of interference bands and balance between the uplink and the downlink. The result shows that they are normal. Immediate assignment success rate is normal while TCH assignment success rate is low. 4. Query the traffic volume and congestion rate on the SDCCH and TCH. The results show that TCH congestion rate is high. The number of initially configured TCHHs is great while that of TCHFs is small. The difference between successful TCH seizures (only full-rate) and TCH seizure requests (only full-rate) is great. Therefore, it is doubted that static half-rate channels may be configured, which makes some MSs supporting only full-rate services unable to seize the TCHFs. 5. Check the data configuration to verify the preceding doubt. Change the channel type and collect the feedback from the subscribers. The fault is rectified. Summary: Obtain detailed information on recent operations before rectifying the faults. The faults may be caused by incorrect configuration of relevant parameters. Detailed analysis of traffic statistics helps locate the faults quickly. 7 Feedbacks on Assignment Success Rate If assignment success rate is too low, send the data described in the following table to the WCDMA&GSM Network Performance Research Dept. Check Item Remarks Purpose Software version of the BSC Software version To check whether the software version is and BTS faulty. Data configuration table To check the optimization parameters and *.dat file power configuration. Alarms related to hardware, clock, and transmission Alarm information To check whether such alarms exist in the (self-check) faulty cell. Such alarms should be cleared.
  • 34. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual INTERNAL Assignment success rate and Traffic statistics Signaling To observe the traffic measurement result and channel availability check for congestion or interference. RSL tracing data To check the reasons of assignment failures. To observe the drive test data and check for Drive test data interference or coverage problems. Engineering parameter tables Others *.log(*.CELL) or *.ant file To check the geographical information of the and E-maps NASTAR. Appendix: Major Parameters Affecting TCH Assignment Success Rate Category Parameter Call Control Assignment Cell Load Judge Enable Directed Retry Load Access Threshold Allow Reassign Max Assignment Retry Times Speech Version Channel Management TCH Traffic Busy Threshold AMR TCH/H Prior Allowed Other Parameters T3107(ms) Cell Directed Retry Forbidden Threshold T3103A(ms) Basic Parameters SDCCH Dynamic Allocation Allowed Directed Retry Allowed MSC Parameters Queuing Allowed
  • 35. GSM BSS Network KPI (TCH Assignment Success Rate) Optimization Manual Preemption Allowed INTERNAL