08 GSM BSS Network KPI (Immediate Assignment Success Rate) Optimization Manual

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

Product Name GSM BSS Product Version

INTERNAL

Confidentiality Level INTERNAL Total 34 pages

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

Prepared by

GSM&UMTS Network Performance Research Department Wang Fei

Reviewed by Reviewed by Granted by

Date 2008-06-18 Date Date Date

Huawei Technologies Co., Ltd All rights reserved

2008-10-25

All Rights Reserved

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Revision Record Date 2008-06-18

Revision Version 0.8

2008-07-18

1.0

2008-10-25

Change Description Draft completed. The document is modified according to review comments.

All Rights Reserved

Author Wang Fei Wang Fei

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References Number 1

2008-10-25

References GSM BSS Network KPI (Immediate Assignment Success Rate) Baseline

Author Liu Xiuyu

All Rights Reserved

Date 2007-03-10

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Content 1 Overview of the Immediate Assignment Success Rate..............................................................7 1.1 Definition.................................................................................................................................7 1.2 Signaling Procedure and Measurement Points......................................................................8 1.3 Symptom Description .............................................................................................................8 2 Factors Concerning Immediate Assignment Success Rate........................................................9 2.1 Signaling Factors....................................................................................................................9 2.2 Factor Analyzing According to the Definition of the KPI.........................................................9 2.3 Factors Analysis....................................................................................................................10 2.3.1 Equipment Faults............................................................................................................10 2.3.2 Um Interface Problems...................................................................................................11 2.3.3 SDCCH Congestion........................................................................................................11 2.3.4 MS Problems..................................................................................................................12 3 Method of Analyzing the Problem of Low Immediate Assignment Success Rate.....................12 3.1 Process of Analyzing the Problem of Low Immediate Assignment Success Rate...............12 3.2 Method of Solving the Problem of Low Immediate Assignment Success Rate....................14 3.2.1 Equipment Faults............................................................................................................14 3.2.2 Um Interface Problem.....................................................................................................14 3.2.3 System Capacity (Congestion) Analysis ........................................................................19 3.2.4 MS Problem Analysis......................................................................................................23 4 Test Method...............................................................................................................................26 5 Cases of Immediate Assignment Success Rate Optimization..................................................26 5.1 Case 1: Decrease in Immediate Assignment Success Rate Because of SDCCH Congestion Caused by Incorrect LAC Setting...........................................................................................26 5.2 Case 2: SDCCH Congestion Caused by a Burst of Location Updates................................27 5.3 Case 3: Low Assignment Success Rate Because the RACH Minimum Access Level Is Set to 0.........................................................................................................................................28 5.4 Case 4: Low Immediate Assignment Success Rate in Some Cells Because of MS Error...29 6 Onsite Information that Needs to Be Obtained.........................................................................33

2008-10-25

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Tables Table 1 Mapping between formulae and factors..........................................................................10

2008-10-25

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GSGSM BSS Network KPI (Immediate Assignment Success Rate) Optimization Manual

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GSM BSS Network KPI (Immediate Assignment Success Rate) Optimization Manual Abstract Key Words: SDCCH, immediate assignment success rate Abstract: This document provides the procedure for analyzing the problems about immediate assignment success rate, describes the troubleshooting procedure, and provides typical cases. Abbreviations: Abbreviation Full Spelling BSC Base Station Controller BTS Base Tranceiver Station SDCCH Stand-alone Dedicated Control Channel

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Huawei Technologies Proprietary

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1 Overview

of

the

Immediate

INTERNAL

Assignment

Success Rate Immediate assignment success rate indicates the success rate of the MS accessing the signaling channel. It concerns the procedure from the MS sending a channel required message to the BTS to the MS sending an establish indication message to the network. Immediate assignment success rate is a key access counter. It directly reflects the success rate of the MS accessing the signaling channel and affects the user experience. The BSC versions concerned in this document are as follows: BSC32——G3BSC32V300R007C01B015 BSC6000——BSC6000V900R001

1.1

Definition The immediate assignment success rate is calculated from traffic statistics. The

recommended formula is as follows: Immediate

Assignment

Success

Rate

=

(Successful

Immediate

Assignments/Immediate Assignment Requests) x 100%

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1.2

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Signaling Procedure and Measurement Points BTS

MSC

BSC

Channel REQ Channl RQD A1 Channel ACT Channel ACT ACK I MM ASS CMD B1

f i rst SABM UA EST I ND( CM Servi ce REQ)

C1

CR( CMP L3 i nf ormat i on) CC CM Servi ce Accepted

Note: A1: Immediate Assignment Requests (Channel Requests (Circuit Service)) B1: Immediate Assignment Commands C1: Successful Immediate Assignments (Call Setup Indications (Circuit Service))

1.3

Symptom Description If the immediate assignment success rate decreases, the following symptoms may

occur: 

Call setup success rate decreases.

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

Congestion occurs frequently on the SDCCH.



Traffic volume on the TCH decreases.



Short messages cannot be successfully sent.

INTERNAL

……

2 Factors Concerning Immediate Assignment

Success Rate 2.1

Signaling Factors According to the signaling flow, the factors concerning the immediate assignment

success rate during each phase are as follows: (1) Random access request phase: whether the small interference signal on the Um interface is regarded as the random access request by the BSC. Affecting factor: Um interface problems; (2) SDCCH assignment phase: whether the SDCCH is available after the random access request is accepted. Affecting factor: SDCCH congestion; (3) Channel activation phase: whether the channel activation is successful after the channel assignment succeeds. Affecting factor: CHAN ACTIV NACK message is received because of equipment faults during SDCCH activation; (4) Immediate assignment command phase: whether the timer for the acknowledgment of the immediate assignment expires. Affecting factor: Um interface problems; (5) Link establishment phase: whether the link can be established on the SDCCH after the immediate assignment command is sent. Affecting factor: MS features. The following analysis is based on these factors.

2.2

Factor Analyzing According to the Definition of the KPI As shown in the previous section, the following four factors are related to immediate

assignment success rate: Um interface problems, SDCCH congestion, reception of 2012-12-12


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CHAN ACTIV NACK because of equipment faults during SDCCH activation, MS problems. These factors are related to the definition of the KPI. There are three formulae that can define the immediate assignment success rate, the mapping between formulae and factors are as follows: Table 1 Mapping between formulae and factors

Formula Um interface problems

Immediate Assignment Success Rate √ = (Successful Immediate Assignments/Immediate Assignment Requests) x 100% Immediate Assignment Success Rate 　 = (Successful Immediate Assignments/ (Immediate Assignment Requests (Immediate Assignment Commands Successful Immediate Assignments))) x 100% Immediate Assignment Success Rate √ = (Successful Immediate Assignments/Immediate Assignment Commands) x 100%

2.3

Affecting Factor Reception SDCCH of CHAN congestion ACTIV NACK because of equipment faults during SDCCH activation √ √

√

√

√

　

　

　

√

MS problems

Factors Analysis

2.3.1 Equipment Faults 2.3.1.1

Low Immediate Assignment Success Rate Caused by TRX Faults

Generally, reception of CHAN ACTIV NACK during the SDCCH activation is caused by equipment faults. If one TRX fails in the cell configured with multiple TRXs, the immediate assignment may fail or the SDCCH may be congested. If you are sure that the fault lies in a TRX, replace the faulty TRX. Otherwise, check the cables in the antenna 2012-12-12


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system and the VSWR. If the cables are properly connected and the VSWR is normal, you need to replace the TRX, and then check whether services are normal again.

2.3.2 Um Interface Problems 2.3.2.1

Mistakenly Regarding Interference as Random Access Request

The BTS may mistakenly regard the interference on the Um interface as the random access signal. This may lead to immediate assignment failure or SDCCH congestion. In this situation, the interference problem should be solved.

2.3.2.2

Channel Immediate Assignment Failure Caused by Coverage Difference Between BCCH TRX and non-BCCH TRX

In a cell configured with six or more than six TRXs, the combination losses of the BCCH TRX and non-BCCH TRXs are different, leading to coverage difference. If the SDCCH is configured on the non-BCCH TRX, a call that is far away from the serving cell may fail to access the SDCCH when it is assigned to the non-BCCH TRX. Thus, the call drop may occur.

2.3.3 SDCCH Congestion 2.3.3.1

SDCCH Congestion Caused by Heavy Traffic

For SDCCH congestion caused by heavy traffic, capacity expansion is necessary. You can also check the parameters concerning location update and SDCCH dynamic configuration. For SDCCH congestion caused by traffic burst, such as group sending of short messages and location update at the portal of a tunnel, the problem cannot be completely solved. You can, however, alleviate the congestion by enabling the functions such as SDCCH dynamic adjustment.

2.3.3.2

Congestion Caused by Inappropriate Data Configuration

① Location area planning: a reasonable location area plan can help alleviate the SDCCH congestion. ② Dynamic SDCCH assignment: the dynamic SDCCH assignment can help 2012-12-12


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alleviate SDCCH congestion. ③ Dual-band network: a reasonable setting of dual-band network parameters (for example, CRO, CBA, CBQ, and Cell Reselection Hysteresis) can help alleviate SDCCH congestion. ④ Check whether the settings of related timers (such as T3101, T3122, T3212, and T3111) are reasonable.

2.3.4 MS Problems In some cases, the location updates performed by the MS are abnormal, leading to a low immediate assignment success rate. These MSs fail to establish a link on the SDCCH after sending the channel request, causing a low immediate assignment success rate. For the detailed analyzing method and case study, see case 4 in section 3.

Method of Analyzing the Problem of Low

3

Immediate Assignment Success Rate 3.1

Process of Analyzing the Problem of Low Immediate Assignment Success Rate When the immediate assignment success rate becomes low, you should firstly check

the range in which the problem exists, and then find out the factors related to the problem according to KPI statistical formulae. For details, see section 2.2 "Factor Analyzing According to the Definition of the KPI". In the case of recommended formula, the general process of analyzing the problem is as follows:

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Start

Analysis of low immediate assignment success rate

Find cells with low immediate assignment success rate

Equipmen t fault?

Um interface fault?

Yes

Ye s

Check TRX availability and TRX alarm

Check SDCCH availablity

Idle interference band/signaling analysis

TRX receive level and quality analysis

Check number of SDCCH activation failures/timeout

TRX fault or transmission failure

Solve Interferenc e problem

RACH overload times

No Ye Insufficient system s capacity? No

SDCCH seizure Normal success rate (location update)

Abnorma l Traffic volume per SDCCH exceeds paln

MS fault?

Analyze signaling and traffic statistics of cells with problem

SDCCH and TCH traffic volume analysis

Incoming and outgoing handovers of the cell

Check timers related to location update

Check SDCCH dynamic configuration

Enable CBA of the cell

Replan the location areas

Hardware expansion

For swapped network, check whether the problem exists in ventor's network

Locate problem and submit report

No Problem solved? Yes End

Figure 1 Process of analyzing the problem of low immediate assignment success rate

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3.2

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Method of Solving the Problem of Low Immediate Assignment Success Rate Before analyzing the cause of low immediate assignment success rate, you should

find out the difference between the actual immediate assignment success rate and the expected value. You should also find out the influence of the problem and the definition of the specific immediate assignment success rate KPI.

3.2.1 Equipment Faults This part aims to solve TRX or transmission problems. Firstly, you should check TRX Availability in BSC Measurement and SDCCH Availability in SDCCH Measurement. Secondly, you should check the number of times that a NACK message is received or the timer expires during the SDCCH activation. In this way, you can determine whether the problem is caused by board faults. You can also check whether hardware is faulty by viewing BTS alarms or by viewing the hardware state on Site Device Panel of the LMT. You can check the following traffic statistics for reference: Cause Equipmen t faults

BSC Level BSC Measurement -> Access measurement per BSC -> SDCCH Availability per BSC Configured SDCCHs per BSC Available SDCCHs per BSC

Cell Level Call Measurement -> Channel Activation Measurement per Cell -> SDCCH Connection Measurement per Cell -> CHAN ACTIV NACK Messages Sent by BTS in Immediate Assignment Procedure (SDCCH) Channel Activation Timeouts in Immediate Assignment Procedure (SDCCH) KPI Measurement per Cell -> TCH Availability

3.2.2 Um Interface Problem 3.2.2.1

Mistakenly Regarding Interference as Random Access Requests

Interference may cause SDCCH congestion, causing low immediate assignment success rate. Especially in the areas with a small space between BTSs and dense BCCH frequency planning, the system allocates SDCCH for each signal if a large number of interference signals are received. Thus, the congestion may occur. In this situation, the immediate assignment success rate and paging success rate decrease, and the RACH 2012-12-12


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may be overloaded. You can locate the problems by analyzing the idle interference bands and the TRX receive quality during the call. If the system mistakenly regards the interference as a random access signal and sends an assignment command, the immediate assignment will fail. You can locate the problem by analyzing the Chan Req message. For the SDCCH congestion caused by interference, you can check the following traffic statistics for reference: Cause SDCCH congestion caused by interference

BSC Level BSC Measurement -> Access measurement per BSC -> Random Access Success Ratio per BSC

Cell Level Call Measurement -> Flow Control Measurement per Cell -> MSG CCCH LOAD IND (RACH) Messages Sent on Abis Interface

TRX Level MR Measurement -> Analyzed Measurement of Interference Band per TRX Receive Quality Measurement per TRX

For the problem of mistakenly regarding interference as random access requests, you can locate the problem by performing signaling analysis on the Abis interface of the cell with worst performance. If the signaling analysis result shows that the random access signals are all from far away, for example, the values of TA are higher than 10, sometimes even higher than 20, and the levels are lower than -100 dBm, the random access success rate is high and the immediate assignment success rate is low, you can infer that the signals are interference. To restrict the access of interference, reduce MAX TA and increase RACH Min Access Level. You can check the following traffic statistics for reference: Cause Mistakenly Access of Interference on the SDCCH

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BSC Level BSC Measurement -> Access measurement per BSC -> Random Access Success Ratio per BSC

Cell Level Call Measurement -> Immediate Assignment Measurement per Cell -> Immediate Assignment Analyzed Measurement per Cell -> Call Setup Indications Timed Out Call Measurement -> Immediate Assignment Measurement per Cell -> Immediate Assignment Analyzed Measurement per Cell -> Success Rate of Random Access


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Figure 2 shows the contents of a channel required message.

Figure 2 Channel request signaling Information element description: 1. The meaning of Random Access Information (8bits) is as follows: Table 9.9/3GPP TS 04.08: CHANNEL REQUEST message content

MS codes bits 8 .... 1

According to Establishment cause:

101xxxxx

Emergency call

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110xxxxx Call re-establishment; TCH/F was in use, or TCH/H was in use but the network does not set NECI bit to 1 011010xx Call re-establishment; TCH/H was in use and the network sets NECI bit to 1 011011xx Call re-establishment; TCH/H + TCH/H was in use and the network sets NECI bit to 1 100xxxxx 0010xxxx 0011xxxx 0001xxxx

Answer to paging See table 9.9a/3GPP TS 04.08

111xxxxx Originating call and TCH/F is needed, or originating call and the network does not set NECI bit to 1, or procedures that can be completed with a SDCCH and the network does not set NECI bit to 1. note 1 0100xxxx Originating speech call from dual-rate mobile station when TCH/H is sufficient and supported by the MS for speech calls and the network sets NECI bit to 1 note 5 0101xxxx Originating data call from dual-rate mobile station when TCH/H is sufficient and supported by the MS for data calls and the network sets NECI bit to 1 note 5 000xxxxx

Location updating and the network does not set NECI bit to 1

0000xxxx

Location updating and the network sets NECI bit to 1

0001xxxx Other procedures which can be completed with note 1 an SDCCH and the network sets NECI bit to 1 011110xx 01111x0x 01111xx0

One phase packet access with request for single timeslot uplink transmission; one PDCH is needed.

01110xxx Single block packet access; one block period on a PDCH is needed for two phase packet access or other RR signalling purpose. 01100111

LMU establishment

01100xx0 01100x01 01100011

Reserved for future use

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note 2

note 2a Huawei Technologies Proprietary

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01111111

Reserved for future use.

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note 2b

NOTE 1: Examples of these procedures are: IMSI detach, Short Message Service (SMS), Supplementary Service management, Location Services. NOTE 2: If such messages are received by a network, an SDCCH shall be allocated. NOTE 2a: If such messages are received by a network, an SDCCH may be allocated. NOTE 2b: This value shall not be used by the mobile station on RACH. If such message is received by the network, it may be ignored.

2. The value of Access Delay indicates the TA. 3. Bits 13, 14, 15, and 16 indicate the RACH level and the number of error bits in the training sequence. In this example, the value of bit 13 is 43. This indicates that the bits followed indicate the AB access level and the number of error bits in the training sequence. The value of bit 14 is 2. This indicates that the following two bits indicate the AB access level and the number of error bits in the training sequence. The value of bit 15 indicates the RACH level (39 – 120 = –81 dBm).

3.2.2.2

Coverage Difference

If the combination mode of the BCCH TRX and the non-BCCH TRX is different, or the losses are inconsistent because of different transmit power or other problems, the assignment on the non-BCCH TRX may fail. The causes can be classified into three types: ① The transmit power of TRXs in the same cell is different. If concentric cell technology is not applied, the power of different TRXs on the antenna input port is different because of different uplink losses. This causes inter-TRX coverage difference. Thus, assignment failure may occur. To solve this problem, check whether the combiner, divider, CDU, or SCU is properly connected. ② Multiple antennae are installed in a cell. When multiple antennae are installed in a cell, the coverage differences of the antennae may cause assignment failure. You can solve this problem through engineering adjustment. ③ The transmit and receive antennae are not on the same horizontal plane or their tilts are not the same. You can solve this problem by adjusting the antennae. 2012-12-12


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3.2.3 System Capacity (Congestion) Analysis 3.2.3.1

Congestion Caused by Heavy Traffic on the SDCCH

Generally, heavy traffic on the SDCCH is caused by frequent location updates. You can check the rate of the SDCCH being occupied by querying the counter Channel Requests (Location Updating) and the total number of channel requests. Figure 3 shows the ratios of different kinds of SDCCH requests to the total number of SDCCH requests on Shanghai Unicom when the traffic is heavy. If the ratios are high, check the location area planning and the drive test result. If the edges of location areas are configured in the area with heavy traffic, reconfigure the location areas. In addition, you can adjust the CHR parameters to reduce location updates. For details, see Case 1. If the SDCCH congestion rate is high (greater than 5%), the traffic volume on each SDCCH may have exceeded the limit as planned. You should check the configuration of the location update and SDCCH dynamic configuration parameters. If the settings of parameters are reasonable, a hardware capacity expansion is required. If the congestion rate and traffic volume of the SDCCH are high, but the traffic volume of the TCH is normal, the SDCCH congestion may be caused by burst traffic. For the sites along the railway, especially for these near the tunnel portal, the capacity configured is generally small. When a train passes through or stops, a large number of location updates occur, which finally lead to SDCCH congestion. In addition, during the period when the volume of short messages is large, SDCCH congestion may occur. This problem cannot be completely solved, but you can take some measures to alleviate the congestion. For example, configure more SDCCHs, or enable the dynamic conversion between the SDCCH and TCH. For details, see Case 2. For the configuration of related parameters, see section 3.2.3.2 "Congestion Caused by Inappropriate Parameter Configuration". You can check the following traffic statistics for reference: Cause 2012-12-12

BSC Level

Cell Level Huawei Technologies Proprietary

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Congestion caused by heavy traffic on the SDCCH

BSC Measurement -> Access measurement per BSC -> Congestion Ratio on SDCCH per BSC

INTERNAL

Call Measurement -> Immediate Assignment Measurement per Cell -> Channel Requests per Cell -> Channel Requests (MOC), Channel Requests (MTC), Channel Requests (Emergency Call), Channel Requests (Call Re-establishment), Channel Requests (Location Updating), Channel Requests (Packet Call), Channel Requests (LMU+Reserved), Channel Requests (Protocol Incompatible) KPI Measurement per Cell -> SDCCH Traffic Volume, SDCCH Seizure Requests, Failed SDCCH Seizures due to Busy SDCCH

Figure 3 shows the ratios of different kinds of SDCCH requests to the total number of SDCCH requests on Shanghai Unicom when the traffic is heavy: Rat i os of Di f f erent Ki nds of SDCCH Request s t o t he Tot al Number of SDCCH Request s on Shanghai Uni comwhen t he Traf f i c i s Heavy Rat i os of di f f erent ki nds of SDCCH request s 50. 000%

47. 003%

45. 000% 40. 000% 35. 000%

31. 059%

30. 000% 25. 000% 20. 000% 9. 309% 0. 001%

Figure 3 Ratios of different kinds of SDCCH requests to the total number of SDCCH requests on Shanghai Unicom when the traffic is heavy

Congestion Caused by Inappropriate Parameter Configuration An inappropriate parameter configuration may lead to the SDCCH congestion. The parameters related to SDCCH congestion and their configurations are as follows: 1. CRO: Generally, it is recommended that an MS should not camp on a cell with high traffic volume or low QoS. The parameters of such cells should be set as follows: firstly, set PT to 31, so that TO becomes invalid. Secondly, set C2 to the same value as CRO subtracted from C1 so that the value of C2 is reduced and thus the possibility of 2012-12-12


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0. 006% A300K：Number of SDCCH r equest s ( pr ot ocol i ncompat i bl e)

A300C：Number of SDCCH r equest s ( mobi l e t er mi nat ed cal l )

A300A：Number of SDCCH r equest s ( mobi l e or i gi nat ed cal l )

0. 030%

A300I ：Number of SDCCH r equest s ( LMU + Reser ved)

0. 055%

0. 000%

A300H：Number of SDCCH r equest s ( packet cal l )

5. 000%

A300F：Number of SDCCH r equest s ( l ocat i on updat i ng)

10. 000%

A300E：Number of SDCCH r equest s ( cal l r eest abl i shment )

12. 538%

A300D：Number of SDCCH r equest s ( emer gency cal l )

15. 000%


INTERNAL

reselecting this cell is reduced. In addition, network operators can set the CRO according to

the actual conditions. The greater the CRO is, the more difficult it is for an MS to access the cell. 2. Cell Reselection Hysteresis (CRH): This parameter is used to determine whether cell reselection is performed between different location areas. This parameter can prevent frequent location update, thus minimize the possibility of losing paging messages. Generally, this parameter is set to 6 dB. For dual-band networks in urban areas not sharing the location areas, this parameter is set to 8–10 dB. 

If the traffic volume in an area is high, and signaling overload occurs frequently, it is recommend that you set the CRH of the neighboring cells belong to different LACs under this area to a larger value.



If the overlapping area of the neighbor cells under different location areas is large, it is recommended that you set the CRH of the cells to a larger value.



If some areas between two neighbor cells under different location areas have poor coverage, or if most of the MSs in the area (such as highways) move at a high speed, it is recommended that you set the CRH of the cells to 2–6 dB.

3. Set the parameters related to SDCCH dynamic adjustment as follows: Parameter SDCCH Dynamic Allocation Allowed Idle SDCCH Threshold N1

Cell SDCCH Channel Maximum TCH Minimum Recovery 2012-12-12

Description If this parameter is set to YES, the dynamic conversion between the SDCCH and the TCH is enabled. According to the channel assignment algorithm, when the number of idle SDCCHs in the cell is smaller than or equal to the value of this parameter, the system tries to find a TCHF that can be converted into an SDCCH. This parameter is one of the conditions for the dynamic conversion from TCHF to SDCCH. According to the channel assignment algorithm, the system determines whether the number of SDCCHs in the cell after the conversion exceeds the value of Cell SDCCH Channel Maximum before initiating the dynamic conversion from TCH to SDCCH. If yes, the conversion is not initiated. This parameter specifies the minimum time for the recovery of a TCH from the SDCCH. Huawei Technologies Proprietary

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Time(s)

4. Related timers and their recommended configurations are as follows: 

T3101: This timer is used for immediate assignment procedure monitoring. Reduce the value of this parameter properly to minimize the congestion caused by dual assignment of the SDCCH. If the length of this timer is too long, the invalid usage of signaling resources is increased. To enhance the usage of the signaling resources, you should shorten the length of this timer, especially when the queuing function is enabled.



T3122: This timer is started by the MS when the MS receives the IMMEDIATE ASSIGN REJECT message. The MS can send a new channel request message only after T3122 expires. If the length of T3122 is too short, the MS may send the channel request message frequently when no system resources are available. This increases the load on the RACH and CCCH.



T3212: This timer is used for the periodic location update. Increase the length of this timer properly to reduce the load on the SDCCH brought by periodic location update.



T3111: This timer delays the deactivation of channels after main signaling links are disconnected to reserve time for repeated link disconnection. T3111 is started during the disconnection of both TCH and SDCCH. The value of T3111 must be the same as the value of T3110 at the MS side. Generally, they are set to 2 seconds. If the length of T3111 is too long, the SDCCH congestion rate is increased.

5. RACH Min Access Level: If the value of this parameter is too small, much interference may access the network, thus leading to SDCCH congestion. If the value of this parameter is too large, it is probable that MSs cannot make the call even though the signal level is high. This parameter should be set on the basis of the actual receiver

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sensitivity of the BTS, the minimum access level of the MS, and the interference condition. 6. Late assignment: This function is set at the MSC side. If this function is enabled, the call is on the SDCCH before the called MS answers. This prolongs the time when the SDCCH is occupied and thus may cause SDCCH congestion. 7. RXLEV_ACCESS_MIN: If this level is too low, a large number of MSs may access the network, and SDCCH congestion may occur during call initiation or location update. In addition, interference signal may be mistakenly regarded as the random access signals. For details, see Case 3. 8. Tx-integer: When the network traffic is heavy, the success rate of immediate assignment is low if the sum of S and T is small. Thus, the value of T should be properly adjusted to make the sum of S and T greater. For details about the value of S, see the description of Tx-integer. 9. Cell Bar Access (CBA): 0: cell access allowed; 1: cell access prohibited. Together with CBQ, CBA determines the priority of a cell. For details, see the GSM 04.08 protocol. CBQ

CBA

0 0 1 1

0 1 0 1

Cell Selection Priority Normal Prohibited Low Low

Cell Reselection Priority Normal Prohibited Normal Normal

10. Cell Bar Qualify (CBQ): CBQ takes effect during cell selection rather than cell reselection.

3.2.4 MS Problem Analysis 3.2.4.1

Checking the Problem Symptoms

Objective: to check the problems that exist on site and solve the problems caused by poor air interface quality and incorrect parameter settings.

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The symptoms are listed as follows: Symptom 1: The number of failed SDCCH seizures due to timeout is almost equal to the number of location updating requests minus the number of link establishment indications for location updating. Symptom 2: The problem happens discontinuously during both busy hours and idle hours. Symptom 3: The call service is normal. Except for low immediate assignment success rate in a few cells during certain periods, the KPIs and drive test result are normal, and no call drop complaint is received. Because when an MS fails in location update, it may try to access other cells or try to access the cell again periodically. Symptom 4: No interference and no cells with the same BCCH frequency and BSIC exist. Symptom 5: No abnormality such as uplink-downlink imbalance exists. Symptom 6: The result of signaling analysis on the Abis interface shows that the number of request retransmissions and the retransmission interval of the failed location updates meet the configuration requirements. If all the previous symptoms exist, you can infer that the problem is low immediate assignment success rate caused by abnormal location update.

3.2.4.2

Problem Handling

As the problem of low immediate assignment success rate is caused by some MSs, and the MSs cannot be located because no layer-3 information is provided in location update procedure, this problem cannot be solved on the network side currently. To improve the user satisfaction, do as follows: (1) Reduce the impact of abnormal MS location updates on the network KPIs. (2) Communicate thoroughly with customers. Problem handling: If any of the previous six symptoms does not exist, initiate the procedures for network optimization and troubleshooting. To perform network optimization according to the RSL signaling, do as follows: 1. If the value of TA in the location update signaling is high and the access level is 2012-12-12


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low during the time when the problem exists, do as follows: 

Adjust the following parameters: Max TA, RACH Min Access Level, and Txinteger (increase S and T).



Increase the length of the following timers: T3101, T3122, T3212, and T3111.



Adjust the cell selection priority by setting the value of CBA and CBQ.

2. For the sites located on the edge of location areas, do as follows to optimize the location update strategy: Adjust the parameter Cell Reselection Hysteresis and adjust the cell selection priority by setting the value of CBA and CBQ. 3. Reduce the MS retransmission times (recommended value: 1) and reduce the impact of retransmission on the network KPIs. Data analysis: 1. For the analysis report of problem location, see the following attachment.

Analysis on Low Immediate Assignment Success Rate Caused by Abnormal MS Location Update-qiuwenfang.rar

Based on the previous analysis, we find that the channel requests received when the problem occurred were sent by some abnormal MSs, and the BTS responded and handled the requests correctly. The analysis is based on the uplink and downlink. The details are described as follows: 2. Uplink – channel requests from the MS (1) When the cell access is prohibited, the immediate assignment success rate of the cell becomes normal. This indicates that the abnormal location updates are caused by the MS rather than the interference. (2) The location update requests are with high level, low error rate, and small TA (except for some sites with wide coverage area). This indicates that the abnormal location updates are initiated by the MS. (3) The number of location update requests sent by an MS complies with the settings of network parameters. This also indicates that the abnormal location updates are initiated by the MS. 3. Downlink – immediate assignment 2012-12-12


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(1) The immediate assignment commands are sent correctly, because the values of T1, T2, and T3 are the same as those in the channel requests. (2) Call access and the supplementary services such as short message are all normal. The problem only exists in the SDCCH access related to location update. This indicates that the immediate assignment commands are sent correctly. 4. Network data comparison (for the swapped offices and the offices with segmental networking) By comparing the data before and after swapping (for swapped offices) and the data of the surrounding cells (for the offices with segmental networking), we find that the problem also exists in the networks of other companies.

4 Test Method Immediate assignment success rate is a traffic measurement counter. As the formulae of this counter in different manufacturers and operators are different currently, you should record the value of other related counters and choose a proper formula according to the actual conditions.

Cases of Immediate Assignment Success

5

Rate Optimization 5.1

Case 1: Decrease in Immediate Assignment Success Rate Because of SDCCH Congestion Caused by Incorrect LAC Setting

Problem description The SDCCH congestion rate of cell 2 in S1/1/1 configuration was higher than 8%, and the immediate assignment success rate of that cell was 90%. Cause analysis and handling (1) The measurement counters of the TCH and SDCCH shows that the traffic volume on 2012-12-12


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the TCH is low, and the traffic volume per cell in busy hours is lower than 2.2 Erl; however,

there are many SDCCH seizure requests (3032 in busy hours), the traffic volume on the SDCCH is 1.86 Erl, and the congestion rate is higher than 8%. (2) The SDCCH is mainly occupied by the signaling before the call is established, the signaling during handover, the signaling for location update in idle mode, and the short messages. (3) As the traffic volume on the TCH, the number of TCH seizure requests (318), and the number of handover requests (146) are all normal, we infer that the high SDCCH seizure rate may be caused by too frequent location updates or too many short messages. (4) The LAC of the cell is 0500, but the LACs of other cells around this cell are all 0520. After the LAC of the cell is changed to 0520, the number of SDCCH seizure requests becomes 298, the traffic volume on the SDCCH becomes 0.27 Erl, the congestion rate decreases to 0, and the immediate assignment success rate reaches up to 95%.

5.2

Case 2: SDCCH Congestion Caused by a Burst of Location Updates

Problem description The immediate assignment success rate was low in a local network. Traffic statistics showed that the problem was mainly caused by SDCCH congestion on some sites.

Cause analysis and handling (1) Traffic statistics shows that the SDCCH is occupied for 300 to 400 times in busy hours. The congested cells are all in S1/1/1 configuration and each cell is configured with eight SDCCHs/8. In normal conditions, 300 to 400 times of SDCCH seizures are acceptable, but SDCCH congestion happens for tens of times in each cell. (2) Traffic statistics show that most of the SDCCH seizures are caused by location update. As most of the congested BTSs are located in the intersection areas of two location areas along the railway, we doubt that the SDCCH congestion may be caused by 2012-12-12


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a burst of location updates. (3) After checking the five-minute traffic statistics, we find that most of the location updates happened within a certain period of five minutes. The train timetable shows that four or five trains had passed within that five minutes. When the trains passed, a burst of location updates were performed during a short period of time, thus, the congestion happened. (4) For the BTSs located in the intersection areas of two location areas along the railway, it is recommended that the SDCCH dynamic assignment function be enabled and adequate SDCCHs be configured.

5.3

Case 3: Low Assignment Success Rate Because the RACH Minimum Access Level Is Set to 0

Problem description Subscribers of the cells controlled by a certain BSC complained that the success rate of random access was low.

Cause analysis and handling: (1) The clock status of the BTSs are normal. (2) The value of parameter Tx-integer (RACH Timeslot) is normal. By analyzing the data and traffic statistics, we find that the parameter RACH Min Access Level of all the cells with low assignment success rate is set to 0. The immediate assignment success rates of cells whose RACH Min Access Level are set to 1, 3, or 5, however, are around 99%. According to the previous data, we infer that the problem is caused by mistaken access. (3) If RACH Min Access Level is set to 0, there is no restriction on the random access; If RACH Min Access Level is set to 1, a call can access the cell only when the level is higher than –109 dBm. In Shantou Unicom, the RACH Min Access Level of most of the cells is set to 0 to improve the paging success rate. This causes a great number of mistaken access 2012-12-12


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and finally causes low assignment access rate. In addition, the parameter Random Access Error Threshold also has error restrictions on the access signals. (4) When RACH Min Access Level and Random Access Error Threshold of all the cells in module 1 of the BSC in Shantou Unicom are set to different values, the results are as follows: 

When Random Access Error Threshold and RACH Min Access Level are set to 200 and 1 respectively, the mean immediate assignment success rate of all the cells is 97.9% an hour later;



When Random Access Error Threshold and RACH Min Access Level are set to 200 and 0 respectively, the mean immediate assignment success rate of all the cells is 91.92% an hour later;



When Random Access Error Threshold and RACH Min Access Level are set to 180 and 1 respectively, the mean immediate assignment success rate of all the cells is 97.24% three hours later. After discussing with the customer, we decide to set Random Access Error Threshold and RACH Min Access Level of all the cells to 180 and 3 respectively. In this way, mean immediate assignment success rate of all the cells is higher than 98.6%.

5.4

Case 4: Low Immediate Assignment Success Rate in Some Cells Because of MS Error

Problem description The personnel on site reported that the immediate assignment success rate of some cells was low.

Cause analysis and handling (1) As the problem exists only in some cells, it may be caused by the MS error. (2) Traffic statistics of the cells with the problem show that the number of link 2012-12-12


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establishing failures during location updates is approximately equal to the number of times that the setup indication timer expires, as shown in the following figure. This indicates that the SDCCH setup failure is mainly caused by location updates.

(3) The signaling of the cells with problem shows that generally the value of TA is small and the level is high. This indicates that the problem is not caused by interference. But after the BSS sends the immediate assignment command and waits for the MS to access, the uplink signal strength detected by the BTS is always lower than –110 dBm. This indicates that the MS does not report the link establishing indication and thus causing the immediate assignment failure.

In addition, we check whether location updates are initiated by a same MS and whether the number of location updates is in compliance with the network setting. The following figure shows the signaling captured in the local network. As can be seen, the 2012-12-12


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location updates are mainly initiated by a same MS and the maximum number of retransmissions of the location update request set on the network is five.

The parameters associated to channel request are configured as follows:

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(4) Cell bar access (CBA) status modification test. After the CBA of a cell is enabled, the system information of the cell will carry a flag bit. The MSs that receive such system information will not consider that cell during cell reselection, and the number of location update requests to the neighboring cells increases. After the CBA is enabled, the immediate assignment KPIs become better, as shown in the following figure.

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(5) For a swapping project, check whether the problem exists in the network before swapping. If the problem existed, then we can infer that the problem is mainly related to the MS.

6

Onsite

Information

that

Needs

to

Be

Obtained 1. Traffic statistics of the network during one week before and after the problem occurred. BSC32: (1) All the counters of the Access Capability Measurement 1 and Access Capability Measurement 2 in BSC Overall Performance Measurement; (2) All the counters of Receive Quality Performance Measurement; (3) All the counters of the SDCCH Assignment Performance Measurement and TCH Assignment Performance Measurement in Channel Assignment Performance Measurement;

(4) All the counters of SDCCH Performance Measurement, TCH Performance Measurement, and Random Access Performance Measurement in Cell Performance Measurement.

BSC6000: (1) All the counters of Access Measurement per BSC in BSC Measurement; (2) All the counters of Interference Band Measurement per TRX and Receive Quality Measurement per TRX in MR Measurement; (3) All the counters of Channel Configuration Measurement per Cell in Channel Measurement; (4) All the counters of KPI Measurement per Cell, Immediate Assignment Measurement per Cell, and Flow Control Measurement per Cell in Call Measurement. 2012-12-12


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2. Data configuration and OMC data operation logs before and after the problem occurred. 3. Data configuration file and engineering parameters table. 4. Whether the network optimization operations, such as antenna adjustment, frequency adjustment, and location area adjustment, are performed after the problem occurred. 5. Alarms generated before and after the problem occurred, including commissioning alarms.

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08 GSM BSS Network KPI (Immediate Assignment Success Rate) Optimization Manual

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