omo333010 bsc6900 gsm call drop problem analysis issue 1.01.pdf

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Copyright 2011 Huawei Technologies Co., Ltd. All rights reserved.

GSM Call Drop

Problem Analysis


Foreword

A too high call drop rate adversely affects the users

experience.

Hence call drop analysis is at the heart of optimization.

The following training addresses the call drop analysis in

depth from the relevant KPIs to practical case studies.


Objectives

Upon completion of this course, you will be able to:

Describe the definition and classification of call drop

Outline the analysis procedure of call drop

Performance the checklist of call drop

Discuss call drop cases


Contents

1. Call Drop Definition and Classification

2. General Troubleshooting Idea of Call Drop Problems

3. Call Drop Troubleshooting Checklists

4. Call Drop Cases

Page4


Call Drops Definition

A call drop indicates that a call ends unexpectedly. During a call drop, the

service of the subscriber is interrupted because the BSC or MS releases the

business channel.

Statistics of call drops by counters: Upon receiving the Connection Failure and

Error Indication messages from the BTS, the BSC sends the Clear Request

message to the MSC to apply for disconnection. In this case, the number of call

drops is counted according to the reason for call drops.

MS BTS BSC MSC

Error Indication

Connection Failure

Clear Request Measurement

point

Page5


Typically, most call drops in a network occur over radio interface (CM33C call drops).

These call drops are classified into call drops that occur in the stable state and call drops

that occur during handover and should be given special attention.

Other types of call drops seldom occur in the network, among which, pay attention to

CM334 and CM333. The loopback function is seldom used in the network, therefore,

loopback call drops seldom occur.

Classification of Traffic Call Drops

CM33:

Call Drops on Traffic Channel

CM33C: Radio

Interface

CM330:

Stable State

CM331:

Handover State

CM332: No MR from MS for a Long

Time

CM333:Abis Terrestrial

Link Failure

CM334: Equipment

Failure

CM335: Forced

Handover

CM397 CM385

Page6


CM33C:

Radio Interface

CM330:

Stable State

CM3300: (Error

Indication)

CM3301: (Connection

Failure)

CM3302: (Release

Indication)

CM331:

Handover State

H3027Ca: (Internal Intra-Cell

Handovers)

H3028Ca: (Internal Intra-Cell

Handovers)

H3127Ca: (Outgoing Internal

Inter-Cell Handovers)

H3128Ca: (Outgoing Internal

Inter-Cell Handovers)

H3327Ca: (Outgoing External Inter-Cell

Handovers)

H3328Ca: (Outgoing External Inter-Cell

Handovers)

Classification of Call Drops That Occur over Um

Interface

CM3300:Call Drops on Traffic

Channel in Stable State (Error

Indication)

M3100A:Call Drops due to ERR IND Received on TCHF (Traffic Channel) in Stable State (T200 Expired)

M3100B:Call Drops due to ERR IND Received on TCHF (Traffic Channel) in Stable State (Unsolicited DM Response)

M3100C:Call Drops due to ERR IND Received on TCHF (Traffic Channel) in Stable State (Sequence Error)

M3200A:Call Drops due to ERR IND Received on TCHH (Traffic Channel) in Stable State (T200 Expired)

M3200B:Call Drops due to ERR IND Received on TCHH (Traffic Channel) in Stable State (Unsolicited DM Response)

M3200C:Call Drops due to ERR IND Received on TCHH (Traffic Channel) in Stable State (Sequence Error)

CM3301:Call Drops on Traffic

Channel in Stable State

(Connection Failure)

M3101A:Call Drops due to CONN FAIL Received on TCHF (Traffic Channel) in Stable State (Radio Link Failure)

M3101B:Call Drops due to CONN FAIL Received on TCHF (Traffic Channel) in Stable State (HO Access Failure)

M3101C:Call Drops due to CONN FAIL Received on TCHF (Traffic Channel) in Stable State (OM Intervention)

M3101D:Call Drops due to CONN FAIL Received on TCHF (Traffic Channel) in Stable State (Radio Resource Unavailable)

M3101E:Call Drops due to CONN FAIL Received on TCHF (Traffic Channel) in Stable State (Other Causes)

M3201A:Call Drops due to CONN FAIL Received on TCHH (Traffic Channel) in Stable State (Radio Link Failure)

M3201B:Call Drops due to CONN FAIL Received on TCHH (Traffic Channel) in Stable State (HO Access Failure)

M3201C:Call Drops due to CONN FAIL Received on TCHH (Traffic Channel) in Stable State (OM Intervention)

M3201D:Call Drops due to CONN FAIL Received on TCHH (Traffic Channel) in Stable State (Radio Resource Unavailable)

M3201E:Call Drops due to CONN FAIL Received on TCHH (Traffic Channel) in Stable State (Other Causes)

CM3302:Call Drops on Traffic Channel

in Stable State (Release Indication)

M3102:Call Drops due to REL IND Received on TCHF (Traffic Channel)

M3202:Call Drops due to REL IND Received on TCHH (Traffic Channel)

Page7


Example of Proportions of Various Types of

Call Drops

In one practical network, the distribution of various types of call drops is

counted as follows:

Call drops that occur over Um interface occupies 98.21%.

Among call drops that occur

over Um interface, many call

drops result from connection

failure CM3301. Among this

type of call drops, call drops

due to radio link

failure(M3101A and M3201A)

occupies the largest proportion.

Page9


Analysis of Proportions of Various Types of

Call Drops

The proportion of a type of call drop can be counted as follows:

(A type of call drop)% = this type of call drop/CM33.

For example, (call drops due to equipment failure) %= CM334/CM33

The call drop rate is counted in two modes

Call drop rate (including handovers)

=CM33/(K3013A+K3013B+K3023)

Call drop rate (excluding handovers)

=CM33/(K3013A+CH323+CH343-CH313-CH333)

Download the call drop counters during busy hours in a

week.( the counters are list on the right table)

Page10


Other KPIs and Related Data Configuration

Detail Call Drops Reasons Counters on Radio Interface %

M3030A: Call Drops on TCH(TA)

M3030B: Call Drops on TCH(Uplink Received Level)

M3030C: Call Drops on TCH(Downlink Received Level)

M3030D: Call Drops on TCH(Uplink and Downlink Received Level)

M3030H: Call Drops on TCH(Uplink Quality)

M3030I: Call Drops on TCH(Downlink Quality)

M3030J: Call Drops on TCH(Uplink and Downlink Quality)

M3030K: Call Drops on TCH(Other)

Page11


Exercises

Please analysis traffic statistic

results files(listed in the notes) of

M2000, and answer the following

questions:

1. Please analysis the date on 24th

August 2011, and fill out the

corresponding percentage of

different counters in the two

tables in the right.

2. Please identify the most possible

reason resulting in the call drop

on 24th August 2011 basing on

the result of question1 .

Page12


Contents




4. Call Drop Cases

Page13


Call Drop Overall Procedure

Does Call drop disappear? No

Yes

End

Detail Call drop reasons

basing on data sources

Filter TOP cells/areas

Perform the checklists

Suggest the solutions

Begin

Process the problem

Chapter 3. Call Drop

Troubleshooting Checklists Refer to

Explained in this chapter

Page14


Filter TOP Cells/Areas

Rules for Filtering TOP Cells

Rule 1: DCR

Put cells in DCR order. Select cells

with the DCR greater than the overall

DCR

Rule 2: Number of call drops

Put cells in DCR order. Select the cells

with the number of call drops greater

than the average number of call .

Filter cells by both DCR and the number of

call drops.

Page15


Filter TOP Cells/Areas

After calculating the DCR of each cells, rank them in the descending order of

DCR(excluding handovers) to get the TOP cells.

Analyze whether a TOP area exists.

Calculate the proportion of different type of DCR of each TOP cell and

determine which type of call drops percentage is the highest one in each cell.

Focus on the specific type of call drop in each cell and analyze accordingly.

For example:

The reasons are the

result of the

analysis of the Call

Drop Measurement

per Cell

Page16


Detail Call Drop Reasons

Get the detail call drop

reasons basing on the

following data source

Traffic

Statistics

Signaling

Trace

Drive

Test

Detail Call Drop Reasons

Page17


Detail Call Drop Reasons by Traffic Statistics

Download the counters related to call drops

and calculate the DCR. Achieve this via

PRS if available.

Calculation of the DCR for each cell

according to the table on the right.

Filter TOP cells according to the DCR,

considering the number of calls at the same

time, because a cell where few calls are

made has small effect on the entire network

even it has a high DCR.

Calculate the DCR of the entire network

after removing the TOP cells and determine

whether the call drop problem is caused by

TOP cells.

Page18


Detail Call Drop Reasons by Signaling

Trace(1/2) Two ways to enable the signaling trace: in M2000 or Web

LMT

The Signaling Trace in M2000

Page19


Detail Call Drop Reasons by Signaling

Trace(2/2) Two ways to enable the signaling trace: in M2000 or Web

LMT

The Signaling Trace in Web LMT

Page20


Detail Call Drop Reasons by DT(1/2) The problem of DT call drop is basically consistent with that of traffic call drop. The only

difference is that DT data cannot be classified according to the call drop reason.

According to the DT data or the log, analyze what causes call drops and count the DCR of

each cells. Then, filter the TOP cells/TOP area according to the DCR of each cells with the

help of the geographical display function of the MAPInfo.

Neighboring cell

relationship

Handover

parameters

Interference

problems

Cross

coverage

Terminal

problems

DT call drops

Due to co-

frequency/adjacent-

frequency

interference, inter-

network interference,

or intermodulation

interference of

equipment, call

drops due to bad

quality occur.

Due to missing

configuration of

neighboring cell

relationship, handovers

cannot be triggered in

time. Due to redundant

neighboring cell

relationship, wrong

handovers occur. This,

then, causes call drops.

Due to improper

configuration of

handover

parameters,

handovers cannot

be triggered in

time or wrong

handovers occur.

This, then, causes

call drops.

As some terminals

do not decode the

level and BSIC of

neighboring cells

in time, handovers

cannot be

triggered in time.

This, then, causes

call drops.

Due to the

geographical

location and

terrain of the

site, cross

coverage is

caused and

then call

drops occur.

Page21


Detail Call Drop Reasons by DT(2/2) The information such as receiving level, quality, CIR, usually used to analyze

call drop , are easily to be got by reviewing the history log files via software

TEMS, Probe

Compare with the signaling and traffic statistics, more information about

downlink such as the neighbor cells in BA2 is easily to be got to analyze the

neighbor cells missing, interference, handover etc.

Page22


Contents




4. Call Drop Cases

Page23


Basic Process of Troubleshooting Call Drop

Problems

In a normal network, most call drops over Um interface usually exist in only few cells .This type of call drops may be caused by

improper parameter settings, cross coverage, improper neighboring cells relationship, interference, and poor coverage.

Call drops over none radio interface seldom occur. When such call drops occur, just analyze the TOPN cell.

Call Drop Problems

Analysis of Proportions of Various

Types of Call Drops

Proportions of CM33C is

high

hard

ware

failu

re tra

nsp

ort fa

ilure

alarm information

parameters

interference

coverage

Proportions of

CM334 and CM333

is high

Channel C

onve

rsion

Mutu

al a

id o

f TR

Xs

Yes

No

Yes

No

entire network /some cell

BSC parameters

core network parameters

network planning

software version problems

TOP cellentire

network

Analysis of Proportions of Various

Types of Call Drops

hardware failure transport failure

neighboring cells

frequency

Page24


Troubleshooting for Call Drops over Um

Interface in the Entire Network The reasons for call drops over Um interface in a large proportion in the entire network

Usually caused by improper parameter settings.

In this case, check the settings of various parameters according to the

recommended value.

Lack of coordination between core network parameters and BSC parameters.

Improper network planning and frequency planning of the entire network.

Call drop in the

entire network

Checking

special

scenario

Checking

core network

para.

Checking cell

network para.

Checking

freq.

planning

T305, T306, T308, T310, T313, T301, T303, etc.

SACCH multi-frame, RLT, T200, N200, T3101A, T3101C,

T8, threshold of different HO, P/N, CS/PS RACH Min.

Access Level

Checking Co-channel or adjacent-channel via Nastar. if

necessary, replanning frequency, or enable PC 3.5 algrithm,

EICC and so on.

For Co-BCCH, confirm call drop is in overlaid or underlaid,

and adjust the assignment and HO para. For AMR, adjust

relative para.

Page25


Troubleshooting for Call Drops over None Um

Interface in the Entire Network For call drops over none Um interface, pay attention to CM333 and CM334.

For CM333, determine whether they are caused by links problem on the Abis interface

according to the proportion and number of call drops.

Disconnection of RSL links may also cause CM333 call drops.

For CM334, determine whether they are caused by equipment failure according to the

proportion and number of call drops.

The following factors may cause increase of call drops due to equipment failure:

Mutual aid of TRXs, dynamic modification of cell attributes, dynamic modification of frequencies

on TRXs, dynamic modification of frequency hopping data of TRXs, dynamic deletion of cells,

dynamic deletion of TRXs, blocking of cells, blocking of TRXs, blocking of channels

If CM333/CM334 increase, check the related alarm

information, locate the problem according to the alarm

information. If necessary, troubleshoot transmission

equipment and hardware on site.

Alarm Name LAPD Link Fault Transmission LAPD Link Interrupt Alarm E1/T1 Local Alarm E1/T1 Remote Alarm Indication Signal E1/T1 Loss of Signal(LOS) E1/T1 Loss of Frame Alignment(LOF) TRX communication alarm TRX VSWR alarm DRU Hardware alarm

Page26


Check Procedure for TOP Cells

TOP Cell/Area

Filtering Find what are TOP cells and determine whether a TOP area exists

Alarm

Check Check the related alarm information

Engineering

Check

Mainly check whether the RF Tunnel quality is eligible

Parameter

Check

Check the parameters according to actual situation

Interference

Check

Check whether internal/external interference/

intermodulation interference exists

Coverage

Check

Check whether any poor coverage area or any blind coverage

area exists

Neighboring

Cell Check

Check the relationship between the neighboring cell and the

serving cell

Page27


Usually, an alarm is reported when hardware and transport problems occur.

Sometimes, however, no alarm is reported when these problems occur. In this

case, analyze the traffic statistics in the tables in notes

Counters Related to Hardware and

Transmission

TRX Usability


Engineering Check: RF Tunnel Faults

Check

Principle

Normally there should be no great difference between the main and

diversity levels of each TRX.

By analyzing the difference between the main and diversity levels, sort

out the TRXs with comparatively great difference between main and

diversity levels.

Then detect specific fault points of an antenna feeder by referring to

the data configuration and distribution rules of the TRXs that have

great difference between the main and diversity levels.

Page30


Engineering Check: RF Tunnel Faults Check

Traffic measurement systems vary with NE types (such as BSC6000 and

BSC6900) and data sources.

NE Version Source Counters ID Calculation

BSC6000 All versions Traffic measurement

data of the entire

network from the

M2000

S4501, S4502 and

S4503

Main level (dBm) = 10 * log10

(S4502/S4501) - 120

Diversity level (dBm) = 10 * log10

(S4503/S4501) - 120

BSC6900 Versions

before

BSC6900V900

R011SPC720

Traffic measurement

data of the entire

network from the

M2000

S4556 and S4557 Main level (dBm) = 10 * log10

(S4556) - 120

Diversity level (dBm) = 10 * log10

(S4557) - 120

BSC6900V900

R011SPC720

and later

versions

Traffic measurement

data of the entire

network from the

M2000

S4556 and S4557 The same as above

Page31


Basic Steps

Step 1 Analyze the data by running the macro in the tool after collecting busy hour traffic

measurement data in the last one or two days.

Step 2 The analysis results indicate the difference between the main and diversity levels of

each TRX based on BSC cells, and mark TRXs with a greater difference by using different

colors, as shown in the following figure.

Step 3 To analyze a specific fault point, refer to data configuration and summarized rules.

Engineering Check: RF Tunnel Faults Check

Ais of Dif

15.7205

10.3244

Page32


Case1: Connecting Line between TRX to the

DFCU/DDPU Fails

Regular Summary: For some TRXs on the DFCU/DDPU,

the main antenna level is more than 6 dB higher than the

diversity antenna level (for dual-antenna sites)

Typical case:

TRX No.502 in cell B. The fault

occurs because the RF connector on

the DFCU that is connected to

No.502 TRX comes off.

Cell TRX Main level Diversity level Dif of Main and Div Abs of Dif

Cell Label=B

TRX Index=502 -84.1285807 -95.4815181 11.35293745 11.35293745

TRX Index=503 -82.7684919 -84.9996951 2.231203188 2.231203188

TRX Index=509 -83.795518 -84.4110702 0.615552168 0.615552168

TRX Index=510 -83.311275 -84.7116505 1.400375472 1.400375472

Connector loosed

11.35293745

Page33


Case2: The Connecting Line Between Two

MRFUs on the BTS3900 Fails Typical case

There are the two modules in one cell, for all TRXs of one module, the main antenna

level is more than 12 dB higher than the diversity antenna level. For the other module,

the levels of the main and diversity antennas are normal.

Connector loosed

Through detection, it is discovered that

the receiving line between the two

modules is not connected. After

connecting the receiving line, the

difference between the main antenna

level and the diversity antenna level

restores to normal.

Cell TRX Main level Diversity level Dif of Main and Div Abs of Dif

Cell Label=A

TRX Index=195 -73.1199495 -75.844143 2.724193511 2.724193511

TRX Index=196 -75.3536322 -76.2385249 0.884892689 0.884892689

TRX Index=215 -75.9703878 -109.387862 33.41747396 33.41747396

TRX Index=216 -75.1245195 -107.852813 32.72829337 32.72829337

33.41747396

32.72829337

Page34


Parameter /Interference/Coverage Checking Parameter check

For details, see page before(Troubleshooting for Call Drops over Um Interface in the Entire Network).

Interference checking

Whether severe interference exists according to interference bands 4&5 at intervals.

According to the DT result, analyze whether internal or external interference exists.

Coverage checking

Analyze the traffic statistics and find whether problems such as high proportion of great TAs, imbalance

between uplink and downlink, and high proportion of low levels exist.

According to the DT result, determine whether poor coverage areas exists or not.

Page35


Neighboring Cells Check

Methods for Optimizing Missing

Neighboring Cells

Perform the optimization based on

the following KPIs H370c:Outgoing Inter-Cell

Handover Requests H380:Incoming Inter-Cell

Handover Requests H375B: Outgoing Inter-Cell

Handover Fail Reconn Fail

Perform the optimization based on

MR data, including the following KPIs

S371:Measurement Reports with

Signal Strength Difference

Between Neighbor Cell and Serving

Cell Greater Than Relative Level

Threshold

S372:Measurement Reports with

Signal Strength of Neighbor Cell

Greater Than Absolute Level

Threshold

S3013:MRs of Serving Cells

Perform neighboring cell optimization

based on topology structures

Perform neighboring cell optimization

Method for Optimizing

Redundant Neighboring Cells

Page36


Practice

Please analyze the traffic statistic results files(listed in the

notes) of M2000 and answer the following questions:

1. Which cell(or cells) has(or have) the call drop problem?

2. Whats the most possible reason resulting in the call

drop of the cell identified in question1?

Page37


Contents




4. Call Drop Cases

Page38


Case 1 Impact of Traffic Sharing on a Dual-Band Network on Call Drops

Fault description

After network swapping at a PT site in country P, the call

drop rate does not meet the requirement. The analysis

shows that the call drop rates are different between

GSM900 and DCS1800 and the handover parameter settings

are inappropriate. The call drop rate decreases significantly

after the traffic on GSM900 and DCS1800 is balanced by

adjusting parameter settings.

Page39



Problem Analysis

Step 1: Determine the scope of the call drop problem.

According to the analysis of traffic statistics, the call drop

problem occurs on the entire network and in top N cells, most of

which work on GSM900.

Step 2: Analyze the call drop causes.

According to the analysis of traffic statistics, most call drops are

caused by Um interface problems due to high levels and poor

quality.

Page40



Required Action Data Analysis Result Conclusion

Check all parameters

Calls are difficult to hand over from GSM900 to

DCS1800 because some parameters such as Inter-

layer HO Hysteresis are set inappropriately. After network

swapping, network

expansion, network

deployment, or

UMTS900 refarming,

the call drop rates

differ greatly between

GSM900 and

DCS1800 due to

inappropriate

parameter settings.

Check network

coverage (power

matching and newly

deployed sites)

Traffic statistics: The proportion of call drops due to high

levels and poor quality is high.

DT data: The downlink interference is strong.

Analyze changes in

traffic and KPIs in

special scenarios

(networks supporting

GSM900 and DCS1800

and configured with co-

BCCH)

Traffic statistics: The receive quality and call drop rate

differ greatly between GSM900 and DCS1800. In

addition, the proportion of call drops due to high levels

and poor quality is high for cells working on GSM900.

DT data: The downlink interference is strong in cells

working on GSM900.

Step 3: Take required actions.

Page41



Step 4: Troubleshoot call drops and evaluate the result.

Required Action Conclusion Solution Evaluation

Check all parameters

After network swapping, network expansion, network deployment, or UMTS900 refarming, the call drop rates in the cells working on GSM900 increase significantly because of heavy load and strong interference caused by inappropriate parameter settings.

Transfer some traffic from GSM900 to DCS1800 by adjusting the settings of parameters such as Inter-layer HO Hysteresis.

The call drop rate decreases significantly after the parameter settings are adjusted. See the following figure.

Check network coverage (power matching and newly deployed sites)

Analyze changes in traffic and KPIs in special scenarios (networks supporting GSM900 and DCS1800 and configured with co-BCCH)

Page42


Case 2 Call Drops Due to Inappropriate Parameter Settings

Fault description

The call drop rate in a cell is high because calls are likely to drop after

being handed over to the cell from cells served by another BSC. This

problem is caused by inappropriate settings of inter-BSC handover

parameters.

Problem Analysis


The call drop problem occurs in top N cells.



caused low levels over the Um interface.

Page43


Case 2 Call Drops Due to Inappropriate Parameter Settings Step 3: Take required actions.

Step 4: Troubleshoot call drops and

evaluate the result.

Required Action

Data Analysis Result Conclusion Solution Evaluation

Check all

parameters

Traffic statistics: Most call drops are caused low levels over

the Um interface.

DT data: A call initiated in cell A is handed over to cell B where

the level is low. As a result, the call drops after the handover.

According to the moving direction of the DT car, the call

should be handed over to cell C where the level is higher than

that in cell B.

Parameters: Cell A and cell C are served by the same BSC

and cell B is served by another BSC. For cells served by the

same BSC, Inter-layer HO Threshold is set to 63 and bit 14

of the 16-bit priority is set to 1. For the neighboring cells

served by another BSC, Inter-layer HO Threshold is set to

25. In this case, if the level is greater than the sum of Inter-

layer HO Threshold and HO Hysteresis, bit 14 of the 16-bit

priority is set to 0. On the current network, if the level of a

neighboring cell served by another BSC is greater than 82 dBm, the neighboring cell has a higher priority than the

neighboring cells served by the same BSC as the serving cell.

As a result, calls in the serving cell are first handed over to the

neighboring cell served by another BSC.

On the current

network, if the level

of a neighboring cell

under another BSC

is greater than 82 dBm, this

neighboring cell has

a higher priority

than the

neighboring cells

served by the same

BSC as the serving

cell. As a result,

calls in the serving

cell are first handed

over to the

neighboring cell

served by another

BSC. This handover

mechanism is

inappropriate.

Adjust the

setting of

Inter-layer

HO

Threshold

for the

neighborin

g cells

served by

another

BSC.

The call drop

rate

becomes

normal.

Analyze

neighboring

relationships

(for newly

deployed sites

and areas

where Huawei

devices are

interconnected

to devices from

other vendors)

Check network

coverage

(power

matching and

newly deployed

sites)

Page44


Case 3 Call Drops Due to Cross Coverage

Fault description

The call drop rate in a cell is high and the proportion of call drops due

to low levels is high. This problem is caused by cross coverage. If

cross coverage occurs, calls are not handed over in time and

therefore drop when MSs move to street corners.

Problem Analysis






Page45


Case 3 Call Drops Due to Cross Coverage Step 3: Take required actions.

Step 4: Troubleshoot call drops

and evaluate the result.

Required Action Data Analysis Result Conclusion Solution Evaluation

Check RF tunnels for main

and diversity faults, cross

connections, and

interference

Acceptable

Check for device faults and

alarms Acceptable

Check all parameters Acceptable

Analyze neighboring

relationships (for newly

deployed sites and areas

where Huawei devices are

interconnected to devices

from other vendors)

Traffic statistics: Most call drops

are caused low levels over the

Um interface.

DT data: A call in cell A is not

handed over to cell B when the

MS is moving to a street corner.

After the MS passes through the

street corner, the call drops

because the level decreases and

the network quality becomes

poor.

The call is not

handed over to

cell B in time

because cross

coverage occurs

in cell A.

Adjust the

antenna tile

and azimuth

of cell A to

control the

coverage.

The call drop

rate becomes

normal.

Check network coverage

(power matching and newly

deployed sites)

Page46


Case 4 Call Drops Due to Missing Neighboring Cells

Fault description

The call drop rate in a cell is high and the proportion of call drops

due to low levels is high. This problem is caused by missing

neighboring cells.

Problem Analysis






Page47


Case 4 Call Drops Due to Missing Neighboring Cells





Check RF tunnels for main

and diversity faults, cross

connections, and

interference

Acceptable

Check for device faults and

alarms Acceptable


Analyze neighboring

relationships (for newly

deployed sites and areas

where Huawei devices are

interconnected to devices

from other vendors)

Traffic statistics: Most call drops are

caused low levels over the Um

interface.

DT data: After the test MS in cell A

passes through the corner of the street,

the network quality becomes poor. The

call initiated by the test MS fails to be

handed over to cell B because cell B is

not configured as a neighboring cell of

cell A and therefore drops. After the call

drops, the test MS initiates another call

and accesses cell B.

The call fails to

be handed over

from cell A to

cell B because

cell B is not

configured as a

neighboring cell

of cell A and

therefore drops.

Add a

neighboring

relationship

between cell

A and cell B.

The call drop

rate becomes

normal.

See the

following figure. Check network coverage

(power matching and newly

deployed sites)

Page48


Case 5 Call Drops Due to Coverage Changes Caused by Addition of

Combiners Fault description

After network swapping at site A, the call drop rates are high in the cells

covering two tunnels. After the network swapping, the network coverage

decreases because some combiners are added to BTSs. As a result, the call

drop rates increase. After the power is adjusted, the call drop rates become

normal.

Problem Analysis




According to the analysis of traffic statistics, most call drops are caused

low levels over the Um interface.

Page49



Check for device faults and alarms

Acceptable


Analyze neighboring relationships (for newly deployed sites and areas where Huawei devices are interconnected to devices from other vendors)

Acceptable

Check RF tunnels for main and diversity faults, cross connections, and interference

Some combiners are added to BTSs, but the power of the BTSs is not increased. After the network swapping,

some combiners are added to BTSs but the power of the BTSs is not increased. As a result, the network coverage becomes weak.

Increase the power of the BTSs to which combiners are added.

The network coverage and call drop rates become normal. Check network coverage

(power matching and newly deployed sites)

Traffic statistics: Most call drops are caused low levels over the Um interface.

DT data: The network coverage becomes weak after the network swapping.

Case 5 Call Drops Due to Coverage Changes Caused by Addition of Combiners




Page50


Case 6 Call Drops Due to Interference

Fault description

The call drop rate in cell 3 of a BTS is 10% accompanied

with high congestion rate, but call drop rate and congestion

rate in cell 1 and cell 2 are normal.

Page51



Analysis

Check the related traffic statistic

Check whether there is high interference band in TCH measurement

function.

Check the situation of call drop in call drop measurement function.

Check whether handover of the cell is normal.

Check whether there is interference through checking frequency

planning, moreover confirm whether there is external interference

with spectrum analyzer.

Drive test

Check the hardware

Page52



Troubleshooting

Block TRX in turn and the congestion rate is always quite high no

matter which TRX is blocked.

Check and analyze the traffic statistic, interference band and

traffic volume and call drop rate, and it is found that the

interference becomes more serious as the traffic gets high.

Change frequency. The frequency of cell 3 is changed to 1MHz

away from the original value. But the problem persists.

Judge whether the equipment is faulty.

Locate external interference.

Page53



Troubleshooting

Make a scanning test with a spectrum analyzer.

A suspect signal with 904.14 MHz center frequency, 300 kHz bandwidth is

found. It is similar to an analog signal and it exists continuously.

At the distributor output port of cell 1,2 and 3, the signal strength is 60 dBm,

40 dBm and 27 dBm respectively. It accords with the interference margin.

Traffic volume is higher in the day time than at night.

Now the problem is found: 904 MHz external interference source.

Conclusion: Reduce interference

Reduce internal interference through checking frequency planning.

After internal interference is excluded, we can locate external interference

with spectrum analyzer.

Page54


Case 7 Call Drops Due to Adjacent Relationship

Fault description

Subscribers complain that call drops often happen on the

5th floor and further up in a building.

Subscriber complaint is also an important source of information

about the network quality.

Page55



Analysis

Perform on-site test

There are call drops and noise on the site

The test mobile phone shows that before the call drop the serving

cell is BTS-B. But this building should be covered by BTS-A.

Check traffic statistic

BTS-B is about 9 kilometers away from this building. It is

determined that the BTS-B signal received in this area is coming

from some obstacles reflection. Thus an isolated island coverage

is formed in this area.

Page56



Analysis

Check data configuration

In BSC data configuration, BTS-A is not configured as the adjacency of

BTS-B

Cause analysis of call drop

When the MS uses the signal of cell 2 of BTS-B in this area, the signal of

cell 3 of BTS-A is strong. But cell 2 of BTS-B and cell 3 of BTS-A are not

adjacent, therefore, handover cannot happen.

The signal in cell 2 of BTS-B is the result of multiple reflections. When the

signal of BTS-B received by the mobile phone gets weak suddenly, an

emergency handover is needed. But there is no adjacent cell of BTS-B, so

call drops will occur.

Page57



Troubleshooting

Modify the data in BA1 table, BA2 table and add adjacent cell

relationship, set cell 3 of BTS-A as an adjacent cell of cell 2 of

BTS-B.

Optimize the network parameters to eliminate the isolated

island.

The test results show that the call drop problem is solved.

Conclusiontwo methods to solve isolated island problem

Adjust the antenna of the isolated cell, to eliminate the

isolated island problem.

Define new adjacent cells for the isolated cell. Page58


Case 8 Call Drops Due to HO Parameters

Fault description

In a drive test from A to B, it is found that there are many

call drops at entrance of a tunnel near a BTS due to slow

handover.

Page59



Analysis

The tunnel is near the BTS. When the MS enters the tunnel, the

power of the target cell is -80dBm. But the signal of source cell

goes down quickly to less than -100dBm. Before the MS enters the

tunnel, the downlink power of the two cells is good and no

handover is triggered. When the MS enters the tunnel, the level of

the source cell goes down rapidly. The call drop occurs before any

handover is triggered.

Think it over: How to solve problems of this type?

Page60



Troubleshooting

The adjusted parameters are listed below.

Parameter name Value before

change Value after

change

PBGT watch time 5 3

PBGT valid time 4 2

PBGT HO threshold 72 68

UL Qual. Thrsh. (Emergency handover)

70 60

Min. DL level on candidate cell

10 15

Page61



Conclusion: optimize and adjust handover parameters to

reduce call drop

On condition that there is no ping-pang handover and

excessive voice interruption, PBGT handover can help to

reduce interference and lower call drop rate.

Set emergency handover thresholds properly, and make

sure the emergency handover can be triggered in time

before the call drop so as to reduce call drops.

Page62


Case 9 Call Drops Due to BTS Hardware

Fault description

In the dialing test, many call drops are found in cell 2.

Analysis

Check the traffic statistic and find out that TCH congestion

rate of this cell is over 10% and internal inter-cell incoming

handover failure rate is high. It is found that one TRX board

of this cell is abnormal in OMC. A preliminary conclusion is

that TRX board problem causes the call drop.

Page63



Troubleshooting

Lock the frequency with a test mobile phone and perform

dialing test for many times. It is found that call drops only

happen in timeslots 1, 3, 5, 7 while communications in

timeslots 2, 4, 6, 8 are normal.

Move this board to another slot, and the problem still exists.

Move another good board to this slot, and the

communication is normal.

Move this defective board to other cabinet, the problem

arises.

When it is replaced, the communication is recovered. Page64



Conclusion

The BTS test should guarantee that communication should

be successful not only in each TRX but also in each timeslot

of the TRX.

It must be ensured that each TCH channel provides

bidirectional high quality communication.

Page65


Summary

What is classification of the call drop?

How to analyze the call drop generally?

What is the main reasons of the call drop and its

corresponding solution?

Page66

Thank you www.huawei.com

omo333010 bsc6900 gsm call drop problem analysis issue 1.01.pdf

Documents

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drop analysis

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drop cases copyright

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drop performance

drop problems