15- omo333010 bsc6900 gsm v9r13 call drop problem analysis issue 1.00

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

GSM Call Drop Problem Analysis


Page2

Foreword A too high call drop rate adversely affects the user’s

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 over the influential factors to practical case studies.


Page3

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

Describe the definition of call drop Outline the analysis procedure of call drop Troubleshooting cause of call drop Main solution methods to call drop Call drop cases analysis

Copyright © 2011 Huawei Technologies Co., Ltd. All rights reserved. Page4

Contents1. Call Drop Definition and Classification2. Specific Causes of Call Drops and its Solutions3. Call Drop Cases


Call Drop Definition

One of retainability KPIs.TCH Call Drop Rate refer to the ratio of

call drop to successful TCH seizures after the BSC successfully assigns TCHs to MSs.

TCH Call Drop Rate (Including Handover)TCH Call Drop Rate (Excluding Handover)

One of accessibility KPIs. SDCCH Call Drop Rate indicates the

probability of call drops when the MS occupies the SDCCH.

TCH call drop

SDCCH call drop


Measurement Point of SDCCH Call Drop Measurement point of SDCCH call drop during IMM ASS

M3000A~C:Call Drops due to ERR IND

M3001A~E:Call Drops due to CONN FAIL

M3002:Call Drops due to REL IND

M302:Call Drops due to No MRs from MS for a Long Time (SDCCH)

M303:Call Drops due to Abis Terrestrial Link Failure (SDCCH)

M304:Call Drops Due to Equipment Failure (SDCCH)

CM400~CM404:Number of Clear Requests Sent on the A Interface

M305:Call Drops due to Forced Handover (SDCCH)

K3003:Successful SDCCH Seizures (Call Type)

CM30A:Call Drops on Radio Interface (SDCCH)


Measurement Point of SDCCH Call Drop Measurement Point related with IMM ASS channel active

MS BTS BSC

CHAN REQ

CHAN RQD

CHAN ACT

CHAN ACT ACK

IMM ASS CMD

R3300A:Channel Activation Attempts in Immediate Assignment Procedure (SDCCH)

CHAN ACT NEGATIVE ACKR3300B: CHAN ACTIV NACK Messages Sent by

BTS in Immediate Assignment Procedure (SDCCH)

R3300C:Channel Activation Timeouts in Immediate Assignment Procedure (SDCCH)


Formula of SDCCH Call Drop RateZTR104C:Call

Drop Rate on

SDCCH

CM30:Call Drops on SDCCH

K3003:Successful SDCCH Seizures=

=

[CM30A:Call Drops on Radio Interface (SDCCH)] + [M302:Call Drops due to No MRs from MS for a Long Time (SDCCH)] + [M303:Call Drops due to Abis Terrestrial Link Failure (SDCCH)] + [M304:Call Drops Due to Equipment Failure (SDCCH)] + [M305:Call Drops due to Forced Handover (SDCCH)]

[R3300A:Channel Activation Attempts in Immediate Assignment Procedure (SDCCH)]-[R3300B:CHAN ACTIV NACK Messages Sent by BTS in Immediate Assignment Procedure (SDCCH)]-[R3300C:Channel Activation Timeouts in Immediate Assignment Procedure (SDCCH)] +[H3040:Internal Intra-Cell Handover Detection Messages Received by BSC (SDCCH)] +[H3240:Incoming Internal Inter-Cell Handover Detection Messages Received by BSC (SDCCH)] +[H3440:Incoming External Inter-Cell Handover Detection Messages Received by BSC (SDCCH)]

x 100%

x 100%


Definition of TCH Call Drop Rate Call Drop Rate on TCH indicates the ratio of the number of call

drops to the number of successful TCH seizures after the BSC successfully assigns TCHs to MSs.

Call Drop Rate on TCH contains the following aspects The probability of call drops after the establishment of TCHs The probability of call drops during call establishment

Call Drop Rate on TCH is one of retainability KPIs. It reflects the probability of call drops due to various reasons after the MSs access the TCHs properly. If Call Drop Rate on TCH is high, user experience is adversely affected.


Measurement Point during (Immediate) Assignment Procedure

MS BTS BSC MSCChannel_req Channel_Required

Channel_ActiveChannel_Active_Ack

IMMEDIATE ASSIGN COMMANDEstablish_IND(CM Service Req)First SABM

CM Service Accepted

CR(Complete_L3_information)CC

SetupCall Processing

ASSIGNMENT COMMANDFirst SABM Establish_IND

Channel_ActiveChannel_Active_Ack

Assignment_Req

Assignment_CMPASSIGNMENT CMP

AlertingConnect

Connect AckConversation

UA

UA

K3023:Successful TCH Seizures (Signaling Channel)

K3013A:Successful TCH Seizures (Traffic Channel)

Error Indication CM3300:Call Drops on Traffic Channel in Stable State (Error Indication) orCM3301:Call Drops on Traffic Channel in Stable State (Connection Failure) orCM3302:Call Drops on Traffic Channel in Stable State (Release Indication) CM332:Call Drops Due to No MR from MS for a Long Time (Traffic Channel)

CM333:Call Drops due to Abis Terrestrial Link Failure (Traffic Channel) CM334:Call Drops due to Equipment Failure (Traffic Channel) Clear Request

Connection Failure IndicationRelease IndicationDISC


Intra-BSC handover

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Measurement Point during Handover Procedure

MS Target BTS1 BSC Original BTS2 MS MSC

Channel ActiveChannel Active Ack

Measure Report

HANDOVER COMMANDHandover Access

Handover DetectPHYINFOPHYINFO

First SABMEstablish_IND

HANDOVER COMPLETE

UA CM335:Call Drops due to Forced Handover (Traffic Channel)]

Clear Request

Handover Performed

H3027Ca:Failed Internal Intra-Cell Handovers (Timer Expired) (TCHF) (Traffic Channel)H3028Ca:Failed Internal Intra-Cell Handovers (Timer Expired) (TCHH) (Traffic Channel)H3127Ca:Failed Outgoing Internal Inter-Cell Handovers (Timer Expired) (TCHF) (Traffic Channel)H3128Ca:Failed Outgoing Internal Inter-Cell Handovers (Timer Expired) (TCHH) (Traffic Channel)

T3103 Time Out

H3047:Internal Intra-Cell Handover Detection Messages Received by BSC (TCHF)H3048:Internal Intra-Cell Handover Detection Messages Received by BSC (TCHH)H3247:Incoming Internal Inter-Cell Handover Detection Messages Received by BSC (TCHF)H3248:Incoming Internal Inter-Cell Handover Detection Messages Received by BSC (TCHH)

K3013B:Successful TCH Seizures in TCH handovers (Traffic Channel)]


Measurement Point during Handover Procedure

Inter-BSC handover

H3448:Incoming External Inter-Cell Handover Detection Messages Received by BSC (TCHH)

MS Original BTS1 Original BSC1 Target BTS2 MSMSC Target BSC2

Handover_Required

Handover_Request

Channel_Active

Channel_Active_AckHandover_Request_AckHandover_Cmd

Handover_Detect

Handover_Detect

Handover CommandHandover Access

PHY INFO

PHY INFO

First SABMEstablish_INO

Handover CompleteHandover_CmpClear_CMD

Clear_CMP

UA

H3447:Incoming External Inter-Cell Handover Detection Messages Received by BSC (TCHF)

Measure Report from MS

T8 Time Out

H3327Ca:Failed Outgoing External Inter-Cell Handovers (T8 Expired) (TCHF) (Traffic Channel)H3328Ca:Failed Outgoing External Inter-Cell Handovers (T8 Expired) (TCHH) (Traffic Channel)

K3013B:Successful TCH Seizures in TCH handovers (Traffic Channel)]


Formula of TCH Call Drop Rate (Including Handover)

ZTR304:Call Drop Rate on TCH per cell (Including Handover)

[CM33:Call Drops on Traffic Channel]

[K3023:Successful TCH Seizures (Signaling Channel)] + [K3013A:Successful TCH Seizures (Traffic Channel)] + [K3013B:Successful TCH Seizures in TCH handovers (Traffic Channel)]

CM33:Call Drops on Traffic Channel=[CM330:Call Drops on Radio Interface in Stable State (Traffic Channel)] + [CM331:Call Drops on Radio Interface in Handover State (Traffic Channel)] + [CM332:Call Drops due to No MRs from MS for a Long Time (Traffic Channel)] + [CM333:Call Drops due to Abis Terrestrial Link Failure (Traffic Channel)] + [CM334:Call Drops due to Equipment Failure (Traffic Channel)] + [CM335:Call Drops due to Forced Handover (Traffic Channel)] + [CM397:Call Drops Due to Loopback Start Failure] + [CM385:Call Drops Due to Failures to Return to Normal Call from Loopback]

=

CM330:Call Drops on Radio Interface in Stable State (Traffic Channel) = [CM3300:Call Drops on TCH in Stable State (Error Indication)] + [CM3301:Call Drops on TCH in Stable State (Connection Failure)] + [CM3302:Call Drops on Traffic Channel in Stable State (Release Indication)]

CM331:Call Drops on Radio Interface in Handover State (Traffic Channel) =[H3027Ca:Failed Internal Intra-Cell Handovers (Timer Expired) (TCHF) (Traffic Channel)] +[H3028Ca:Failed Internal Intra-Cell Handovers (Timer Expired) (TCHH) (Traffic Channel)] +[H3127Ca:Failed Outgoing Internal Inter-Cell Handovers (Timer Expired) (TCHF) (Traffic Channel)] +[H3128Ca:Failed Outgoing Internal Inter-Cell Handovers (Timer Expired) (TCHH) (Traffic Channel)] +[H3327Ca:Failed Outgoing External Inter-Cell Handovers (T8 Expired) (TCHF) (Traffic Channel)] +[H3328Ca:Failed Outgoing External Inter-Cell Handovers (T8 Expired) (TCHH) (Traffic Channel)]

1

1

×100%


ZTR304A:Call Drop Rate on TCH per cell (Excluding Handover)

[CM33:Call Drops on Traffic Channel][K3013A:Successful TCH Seizures (Traffic Channel)] + [CH323:Successful Incoming Internal Inter-Cell Handovers] + [CH343:Successful Incoming External Inter-Cell Handovers] – [CH313:Successful Outgoing Internal Inter-Cell Handover] – [CH333:Successful Outgoing External Inter-Cell Handovers]

=

×100%

CH323:Successful Incoming Internal Inter-Cell Handovers =

[CH320:Incoming Internal Inter-Cell Handover Requests] - [CH322:Failed Incoming Internal Inter-Cell Handovers]

CH320:Incoming Internal Inter-Cell Handover Requests = [H3200W:Incoming Internal Inter-Cell Handover Requests (SDCCH) (900/850-900/850)] + [H3200X:Incoming Internal Inter-Cell Handover Requests (SDCCH) (1800/1900-1800/1900)] + [H3200Y:Incoming Internal Inter-Cell Handover Requests (SDCCH) (900/850-1800/1900)] + [H3200Z:Incoming Internal Inter-Cell Handover Requests (SDCCH) (1800/1900-900/850)] +[H3209W:Incoming Internal Inter-Cell Handover Requests (TCH) (900/850-900/850)] + [H3209X:Incoming Internal Inter-Cell Handover Requests (TCH) (1800/1900-1800/1900)] +[H3209Y:Incoming Internal Inter-Cell Handover Requests (TCH) (900/850-1800/1900)] + [H3209Z:Incoming Internal Inter-Cell Handover Requests (TCH) (1800/1900-900/850)]

CH322:Failed Incoming Internal Inter-Cell Handovers = [H3220W:Failed Incoming Internal Inter-Cell Handovers (SDCCH) (900/850-900/850)] +[H3220X:Failed Incoming Internal Inter-Cell Handovers (SDCCH) (1800/1900-1800/1900)] + [H3220Y:Failed Incoming Internal Inter-Cell Handovers (SDCCH) (900/850-1800/1900)] + [H3220Z:Failed Incoming Internal Inter-Cell Handovers (SDCCH) (1800/1900-900/850)] + [H3229W:Failed Incoming Internal Inter-Cell Handovers (TCH) (900/850-900/850)] + [H3229X:Failed Incoming Internal Inter-Cell Handovers (TCH) (1800/1900-1800/1900)] + [H3229Y:Failed Incoming Internal Inter-Cell Handovers (TCH) (900/850-1800/1900)] +[H3229Z:Failed Incoming Internal Inter-Cell Handovers (TCH) (1800/1900-900/850)]

1

1

1

Formula of TCH Call Drop Rate (Excluding Handover)


Call Drop Analysis ProcedureCommon

phenomenonAnalyzing the frequency planning, coverage, and

cell parameters

Times of A interface failures during TCH

occupation

Radio problem

interference band analysis

TRX received level and received quality

RF call drop rate

Analyzing a call drop rate

Filtering for the cell with a high call drop rate

TCH assignment failure (equipment

fault)

TCH availability

Times of call drops on the terrestrial link

Equipment fault

Y

Power control average level

Ratio of poor handovers between

the uplink and downlink

Interference

Times of reestablishment

failures upon handover failure

Average received level during a TCH call drop

Average distance during a TCH call drop

Measurement of uplink/downlink balance

performance

Outgoing handover success rate and average level

Coverage

N

YN

Average received level and quality during the

handover

Which cell has a lower incoming handover

success rate

Times of handovers and times of call occupation

success

Outgoing handover success rate

The ratio of inter-office handovers is high.

Handover


Transmission

Coverage HO

Interference

Antenna

Parameter

Specific Causes of Call Drops and its Solutions


Call Drops Reasons for Coverage

Discontinuous coverage (blind area)

Bad indoor coverage

Cross coverage (isolated island)

Small coverage

1 2 3 4


Methods to Identify Call Drop Caused by Coverage

Drive Test

Traffic Measurement For detail, please refer

to the next page

Call Drop for Coverage


Traffic Statistic Tasks Related to Coverage Analysis

Call Drop Result fromCoverage Problem

Power control performance measurement

Receiving level performance measurement

1

2

3Cell performance measurement / inter-cell handover performance measurement

4 Call drop performance measurement

5 Neighbor cell performance measurement

6 Uplink/downlink balance performance measurement

Traffic Statistic Tasks


Solution to Call Drop Caused by Coverage

Area with insufficient coverage

Ensure the indoor

communication qualityEliminate

overshooting coverage

Remove hardware problem


Call Drops Caused by Handover

Main Reasons

Unreasonable parametersT3103 timer overflows

Incomplete neighbor cells

Co-BCCH and Co-BSIC

BTS clock is out of

synchronization

Traffic congestion


Methods to Identify Call Drop Caused by Handover

Drive testCheck

the alarms

Analyze the traffic

measurement

Method to

Identify


Traffic Statistic Tasks Related to Handover Analysis

Inter-cell handover performance measurementNeighbor cells performance measurementOutgoing cell handover performance measurementIncoming cell handover success rate is low

TCH performance measurement

1

2

3

4

5

Call Drop Result fromHandover Problem



Solution to Call Drop Caused by Handover

Check handoverparameters

Adjust the traffic

Provide good clock synchronization

1 2 3


Call Drops Reasons for Interference

Inter-modulation interf. and

Other Outside interfe.

Adjacent-channel

interference

Co-channel interference

Main Reasons


Methods to Identify Call Drop Caused by Interference

Analyze the traffic statistic

Use spectrum analyzer to scan the frequencies

Eliminate causes resulting from equipment

Analyze basing on user complaint

Check the frequency planning

Adjust ARFCNs

Method to

Identify


Traffic Statistic Tasks Related to Interference Analysis

Interference Analysis

Interference band in MR

Receiving quality

performance measurement

Call drop performance measurement

Many handover

failures and reestablishment failures

BQ handover ratio

Receiving level

performance measurement


Solution to Call Drop Caused by Interference

Interference from outside the network and inside the network are solved by different methods. Invalid interference from outside the network is solved by the Radio Committee interference from inside the network is solved by adjusting the network.

1

Drive test

2

Use DTX, FH, PC, and diversity technologies

3

Exclude the interference caused by the equipment


Call Drops caused by Antenna Feeder

Antenna feeder problem

Different downtilt angles

for two sets of

antenna

Main Reasons

Differentazimuth angles

for two sets of antenna

Antenna feeders are

wrongly connected


Methods to Identify Call Drop Caused by Antenna Feeder

Whether there is alarm related to

antenna unit

check whether the BCCH of the serving

cell is consistent with the planning

BTS on-site check and test

Check whether there is BTS

hardware fault

whether the boards are normal

from remote maintenance

consolewhether the uplink and

downlink are balanced in

MRs

Method to

Identify


Traffic Statistic Tasks Related to Antenna Feeder Analysis

Uplink and downlink balance PM

Call drop PM

Power control PM

1

2

3

Analyze whether the uplink and downlink are balanced in order to identify antenna feeders problems if any exist



Call Drops Reasons for Transmission

Bad transmission quality and unstable transmission link Abis interface and the A interface links in

Observe the transmission and board

alarms

Check transmission

paths

Observe traffic

statistics

TCH PM shows ： 1 ） A interface failures during TCH

seizure 2 ） Whether the TCH availability is

normal 3 ） Call drops due to interruption of

terrestrial links

Main Reasons


Radio Parameters That Affect Call Drop

CallDrop Problem

RLT/SACCH Multi-frames

T200 parameter

1

2

3 RXLEV_ACCESS_MIN

4 RACH Min Access Level

5 NCC Permitted

6 Handover parameters

7 Power control parameters

MainRadioParameters

8 Frequency planning parameters


Call Drop Case 1 Fault Description

The BTS distribution of an area is as illustrated in the diagram (red numbers stand for BCCH frequencies. No hopping, no DTX). Some subscribers complain that call drop in second sector of base station C is serious. Hardware fault has been ruled out.

Please confirm whether the frequency distribution in the

cells are reasonable?


Call Drop Case 1 Analysis

From the analysis of BTS topology, it can be concluded that the frequencies are well planned.

Next, check the interference band of traffic statistic.



09:00~10:00

IB1 IB2 IB3 IB4 IB5

cell 1 2.85 14.25 1.14 0.27 0.54

cell 2 4.09 12.57 3.14 0.03 0.01

cell 3 0 2.92 13.27 0.25 0.3703:00~04:00

IB1 IB2 IB3 IB4 IB5

cell 1 2.85 4.28 0.00 0.00 0.00

cell 2 4.09 2.89 0.00 0.00 0.00

cell 3 0 2.12 0.00 0.00 0.00


Call Drop Case 1 Troubleshooting

Conducting a drive tests, it is found that the quality is bad when the receiving strength is high.

Traffic statistic show that during periods of high call drop rate, handovers are mostly caused by quality reasons, and channel assignment failure rate is also high.

Conclusion: Interference is high.



A site investigation shows that the operator has a repeater. It is a broadband repeater. It transmits the signals from a remote TACS site. TACS signals are amplified and then there is interference in second sector of base station C.

Problem has been located: interference causes the call drop. Finally, lower the power of the repeater. The interference

band reduce to IB1. Now the high call drop rate problem at site C is solved.


Call Drop Case 1 Common methods of checking and clearing call drop due to

interference Rule out the internal interference caused by equipment problems and

check the separation of BTS transceivers, antenna feeder installation, and so on.

Check the interference band Drive test Check traffic statistic of handover causes for characteristic footprints Clear uplink interference Clear downlink interference Check whether DTX, frequency hopping technology and power control

application are reasonable Use PBGT handover algorithm flexibly to avoid co-channel and adjacent-

channel interference effectively.


Call Drop Case 2 Fault description

The call drop rate in cell 3 of a BTS is 10% accompanied by high congestion rate, but call drop rate and congestion rate in cell 1 and cell 2 are normal.



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



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.



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.


Call Drop Case 2 Conclusion: Reduce interference

Reduce internal interference through checking frequency planning.

After internal interference is excluded, we can locate external interference with spectrum analyzer.



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 informationabout the network quality.



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.



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.



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.


Call Drop Case 3 Conclusion ： two 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.



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.



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?



The adjusted parameters are listed below.

Parameter nameValue before change

Value after

changePBGT watch time 5 3PBGT valid time 4 2PBGT HO threshold 72 68UL Qual. Thrsh. (Emergency handover) 70 60Min. DL level on candidate cell 10 15


Call Drop Case 4 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.



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.



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.


Call Drop Case 5 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.



In dual-band network, when a call is setup in a GSM1800 cell and being handed over to a GSM900 cell in the same site, the call drops in the GSM900 cell in 2 to 5 seconds. The call drop rate in the GSM900 cell is quite high.



In the test it is found that the clock of GSM900 cell and GSM1800 cell are not synchronized.

When a call set up in a GSM1800 cell and is handed over to a GSM900 cell, the drive test tool shows that FER increases to the maximum value suddenly and then it goes down to zero gradually.

It is the same with the handover from GSM900 to GSM1800.



After adjusting GSM900 clock system, the abnormal call drop problem is solved.


Call Drop Case 6 Conclusion

Clocks of GSM900 and GSM1800 should be exactly synchronized with each other in a dual band network, otherwise there will be call drops and handover failures.


Summary What is classification of the call drop? How to analyze the call drop? What is the main reasons of the call drop and its

corresponding solution?

Thank youwww.huawei.com

15- omo333010 bsc6900 gsm v9r13 call drop problem analysis issue 1.00

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