OMF000404 Case Analysis-Call Drop ISSUE2.0

8/8/2019 OMF000404 Case Analysis-Call Drop ISSUE2.0

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OMF000404

Case Study Call Drop

Issue 2.0


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HUAWEI TECHNOLOGIES CO., LTD. All rights reserved Page 1

References

31160978-BSC Traffic StatisticManual Volume I

31033203-BSS Troubleshooting

Manual


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Upon completion of this course, you are

supposed to be able to:> Understand the principles of call drop.

> Analyze and solve call drop problems


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Ch apter 1Ch apter 1 Principle of call dropPrinciple of call drop

Ch apter 2 Analysis of call drop

Ch apter 3 C all drop cases


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Chapter 1Chapter 1 Principle of call dropPrinciple of call drop

Section 1 Definition of call drop

Section 2 TCH call drop

Section 3 SDCCH call drop


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Definition of Call Drop

There are two types of call drop: TCH call drop and SDCCHcall drop:

> TCH call drop means TCH channel is released abnormallyafter it is occupied successfully.

> SDCCH call drop means SDCCH channel is releasedabnormally after it is occupied successfully.


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Ch apter 1Ch apter 1 Principle of call dropPrinciple of call drop

Section 1 Definition of call dropSection 2 T CH call drop

Section 3 SD CCH call drop


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Causes of TCH Call Drop

Usually, the typical causes for sending the Clear_Requestmessage are as follows:

> Radio interface failure

> O&M intervention> Equipment failure

> Protocol error

> Preemption


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Formula of TCH Call Drop Rate

Formula of TCH call drop rate

> TCH call drop rate=TCH call drops / Successful TCHseizures (all)


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Measurement Point of TCH Call Drop


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Measurement Point of TCH Call DropTCH serves as traffic channel, BSC receives ERROR INDICATIONmessage from BTS.

TCH is seized as traffic channel, BSC receives CONNECT FAILUREINDICATION message from BTS.

In assignment procedure and handover procedure, failure of decoding

HANDOVER DETECTION and HANDOVER COMPLETE message.TCH serves as traffic channel, incoming BSC handover initiated and thetimer for the target cell to wait for HANDOVER COMPLETE messageexpires.

TCH serves as traffic channel, outgoing BSC handover initiated and thetimer for the source cell to wait for CLEAR COMMAND message from MSC

expires (T8 expires).In intra-BSC handover procedure, the target cell sends Inter Clear Requestto the source cell when the timer for the target cell to wait for HANDOVERCOMPLETE from MS expires, in this case, this item is measured in thesource cell.


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Measurement Point of TCH Call DropIn Intra-BSC handover procedure, the source cell, excluding the source cellfor directed retry, measures the item when the timer for the source cell to waitfor Inter Clear Request with cause value HANDOVER COMPLETE from thetarget cell expires.

In Intra-BSC handover procedure, when the target cell AM/CM net-drive fails(due to timeout or negative acknowledgement), the target cell sends Inter Clear Request to source cell, in this case, this item is measured in the sourcecell.

In the case that MS reverses to the original channel after intra BSC handover fails, the source cell first releases the terrestrial connection but the AM/CMre-net-drive fails (due to timeout or negative acknowledgement). In this casethis item is measured in the source cell.

The resource of the lower_priority call will be preempted by thehigher_priority call if MSC and BSC both supports preemption, which willcause call drop.

This item is measured when the RSL link of the TRX that the call is usingdisconnects, which will cause call drop.


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TCH Call Drop - Assignment

MS BTS BSC

ASSIGN CMM AND

MSC

ASSIGNMENT REQUEST

SABM

UAESTABLISH INDICATION

CHANNEL ACTICATION

CHANNEL ACTIVATION ACK

A1

ERROR INDICATION

CONNECTION FAILURE INDICATION

A2


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TCH Call Drop Intra-BSC Handover

S BSC' BSC Ori-Cell

HANDO

E ACCESS

UA

Intercell Handover e est

BSC Tag-Cell BSC''

HANDO

E CO AND

Intercell Handover esponse

HANDO

E DETECT

CH ACT

CH ACT ACK

A1

E

O

INDICATION

CONNECTION AILU E INDICATION

A

SAB


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TCH Call Drop - Incoming BSC Handover

MS Other BSC

HANDOVER ACCESS

UA

HANDOVER REQUIRED

HUAWEI BSC HUAWEI BTS

HANDOVER COMMAND

HANDOVER REQUEST ACK

HANDOVER DETECT

CH ACTCH ACT ACK

MSC

A1

ERROR INDICATION

CONNECTION FAILURE INDICATION

A2

SABM

HANDOVER REQUEST


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Measurement Point of Successful TCH Seizures

Measurement point of Successful TCH seizures

> Upon BSCs reception of CHANNEL ACTIVATION AC NOWLEDGE message from BTS in very early assignment TCHprocess.

> In the case the target cell of directed retry is located in other BSCand directed retry succeeds, MSC sends CLEAR COMMANDmessage to the originating BSC to release the original connection.This item is measured when the originating BSC receives thismessage.

> In the case the directed retry target cell is located in the same BSCand the directed retry succeeds, target cell sends Inter Clear Request message to the source cell to request to release theresource and the original connection. This item is measured whenthe source cell receives this message.


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Measurement Point of Successful TCH

seizuresMeasurement point of Successful TCH seizures

> This item is measured when BSC sends ASSIGNMENT COMPLETEmessage to MSC after the assignment procedure is successfullyimplemented.

> In incoming BSC handover procedure, MS sends HANDOVER ACCESS message to the BSC. This item is measured when BSCreceives HANDOVER DETECT message from BTS.

> In the process of incoming internal inter cell handover and intracell

handover in BSC, MS sends HANDOVER ACCESS message to BSC.BSC measures this item in the target cell when receiving HANDOVERDETECTION message from BTS.


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Successful TCH Seizure Very Early Assignment

Very early assignment process

S TS SC SCChannel e uest ( ACH) Channel e uired

Channel Activation (TCH or SDCCH)

Channel Activation Ackno ledge

Immediate Assignment Command

Immediate Assignment (AGCH)


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Successful TCH Seizure Assignment

Assignment process

S TS SC SC ASS EQ

CH ACTCH ACT ACK

ASS C D ASS C D

EST IND

ASS C P ASS C P ASS C P


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Successful TCH Seizure Intra-BSC Handover

Intra-BSC handover process

MS MSBTS1 BTS2BSCMeasurement Report from MS

Channel_Active

Channel_Active ACHANDOVER COMMAND

HO_ Access

HO_DetectPHY INFO

First SABM Establish_IND

PHY INFO

Handover Complete HO_Performed

UA

MSC


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Successful TCH Seizure Inter-BSC

Handover Inter BSC handover process

S SBTS BTS2BSC BSC2SCeasure eport fro S

HO e uiredHO e uest

CH ACT

CH ACT ACKHO e uest ACK

HO C DHandover Co and HO Access

HO DetectHO Detect

PHY IN O

PHY IN O

irst SABEsta lish IND

Handover Co pleteHO C PClear C D

Clear C P


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Chapter 1Chapter 1 Principle of call dropPrinciple of call drop

Section 1 Definition of call drop

Section 2 TCH call drop

Section 3 SDCCH call drop


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Formula of SDCCH Call Drop Rate

Formula of SDCCH call drop rate:

> SDCCH call drop rate = SDCCH call drops/ successfulSDCCH seizures*100%


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Measurement Point of SDCCH Call Drop

This item is measured when BSC receives ERROR INDICATIONmessage from BTS due to an abnormal case for a radio link layer connection.

This item is measured when BSC receives CONNECTION FAILURE

INDICATION message from BTS because an active connection has beenbroken for some reason such as SDCCH link failure or hardware failure(see GSM 0508 for details).

In incoming BSC handover procedure on SDCCH, this item is measuredin the target cell in the case of the failure of decoding HANDOVER

DETECTION and HANDOVER COMPLETE message.In the process of incoming BSC handover on SDCCH, this item ismeasured in the target cell when the timer for the target cell to wait for the HANDOVER COMPLETE message expires.


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In the process outgoing BSC handover on SDCCH, this item ismeasured when the timer for the source cell to wait for CLEARCOMMAND message from MSC expires (T8 expires).

In the process of intra BSC handover on SDCCH, this item is measuredin the source cell when the timer for the source cell or the target cell towait for HANDOVER COMPLETE message expires.

This item is measured when the RSL link of the TRX that the call isrunning on disconnect, which will cause call drop, this item measures

call drop on SDCCH for RSL disconnection.


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MS TS SC MSC

O r:

O r:

Channel e uest Channel e uired

Channel Activation (SDCCH)

Channel Activation Ackno ledge

Immediate Assignment CommandImmediate Assignment

Esta lish Indication (L Info)

C ti F ilur

Err r I i ti

bi F ilur C ll CCH C ll r ( ubj t t iff r t )


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Measurement Point of Successful SDCCHSeizures

In immediate SDCCH assignment procedure, this item ismeasured when BSC receives CHANNEL ACTIVATION

AC NOWLEDGE message from BTS.

In the process of incoming BSC handover on SDCCH. Thisitem is measured when BSC receives HANDOVERDETECTION from BTS.

In the process of incoming internal inter cell handover andintracell handover in BSC.BSC measures this item in the targetcell when receiving HANDOVER DETECTION from BTS.


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Ch apter 1 Principle of call drop

Ch apter 2Ch apter 2 Analysis of call dropAnalysis of call drop

Ch apter 3 C all drop cases


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Analysis of Call Drop

content

> main causes of high call drop rate

> troubleshooting of high call drop rate


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Analysis of Call Drop

According to the definition of call drop measurement point, call drop is usuallycaused by the following:

> Radio link fault. During the communication, messages can not be receivedcorrectly.

> Abis link broken during conversation.

> Call drop during handover.

> Other system faults.

Timers that may cause call drops (BSC timer):

> T3103: starting from sending HANDOVER CMD and ending at receivingHANDOVER CMP. Time out of the timer will cause call drop.

> T3109: starting from sending CHAN REL and ending at receiving REL IND.


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Radio Link Fault

Signaling process chart of radio link fault

S TS SC SC

easure ent Report

easure ent ResultConnection Failure

Clear REQ (Radio Interface Failure)

( )

( )

(3)

( ) Dadicated ode is created. (SDCCH/TCH)( ) Activate A is onitoring function.(3)SACCH essage lock can not e decoded(uplink/do nlink),

resulting in radio link ti eout.


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Radio Link Fault

Diagram of radio link timeout


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T3103 Times Out

Handover process

S TS1 SC SCTS2Handover Indication

CH ACT

CH ACT ACK

Handover CommandHandover Command

Handover Access HO Detect

SA

Ph sical Information (TA)

UA

Handover CompleteHandover Complete

EST IND

Set T3103

eset T3103


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Causes of Radio Link FaultThe causes of radio link fault can be

> Interference Internal interference External interference Equipment interference

> Poor coverage Coverage hole Isolated island Uplink/downlink imbalance

> Improper parameter setting Radio link timeout, SACCH multi-frames Handover parameters Power control parameters

> Equipment problem (Antenna, feeder, combiner, TRX)> Clock problem> Transmission problem


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Radio Link Fault - Interference

Category

> Co-channel interference

> Adjacent-channel interference

> Inter-modulation interference and other externalinterference


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Solution

> First check equipment problems.

> Make an drive test, check the interference area and distribution of

signal quality. Find the interference frequency.> Further search for the interference source with the spectrum analyzer.

> Activate hopping, DTX and power control functions to lower theinternal interference of the system


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Judgment Process

> Analyze the occurrence regularity of interference band in thetraffic measurement.

>Observe the receiving level performance

> Find the poor quality handover rate

> Observe receiving quality performance

> Observe call drop performance

> Observe whether there are many handover failures and reversionfailures.


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Radio Link Fault - Coverage

Coverage:

> Overshooting

> Coverage hole

> Signal attenuation> Incomplete definition of adjacent cells

> Imbalance of uplink/downlink


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Judgment Process

> Power control measurement function

> Receiving level measurement function

> Cell measurement function/inter-cell handover measurement function> Call drop measurement function

> Defined adjacent cell measurement function

> Undefined adjacent cell measurement function

> Outgoing inter-cell handover measurement function

> Up-down link balance measurement function


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Solution

> Adjust network parameters

> Add BTS


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Radio Link Fault - Parameters

Main parameters that may affect the call drop rate:

> Radio link timeout and SACCH multi-frames

> RACH busy threshold and RACH minimum access level.

> MS minimum receiving signal level> Call re-establishment permitted.


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Radio Link Fault - Parameters

Main parameters that may affect the call drop rate:

> NCC permitted

> Handover related parameters.

> Power control related parameters.> Frequency planning parameters


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Radio Link Fault

Judgment process

> System information data

> Cell data

> Radio link connection timer (T3105)> Maximum times of physical information

> Call drop performance measurement function

> Judge from the cause of call drop

error indication

connection failure.


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Handover Problem

Judgment process :

> In inter-cell handover measurement function, it occursfrequently that the handover fails and the reversion alsofails.

> In inter-cell handover measurement function, handover failures with successful reversions occur many times.

> In undefined adjacent cell measurement function, observethe receiving level of the undefined adjacent cells and

number of the measurement reports.


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Handover Problem

Judgment process

> In outgoing inter-cell handover measurement function, theoutgoing inter-cell handover success rate is low (for acertain cell). Find the adjacent cell to which the handover success rate is low and find the cause.

> Incoming inter-cell handover success rate is low. Thehandover judgment parameter setting of the target cell maybe improper.

> In TCH measurement function, handover times are not innormal proportion to successful TCH seizures for call.(handovers/calls>3)


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Equipment Problem

Call drop arising from equipment problem

> Hardware problem

> Transmission problem

> Antenna and feeder fault> Other causes


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Ch apter 1 Principle of call drop

Ch apter 2 Analysis of call drop

Ch apter 3Ch apter 3 C all drop casesC all drop cases


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Call Drop Case 1Fault Description

> The BTS distribution of an area is as illustrated in thediagram (red numbers stand for BCCH frequencies. Nohopping, no DTX). Some subscribers complain that calldrop in second sector of base station C is serious.Hardware fault has been ruled out.

please confirm whether thefrequency distribution in the

cells are resonable?


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Call Drop Case 1

Analysis

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

> Next, check the interference band of traffic statistic.


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Call Drop Case 1

Analysis

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 0.01cell 3 0 2.92 13.27 0.25 0.37

03:00~04:00 IB1 IB2 IB3 IB4 IB5

cell 1 2.85 4.28 0.00 0.00 0.00cell 2 4.09 2.89 0.00 0.00 0.00

cell 3 0 2.12 0.00 0.00 0.00


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Call Drop Case 1

Troubleshooting

> Conducting a drive tests, it is found that the quality is badwhen the receiving strength is high.

> Check traffic statistic and it is found that when the call droprate is high, handovers are mostly caused by qualityreasons, and channel assignment failure rate is also high.

> The conclusion is that there is interference from theanalysis of comprehensive traffic statistic and drive test.


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Call Drop Case 1

Troubleshooting

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

> Problem has been located: interference causes the calldrop.

> Finally, lower the power of the repeater. The interference

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


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

> Check the interference band

> Drive test

> Check traffic statistic of handover causes to get judgment

> Clear uplink interference

> Clear downlink interference

> Check whether DTX, frequency hopping technology and power controlapplication are reasonable

> Use PBGT handover algorithm flexibly to avoid co-channel and adjacent-channel interference effectively.


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Call Drop Case 2

Fault description

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


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Call Drop Case 2

Analysis

> Check the related traffic statistic

Check whether there is high interference band in TCHmeasurement 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 frequencyplanning, moreover confirm whether there is external interferencewith spectrum analyzer.

> Drive test

> Check the hardware


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Call Drop Case 2

Troubleshooting

> Block TRX in turn and the congestion rate is always quite high nomatter which TRX is blocked.

> Check and analyze the traffic statistic, interference band and trafficvolume and call drop rate, and it is found that the interferencebecomes more serious as the traffic gets high.

> Change frequency. The frequency of cell 3 is changed to 1MHz awayfrom the original value. But the problem persists.

> Judge whether the equipment is faulty.> Locate external interference.


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Call Drop Case 2

Troubleshooting

> Make a scanning test with a spectrum analyzer.

A suspect signal with 904.14MHz center frequency, 300bandwidth is found. It is similar to an analog signal and it existscontinuously.

At the distributor output port of cell 3, the signal strength is 27dBm.cell 2 is 40dBm, cell 1 is 60dBm. It accords with the degree of interference.

Traffic volume is higher in the day time than that at night.> Now the problem is found: 904MHz external interference source.


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Call Drop Case 2

Conclusion: solution of interference

> Solve internal interference through checking frequency planning.

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


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Call Drop Case 3

Fault description

> Subscribers complain that call drops often happen from onthe 5th floor and above in a building.

S scri er complaint is also an important so rce of informationa o t t h e network q ality.


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Call Drop Case 3

Analysis

> Perform on-site test

There are call drops and noise on the site

The test mobile phone shows that before the call drop theserving cell is BTS-B. But this building should be covered byBTS-A.

> Check traffic statistic

BTS-B is about 9 kilometers away from this building. It isdetermined that the BTS-B signal received in this area iscoming from some obstacles reflection. Thus an isolatedisland coverage is formed in this area.


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Call Drop Case 3

Analysis

> Check data configuration

In BSC data configuration, BTS-A is not configured as the adjacencyof BTS-B

> Cause analysis of call drop

When the MS uses the signal of cell 2 of BTS-B in this area, the signalof cell 3 of BTS-A is strong. But cell 2 of BTS-B and cell 3 of BTS-Aare not adjacent, therefore, handover cannot happen.

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


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Call Drop Case 3

Troubleshooting

> Modify the data in BA1 table, BA2 table and add adjacent cellrelationship, 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.


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Call Drop Case 3

Conclusion two methods to solve isolated island problem

> Adjust the antenna of the isolated cell, to eliminate the isolated islandproblem.

> Define new adjacent cells for the isolated cell.


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Call Drop Case 4

Fault description

> In a drive test from A to B, it is found that there are many call dropsat entrance of a tunnel near a BTS due to slow handover.


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Call Drop Case 4

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 downquickly 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 downrapidly. The call drop occurs before any handover is triggered.

Th ink it over H ow to solve pro lems of t h is type?


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Call Drop Case 4

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 oncandidate cell 10 15


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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 voiceinterruption, PBGT handover can help to reduce interference and lower call drop rate.

> Set emergency handover thresholds properly, and make sure theemergency handover can be triggered in time before the call drop so asto reduce call drops.


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Call Drop Case 5

Fault description

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

Analysis

> Check the traffic statistic and find out that TCH congestion rate of this cellis over 10% and internal inter-cell handover failure rate is high. It is foundthat one TRX board of this cell is abnormal in OMC. A preliminaryconclusion is that TRX board problem causes the call drop.


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Call Drop Case 5

Troubleshooting

> Lock the frequency with a test mobile phone and perform dial test for many times. It is found that call drops only happen in timeslots 1, 3, 5, 7while 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.


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Call Drop Case 5

Conclusion

> The BTS test should guarantee that communication should be successfulnot only in each TRX but also in each timeslot of the TRX.

> It must be ensured that each TCH channel provides bidirectional highquality communication.


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Call Drop Case 6

Fault description

> In dual-band network, when a call is setup in a GSM1800 cell and beinghanded over to a GSM900 cell in the same site, the call drops in theGSM900 cell in 2 to 5 seconds. The call drop rate in the GSM900 cell is

quite high.


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Call Drop Case 6

Analysis

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

> When a call set up in a GSM1800 cell and is handed over to a GSM900cell, the drive test tool shows that FER increases to the maximum valuesuddenly and then it goes down to zero gradually.

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


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Call Drop Case 6

Troubleshooting

> After adjusting GSM900 clock system, the abnormal call dropproblem is solved.


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Call Drop Case 6

Conclusion

> Clocks of GSM900 and GSM1800 should be exactly synchronizedwith each other in a dual band network, otherwise there will be calldrops and handover failures.


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Summary

Types of call drop

Measurement points of TCH call drop

Measurement points of SDCCH calldrop

Causes of call drop

Cases


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OMF000404 Case Analysis-Call Drop ISSUE2.0

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