2g drop analysis

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OMF000404Case Study – Call Drop

Issue 2.0


References

31160978-BSC Traffic Statistic Manual

Volume I

31033203-BSS Troubleshooting

Manual


Upon completion of this course, you are

supposed to be able to:

Understand the principles of call drop.

Analyze and solve call drop problems


Chapter 1 Chapter 1 Principle of call dropPrinciple of call drop

Chapter 2 Analysis of call drop

Chapter 3 Call drop cases


Chapter 1 Chapter 1 Principle of call Principle of call

dropdrop

Section 1 Definition of

call drop

Section 2 TCH call drop

Section 3 SDCCH call

drop


Definition of Call Drop

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

call drop:

TCH call drop means TCH channel is released abnormally

after it is occupied successfully.

SDCCH call drop means SDCCH channel is released

abnormally after it is occupied successfully.


Chapter 1 Chapter 1 Principle of call dropPrinciple of call drop

Section 1 Definition of call drop


Section 3 SDCCH call drop


Causes of TCH Call Drop

Usually, the typical causes for sending the Clear_Request

message are as follows:

Radio interface failure

O&M intervention

Equipment failure

Protocol error

Preemption


Formula of TCH Call Drop Rate

Formula of TCH call drop rate

TCH call drop rate=TCH call drops / Successful TCH

seizures (all)


Measurement Point of TCH Call Drop


Measurement Point of TCH Call Drop TCH serves as traffic channel, BSC receives ERROR INDICATION

message from BTS.

TCH is seized as traffic channel, BSC receives CONNECT FAILURE INDICATION 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 the timer for the target cell to wait for HANDOVER COMPLETE message expires.

TCH serves as traffic channel, outgoing BSC handover initiated and the timer 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 Request to the source cell when the timer for the target cell to wait for HANDOVER COMPLETE from MS expires, in this case, this item is measured in the source cell.


Measurement Point of TCH Call Drop In Intra-BSC handover procedure, the source cell, excluding the source cell

for directed retry, measures the item when the timer for the source cell to wait for Inter Clear Request with cause value HANDOVER COMPLETE from the target 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 source cell.

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/CM re-net-drive fails (due to timeout or negative acknowledgement). In this case this item is measured in the source cell.

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

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


TCH Call Drop - Assignment

MS BTS BSC

ASSIGN CMMAND

MSC

ASSIGNMENT REQUEST

SABM

UAESTABLISH INDICATION

CHANNEL ACTICATION

CHANNEL ACTIVATION ACK

A1

ERROR INDICATION

CONNECTION FAILURE INDICATION

A2


TCH Call Drop – Intra-BSC Handover

MS BSC' BSC Ori-Cell

HANDOVER ACCESS

UA

Intercell Handover Request

BSC Tag-Cell BSC''

HANDOVER COMMAND

Intercell Handover Response

HANDOVER DETECT

CH ACT

CH ACT ACK

A1

ERROR INDICATION


A2

SABM


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


A2

SABM

HANDOVER REQUEST


Measurement Point of Successful TCH Seizures

Measurement point of Successful TCH seizures

Upon BSC’s reception of CHANNEL ACTIVATION ACKNOWLEDGE message from BTS in very early assignment TCH process.

In the case the target cell of directed retry is located in other BSC and directed retry succeeds, MSC sends CLEAR COMMAND message to the originating BSC to release the original connection. This item is measured when the originating BSC receives this message.

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


Measurement Point of Successful TCH seizures

Measurement point of Successful TCH seizures

This item is measured when BSC sends ASSIGNMENT COMPLETE

message to MSC after the assignment procedure is successfully

implemented.

In incoming BSC handover procedure, MS sends HANDOVER ACCESS

message to the BSC. This item is measured when BSC receives

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 HANDOVER

DETECTION message from BTS.


Successful TCH Seizure – Very Early Assignment

Very early assignment process

MS BTS BSC MSCChannel Request (RACH) Channel Required

Channel Activation (TCH or SDCCH)

Channel Activation Acknowledge

Immediate Assignment Command

Immediate Assignment (AGCH)


Successful TCH Seizure – Assignment

Assignment process

MS BTS BSC MSC

ASS_REQCH_ACT

CH_ACT_ACK

ASS_CMDASS_CMD

EST_IND

ASS_CMP ASS_CMPASS_CMP


Successful TCH Seizure – Intra-BSC Handover

Intra-BSC handover process

MS MSBTS1 BTS2BSC

Measurement Report from MS

Channel_Active

Channel_Active ACK

HANDOVER COMMAND

HO_ Access

HO_DetectPHY INFO

First SABMEstablish_IND

PHY INFO

Handover Complete HO_Performed

UA

MSC


Successful TCH Seizure – Inter-BSC Handover Inter BSC handover process

MS MSBTS1 BTS2BSC1 BSC2MSC

Measure Report from MS

HO_Required

HO_Request

CH_ACT

CH_ACT_ACKHO_Request_ACK

HO_CMDHandover Command HO_Access

HO_DetectHO_Detect

PHY INFO

PHY INFO

First SABMEstablish_IND

Handover CompleteHO_CMPClear_CMD

Clear_CMP


Chapter 1 Chapter 1 Principle of call Principle of call

dropdrop

Section 1 Definition of call drop


Section 3 SDCCH call drop


Formula of SDCCH Call Drop Rate

Formula of SDCCH call drop rate:

SDCCH call drop rate = SDCCH call drops/ successful

SDCCH seizures*100%


Measurement Point of SDCCH Call Drop

This item is measured when BSC receives ERROR INDICATION

message 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 been

broken 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 measured

in 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 is

measured in the target cell when the timer for the target cell to wait for

the HANDOVER COMPLETE message expires.



In the process outgoing BSC handover on SDCCH, this item is

measured when the timer for the source cell to wait for CLEAR

COMMAND message from MSC expires (T8 expires).

In the process of intra BSC handover on SDCCH, this item is measured

in the source cell when the timer for the source cell or the target cell to

wait for HANDOVER COMPLETE message expires.

This item is measured when the RSL link of the TRX that the call is

running on disconnect, which will cause call drop, this item measures

call drop on SDCCH for RSL disconnection.



MS BTS BSC MSC

Or:

Or:

Channel Request Channel Required

Channel Activation (SDCCH)

Channel Activation Acknowledge

Immediate Assignment CommandImmediate Assignment

Establish Indication (L3 Info)

Connection Failure

Error Indication

Abis Failure Cell SDCCH Call Drop (Subject to different cases)


Measurement Point of Successful SDCCH Seizures

In immediate SDCCH assignment procedure, this item is

measured when BSC receives CHANNEL ACTIVATION

ACKNOWLEDGE message from BTS.

In the process of incoming BSC handover on SDCCH. This

item is measured when BSC receives HANDOVER

DETECTION from BTS.

In the process of incoming internal inter cell handover and

intracell handover in BSC.BSC measures this item in the target

cell when receiving HANDOVER DETECTION from BTS.


Chapter 1 Principle of call drop

Chapter 2 Chapter 2 Analysis of call dropAnalysis of call drop

Chapter 3 Call drop cases


Analysis of Call Drop

content

main causes of high call drop rate

troubleshooting of high call drop rate


Analysis of Call Drop

According to the definition of call drop measurement point, call drop is usually

caused by the following:

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

correctly.

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 receiving

HANDOVER CMP. Time out of the timer will cause call drop.

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


Radio Link Fault

Signaling process chart of radio link fault

MS BTS BSC MSC

Measurement Report

Measurement Result

Connection Failure

Clear_REQ (Radio Interface Failure)

(1)

(2)

(3)

(1) Dadicated mode is created. (SDCCH/TCH)(2) Activate Abis monitoring function.(3)SACCH message block can not be decoded(uplink/downlink), resulting in radio link timeout.


Radio Link Fault

Diagram of radio link timeout


T3103 Times Out

Handover process

MS BTS1 BSC MSCBTS2

Handover Indication

CH_ACT

CH_ACT_ACK

Handover CommandHandover Command

Handover Access HO_Detect

SABM

Physical Information (TA)

UA

Handover CompleteHandover Complete

EST_IND

Set T3103

Reset T3103


Causes of Radio Link Fault The 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


Radio Link Fault - Interference

Category

Co-channel interference

Adjacent-channel interference

Inter-modulation interference and other external

interference



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 the

internal interference of the system



Judgment Process

Analyze the occurrence regularity of interference band in the

traffic 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 reversion

failures.


Radio Link Fault - Coverage

Coverage:

Overshooting

Coverage hole

Signal attenuation

Incomplete definition of adjacent cells

Imbalance of uplink/downlink



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



Solution

Adjust network parameters

Add BTS


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.


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


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.


Handover Problem

Judgment process :

In inter-cell handover measurement function, it occurs

frequently that the handover fails and the reversion also

fails.

In inter-cell handover measurement function, handover

failures with successful reversions occur many times.

In undefined adjacent cell measurement function, observe

the receiving level of the undefined adjacent cells and

number of the measurement reports.


Handover Problem

Judgment process

In outgoing inter-cell handover measurement function, the

outgoing inter-cell handover success rate is low (for a

certain 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. The

handover judgment parameter setting of the target cell may

be improper.

In TCH measurement function, handover times are not in

normal proportion to successful TCH seizures for call.

(handovers/calls>3)


Equipment Problem

Call drop arising from equipment problem

Hardware problem

Transmission problem

Antenna and feeder fault

Other causes


Chapter 1 Principle of call drop

Chapter 2 Analysis of call drop

Chapter 3 Chapter 3 Call drop casesCall drop cases


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 resonable?


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.


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 0.01

cell 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 0.00

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.

Check traffic statistic and it is found that when the call drop

rate is high, handovers are mostly caused by quality

reasons, and channel assignment failure rate is also high.

The conclusion is that there is interference from the

analysis of comprehensive traffic statistic and drive test.


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 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 to get judgment

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

with high congestion rate, but call drop rate and congestion

rate in cell 1 and cell 2 are normal.


Call Drop Case 2

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


Call Drop Case 2

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.


Call Drop Case 2

Troubleshooting

Make a scanning test with a spectrum analyzer.

− A suspect signal with 904.14MHz center frequency, 300K

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

continuously.

− 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.


Call Drop Case 2

Conclusion: solution of interference

Solve internal interference through checking frequency planning.

After internal interference is excluded, we can locate external

interference with spectrum analyzer.


Call Drop Case 3

Fault description

Subscribers complain that call drops often happen from on

the 5th floor and above in a building.

Subscriber complaint is also an important source of information

about the network quality.


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 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.


Call Drop Case 3

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.


Call Drop Case 3

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.


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.


Call Drop Case 4

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.


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 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?


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 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.


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 cell

is over 10% and internal inter-cell 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.


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, 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.


Call Drop Case 6

Fault description

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.


Call Drop Case 6

Analysis

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.


Call Drop Case 6

Troubleshooting

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

Types of call drop

Measurement points of TCH call drop

Measurement points of SDCCH call

drop

Causes of call drop

Cases

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