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www.huawei.com
Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Case Study – Call Drop
Page1Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
References
� 31160978-BSC Traffic Statistic Manual Volume I
� 31033203-BSS Troubleshooting Manual
Page2Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Objectives
� Upon completion of this course, you will be able to:
� 31160978-BSC Traffic Statistic Manual Volume I
� 31033203-BSS Troubleshooting Manual
Page3Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Contents
1. Principle of Call Drop
2. Analysis of Call Drop
3. Call Drop Cases
Page4Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Contents
1. Principle of Call Drop
1.1 Definition of Call Drop
1.2 TCH Call Drop
1.3 SDCCH Call Drop
Page5Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page6Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Contents
1. Principle of Call Drop
1.1 Definition of Call Drop
1.2 TCH Call Drop
1.3 SDCCH Call Drop
Page7Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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
Page8Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Formula of TCH Call Drop Rate
� Formula of TCH call drop rate
� TCH call drop rate=TCH call drops / Successful TCH seizures (all)
Page9Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Measurement Point of TCH Call Drop
Page10Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page11Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page12Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
TCH Call Drop - Assignment
MS BTS BSC
ASSIGN CMMAND
MSC
ASSIGNMENT REQUEST
SABM
UA
ESTABLISH INDICATION
CHANNEL ACTICATION
CHANNEL ACTIVATION ACK
A1
ERROR INDICATION
CONNECTION FAILURE INDICATION
A2
Page13Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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
CONNECTION FAILURE INDICATION
A2
SABM
Page14Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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 ACT
CH ACT ACK
MSC
A1
ERROR INDICATION
CONNECTION FAILURE INDICATION
A2
SABM
HANDOVER REQUEST
Page15Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page16Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page17Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Successful TCH Seizure–Very Early Assignment
� Very early assignment process
MS BTS BSC MSC
Channel Request (RACH) Channel Required
Channel Activation (TCH or SDCCH)
Channel Activation Acknowledge
Immediate Assignment Command
Immediate Assignment (AGCH)
Page18Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Successful TCH Seizure – Assignment
� Assignment process
MS BTS BSC MSC
ASS_REQ
CH_ACT
CH_ACT_ACK
ASS_CMDASS_CMD
EST_IND
ASS_CMP ASS_CMPASS_CMP
Page19Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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 SABM
Establish_IND
PHY INFO
Handover CompleteHO_Performed
UA
MSC
Page20Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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
Page21Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Contents
1. Principle of Call Drop
1.1 Definition of Call Drop
1.2 TCH Call Drop
1.3 SDCCH Call Drop
Page22Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Formula of SDCCH Call Drop Rate
� Formula of SDCCH call drop rate:
� SDCCH call drop rate = SDCCH call drops/ successful SDCCH
seizures*100%
Page23Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page24Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Measurement Point of SDCCH Call Drop
� 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.
Page25Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Measurement Point of SDCCH Call Drop
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)
Page26Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page27Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Contents
1. Principle of Call Drop
2. Analysis of Call Drop
3. Call Drop Cases
Page28Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Analysis of Call Drop
� content
� main causes of high call drop rate
� troubleshooting of high call drop rate
Page29Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page30Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page31Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Radio Link Fault
� Diagram of radio link timeout
Page32Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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
Page33Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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
Page34Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Radio Link Fault - Interference
� Category
� Co-channel interference
� Adjacent-channel interference
� Inter-modulation interference and other external interference
Page35Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Radio Link Fault - 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
Page36Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Radio Link Fault - Interference
� 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.
Page37Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Radio Link Fault - Coverage
� Coverage:
� Overshooting
� Coverage hole
� Signal attenuation
� Incomplete definition of adjacent cells
� Imbalance of uplink/downlink
Page38Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Radio Link Fault - Coverage
� 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
Page39Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Radio Link Fault - Coverage
� Solution
� Adjust network parameters
� Add BTS
Page40Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page41Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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
Page42Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page43Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page44Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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)
Page45Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Equipment Problem
� Call drop arising from equipment problem
� Hardware problem
� Transmission problem
� Antenna and feeder fault
� Other causes
Page46Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Contents
1. Principle of Call Drop
2. Analysis of Call Drop
3. Call Drop Cases
Page47Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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?
Page48Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page49Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Call Drop Case 1
� Analysis
2.92
12.57
14.25
IB2
13.27
3.14
1.14
IB3
0.25
0.03
0.27
IB4
cell 3
cell 2
cell 1
09:00~10:00
0
4.09
2.85
IB1
0.37
0.01
0.54
IB5
2.12
2.89
4.28
IB2
0.00
0.00
0.00
IB3
0.00
0.00
0.00
IB4
cell 3
cell 2
cell 1
03:00~04:00
0
4.09
2.85
IB1
0.00
0.00
0.00
IB5
Page50Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page51Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page52Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page53Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page54Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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
Page55Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page56Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page57Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page58Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page59Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page60Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page61Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page62Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page63Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page64Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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?
Page65Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Call Drop Case 4
� Troubleshooting
� The adjusted parameters are listed below.
1510Min. DL level on candidate cell
6070UL Qual. Thrsh. (Emergency handover)
6872PBGT HO threshold
24PBGT valid time
35PBGT watch time
Value after change
Value before change
Parameter name
Page66Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page67Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page68Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page69Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page70Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page71Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page72Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Call Drop Case 6
� Troubleshooting
� After adjusting GSM900 clock system, the abnormal call drop
problem is solved.
Page73Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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.
Page74Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
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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