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Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Case Analysis --
Handover
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Page1Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
References 31160978-BSC Traffic Statistic Manual Volume I
31033203-BSS Troubleshooting Manual
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Page2Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Objectives Upon completion of this course, you will be able to:
Understand the principles of handover
Analyze and solve handover problems
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Page3Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Contents1. Measurement Points of Handover
2. Handover Data Process
3. Analysis of Handover Problem
4. Handover Cases
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Page4Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Measurement Points of Intra BSC Handover MS BTS(Source) BSC BTS(Target) MSC
Measurement Report
Measurement Report
Handover decision
Channel Activation
Channel Activation ACK
Handover Command (Old FACCH)
Handover Access (New FACCH)
Handover Complete (New FACCH)
RF Channel Release
Handover Performed
Attempted internal inter cell H.O
Internal inter cell H.O
Successful internal inter cell H.O
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Page5Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Measurement Points of Intra BSC Handover Handover formula definition
Inter cell radio handover success rate
=(Successful incoming internal inter cell handovers + Successful
outgoing internal inter cell handovers) / (Incoming internal inter cell
handovers + Outgoing internal inter cell handovers )
Internal inter cell handover success rate
=(Successful incoming internal inter cell handovers + Successful
outgoing internal inter cell handovers) / (Attempted incoming internal
inter cell handovers + Attempted outgoing internal inter cell handovers)
Inter cell radio handover success rate >= Internal inter cellhandover success rate
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Page6Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
MS BTS(Source) BSC MSC BSC BTS(Target)
Measurement Report
Handover Required
Handover Request
Channel ACT
Channel ACT ACK
Handover Request ACK
Handover Command
Handover Access
Handover Detect
Handover Complete
Handover Complete
Clear Command (HO successful)
Attempted incoming inter BSC inter cell H.O
Successful outgoing inter BSC H.O
Attempted outgoing inter BSC inter cell H.O
Successful incoming inter BSC H.O
Measurement Points of Inter-BSC Handover
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Page7Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
MSC-BMSC-A VLR-BBSC-A BSC-B
HOHO--RequiredRequired Prepare_HO
Prepare_HO_ACK
Allocate_HO_NUM
Send_HO_Report
Send_HO_Report_ACK
MS
HOHO--RequestRequest
HO-Request-ACK
HOHO--CommandCommand
MS
HO-AccessProcess_Access_Signalling
HOHO--CompleteCompleteSend_End_Signal
ClearClear--CommandCommand
Clear-Complete
Send_End_Signal_ACK
Some intermediate steps are omitted
IAIACM
Measurement Points of Inter-BSC Handover
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Page8Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Measurement Points of Inter-BSC Handover
Inter BSC handover Success rate
=(Successful incoming inter BSC inter cell handovers + Successful outgoing inter BSC
inter cell handovers) / (Attempted incoming inter BSC inter cell handovers + Attempted
outgoing inter BSC inter cell handovers )
Incoming BSC handover Success rate
=(Successful incoming inter BSC inter cell handovers) / (Attempted incoming inter BSC
inter cell handovers)
Outgoing BSC handover Success rate
=(Successful outgoing inter BSC inter cell handovers ) / (Attempted outgoing inter BSC
inter cell handovers)
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Page9Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Contents1. Measurement Points of Handover
2. Handover Data Process
3. Analysis of Handover Problem
4. Handover Cases
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Page10Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Handover Data Process
BA table(1)
MR(2)MS
BTS1
BSC1
Preprocessed
MR(3)
Handover
judgement(
4)
Channel active(5)
MSC1
CGI(6)
BTS2
MSC2
BSC2
H.O request(7)
Inter-MSC H.O(8)
Searchtarget cell(7)
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Page11Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Handover Data Process BCCH frequencies of all adjacent cells in BA2 table are sent to
MS on system message 5.
MS reports measurement report to BSS. It includes the BCCH ,
BSIC and signal level of the adjacent cells and serving cells.
When the measurement report is preprocessed, BSC determines
the CGI of all adjacent cells through BCCH frequency and BSIC .
BSC executes handover judgment flow such as basic cell
ranking . Once a proper target cell is found, the handover request
message which includes the target cell CGI will be sent to MPU of
BSC , and then MPU will confirm the module ID of the cell based
on the CGI.
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Page12Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Handover Data Process MPU sends handover request message to the module and makes
statistics of "outgoing inter cell handover request". At the same time BSC
send the “channel active” to the BTS. If the target cell triggered by BSC is an external cell, the CGI of the target
cell and service cell is sent to MSC on the handover required.
By matching the CGI of the target cell, MSC searches target cell . Once
the cell is found, MSC will confirm which BSC is belonged to, and send
the handover request message to this BSC.
If there is no CGI of the target cell , MSC will check “Adjacent MSC
Table" and find the target MSC, then send the handover request
message to that MSC.
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Page13Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Contents1. Measurement Points of Handover
2. Handover Data Process
3. Analysis of Handover Problem
4. Handover Cases
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Page14Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Analysis of Handover Problem Types of handover problems
Locating handover problem
Causes of handover problem
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Page15Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Types of Handover Problems Types- Possible Results
No handover-
Cause call drop Handover failure- Affect the conversation quality, and call drop.
Frequent handover- Affect the conversation quality and increase
load of the system
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Page16Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Locating Handover Problem Traffic statistics analysis
Measurement of BSC overall performance
Measurement of inter cell handover performance
Measurement of outgoing/incoming inter cell handover performance
Measurement of performance of undefined adjacent cell
View alarm Board fault, transmission, clock, etc.
Drive test
Signaling analysis A interface, E interface, Abis interface
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Page17Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Causes of Handover Problem Coverage
Interference
Antenna and feeder system
Base station hardware
Transmission
BSC hardware
Data Configuration
A interface
Target cell congestion Cooperation with equipment of other manufacturers
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Page18Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Causes of Handover Problem
√H.O between equipment ofdifferent manufactures
√√Target cell congestion
√√√A interface
√√√Data configuration
√BSC hardware
√transmission
√BTS hardware
√Antenna and feeder system
√Interference
√√√Coverage
Frequent H.OH.O failureNo H.OTypescauses
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Page19Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Coverage and Interference at Radio Interface Coverage:
Poor coverage: forest, complicated topography, building direction
and indoor coverage
Isolated site: no adjacent cell
Over shooting: island effect result in no adjacent cell
Interference: MS can not access network or receive any signal.
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Page20Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Adjacent
Cell N3
Adjacent
Cell N2
Adjacent
cell N1
Non-adjacentcell
Non-adjacent
cell
Non-adjacent
cell
Service cell
There is noadjacent cell, sohandover becomes
impossible.
Isolated island
resulting fromover shooting
Island Effect Results in Handover Failure
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Page21Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Antenna and Feeder System High Voltage Standing Wave Ratio (VSWR)
Antenna is not properly installed
Antenna is not parallel
The azimuth and downtilt are not correct
Poor antenna isolation value
RF cables, connection are loose or incorrect
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Page22Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
BTS Hardware CDU, splitter/combiner failure
TRX failure
TMU failure
Clock failure
Internal communication cable
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Page23Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Analysis of Handover Problem Transmission
Transmission is not stable
Serious BER in transmission
Fault of BSC Hardware
Clock board: the faulty clock board causes clock inconsistency
between base stations.
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Page24Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Data Configuration Unsuitable setting of handover hysteresis and handover priority.
Unsuitable setting of P, N value of statistic time
Unsuitable frequency and adjacency relation configured in
BA1/BA2 table
CGI, BCCH and BSIC in "External Cell Description Table" aredifferent from those in the opposite BSC.
The DPC of BSC in MSC "LAI and GCI Table" is incorrect.
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Page25Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Analysis of Handover Problem A interface problem
Basically, the insufficient link resource results in the abnormal
handover, as well as abnormal communication.
Circuit pool numbers are different ,causing the handover failure.
Target cell congestion
The target cell is congested, which causes the handover failure.
Then the target cell should be expanded or reduce its traffic .
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Page26Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Analysis of Handover Problem Handover between equipment of different manufacturers
The signaling at A interface, E interface of the opposite
equipment are not matched to our equipment and can not be
recognized or supported, which causes the handover failure,
such as voice version, handover number, TUP circuit,
addressing mode (CGI or LAI).
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Page27Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Contents1. Measurement Points of Handover
2. Handover Data Process3. Analysis of Handover Problem
4. Handover Cases
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Page28Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Handover Case 1 Fault Description
A 1800 cell of a dual-band network (all the 1800 cells belong
to one BSC), the incoming handover success rate of incoming
BSC and intra BSC are low from the beginning of the service,
while the outgoing handover success rate of intra-BSC and
inter-BSC are normal.
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Page29Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Handover Case 1 Analysis:
Register the traffic statistics and analyze whether the low handover
success rate is due to the failure of handover from all cells to this cell
or from some few cells.
If handover fails from some few cells to this cell, check the handover
data and see whether there is co-channel and co-BSIC problem. If handover fails from all other cells to this cell, check the data of this
cell.
If data cause is ruled out, check the hardware carefully. Check the
alarm or perform drive test to locate uplink fault or downlink fault.
Check step by step and find out the cause.
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Page30Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Handover Case 1 Solution:
Register the incoming inter cell handover measurement function and find that
the successful rate of handover from all other cells to this cell is low, although itis not always 0 percent. Based on careful data checking, the data of this cell is
correct.
Perform drive test and find that the downlink signal is normal but almost all
handovers to this cell are failure. Near the BTS, the handover is successfuloccasionally. Perhaps the problem is with the uplink signal.
Check the uplink channel, antenna, CDU, they are no problem. Change the
TRX , everything is normal.
Conclusion:
The symptom is that the uplink and downlink at UM interface are unbalanced
so uplink voice quality is bad.
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Page31Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Handover Case 2 Fault Description
After LAPD software upgrade of a dual-band network on 19th
September, it was found that some cells in module 4 under
1800 network are seriously congested (40%~50%), and
overall handover indices of BSC come down from 95% to 90%.
From "Inter -cell Hanover Measurement Function, it was found
that besides module 4, some cells of other modules also suffer
from the problem that inter cell handover success rate is low.
After 19th September, the situation became worse.
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Page32Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Handover Case 2 Analysis:
Check whether it is attributed to LAPD software upgrade.
Analyze traffic statistics and the main causes of the failure to
locate the problem.
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Page33Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Handover Case 2 Solution:
Check whether it is attributed to LAPD software upgrade. After the
upgrade, not all cells under module 4 are congested and the handover
indices of some cells are not low. Upgrade isn’t the cause.
Analyze traffic statistics:
Congested cells are mostly at site A and site B. Their traffics are
lower than before the upgrade. It is a pseudo congestion.
The cells where handover success rate is low are mostly at site A,
B, C, D and E, the main cause of the failure is timer timeout.
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Page34Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 2 Solution:
Check alarm: At 19:31 of 18th September, the clock system
alarms of site A and site B occurred. It is found that 13M clock
isn’t synchronous. The clock problem result in low handover
success rate of the two sites. Such impact spread to the
adjacent cells of these two cells and even affected the whole
network.
Analyze handover data and traffic statistics: All cells where
handover success rate is low are adjacent to A and B except A
and B themselves.
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Page35Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 2 Solution:
Reset site A and site B, the clock system is recovered, and the
handover success rate inter/intra BSC goes up to 93%. The problem
is solved.
Conclusion: The problem in clock system will result in low handover success
rate. Pay attention to the alarm console and the excursion of the
clock system.
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Page36Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 3 Fault Description:
Huawei BSC and another manufacturer’s BSC (S vender) are
connected to the same MSC. After cutover, MS can not handover
from S BSC to Huawei BSC but opposite is normal.
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Page37Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 3 Analysis:
Since incoming BSC handover request times is 0 but the
outgoing handover is normal. Carefully check the outgoing
inter-cell handover signaling and handover data at S BSC.
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Page38Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 3 Solution:
Trace interface message. After receiving HO-REQUEST
message, Huawei BSC returns HO-FAILURE message
immediately. In normal conditions Huawei BSC should return
HO-REQ-ACK message. Carefully analyze HO-REQ message
and HO-FAILURE message.
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Page39Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 3Handover failure
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Page40Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 3
HO-REQUEST
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Page41Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 3
Solution:
HO-FALUER: The cause of the failure is invalid message
content
HO-REQUEST: The difference is basically the Address
Indicator in comparison with the normal handover request
message. HUAWEI BSC does not recognize case 41. The
system thinks that Address Indicator must be 0x43. Inform the
owner of network, modify the relation parameter, and this
problem is solved.
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Page42Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 3
Conclusion:
In cooperation with equipment of other manufacturers, faults
can be located by analyzing the signaling.
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Page43Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 4
Fault Description:
Huawei MSC cooperate with MSC of N (vender). The
communication is normal. The Huawei intra BSC handover
and incoming BSC handover are normal, but outgoing BSC
handover success rate is about 25%. The handover from the
N BSC to Huawei BSC is successful, but opposite is failure
sometimes.
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Page44Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 4
Analysis:
Check handover data, including the external cell description
table, BA2 table, cell adjacency relation table inside Huawei
BSC and N BSC and CGI at N MSC. If the data are correct,
check outgoing BSC handover signaling.
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Page45Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 4
Solution:
Check data: There is no fault in BSC and MSC of the two
venders.
Check alarm: BTS maintenance console, No.7 link and A
interface circuit are all normal.
Trace Huawei A interface signaling. After "HO-REQUIRED"
message is sent, "HO-COMD" command is not received from
MSC.
Trace MAP message at E interface (inter-MSC) with a
signaling meter. It is found that after receiving HO-Number,
Huawei MSC never sends IAI (Initial Address Information).
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Page46Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 4
Solution:
Check HO-Number message from the opposite and find that the
handover code format is 130********(only a number of mobile
telephone, no any prefix). But this mobile company requires that
roaming/handover between exchanges should be in the format:
00+country code + roaming/handover number. In addition, Huaweiequipment does not recognize the handover number without
00+country code. Therefore the signaling is halted.
Discuss with N, asking them to add "00+country code" before the
handover number. Then outgoing BSC handover is normal.
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Page47Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 4
Conclusion:
Problems can be located quickly and accurately if you are
familiar with the signaling flow of handovers.
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Handover Cases 5
Fault Description:
In the independent MSC networking of a dual band network,
Huawei 1800 network cooperate with 900 network of vender A .
After cutover, the successful rate of handover from M1800 to
M900 is low (about 60%), while the handover from M900 to
M1800 are normal and the handover data are correct.
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Analysis 1: Trace the signalings at A interface and E interface.
Normal signaling is as follow:
MSC-BMSC-HW VLR-BBSC-HW BSC-B
HO-REQUIRED MAP_Prepare_HO
MAP_Prepare_HO_ACK
MAP_Allocate_HO_NUM
MAP_Send_HO_Report
MAP_Send_HO_Report_ACK
MS
HO-REQUEST
HO-REQUEST-ACK
HO-Command
MS
HO-AccessMAP_Process_Access_Signaling
HO-Complete
MAP_Send_End_SignalClear-Command
Clear-Complete
MAP_Send_End_Signal_ACK
Intermediate stepsare omitted.
IAIACM
Handover Cases 5
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Page50Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Analysis 2: Abnormally signaling is as follow:
MSC-BMSC-HW VLR-BBSC-HW BSC-B
HO-REQUIRED MAP_Prepare_HO
MS
HO-REJECT
MS
MAP_Abort
Handover Cases 5
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Page51Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 5
Solution:
Analyze the Prepare-HO message sent from Huawei MSC to the
opposite. The voice version is full-rate version 1, 2 and half-rate
version 1. It is a PHASE 2+ version. However there is only full-rate
version 1 in the message sent from M900 to Huawei M1800. It turns
out that the opposite does not support half-rate version, so thehandover fails.
Modify A interface circuit pool table of Huawei MSC data, only using
full-rate version 1.
Then the outgoing BSC handover is normal.
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Page52Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 5
Conclusion:
A GSM network, where one MSC is attached with two BSC, is
all configured with Huawei equipment. When the two BSC are
cutover, the successful rate of handover from BSC1 to BSC2
is 0, but opposite is normal. Intra BSC handover of BSC1 and
BSC2 are normal.
C
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Page53Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 6
Fault Description:
A GSM network, where one MSC is attached with two BSC, is
all configured with Huawei equipment. When the two BSC are
cutover, the successful rate of handover from BSC1 to BSC2
is 0, but opposite is normal. Intra BSC handover of BSC1 and
BSC2 are normal.
H d C 6
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Page54Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 6
Analysis:
Register “outgoing inter-cell handover measurement function”
of BSC1 and “incoming inter-cell handover measurement
function” of BSC2.
Check the data related to handover of BSC1 and BSC2.
Analyze signaling of handover failure.
H d C 6
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Page55Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 6
Solution:
Check all data of inter-BSC handover: external cell description
data table, BA2 table, CGI of MSC. No problem is found.
Trace A interface message of BSC1 and BSC2. After BSC1
sends "HO-REQUIRED", BSC2 has not received "HO-
REQUEST" message, but opposite is normal.
The path of data search: MSC goes to "LAI and GCI Table"
according to the CGI of the target cell in "HO-REQUIRED"
message, and sends HO request to the correct BSC base on
the description of the DPC of the cell given in the table.
H d C 6
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Handover Cases 6
Solution:
The HO-REQUEST message, which should have been sent to
BSC2, is sent to BSC1. It is DPC error. After correction, the
problem is solved.
H d C 6
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Handover Cases 6
Conclusion:
For the data of inter-BSC handover, besides checking whether
CGI at MSC side is consistent with that of opposite and BSC,
check whether the DPC is correct. MSC look up target DPC
based on the CGI of the target cell. When the DPC is incorrect,
"HO-REQUEST" will be sent to wrong BSC.
Hando er Cases 7
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Handover Cases 7
Fault Description:
A dual band network is a mixed networking of Huawei BSC
(M1800) and vender N BSC (M900), attached to MSC of
vender S. After the cutover, the successful rate of Huawei
incoming BSC handover is always 88%~92%, while outgoing
BSC and intra-BSC handovers are normal (above 92%).
Furthermore, the cells with low successful rate of incoming
BSC handover are randomly distributed.
Handover Cases 7
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Handover Cases 7
Analysis:
Since the handover success rate is not very low and the low
ratio is not in some few cells, data problem is excluded.
Perhaps it is due to some interference at the radio interface
coverage hole. So, trace the signaling.
Handover Cases 7
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Handover Cases 7
Solution:
Perform forced handover with a test mobile between several
cells where handover success rate is low. The forced
handovers are all successful. Data problem is excluded.
The interference band of cells is ideal, call drop and congestion
rate are normal. It is unlikely that the coverage and interference
at the radio interface causes the problem.
Trace A interface signaling and compare the failure signaling
with success signaling:
Handover Cases 7
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Page61Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Failure signaling:
Success signaling:
Handover Cases 7
A interface signaling analysis of inter-BSC handover in MSC:
Handover Cases 7
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Page62Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 7 Solution:
The comparison result is that Huawei BTS has not detected
any MS access information (No HO DETECT signaling occurs).It shows that there is problem when the MS is accessing
M1800 cells.
Reconfirm that there is no problem with the handover data,otherwise the handover success rate will be very low.
Re-trace large number of handover signaling to find that all
failures are attributed to the same cause. In addition, theproblem is the same: there are several handover requests in a
call duration but all requests fail.
Handover Cases 7
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Page63Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Handover Cases 7
Solution:
Analyze other similarities of the failed message, to find that the
first 6 digits of IMEI numbers of these MS are all 449684. It
shows that the handover problem is related to the MS.
Find the MS owners with the IMSI numbers and find that all MS
with handover problem are F MS. Not long ago it was proved
that this MS has poor insulation performance between M900
and M1800, so it can not access M1800 network. Now the
cause is found.
Handover Cases 7
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Handover Cases 7
Conclusion: MS problem will also causes handover failure.
In this case the handover success rate is not very low, therefore
coordination problem is unlikely.
Trace the signaling to find the cause of the handover failure: BTS has
not received handover access message from MS.
Find the similarity of these MS through large amount of signaling
tracing: IMEI numbers are the same. Then it is found that the problemis on F MS.
The coverage of M900 is very good, F MS is always under M900 in
idle status. Therefore in a call, TCH assignment seldom fail, which will
not result in high congestion rate. But in handover, problem is very
likely to happen.
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