The document discusses handover analysis and troubleshooting. It covers measurement points of intra-BSC and inter-BSC handovers, the handover data process, analysis of common handover problems including their causes, and a case study of low incoming handover success rates. The goal is to understand handover principles, analyze issues, and solve problems.
2 g case analsyis handover training-20060901-a-2.0
1. Internal
OMF000401
Case Analysis -- Handover
ISSUE 2.0
www.huawei.com
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2. References
31160978-BSC Traffic Statistic Manual Volume I
31033203-BSS Troubleshooting Manual
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3. Upon completion of this course, you are
supposed to be able to:
Understand the principles of handover
Analyze and solve handover problems
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4. Chapter 1 Measurement Points of Handover
Chapter 2 Handover Data Process
Chapter 3 Analysis of Handover Problem
Chapter 4 Handover Cases
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5. Measurement Points of Intra BSC Handover
MS BTS(Source) BSC BTS(Target) MSC
Measurement Report
Measurement Report Attempted internal inter cell H.O
Handover decision
Channel Activation
Channel Activation ACK
Internal inter cell H.O
Handover Command (Old FACCH)
Handover Access (New FACCH)
Handover Complete (New FACCH) Successful internal inter cell H.O
RF Channel Release
Handover Performed
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6. 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 cell handover
success rate
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7. Measurement Points of Inter-BSC Handover
MS BTS(Source) BSC MSC BSC BTS(Target)
Attempted outgoing inter BSC inter cell H.O
Measurement Report
Attempted incoming inter BSC inter cell H.O
Handover Required
Handover Request
Channel ACT
Channel ACT ACK
Handover Request ACK
Handover Command
Handover Access
Handover Detect
Handover Complete
Successful incoming inter BSC H.O
Handover Complete
Clear Command (HO successful)
Successful outgoing inter BSC H.O
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8. Measurement Points of Inter-BSC Handover
MS BSC-A MSC-A MSC-B VLR-B BSC-B MS
HO-Required Prepare_HO Allocate_HO_NUM
Send_HO_Report
Send_HO_Report_ACK
HO-Request
Prepare_HO_ACK HO-Request-ACK
IAI
HO-Command ACM
HO-Access
Process_Access_Signalling Some intermediate steps are omitted
HO-Complete
Send_End_Signal
Clear-Command
Clear-Complete
Send_End_Signal_ACK
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9. 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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10. Chapter 1 Measurement Points of Handover
Chapter 2 Handover Data Process
Chapter 3 Analysis of Handover Problem
Chapter 4 Handover Cases
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11. Handover Data Process
Search MSC2
Handover target cell
judgement ( 4 (7) MSC1
)
Inter-MSC
BSC1 CGI ( 6 ) H.O ( 8 )
H.O request ( 7 )
BTS1 Channel BSC2
active ( 5 )
Preprocessed BTS2
BA
table ( 1 ) MR ( 3 )
MS
MR ( 2 )
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12. 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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13. 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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14. Chapter 1 Measurement Points of Handover
Chapter 2 Searching Process of Handover Data
Chapter 3 Analysis of Handover Problem
Chapter 4 Handover Cases
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15. Analysis of Handover Problem
Types of handover problems
Locating handover problem
Causes of handover problem
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16. 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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17. 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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18. 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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19. Causes of Handover Problem
causes Types No H.O H.O failure Frequent H.O
Coverage √ √ √
Interference √
Antenna and feeder √
system
BTS hardware √
transmission √
BSC hardware √
Data configuration √ √ √
A interface √ √ √
Target cell congestion √ √
H.O between equipment of √
different manufactures
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20. 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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21. Island effect results in handover failure
There is no
adjacent cell, so
handover becomes Non-adjacent
impossible. Adjacent
cell
cell N1
Service cell
Non-adjacent
cell Adjacent
Cell N2
Non-adjacent Adjacent
cell Cell N3
Isolated island
resulting from
over shooting
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22. 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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23. BTS hardware
CDU, splitter/combiner failure
TRX failure
TMU failure
Clock failure
Internal communication cable
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24. 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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25. 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" are
different from those in the opposite BSC.
The DPC of BSC in MSC "LAI and GCI Table" is incorrect.
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26. 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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27. 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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28. Chapter 1 Measurement Points of Handover
Chapter 2 Searching Process of Handover Data
Chapter 3 Analysis of Handover Problem
Chapter 4 Handover Cases
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29. 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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30. 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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31. 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 it is 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 successful
occasionally. 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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32. 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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33. 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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34. 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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35. 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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36. 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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37. 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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38. 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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39. 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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40. Handover Cases 3
Handover failure
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41. Handover Cases 3
HO-REQUEST
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42. 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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43. Handover Cases 3
Conclusion:
In cooperation with equipment of other manufacturers, faults
can be located by analyzing the signaling.
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44. 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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45. 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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46. 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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47. 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, Huawei equipment
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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48. 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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49. 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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50. Handover Cases 5
Analysis 1: Trace the signalings at A interface and E interface.
Normal signaling is as follow:
MS BSC-HW MSC-HW MSC-B VLR-B BSC-B MS
HO-REQUIRED MAP_Prepare_HO MAP_Allocate_HO_NUM
MAP_Send_HO_Report
MAP_Send_HO_Report_ACK
HO-REQUEST
HO-REQUEST-ACK
MAP_Prepare_HO_ACK
IAI
HO-Command ACM
HO-Access
MAP_Process_Access_Signaling Intermediate steps
HO-Complete are omitted.
MAP_Send_End_Signal
Clear-Command
Clear-Complete
MAP_Send_End_Signal_ACK
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51. Handover Cases 5
Analysis 2: Abnormally signaling is as follow:
MS BSC-HW MSC-HW MSC-B VLR-B BSC-B MS
HO-REQUIRED MAP_Prepare_HO
MAP_Abort
HO-REJECT
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52. 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 the handover 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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53. Handover Cases 5
Conclusion:
There are often many problems in coordination between the
equipment of different manufacturers. However, problems
can be located accurately by tracing signaling.
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54. 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.
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55. 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.
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56. 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.
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57. 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.
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58. 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.
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59. 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.
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60. 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.
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61. 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:
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62. Handover Cases 7
A interface signaling analysis of inter-BSC handover in MSC:
Failure signaling:
Success signaling:
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63. 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, the
problem is the same: there are several handover requests
in a call duration but all requests fail.
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64. 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.
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65. 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 problem is 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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Difference between "Internal inter cell handover success rate " and "Cell radio handover success rate " : As viewed from formulas, both numerators are success times of handover, while the denominators are different. While viewed from the measurement points of the counter, Attempted inter cell handovers >= inter cell handovers, so intra BSC handover success rate <= cell radio handover success rate. In practice: If "Internal inter cell handover success rate "="cell radio handover success rate ", this indicates that there is no problem with data, the unsuccessful handovers are caused by radio interfaces, and the radio reasons like interference and coverage should be checked in the first place. If "Internal inter cell handover success rate " < "cell radio handover success rate", this indicates that failures may exist in the process from “Channel_Activate” to "HO-COMD", and there might be data problem or congestion.
The key measurement points are as follows, same as those of intra BSC handover: After sending "HO-REQUIRED", the original BSC measures "Attempted outgoing inter BSC inter cell handovers". After receiving "HO-REQUEST", the target BSC measures "Attempted incoming inter BSC handovers". After sending "HO-REQUEST ACK", the target BSC measures "incoming inter BSC handovers". After receiving "HO-Command" , the original BSC measures "outgoing inter BSC handovers". After receiving "HO-Complete", the target BSC measures "Successful incoming inter BSC handover" After receiving "Clear-COM" and the cause value is "HO-Successful", the original BSC measures "Successful outgoing inter BSC inter cell handover". The difference between "handover times" and "handover request times": Handover times - After "HO-COM" is received or "HO-REQ-ACK" is sent Handover request times - After "HO-REQUIRED" is sent or "HO-REQUEST" is received Therefore, inter BSC inter cell radio handover success rate >= inter BSC inter cell handover success rate.
Note: Signaling flow of A interface and Abis interface of inter MSC handover is the same as that of intra MSC handover, only the signaling between two MSC is particular for the inter-MSC handover. As shown in above figure the signaling with "MAP" is of the MAP layer, and signaling of A and Abis interfaces are omitted. After receiving "HO-REQUIRED" of BSC-A (the request message includes CGI of the original cell and target cell), if MSC-A finds that LAC of the target cell doesn’t belong to this MSC, MSC-A will query the "REMOT LAC Table“ (including the LAC and route of the adjacent MSC), and send "Prepare-HO" message to MSC-B according to the route. CGI of the target cell and the indicator of whether to allocate the handover number are included in this Prepare-HO message. According to the received "Prepare-HO" message, if the handover number needs allocating, MSC-B will request the local VLR to allocate the handover number. If VLR has the free handover number, the handover number will be sent to MSC-B through "Send-HO-Report". If no handover number is needed, proceed to the next step. After SCCP link between MSC-B and BSC-B is established, MSC-B sends "HO-REQUEST" to BSC-B. After that, BSC-B activates the target cell’s channel, and returns "HO-REQUEST ACK" to MSC-B after receiving the channel activation acknowledgement. According to this message, MSC-B sends "Prepare-HO ACK" to MSC-A. MSC-A establishes the route to MSC-B according to the handover number, and sends "Initialize-Address" (IAI) to MSC-B to help the latter to identify which voice channel is reserved for MS. While MSC-B returns "Address-Complete" (ACM) to MSC-A.
Frequent handover: Handover in GSM is hard handover, it is meaning, before handover to the new channel, MS must release the original channel firstly. So frequent handover will result in word-loss in handover and break-make of conversation, thus affecting the conversation quality.
In isolated cell coverage area, when MS moves towards the cell edge, the signal becomes weaker and weaker, and since there is no adjacent cell available around, handover cannot be triggered, thus call drop will occur.
For BTS clock, BSC clock, and MSC clock, the following standards are used to judge whether they are out of synchronization: MSC: △f/f (frequency deviation) ≤ 1E-8 BSC/BTS: △f/f (frequency deviation) ≤5E-8 Inaccurate clock will cause that MS cannot decode BSIC of the adjacent cell correctly.
For service cell: Outgoing inter cell handover will fail. For illusory target cell: Incoming inter-cell handover will fail, and successful rate of handover = successful rate of radio handover. Notes: CI can not configure as "FFFF". "Transmitting BS/MS Power Level": If the measurement report preprocess is enabled, this parameter must be set to "Yes".
The cause value of the handover failure message: Invalid message content.
"Address Indicator" differs from the normal "HO-REQUEST".
Signaling tracing is very helpful if there are problems in cooperation with other manufacturers’ equipment.
After BSC-HW sends "HO-REQUIRED" to MSC-HW, MSC-HW sends "Prepare-HO" to the peer, however MSC-B directly returns abort message, causing that BSC-HW receives "HO-Reject" from MSC-HW. The handover fails.
Failure signaling: MSC receives "HO-REQUEST-ACK" sent by Huawei BSC, and sends "HO-COMD" to N vender BSC. While MS doesn’t perform "HO-ACCESS", so Huawei BSC do not send "HO-Detect" to MSC, and MSC sends "Clear" command to N vender BSC, with the reason "Radio interface error, return to the original channel".