39920096 omd6046 case analsyis handover issue1 1
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OMD6046Case Analsyis--Handover
ISSUE1.1
Wireless Curriculum
Development Section
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Outline of Handover Topic Statistics by handover counter
Searching process of handover data
Analysis of causes of handoverproblem
Handover cases
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Statistics of handover counter
Intra-BSC handover
Inter-BSC handover
Statistic Point of Handover Counter
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Statistic Point of Handover CounterIntra-BSC Handover
MS BTS(Source) BSC BTS(Target) MSC
Measurement Report
Measurement Report
Handover algorithm verdict on the cell handover request T09++
(In-cell handover request get to)T12++
Channel Activation
Channel Activation ACK
Handover Command (Old FACCH)
Handover Access (New FACCH)
Handover Complete (New FACCH) T10++ T13++
RF Channel Release
Handover Performed
Attempted incoming internalinter cell handovers
Successful incominginternal in-cellhandover
Attempted outgoing internalinter cell handovers
Successful outgoing internal in-cellhandover
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Possible causes of failure of intra-BSC handover
No available channel in the destination cell
Channel activation negative acknowledgementCHAN ACK NACK Transmitting channel activation message failure
Waiting channel activation timeout
MS access failureRadio interface may be the cause
Max repeat time of physical information*radio link connected withthe timer < time interval of EST IND~HO DETECT (120~180ms)
Waiting to timeout after establish indication (MS has not receivedUA frame or has not sent handover complete message)
Statistic Point of Handover Counter Intra-BSCHandover
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Thesa
measint
ra-MSC
Statistic Point of Handover Counter Inter-BSC HandoverMS BTS(Source) BSC MSC BSC BTS(Target)
Measurement Report
MeasurementReport
Handover Required T17++
HandoverRequest T14++
Channel ACT
Channel ACT ACK
Handover Request ACK
Handover Command
Handover Access
Handover Detect
Handover Complete
Handover Complete T15++
Clear Command(HO successful) T18++
RF Channel Release
Clear Complete
Attempted outgoing inter-BSCinter cell handovers
Attempted incominginter-BSC inter cell handovers
Successful outgoing interBSCinter cell handover
Successful incoming interBSCinter cell handover
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Signaling flow between MSCs
MSC-BMSC-A VLR-BBSC-A BSC-B
HO-RequiredMAP_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_Signalling
HO-CompleteMAP_Send_End_SignalClear-Command
Clear-Complete
MAP_Send_End_Signal_ACK
Some intermediate steps are omitted
IAIACM
Statistic Point of Handover Counter Inter-BSC Handover
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Success ratio of
inter-BSC handover--------------100Success times of inter-cell handover between BSCsRequest times of inter-cell handover between BSCs
Success ratio of
In-BSC handover----------------100Destination BSC receives HO-COMP from MSAfter destination BSC receives HO-REQ
Success ratio ofOut-BSC handover------------100Source BSC receives Clear-COMD from MSC
After BSC sends HO-Required
Statistic Point of Handover Counter Inter-BSC Handover
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Statistics by handover counter
Searching process of handoverdata
Analysis of causes of handoverproblem
Handover cases
Outline of Handover Topic
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Searching Process of Handover Data
1. BCCH of all adjacent cells in BA2 table are sent to MS on system
message 5.2. MS reports service cell and six adjacent cell where level is the most
powerful (through measurement report) to BSS, including the
BCCH , BSIC and signal level of the adjacent cells and service
cells.
3. When the measurement report is pre-processed, BSC determines themodule ID, cell ID and CGI of all adjacent cells through BCCH
frequency and BSIC in Cell Adjacency Relation Table and Cell
Description Data Table (or External Cell Description Data Table).
4. BSC executes handover verdict flow such as basic cell sorting (on
LAPD board). Once a proper destination is found, the handover
request message of the destination cell CGI will be sent to MPU of
BSC host, and then MPU will confirm the module ID of the cell in
Cell Module Information Table based on the CGI.
5. MPU sends handover request message to the module and makes
statistics of Out-cell handover request.
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6. If the destination cell triggered by BSC is an external cell, the
CGI of the destination cell and service cell is sent to MSC with the
handover request.
7. MSC first goes to Location Area Cell Table to search the cell
that matches the CGI of the destination cell. Once the cell is found,confirm which BSC is belong to. Send the handover request
message to this BSC.
8. If there is no CGI of the destination cell in Location Area Cell
Table, go to adjacent MSC table and find the destination MSC,then send the handover request message to this MSC.
Searching Processing of Handover Data
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Outline of Handover Topic
Statistics by handover counter
Searching process of handoverdata
Analysis of causes of handoverproblem
Handover cases
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Analysis of Handover Problem
Types of Handover Problems
Locating problem
Causes of Handover Problem
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Type of Handover Problem
No handoverDropout occurs Handover failureAffect the communication quality,hence the dropout.
Frequent handoverAffect the communication qualityand increase the system load
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Locating Handover
Problem
Analyze traffic statistics
Measurement of BSC overall performance
Measurement of inter-cell handover performance
Measurement of out-cell/in-cell handover performanceMeasurement of performance of undefined adjacent cells
View alarm: Board fault, transmission, clock, etc.
Drive test
Analyze signaling: A interface, E interface, Abis interface
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Coverage and interference at radiointerface
Antenna & feeder system
BS software/hardware Transmission
BSC software/hardware/ DataConfiguration
A interface problem
Destination cell is busy
Interconnection with equipment of othermanufacturers
Analysis of Handover Problem
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I. Coverage and interference at radio interface
1. Coverage:
Poor coverage: forest, complicated topography,building direction and indoor coverage
Isolated site: no-neighbor cell
Over coverage: island effect result in a no-neighbor
cell
2. Interference: consequently, MS uplink can not beaccessed or the downlink can not receive any signal.
Analysis of Causes of Handover Problem
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Island effect results in handover failure
Adjacent
Cell N3
Adjacent
Cell N2
Adjacent
cell N1
Non-adjacent
cell
Non-adjacent
cell
Non-adjacent
cell
Service cell
There is noadjacent cells,so handoverbecomeimpossible.
Isolated islandresulting fromcross-cellcoverage
Analysis of Causes of Handover Problem
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II. Antenna & Feeder System
1. High voltage standing wave ratio(VSWR)
2. Antennae are not properly installed
3. Antennae are not parallel
4. The azimuth and obliquity are not correct
5. Poor antenna isolation
6. RF cables ,connection are loose or incorrectconnection
Analysis of Causes of Handover Problem
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III. BTS software/hardware
1. CDU, splitter/combiner failure
2. TRX, RC board failure
3. TMU failure
4. FPU failure
5. Clock failure
6. Internal communication cable
7. BTS software
Analysis of Causes of Handover Problem
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IV. Transmission1. Transmission is not stable
2. Serious BER in transmission
V. Fault BSC Software/Hardware
Example for:Clock board: the defective clock board
causes clock inconsistency between BSs.
Analysis of Causes of Handover Problem
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Data ConfigurationCause handover failure, delay, frequenthandover, no handover, unreasonable in/out handover ratio Improper setting of handover threshold (higher, lower, the edge handover
threshold is higher than the power control threshold.)
Improper setting of hysteresis handover and priority handover
Improper setting of P, N value of statistic time
Incomplete frequency and adjacency relation configured in BA2 table
There are adjacent cells with the same frequency and the same BSIC
CGI and module ID in Cell Description Data Table are different from those
in Cell Module Information Table.
CGI, BCCH and BSIC in External Cell Description Table are different fromthose in the opposite BSC.
The Destination signaling point of BSC in MSC Location Area Cell Table isincorrect.
Analysis of Causes of Handover Problem
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Precautions:
Frequency of Preprocess Measurement Report: when Measurement
Report Preprocess is enable , if the link configuration is 15:1, the
setting is once per second. At this time, for the handovers that needs
P/N verdict, such as boundary handover, layer handover, PBGT
handover and concentric handover, the P/N verdict time should be
shortened.
BSC can not process the CGI in lower case. Therefore it must be input
in upper case, otherwise the handover would fail.
BSC does not recognize CI as FFFF cell, otherwise the handover
would not happen
Transmitting BS/MS Power Level: If the measurement report
preprocess is enabled, this parameter must be set to Yes.
Analysis of Causes of Handover Problem
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VI. A interface Problem
Basically the insufficient link resource results in the abnormal handover,
as well as abnormal communication.
VII. Destination Cell Busy
The destination cell is busy and there is no available channel, whichcauses the handover failure. Then the destination cell should be
expanded or reduce its traffic .
VIII. Handover between Equipment of Different
Manufacturers
The signalings at A interface, E interface of the opposite end are not
matching to our equipment and they can not be recognized or support,
which causes the handover failure, such as voice version, handover
number, TUP circuit, addressing mode (CGI or LAI).
Analysis of Causes of Handover Problem
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Outline of Handover Topic
Statistics by handover counter
Searching process of handover
data
Analysis of causes of handoverproblem
Handover cases
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Case 1
Fault Description: a 1800 cell of a dual-band network
(all the cells of this are 1800), the success ratio of in-
BSC handover and in-cell handover in BSC are low
from the beginning of the service, while out-cell
handovers inside BSC and between BSCs are normal.
Typical Handover Cases
Typical Handover
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Case 1
Typical HandoverCase
Tips for troubleshooting:
1. Register the traffic statistics and analyze whether the low
handover ratio is due to the failure of handover from all cells to this
cell or from some few cells to this cell.
2. If handover fails from some few cells to this cell, check thehandover data and see whether there is co-channel or same BSIC
problem.
3. If handover fails from all other cells to this cell, check the data of
this cell.
4. 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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Typical Handover Case
Solution:
1. Register the in-cell handover performance measurement and find that
the success ratio of handover from all other cells to this cell is low,
although it is not always 0 percent. Check the data and the data are
correct.
2. Perform drive test and find that the downlink signal is normal but
almost all handovers to this cell fail. In the BS, the handover is
successful occasionally.Perhaps the problem is with the uplinksignal.
3. Check the uplink, antenna, CDU are no problem. Change the TRX and
everything is normal.
Conclusion: The symptom is that the uplink and downlink atUM port are not balanced so the opposite hears low qualityvoice.
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Typical Handover Case
Fault Description: in a cell of a 900M mono-
frequency network, the success ratio of in-cell
wireless handover in BSC is low (1030), whichis the same as handover success ratio.
Case 2
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Typical Handover Cases
Solution:
1. Check data: register In-cell Handover Performance Measurement and
find that the success ratio of handover into all adjacent cells is low, but is not
0. Check the data to find that there is no co-frequency 0f BCCH and co-BSIC
problem. Data cause is ruled out.
2. Check coverage:
The traffic in this cell is relatively low. Check BS Maintenance to find that
TCH channel can be occupied, but occupation times are few.
Perform a drive test. In a place 2 km from the BS, the downlink receiving
signal is85dbm, but in-cell handovers always fail or dropout always occursat handover.
Make dial test in this cell by locking frequency, the caller can not be
created. The called can be created but can not call out. A rough conclusion is
that poor uplink causes the problem.
3. Check the installation of uplink hardware, nothing abnormal is identified.
Maybe a fault has occurred to the uplink path of TRX or CDU. Replace CDU,
the problem is solved.
Conclusion: hardware fault results in poor signal,and the success ratio of wireless handover is low.
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Solution:
1. Check whether it is attributed to LAPD updating: after the upgdating,
not all cells under module 4 are congested and the handover indexes of
some indexes are not lowered. Upgrading is not the cause.
2. Analyze traffic statistics:
Congested cells are mostly at Site A and Site B. Their traffics are
lowered than those before the upgrading. It is a pseudo congestion.
The cells where handover success ratio is lowered are mostly at Site A,
B, C, D and E and the main cause of the failure is timer timeout.
3. Check alarm:
At 19:31 of September 18, the clock systems of Site A and Site B gave
alarms. It was found that 13M loses lock.
the clock problem lowers the handover success ratio of the two sites.
Such impact is spread to the adjacent cells of these two cells and even
to the whole network.
Typical Handover Cases
T i l H d C
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Solution:
4. Analyze handover data and traffic statistics:All cells where handover success ratio is lowered are adjacent
to A and B except A and B themselves.
5. Reset Site A and Site B, the clock system is recovered, and
the handover success ratio inter/intra-BSC goes up to 93. Theproblem is solved.
Conclusion: the problem in clock system
will lower the handover success ratio. Pay
attention to the alarm console and the
slow switchover of the clock system.
Typical Handover Cases
T i l H d C
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Case 4
Fault Description: Huawei BSC and the another
manufacturerBSC(for short S BSC) are connected to the same
MSC. After the cutover, S BSCBS can not handover into HuaweiBS but Huawei BS can handover into SBSC BS. In other words,
In-BSC handover request times of Huawei BSC is 0.
Typical Handover Cases
T i l H d C
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Case 4
Analysis: Since in-BSC handover request
times is 0 but the out handover is normal.
Carefully check the out-cell handover
signaling and handover data at S BSC side
Typical Handover Cases
T i l H d C
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Case 4
Solution:
1. Trace interface message. After receiving HO-REQUEST
message, Huawei BSC returns HO-FAILURE message
immediately but Huawei BSC should have returned HO-
REQ-ACK message as in normal conditions.
Carefully analyze HO-REQ message and HO-FAILURE
message
Typical Handover Cases
T i l H d C
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Typical Handover Cases
T i l H d C
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HO-REQUEST
Typical Handover Cases
T i l H d C
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Solution2. HO-FALUER: The cause of the failure is invalid message
content
3. HO-REQthe difference is basically the Address Indicator incomparison 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.
Case 4
Typical Handover Cases
T i l H d C
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Conclusion:In interworking with equipment of other manufacturers, faults
can be located by analyzing the signaling.
Case 4
Typical Handover Cases
T i l H d C
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Fault Description: Huawei MSC interworks with MSC of N(manufacturer) with Ns BSC attached. The communication isnormal. The handovers inside Huawei BSC and in-BSC are
normal, but out-BSC handover success ratio is about 25%.
The handover from the BSC of N to Huawei BSC is successful
and the subsequent handover is also successful (In the same
communication, the handover goes back to BSC of N).
Case 5Typical Handover Cases
T i l H d C
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Analysis: Check handover data, including theexternal cell description table, BA2 table, cell
adjacency relation table inside Huawei BSC
and opposite BSC and CGI at MSC side. If the
data are correct, check out-BSC handover
signaling.
Case 5Typical Handover Cases
Typical Handover Cases
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Solution:
1. Check data: BSC and MSC of Huawei and N. No problem is found.
2. Check alarm: BS maintenance console, No.7 link and A interface circuit are all
normal.
3. Trace Huawei A interface signaling. After HO-Required message is received,
HO-COMD command is not received from MSC.
4. 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).
5. Check HO-Number message from the opposite end to 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 inthe 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.
6. Coordinate with N, asking them to add 00+country code before the handover
number. Then out-BSC handover is normal.
Case 5
Typical Handover Cases
Typical Handover Cases
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Conclusion: Problems can be located quickly and accurately
if you are familiar with the signaling flow of handovers.
Case 5Typical Handover Cases
Typical Handover Cases
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Fault Description: In the independent MSC networking of adual frequency network, Huawei 1800M coordinates with
900M of manufacturer A . After the cutover, the success ratioof handover from M1800 to M900 is low (about 60), while thehandover from M900 to M1800 are normal and the handover
data are correct.
Case 6
Typical Handover Cases
Typical Handover Cases
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Analysis 1: Trace the signalings at A interface and E
interface. Normally:
MSC-BMSC-HW VLR-BBSC-HW BSC-B
HO-RequiredMAP_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-CompleteMAP_Send_End_Signal
Clear-Command
Clear-Complete
MAP_Send_End_Signal_ACK
Intermediate stepsare omitted.
IAIACM
Typical Handover Cases
Typical Handover Cases
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Analysis 2: Abnormally:
MSC-BMSC-HW VLR-BBSC-HW BSC-BHO-Required MAP_Prepare_HO
MS
HO-REJECT
MS
MAP_Abort
Typical Handover Cases
Typical Handover Cases
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Analysis 2:
1. Analyze the Prepare-HO message sent from Huawei MSC to the opposite end.
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.
2. Modify A interface circuit pool table of Huawei MSC data, only using full-rate
version 1.
3. Then the out-BSC handover is normal.
Case 6
Conclusion: There are often many problems in coordination
between the equipment of different manufacturers. However,
problems can be located accurately by tracing signaling.
Typical Handover Cases
Typical Handover Cases
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Case 7
Fault Description: A specific GSM network, where one MSC is attached with
two BSC, is all configured with Huawei equipment. When the two BSCs are
cut over one after another, the success ratio of handover from BSC1 to
BSC2 is very low, but the success ratio of handover from BSC2 to BSC1 is
normal. Intra-BSC handover inside BSC1 and BSC2 is normal.
Typical Handover Cases
Typical Handover Cases
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Case 7
Analysis: check handover data and trace signaling.
Solution:
1. Check all data of inter-BSC handover: external cell description data table,
BA2 table, CGI of MSC. No problem is found.
2. Trace A interface message of BSC1 and BSC2. After BSC1 sends HO-
Required, BSC2 has not received HO-Request message, but BSC1 has.
3. The path of data search: MSC goes to Location Area Cell Table according to
the CGI of the destination cell in HO-Required message, refers to the
description of the DSP of the cell given in the table and sends HO request to the
correct BSC.
4. The Ho-request message, which should have been sent to BSC2, is sent to
BSC1, so it is DSP error. After correction, the problem is solved.
Typical Handover Cases
Typical Handover Cases
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Case 7
Conclusion:
For the data of inter-BSC handover, besides checking
whether CGI at MSC side is consistent with that at the
opposite and at BSC, check whether the DSP is correct. MSCindexes to the CGI of the destination cell with the existing
CGI.. When the DSP is incorrect, HO-Request will be sent to
wrong BSC.
Typical Handover Cases
Typical Handover Cases
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Case 7
Fault Description: A specific dual frequency network is a mixed
networking of Huawei BSC (M1800) and manufacturer N BSC (M900),
attached to MSC of manufacturer S. After the cutover, the success
ratio of Huawei in-BSC handover is always 8892, while out-BSCand intra-BSC handovers are normal (above 92). Furthermore, thecells with low success ratio of in-BSC handover are randomly
distributed. When there is no fixed BM, there is no fixed LAC.
Typical Handover Cases
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Typical Handover Cases
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Case 7:
Solution:
1. Perform forced handover with a tester between several cells where
handovers have failed. The forced handovers are all successful. Data
problem is ruled out.2. The interference band of cells is ideal, dropout and congestion ratio are
normal. It is unlikely that the coverage and interference at the radio port
causes the problem.
3. Trace A interface signal and compare the failure signaling and success
signaling:
Typical Handover Cases
Typical Handover Cases
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A interface signaling analysis of inter-BSChandover in MSC:
Failure signaling:
Success signaling:
Typical Handover Cases
Typical Handover Cases
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Case 7:
Solution:
4. 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 cell M1800 or the MS does not access cell 1800.
5. Reconfirm that there is no problem with the handover data, otherwise the handover
success ratio will be very low.
6. Re-trace large number of handover signalings to find that all failures are attributed
to the same cause. In addition, the problem mode is the same: one MS makes multiple
handover requests in one communication and all requests fail.
7. Analyze other similarities of the failed message, to find that the first 6 digits of IMEI
numbers of these MSs are all 449684. It shows that the MSs that have handoverproblem are of the same model.
8. Find the MS owners with the IMSI numbers and find that all MSs with handover
problem are F-MS. Not long before it was proved that this MS has poor insulation
performance between 900 and 1800, so it can not access 1800 network. Now the cause
is found. .
Typical Handover Cases
Typical Handover Cases
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Case 7:
Conclusion: MS problem will also causes handover failure.
1. In this case the handover success ratio is not very low,
therefore coordination problem is unlikely.
2. Trace the signaling to find the cause of the handover failure:
BTS has not received handover access message from MS.
3. Find the similarity of these MSs through large amount of
signaling tracing: IMEI numbers are the same. Then it is
found that the problem is on F-MS.
4. In this case, 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 ratio. But in handover, problem is very likely to
happen.
Typical Handover Cases
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