omf000401 case analsyis handover training 20031001 a 1.4

Huawei Confidential

OMF000401 Case Study -Handover ISSUE1.4

OMF000401 Case Study -Handover ISSUE1.4

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• Possible causes of intra BSC handover failure No channel available in the target cell Channel activation negative acknowledgement － CHAN ACK

NACK Transmitting channel activation message failure Channel activation timeout MS access failure (It may be caused by radio interface.) Max resend time of physical information * radio link

connected with the timer < time interval of EST IND ~ HO DETECT (120~180ms)

Waiting until timeout after establish indication (MS has not received UA frame or has not sent handover complete message)

Measurement Points of Intra BSC Handover

Measurement Points of Intra BSC Handover

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Measurement Points of Inter-BSC Handover

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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Searching Process of Handover Data (1)


• BCCH frequencies of all adjacent cells in BA2 table are sent to

MS on system message 5.

• MS reports measurement report to BSS. It including the BCCH ,

BSIC and signal level of the adjacent cells and serving cells.

• When the measurement report is preprocessed, BSC

determines the module ID, cell ID and CGI of all adjacent cells

through BCCH frequency and BSIC in Adjacent Cell Relation

Table and Cell Description Table (or External Cell Description

Table).

• BSC executes handover judgement flow such as basic cell

ranking (on LAPD board). Once a proper target cell is found,

the handover request message which includes the target 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.

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• MPU sends handover request message to the module and makes statistics of "outgoing inter cell handover request".

• 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 search target cell in “LAI And GCI Table ”. Once the cell is found, MSC will confirm which BSC is belong to, and send the handover request message to this BSC.

• If there is no CGI of the target cell in " LAI And GCI Table ", MSC will go to “Adjacent MSC Table" and find the target MSC, then send the handover request message to that MSC.

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Analysis of Handover Problem

• Types of handover problems• Locating handover problem• Causes of handover problem

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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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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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Causes of Handover Problem

• Coverage and interference at radio interface

• Antenna and feeder system

• Base station software or hardware

• Transmission

• BSC software, hardware, Data Configuration

• A interface

• target cell congestion

• Cooperation with equipment of other manufacturers

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• 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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Adjacent

Cell N3

Adjacent

Cell N2

Adjacent

cell N1

Non-adjacent

cell

Non-adjacent

cell

Non-adjacent

cell

Service cell

There is no adjacent cell, so handover become impossible.

Isolated island resulting from over shooting

Analysis of Handover ProblemAnalysis of Handover Problem

Island effect results in handover failure

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• Antenna & Feeder System High voltage standing wave ratio (VSWR) Antenna are not properly installed Antenna are not parallel The azimuth and downtilt are not correct Poor antenna isolation value RF cables, connection are loose or incorrect

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• BTS hardware CDU, splitter/combiner failure

TRX failure

TMU failure

FPU failure

Clock failure

Internal communication cable

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• Transmission Transmission is not stable

Serious BER in transmission

• Fault of BSC Hardware Example:

• Clock board: the faulty clock board causes clock inconsistency between base stations.

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• Data Configuration Improper setting of handover threshold (higher, lower, the edg

e handover threshold is higher than the power control threshold.)

Improper setting of handover hysteresis and handover priority. Improper setting of P, N value of statistic time Incomplete frequency and adjacency relation configured in BA

1/BA2 table There are adjacent cells with the same frequency and the same

BSIC CGI and module ID in "Cell Description Table" are different fro

m those in "Cell Module Information Table". CGI, BCCH and BSIC in "External Cell Description Table" are diff

erent from those in the opposite BSC. The DPC of BSC in MSC "LAI And GCI Table" is incorrect.

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• A interface problem Basically the insufficient link resource results in the a

bnormal handover, as well as abnormal communication.

• Target cell congestion The target cell is congested, which causes the handov

er failure. Then the target cell should be expanded or reduce its traffic .

• Handover between equipment of different manufacturers

The signaling at A interface, E interface of the opposite equipment 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).



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Course Contents

Measurement points of handover

Searching process of handover data

Analysis of handover problem

Handover cases

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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.

Typical Handover Cases


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Case 1

Typical Handover Case

Typical Handover Case

Analysis:1. 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.

2. 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.

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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•Solution: 1. 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. Base on careful data checking, the data of this cell is correct.

2. 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.

3. 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.

Typical Handover CaseTypical Handover Case

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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 beside 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.

Case 2



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Case 2Analysis:1. Check whether it is attributed to LAPD software upgrade.2. Analyze traffic statistics and the main causes of the failure to locate the problem.



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• Solution: 1. 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.

2. 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 and the main cause of the failure is timer timeout.



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Typical Handover CasesTypical Handover Cases

• Solution: 3. 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 is spread to the adjacent cells of these two cells and even to the whole network.

4. 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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• Solution: 5. Reset site A and site B, the clock system is recovered, and t

he 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 succ

ess rate. Pay attention to the alarm console and the excursion of the clock system.

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Case 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. In other words, “Attempted incoming interBSC inter cell handover" of Huawei BSC is 0.


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Case 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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• Solution: 1. 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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• Handover failure

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• HO-REQUEST

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• Solution ： 2. HO-FALUER: The cause of the failure is invalid message con

tent 3. HO-REQUEST: The difference is basically the Address Indic

ator 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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• Conclusion: In cooperation with equipment of other manufacturers,

faults can be located by analyzing the signaling.

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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.

Case 4


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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.

Case 4


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• Solution: 1. Check data: There is no fault in BSC and MSC of the

two venders.

2. Check alarm: BTS maintenance console, No.7 link and A interface circuit are all normal.

3. Trace Huawei A interface signaling. After "HO-REQUIRED" message is sent, "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).

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• Solution: 5. 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.

6. Discuss with N, asking them to add "00+country code" before the handover number. Then outgoing BSC handover is normal.

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• Conclusion: Problems can be located quickly and accurately if you

are familiar with the signaling flow of handovers.

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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.

Case 5


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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-CompleteMAP_Send_End_Signal

Clear-Command

Clear-Complete

MAP_Send_End_Signal_ACK

Intermediate steps are omitted.

IAIACM


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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


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• Solution: 1. 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.

2. Modify A interface circuit pool table of Huawei MSC data, only using full-rate version 1.

3. Then the outgoing BSC handover is normal.

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• 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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Case 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 very low, but opposite is

normal. Intra BSC handover of BSC1 and BSC2 are normal.


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Case 6


• Analysis: Register “outgoing inter-cell handover measuremen

t function” of BSC1 and “incoming inter-cell handover measurement function” of BSC2.

Check the data relate to handover of BSC1 and BSC2. Analyze signaling of handover failure.

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• 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 opposite is normal.

3. 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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• Solution: 4. 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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• 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 base on the CGI of the target cell. When the DPC is incorrect, "HO-REQUEST" will be sent to wrong BSC.

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Case 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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Case 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. Next, trace the signaling.


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• Solution: 1. 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.

2. 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.

3. Trace A interface signaling and compare the failure signaling with success signaling:

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Failure signaling:

Success signaling:


• A interface signaling analysis of inter-BSC handover in MSC:

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• 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 M1800 cells.

5. Reconfirm that there is no problem with the handover data, otherwise the handover success rate will be very low.

6. 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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• Solution: 7. 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 MS that have handover problem are of the same model.

8. Find the MS owners with the IMSI numbers and find that all MS with handover problem are F MS. Not long before 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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• Conclusion: MS problem will also causes handover failure.

1. In this case the handover success rate 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 MS through large amount of signaling tracing: IMEI numbers are the same. Then it is found that the problem is on F MS.

4. 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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Thank You

omf000401 case analsyis handover training 20031001 a 1.4

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