2 g case analsyis handover training-20060901-a-2.0

HUAWEI TECHNOLOGIES CO., LTD.

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OMF000401 Case Analysis -- Handover

ISSUE 2.0

HUAWEI TECHNOLOGIES CO., LTD. Page 2All rights reserved

References

31160978-BSC Traffic Statistic Manual Volume I

31033203-BSS Troubleshooting Manual


Upon completion of this course, you are

supposed to be able to:

Understand the principles of handover

Analyze and solve handover problems


Chapter 1 Measurement Points of Handover Chapter 1 Measurement Points of Handover

Chapter 2 Handover Data ProcessChapter 2 Handover Data Process

Chapter 3 Analysis of Handover ProblemChapter 3 Analysis of Handover Problem

Chapter 4 Handover CasesChapter 4 Handover Cases


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


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


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


MSC-BMSC-A VLR-BBSC-A BSC-B

HO-RequiredHO-Required Prepare_HO

Prepare_HO_ACK

Allocate_HO_NUM

Send_HO_Report

Send_HO_Report_ACK

MS

HO-RequestHO-Request

HO-Request-ACK

HO-CommandHO-Command

MS

HO-AccessProcess_Access_Signalling

HO-CompleteHO-CompleteSend_End_Signal

Clear-CommandClear-Command

Clear-Complete

Send_End_Signal_ACK

Some intermediate steps are omitted

IAIACM



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)



Chapter 1 Measurement Points of HandoverChapter 1 Measurement Points of Handover

Chapter 2 Handover Data Process Chapter 2 Handover Data Process




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 ）

Search target cell （ 7 ）



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.



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.


Chapter 1 Measurement Points of HandoverChapter 1 Measurement Points of Handover

Chapter 2 Searching Process of Handover DataChapter 2 Searching Process of Handover Data




Analysis of Handover Problem

Types of handover problems

Locating handover problem

Causes of handover problem


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 l

oad of the system


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


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


Causes of Handover Problem

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

√

Typescauses


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.


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 becomes impossible.

Isolated island resulting from over shooting

Island effect results in handover failure


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


BTS hardware

CDU, splitter/combiner failure

TRX failure

TMU failure

Clock failure

Internal communication cable



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.


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/B

A2 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.



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 .



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).


Chapter 1 Measurement Points of Handover Chapter 1 Measurement Points of Handover

Chapter 2 Searching Process of Handover DataChapter 2 Searching Process of Handover Data




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.


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.


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.


Handover Case 2

Fault Description:

After LAPD software upgrade of a dual-band network on 19th Septem

ber, 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 Mea

surement Function, it was found that besides module 4, some cells of

other modules also suffer from the problem that inter cell handover su

ccess rate is low. After 19th September, the situation became worse.


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.


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.


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.


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.


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.

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.

Handover Cases 3


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.


Handover Cases 3

Handover failure


Handover Cases 3

HO-REQUEST


Handover Cases 3

Solution： HO-FALUER: The cause of the failure is invalid message co

ntent

HO-REQUEST: The difference is basically the Address Indic

ator in comparison with the normal handover request mess

age. HUAWEI BSC does not recognize case 41. The syste

m thinks that Address Indicator must be 0x43. Inform the o

wner of network, modify the relation parameter, and this pr

oblem is solved.


Handover Cases 3

Conclusion:

In cooperation with equipment of other manufacturers, faults

can be located by analyzing the signaling.


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.

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.

Handover Cases 4


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).


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.


Handover Cases 4

Conclusion:

Problems can be located quickly and accurately if you are

familiar with the signaling flow of handovers.


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 han

dover data are correct.

Handover Cases 5


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

Handover Cases 5


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


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.


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.


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.

Handover Cases 6


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.


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.


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.


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.


Fault Description:

A dual band network is a mixed networking of Huawei BSC (M1800) a

nd vender N BSC (M900), attached to MSC of vender S. After the cut

over, the successful rate of Huawei incoming BSC handover is alway

s 88 ％～ 92 ％ , while outgoing BSC and intra-BSC handovers are no

rmal (above 92 ％ ). Furthermore, the cells with low successful rate of

incoming BSC handover are randomly distributed.

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


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:


Failure signaling:

Success signaling:

Handover Cases 7

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


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.


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

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.

www.huawei.com

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