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

Internal

OMF000401
Case Analysis -- Handover
ISSUE 2.0

www.huawei.com

HUAW I T CH
E E NOL OGIE CO., L D.
S T All rights reserved

References

 31160978-BSC Traffic Statistic Manual Volume I

 31033203-BSS Troubleshooting Manual

HUAW I T CH
S T All rights reserved Page 2

Upon completion of this course, you are
supposed to be able to:
 Understand the principles of handover
 Analyze and solve handover problems

HUAW I T CH

Chapter 1 Measurement Points of Handover

Chapter 2 Handover Data Process

Chapter 3 Analysis of Handover Problem

Chapter 4 Handover Cases

HUAW I T CH

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

HUAW I T CH

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

HUAW I T CH

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

HUAW I T CH


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

HUAW I T CH

 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)
HUAW I T CH


Chapter 2 Handover Data Process



HUAW I T CH

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 ）

HUAW I T CH

 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.

HUAW I T CH


 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.

HUAW I T CH


Chapter 2 Searching Process of Handover Data



HUAW I T CH

Analysis of Handover Problem

 Types of handover problems

 Locating handover problem

 Causes of handover problem

HUAW I T CH

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

HUAW I T CH

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
HUAW I T CH

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

HUAW I T CH

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
HUAW I T CH

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.

HUAW I T CH

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

HUAW I T CH

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

HUAW I T CH

BTS hardware

 CDU, splitter/combiner failure
 TRX failure
 TMU failure
 Clock failure
 Internal communication cable

HUAW I T CH


 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.

HUAW I T CH

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.

HUAW I T CH


 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 .

HUAW I T CH


 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).

HUAW I T CH


Chapter 2 Searching Process of Handover Data



HUAW I T CH

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.

HUAW I T CH

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.

HUAW I T CH

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.

HUAW I T CH

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.

HUAW I T CH

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.

HUAW I T CH

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.

HUAW I T CH

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.

HUAW I T CH

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.

HUAW I T CH

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.

HUAW I T CH

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.

HUAW I T CH

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.

HUAW I T CH

Handover Cases 3
Handover failure

HUAW I T CH

Handover Cases 3
HO-REQUEST

HUAW I T CH

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.

HUAW I T CH

Handover Cases 3

Conclusion:

In cooperation with equipment of other manufacturers, faults
can be located by analyzing the signaling.

HUAW I T CH

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.

HUAW I T CH

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.

HUAW I T CH

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).

HUAW I T CH

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.

HUAW I T CH

Handover Cases 4

Conclusion:

Problems can be located quickly and accurately if you are
familiar with the signaling flow of handovers.

HUAW I T CH

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.

HUAW I T CH

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

HUAW I T CH

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

HUAW I T CH

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.

HUAW I T CH

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.

HUAW I T CH

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.

HUAW I T CH

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.

HUAW I T CH

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.

HUAW I T CH

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.

HUAW I T CH

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.

HUAW I T CH

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.

HUAW I T CH

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.

HUAW I T CH

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:

HUAW I T CH

Handover Cases 7
A interface signaling analysis of inter-BSC handover in MSC:
Failure signaling:

Success signaling:

HUAW I T CH

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.

HUAW I T CH

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.

HUAW I T CH

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.

HUAW I T CH

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

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