microsoft power point - 12 omf000401 case analysis-handover issue2

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Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.

Case Analysis --Handover


References

� 31160978-BSC Traffic Statistic Manual Volume I

� 31033203-BSS Troubleshooting Manual


Objectives

� Upon completion of this course, you will be able to:

� Understand the principles of handover

� Analyze and solve handover problems


Contents

1. Measurement Points of Handover

2. Handover Data Process

3. Analysis of Handover Problem

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

HOHO--RequiredRequired Prepare_HO

Prepare_HO_ACK

Allocate_HO_NUM

Send_HO_Report

Send_HO_Report_ACK

MS

HOHO--RequestRequest

HO-Request-ACK

HOHO--CommandCommand

MS

HO-AccessProcess_Access_Signalling

HOHO--CompleteCompleteSend_End_Signal

ClearClear--CommandCommand

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)


Contents




4. Handover Cases


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.


Contents




4. Handover Cases


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

load 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

√H.O between equipment of different manufactures

√√Target cell congestion

√√√A interface

√√√Data configuration

√BSC hardware

√transmission

√BTS hardware

√Antenna and feeder system

√Interference

√√√Coverage

Frequent H.OH.O failureNo H.OTypescauses


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/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.



� 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).


Contents




4. Handover Cases


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

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.


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.


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.


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

� 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

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.


Handover Cases 3

� Conclusion:

� In cooperation with equipment of other manufacturers, faults

can be located by analyzing the signaling.


Handover Cases 4


� 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

� 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.


Handover Cases 5


� 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.


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 stepsare 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:

� 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


� 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

� 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.


Handover Cases 7


� 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.


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

� 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.

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microsoft power point - 12 omf000401 case analysis-handover issue2

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