GSM Call Drop
Problem Analysis
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Copyright © 2010 Huawei Technologies Co., Ltd. All rights reserved.
Objectives

Upon completion of this course, you will be able to:

Describe the definition of call drop

Outline the analysis procedure of call drop

Troubleshooting location of call drop

Main solution methods to call drop
Copyright © 2010 Huawei Technologies Co., Ltd. All rights reserved.
Page1
目 录
1. Analysis Procedure of Call Drop
2. Reasons Analyze of Call Drop
3. Main Reasons Location and Solution for Call Drop
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Page2
Call Drop Classification
One of retainability KPIs.
 TCH Call Drop Ratio refer to the ratio of call
drop to successful TCH seizures after the BSC
successfully assigns TCHs to MSs.

TCH
call drop
SDCCH
call drop
One of accessibility KPIs.
 SDCCH Call Drop Rate indicates the probability
of call drops when the MS occupies the SDCCH.

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Page3
Call Drop Analysis Procedure
Analyzing a call drop rate
Common
phenomenon
N
Y
Analyzing the frequency
planning, coverage, and
cell parameters
Filtering for the cell with a
high call drop rate
N
RF call drop
rate
Times of A interface
failures during TCH
occupation
TCH assignment
failure (equipment
fault)
Times of call drops on
the terrestrial link
TCH availability
Equipment fault
Y
Radio problem
Average received level
and quality during the
handover
interference band
analysis
Power control average level
Outgoing handover
success rate
TRX received level
and received quality
Average received level
during a TCH call drop
Ratio of poor
handovers between
the uplink and
downlink
Average distance during a
TCH call drop
Times of
reestablishment
failures upon
handover failure
Interference
Times of handovers and
times of call occupation
success
Which cell has a lower
incoming handover
success rate
Outgoing handover success
rate and average level
The ratio of inter-office
handovers is high.
Measurement of
uplink/downlink balance
performance
Handover
Coverage
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Page4
Information for Analyzing Call Drop Rate
Cause
Sub-Cause
Check on Traffic Statistics
BSC overall performance measurement -> Times of TCH call drops
BSC overall performance measurement -> Times of SDCCH call
drops
A interface trunk performance measurement -> Average occupation
duration of timeslot n.
Overall
call drop
rate is high.
Cell outgoing handover fail reconn fail
Cell internal handover fail reconn fail
Cell Outgoing Handover Fail Reconn Fail
The call drop rate during Unsuccessful BSC internal handovers with unsuccessful reversion
the handover is high.
Unsuccessful incoming BSC handovers with unsuccessful reversion
Unsuccessful outgoing system handovers with unsuccessful reversion
The call
drop rates
of certain
cells are
high.
Interference band measurement of the measurement report;
Full/half-rate channel received level measurement of the
measurement report;
Interference Received quality measurement of the measurement report;
Times of initiated handover attempts (uplink/downlink);
The call
drop rate
of the
radio
network
is high.
Coverage
The
configuration
of the
adjacent cell
relationship is
missing.
Check on Data
Configuration
T305, T308
Transmission,
communication
between boards,
SWR, clock alarms
BSC clock check
Handover-related timers: T3103,
T8;
Handover-related threshold
parameters that ensure handovers
are triggered timely and that
power control is triggered before
the handovers. This allows the
handovers to be completed under
the optimal power condition.
SACCH multi-frames;
Radio link timeout;
T200, N200
Full/half-rate channel received level measurement of the
measurement report;
Received quality measurement of the measurement report;
Call drop measurement;
RXLEV_ACCESS_MIN ;
RACH Min. Access Level ;
Power level of TRX
Mean uplink received level during handover initiation, mean uplink
received quality during handover initiation;
mean TA during radio link failure (SDCCH/TCHF/TCHH);
Mean uplink/downlink level during radio link failure
(SDCCH/TCHF/TCHH);
Incoming/outgoing cell handover performance measurement;
Check the adjacent cell
relationship table.
Copyright © 2010 Huawei Technologies Co., Ltd. All rights reserved.
Others
Page6
For out-of-band
interference, you
need to eliminate
the interference
source through
frequency sweep in
drive tests.
Supplement
adjacent cells with
the help of drive
tests.
目 录
1. Analysis Procedure of Call Drop
2. Reasons Analyze of Call Drop
3. Main Reasons Location and Solution for Call Drop
Copyright © 2010 Huawei Technologies Co., Ltd. All rights reserved.
Page7
Main Reason for Call Drop

From the perspective of the whole network, there are three
reasons for call drops
Radio link
fault
Main
Reasons
System
fault
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T3103
timeout
Page8
Radio Link Fault

The main reason for a call drop is radio link fault.
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Page9
Radio Link Fault Signaling Flow
MS
BTS
BSC
MSC
(1)
Measurement report
(2)
Measurement result
Connection failure
(3)
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Clear REQ(radio interface failure)
Page10
HO Signaling Flow(T3103 Timer)
MS
BTS1
BTS2
Handover
BSC
MSC
Indication
Ch_Act
Ch_Ack
Handover command
Handover
Handover
Access
Physical info(TA)
SABM
UA
Handover
Set T3103
command
HO_DETECT
EST Indication
Complete
Reset T3103
HO_Complete
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Page11
目 录
1. Analysis Procedure of Call Drop
2. Reasons Analyze of Call Drop
3. Main Reasons Location and Solution for Call Drop
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Page12
Main Reasons for Call Drop
HO
Coverage
Parameter
Interference
Transmis
sion
Antenna
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Page13
Call Drops Reasons for Coverage
1
2
3
4
Discontinuous coverage
(blind area)
Bad indoor coverage
Cross coverage (isolated island)
Small coverage
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Page14
Methods to Identify Call Drop Caused
by Coverage

Drive Test
Traffic Measurement


For detail, please refer to
the next page
Call Drop for Coverage
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Page15
Traffic Statistic Tasks Related to Coverage Analysis
Traffic
Statistic
Tasks
1
Power control performance measurement
2
Receiving level performance measurement
3
Cell performance measurement / inter-cell
handover performance measurement
4
Call drop performance measurement
5
Neighbor cell performance measurement
6
Uplink/downlink balance performance
measurement
Copyright © 2010 Huawei Technologies Co., Ltd. All rights reserved.
Call
Drop
Result
from
Coverage
Problem
Page16
Solution to Call Drop Caused by Coverage
Area with insufficient
coverage
Remove
hardware
problem
Eliminate
overshooting
coverage
Ensure the indoor
communication
quality
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Page17
Call Drops Reasons for Handover
Unreasonable
parameters
T3103 timer overflows
BTS clock is out of
synchronization
Main
Reasons
Traffic congestion
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Incomplete
neighbor cells
Co-BCCH and Co-BSIC
Page18
Methods to Identify Call Drop Caused
by Handover
Analyze the
traffic
measurement
Method
to
Identify
Drive test
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Check
the
alarms
Page19
Traffic Statistic Tasks Related to Handover
Analysis
Traffic
Statistic
Tasks
1
Inter-cell handover performance
measurement
2
Neighbor cells performance
measurement
3
Outgoing cell handover performance
measurement
4
Incoming cell handover success
rate is low
5
TCH performance measurement
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Page20
Call
Drop
Result
from
Handover
Problem
Solution to Call Drop Caused by Handover
1
2
3
Check handover
parameters
Adjust the traffic
Provide good clock
synchronization
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Page21
Call Drops Reasons for Interference
Co-channel
interference
Main
Reasons
Crossmodulation
interf. and
Other Outside
interfe.
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Adjacentchannel
interference
Page22
Methods to Identify Call Drop Caused
by Interference
Analyze the traffic
statistic
Use spectrum analyzer
to scan the frequencies
Analyze basing
on user complaint
Method
to
Identify
Eliminate reasons
result from equipment
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Check the
frequency planning
Adjust ARFCNs
Page23
Traffic Statistic Tasks Related to Interference Analysis
Interference band
in MR
Many handover
Receiving level
failures and
performance
reestablishment
measurement
failures
Interference
Analysis
Call drop
performance
measurement
BQ handover
ratio
Receiving
quality
performance
measurement
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Page24
Solution to Call Drop Caused by Interference

Interference from outside the network and inside the network are solved by
different methods.

Invalid interference from outside the network is solved by the Radio Committee

interference from inside the network is solved by adjusting the network.
1
Drive test
2
3
Use DTX, FH, PC,
and diversity
technologies
Exclude the
interference caused
by the equipment
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Page26
Call Drops Reasons for Antenna Feeder
Antenna feeders
are wrongly
connected
Antenna
feeder
problem
Main
Reasons
Difference
downtilt angles
for two sets of
antenna
Difference
azimuth angles
for two sets of
antenna
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Page27
Methods to Identify Call Drop Caused
by Antenna Feeder
Whether there is
alarm related to
antenna unit
whether the
Check whether
boards are normal
there is BTS
from remote
hardware fault
maintenance
Method
console
to
Identify
whether the
uplink and
BTS on-site
downlink are
check and test
balanced in
check whether
MRs
the BCCH of the
serving cell is
consistent with
the planning
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Page28
Traffic Statistic Tasks Related to Antenna
Feeder Analysis
1
Traffic
Statistic
Tasks
Uplink and downlink balance PM
2
Call drop PM
3
Power control PM
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Analyze whether
the uplink and
downlink are
balanced in order
to identify there
is antenna
feeders problem
or not
Page29
Call Drops Reasons for Transmission

Observe the transmission
and board alarms

Main
Reasons


Check
transmission
paths


Observe
traffic
statistics

Copyright © 2010 Huawei Technologies Co., Ltd. All rights reserved.
Bad transmission
quality and
unstable
transmission link
Abis interface and
the A interface
links in
TCH PM shows：
1） A interface failures during TCH seizure
2） Whether the TCH availability is normal
3） Call drops due to interruption of
terrestrial links
Page30
Radio Parameters That Affect Call Drop
Main
Radio
Parameters
1
RLT/SACCH Multi-frames
2
T200 parameter
3
RXLEV_ACCESS_MIN
4
RACH Min Access Level
5
NCC Permitted
6
handover parameters
7
power control parameters
8
Frequency planning parameters
Copyright © 2010 Huawei Technologies Co., Ltd. All rights reserved.
Call
Drop
Problem
Page31
Summary

What is classification of the call drop?

How to analyze the call drop?

What is the main reasons of the call drop and its
corresponding solution?
Copyright © 2010 Huawei Technologies Co., Ltd. All rights reserved.
Page33
Thank you
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