Gsm bss-network-kpi-tch-call-drop-rate-optimization-manual(cdr)

GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual

Product Name

Confidentiality Level

G3BSC

INTERNAL

Product Version

INTERNAL

Total 44 pages

GSM BSS Network KPI (TCH Call Drop
Rate) Optimization Manual
(For internal use only)

Prepared by

WCDMA & GSM Network
Performance Research Dept.

Date
2008-6-28

Su Shi
Reviewed by

Date

Reviewed by

Date

Granted by

Date

Huawei Technologies Co., Ltd.
All rights reserved


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Contents
1 Overview of the TCH Call Drop Rate....................................6
1.1 Meaning.................................................................................................................................................6
1.2 Recommended Formulas.......................................................................................................................6
1.3 Signaling Procedure and Measurement Points......................................................................................8

2 Factors That Affect the TCH Call Drop Rate........................11
2.1 Hardware Failure.................................................................................................................................11
2.2 Transmission Problem.........................................................................................................................11
2.3 Version Upgrade..................................................................................................................................12
2.4 Parameter Setting.................................................................................................................................12
2.5 Intra-Network and Inter-Network Interference...................................................................................13
2.6 Coverage Problem..............................................................................................................................13
2.7 Antenna System Problem....................................................................................................................14
2.8 Imbalance Between Uplink and Downlink..........................................................................................14
2.9 Repeater Problem................................................................................................................................14

3 Analysis of and Solutions to High TCH Call Drop Rate.........15
3.1 Solutions to High TCH Call Drop Rate...............................................................................................18
3.1.1 Checking the Hardware.................................................................................................................18
3.1.2 Checking the Transmission............................................................................................................19
3.1.3 Checking the BSC and BTS Version Upgrade..............................................................................20
3.1.4 Checking the Parameter Settings...................................................................................................20
3.1.5 Checking the Interference..............................................................................................................25
3.1.6 Checking the Coverage..................................................................................................................26
3.1.7 Checking the Antenna System.......................................................................................................27
3.1.8 Checking the Balance Between Uplink and Downlink.................................................................28
3.1.9 Checking the Repeaters.................................................................................................................29

4 Test Methods.................................................................... 30
5 Remarks About the Signaling Analysis of the TCH Call Drop
Rate.................................................................................... 30
6 Cases for TCH Call Drop Rate Optimization.........................33
6.1 Case 1: Call Drop Due to Interference................................................................................................33
6.2 Case 2: Call Drop Due to Imbalance Between Uplink and Downlink................................................34
6.3 Case 3: Call Drop Due to Repeater Problem.......................................................................................34
6.4 Case 4: Call Drop Due to Coverage....................................................................................................35


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6.5 Case 5: Reduction of Call Drops by Optimizing Handover Parameters.............................................36
6.6 Case 6: Call Drop Due to Inappropriate Parameter Setting................................................................36
6.7 Case 7: Call Drop Due to TRX Board Fault........................................................................................37
6.8 Case 8: Call Drop Due to Antenna System Problem...........................................................................38
6.9 Case 9: Call Drop Due to Transmission Problem...............................................................................38
6.10 Case 10: Call Drop Rate Doubled in a CoBCCH Network After Upgrade from V9R1 to V9R3.....39
6.11 Case 11: Increase in Call Drop Rate Due to Inactivity of T305 and T308........................................40
6.12 Case 12: Increase in Call Drop Rate Due to Change of TR1N on the MSC Side.............................40

7 Feedback Form for the TCH Call Drop Rate ........................41


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Figures
Figure 1.1Immediate assignment procedure............................8
Figure 1.2Assignment procedure............................................8
Figure 1.3 Intra-BSC handover procedure................................9
Figure 1.4 Incoming BSC handover procedure..........................9
Figure 1.5Procedures for analyzing high TCH call drop rate....16


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Revision Record
Date

Revision Version

Change Description

Author

2008-6-28

0.8

Draft completed.

Su Shi

2008-7-26

1.0

The document is modified according
to review comments.

Su Shi

References
S
N

Document Name

Author

Date

1

G-Guide to Eliminating Interference - 20050311-A-1.0

Chen
Baolin

2005-3-11

2

GSM BSS Network KPI (Network Coverage) Optimization
Manual

Xie Haibin

2008-6-18

3

GSM BSS Network KPI (TCH Call Drop Rate) Baseline

Wu Zhen

2007-6-22

4

GSM BSS Network KPI (Uplink and Downlink Balance)
Optimization Manual

Yang
Jixiang

2008-3-26

5

Guide to Solving Call Drop Problems

Yang Bin

2002-3-7


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GSM BSS Network KPI (TCH Call Drop Rate)
Optimization Manual
Keywords: TCH call drop rate, KPI
Abstract: This document provides the definition and describes the test method and
optimization method of TCH call drop rate.
Acronyms:
The following table lists the acronyms and their expansion:
Acronym

Expansion

TCH

Traffic Channel

MS

Mobile Station

BSC

Base Station Controller

KPI

Key Performance Indicator


1

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Overview of the TCH Call
Drop Rate

1.1 Meaning
The TCH call drop rate refers to the ratio of call drops to successful TCH seizures
after the BSC successfully assigns TCHs to MSs. The TCH call drop rate can be
measured from the following aspects:


TCH call drop rate (including handover)



TCH call drop rate (excluding handover)

The TCH call drop rate, one of the most significant KPIs for telecom operators, is
related to retainability. It indicates the probability of call drops due to various
reasons after MSs access TCHs. A too high TCH call drop rate adversely affects
the user's experience.

1.2 Recommended Formulas
BSC32:
TCH call drop rate (including handover) = (Number of TCH call drops + Number
of TCH call drops during very early assignment)/Number of successful TCH
seizures x 100%
TCH call drop rate (excluding handover) = Number of call drops on TCH/(Number
of successful TCH seizures + Number of successful incoming internal inter-cell
handovers + Number of successful incoming external inter-cell handovers –
Number of successful outgoing internal inter-cell handover] – Number of
successful outgoing external inter-cell handovers) x 100%
BSC6000:
TCH call drop rate (including handover) = Number of call drops on TCH/(Number
of successful TCH seizures (signaling channel) + Number of successful TCH
seizures (TCH) + Number of successful TCH Seizures in TCH handovers (TCH))


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x 100%
TCH call drop rate (excluding handover) = Number of call drops on TCH/Number
of successful TCH seizures (TCH) x 100%
Through the analysis of each pair of formulas, you can find out that the TCH call
drop rate (including handover) is lower than the TCH call drop rate (excluding
handover) irrespective of the BSC model (BSC32 or BSC6000). The following
takes the formulas for the BSC32 as an example. The number of call drops on TCH
is small during the very early assignment procedure. Therefore, the difference
between the numerator of the formula for the TCH call drop rate (including
handover) and that of the formula for the TCH call drop rate (excluding handover)
can be omitted. Including the TCH seizures in the case of handovers, the
denominator of the formula for the TCH call drop rate (including handover) is
greater than the denominator of the formula for the TCH call drop rate (excluding
handover). Therefore, the result of the formula for the TCH call drop rate
(including handover) is smaller than that of the formula for the TCH call drop rate
(excluding handover).
For details, refer to the GSM BSS Network KPI (TCH Call Drop Rate) Baseline.


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1.3 Signaling Procedure and Measurement
Points

Figure 1.1 Immediate assignment procedure

Figure 1.2 Assignment procedure

SABM: Set Asynchronous Balanced Mode. A message which establishes the signalling link over the air interface.
UA : Unnumbered Acknowledgment. A message sent from the MS to the BSS to acknowledge release of radio resources
when a call is being cleared.


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Figure 1.3 Intra-BSC handover procedure

Figure 1.4 Incoming BSC handover procedure

The meanings of the measurement points in the these figures are as follows:
TCH-SUCC-A: indicates the number of successful TCH seizures.
TCH-SUCC-B: indicates the number of successful incoming internal inter-cell handovers
plus the number of successful internal intra-cell handovers.

ACT :activation


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TCH-SUCC-C: indicates the number of successful incoming external inter-cell
handovers.
TCH-SUCC: indicates the number of successful TCH seizures during the very early
assignment procedure.


2

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Factors That Affect the TCH Call
Drop Rate
According to user complaints and network optimization experience, the
major factors that affect the TCH call drop rate are as follows:


Hardware failure



Transmission problem



Version upgrade



Parameter setting



Intra-network and inter-network interference



Coverage problem



Antenna system problem



Imbalance between uplink and downlink



Repeater problem

2.1 Hardware Failure
When a TRX or a combiner is faulty, seizing the TCH becomes difficult, and
thus the TCH call drop rate increases.

2.2 Transmission Problem
The TCH call drop rate increases in the following conditions:


The transmission quality on the A or Abis interface is poor for various
reasons.



Transmission links are unstable.


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2.3 Version Upgrade
After the BTS version or BSC version is upgraded, the BTS version may be
incompatible with the BSC version, and the parameters and algorithms in
the new version may be changed. In this case, the TCH call drop rate
increases.

2.4 Parameter Setting
The settings of some parameters on the BSC and MSC sides may affect
the TCH call drop rate. If the following situations occur, the TCH call drop
rate may increase:
1.

The parameters SACCH Multi-Frames and Radio Link Timeout are set to
too small values.

2.

The parameter RXLEV_ACCESS_MIN is set to a too small value.

3.

The parameter RACH Min.Access Level is set to a too small value.

4.

The parameters Min DL Power on HO Candidate Cell and Min Access
Level Offset are inappropriately set.

5.

The length of timer T3103 (this timer is set to wait for a Handover
Complete message) is set to a too small value.

6.

The length of timer T3109 (this timer is set to wait for a Release
Indication message) is set to a too small value.

7.

The length of timer T3111 (this timer specifies the connection release
delay) is set to a too small value.

8.

The length of timer T305/T308 is set to an invalid or too great value.

9.

The parameter TCH Traffic Busy Threshold is set to a too small value.

10. The parameter Call Reestablishment Forbidden is set to Yes.
11. The parameters related to edge handover are inappropriately set.
12. The parameters related to BQ handover are inappropriately set.
13. The parameters related to interference handover are inappropriately
set.
14. The parameters related to concentric cell handover are inappropriately
set.
15. The parameters related to power control are inappropriately set.
16. T200 and N200 are set to too small values.
17. Some neighboring cell relations are not configured.
18. The parameter MAIO is inappropriately set.
19. The parameter Disconnect Handover Protect Timer is set to a too small
value.
20. The parameter TR1N is set to a too small value.
21. The parameters Software Parameter 13 and MAX TA are set to too
small values.
22. If a repeater is used, the parameter Directly Magnifier Site Flag is set


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to No.

2.5 Intra-Network and Inter-Network Interference
If inter-network interference and repeater interference exist, or if severe
intra-network interference occurs because of tight frequency reuse, call
drops may occur on TCHs because of poor QoS. This adversely affects the
TCH call drop rate.
The following types of interference may occur:
1.

Inter-network interference from scramblers or privately installed
antennas

2.

Interference from the CDMA network of China Unicom

3.

Repeater interference

4.

Intermodulation interference from BTSs

5.

Intra-network co-channel and adjacent-channel interference

2.6 Coverage Problem
The following coverage problems may affect the TCH call drop rate.
1.

Discontinuous coverage (blind areas)

The voice quality at the edge of an isolated BTS is poor and calls cannot be
handed over to other cells. In this case, call drops may occur.
In complex terrains such as mountainous regions, the signals are blocked and thus
the transmission is discontinuous, leading to call drops.
2.

Poor indoor coverage

Densely distributed buildings and thick walls cause great attenuation and low
indoor signal level, which causes call drops.
3.

Cross coverage (isolated BTS)

The serving cell causes cross coverage due to various reasons (such as
excess power). An MS cannot be handed over to another cell due to no
suitable neighboring cells. In this case, the signal level becomes low and
the voice quality of the MS deteriorates. Thus, call drops occur.
4.

Insufficient coverage

If the signal from an antenna is blocked or the BCCH TRX is faulty, call drops may
occur because of discontinuous coverage.


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2.7 Antenna System Problem
The following antenna system problems may affect the TCH call drop rate
1.

If the transmit antennas of two cells are improperly connected, the
uplink signal level in each cell is much lower than the downlink signal
level in the cell. Therefore, call drops are likely to occur at places far
away from the BTS.

2.

If a directional cell has main and diversity antennas, the BCCH and
SDCCH of the cell may be transmitted from different antennas. If the
two antennas have different pitch angles or azimuths, the coverage
areas of the two antennas are different. In this case, the following
result may occur: An MS can receive the BCCH signals from one
antenna; when a call is made, the MS cannot seize the SDCCH
transmitted by the other antenna and thus a call drop occurs.

3.

If the feeder is damaged, water leaks in the feeder, or the feeder and
the connector are not securely connected, both the transmit power and
receiver sensitivity of the antenna are reduced. Thus, call drops may
occur.

2.8 Imbalance Between Uplink and Downlink
The difference between the uplink signal level and the downlink signal level
may be great in the following conditions:


The transmit power of the BTS is high.



The tower mounted amplifier (TMA) or BTS amplifier does not work
properly.



The antenna and the connector are not securely connected.

As a result, call drops may occur at the edge of the BTS coverage area.

2.9 Repeater Problem
If a cell is installed with a repeater, BTS coverage problems may occur in
the case that the repeater is faulty or that the uplink and downlink gain is
inappropriately set. Therefore, the TCH call drop rate increases.
If a wide-frequency repeater is used and the gain is set to a great value,
strong interference may be caused. As a result, the network quality is
adversely affected and the TCH call drop rate increases.


3

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Analysis of and Solutions to High
TCH Call Drop Rate
Figure 1.5 shows the procedures for analyzing high TCH call drop rate.


Figure 1.5 Procedures for analyzing high TCH call drop rate

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3.1 Solutions to High TCH Call Drop Rate
Before analyzing the causes of high TCH call drop rates, you should find
out the difference between the actual TCH call drop rate and the expected
value. You should also find out the impact of the problems and the related
KPIs.
You can analyze the distribution of call drops based on the traffic
measurement. If a certain type of call drop accounts for a large proportion
of total call drops, you can locate the fault by performing the corresponding
procedure. If no obvious causes are found, you can perform the
procedures shown in Figure 1.5. The following table lists the mapping
between the traffic measurement counters and the troubleshooting
procedures.
Traffic Measurement Counter

Troubleshooting Procedure

M3100A (T200 expiry)

3.1.4Checking the Parameter Settings

M3101A (radio link failure)

3.1.4Checking the Parameter Settings

M3101D (radio resource unavailable)

3.1.1Checking the Hardware

CM333 (Abis terrestrial link
failure)

3.1.2Checking the Transmission

M314 (device failure)

3.1.1Checking the Hardware

The following sections describe the solutions to high TCH call drop rates.
The traffic measurement counters and KPIs in this document are the same
as those in the BSC6000V9R8C01B051 version.

3.1.1 Checking the Hardware
If a TRX or a combiner is faulty or if an RF cable is incorrectly connected,
seizing the TCH becomes difficult, and thus the TCH call drop rate
increases. See Case 7: Call Drop Due to TRX Board Fault.
You can check whether hardware is faulty by viewing BTS alarms or viewing
the hardware state on the Site Device Panel of the LMT. Table 1.1 lists the
major BSC alarms related to hardware failures.
Table 1.1 Major BSC alarms related to hardware failure
Alarm ID

Alarm Name

1000

LAPD_OML Fault Alarm

2204

TRX Communication Alarm

4414

TRX VSWR Alarm


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DRU Hardware Alarm

3606

In addition, you can locate the fault by checking the traffic measurement
related to hardware failures.

Cause

BSC Level

Cell Level

Equipmen
t failure

Access Measurement per BSC
in BSC Measurement:

KPI Measurement per Cell:

Available TCHs per BSC

Available TCHs

Configured TCHs per BSC

Configured TCHs

Available TCHs per BSC

TRX Measurement per Cell:

TCH Availability

Number of configured TRXs in a
cell
Number of available TRXs in a
cell
Call Drop Measurement per Cell
in Call Measurement:
Call Drops due to Equipment
Failure (TCH)

3.1.2 Checking the Transmission
Poor transmission quality, unstable transmission links, or insufficient
resources on the Abis and A interface may lead to the increase of the TCH
call drop rate. You can check the transmission conditions by viewing the
alarms related to transmission. If a large number of transmission alarms
are generated, you can infer that transmission failure has occurred. Then,
you should check the transmission connections. See Case 9: Call Drop Due
to Transmission Problem.
Table 1.1 BSC alarms related to transmission
Alarm ID

Alarm Name

1000

LAPD_OML Fault Alarm

11270

LAPD Alarm

11278

E1 Local Alarm

11280

E1 Remote Alarm

20081

Loss of E1/T1 Signals (LOS)

20082

E1/T1 Frame Out-of-Synchronization (LOF)


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In addition, you can locate the fault by checking the traffic measurement
related to transmission failures.
Cause

BSC Level

Cell Level

Transmission failure

LAPD Measurement in
BSC Measurement

Channel Activation
Measurement per Cell in Call
Measurement:
CHAN ACTIV NACK
Messages Sent by BTS
Channel Activation Timeouts
Call Drop Measurement per
Cell in Call Measurement:
Measurement of Call Drops
Due to Abis Terrestrial Link
Failure

3.1.3 Checking the BSC and BTS Version Upgrade
If TCH call drop increases after the BSC version or BTS version is
upgraded, you should check whether the BTS version is compatible with
the BSC version and whether the parameters and algorithms in the new
version are changed. See Case 6: Call Drop Due to Inappropriate Parameter
Setting and Case 10: Call Drop Rate Doubled in a CoBCCH Network After
Upgrade from V9R1 to V9R3.
To locate the problem, you can check the version description document
and the related documents, or provide feedback for the R&D department to
learn whether the new version has known defects. If the new version has
defects, you should replace it with another version or install the requisite
patch.
For details, refer to the BSC6000 Upgrade Guide.

3.1.4 Checking the Parameter Settings
The parameter settings on the BSC side and MSC side may affect the TCH
call drop rate. You should check the settings of the following parameters for
a cell with a high TCH call drop rate. See Case 5: Reduction of Call Drops by
Optimizing Handover Parameters and Case 12: Increase in Call Drop Rate Due to
Change of TR1N on the MSC Side.
1.

SACCH Multi-Frames

This parameter determines whether an uplink radio link is faulty. Each time the
BTS fails to decode the measurement report on the SACCH from the MS, the
counter decreases by 1. Each time the BTS successfully decodes the


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measurement report on the SACCH, the counter increases by 2. When the

value of this counter is 0, the BTS regards the radio link as faulty. In the
traffic measurement, if there are many call drops (M3101A) related to radio
link failure, you can infer that the radio propagation conditions are poor. In
this case, you can set this parameter to a greater value.
2.

Radio Link Timeout

This parameter determines whether a downlink radio link is faulty. Each
time the BTS fails to decode the measurement report sent over the SACCH
by the MS, the counter decreases by 1. Each time the BTS successfully
decodes the measurement report sent over the SACCH, the counter
increases by 2. When the value of this parameter is 0, the BTS regards the
radio link as faulty. In the traffic measurement, if there are many call drops
(M3101A) related to radio link failure, you can infer that the radio
propagation conditions are poor. In this case, you can set this parameter to
a greater value.
3.

RXLEV_ACCESS_MIN

This parameter specifies the minimum receive level of an MS to access the BSS. If
this parameter is set to a too small value, some MSs with low receive levels
may access the network and call drops are likely to occur. You can set this
parameter to a great value to reduce the TCH call drop rate. The counters
such as call setup success rate and the counters related to traffic volume,
however, are accordingly affected.
4.

RACH Min.Access Level

This parameter determines whether an MS can access the network over the RACH.
If this parameter is set to a too small value, some MSs with low signal levels
may access the network and call drops are likely to occur. You can set this
parameter to a great value to reduce the TCH call drop rate. The counters
such as call setup success rate and paging success rate, however, are
affected.
5.

Min DL Power on HO Candidate Cell and Min Access Level Offset

The sum of the values of the two parameters specifies the minimum
downlink receive level of a candidate neighboring cell for a handover. If this
parameter is set to a too great value, some desired cells may be excluded from
the candidate cells; if this parameter is set to a too small value, an unwanted cell
may become the candidate cell. Both conditions may lead to the increase of call
drops.
6.

Timer T3103 series

Timer T3101 series consists of T3103A, T3103C, and T8. These timers are
started to wait for a handover complete message. If the lengths of the
timers are set to small values, probably no message is received when timer
T3103 series expires. In this case, the BSC considers that the radio link in
the originating cell is faulty. Then, the BSC releases the channel in the
originating cell. Thus, call drops occur. In the traffic measurement, if many
call drops are related to handovers (CM331: Call Drops on Radio Interface
in Handover State), you can set this parameter to a greater value. If this
parameter is set to a too great value, channel resources are wasted and


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TCH congestion occurs.
7.

Timer T3109

This parameter specifies the period for waiting for a Release Indication
message after the BSC sends a Channel Release message to the BTS. If
this parameter is set to a too small value, the link may be released before
the Release Indication message is received. As a result, a call drop occurs.
You can set this parameter to a greater value to reduce the TCH call drop
rate. It is recommended that timer T3109 be set to 1–2 seconds longer than
timer Radio Link Timeout.
8.

Timer T3111

This parameter specifies the interval between the time that the main
signaling link is disconnected and the time that a channel is deactivated.
The purpose is to reserve a period of time for repeated link disconnections. If this
timer is set to a too small value, a channel may be deactivated too early.
Thus, call drops increase.
9.

Timers T305 and T308

Timers T305 and T308 are used on the MSC side. Timer T305 specifies the
period during which the MSC monitors the on-hook procedure. Timer T308
specifies the period during which the MSC monitors the resource release
procedure. You should set the two parameters when adding BSC data. Note
that the modification of the data in the timer table does not take effect. If
timers T305 and T308 are set to invalid or great values, the MSC clears the
call a long time after the MS hangs up. After the T3103 and Radio Link
Timeout timers expire, the number of call drops is increased and thus the
TCH call drop rate is significantly affected.
10. TCH Traffic Busy Threshold
If the current channel seizure ratio exceeds the value of this parameter, the
BSC preferentially assigns a half-rate channel to a dualrate-enabled call.
Otherwise, the BSC assigns a full-rate channel to the dualrate-enabled call.
Compared with a full-rate channel, a half-rate channel has weak antiinterference capabilities. Therefore, if a large number of half-rate channels
are assigned, the TCH call drop rate increases. It is recommended that this
parameter should not be set to a too small value if congestion is unlikely to
occur.
11.

Call Reestablishment Forbidden

This parameter specifies whether to allow call reestablishment. In case of
burst interference or radio link failure due to blind areas caused by high
buildings, call drops occur. In this case, MSs can initiate the call
reestablishment procedure to restore communication. To reduce the TCH
call drop rate, you can set this parameter to No to allow call
reestablishment. In certain conditions, allowing call reestablishment greatly
reduces the TCH call drop rate. Call reestablishment lasts for a long time, and
therefore some subscribers cannot wait and hang up. This affects user experience.
12.

Parameters related to edge handover

When the receive level drops greatly, an edge handover cannot be


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performed in time in any of the following conditions: The parameter Edge
HO UL RX_LEV Threshold or Edge HO DL RX_LEV Threshold is set to a
small value; the parameter Inter-cell HO Hysteresis is set to a great value;
the parameters Edge HO Watch Time and Edge HO AdjCell Watch Time
are set to great values; the parameters Edge HO Valid Time and Edge HO
AdjCell Valid Time are set to great values. As a result, a call drop occurs.
To reduce the TCH call drop rate, you can appropriately set these
parameters so that edge handovers can be performed in time to avoid call
drops.
13. Parameters related to BQ handover
When the signal quality deteriorates, a BQ handover cannot be performed
in time in any of the following conditions: The parameters
ULQuaLimitAMRFR, ULQuaLimitAMRHR, UL Qual. Threshold,
DLQuaLimitAMRFR, DLQuaLimitAMRHR, and DL Qual. Threshold are
set to great values; the parameter BQ HO Margin is set to a small value;
the parameter Inter-cell HO Hysteresis is set to a great value. As a result,
call drops occur. To reduce the TCH call drop rate, you should appropriately
set these parameters so that BQ handovers can be performed in time to
avoid call drops.
14. Parameters related to interference handover
If the parameters RXQUAL1 to RXQUAL12 are set to great values or if the
RXLEVOff parameter is set to a great value, strong interference may occur.
In this case, if interference handovers are not performed in time, call drops
occur. To reduce the TCH call drop rate, you can appropriately set these
parameters so that interference handovers can be performed in time to
avoid call drops. If the parameters RXQUAL1 to RXQUAL12 are set to
small values, the number of handovers due to other causes increases
greatly, thus affecting the handover success rate.
15. Parameters related to concentric cell handover
A call at the edge of the overlaid subcell cannot be handed over to the
underlaid subcell in any of the following conditions: In the case of a normal
concentric cell, the parameters RX_LEV Threshold and RX_LEV
Hysteresis are set to great values; in the case of an enhanced concentric
cell, the parameter OtoU HO Received Level Threshold is set to a great
value. As a result, a call drop is likely to occur. If the Call Drop Ratio on TCH
on the TRX in the OverLaid Subcell (RM330a) is high, you can
appropriately set these parameters so that calls at the edge of the overlaid
subcell can be handed over to the underlaid subcell in time.
When a call in the underlaid subcell has interference, the call cannot be
handed over to the overlaid subcell if the RX_QUAL for UO HO Allowed
parameter is set to Yes and the RX_QUAL Threshold parameter is set to a
great value. Thus, a call drop occurs. If the Call Drop Ratio on TCH on the
TRX in the Underlaid Subcell (RM330) is high, you can set these
parameters properly so that the call can be handed over to the overlaid
subcell at the earliest.
16.

Parameters related to power control

If the power control level and quality threshold are set to small values, call


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drops are likely to occur because of low signal level or bad voice quality.
17. T200 and N200
If the parameters T200 FACCH/F, T200 FACCH/H, N200 of FACCH/Full
rate, and N200 of FACCH/Half rate are set to small values, data links are
disconnected too early. Thus, all drops are likely to occur. If call drops occur
because of T200 expiry, you can increase the values of T200 and N200
properly.
18. Neighboring cell relations
If the neighboring cells configured in the BA2 table are incomplete, call
drops are likely to occur in the case of no suitable neighboring cell for
handover and progressive deterioration in the voice quality. Neighboring cell
relations should be configured completely on the basis of the drive test data
and electronic map (for example, Nastar) to minimize the call drops due to
no available neighboring cells.
19. MAIO
If frequency hopping (FH) is applied in a cell and the MAIO is set
inappropriately (for example, different TRXs serving the same cell have the
same MAIO), frequency collision may occur during FH. Thus, the TCH call
drop rate increases.
20. Disconnect Handover Protect Timer
This parameter is a software parameter of the BSC. After receiving a
DISCONNECT message from an MS, the BSC cannot hand over the MS
within the period specified by this parameter. Therefore, the following case
can be avoided: After being handed over to the target cell, the MS cannot
be put on hook because it does not receive a release acknowledgement
message. You are advised to set this parameter properly.
21. TR1N
This parameter should be set on the MSC side. It is used to avoid the
retransmission of short messages. When this parameter is set to a too great
value, the MSC does not send a CLEAR CMD message if the MS receives
a short message during link disconnection. As a result, the MS sends the
BTS a DISC message to disconnect layer 2 connection. After receiving the
DISC message, the BTS sends a REL_IND message to the BSC. Then, the
BSC sends a CLEAR REQ message to the MSC and the number of call
drops is incremented by one.
22. Software Parameter 13 and MAX TA
When the parameter Software Parameter 13 is enabled and the parameter MAX
TA is set to a too small value, the channel is released when the TA of a call exceeds
the MAX TA. In this case, call drops occur. It is recommended that the
parameter Software Parameter 13 should not be enabled.
23. Directly Magnifier Site Flag
If a BTS is installed with repeaters, the handover between repeaters can only
be asynchronous because the distance between repeaters is long. If


INTERNAL

synchronous handovers are performed, the handovers may fail and thus many
call drops occur. Therefore, when a BTS is installed with repeaters, the

parameter Directly Magnifier Site Flag should be set to Yes to avoid
synchronous handovers between cells under the same BTS.

You can check whether the data configuration is correct by viewing the
traffic measurement results.
The following table lists the traffic measurement counters related to
repeaters.
Cause

Cell Level

Parameters

Call Drop Measurement per Cell
in Call Measurement:
Call Drops on TCH in Stable State
(Error Indication)
(Connection Failure)
(Release Indication)

TRX Level
Measurement of Power Control
Messages in MR
Measurement
Radio Link Failure
Measurement in MR
Measurement

Call Drop Ratio on TCH on the
TRX in the Underlaid Subcell
Call Drop Ratio on TCH on the
TRX in the Overlaid Subcell
KPI Measurement per Cell:
Call Drops in TCH Handovers
(Traffic Channel)
(Traffic Channel)

3.1.5 Checking the Interference
If inter-network interference and repeater interference exist, or if severe
intra-network interference occurs because of tight frequency reuse, call
drops may occur on TCHs due to bad QoS. This affects the TCH call drop
rate. See Case 1: Call Drop Due to Interference.
The uplink interference information can be obtained on the basis of the
interference band distribution in the traffic measurement results. A large
proportion of interference levels belong to interference bands 3–5, you can
infer that the uplink has strong interference. You can obtain the downlink
interference information by performing drive tests or by analyzing the traffic
measurement results related to receive quality.
For details on how to check the intra-network and inter-network
interference, refer to the G-Guide to Eliminating Interference.


INTERNAL

The following table lists the traffic measurement counters related to
interference. (If a cell has interference, the TCH call drop rate is high. In
addition, the handover success rate decreases and the handovers due to
bad quality account for a large proportion of all handovers.)
Cause

Cell Level

TRX Level

Interference

Outgoing Internal Inter-Cell
Handover Measurement per
Cell in Call Measurement:

Interference Band Measurement per
TRX in MR Measurement

Number of Outgoing Internal
Inter-Cell Handover Requests
(Signal Quality)
Number of Outgoing Internal
Inter-Cell Handover Requests
(Other Causes)

Mean Number of TCHs in
Interference Band 1
Interference Band 2
Interference Band 3
Interference Band 4
Interference Band 5
Measurement of Receive Quality in
MR Measurement

3.1.6 Checking the Coverage
You can check the network coverage by conducting outdoor drive tests or
indoor dialing tests. During the tests, you can infer that the network
coverage is insufficient if the following conditions occur: The downlink
receive level is low (lower than –110 dBm) so that the test MS cannot
access the network or the voice quality is bad; a handover cannot be
performed because of no suitable neighboring cells and the signal quality
gradually deteriorates. See Case 4: Call Drop Due to Coverage.
If the network coverage in a cell is insufficient, the TCH call drop rate is
high. In addition, the handover success rate is low, and a large proportion
of handovers are performed because of weak signal strength. You can
check whether a cell has coverage problems by viewing the traffic
measurement results. The following table lists the traffic measurement
counters related to coverage.


Cause

Cell Level

Coverage

Outgoing Internal Inter-Cell Handover
Measurement:
Number of Outgoing Internal Inter-Cell
Handover Requests (Signal Strength)
Outgoing External Inter-Cell Handover
Measurement:

INTERNAL

TRX Level
TCHF Receive Level
Measurement per TRX
in MR Measurement
TCHH Receive Level
Measurement per TRX
in MR Measurement

Requests (Uplink Strength)
Requests (Downlink Strength)

If a coverage problem exists, you can solve the problem through the
following methods: adjusting the tilt of the antenna, increasing the transmit
power, adding repeaters, or changing the combining mode. For details,
refer to the GSM BSS Network Performance KPI (Coverage) Optimization
Manual.

3.1.7 Checking the Antenna System
In the case of dual-transmit antennas, ensure that the tilt and azimuth of one
antenna are the same as those of the other antenna.
In addition, you should check whether the jumpers are improperly
connected (for example, by analyzing drive test data). If a jumper is
improperly connected, the uplink signal level in the cell is significantly lower
than the downlink signal level, and call drops are likely to occur at a place
far away from the BTS. You should ensure that the jumpers are connected
correctly. See Case 8: Call Drop Due to Antenna System Problem.
If the feeder is damaged, water runs into the feeder, or the feeder and the
connector are not securely connected, both the transmit power and
receiver sensitivity of the antenna are reduced. Thus, call drops probably
occur. You can locate these problems by checking the VSWR alarms. If any
feeder is faulty, you should replace it in time.
If the antenna system has problems, the TCH call drop rate and handover
failure rate are high. In addition, the difference between the uplink quality
and the downlink quality is great, or both the uplink quality and the
downlink quality are bad. You can check whether the antenna system is
faulty by viewing the traffic measurement results. The following table lists
the traffic measurement counters related to the antenna system.


Cause

Cell Level

Antenna
system

KPI Measurement per Cell in Call
Measurement:
Success Rate of Radio Handover
Measurement:
Requests (Uplink Strength)

INTERNAL

TRX Level
Measurement of Receive
Quality in MR
Measurement
Uplink-and-Downlink
Balance Measurement per
TRX in MR Measurement

Requests (Downlink Strength)

3.1.8 Checking the Balance Between Uplink and
Downlink
The difference between the uplink signal level and the downlink signal level
may be great in the following conditions: The transmit power of the BTS is
high; the tower mounted amplifier (TMA) or BTS amplifier does not work
properly; the antenna and the connector are not securely connected. As a
result, call drops may occur at the edge of the BTS coverage area. See
Case 2: Call Drop Due to Imbalance Between Uplink and Downlink.
To analyze the balance between the uplink and the downlink, check
whether the transmit power of the BTS is too high. Then, you should check
whether such components as the TMA, BTS amplifier, and antenna port
that affect downlink receive level have problems. For details, refer to the
GSM BSS Network Performance KPI (Uplink and Downlink Balance)
Optimization Manual.
If the uplink and downlink are imbalanced, the following conditions may
occur: The difference between the mean uplink receive level and the mean
downlink receive level is great; the uplink and downlink balance level is
high; the immediate assignment success rate and the assignment success
rate are low. The following table lists the traffic measurement counters
related to the balance between the uplink and the downlink.


INTERNAL

Cause

Cell Level

TRX Level

Balance between uplink and
downlink

Assignment
Measurement
per Cell in Call
Measurement:

Uplink-and-Downlink
Balance Measurement
per TRX in MR
Measurement

TCH
Assignment
Success Ratio

TCHF Receive Level
Measurement per
TRX in MR
Measurement

Success Rate of
Call
Establishment
Immediate
Assignment
Measurement
per Cell in Call
Measurement:

TCHH Receive Level
Measurement per
TRX in MR
Measurement

Success Rate of
Immediate
Assignments

3.1.9 Checking the Repeaters
Check whether the parameter Directly Magnifier Site Flag is set to Yes in
the data configuration on the LMT. If this parameter is set to Yes, you can
infer that the cell is configured with repeaters. If this parameter is set to No,
you should check whether other operators' repeaters are installed near the
cell.
If repeaters are installed, you should check whether they are widefrequency repeaters, and whether the uplink/downlink amplification
coefficient is too great. Ensure that the amplification coefficient is properly
set. If a repeater has an impact on the TCH call drop rate, you should
switch off the repeater.
In addition, you should check whether a repeater is faulty and whether the
uplink/downlink gain is set to a too great/small value. If such problems
exist, the coverage area of the BTS changes. Thus, the TCH call drop rate
increases. See Case 3: Call Drop Due to Repeater Problem.
If repeater problems exist in a cell, the TA distribution varies greatly in the
traffic measurement results. The following table lists the traffic
measurement counters related to repeaters.
Cause

Cell
Level

TRX Level

Repeate
r

None

Number of MRs based on TA per TRX in MR
Measurement


4

INTERNAL

Test Methods

The TCH call drop rate is one of retainability KPIs, which can be obtained
through registration of or reporting of the related traffic measurement
counters. In addition, the TCH call drop rate is one of key drive test
counters and it can be obtained through drive tests.
At present, the formula for the TCH call drop rate varies with device
manufacturer and with telecom operator, thus affecting the value of the
TCH call drop rate. During actual tests, you should register the specific
counters and use an appropriate formula to calculate the value of the TCH
call drop rate.

5

Remarks About the Signaling

Analysis of the TCH Call Drop Rate
Trace the RSL signaling on the Abis interface. Then, generate the signaling
tracing file on the LMT or through the Signal Analyze Tool. Obtain the
CONN_FAIL and ERROR_INC signaling from the file. Then, right-click a
piece of signaling and choose Call Trace from the shortcut menu, as
shown in the following figure.


INTERNAL

Then, right-click the signaling of a call and choose Show Chart from the
shortcut menu, as shown in the following figure.

From the following figure, you can view such information as the uplink and
downlink receive level of the serving cell, uplink and downlink signal quality
of the serving cell, downlink receive level of neighboring cells, TA, MS
power, and BTS power. Based on the information, you can find out the
causes of call drops, such as insufficient downlink coverage and
interference.


6

INTERNAL

Cases for TCH Call Drop Rate
Optimization

6.1 Case 1: Call Drop Due to Interference
Symptom description:
A certain BTS adopted the 1x3 RF FH mode. After the capacity of the BTS
was expanded, the TCH assignment failure rate remained high (because of
radio link failure), and the TCH drop rate and handover failure rate were
high. The SDCCH call drop rate, however, was normal.
Cause analysis and handling:
In the case of high TCH assignment failure rate, TCH call drop rate, and
handover failure rate, you can infer that there are two probabilities: Failure
occurs during the TCH assignment, and the frequency or timeslot used by
the call has interference or is unstable. In the case of normal SDCCH call
drop rate, there is a low probability that the BCCH TRX and the BCCH
frequency have interference. Therefore, there is a high probability that the
non-BCCH TRX and the FH frequency have interference.
The check of the device hardware, antenna system, and transmission
stability finds no problems. Through drive tests, you can find a high ratio of
high signal level and low voice quality. Dialing tests show that the voice
quality is bad. When checking the parameter settings, you can find that the
MAIO of the new TRX is the same as the MAIO of another TRX.
The cause of the fault is frequency collision because the same MAIO is
used.


INTERNAL

6.2 Case 2: Call Drop Due to Imbalance Between
Uplink and Downlink
The following symptoms occurred during drive tests: After the test MS
camped on a cell, it could not make calls; the MS can only receive calls;
call drops occurred frequently at a certain distance from the antenna; a call
drop occurred after frequent handovers.
The cause may be the uplink signal level is much lower than downlink
signal level. During drive tests, move the test MS towards the edge of the
cell, and use the MA10 signaling analysis tool to trace the signaling on the
BTS side.

The tracing result (as shown in the previous figure) shows that the uplink
signal level is much lower than the downlink signal level. Therefore, call
drops occur.

6.3 Case 3: Call Drop Due to Repeater Problem
Under a BTS, the TCH call drop rate in cell 3 reached 10%; however, the
call drop rate and congestion rate in cell 1 and cell 2 remained normal.
1. Block the channels in the cell. The congestion rate in cell 3, however,
remains high.
2. Check the traffic measurement results. The distribution of interference
bands is regular, that is, the interference is high during peak traffic hours
and is low during low traffic hours.
3. Change the frequency of cell 3 so that the spacing between the
frequency and the original one is at least 1 MHz. The interference,
however, persists. Therefore, the probability of co-channel interference and
adjacent-channel interference is eliminated.
4. Ensure that the devices are not faulty.
5. Find the external interference.
6. Use a spectrum analyzer to perform frequency scan tests. The signal
from a certain frequency (the central frequency is 904.14 MHz and the
spectrum bandwidth is 300 kHz) exists continuously and it is similar to the
signal from an analog spectrum. The strength of the signal at the divider


INTERNAL

port of cell 3, cell 2, and cell 1 is –27 dBm, –40 dBm, and –60 dBm
respectively, and the signal strength is consistent with the interference
level. The traffic volume in daytime is greater than that at night, and thus
the probability of intermodulation is high. It can be concluded that the 904.14
MHz frequency is the interference source. When a spectrum analyzer is
used to perform drive tests, the interference source cannot be located.
When tests are performed on a rooftop, it is found that the interference is
generated by a small antenna of a repeater. If the signal from the antenna
is blocked, all the test results are normal. Therefore, the interference signal
is generated by the antenna.

6.4 Case 4: Call Drop Due to Coverage
Subscribers complained that call drops occurred frequently when calls
were made on the fifth or the higher floors of a building.
Cause analysis:
Step 1: Perform onsite tests. Call drops occur and interference exists.
When a call drop occurs, the MS is located in a cell that does not belong to
local BTS A. Step 2: Confirm that the cell belongs to BTS B, which is about
three or four kilometers away from the building. Therefore, the signal from
a cell under BTS B is reflected by an obstruction and then is received by
the MS. A cross coverage area is formed on the fifth or above floor of the
building. Step 3: Check the data configuration. In the BSC data
configuration, cell 2 of BTS A is configured as a neighboring cell of BTS B,
but cell 3 of BTS A is not. When the MS in the area uses the signal from
cell 2 of BTS B, the signal from cell 3 of BTS A is stronger than that from
cell 2 of BTS B. In this case, handovers cannot be performed because cell
3 of BTS A is not a neighboring cell of BTS B.
The signal from cell 2 of BTS B may be reflected multiple times before it is
received by the MS. If the signal becomes weak suddenly, an emergency
handover is required. In this case, if both cells 2 and 3 are not the best
candidate cells for the handover, the MS may be handed over to a cell
under BTS C. The MS, however, cannot receive signals from BTS C. Thus,
a call drop occurs.
Handling:
Modify the BA1 (BCCH) table, BA2 (SACCH) table, and neighboring cell
relation table in the BSC data configuration. Ensure that cell 3 under BTS A
is a neighboring cell of cell 2 under BTS B. Network engineering
parameters are further optimized to eliminate the cross coverage problem.
Subsequent tests show that the call drop problem is solved.
Conclusion:
You can use the following methods to solve the cross coverage problem: 1.
Adjusting the antenna of the cross coverage cell to eliminate cross
coverage 2. Defining new neighboring cells for the cross coverage cell


INTERNAL

6.5 Case 5: Reduction of Call Drops by
Optimizing Handover Parameters
During drive tests, call drops occurred frequently at a cave entrance near
the BTS because handovers were not performed in time. Before the MS
entered the cave, the signal level of both the serving cell and the target cell
was high, and thus a handover was not triggered. After the MS entered the
cave, the signal level of the target cell was proper (about 80 dBm), but the
signal level of the original serving cell rapidly decreased to below 100 dBm.
Therefore, a call drop occurred before the measurement period ended.
Modify the settings of the related parameters.
Parameter Name

Before
Modification

After
Modification

PBGT Watch Time

5

3

PBGT Valid Time

4

2

PBGT HO Threshold

72

68

UL Qual. Threshold

70

60

Min DL Power on HO
Candidate Cell

10

15

You can adjust the handover parameters to reduce call drops in the
following ways:
1. If there is no frequent audio discontinuity or ping-pong handover, set the
parameters properly so that PBGT handovers can be easily performed,
thus minimizing the interference and reducing the call drop rate.
2. Set the emergency handover threshold properly so that emergency
handovers are triggered before call drops occur.

6.6 Case 6: Call Drop Due to Inappropriate
Parameter Setting
After a cutover of five BTSs for capacity expansion, the TCH call drop rates
in the cells under these BTSs were high (reached 5%). The number of call
drops in each cell was about 100. Among the five BTSs, one BTS that had
no capacity change also had a high TCH call drop rate. The causes of all
call drops were related to radio frequency. There was no interference, and


INTERNAL

the BTS hardware was not faulty.
Check the data configuration, frequency planning, BSIC planning, and
traffic measurement results. All the interference bands are normal and no
interference exists. The handover success rate is over 93% and thus
handovers are performed normally. Check the versions of each TRX and
FPU. It is found that the TRX version is inconsistent with the FPU version.
Upgrade the TRX and FPU to ensure that their versions are compatible,
but the problem persists. Check the data configuration again. It is found
that the BTS after capacity expansion adopts the 15:1 multiplexing mode,
and that the measurement report preprocessing function is enabled for the
BTS2X. The BTS2X in some versions, however, do not support the
measurement report preprocessing function. Therefore, the TCH call drop
rate is too high.
After a large-scale adjustment is performed on the system, for example,
BTS cutover, BTS capacity expansion, frequency replanning, upgrade,
patch installation, the related system parameters should be checked
completely and adjusted if required. The following parameters should be
checked: neighboring cell relations, frequency interference conditions, FH
parameters, and cell parameters. Special attention should be paid to the
BTS version.

6.7 Case 7: Call Drop Due to TRX Board Fault
During dialing tests, call drops occurred frequently in cell 2 of a BTS.
Cause analysis:
The traffic measurement results show that the TCH congestion rate in the
cell exceeds 10% and that the incoming handover failure rate is high. The
remote maintenance terminal shows that one TRX board in the cell is not
normal. Thus, the TRX board may be faulty.
Handling:
Use the test MS to make calls repeatedly on only the frequency of the
faulty TRX board. It is found that call drops occur on timeslots 1, 3, 5, and 7
and that calls are made normally on timeslots 2, 4, 6, and 8. Remove the
TRX board and then insert it into another slot. The problem persists. Insert
a functional TRX board into the slot of the faulty TRX board. Calls are
made normally. Then, insert the faulty TRX board into another cabinet. The
problem persists. As a conclusion, the TRX board is faulty. Insert a spare
board into the slot of the faulty TRX board, calls are made normally.
Summary:
When tests are performed on the BTS side, each TRX and each timeslot
on the TRX should be tested. You should ensure that bi-directional calls
can be made on each TCH and that the voice quality is good.


INTERNAL

6.8 Case 8: Call Drop Due to Antenna System
Problem
A new BTS3012 was deployed at a site and the cell configuration was
S2/2/2. After the BTS3012 was put into operation, the number of TCH call
drops in both cell 1 and cell 2 in busy hours reached 20, the number of
SDCCH call drops in cell 3 in busy hours reached 1,000. These counters in
cell 3 were normal.
Analyze the traffic measurement results of TRX-level radio link
performance in busy hours. It is found that the number of abnormal radio
links on both TRX 3 (TRX 2 and TRX 3 are configured for cell 1) and TRX 7
(TRX 6 and TRX 7 are configured for cell 2) is great. TRX 3 and TRX 7 are
the second TRX in cell 1 and cell 2 respectively; therefore, they are
connected to the TXB channel of the DDPU in the corresponding cell. The
jumpers of the two non-BCCH TRXs may be improperly connected.
Analyze the traffic measurement results related to the uplink and downlink
balance performance. It is found that uplink and downlink imbalance levels
1, 2, and 3 account for a large proportion of all imbalance levels for both
TRX 3 and TRX 7. This indicates that the downlink loss is too great or the
downlink transmit power is too low. TRX 2 (main BCCH TRX) and TRX 6
(main BCCH TRX) are connected to the TXA channel of the DDPU in cell 1
and cell 2 respectively. When calls are assigned with the channels on the
non-BCCH TRX, transmit power decreases sharply because the feeders of
TRX 3 and TRX 7 are improperly connected. Thus, call drops occur. Rectify
the misconnection and find that the TCH call drop rate and SDCCH call
drop rate in both cell 1 and cell 2 become normal.

6.9 Case 9: Call Drop Due to Transmission
Problem
At a certain site, the MOTO BTS was replaced by the Huawei BTS and the
cell configuration was S2/2/2. On the night of the replacement, calls were
made normally and drive tests showed that all performance counters were
normal. The traffic measurement results within a period of 15 minutes
showed that MS-originated and MS-terminated calls were made normally
and handovers were performed normally. After a week of operation, the
traffic measurement results showed that the value of the counter SDCCH
Seizure Request was not normal: The maximum number of SDCCH
seizure requests reached 9000, the number of Successful SDCCH Seizure
Requests was over 7000, and the number of Failed SDCCH Seizures due
to Busy SDCCH was over 900. Compared with the similar SDCCH
counters, the TCH traffic volume is small and the TCH call drop rate is high.


INTERNAL

Check the hardware on site. It is found that making a call is difficult on site.
In addition, some subscribers complained that it was difficult to make calls
after the replacement. After obtaining the consent from the customer, the
onsite engineers power off the BTS and load the data again. During the
initialization of the BTS, a message is displayed, indicating that the process
is disrupted and that the communication is timed out. Some parameters of
the BTS cannot be initialized. The BTS hardware is normal and the cable
connections are proper.
Before the replacement, the transmission is normal. After the replacement,
most of the transmission cables are the original ones. Huawei engineers
replace only the transmission cable between the transmission equipment
room and the Huawei BSC and use a new E1 connector to fix the DDF
transmission cable to the E1 port on top of the BTS cabinet. Therefore, the
E1 connector may be made improperly so that the transmission bit error
rate is high and that the BTS cannot be completely initialized. As a result,
when a subscriber makes a call, assigning a TCH is difficult. A detailed
check shows that the E1 connector on top of the BTS cabinet is made
improperly. After a proper E1 connector is used, the problem is solved.

6.10 Case 10: Call Drop Rate Doubled in a
CoBCCH Network After Upgrade from V9R1 to
V9R3
In the Egypt 3rd license project, after the BSC was upgraded from V9R1 to V9R3,
the number of call drops in the CoBCCH network doubled.
Compare V9R1 with V9R3. It is found that the function of configuring the
BCCH in the overlaid subcell is added to V9R3 and that a new TRX-level
parameter HW_Concentric Attribute (with the default value None) is add.
Check the code. It is found that if the parameter HW_Concentric Attribute
is set to None, the operating frequency band of the MS may be wrongly
determined. As a consequence, power control may be performed
improperly. For example, the 900 MHz frequency band may be mistakenly
regarded as the 1800 MHz frequency band. In this case, if power control is
performed, the power control amplitude becomes large and the signal level
is adjusted to a low level. Thus, call drops increase. Manually set the
HW_Concentric Attribute of the main BCCH to Underlay cell. The
problem is solved and the call drop rate becomes normal.


INTERNAL

6.11 Case 11: Increase in Call Drop Rate Due to
Inactivity of T305 and T308
After a replacement was performed in Hainan Mobile project, the TCH call
drop rate increased. In urban areas, the TCH call drop rate increased from
0.4% to 0.7%; in suburban areas, it increased from 0.7% to 1.1%.
Analyze the A interface signaling and the version change. A version change
is found, that is, timers T305 and T308 must be set during the addition of
the BSC data, and the data modification in the timer table does not take
effect. Timer T305 and T308 are inactive; therefore, the MSC does not
initiate the call release procedure. As a result, the number of call drops
increases greatly. After the settings of the two parameters are modified, the
call drop rate becomes lower than that in the original network. The problem
is solved.

6.12 Case 12: Increase in Call Drop Rate Due to
Change of TR1N on the MSC Side
The value of the TR1N parameter was changed from 20s to 60s to avoid
retransmission of short messages and to improve user experience. After
the change, the number of call drops with the cause value Release
Indication increased greatly.
Analyze the signaling on the A interface. After the value of TR1N is changed, the
following signaling flow takes place: After the MS sends a DISCONNECT
message to the network, the MSC does not send a CLEAR CMD message to
release the terrestrial resources and the TCH. In this case, the MS sends the BTS a
DISC message to disconnect layer 2 connection. After receiving the DISC
message, the BTS sends a REL_IND message to the BSC. Then, the BSC sends a
CLEAR REQ message to the MSC and the number of call drops is incremented by
one. After the TR1N parameter is set to 20s again, the TCH call drop rate
decreases greatly and returns normal.


7

INTERNAL

Feedback Form for the TCH Call
Drop Rate
If the TCH call drop rate is high and technical support is required, fill in the
following form.
Check Item

Example

Description

Software version

BSC and BTS software
versions

Check whether the software
version is faulty.

Data
configuration
table

*.dat file

Check whether the network
optimization parameters and
power settings are proper.

Alarm
information

Hardware, clock, and
transmission (selfcheck)

Check whether alarms related to
the hardware, clock, and
transmission are generated in a
cell with a high TCH call drop
rate.

Traffic
measurement

Traffic measurement
results related to the
voice quality and the
balance between
uplink and downlink

Based on traffic measurement
results, check whether
interference and imbalance
between uplink and downlink
exist.

Signaling

RSL signaling tracing
data

Check the causes of call drops.

Drive test data

*.log (*.cell site) or
*.ant file

Based on the drive test data,
determine whether interference or
coverage problems exist.


INTERNAL

Check Item

Example

Description

Others

Engineering parameter
table and electronic
map

The NASTAR software can be
used to import the electronic map
to facilitate the geographical
information check.

Gsm bss-network-kpi-tch-call-drop-rate-optimization-manual(cdr)

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Gsm bss-network-kpi-tch-call-drop-rate-optimization-manual(cdr)