Huawei case analysis call drop

Internal

OMF000404
Case Study – Call Drop

Issue 2.0 www.huawei.com

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

References

 31160978-BSC Traffic Statistic Manual
Volume I
 31033203-BSS Troubleshooting
Manual

HUAW I T CH
S T All rights reserved Page 2

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

HUAW I T CH

Chapter 1 Principle of call drop

Chapter 2 Analysis of call drop

Chapter 3 Call drop cases

HUAW I T CH


Section 1 Definition of call drop

Section 2 TCH call drop

Section 3 SDCCH call drop

HUAW I T CH

Definition of Call Drop

 There are two types of call drop: TCH call drop and SDCCH
call drop:
 TCH call drop means TCH channel is released abnormally
after it is occupied successfully.
 SDCCH call drop means SDCCH channel is released
abnormally after it is occupied successfully.

HUAW I T CH


HUAW I T CH

Causes of TCH Call Drop

 Usually, the typical causes for sending the Clear_Request
message are as follows:
 Radio interface failure
 O&M intervention
 Equipment failure
 Protocol error
 Preemption

HUAW I T CH

Formula of TCH Call Drop Rate

 Formula of TCH call drop rate

 TCH call drop rate=TCH call drops / Successful TCH
seizures (all)

HUAW I T CH

Measurement Point of TCH Call Drop

HUAW I T CH

 TCH serves as traffic channel, BSC receives ERROR INDICATION
message from BTS.
 TCH is seized as traffic channel, BSC receives CONNECT FAILURE
INDICATION message from BTS.
 In assignment procedure and handover procedure, failure of decoding
HANDOVER DETECTION and HANDOVER COMPLETE message.
 TCH serves as traffic channel, incoming BSC handover initiated and the
timer for the target cell to wait for HANDOVER COMPLETE message
expires.
 TCH serves as traffic channel, outgoing BSC handover initiated and the
timer for the source cell to wait for CLEAR COMMAND message from MSC
expires (T8 expires).
 In intra-BSC handover procedure, the target cell sends Inter Clear Request
to the source cell when the timer for the target cell to wait for HANDOVER
COMPLETE from MS expires, in this case, this item is measured in the
source cell.

HUAW I T CH

 In Intra-BSC handover procedure, the source cell, excluding the source cell
for directed retry, measures the item when the timer for the source cell to wait
for Inter Clear Request with cause value HANDOVER COMPLETE from the
target cell expires.
 In Intra-BSC handover procedure, when the target cell AM/CM net-drive fails
(due to timeout or negative acknowledgement), the target cell sends Inter
Clear Request to source cell, in this case, this item is measured in the source
cell.
 In the case that MS reverses to the original channel after intra BSC handover
fails, the source cell first releases the terrestrial connection but the AM/CM re-
net-drive fails (due to timeout or negative acknowledgement). In this case this
item is measured in the source cell.
 The resource of the lower_priority call will be preempted by the higher_priority
call if MSC and BSC both supports preemption, which will cause call drop.
 This item is measured when the RSL link of the TRX that the call is using
disconnects, which will cause call drop.

HUAW I T CH

TCH Call Drop - Assignment

MS BTS BSC MSC

ASSIGNMENT REQUEST

CHANNEL ACTICATION

CHANNEL ACTIVATION ACK

ASSIGN CMMAND
SABM
ESTABLISH INDICATION
UA

ERROR INDICATION
CONNECTION FAILURE INDICATION A1

A2

HUAW I T CH

TCH Call Drop – Intra-BSC Handover
MS BSC' BSC Ori-Cell BSC Tag-Cell BSC''

Intercell Handover Request

CH ACT
CH ACT ACK

Intercell Handover Response

HANDOVER COMMAND

HANDOVER ACCESS

HANDOVER DETECT
SABM

UA

ERROR INDICATION

A1
CONNECTION FAILURE INDICATION

A2

HUAW I T CH

TCH Call Drop - Incoming BSC Handover

MS Other BSC HUAWEI BSC HUAWEI BTS MSC

HANDOVER REQUIRED
HANDOVER REQUEST
CH ACT
CH ACT ACK
HANDOVER REQUEST ACK
HANDOVER COMMAND

HANDOVER ACCESS
HANDOVER DETECT
SABM
UA

ERROR INDICATION
A1
CONNECTION FAILURE INDICATION

A2

HUAW I T CH

Measurement Point of Successful TCH Seizures
 Measurement point of Successful TCH seizures

 Upon BSC’s reception of CHANNEL ACTIVATION ACKNOWLEDGE
message from BTS in very early assignment TCH process.
 In the case the target cell of directed retry is located in other BSC
and directed retry succeeds, MSC sends CLEAR COMMAND
message to the originating BSC to release the original connection.
This item is measured when the originating BSC receives this
message.
 In the case the directed retry target cell is located in the same BSC
and the directed retry succeeds, target cell sends Inter Clear
Request message to the source cell to request to release the
resource and the original connection. This item is measured when
the source cell receives this message.

HUAW I T CH

Measurement Point of Successful TCH
seizures
 Measurement point of Successful TCH seizures

 This item is measured when BSC sends ASSIGNMENT COMPLETE
message to MSC after the assignment procedure is successfully
implemented.
 In incoming BSC handover procedure, MS sends HANDOVER ACCESS
message to the BSC. This item is measured when BSC receives
HANDOVER DETECT message from BTS.
 In the process of incoming internal inter cell handover and intracell
handover in BSC, MS sends HANDOVER ACCESS message to BSC.
BSC measures this item in the target cell when receiving HANDOVER
DETECTION message from BTS.

HUAW I T CH

Successful TCH Seizure – Very Early Assignment
 Very early assignment process

MS BTS BSC MSC
Channel Request (RACH) Channel Required

Channel Activation (TCH or SDCCH)

Channel Activation Acknowledge

Immediate Assignment Command

Immediate Assignment (AGCH)

HUAW I T CH

Successful TCH Seizure – Assignment
 Assignment process

MS BTS BSC MSC
ASS_REQ
CH_ACT
CH_ACT_ACK

ASS_CMD
ASS_CMD
EST_IND

ASS_CMP ASS_CMP ASS_CMP

HUAW I T CH

Successful TCH Seizure – Intra-BSC Handover
 Intra-BSC handover process

MS BTS1 BSC BTS2 MS MSC
Measurement Report from MS
Channel_Active

Channel_Active ACK
HANDOVER COMMAND
HO_ Access
HO_Detect
PHY INFO
PHY INFO
First SABM
Establish_IND
UA

Handover Complete HO_Performed

HUAW I T CH

Successful TCH Seizure – Inter-BSC
Handover
 Inter BSC handover process

MS BTS1 BSC1 MSC BSC2 BTS2 MS
Measure Report from MS

HO_Required
HO_Request
CH_ACT
CH_ACT_ACK
HO_Request_ACK
HO_CMD
Handover Command HO_Access
HO_Detect
HO_Detect
PHY INFO
PHY INFO

Establish_IND First SABM
HO_CMP Handover Complete
Clear_CMD
Clear_CMP

HUAW I T CH


HUAW I T CH

Formula of SDCCH Call Drop Rate

 Formula of SDCCH call drop rate:

 SDCCH call drop rate = SDCCH call drops/ successful
SDCCH seizures*100%

HUAW I T CH

Measurement Point of SDCCH Call Drop
 This item is measured when BSC receives ERROR INDICATION
message from BTS due to an abnormal case for a radio link layer
connection.
 This item is measured when BSC receives CONNECTION FAILURE
INDICATION message from BTS because an active connection has been
broken for some reason such as SDCCH link failure or hardware failure
(see GSM 0508 for details).
 In incoming BSC handover procedure on SDCCH, this item is measured
in the target cell in the case of the failure of decoding HANDOVER
DETECTION and HANDOVER COMPLETE message.
 In the process of incoming BSC handover on SDCCH, this item is
measured in the target cell when the timer for the target cell to wait for
the HANDOVER COMPLETE message expires.

HUAW I T CH


 In the process outgoing BSC handover on SDCCH, this item is
measured when the timer for the source cell to wait for CLEAR
COMMAND message from MSC expires (T8 expires).
 In the process of intra BSC handover on SDCCH, this item is measured
in the source cell when the timer for the source cell or the target cell to
wait for HANDOVER COMPLETE message expires.
 This item is measured when the RSL link of the TRX that the call is
running on disconnect, which will cause call drop, this item measures
call drop on SDCCH for RSL disconnection.

HUAW I T CH

MS BTS BSC MSC
Channel Request Channel Required

Channel Activation (SDCCH)

Channel Activation Acknowledge

Immediate Assignment Command
Immediate Assignment
Establish Indication (L3 Info)

Connection Failure
Or:
Or: Error Indication

Abis Failure
Cell SDCCH Call Drop
(Subject to different cases)

HUAW I T CH

Measurement Point of Successful SDCCH
Seizures
 In immediate SDCCH assignment procedure, this item is
measured when BSC receives CHANNEL ACTIVATION
ACKNOWLEDGE message from BTS.
 In the process of incoming BSC handover on SDCCH. This
item is measured when BSC receives HANDOVER
DETECTION from BTS.
 In the process of incoming internal inter cell handover and
intracell handover in BSC.BSC measures this item in the target
cell when receiving HANDOVER DETECTION from BTS.

HUAW I T CH




HUAW I T CH

Analysis of Call Drop

 content

 main causes of high call drop rate
 troubleshooting of high call drop rate

HUAW I T CH

Analysis of Call Drop

 According to the definition of call drop measurement point, call drop is usually
caused by the following:
 Radio link fault. During the communication, messages can not be received
correctly.
 Abis link broken during conversation.
 Call drop during handover.
 Other system faults.
 Timers that may cause call drops (BSC timer):

 T3103: starting from sending HANDOVER CMD and ending at receiving
HANDOVER CMP. Time out of the timer will cause call drop.
 T3109: starting from sending CHAN REL and ending at receiving REL IND.

HUAW I T CH

Radio Link Fault
 Signaling process chart of radio link fault

MS BTS BSC MSC
(1)
Measurement Report
(2)
Measurement Result

Connection Failure

Clear_REQ (Radio Interface Failure)
(3)

(1) Dadicated mode is created. (SDCCH/TCH)
(2) Activate Abis monitoring function.
(3)SACCH message block can not be decoded(uplink/downlink),
resulting in radio link timeout.

HUAW I T CH

Radio Link Fault

 Diagram of radio link timeout

HUAW I T CH

T3103 Times Out
 Handover process

MS BTS1 BTS2 BSC MSC
Handover Indication

CH_ACT
CH_ACT_ACK

Handover Command
Handover Command Set T3103
Handover Access HO_Detect
Physical Information (TA)

SABM
EST_IND
UA

Handover Complete
Handover Complete
Reset T3103

HUAW I T CH

Causes of Radio Link Fault
 The causes of radio link fault can be
 Interference
− Internal interference
− External interference
− Equipment interference
 Poor coverage
− Coverage hole
− Isolated island
− Uplink/downlink imbalance
 Improper parameter setting
− Radio link timeout, SACCH multi-frames
− Handover parameters
− Power control parameters
 Equipment problem (Antenna, feeder, combiner, TRX)
 Clock problem
 Transmission problem
HUAW I T CH

Radio Link Fault - Interference

 Category

 Co-channel interference
 Adjacent-channel interference
 Inter-modulation interference and other external
interference

HUAW I T CH


 Solution

 First check equipment problems.
 Make an drive test, check the interference area and distribution of
signal quality. Find the interference frequency.
 Further search for the interference source with the spectrum analyzer.
 Activate hopping, DTX and power control functions to lower the
internal interference of the system

HUAW I T CH


 Judgment Process

 Analyze the occurrence regularity of interference band in the
traffic measurement.
 Observe the receiving level performance
 Find the poor quality handover rate
 Observe receiving quality performance
 Observe call drop performance
 Observe whether there are many handover failures and reversion
failures.

HUAW I T CH

Radio Link Fault - Coverage

 Coverage:

 Overshooting
 Coverage hole
 Signal attenuation
 Incomplete definition of adjacent cells
 Imbalance of uplink/downlink

HUAW I T CH


 Judgment Process

 Power control measurement function
 Receiving level measurement function
 Cell measurement function/inter-cell handover measurement function
 Call drop measurement function
 Defined adjacent cell measurement function
 Undefined adjacent cell measurement function
 Outgoing inter-cell handover measurement function
 Up-down link balance measurement function

HUAW I T CH


 Solution

 Adjust network parameters
 Add BTS

HUAW I T CH

Radio Link Fault - Parameters

 Main parameters that may affect the call drop rate:

 Radio link timeout and SACCH multi-frames
 RACH busy threshold and RACH minimum access level.
 MS minimum receiving signal level
 Call re-establishment permitted.

HUAW I T CH

Radio Link Fault - Parameters

 Main parameters that may affect the call drop rate:

 NCC permitted
 Handover related parameters.
 Power control related parameters.
 Frequency planning parameters

HUAW I T CH

Radio Link Fault

 Judgment process

 System information data
 Cell data
 Radio link connection timer (T3105)
 Maximum times of physical information
 Call drop performance measurement function
 Judge from the cause of call drop
− error indication
− connection failure.

HUAW I T CH

Handover Problem

 Judgment process :

 In inter-cell handover measurement function, it occurs
frequently that the handover fails and the reversion also
fails.
 In inter-cell handover measurement function, handover
failures with successful reversions occur many times.
 In undefined adjacent cell measurement function, observe
the receiving level of the undefined adjacent cells and
number of the measurement reports.

HUAW I T CH

Handover Problem

 Judgment process

 In outgoing inter-cell handover measurement function, the
outgoing inter-cell handover success rate is low (for a
certain cell). Find the adjacent cell to which the handover
success rate is low and find the cause.
 Incoming inter-cell handover success rate is low. The
handover judgment parameter setting of the target cell may
be improper.
 In TCH measurement function, handover times are not in
normal proportion to successful TCH seizures for call.
(handovers/calls>3)

HUAW I T CH

Equipment Problem

 Call drop arising from equipment problem

 Hardware problem
 Transmission problem
 Antenna and feeder fault
 Other causes

HUAW I T CH




HUAW I T CH

Call Drop Case 1
 Fault Description
 The BTS distribution of an area is as illustrated in the
diagram (red numbers stand for BCCH frequencies. No
hopping, no DTX). Some subscribers complain that call
drop in second sector of base station C is serious.
Hardware fault has been ruled out.

please confirm whether the
frequency distribution in the
cells are resonable?

HUAW I T CH

Call Drop Case 1

 Analysis

 From the analysis of BTS topology, it can be concluded that
the frequencies are well planned.
 Next, check the interference band of traffic statistic.

HUAW I T CH

Call Drop Case 1

 Analysis

09:00-10:00 IB1 IB2 IB3 IB4 IB5
cell 1 2.85 14.25 1.14 0.27 0.54

cell 2 4.09 12.57 3.14 0.03 0.01

cell 3 0 2.92 13.27 0.25 0.37

03:00~04:00 IB1 IB2 IB3 IB4 IB5
cell 1 2.85 4.28 0.00 0.00 0.00

cell 2 4.09 2.89 0.00 0.00 0.00

cell 3 0 2.12 0.00 0.00 0.00

HUAW I T CH

Call Drop Case 1

 Troubleshooting

 Conducting a drive tests, it is found that the quality is bad
when the receiving strength is high.
 Check traffic statistic and it is found that when the call drop
rate is high, handovers are mostly caused by quality
reasons, and channel assignment failure rate is also high.
 The conclusion is that there is interference from the
analysis of comprehensive traffic statistic and drive test.

HUAW I T CH

Call Drop Case 1

 Troubleshooting

 A site investigation shows that the operator has a repeater.
It is a broadband repeater. It transmits the signals from a
remote TACS site. TACS signals are amplified and then
there is interference in second sector of base station C.
 Problem has been located: interference causes the call
drop.
 Finally, lower the power of the repeater. The interference
band reduce to IB1. Now the high call drop rate problem at
site C is solved.

HUAW I T CH

Call Drop Case 1
 Common methods of checking and clearing call drop due to interference

 Rule out the internal interference caused by equipment problems and check
the separation of BTS transceivers, antenna feeder installation, and so on.
 Check the interference band
 Drive test
 Check traffic statistic of handover causes to get judgment
 Clear uplink interference
 Clear downlink interference
 Check whether DTX, frequency hopping technology and power control
application are reasonable
 Use PBGT handover algorithm flexibly to avoid co-channel and adjacent-
channel interference effectively.

HUAW I T CH

Call Drop Case 2

 Fault description

 The call drop rate in cell 3 of a BTS is 10% accompanied
with high congestion rate, but call drop rate and congestion
rate in cell 1 and cell 2 are normal.

HUAW I T CH

Call Drop Case 2

 Analysis

 Check the related traffic statistic
− Check whether there is high interference band in TCH
measurement function.
− Check the situation of call drop in call drop measurement
function.
− Check whether handover of the cell is normal.
 Check whether there is interference through checking frequency
planning, moreover confirm whether there is external interference
with spectrum analyzer.
 Drive test
 Check the hardware
HUAW I T CH

Call Drop Case 2

 Troubleshooting

 Block TRX in turn and the congestion rate is always quite high no
matter which TRX is blocked.
 Check and analyze the traffic statistic, interference band and traffic
volume and call drop rate, and it is found that the interference
becomes more serious as the traffic gets high.
 Change frequency. The frequency of cell 3 is changed to 1MHz away
from the original value. But the problem persists.
 Judge whether the equipment is faulty.
 Locate external interference.

HUAW I T CH

Call Drop Case 2

 Troubleshooting

 Make a scanning test with a spectrum analyzer.
− A suspect signal with 904.14MHz center frequency, 300K
bandwidth is found. It is similar to an analog signal and it exists
continuously.
− At the distributor output port of cell 3, the signal strength is –
27dBm. cell 2 is –40dBm, cell 1 is –60dBm. It accords with the
degree of interference.
− Traffic volume is higher in the day time than that at night.
 Now the problem is found: 904MHz external interference source.

HUAW I T CH

Call Drop Case 2

 Conclusion: solution of interference

 Solve internal interference through checking frequency planning.
 After internal interference is excluded, we can locate external
interference with spectrum analyzer.

HUAW I T CH

Call Drop Case 3


 Subscribers complain that call drops often happen from on
the 5th floor and above in a building.

Subscriber complaint is also an important source of information
about the network quality.

HUAW I T CH

Call Drop Case 3

 Analysis

 Perform on-site test
− There are call drops and noise on the site
− The test mobile phone shows that before the call drop the
serving cell is BTS-B. But this building should be covered by
BTS-A.
 Check traffic statistic
− BTS-B is about 9 kilometers away from this building. It is
determined that the BTS-B signal received in this area is
coming from some obstacles’ reflection. Thus an isolated
island coverage is formed in this area.

HUAW I T CH

Call Drop Case 3

 Analysis

 Check data configuration
− In BSC data configuration, BTS-A is not configured as the adjacency of
BTS-B
 Cause analysis of call drop
− When the MS uses the signal of cell 2 of BTS-B in this area, the signal
of cell 3 of BTS-A is strong. But cell 2 of BTS-B and cell 3 of BTS-A are
not adjacent, therefore, handover cannot happen.
− The signal in cell 2 of BTS-B is the result of multiple reflections. When
the signal of BTS-B received by the mobile phone gets weak suddenly,
an emergency handover is needed. But there is no adjacent cell of
BTS-B, so call drops will occur.

HUAW I T CH

Call Drop Case 3

 Troubleshooting

 Modify the data in BA1 table, BA2 table and add adjacent cell
relationship, set cell 3 of BTS-A as an adjacent cell of cell 2 of BTS-B.
 Optimize the network parameters to eliminate the isolated island.
 The test results show that the call drop problem is solved.

HUAW I T CH

Call Drop Case 3

 Conclusion ： two methods to solve isolated island problem

 Adjust the antenna of the isolated cell, to eliminate the isolated island
problem.
 Define new adjacent cells for the isolated cell.

HUAW I T CH

Call Drop Case 4


 In a drive test from A to B, it is found that there are many call drops
at entrance of a tunnel near a BTS due to slow handover.

HUAW I T CH

Call Drop Case 4

 Analysis

 The tunnel is near the BTS. When the MS enters the tunnel, the power
of the target cell is -80dBm. But the signal of source cell goes down
quickly to less than -100dBm. Before the MS enters the tunnel, the
downlink power of the two cells is good and no handover is triggered.
When the MS enters the tunnel, the level of the source cell goes down
rapidly. The call drop occurs before any handover is triggered.

Think it over: How to solve problems of this type?

HUAW I T CH

Call Drop Case 4

 Troubleshooting

 The adjusted parameters are listed below.

Parameter name Value before Value after
change change

PBGT watch time 5 3

PBGT valid time 4 2

PBGT HO threshold 72 68
UL Qual. Thrsh.
(Emergency handover) 70 60

Min. DL level on 10 15
candidate cell

HUAW I T CH

Call Drop Case 4

 Conclusion: optimize and adjust handover parameters to reduce call drop

 On condition that there is no ping-pang handover and excessive voice
interruption, PBGT handover can help to reduce interference and lower
call drop rate.
 Set emergency handover thresholds properly, and make sure the
emergency handover can be triggered in time before the call drop so as
to reduce call drops.

HUAW I T CH

Call Drop Case 5


 In the dial test, many call drops are found in cell 2.
 Analysis

 Check the traffic statistic and find out that TCH congestion rate of this cell
is over 10% and internal inter-cell handover failure rate is high. It is found
that one TRX board of this cell is abnormal in OMC. A preliminary
conclusion is that TRX board problem causes the call drop.

HUAW I T CH

Call Drop Case 5

 Troubleshooting

 Lock the frequency with a test mobile phone and perform dial test for
many times. It is found that call drops only happen in timeslots 1, 3, 5, 7
while communications in timeslots 2, 4, 6, 8 are normal.
 Move this board to another slot, and the problem still exists.
 Move another good board to this slot, and the communication is normal.
 Move this defective board to other cabinet, the problem arises.
 When it is replaced, the communication is recovered.

HUAW I T CH

Call Drop Case 5

 Conclusion

 The BTS test should guarantee that communication should be successful
not only in each TRX but also in each timeslot of the TRX.
 It must be ensured that each TCH channel provides bidirectional high
quality communication.

HUAW I T CH

Call Drop Case 6


 In dual-band network, when a call is setup in a GSM1800 cell and being
handed over to a GSM900 cell in the same site, the call drops in the
GSM900 cell in 2 to 5 seconds. The call drop rate in the GSM900 cell is
quite high.

HUAW I T CH

Call Drop Case 6

 Analysis

 In the test it is found that the clock of GSM900 cell and GSM1800 cell
are not synchronized.
 When a call set up in a GSM1800 cell and is handed over to a GSM900
cell, the drive test tool shows that FER increases to the maximum value
suddenly and then it goes down to zero gradually.
 It is the same with the handover from GSM900 to GSM1800.

HUAW I T CH

Call Drop Case 6

 Troubleshooting

 After adjusting GSM900 clock system, the abnormal call drop
problem is solved.

HUAW I T CH

Call Drop Case 6

 Conclusion

 Clocks of GSM900 and GSM1800 should be exactly synchronized
with each other in a dual band network, otherwise there will be call
drops and handover failures.

HUAW I T CH

Summary

 Types of call drop

 Measurement points of TCH call drop

 Measurement points of SDCCH call
drop
 Causes of call drop

 Cases

HUAW I T CH

Huawei case analysis call drop

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