Case analysis call drop

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ISSUEISSUE
OMF000404
Case Study – Call Drop
OMF000404OMF000404
Case StudyCase Study –– Call DropCall Drop
1.51.5

Principle of call drop
Analysis of call drop
Call drop cases
Course ContentsCourse Contents

Principle of Call DropPrinciple of Call Drop
Content:
Calculation formula of TCH call drop rate and measurement
points.
Calculation formula of SDCCH call drop rate and measurement
points.

Principle of Call DropPrinciple 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.

Formula of TCH Call Drop RateFormula of TCH Call Drop Rate
Formula of TCH call drop rate
TCH call drop rate=TCH call drops / Successful TCH seizures
(all)
Measurement points of TCH call drop
When the channel seizures is TCH, BSC originates
CLEAR_REQ to MSC.

Causes of TCH Call DropCauses 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

Measurement Point of TCH Call DropMeasurement Point of TCH Call Drop

Measurement point of Successful TCH seizures
When CH_ACT_ACK message is received in the immediate
assignment process. Since there is no SDCCH available, TCH
channel is directly assigned.
When CH_ACT_ACK message is received, the calling status is
CS_WAIT_RR_EST(Waiting RR setup status) and the present
channel is TCH.
When assignment complete message is sent in process of
assignment.

Measurement point of Successful TCH seizures
When the MSG_ABIS_HO_DETECT message is received
during incoming inter BSC inter cell handover, and the handover
type is non-SDCCH handover.
When the MSG_ABIS_HO_DETECT message is received
during internal inter cell handover, and the handover type is non-
SDCCH handover.
When the CLEAR-CMD message with the cause of HO_SUCC
or CALL_CTRL is received from the MSC during inter-BSC
handover , and the handover cause is directed retry.

Immediate 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)

MS BTS BSC MSC
CH_REQ
CH_REQ
CH_ACT
CH_ACT_ACK
IMM_ASS
IMM_ASS
SABM
UA EST_IND CONN_REQ
IDLE
WAIT_RR_EST

Assignment process
MS BTS BSC MSC
ASS_REQ
CH_ACT
CH_ACT_ACK
ASS_CMD
ASS_CMD
EST_IND
ASS_CMP ASS_CMPASS_CMP

Intra-BSC handover process
MS MSBTS1 BTS2BSC
Measurement Report from MS
Channel_Active
Channel_Active ACK
HANDOVER COMMAND
HO_ Access
HO_Detect
PHY INFO
First SABM
Establish_IND
PHY INFO
Handover Complete HO_Performed
UA
MSC

Inter BSC handover process
MS MSBTS1 BTS2BSC1 BSC2MSC
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
First SABMEstablish_IND
Handover CompleteHO_CMP
Clear_CMD
Clear_CMP
Measurement point of TCH Call DropMeasurement point of TCH Call Drop

Direct retry process
When the CLEAR-CMD message, whose cause values is
HO_SUCC or CALL_CTRL, is received from the MSC during
inter BSC handover , and at the same time the handover cause
is direct retry, one measurement point will be counted and
added to successful TCH seizures times
Measurement point of TCH Call DropMeasurement point of TCH Call Drop

Formula of SDCCH Call Drop RateFormula of SDCCH Call Drop Rate
Formula of SDCCH call drop rate:
SDCCH call drop rate=SDCCH call drops/ successful SDCCH
seizures*100%
SDCCH call drop rate = (Number of SDCCH lost connections
(connect failure) + Number of SDCCH lost connections (error
indications) + Number of unsuccessful SDCCH seizures due
to terrestrial links (ABIS)) / successful SDCCH seizures *
100%

Measurement Point of SDCCH
Call Drop
Call Drop
Measurement point of SDCCH call drop
When the CLEAR_REQ and ERR_IND messages are sent to
the MSC, and the channel currently seized is the SDCCH.
Measurement point of successful SDCCH seizures
CH_ACT_ACK is received in the immediate assignment process
and the channel type is SDCCH.
CH_ACT_ACK is received in CS_WAIT_RR_EST status and the
current channel is SDCCH
HO_DETECT is received during incoming inter BSC SDCCH
handover.
HO_DETECT is received during intra-BSC SDCCH handover.

MS BTS BSC MSC
Or:
Or:
Channel Request Channel Required
Channel Activation (SDCCH)
Channel Activation Acknowledge
Immediate Assignment Command
Immediate Assignment
Establish Indication (L3 Info)
Connection Failure
Error Indication
Abis Failure
Cell SDCCH Call Drop
(Subject to different cases)
Call Drop
Call Drop

The following conditions can lead to SDCCH call drop:
When the HO_DETECT message is illegal during incoming
SDCCH handover
When the HO_CMP message is illegal during incoming SDCCH
handover
When HO_CMP message transfer fails during incoming SDCCH
handover
When TN_WAIT_HO_DETECT and TN_WAIT_HO_CMP
(SDCCH handover) are timeout
When TN_WAIT_INTER_HO_CMP(SDCCH handover) is
timeout
When TN_T8 (Out-BSC handover complete) is timeout
When internal clearing is caused by other causes
Call Drop
Call Drop

Analysis of Call DropAnalysis of Call Drop
content
main causes of high call drop rate
troubleshooting of high call drop rate

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.
T3103 counter is timeout.
Other system faults (for example, the cooperation between BSC
timer and MSC timer)
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. Time out of the timer will cause call drop.
Analysis of Call DropAnalysis of Call Drop

Radio Link FaultRadio Link Fault
Signaling process chart of radio link fault
MS BTS BSC MSC
Measurement Report
Measurement Result
Connection Failure
Clear_REQ (Radio Interface Failure)
(1)
(2)
(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.

Diagram of radio link timeout

T3103 is TimeoutT3103 is Timeout
Handover process
MS BTS1 BSC MSCBTS2
Handover Indication
CH_ACT
CH_ACT_ACK
Handover Command
Handover Command
Handover Access HO_Detect
SABM
Physical Information (TA)
UA
Handover Complete
Handover Complete
EST_IND
Set T3103
Reset T3103

Analysis of Causes of Radio Link FaultAnalysis of Causes of Radio Link Fault
Analyze the causes of the fault of radio links. The causes can
be:
Interference
Internal interference, external interference and the equipment
interference
Poor coverage
Coverage hole, isolated island, uplink/downlink unbalance
parameter setting not proper
radio link timeout counter, SACCH multi-frame number, handover,
power control etc.
Equipment problem (Antenna---Feeder---CDU---TRX)
Clock problem
Transmission problem

Radio Link Fault - interferenceRadio Link Fault - interference
Interference
Co-channel interference
Adjacent-channel interference
Inter-modulation interference and other external interference

Solution
First check equipment problems.
Make an actual 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 rapidly
lower the internal interference of the system

Judgment Process
Analyze the occurring laws 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
The handover fails, calling re-establishment also fail too many
times.

Radio Link Fault - CoverageRadio Link Fault - Coverage
Coverage:
Coverage over shooting
Coverage hole
Signal attenuation
Incomplete definition of adjacent cells
unbalance of uplink/downlink

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

Solution
Adjust network parameter
Add BTS

Main parameters that may affect the call drop rate:
Radio link timeout counter and SACCH Multi-Frames
RACH busy threshold and RACH minimum access level.
MS minimum receiving signal level
Call re-establishment permitted.
Network color code (NCC) permitted
Frequency planning parameters
Handover related parameters.
Power control related parameters.

Judgment process
System information table
Cell attribute table
Radio link connection counter(T3105)
Max. retrans times of physical information
Call drop measurement function
Judge from the cause of call drop: error indication and
connection failure.

Handover ProblemHandover Problem
Judgment process :
1. Inter-cell handover measurement function: it occurs frequently
that the handover fails and the calling re-establishment also fails.
2. Inter-cell handover measurement function: handover occurs
many times and re-establishment succeeds many times.
3.Undefined adjacent cell measurement function: level of the
undefined adjacent cells and number of reports.

Judgment process
4. Outgoing inter-cell handover measurement function: the
successful rate of outgoing inter-cell handover is low (for a
certain cell). Find the adjacent cell where the handover
successful rate is low and find the cause.
5. Incoming inter-cell handover successful rate is low. The
handover judgment parameter setting of the target cell is
improper.
6. TCH measurement function: handover times are not in
proportion to the successful times of TCH call attempt.
(handover/call>3)

Solution:
Add adjacent cells as necessary
Adjust improper handover parameters
1510Min. DL level candidate cell
6070UL Qual. Thrsh.
6872PBGT HO Thrsh.
34PBGT valid time
45PBGT watch time
Handover
Value after
change
Value before
change
Parameter name

Power Control ProblemPower Control Problem
610
Filter Length for Stable
RX_LEV
58PC interval
8050
UL RX_LEV
compensationPower
control
Value after
change
Value before
change
Parameter name

Equipment ProblemEquipment Problem
Call drop arising from equipment problem
Hardware problem
Antenna and feeder fault
Other causes

Call drop arising from equipment problem
Hardware problem
Antenna and feeder fault
Other causes

Judgment process
TCH measurement function
TCH availability abnormal.
Excessive call drop and disconnection times of terrestrial links.
If a cell always suffers from high call drop rate and congestion
rate, some equipment in this cell may be faulty.

Call Drop Case 1Call Drop Case 1
Fault Description
The BTS distribution of an area is as illustrated in the diagram
(red numbers stand for BCCH frequency. No hopping, DTX).
Some subscribers complained that call drop in second sector of
base station C is serious. (Hardware fault is ruled out).
please confirm
whether the
frequency
distribution in
the BTS cells
are correct?

Analysis
From the analysis of BTS topology, it can be conclude that the
frequencies are well planned.
Next step: Check the interference band of traffic statistic.

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） IB 1 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

Troubleshooting
1.Actual drive tests: It is found that the quality is bad when the
receiving strength is high.
2.Check traffic statistic: It is found that when the call drop rate is
high, the handover is mostly caused by quality problems and
channel assignment failure rate is also high.
3.The conclusion is interference from the analysis of
comprehensive traffic statistic and drive test.

Troubleshooting
4.A site investigation shows that the operator has a repeater . It
is a broadband repeater . It transmits the signals from a remote
TACS site through optical fiber for amplification and sends it. In
this way, digital signals are amplified and then there is
interference in second sector of base station C.
5.Fault has been located: Interference causes the call drop.

Troubleshooting
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.

Common methods of checking and clearing call drop due to
interference
1. Rule out the internal interference caused by equipment
problems and check the separation of BTS transceivers,
antenna feeder installation, and so on.
2. Check the interference band
3. Driving test
4. Check traffic statistic of handover causes to get judgment
5. Clear uplink interference
6. Clear downlink interference

Common methods of checking and clearing call drop due to
interference
7. Check whether DTX, frequency hopping technology and
power control application are reasonable
8. Use PBGT handover algorithm flexibly to avoid co-channel
and adjacent-channel interference effectively.

Fault Description
1×3 RF hopping is used in a specific site. After expansion, TCH
channel assignment failure rate is continuously high (due to
radio link fault), accompanied by high TCH call drop rate and
incoming inter cell handover failure rate. SDCCH call drop rate
is normal.
The channel assignment failure rate and incoming handover
failure rate are high, what are the causes?

Analysis
Since assignment failure is accompanied with high call drop rate
and incoming inter cell handover failure rate, the causes may
be as follows:
A problem occurs when TCH channel is assigned
The timeslot seizures in communication is not stable or affected by
interference
Since SDCCH call drop rate is normal, it is unlikely that the
interference comes from the carriers of BCCH. Accordingly, the
TCH carrier of and hopping frequency may attribute to the
interference.

Troubleshooting
Check the equipment hardware, antenna and feeder, the
transmission stability. No problem is found.
In the driving test, it is found that high level and bad quality
problem is very serious.
Make dialing test nearby this site , it is found that
communication quality is bad.
Check the parameter, it is found that the MAIO of the new
carrier is the same as that of another carrier.
The fault: There exists conflict of hopping.

Troubleshooting
After setting a new value for the MAIO of the new carrier, the
related indices such as call drop rate is normal.
What other hopping parameters may cause the
co-channel or adjacent-channel conflict?

Conclusion
It is important to check frequency planning and parameter
configuration for solving frequency interference.

Fault description
In a driving test, it is found that an MS occupies a cell, but it can
not make an original call. Communication is unidirectional. Call
drop also occurs frequently at a specific distance from the cell
after frequently handover.
Analysis
It may be caused by ……?

Analysis
The problem as mentioned above is usually caused by
uplink/downlink unbalance
Troubleshooting
Make a driving test : MS move to the cell boundary. At the same
time, trace and capture data at the BTS side with a MA10
signaling analyzer. (see the figure below).

Capture data with MA10

Troubleshooting
Check whether the service area of the cell is too large.
When the uplink power control is enabled, improper setting of
power control parameters will also cause obvious unbalanced
link.
First confirm that the static power level of MS is set properly (900 is
level 5 and 1800 is level 0).
An investigation shows that, in the System Information Table, no
matter it is a 900 cell or a 1800 cell, the corresponding MS “Max.
transmitting power levels” are all set as 5. In this case, the
corresponding DCS1800 MS output power is much more lower than
its MAX. transmitting power 1W (30dBm).

Troubleshooting
Adjust “Max. transmitting power levels” of GSM 1800 cell,
the parameter is changed to “0”, the problem is solved.

Conclusion: solution for call drop due to unbalanced
uplink/downlink
If the cell coverage is too large, reduce the BTS transmitting
power or increase MS access threshold and handover threshold
of the cell.
Higher the uplink compensation factor and shorten the stable
signal filter.

Fault description
The call drop rate in cell 3 of a BTS is 10%, but call drop rate
and congestion rate in cell 1 and cell 2 are normal.

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.
Driver test
Check the hardware

Troubleshooting
1.The congestion rate always is quite high no matter which
channel is blocked.
2.Check and analyze the traffic statistic, interference band and
traffic volume and call drop rate. They are all regular.
3. Change frequency. The frequency interval of cell 3 is changed
to 1M . But the problem persists.
4. Judge whether the equipment is faulty.
5. Locate external interference.

Troubleshooting
6. Make a scanning test with a spectrum analyzer.
A suspect signal with 904.14 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: 904M external interference source.

Conclusion: solution of interference
Solve internal interference through checking frequency
planning.
After internal interference is excluded, we can locate external
interference with spectrum analyzer.

Fault description
Subscriber complained it is often call drop from the 5th floor and
above in a building.
Subscriber complaint is also an important source of information
about the network quality.

analysis
Step 1: 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. we have known this building should be covered by
BTS-A.
Step 2: Check traffic statistic
Make sure that the BTS-B cell 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.

Analysis
Step 3: 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 fails.
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 is reduced
suddenly, an emergency handover is needed. But there is no
adjacent cell of BTS-B, so call drops will occur.

Troubleshooting
Modify the data in BA1 table, BA2 table and Adjacent Cell
Relation Table.
Set cell 3 of BTS-A as an adjacent cell of cell 2 of BTS-B.
Optimize the network parameter to eliminate the isolated island.
The test results show that the call drop problem is solved.

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.

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

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?

Troubleshooting
The adjusted parameter tables are as follows
6872PBGT HO Thrsh.
24PBGT valid time
35PBGT watch timeHandover
Value after
change
Value before
change
Parameter name

Troubleshooting
The adjusted parameter tables are as follows
1510Min. DL Level on
Candidate Cell
6070UL Qual. Thrsh.
(Emergency handover)
Handover
Value after
change
Value before
change
Parameter name

Conclusion: optimize and adjust handover parameter to
reduce call drop
On condition that there is no ping-pang handover and excessive
voice interruption, PBGT handover will help to reduce
interference and lower call drop rate.
Set emergency handover trigger threshold properly, make sure
the emergency handover is triggered in time before the call drop
so as to reduce call drops.

Fault description
In the dialing 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 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.

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 other good boards to this slot, and the
communication is normal. Move this defective board to other
cabinet, the problem arises. This TRX proves defective. When it
is replaced with a standby board, the communication is
recovered.

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

Fault description
In dual-band network, When a call is setup a GSM1800 cell, the
call is handed over to a GSM900 cell from the same site. After
2~5 seconds, the call dropped in the GSM900 cell. The call drop
rate in the GSM900 cell is quite high.

Analysis
In the test it is found that the clock of GSM900 and GSM1800 is
not synchronized. When a call set up in a GSM1800 cell is
handed over to a GSM900 cell, the drive test tool shows that
FER increase to the maximum value suddenly and then it goes
down to zero gradually. And it is the same with the handover
from GSM900 to GSM1800.
Trace the signaling and find that the conversation before the call
drop for several seconds is actually call re-establish, but test
mobile phone indicates the call is already handover to cell
GSM900 successfully. The clock synchronization problem is
serious.

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

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.

Case analysis call drop

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