Post on 04-Jun-2018
8/13/2019 OMD6068 Case Analysis--Call Drop ISSUE1.1
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OMD6068Case Analysis--Call Drop
ISSUE1.1
Wireless Curriculum
Development Section
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Summary
Principle of call drop rate calculationThe cause analysis of call dropTypical call drop cases
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Principle of Call Drop Rate Calculation
P key points:
Master the calculation formula of TCH call drop rate and
statistic points.Master the calculation formula of SDCCH call drop rate and
statistic points.
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Overview
There are two types of call drops: 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.
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General Concepts of TCH call drop
Formula of TCH call drop rate
TCH call drop rate=Number of TCH call drops/Number of
TCH occupied successfully 100%
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General Concepts of TCH call drop Rate
Statistic point of TCH call drops
When BSC originates CLEAR_REQ message to MSC, the channeloccupied is TCH.
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General Concepts of TCH call drop Rate
Usually,The typical cause values for sending the Clear_Request message
are:
Radio interface message failure
O&M intervention
Equipment failure
Protocol error between BSS and MSC
Preemption
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General Concepts of TCH call drop Rate
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Analysis of TCH call drop RateStatistic Points
Statistic Point of TCH Occupied Successfully
(a)when CH_ACT_ACK message is received in the immediate
assignment process. Since there is no available SDCCH channel,
TCH channel is directly assigned.
(b) When CH_ACT_ACK message is received when the calling
status is CS_WAIT_RR_EST(Waiting RR setup status), and the
present channel is TCH.
(c) When Assignment complete message is sent in process of
assignment.
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Analysis of TCH call drop Rate StatisticPoints
Statistic Point of TCH Occupied Successfully
d When the MSG_ABIS_HO_DETECT message is received during inter-
coming BSC handover, and the handover type is non-SDCCH handover;
e When the MSG_ABIS_HO_DETECT message is received during intra-
BSC handover, and the handover type is non-SDCCH handover;
f When the CLEAR-CMD message with the cause values of HO_SUCC or
CALL_CTRL is received from the MSC during inter-BTSC handover , and thehandover cause is directed retry;
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Analysis of Call Flow Chart and TCH call dropStatistic Points (I)
Immediate assignment flow chart:
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Analysis of Call Flow Chart and TCH call dropStatistic Points (II)
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Analysis of Call Flow Chart and TCH call drop Statistic Points(III)
Assignment flow chart
MS BTS BSC MSC
Est_Ind
Ch_Act
Ch_Act_Ack
Ass_Req
Ass_Cmd
Ass_Cmp
Ass_Cmd
Ass_Cmp Ass_cmp
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Analysis of Call Flow Chart and TCH call drop Statistic Points(IV)
Intra-BTSC handover flow
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Analysis of Call Flow Chart and TCH call drop Statistic Points(V)
Inter-BTSC handover flow
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Analysis of Call Flow Chart and TCH call drop Statistic Points(VI)
Directional re-try flow
When the CLEAR-CMD message with the cause values of
HO_SUCC or CALL_CTRL is received from the MSC during
inter-BSC handover , and the handover cause is directed retry,one statistic point is counted and added to the times of TCH
occupation success.
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General Concepts of SDCCH call dropRate
Formula of SDCCH call drop rate
SDCCH call drop include:Number of SDCCH lost connections (connect failure)Number of SDCCH lost connections (error indications)Number of unsuccessful SDCCH seizures due to terrestrial (ABIS)
SDCCH call drop rate=Number of SDCCH call drops/Totalsuccessful SDCCH seizures*100
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Analysis of SDCCH call drop Rate StatisticPoint (I)
Statistic point of SDCCH call dropsWhen the CLEAR_REQ and ERR_IND messages are originated to the MSC,
the channel currently seized is the SDCCH.
Statistic point of successful SDCCH seizures (1) CH_ACT_ACK is received in the immediate assignment process. Thechannel type is SDCCH(2) CH_ACT_ACK message is received in CS_WAIT_RR_EST status and thecurrent channel is SDCCH(3) HO_DETECT is received when incoming SDCCH handover is performed.(4) HO_DETECT is received when intra-BTSC SDCCH handover isperformed.
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Analysis of SDCCH call drop Rate StatisticPoint (II)
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Analysis of SDCCH call drop Rate StatisticPoint (III)
The following conditions can lead to SDCCH call drop:When the HO_DETECT message is illegal during incoming SDCCH
handoverWhen the HO_CMP message is illegal during incoming SDCCH
handoverWhen HO_CMP message transfer fails during incoming SDCCH
handoverWhen TN_WAIT_HO_DETECT, and TN_WAIT_HO_CMP (SDCCHhandover) are overtime
When TN_WAIT_INTER_HO_CMP(SDCCH handover) is overtimeWhen TN_T8 (Out-BTSC handover complete) is overtimeWhen internal clearing is caused by other causes
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Overview of Causes of call drop
According to the definition of call drop statistic point, call drop is usuallycaused by the following:
Radio link fault. During the communication, messages can not be receivedcorrectly. T3103 counter gives no response after timeout Other system faults (for example, the matching between BSC timer and MSCtimer.
Timers that may cause call drops (BSC timer):T3109 T3103
T3103Time counting from sending up HANDOVER CMD start to receive HANDOVERCMP end or overtime
T3109
Time counting from sending up CHAN REL to receive REL IND end or overtime
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Wireless Link Fault
Signaling flow chart of radio link fault:
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Wireless Link Fault
Diagram of radio link timeout
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T3103 Counter Has No Response After Timeout
Handover flow (T3103 time counting):
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Analysis of Causes of Wireless Link Fault---Interference
Interference
Co-channel interference
Adjacent-channel interference
Intermodulation interference and other external
interferences
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Analyze and Locate TCH call drop Problem (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 cross-interference of the system
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Analyze and Locate TCH call drop Problem (Interference)
Process of judgment
1. Analyze the occurring laws of interference band in the
traffic measurement.
2. Observe the receiving level performance3. Find the poor quality handover ratio
4. Observe receiving quality performance
5. Observe call drop performance
6. The handover fails, calling re-establishment
also fail too many times.
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Analysis of Causes of Wireless Link Fault---Coverage
Coverage:
1. Cross-cell coverage
2. Blind area coverage
3. Signal attenuation
4. Incomplete definition of adjacent cells
5. Imbalance of uplink/downlink
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Analyze and Locate TCH call drop Problem (Coverage)
Process of judgment
Power control performance measurement
Receiving level performance measurement
Cell performance measurement/inter-cell handover performancemeasurement
call drop performance measurement
Adjacent cell performance measurement
Measurement of undefined adjacent cell average levelPower control performance measurement
Out-cell handover performance measurement
Uplink/downlink balance performance measurement
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Analyze and Locate TCH call drop Problem (Coverage)
Solution
Adjust network parameter
Add BTS
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Analysis of Causes of Wireless Link Fault---ParameterSetting
Main parameters that may affect the call drop rate:1. Radio link timeout counter and number of SACCH multi-frames2. RACH busy threshold and RACH minimum access level.3. MS minimum receiving signal level4. Call re-establishment permission.5. Nation color code (NCC) permission
6. Frequency planning parameters7. Handover related parameters. Refer to Case 6 8. Power control related parameters. Refer to Case 5 9. Version related parameters. Refer to Case 7
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Analyze and Locate TCH call drop Problem (Radio Parameter Setting)
Locate and Clear Fault:
System message data table
Cell attribute table
Radio link connection counter
max. repeat times of Physical information
call drop rate performance measurement
Judge from the cause of call drop: error indication and connection failure.
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Analyze and Locate TCH call drop Problem (Handover ParameterSetting)
Process of Judgment
Inter-cell handover performance measurement: it occurs frequently thatthe handover fails and the calling re-establishment also fails.
Inter-cell handover performance measurement: handover occurs many
times and re-establishment succeeds many times.
Measurement of undefined adjacent cell performance: level of the
undefined adjacent cells and number of reports.
Out-cell handover performance measurement: the success ratio of out-
cell handover is low (for a specific cell). Find the adjacent cell where the
handover success ratio is low and find the cause.
In-cell handover success ratio is low. The handover verdict parameter
setting of the opposite cell is not proper.
TCH performance measurement: handover times are not in proportion tothe success times of TCH call occupation.(handover/call>3)
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Analyze and Locate TCH call drop Problem (Handover ParameterSetting)
Solution:
Add adjacent cells as necessary
Adjust improper handover parameters
Hando
ver
Parameter name
Value before
change
Value after
change
PBGT watch time 5 4
PBGT valid time 4 3
PBGT HO threshold 72 68
UL qual. threshold 70 60
Min. DL level candidate cell 10 15
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Analyze and Locate TCH call drop Problem (Power ControlParameter Setting)
Power
control
Parameter nameValue before
change
Value after
change
UL Qual. Or RX_LEV
compensation50 80
PC interval 8 5
filter length for stable 10 6
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Analysis of Causes of Wireless Link Fault---Equipment Problem
call drop arising from equipment problem
Hardware problem
Transmission problem
Antenna & feeder fault
Other causes
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Analyze and Locate TCH call drop Problem by TrafficMeasurement (Equipment Fault)
Process of judgment
TCH performance measurement
Excessive failure times of A interface when TCH is occupied
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.
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Typical call drop Cases
Case 1
l Fault Description
The BTS distribution of an area is as illustrated in the diagram (red
numbers stand for BCCH frequency. No hopping, DTX). Some users
complained that call drop in cell 2 of base station C is serious.
(hardware fault is ruled out).Step 1: please
confirm whetherthe frequency
distribution in theBTS cells are
correct?
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Case Analysis
Here is the conclusion from the analysis of BTS topology: the frequency are well
planned.
Next step: View the interference band of traffic measurement:
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
Is there anything abnormalindicated by the interference?
What does it imply?
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Troubleshooting Procedure
Finally, lower the power of the repeater and it works. The
interference band is lowered immediately from IB2, IB3 to IB1.
Now the high call drop rate problem at site C is solved.
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Summary
Discussion
Common methods and procedures ofChecking and clearing call drop due to interference.
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Typical call drop Cases
Case 2
Fault Description
1 3 RF hopping is used in a specific site. After expansion, TCH channel allocation
failure rate is continuously high (due to radio link fault), accompanied by high TCH
call drop rate and incoming handover failure rate. SDCCH call drop rate is normal.
Since the channel allocation failure rate and
Incoming handover failure rate are high, what are the causes?
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Case Analysis
Since assignment failure is accompanied with high call drop
rate and incoming handover failure rate, basically the causes may
be as follows:
A problem occurs when TCH channel is assigned
The band or timeslot occupied by this communication is
not stable or affected by interference
Since SDCCH call drop rate is normal, it is unlikely that the
interference comes from the carrier of BCCH band and the BCCH
band. Accordingly, the carrier of non-BCCH band and hopping band
may attribute to the interference.
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Case Analysis
Find the problem:1. Check the equipment hardware, antenna & feeder and the
transmission stability. No problem is found.2. In the drive test, it is found that high level and low quality
problem is very serious.3. Make a site calling test to find that the communication quality is
poor.4. In checking 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 bands.
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Case Analysis
Troubleshooting:
Set a new value for the MAIO of the new carrier. The related
indexes such as call drop rate are normal.
What other hopping parameters may cause theco-channel or adjacent-channel conflict?
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Typical call drop Cases
Case III
Fault description
In a drive test, it is found that an MS occupies a cell but can not
call out. Communication is unidirectional. call drop occurs
frequently at a specific distance from the cell and after
frequently handover.
AnalysisIt may be caused by ?
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Case Analysis
The drive test phenomena as mentioned above is usuallycaused by:Uplink/downlink imbalanceIt is possible that the power is unbalanced because uplink signalis much lower than the downlink signal.
Make a driving testDuring the test, move the MS to the cell boundary. At the sametime, trace and capture data at the BTS side with an MA10signaling analyzer. (see the figure below).
l
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Case Analysis
Capture data with MA10:
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Case Analysis
Judge, check and clear the problem of unlink/downlink
imbalance
1. Check whether the service area of the cell is too large.2. When the unlink power control is enabled, improper settingOf power control parameters will also cause obvious unbalancedPower.
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Summary
call drop due to unbalanced uplink/downlink
1. If the cell coverage is tooLarge, reduce the BTS transmit power or increase MS access threshold
And handover threshold of the cell.
2. AdjustHigher the uplink compensation factor and shorten the stable signalFilter.
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Case Analysis
Fault analysis and clearing1. The congestion rate remains high even if the RC channel of the cell is
blocked in rotation.
2. Check and analyze the traffic measurement task data, interference band
and traffic and call drop rate. They are all regular.
3. Change band. Change the band of CELL3 to 1M away from the previous
band. But the problem persists.
4. Judge whether The equipment itself is faulty.
5. Locate external interference.
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Case Analysis
5. Make a scanning test with a spectrum analyzer.
A suspect signal with 904.14 center frequency, 300K spectrum
bandwidth is found. It is similar to an analog spectrum and it exists
continuously.
At the distributor output port of CELL3, the signal intensity is 27dBm. CELL2 is 40dBm, CELL3 is 60dBm. It accord with the
degree of interference.
Traffic is higher in the day time than that at night.
Now the problem is found: 904M external interference source.
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Case Analysis
Perform a drive test with a spectrum analyzer to locate the problem.
Perform all tests on top of buildings.
Acknowledge interference signal by interrupting test.
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l
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Case Analysis
Step 1: Perform site test
There is call drop and noise on the site
The test mobile phone shows that before the call drop the phone
is always in a service zone of a remote BTS-A.
Step 2: Check traffic measurementMake sure the cell in the BTS-B is 3-4 kilometers from this
building. It is determined that the BTS cell signal received in this
area is the reflection of a obstacle . Thus an isolated island
coverage is formed in this area.
C A l i
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Case Analysis
Step 3: Check data configurationIn BSC data configuration, only cell 2 but not cell 3 of BTS-A is configuredas the adjacency of BTS-B
Analyze the call dropWhen the MS uses the signal of cell 2 of BTS-B in this area, the signal ofcell 3 of BTS-A is strong. But cell2 of BTS-B and cell3 of BTS-A are notadjacent, therefore, handover fails.
The signal in cell 2 of BTS-B is the result of multiple reflections. When thesignal of BTS-B received by the mobile phone is weakened suddenly, anemergency handover is needed. However, cell2, cell3 of BTS-A are notbest candidate cells for cells of BTS-B, so the signal may be handed overother BTS-C. But the mobile phone can not receive signals from BTS-C,hence the call drop occur.
C A l i
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Case Analysis
SolutionModify the data in BA1 (BCCH) table, BA2 (SCCH) table and cell
adjacency relation table in BSC data configuration.
Set cell 3 of BTS-A as an adjacent cell of cell 2 of BTS-B.
Optimize the network parameter to eliminate the island effect.
The test results show that the call drop problem is solved.
S
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Summary
Commentary
Two methods to solve island problem:
Adjust the antenna of the isolated cell, to eliminate the island problem.
Define new adjacent cells for the isolated cell.
T i l ll d C
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Typical call drop Cases
Case 6 Optimize handover parameter to reduce call drop
In a drive test from A to B, it is found that the call drop occurs many
times at the tunnel near the BTS due to the failure of handover
immediately.
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C A l i
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Case Analysis
Modify related parameter table as follows:
hand
over
Parameter name Value before
change
Value after
change
PBGT watch time 5 3PBGT valid time 4 2
PBGT threshold 72 68
C A l i
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Case Analysis
Hand
over
Parameter name Value before
change
Value after
change
UL qual. threshold of
emergency handover
70 60
Min. DL level on candidate cell 10 15
Modify related parameter table as follows:
S
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Summary
Optimize and adjust handover parameter to reduce call drop
On condition that there is no ping-pong handover and excessive voice
interruption, PBGT handover take places easily, so as to resist
interference and lower call drop rate.Set emergency handover trigger threshold properly, make sure the
emergency handover is triggered correctly before the call drop so as to
reduce call drops.
Typical call drop Cases
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Typical call drop Cases
Case 7 call drop due to faulty TRX board
Fault description
In the calling test, it is found that call drop occurs frequently in cell 2.
Fault analysis
Check the traffic measurement to find that TCH congestion ratio of this cell is
over 10% and inward handover failure rate is high. Check the remote
maintenance console to find that one TRX board of this cell is abnormal. A
preliminary conclusion is that board problem causes the call drop.
Typical call drop Cases
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Typical call drop Cases
Fault clearingLock the band with a test mobile phone and perform dial test formany times. It is found that call drop only occurs in timeslots 1, 3, 5,7while communication is timeslots 2, 4, 6, 8 are normal. Move thisboard to another slot, and the problem exists. Move other goodboards to this slot, and the communication is normal.Move this
defective board to other cabinet, the problem arises. This TRXproves defective. When it is replaced with a standby board, thecommunication is recovered.
Typical call drop Cases
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Typical call drop Cases
Comments:The BTS side test should guarantee that communication should
be successful not only in each RC but also in each timeslot of
each RC. It must ensured that each TCH channel provides
bidirectional communication and the communication is of highquality.
Typical call drop Cases
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Typical call drop Cases
Case 8 call drop due to clock problem
Fault description
A cell 1800A in an area is connected to the network. When
communication is set up in a cell of Site B, the call is handed over
to the 900M cell located the same site address. After 2~5
seconds, call drop is found in 900M, and at a high call drop rate.
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Typical call drop Cases
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Typical call drop Cases
Comments
Dual frequency requires that the clocks of 900 and 1800
should be exactly synchronous.
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