Call drop case study

1. Internal OMF000404 Case Study – Call Drop Issue 2.0 www.huawei.comHUAW I T CH E E NOL
OGIE CO., L D. S T All rights reserved
2. References  31160978-BSC Traffic Statistic Manual Volume I  31033203-BSS Troubleshooting
ManualHUAW I T CH E E NOL OGIE CO., L D. S T All rights reserved Page 2
3. Upon completion of this course, you are supposed to be able to:  Understand the principles of call
drop.  Analyze and solve call drop problemsHUAW I T CH E E NOL OGIE CO., L D. S T All rights
reserved Page 3
4. Chapter 1 Principle of call drop Chapter 2 Analysis of call drop Chapter 3 Call drop casesHUAW I T
CH E E NOL OGIE CO., L D. S T All rights reserved Page 4
5. Chapter 1 Principle of call drop Section 1 Definition of call drop Section 2 TCH call drop Section 3
SDCCH call dropHUAW I T CH E E NOL OGIE CO., L D. S T All rights reserved Page 5
6. 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
E E NOL OGIE CO., L D. S T All rights reserved Page 6
8. 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 
PreemptionHUAW I T CH E E NOL OGIE CO., L D. S T All rights reserved Page 8
9. 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 E E NOL OGIE CO., L D. S T All rights reserved Page
9
10. Measurement Point of TCH Call DropHUAW I T CH E E NOL OGIE CO., L D. S T All rights reserved
Page 10
11. Measurement Point of TCH Call Drop  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 E E NOL OGIE
CO., L D. S T All rights reserved Page 11
12. Measurement Point of TCH Call Drop 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 E
E NOL OGIE CO., L D. S T All rights reserved Page 12
13. 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 A2HUAW I T CH E E NOL
OGIE CO., L D. S T All rights reserved Page 13
14. 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 A2HUAW I T CH E E NOL OGIE CO., L D. S T All rights reserved Page 14
15. 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 A2HUAW I T CH E E NOL OGIE CO., L D.
S T All rights reserved Page 15
16. 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 E E NOL OGIE CO., L D. S T All rights reserved Page 16
17. 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
18. 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 E E NOL OGIE CO., L D. S T All rights reserved Page 18
19. 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_CMPHUAW I
T CH E E NOL OGIE CO., L D. S T All rights reserved Page 19
20. 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_PerformedHUAW I T CH E E NOL OGIE CO., L D. S T All rights reserved Page 20
21. 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_CMPHUAW
I T CH E E NOL OGIE CO., L D. S T All rights reserved Page 21
23. 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 E E NOL OGIE CO., L D. S T
All rights reserved Page 23
24. 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 E E NOL OGIE CO., L D. S T All rights
reserved Page 24
25. Measurement Point of SDCCH Call Drop  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 E E NOL OGIE CO., L D. S T All rights reserved Page 25
26. Measurement Point of SDCCH Call Drop 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 E E NOL OGIE CO., L D. S
T All rights reserved Page 26
27. 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 E E NOL OGIE CO., L D. S T All rights reserved
Page 27

29. Analysis of Call Drop  content  main causes of high call drop rate  troubleshooting of high call
drop rateHUAW I T CH E E NOL OGIE CO., L D. S T All rights reserved Page 29
30. 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 E E NOL OGIE CO., L D.
31. 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 E
32. Radio Link Fault  Diagram of radio link timeoutHUAW I T CH E E NOL OGIE CO., L D. S T All
rights reserved Page 32
33. 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
T3103HUAW I T CH E E NOL OGIE CO., L D. S T All rights reserved Page 33
34. 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 problemHUAW I T CH E E NOL
35. Radio Link Fault - Interference  Category  Co-channel interference  Adjacent-channel interference
 Inter-modulation interference and other external interferenceHUAW I T CH E E NOL OGIE CO., L
D. S T All rights reserved Page 35
36. Radio Link Fault - Interference  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 systemHUAW I T CH E E NOL OGIE
37. Radio Link Fault - Interference  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 E E NOL
38. Radio Link Fault - Coverage  Coverage:  Overshooting  Coverage hole  Signal attenuation 
Incomplete definition of adjacent cells  Imbalance of uplink/downlinkHUAW I T CH E E NOL OGIE

39. Radio Link Fault - Coverage  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 functionHUAW I T CH E E NOL OGIE CO., L D. S T All rights reserved Page
39
40. Radio Link Fault - Coverage  Solution  Adjust network parameters  Add BTSHUAW I T CH E E
NOL OGIE CO., L D. S T All rights reserved Page 40
41. 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 E E NOL OGIE CO.,
L D. S T All rights reserved Page 41
42. Radio Link Fault - Parameters  Main parameters that may affect the call drop rate:  NCC permitted
 Handover related parameters.  Power control related parameters.  Frequency planning
parametersHUAW I T CH E E NOL OGIE CO., L D. S T All rights reserved Page 42
43. 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 E E
44. 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 E E NOL OGIE CO., L D. S T All rights reserved Page 44
45. 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 E E NOL OGIE CO., L D. S T All rights reserved Page 45
46. Equipment Problem  Call drop arising from equipment problem  Hardware problem  Transmission
problem  Antenna and feeder fault  Other causesHUAW I T CH E E NOL OGIE CO., L D. S T All
48. 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 E E NOL OGIE

49. 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 E
50. 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 0.00HUAW I T CH E E
51. 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 E E NOL OGIE CO., L D. S T All rights reserved Page 51
52. 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 E E NOL
53. 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 E E NOL OGIE CO., L D. S T All rights reserved Page 53
54. 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
55. 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 hardwareHUAW I T CH E E NOL OGIE
56. 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 E E NOL OGIE CO., L D. S T All rights reserved Page 56
57. 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 E E NOL OGIE CO., L D. S T All rights reserved Page 57
58. 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 E E NOL OGIE CO., L D. S T All rights reserved
Page 58
59. Call Drop Case 3  Fault description  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 E E NOL OGIE CO., L D. S T All rights reserved Page 59
60. 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 E E NOL OGIE
61. 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 E E NOL OGIE CO., L D. S T All rights reserved Page 61
62. 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 E E NOL OGIE CO., L D. S T All rights reserved Page 62
63. 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 E E NOL OGIE CO., L D. S T All rights reserved Page 63
64. Call Drop Case 4  Fault description  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 E E NOL OGIE CO., L
65. 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 E E NOL OGIE CO., L D. S T All rights reserved Page 65
66. 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
cellHUAW I T CH E E NOL OGIE CO., L D. S T All rights reserved Page 66

67. 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 E E NOL OGIE CO., L D. S T All rights reserved Page 67
68. Call Drop Case 5 Fault description  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 E E NOL
69. 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 E E NOL OGIE CO., L D. S T All rights reserved Page 69
70. 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 E E NOL OGIE CO., L
71. Call Drop Case 6  Fault description  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 E E NOL OGIE CO., L D.
72. 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 E E
73. Call Drop Case 6  Troubleshooting  After adjusting GSM900 clock system, the abnormal call drop
problem is solved.HUAW I T CH E E NOL OGIE CO., L D. S T All rights reserved Page 73
74. 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 E E NOL OGIE CO., L D. S T All rights reserved Page 74
75. Summary  Types of call drop  Measurement points of TCH call drop  Measurement points of
SDCCH call drop  Causes of call drop  CasesHUAW I T CH E E NOL OGIE CO., L D. S T All
76. Thank Youwww.huawei.com

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