Internal
OMF000404 Case Study – Call Drop Issue 2.0
HUAWEI TECHNOLOGIES CO., LTD.
www.huawei.com
All rights reserved
References 31160978-BSC Traffic Statistic Manual
Volume I 31033203-BSS Troubleshooting
Manual
HUAWEI TECHNOLOGIES CO., LTD.
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
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 3
Chapter 1 Principle of call drop Chapter 2 Analysis of call drop Chapter 3 Call drop cases
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 4
Chapter 1 Principle of call drop Section 1 Definition of call drop Section 2 TCH call drop Section 3 SDCCH call drop
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 5
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.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 6
Chapter 1 Principle of call drop Section 1 Definition of call drop Section 2 TCH call drop Section 3 SDCCH call drop
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 7
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
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 8
Formula of TCH Call Drop Rate
Formula of TCH call drop rate TCH call drop rate=TCH call drops / Successful TCH seizures (all)
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 9
Measurement Point of TCH Call Drop
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 10
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.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 11
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 renet-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.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 12
TCH Call Drop - Assignment MS
BSC
BTS
MSC
ASSIGNMENT REQUEST CHANNEL ACTICATION CHANNEL ACTIVATION ACK ASSIGN CMMAND SABM
ESTABLISH INDICATION
UA ERROR INDICATION CONNECTION FAILURE INDICATION
A1 A2
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 13
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
CONNECTION FAILURE INDICATION
A1
A2
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 14
TCH Call Drop - Incoming BSC Handover MS
HUAWEI BSC
Other BSC
HUAWEI BTS
MSC
HANDOVER REQUIRED
HANDOVER REQUEST ACK
HANDOVER REQUEST CH ACT CH ACT ACK
HANDOVER COMMAND HANDOVER ACCESS HANDOVER DETECT
SABM UA
ERROR INDICATION A1
CONNECTION FAILURE INDICATION
A2
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 15
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.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 16
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.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 17
Successful TCH Seizure – Very Early Assignment Very early assignment process
MS
BTS
Channel Request (RACH)
BSC
MSC
Channel Required Channel Activation (TCH or SDCCH) Channel Activation Acknowledge Immediate Assignment Command
Immediate Assignment (AGCH)
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 18
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
HUAWEI TECHNOLOGIES CO., LTD.
ASS_CMP
All rights reserved
ASS_CMP
Page 19
Successful TCH Seizure – Intra-BSC Handover
Intra-BSC handover process MS
BTS1
BSC
BTS2
MS
Measurement Report from MS Channel_Active Channel_Active ACK HANDOVER COMMAND HO_ Access
PHY INFO
HO_Detect
PHY INFO First SABM UA
Establish_IND
Handover Complete
HUAWEI TECHNOLOGIES CO., LTD.
HO_Performed
All rights reserved
Page 20
MSC
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
Handover Command
HO_Request_ACK HO_CMD
CH_ACT_ACK
HO_Detect HO_Detect
HO_Access PHY INFO PHY INFO
Establish_IND First SABM HO_CMP
Handover Complete
Clear_CMD Clear_CMP
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 21
Chapter 1 Principle of call drop Section 1 Definition of call drop Section 2 TCH call drop Section 3 SDCCH call drop
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 22
Formula of SDCCH Call Drop Rate Formula of SDCCH call drop rate:
SDCCH call drop rate = SDCCH call drops/ successful SDCCH seizures*100%
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 23
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. HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 24
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.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 25
Measurement Point of SDCCH Call Drop MS
BTS
BSC
MSC
Channel Request Channel Required Channel Activation (SDCCH) Channel Activation Acknowledge Immediate Assignment
Immediate Assignment Command Establish Indication (L3 Info)
Or: Or:
Connection Failure Error Indication
Abis Failure
HUAWEI TECHNOLOGIES CO., LTD.
Cell SDCCH Call Drop (Subject to different cases) All rights reserved
Page 26
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.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 27
Chapter 1 Principle of call drop Chapter 2 Analysis of call drop Chapter 3 Call drop cases
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 28
Analysis of Call Drop
content main causes of high call drop rate troubleshooting of high call drop rate
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 29
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. HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 30
Radio Link Fault ď Ź Signaling process chart of radio link fault
MS
BTS
(1) Measurement Report
BSC
MSC
(2) Measurement Result Connection Failure
(3)
Clear_REQ (Radio Interface Failure)
(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. HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 31
Radio Link Fault ď Ź Diagram of radio link timeout
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 32
T3103 Times Out ď Ź Handover process
MS
BTS1
BTS2
BSC
MSC
Handover Indication CH_ACT CH_ACT_ACK Handover Command
Handover Command Handover Access
Set T3103
HO_Detect
Physical Information (TA) SABM
EST_IND
UA Handover Complete
HUAWEI TECHNOLOGIES CO., LTD.
Handover Complete Reset T3103
All rights reserved
Page 33
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 HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 34
Radio Link Fault - Interference Category
Co-channel interference Adjacent-channel interference Inter-modulation interference and other external interference
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 35
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 system
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 36
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.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 37
Radio Link Fault - Coverage Coverage:
Overshooting Coverage hole Signal attenuation Incomplete definition of adjacent cells Imbalance of uplink/downlink
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 38
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 function
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 39
Radio Link Fault - Coverage Solution
Adjust network parameters Add BTS
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 40
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.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 41
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
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 42
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.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 43
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.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 44
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)
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 45
Equipment Problem Call drop arising from equipment problem
Hardware problem Transmission problem Antenna and feeder fault Other causes
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 46
Chapter 1 Principle of call drop Chapter 2 Analysis of call drop Chapter 3 Call drop cases
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 47
Call Drop Case 1 ď Ź Fault Description
ď › 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? HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 48
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.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 49
Call Drop Case 1 ď Ź 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
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 50
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.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 51
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.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 52
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 adjacentchannel interference effectively. HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 53
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.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 54
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 HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 55
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.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 56
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.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 57
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.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 58
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.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 59
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.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 60
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. HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 61
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.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 62
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.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 63
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.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 64
Call Drop Case 4 ď Ź Analysis
ď › 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?
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 65
Call Drop Case 4 Troubleshooting
The adjusted parameters are listed below. Parameter name
Value before change
Value after 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 candidate cell
10
15
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 66
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.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 67
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.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 68
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.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 69
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.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 70
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.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 71
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.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 72
Call Drop Case 6 Troubleshooting
After adjusting GSM900 clock system, the abnormal call drop problem is solved.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 73
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.
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 74
Summary
Types of call drop
Measurement points of TCH call drop
Measurement points of SDCCH call drop
Causes of call drop
Cases
HUAWEI TECHNOLOGIES CO., LTD.
All rights reserved
Page 75
Thank You www.huawei.com