OMF000404 Case Study – Call Drop Issue 2.0
Huawei Confidential. All Rights Reserved
References
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31160978-BSC Traffic Statistic Manual Volume I
31033203-BSS Troubleshooting Manual
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Objectives
Upon completion of this course, you are supposed to be able to:
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Understand the principles of call drop.
Analyze and solve call drop problems
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Chapter 1 Principle of call drop
Chapter 2 Analysis of call drop
Chapter 3 Call drop cases
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Chapter 1 Principle of call drop Section 1 Definition of call drop Section 2 TCH call drop Section 3 SDCCH call drop
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Definition of Call Drop
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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.
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Chapter 1 Principle of call drop Section 1 Definition of call drop Section 2 TCH call drop Section 3 SDCCH call drop
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Causes of TCH Call Drop
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Usually, the typical causes for sending the Clear_Request message are as follows: Radio interface failure O&M intervention Equipment failure Protocol error Preemption
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Formula of TCH Call Drop Rate
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Formula of TCH call drop rate TCH call drop rate=TCH call drops / Successful TCH seizures (all)
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Measurement Point of TCH Call Drop
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Measurement Point of TCH Call Drop
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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.
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Measurement Point of TCH Call Drop
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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. Internal Use
TCH Call Drop - Assignment
MS
BSC
BTS
MSC
ASSIGNMENT REQUEST CHANNEL ACTICATION CHANNEL ACTIVATION ACK ASSIGN CMMAND SABM UA
ESTABLISH INDICATION
ERROR INDICATION CONNECTION FAILURE INDICATION
A1 A2
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TCH Call Drop – Intra-BSC Handover MS
BSC'
BSC Ori-Cell
BSC Tag-Cell
BSC''
Intercell Handover Request CH ACT CH ACT ACK Intercell Handover Response HANDOVER COMMAND
HANDOVER ACCESS HANDOVER DETECT SABM UA ERROR INDICATION
A1 CONNECTION FAILURE INDICATION
A2
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TCH Call Drop - Incoming BSC Handover
MS
Other BSC
HUAWEI 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
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Measurement Point of Successful TCH Seizures
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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.
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Measurement Point of Successful TCH seizures
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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.
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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)
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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
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ASS_CMP
ASS_CMP
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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 UA
Establish_IND
Handover Complete
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HO_Performed
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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_Access HO_Detect
HO_Detect
PHY INFO PHY INFO
Establish_IND First SABM HO_CMP
Handover Complete
Clear_CMD Clear_CMP
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Chapter 1 Principle of call drop Section 1 Definition of call drop Section 2 TCH call drop Section 3 SDCCH call drop
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Formula of SDCCH Call Drop Rate
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Formula of SDCCH call drop rate: SDCCH call drop rate = SDCCH call drops/ successful SDCCH seizures*100%
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Measurement Point of SDCCH Call Drop
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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.
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Measurement Point of SDCCH Call Drop
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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.
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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 Cell SDCCH Call Drop (Subject to different cases) 26
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Measurement Point of Successful SDCCH Seizures
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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.
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Chapter 1 Principle of call drop
Chapter 2 Analysis of call drop
Chapter 3 Call drop cases
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Analysis of Call Drop
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content main causes of high call drop rate troubleshooting of high call drop rate
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Analysis of Call Drop
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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.
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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.
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Radio Link Fault
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Diagram of radio link timeout
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T3103 Times Out
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
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Handover Complete Reset T3103
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Causes of Radio Link Fault
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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 Internal Use
Radio Link Fault - Interference
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Category Co-channel interference Adjacent-channel interference Inter-modulation interference and other external interference
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Radio Link Fault - Interference
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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
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Radio Link Fault - Interference
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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.
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Radio Link Fault - Coverage
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Coverage: Overshooting Coverage hole Signal attenuation Incomplete definition of adjacent cells Imbalance of uplink/downlink
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Radio Link Fault - Coverage
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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
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Radio Link Fault - Coverage
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Solution Adjust network parameters Add BTS
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Radio Link Fault - Parameters
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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.
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Radio Link Fault - Parameters
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Main parameters that may affect the call drop rate: NCC permitted Handover related parameters. Power control related parameters. Frequency planning parameters
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Radio Link Fault
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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.
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Handover Problem
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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.
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Handover Problem
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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)
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Equipment Problem
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Call drop arising from equipment problem Hardware problem Transmission problem Antenna and feeder fault Other causes
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Chapter 1 Principle of call drop
Chapter 2 Analysis of call drop
Chapter 3 Call drop cases
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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?
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Call Drop Case 1
Analysis
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From the analysis of BTS topology, it can be concluded that the frequencies are well planned. Next, check the interference band of traffic statistic.
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Call Drop Case 1
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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
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Call Drop Case 1
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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.
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Call Drop Case 1
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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.
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Call Drop Case 1
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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.
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Call Drop Case 2
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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.
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Call Drop Case 2
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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
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Call Drop Case 2
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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.
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Call Drop Case 2
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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.
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Call Drop Case 2
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Conclusion: solution of interference Solve internal interference through checking frequency planning. After internal interference is excluded, we can locate external interference with spectrum analyzer.
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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.
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Call Drop Case 3
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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.
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Call Drop Case 3
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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.
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Call Drop Case 3
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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.
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Call Drop Case 3
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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.
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Call Drop Case 4
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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.
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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?
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Call Drop Case 4
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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
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Call Drop Case 4
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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.
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Call Drop Case 5
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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.
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Call Drop Case 5
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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.
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Call Drop Case 5
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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.
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Call Drop Case 6
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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.
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Call Drop Case 6
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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.
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Call Drop Case 6
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Troubleshooting After adjusting GSM900 clock system, the abnormal call drop problem is solved.
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Call Drop Case 6
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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.
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Summary
Summary
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Types of call drop Measurement points of TCH call drop Measurement points of SDCCH call drop Causes of call drop Cases
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