Case Study-Call Drops

Case Study: Call Drop BSC Traffic Statistic Manual Volume 1BSS Troubleshooting Manual References:

1. Principle of Call Drop 2. Analysis of Call Drop 3. Call Drop Cases Contents:

1.Principle of Call Drop Definition of Call Drop TCH Call Drop SDCCH Call Drop

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.

Causes of TCH Call Drop:

Radio interface failure

O&M intervention

Equipment failure

Protocol error

Preemption

Formula of TCH Call Drop Rate

TCH call drop rate=TCH call drops / Successful TCH seizures (all)

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 (Timer 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.

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

TCH Call Drop - Assignment

TCH Call Drop Intra-BSC Handover

TCH Call Drop - Incoming BSC Handover

Measurement Point of Successful TCH Seizures

Measurement point of Successful TCH seizures

Upon BSCs 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.

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.

Successful TCH Seizure Assignment

Successful TCH Seizure Inter-BSC Handover

SDCCH Call Drop

Formula of SDCCH call drop rate:

SDCCH call drop rate = SDCCH call drops/ successful SDCCH seizures*100%

Measurement Point of SDCCH Call Drop

This Stats is measured when BSC receives ERROR INDICATION message from BTS due to an abnormal case for a radio link layer connection.

This Stats 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.

In incoming BSC handover procedure on SDCCH, this stats 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

Measurement Point of SDCCH Call Drop

In the process outgoing BSC handover on SDCCH, this stats is measured when the timer for the source cell to wait for CLEAR COMMAND message from MSC expires (Timer expires).

In the process of intra BSC handover on SDCCH, this stats 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.

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

Analysis of Call Drop

Main causes of high call drop rate

Troubleshooting of high call drop rate


According to the definition of call drop measurement point, call drop is usually caused by the following: Radio link fault. During the communication, messages can not be received correctly. Abis link broken during conversation. Call drop during handover. Other system faults. Timers that may cause call drops (BSC timer): T: starting from sending HANDOVER CMD and ending at receiving HANDOVER CMP. Time out of the timer will cause call drop. T: starting from sending CHAN REL and ending at receiving REL IND

Radio Link Fault

Timer Times Out

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

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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


Improper parameter setting Radio link timeout, SACCH multi-frames Handover parameters Power control parameters Equipment problem (Antenna, feeder, combiner, TRX) Clock problem Transmission problem

Radio Link Fault - Interference

Co-channel interference Adjacent-channel interference Inter-modulation interference and other external 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


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 performanceObserve call drop performance Observe whether there are many handover failures and reversion failures.

Radio Link Fault - Coverage

Coverage:

Overshooting Coverage hole Signal attenuation Incomplete definition of adjacent cells Imbalance of uplink/downlink

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

Radio Link Fault Coverage

Solution

Adjust network parameters

Add BTS

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.

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

Radio Link Fault Judgment process System information data Cell data Radio link connection timer Maximum times of physical information Call drop performance measurement function Judge from the cause of call drop error indication

connection failure.

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.

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.

Equipment Problem

Call drop arising from equipment problem

Hardware problem

Transmission problem Antenna and feeder fault

Other causes

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

Call Drop Case 1

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.

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.

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 effectivelyCommon methods of checking and clearing call drop due to interference

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

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

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.

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.

Call Drop Case 3

Fault description

Subscribers complain that call drops often happen from on the 5th floor and above in a building.

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.

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. Call Drop Case 3

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.

Drop Case 3

Conclusiontwo 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.

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.

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.

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.

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.

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.

Summary

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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Case Study-Call Drops

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Transcript of Case Study-Call Drops