NETWORK PERFORMANCE IMPROVEMENTDESCRIPTIONFOR THE AREA DROPPED CALLS
Abstract
The purpose of this document is to describe how an investigation of droppedcalls can be performed.The dropped call description is a part of the retainability performance area inthe Network Performance Improvement services.
Table of Contents1 SPECIFICATION............................................................................................... 2
3.1 WORK FLOW.................................................................................................. 33.2 USED FORMULAS........................................................................................... 43.3 HOW TO ANALYSE ......................................................................................... 6
3.3.1 SDCCH drop ..................................................................................... 63.3.2 TCH drop .......................................................................................... 7
4 ON SITE ACTIVITIES .................................................................................... 10
4.1 ON SITE PREPARATIONS .............................................................................. 104.1.1 Tools................................................................................................ 114.1.2 Prerequisites ................................................................................... 11
4.2 WORK FLOW................................................................................................ 114.3 TROUBLE SHOOTING.................................................................................... 12
4.3.1 Dropped Calls Due To Bad Quality................................................ 124.3.2 Dropped Calls Due To Low Signal Strength................................... 134.3.3 Dropped Calls Due To Other Reasons............................................ 14
1.1 DescriptionThe retainability performance evaluates the systems ability to handleestablished connections. Dropped calls are probably the single most importantquality item to control in the system. The level of dropped calls in the systemis in high extent depending on the initial RF planning, optimisation and alsothe system growth.
Since dropped calls is one key factor for the subscriber perceived quality thisarea should be given high priority. The dropped call area is divided into:
• Dropped calls, BSC level.
• Dropped calls, cell level.
The following data shall be evaluated on BSC level:
• Dropped SDCCH connections.
• Erlang minutes per dropped TCH connection.
• Subscriber perceived dropped call rate on TCH.
• Drop reasons for the BSC.
The value should be calculated as an average over the whole measurementperiod or as an average value per day. Since the purpose for investigatingdropped calls on a BSC level mainly is to get an overview of the performance,a longer time period (day, week etc.) is recommended.
The following data shall be evaluated on cell level for the worst 10-15 cells:
• Dropped SDCCH connections during busiest period or average 24-hourvalue.
• Erlang minutes per dropped TCH connection during busiest period oraverage 24-hour value.
• Subscriber perceived dropped call rate during busiest period or average 24-hour value.
• Drop reasons for the cells.
Dropped SDCCH connections shall be analysed together with the TCHcongestion measurements.
The TCH drop rate analysis for the 10-15 worst cells shall also includesubscriber perceived dropped call rate with drop call reasons. A minimumnumber of drops for the investigated cells must apply.
1.2 PurposeThe purpose is to measure the drop call rate, to find the worst cells and thereason for the dropped calls for these cells. Recommendations shall be givenon how the problems can be solved or the problem should be furtherinvestigated and solved by on site activities.
2 Preparation
2.1 PrerequisitesCell data and STS data according to [3], [4] with the object types: CELLTCH,CELLSDCCH, CELLTCHDR, CELLCCHDR, NCELLREL, NECELLREL,NCELLRXT and NECELLRXT. The object types in STS must be setupcorrectly.
CDD printouts.
Maps with cell positions and antenna directions.
2.2 ToolsNecessary tools if PC is used IPOS, SPOS, Turtle and Excel macro. If OSS isused SRP, CNA and Applixware. Make sure that the latest versions of thetools are used.
3 Analysis
3.1 Work flow1. Run SPOS for the days required.
2. Run Turtle and create files per hour and per day on BSC and CELL level.
3. Create a report on BSC level in Turtle.
4. Store the files for each day in separate directories.
5. Start Excel with Turtle macro.
6. Run all SDCCH and TCH macros in Excel or create drop tables and dropcharts for the worst 10-15 cells according to 3.2.
7. Analyse the result from Excel.
8. Write the report or continue with on site activities.
3.2 Used formulasFormulas [1] used for dropped calls (CME20 R6, CMS40 R2).
S_DR: Dropped SDCCH Connections of Total Number ofSDCCH Connections.
S_DR_ERLM: Erlang Minutes per Dropped SDCCH Connection.
S_DR_SS: Dropped SDCCH Connections due to Low SignalStrength of Total Number of Dropped SDCCHConnections.
S_DR_QA: Dropped SDCCH Connections due to Bad Quality ofTotal Number of Dropped SDCCH Connections.
S_DR_TA: Dropped SDCCH Connections due to Excessive TimingAdvance of Total Number of Dropped SDCCHConnections.
S_DR_OTHER: Dropped SDCCH Connections due to Other Reasonsthan Low Signal Strength, Bad Quality or ExcessiveTiming Advance of Total Number of Dropped SDCCHConnections.
T_TRAFF: Average TCH Traffic Level.
T_CONGT: TCH Time Congestion of Total Measurement Interval.
T_AVAIL: Available TCHs (not blocked) of Total Number ofDefined TCHs.
T_DR-S: Dropped TCH Connections of Total Number of CallsTerminated in the Cell.
T_DR_ERLM: Erlang Minutes per Dropped TCH Connection.
T_DR_SS: Dropped TCH Connections due to Low Signal Strengthon Uplink or Downlink of Total Number of DroppedTCH Connections.
T_DR_QA: Dropped TCH Connections due to Bad Quality onUplink or Downlink of Total Number of Dropped TCHConnections.
T_DR_TA: Dropped TCH Connections due to Excessive TimingAdvance of Total Number of Dropped TCHConnections.
T_DR_OTHER: Dropped Calls due to Other Reasons than Low SignalStrength, Bad Quality or Excessive Timing Advance ofTotal Number of Dropped TCH Connections.
H_SUC: Successful Handovers of Total Number of HandoverAttempts.
Formulas [2] used for dropped calls (CME 20 R7). For no overlaid subcells.
S_DR-C: Dropped SDCCH Connections of Total Number ofSDCCH Connections.
S_DR_ERLM: Erlang Minutes per Dropped SDCCH Connection.
S_DR_SS: Dropped SDCCH Connections due to Low SignalStrength of Total Number of Dropped SDCCHConnections.
S_DR_BQ: Dropped SDCCH Connections due to Bad Quality ofTotal Number of Dropped SDCCH Connections.
S_DR_TA: Dropped SDCCH Connections due to Excessive TimingAdvance of Total Number of Dropped SDCCHConnections.
S_DR_OTH: Dropped SDCCH Connections due to Other Reasonsthan Low Signal Strength, Bad Quality or ExcessiveTiming Advance of Total Number of Dropped SDCCHConnections.
T_TRAF: Average TCH Traffic Level.
T_CONGT: TCH Time Congestion of Total Measurement Interval.
T_AVAIL: Available TCHs (not blocked) of Total Number ofDefined TCHs.
T_DWN: Average Cell downtime for active cells
H_SUC: Successful Handovers of Total Number of HandoverAttempts.
T_DR-S: Dropped TCH Connections of Total Number of CallsTerminated in the Cell.
T_DR_ERLM: Erlang Minutes per Dropped TCH Connection.
T_DR_SS_DL: Dropped TCH Connections due to Low Signal Strengthon Downlink of Total Number of Dropped TCHConnections.
T_DR_SS_UL: Dropped TCH Connections due to Low Signal Strengthon Uplink of Total Number of Dropped TCHConnections.
T_DR_SS_BL: Dropped TCH Connections due to Low Signal Strengthon both links of Total Number of Dropped TCHConnections.
T_DR_SUD: Suddenly lost connections of Total Number of DroppedTCH Connections.
T_DR_BQ_DL: Dropped TCH Connections at Bad Quality on Downlinkof Total Number of Dropped TCH Connections.
T_DR_BQ_UL: Dropped TCH Connections at Bad Quality on Uplink ofTotal Number of Dropped TCH Connections.
T_DR_BQ_BL: Dropped TCH Connections at Bad Quality on both linksof Total Number of Dropped TCH Connections.
T_DR_TA: Dropped TCH Connections due to Excessive TimingAdvance of Total Number of Dropped TCHConnections.
T_DR_OTH: Dropped Calls due to Other Reasons than Low SignalStrength, Bad Quality or Excessive Timing Advance ofTotal Number of Dropped TCH Connections.
3.3 How to analyse
3.3.1 SDCCH drop
• Create a table on BSC level including average drop on SDCCH, averagevalue, for benchmark use.
• Create a table with the 10-15 cells with the lowest Erlang minutes perdrop on SDCCH. Make sure that there are at least some dropped calls inthe cell. Add the dropped call per SDCCH establishment and SDCCHdropped reasons to the table. Also add the TCH Erlang minutes per dropand TCH time congestion. Sort on Erlang minutes per drop (ascending).
• Make a chart with the 10-15 worst performing cells according to Erlangminutes per drop on SDCCH or drop call rate on SDCCH and the TCHtime congestion.
• Make a chart with the 10-15 worst performing cells according to Erlangminutes per drop on SDCCH or drop call rate on SDCCH and the dropcall rate on TCH.
• Make a chart with the drop call reason on SDCCH for the same cells thathave the highest drop call rate.
⇒ Compare the drop call rate on SDCCH with the TCH time congestion andTCH drop call rate. Is there any correlation? Congestion on TCH will
increase the drop call rate if assignment to worse cell is not used. Thedrop on SDCCH will then be counted as a drop to other reasons.
⇒ Compere the reasons for the SDCCH drops with the TCH drops. Is thereany correlation? A lot of bad quality drops on both TCH and SDCCHcould indicate interference problems in the cell. A lot of drops on theSDCCH but not on the TCH could indicate problems with the mobilestations, MAXTA to low, SMS problems, MSC/VLR problems etc.
3.3.2 TCH drop
• Create a table for TCH on BSC level including dropped connections,dropped calls, Erlang minutes per drop and percentage drop due to lowSS, Quality and other reasons, for benchmark use.
• Create a table with the 10-15 cells with the lowest Erlang minutes perdrop on TCH and/or a table with subscriber perceived dropped call rate.Add the absolute number of drops on TCH, the highest subscriberperceived dropped call rate on TCH and the dropped call reasons to thetable. Also add the TCH traffic, the highest TCH time congestion, TCHavailability, SDCCH drop call rate and handover success rate (incomingand outgoing) to the table.
• Make a chart with the 10-15 cells with the lowest number of Erlangminutes between dropped calls on TCH. Include the subscriber perceiveddropped call rate on TCH.
• Make a chart with the drop call reason on TCH for the same cells thathave the highest drop call rate.
⇒ Any cell downtime or low TCH availability? If possible check if there hasbeen any alarms, HW problems, transmission problems or maintenancework on the affected cell or its neighbour cells.
⇒ High percentage of quality drops? Check parameter settings. For instanceQLIMUL/QLIMDL specifies if a cell has dropped calls due to badquality. Is the cell situated in an interference area? Drop due bad quality, alot of IHO or many BQ urgency handovers? Could also indicate missingneighbour relations. Proceed with the area Interference.
⇒ High percentage of signal strength drops? Check where the cell is situated,is it in the outskirts of the network? Is there any missing neighbourrelations? Correct power settings? Is it missing indoor or outdoorcoverage, any tunnels, and hilly terrain? Potential obstacles hiding theantennas, wrong azimuth or other antenna problems?
⇒ High percentage of suddenly lost connections? (only for CME 20 R7)Check where the cell is situated. Any tunnels underground parking lots,
buildings with no indoor coverage etc? Can indicate coverage gaps whereyou loose coverage rapidly somewhere in the cell.
⇒ High percentage of excessive timing advance drops? Are the parametersMAXTA and TALIM set to correct values? Is the site located close to anyopen water, desert area or mountain area?
⇒ High percentage of drops to other reasons? Any HW or transmissionproblems on the site? Any uplink (external or internal) interferenceproblems?
⇒ Check the handover performance for the cell. Check if there is a high rateof MS lost at handover to any neighbour relations (incoming or outgoing).The reason is to determine if the high dropped call rate is related tohandover performance. If related to handover performance, proceed withthe area Handover Performance to investigate the problem further.
3.4 ResultsThe results of the STS analysis can be used for reporting or for troubleshooting depending on the service performed. Dropped calls show the numberof abnormal disconnection’s during call setup, SMS, supplementary serviceactivation or during conversation. The different counters for dropped calls, i.e.dropped calls due to low signal strength, bad quality, too high timing advanceand miscellaneous is used to get an indication of the reason for possible badperformance.
From a subscriber point of view, the most serious dropped calls are those thatinterrupt an ongoing conversation, i.e. a call dropped on the TCH. If the call isdropped on the SDCCH the user simply re-dials again and hopefully succeedswith the new call setup. From a system point of view, the dropped calls on theSDCCH are more serious. A radio link time-out on the SDCCH will occupy anSDCCH sub-channel for (RLINKUP+RLINKT)/2 seconds and increase therisk for SDCCH congestion.
If there are any problems with dropped calls the result of the analysis isrecommendations on how the dropped call rate can be improved. Thefollowing are some examples on recommended actions that can be used forreporting.
3.4.1 SDCCH ResultsIf a high drop rate on SDCCH has been noticed the following actions isrecommended in order to proceed and solve the problems.
• Improvements for dropped calls on TCH will improve the drop call rate onSDCCH. I.e. recommend to trouble shoot the TCH drop calls first if thereis poor performance on both TCH and SDCCH.
• The drop call rate on SDCCH can be improved if the congestion on TCH isdecreased. Recommend to use the feature assignment to worse cell orincrease the capacity on TCH.
• The reasons for low SS drops could be too few sites, wrong output power,shadowing, no indoor coverage or network equipment failure.
• The reasons for dropped calls due to bad quality on the uplink or downlinkare related to internal or external interference and trouble shooting isneeded to find the interferrers. The situation could be temporary improvedby means of applicable features. Recommend features that are not activatedor recommend alternative parameter setting.
• The reasons for drops on to high timing advance is related to the sitelocation i.e. close to open water, desert or hilly terrain and the setting ofMAXTA and TALIM. Setting MAXTA to 63 and TALIM to 62 couldsolve the problem and/or tilt the antennas, reduce antenna height, changeantenna or reduce output power.
• Miscellaneous problems could for example be mobile errors which canoccur when old mobiles may cause dropped calls if certain radio networkfeatures are used. Another reasons could be that the MS is damaged andnot working properly.
3.4.2 TCH ResultsIf a high drop rate on TCH has been noticed the following actions isrecommended in order to proceed and solve the problems.
• High drop rate due to high outage time or low availability. Inform theoperation and maintenance department about the problems or check thereasons for the downtime. Check also the alarm list or BTS error log.Another way is also to check the resolvation time for the different alarmcategories. Many problems with dropped calls are often related toinsufficient O&M routines and not to radio problems.
• Dropped TCH due to bad quality are often due to interference problemsand/or low signal strength in areas where there is no dominant server. Thereason could also be missing neighbour relations so that the mobile is notconnected to the strongest server and therefore perceives bad quality. Theinterfered cell should be investigated in order to find the source for theinterference. In most cases the interference is internal but can alsooriginate from external sources such as other networks, radio stations,microwave links etc. The interference could also be reduced by means ofapplicable features such as frequency hopping, assignment to better cell,DTX, MS/BTS power control etc. Recommend applicable features oralternative parameter setting if founded incorrect.
• Dropped TCH due to low signal strength are in most cases related tocoverage gaps but can also be a result of missing neighbour relations,hidden antennas, wrong antenna type (to low gain), antenna or feederproblems, incorrect power settings, etc. The reason can also be unforeseen
subscriber behaviour i.e. the subscribers use their mobiles indoor, inelevators, parkinglots etc. This can also be indicated if there is a highpercentage of suddenly lost connections.
• Dropped TCH due to excessive timing advance should in normal cases notoccur in the network. The reasons for timing advance drops are sitelocation (close to open water, desert or hilly terrain) and the setting ofMAXTA and TALIM. Setting MAXTA to 63 and TALIM to 62 couldsolve the problem. Or reduce the coverage by down tilting the antennas,reduce antenna height, change antenna or reduce output power. If the site islocated close to open water etc. the extended range feature could beconsidered.
• TCH drops due to other reasons than low SS, bad quality, excessive timingadvance could for example be because of BTS HW problems, transmissionproblems, service affecting maintenance work, uplink interferenceproblems (external or internal), mobile station problems etc.
4 On site activitiesThis section describes the activities that should be performed on site for thedropped call analysis.
4.1 GeneralWhen investigating dropped call performance on cell level, some kind ofprioritisation needs to be done in order to find the cells that is interesting toinvestigate further. It is recommended to start with a list for the worst 10–15cells and analyse and solve the problems with these cells and then continuewith the next 10–15 worst performing cells and so on.
One way to make the 10–15 worst cell list is to include cells with low trafficlevel and a very high drop rate in percentage (for example one third of thecalls) and cells with high traffic level and high number of dropped calls.
The cells with the largest number of dropped calls should always beprioritised and included in the top 10-15 list, since they affects the systemperformance and the subscriber perceived quality the most.
The cells with a high drop rate in percentage should also be included if thenumber of calls reaches a sufficient level. The exact number is depending onthe traffic in the system and where the sites are located but a rough figure is atleast 30 to 50 calls.
4.2 On site preparationsPreparations that is needed to perform before the analysis can start.
Check also if there has been any maintenance or configuration work in thearea during the time period.
9. Check the locating parameters and radio network feature settings.Compere the parameter settings for the concerned cell with the other cellsin the network. Any differences? Investigate why.
10. Check the neighbour definitions for the cell. Compare with a plot with sitelocations. Check for any obviously missing relations.
11. Check the cell in EET or TCP. Check the location, height, coverage, C/Iand C/A in order to locate the problem in the cell.
12. Perform drive tests in the suspected area. Try to find the area in the cellwhere it is most likely that the subscribers will perceive the problem. Usethe results from MRR and/or handover analysis.
13. Implement the changes or fault correction actions after the applicableanalysis.
14. Collect STS data and run MRR to verify the result of the changes. If theresult is not satisfactory continue the trouble shooting and try alternativeactions. Save the collected data for reporting and performancecomparisons before and after on site activities.
15. Write the report.
4.4 Trouble shootingThe trouble shooting of dropped calls is divided into three areas, dropped callsdue to bad quality, low SS and other reasons.
4.4.1 Dropped Calls Due To Bad Quality
If the cell suffers from dropped calls due to bad quality the first step is tocheck that the parameters QLIMDL and QLIMUL are set to correct values.
Additional data should be collected in order to make correct conclusions.Check if there is any normal disconnections at bad quality in the cell, thiscould give indications on that there really is a bad quality problem in the cell.
Run MRR on the cell and check the average RXQUAL value in the cell.
Display C/I and C/A predections in EET or TCP. Check if the cell is locatedin any interference area. Rember that there might be interference in the celleven if it is not displayed in the planning tool. Check idle channelmeasurements (ICM) for the cell to see if there exists uplink interference inthe cell.
Check the handover statistics on a neighbour relation level for the cell. See ifthere is any neighbour relation with a high number of bad quality handovers.
This information might give you the location for the bad quality area in thecell.
Frequency Allocation Support (FAS) can be used in order to find theinterferred frequency. This can be useful especially for frequency hoppingsystems. FAS gives however only the uplink information.
Perform TEMS drive tests in the suspected area. Try to located the interferrer,one way for C/I problems is to halt the affected cell and perform a frequencyscanning in order to locate the interfering cell.
See if it is possible to change frequency or reduce the signal strength of theinterferer. For example downtilting the antenna. Check the antenna diagram inorder to see the effect of different tilt angels. A second alternative is to changethe frequency or increase the signal strength in the interferred cell. Check ifthere is any missing neighbour relations causing low SS and by that badquality drops.
4.4.2 Dropped Calls Due To Low Signal StrengthIf the cell suffers from TCH drop due to low signal strength the first step is tocheck the power setting, power balance and neighbour relations in the affectedcell. Check for example on a map with site positions or in the cell-planningtool for any obviously missing neighbour relations. Check also the amount ofnormal disconnections at low SS for the cell.
Check the alarms on the RBS to verify that there is no VSWR alarms causingthe reduction in output power.
MRR can be used for checking the timing advance and RXLEV distribution inthe cell. This can give an indication if the subscribers are close to the basestation or in the outskirts of the cell. If most of the subscribers are on low TAvalues the low SS drops might be lack of indoor coverage or if most of thesubscribers are on high TA values the problem might be a missing site orneighbour relation. By checking RXLEV for the cell indication and comperewith coverage plots in EET or TCP indications of obstacles covering theantenna, feeder problems or other reasons for low SS might be found.
Use the handover statistics on a neighbour relation level to get an indicationon where in the cell the problem might be. Check also if any of the targetneighbours suffers from severe congestion.
If there is a lot of suddenly lost connections in the cell this could indicate thatthere is a tunnel, underground parking lot, high indoor usage etc. Try to findthe most likely position in the cell where this kind of drops might occur.
Check in EET or TCP to see if any coverage gaps or areas with low coveragecan be found in the cell. Verify that the different clutter values make sense inthe planing tool. Is the site position OK? Is the antenna azimuth correct, is itshooting to the correct location, road, building etc.
Perform drive tests in the cell and check for missing neighbours, swappedantennas etc. Perform the drive test close to the site and try to see if it is lineof sight or of the antennas are hidden by any obstacles. Make a site visit andcheck the antennas if necessary.
If the low signal strength is not related to any faults or missing configurationprobably a new site or improved indoor coverage is needed and the problemshould be passed on to the cell planning department.
4.4.3 Dropped Calls Due To Other ReasonsIf the cell suffers from dropped calls besides the reasons low SS, bad qualityand excessive timing advance the dropped calls will be counted as otherreasons. That is that the counters for SS, quality and timing advance are notincremented and only the CNDROP, TFNDROP or THNDROP are stepped.This could for example be the case in cells with uplink interference.Check if ICM is indicating uplink interference in the cell.
Other possible reasons could be problems with the mobile stations of BTSHW problems, transmission problems, and service affecting maintenancework.
Check with the operation and maintenance department or check the applicablealarm logs if there has been any HW problems, transmission problems, andservice affecting maintenance work during the time period. The average celldowntime and TCH and SDCCH availability should also be checked.
If mobile station problems are suspected in the network this needs to be raisedwith the Customer and his customer care department to investigate theproblem further.
5.1 GeneralThe report examples shown below are related to services without any on-siteactivities. In case of services including on-site activities the parts describingthe performance can be kept. Recommendations shall not exist in such report.Instead it is expected that the recommendations, such as parameter changes,are already performed on-site, that the performance of the different changes iswell documented (e.g. by comparison of the performance of the differentsettings) and that a conclusion is drawn in the report, what the problem hasbeen and how it was solved. In opposite to a service without an on-siteactivity, the configuration of a network might be changed during a servicewith on-site activities and the best configuration will be left, when the serviceis finished.
The report should cover dropped calls on BSC level for both TCH andSDCCH. The reasons for dropped calls on TCH should also be shown. On celllevel the worst cells shall be presented both on SDCCH and on TCH. Ananalyse of why these cells are the worst should also be presented andreferences to other areas that might have influence on drop call rate can bemade if applicable. Recommendations on how the drop call rate can bedecreased shall be made for those cells where a problem source has beenfound.
5.2 Dropped calls on BSC level
5.2.1 SDCCH Drop rateThe SDCCH drop rate as a percentage of all SDCCH establishments is 3.0%.The drop rate on SDCCH is high and is considered to be below average.
5.2.2 TCH Drop rateFor the TCH drop rate two different formulas has been used. The first oneshows the dropped calls as a percentage of all calls and is 3.7%. The secondone shows the average time in minutes between dropped calls and the value is34 minutes. Both figures concerning dropped calls are very low and are betterthan average. Many of the calls are however dropped due to bad quality ormiscellaneous reason see Figure 5-1.
5.3 Dropped calls on SDCCHFigure 5-2 shows the 15 worst cells concerning dropped calls on SDCCH. Thereason to the dropped calls are shown in Figure 5-3.
Dropped calls on SDCCH
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Figure 5-2 Dropped calls on SDCCH for the worst 15 cells,Friday YYMMDD.
Figure 5-3 Reasons to dropped calls on SDCCH, sortedaccording to Figure 5-2, Friday YYMMDD.
Most of the calls are dropped due to low signal strength. Some cells also dropcalls due to bad quality. Three cells also drop calls due to other reason. Theseare calls dropped on SDCCH due to the high congestion on TCH. If themobile can not find a TCH in its own cell or another worse cell (assignment toworse cell) then will this be counted as a drop on SDCCH.
Some cells appear on the worst list for dropped calls on TCH and on droppedcalls on SDCCH. The worst cells, for dropped calls on TCH, are also usuallythe worst on SDCCH as well. If dropped calls on TCH can be decreased,dropped calls on SDCCH might also decrease.
5.4 Dropped calls on TCHDropped calls are here measured in two ways. First dropped calls from thesubscriber point of view and secondly the time in minutes between droppedcalls. Figure 5-4 shows the cells with the highest dropped call rate and Figure5-5 shows the reason for the dropped calls for these cells. All cells have atleast 8% dropped call rate and less than 30 minutes between dropped calls.
Figure 5-4 Dropped calls on TCH for the worst 15 cells, FridayYYMMDD.
Reasons to dropped calls
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Figure 5-5 Reasons to dropped calls, sorted according toFigure 5-4, Friday YYMMDD.
Five of the cells drop calls due to low signal strength. These are mainly cellsin the border areas of the BSC and the reason for the dropped calls is probablypoor coverage.
Seven of the cells drop 20% or more of the calls due to bad quality. Two ofthem are highly positioned cells (Cell10 and Cell7). These cells have a lot ofneighbours (16-28) and many MBCCH (13-17) and they cover a very largearea. These two cells also have problems with handover. They are among the
15 worst cells concerning failed handover and handover reversion from othercells into these ones see Figure x-x.
For the remaining 5 cells, Cell11 has the same problem as the highlypositioned cells with handover from other cells into this one, mainly fromCell12. Cell11 also has a co-channel on BCCH with Cell24. Cell15 has a co-channel on TCH with another neighbouring cell Cell75.
Cell12 has a lot of failed handovers to other cells. This is mainly valid forhandovers to Cell11 and Cell33. The problem with Cell12 seems to be failedhandovers to other interfered cells.
