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Ericsson optimization opti

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Ericsson optimization opti

  3. 3. Probable reasons to various performance problems in a mobile network and instructions on how to solve and identify the problem are given. Analysis Flow A proposed analysis flow is given below: 1 Analyse the characteristics of the problem 2 Identify Base Line Requirements 3 Calculate Overall System Performance 4 Do a Benchmarking - Compare with other Operators 5 Identify Problem Areas 6 Retrieve Detailed Statistics on the Problem Areas 7 Initiate and Perform further Measurements 8 Perform a Careful and Detailed Analysis 9 Take Actions to Solve the Problems 10 Verify improvements 11 Give Recommendations - Short term (e.g. parameter changes, etc)
  4. 4. Availability A low availability may lead to congestion and should thus be monitored regularly. Faulty equipment should as soon as possible be removed and replaced. Rule 1: Check the amount of available control channels. If the percentage of available channels is not sufficient then check if any channels are blocked or not installed. See flow chart SDCCH & TCH Availability in the next Slide Formula: Available (not blocked) SDCCH’s of Total Number of Defined SDCCH’s – See CDD Channel Admin worksheet Total Number of Defined SDCCH’s See kpi S_AV_NR , CNUCHCNT and S_AVAIL at MS ACCESS DB or OPTIMA Available (not blocked) TCH’s of Total Number of Defined TCH’s - See CDD Channel Admin worksheet Total Number of Defined TCH’s Check T_AV_NR , TCH_AVG , TNUCHCNT 1. Probable Reasons- Hardware Fault Action- Check BTS error log Solution- Replace and repair faulty hardware.
  5. 5. 3.Using of Synthesizer Hopping When using of synthesizer hopping, one dedicated TRU will be used for BCCH, this is one reason for low TCH availability. Another reason is when using more frequencies than actual TRUs, the system will count more defined channels. Action: . Check using of Synthesizer Hopping. . Check the number of required basic physical channel in a channel group, NUMREQBPC Solution: Define 2 channel groups, one for BCCH and one for TCH Change NUMREQBPC to the actual value. 2. Probable Reasons- Transmission Problem Action- Check transmission performance. Solution- Add transmission capacity. Change transmission equipment. Change transmission lines.
  7. 7. RXTCP:CELL=AFORD,MOTY=RXOTG; RXCDP:MO=RXOTG-22; RXMFP:MOTY=RXOTS-22-10-0; RXMFP:MO=RXOTS-1-3-0&&-4,FAULTY; RXMOP:MO=RXOTX-22-10; RXMOP:MO=RXOTG-22; RXELP:MO=RXOTG-22; Step by Step procedure to check BTS alarm logs !!! Step by Step procedure to check digital path quality high slips!!! RXTCP:MOTY=RXOTG; RXCDP:MO=RXOTG-22; RXAPP:MO=RXOTG-22; RADEP:DEV=RBLT-xx; DTSTP:DIP=ALL; DTQUP:DIP=RTLTBx; RLCFP:CELL=CSTABAR;(ARFCN List) RLCRP:CELL=CSTABAR;(Cell resources) RLCDP:CELL=CSTABAR;(Cell Multiple Channel Alloc Prof) RLDEP:CELL=CSTABAR;(Cell Design Data) RLMFP:CELL=CSTABAR;(Measurement Freq) RLNCP:CELL=CSTABAR;(neighbour relations) RLNRP:CELL=CSTABAR;(cell relations) RLLHP:CELL=CSTABAR;(HCS) RLLCP:CELL=CSTABAR;(Cell Load Sharing) RLSSP:CELL=CSTABAR;(SYSINFO) RLSLP:CELL=CSTABAR;(# of SDCCH resources) RAEPP:ID=ALL;(BSC parameters) RRTPP:TRAPOOL=ALL;(per BSC command) EXRPP:RP=ALL;(check number of rpps1 in the bsc) EXEMP:RP=ALL,EM=ALL;(check rpps) RLGRP:CELL=ALL,DETAIL;(check B-PDCH,E-PDCH) RLGRP:CELL=CSTABAR;(GPRS resources) RLGSP:CELL=CSTABAR;(GPRS Config Data) STDEP: DEV=RTGPHDV-0&&-127;(Check RPP) First RPP is from 0 - 63 RTPGPHDV, Second RPP is from 64 - 127 RTPGHDV Per RPP should have 37 "BUSY" V STATE For two RPPs there must have 75 "BUSY" V STATE Common Cell Resources MML Commands TO RESET the DIP Q DTQSR:DIP=RTLTBx,SES2,ES2,SF; DTQSR:DIP=RTLTBx,SES,ES; DTQSR:DIP=RTLTBx,DEGR;
  8. 8. Congestion Traffic congestion is one of the major network problems in a mobile system. A high congestion deteriorates the overall performance of the network and should be minimized. ∀ Short term growth If the high traffic related to an occasional event, like sport event, fairs, conference, a temporary solution might be considered. ∀. Long term growth If there is a long term growth the network capacity has to grow according to the demand. Cellular System Capacity depends on a number of different factors namely: 1. The number of channels available for voice and data 2. The GOS Grade of Service the subscribers are encountering in the system. What is TRAFFIC ? It refers to to the usage of channels and is usually thought of as the holding time per time unit or the number of “call hours” per hour How MUCH Traffic Can One Cell Carry? It depends on the number of traffic channels available and the amount of congestion which is acceptable ( by both the subscriber and the operator) GOS = Grade of Service - is the percent probability that a call is blocked due to congestion.
  9. 9. Low TCH Assignment Success Rate Successful assignments show the number of successful TCH allocations at call setup.At unsuccessful assignment, the Assignment Complete message, sent by the MS, was never received by the BTS. Probable Reasons The reason for a low number of successful assignments might be the following items: . No Dominant Serving Cell Action: Check coverage plots, perform drive tests. Solution:Add new base station. . Severe Congestion on the TCH. Action: Check failed TCH allocation for assignment due to congestion. Check failed TCH allocation for handover due to congestion Check TCH congestion. Low Signal Strength for Call Access. The signal strength might be higher on the BCCH than on the TCH. Action: Check signal strength of BCCH and TCH. Perform MTR recordings. Solution: Adjust and increase TCH output power.
  10. 10. . Interference Interference, disturbance on SDCCH or target TCH frequency. Action: Check Interference. Solution:Improve frequency plan and decrease interference. . Faulty Transceiver Action: Check BTS Error Log. Solution:Replace or repair faulty equipment. . Low Output Power Action: Check set output power parameters and the actual transceiver output power. Solution: Increase output power or replace faulty transceiver.
  11. 11. Type of Congestion •Identify if there is only SDCCH or TCH congestion or both. Congestion on both SDCCH and TCH may mean that the only way to get rid of the congestion is to add more physical capacity in terms of transceivers or sites. •Consider how many channels that are allocated in the cell. If possible, expand the capacity with new transceivers, otherwise a new site must be implemented. •Frequency planning schemes such as Multiple Reuse Pattern could be used to relieve congestion. Micro cells could be used to take traffic in severe congested areas. SDCCH Congestion The time congestion should be used instead of congestion based on access attempts as there is no way to estimate the number of access attempts a single mobile does. See also flow chart ‘SDCCH Congestion in the next slide. SDCCH Time Congestion - See kpi counter S_CNGT 100 * CTCONGS / ( PERLEN * 60 ) -SDCCH Time Congestion of Total Measurement Interval SDCCH Congestion in Total Number of SDCCH Seizure – See counter CMSESTAB CTCONGS-Total SDCCH Time Congestion CMSESTAB -Total SDCCH Connections As a WAY Forward : Compute 0.5% GOS for SDCCH resources using PEAK traffic. NOTE : The most accurate SDCCH Dimensioning is achieved by looking at the Congestion on TCH and SDCCH for the specific cell . The optimum configuration is achieved by selecting a configuration with as many as TCHs as possible without letting SDCCH GOS exceed 0.5% GOS or ¼ of TCH GOS if TCH GOS is 2%.
  12. 12. Probable Reasons - Increasing Traffic Demand The high traffic could be related to an occasional event or due to a long term growth. Action - Check if short term traffic growth. Make trend comparisons. Check if combined SDCCH is used. Check SDCCH dimensioning. Solution: Increase the number of SDCCH channels. Note, that an increase may lead to the need for new transceivers.If combined SDCCH is used, non-combined channel configuration should be introduced. Probable Reasons - Bad use of Adaptive configuration of logical channels By using the Adaptive configuration of logical channels feature, the basic SDCCH configuration in a cell will be under-dimensioned. If this feature is not used correctly, it will cause SDCCH congestion. Action: Check if ACSTATE is on. Check parameters related to Adaptive configuration of logical channels Solution: If ACSTATE is off, it is suggested to switch on. Probable Reasons - Long Mean Holding time If the mean holding time is long, this generates a higher traffic load. Action: Check SDCCH Mean Holding Time
  13. 13. SDCCH Mean Holding Time Probable Reasons- False Accesses No response from MS after Channel Request. The system waits about 5 seconds before performing a disconnection and the channel is available again. Action:Check frequency plan.Check interference. Solution:Improve frequency plan and reduce interference. . Probable Reasons- Faulty Transceiver Action: Check BTS error log. Solution:Change & repair faulty equipment Probable Reasons- Congestion on signalling routes Action: Check signalling performance & tranmission capacity. Solution: Add more transmission capacity or re-route traffic if possible. Probable Reasons- Many SMS Messages Action:Check SMS Performance. Solution:Reconsider the SDCCH dimensioning and the amount of SMS messages if congestion. Probable Reasons-Long mean holding time A long mean holding time may indicate: . Congestion on TCH Action: Check TCH Congestion Solution:Increase the TCH capacity. See also flow chart ‘SDCCH Mean Holding Time in the next slide.
  14. 14. Probable Reasons - TCH Congestion TCH congestion causes the mobiles to stay extra long time on the SDCCH. Check if there exists TCH congestion and if the SDCCH mean holding time is high. For immediate assignment the time is 2-2,5 seconds. Action- Check TCH Congestion Check SDCCH Mean Holding Time Check if Assignment to Worse cell is used and existing parameter setting. Check if Cell Load Sharing is used. Solution:Increase TCH capacity.Use the features for traffic distribition such as Cell Load Sharing and Assignment to Worse Cell. Probable Reasons - Low Availability Action: Check SDCCH Availability Check if the channels are manual, control or automatic blocked. Solution:Change and repair faulty equipment.Review the O&M procedures. Probable Reasons - Too Frequent Periodic Registration Action: Check Random Access Distribution Check the timer T3212 in the BSC and the parameters BTDM and GTDM in the MSC Solution: Decrease the periodic registration. .
  15. 15. Probable Reasons -Wrong SDCCH Dimensioning Action: Check SDCCH dimensioning. Location Area Border Cell. If the cell is situated on a misplaced Location Area border, this means that unnecessary many normal LUs are performed. Action: Check site position and location area border. Check Location Update Performance Check parameter CRH etc. Solution: If the site is located close to major road or railway, consider to move the Location Area border. Increase the hysteresis CRH. The CRH is the hysteresis value used when the MS in idle mode crosses a LA border. The default value for this parameter is 4. If a high number of Location Updatings occurs in a Location Area border cell, a higher CRH can be set in order to reduce the number of LUs. Probable Reasons -SMS Usage. Extensive SMS usage increases the SDCCH traffic and could cause congestion if badly dimensioned SDCCH channels. Action: Check SMS activity. Solution: Redimension the SDCCH channels with consideration taken to SMS usage. Probable Reasons -Cell Broadcast Used Action: Check if Cell Broadcast is active. If active, check if it is used by the operator. Solution: Remove Cell Broadcast if not used. Probable Reasons - IMSI Attach/Detach in Use. An introduction of IMSI attach/detach will increase the traffic on SDCCH. However, the benefits are that the paging success rate will increase. The recommendation is to use Attach/Detach. Probable Reasons -Software File Congestion Action: Check SAE setting. High Ratio of Random Accesses Action: Check Random Access performance
  16. 16. TCH Mean Holding Time See also flow chart ‘TCH Mean Holding Time’ Formulas TCH Mean Holding Time TCH Mean Holding Time in Underlaid Subcell Probable Reasons-Short mean holding time Short TCH mean holding time might be the case for micro cells or cells close to highways. A short mean holding time may indicate: •Abnormal Amount of Disconnections Action: Check Dropped Calls Solution:Investigate further the reason for the short mean holding time. Many Handovers out from the Cell Action: Check Handover Performance. Solution:The handover related cell parameters may need to be changed. ∀ Many Intra-cell Handovers Action: Check Intra Cell Handover parameters. Solution:Check and change parameter setting. The reason may be interference. ∀ Faulty Transceiver Action: Check BTS error log. Solution:Change & repair faulty equipment.
  17. 17. Long mean holding time A long mean holding time may indicate: Hanging Subscribers Action: Check channel release time. Check if any devices are busy more than 1 hour. Solution:Remove hanging subscribers. Inbuilding Cells (repeater or micro cells) Action: Check site location. No further action.
  18. 18. TCH Congestion The time congestion should be used instead of congestion based on access attempts as there is no way to estimate the number of access attempts a single mobile does. Formulas The following formulas should be used to estimate the congestion. TCH Time Congestion of Total measurement Interval Number of dropped calls due to TCH Congestion or transcoder congestion of Total Number of TCH Assignment Attempts Probable Reasons . Increasing Traffic Demand The high traffic could be related to an occasional event or due to a long term growth. Action: Check if short term traffic growth. Make trend comparisons. Check TCH dimensioning Check the use of congestion relieving features such as Assignment to Worse cell, Cell Load Sharing and HCS. Solution: Increase the number of transceivers. This may lead to problems with floor space, antenna installations, CDU type, expansion cabinets and combiner type. If not used, introduce Assignment to Worse cell and Cell Load Sharing. Note that the interference level will increase if Assignment to Worse cell is used as some mobiles will be closer to a co-channel cell than was intended in the frequency plan. The feature will be more effective if the neighbours are not congested. In a tight network with a high reuse and congestion in a larger area, the feature might only make the situation worse.
  19. 19. Bad Dimensioning Bad allocation of TCH in a system may cause unnecessary congestion. Investigate if possible to move transceivers from non-congested areas to congested areas. Of course, the base station type, CDU-type, current number of transceivers, floor space, combiner type, etc., should be considered before a recommendation to move transceiver could be made. Action: Check TCH traffic and dimensioning. Solution: Redimension the TCHs. . Hardware Fault & Installation Fault Faulty equipment will lead to that all time slots could not be used for handling traffic causing congestion. Low availability can happen if the channels have been manually or automatically blocked and taken out of service. Action: Check TCH Availability.Check TCH blocking. Solution:Change and repair faulty equipment.Review the O&M procedures. . High Antenna Position A high antenna position could mean a too large service area. Also antennas placed on hilltops will cover large areas. A large coverage area might mean that the cell takes a lot of traffic. Action: Check antenna height. Check antenna type Check antenna tilt Solution: Lower antenna if there is no risk for loss of coverage (no coverage at all). Tilting of the antenna or changing antenna type may also decrease the coverage area.
  20. 20. Wrong Use of Hierarchical Cell Structure, HCS If the cell is incorrectly defined as higher priority level of Hierarchical Cell Structure or HCS parameters are not being used properly, it will draw in more traffic than other cells. Action: Check HCS related parameters. Solution:Change to normal value. . Low Handover Activity A low handover activity may lead to congestion if the MS is forced to stay on a cell longer than necessary. Action: Check if congestion in neighbouring cell.Check handover performance Check neighbouring cell definitions. Missing relations could cause handover problems. Solution: Correct handover parameters such as too high or too low hysteresis values, missing neighbour relations, one-way handovers. Congestion in neighbouring cell need to be decreased. . Congestion in Surrounding Cells Action: Check congestion in neighbouring cells.Review neighbour cell list. New relation could relieve the congestion.Check if Assignment to Worse cell is used. If assignment handover to worse cell is used (directed retry). Check the setting of the parameter AWOFFSET. Solution: Add new neighbour cells if appropriate.
  21. 21. There is a wide range of factors can result in that a subscriber fails to complete a call satisfactorily. The only problem many subscribers will tolerate in a public network is a busy tone from the called party. Unfortunately, reality does not always match expectations when it comes to mobile network, which results in customers complaining about poor performance of the service. They will eventually change operator if the perceived bad performance continues.A subscriber should be able to place calls satisfactorily and hang up in its own time, regardless of the technical reasons for call-completion problems. Dropped calls show the number of abnormal disconnections during call setup or during conversation.Use the different counters for dropped calls, i.e. dropped calls due to low signal strength, bad quality, too high timing advance and miscellaneous to get an indication of the reason for possible bad performance.The mechanisms behind dropped calls on TCH and SDCCH are described below: 1. Radio Link Time-Out Every time a SACCH message can not be decoded the radio link time-out counter is decreased by 1. If the message can be decoded the counter is incremented by 2. However, the value can not exceed the initial value. The initial value is set by the parameter RLINKT for radio link time-out in the mobile station and by RLINKUP for time-out in the BSC.If the mobile moves out of coverage and no measurement reports are received in the BSC, there will be a radio link time-out and the message Channel Release (cause: abnormal release, unspecified) is sent to the mobile station and the SACCH is deactivated in the BTS. A Clear Request message is sent to the MSC. To be sure that the mobile has stopped transmitting the BSC now waits RLINKT SACCH periods before the timeslot is released and a new call can be established on the channel. Dropped Calls
  22. 22. 2. Layer 2 Time-Out If the BTS never get an acknowledge on a Layer 2 message after the time T200XN200, the BTS will send Error Indication (cause: T200 expired) to the BSC, which will send Channel Release (cause: abnormal release, timer expired) to the mobile station and a Clear Request to the MSC. The SACCH is deactivated and the BSC waits RLINKT SACCH periods before the timeslot is released and a new call can use the channel. This is only valid if the call is in steady state, i.e. not during handover or assignment. 3. Release Indication When the BTS received a layer 2 DISC frame from the mobile it replies with a Layer 2 UA frame to the mobile station and a Release Indication to the BSC.The system does only react on Release Indication if it is received during a normal disconnection situation. If such a message is received unexpectedly this will usually cause radio link time-out or timer T200 expiration as the mobile station stops the transmitting of measurement reports. It is also possible that the release will be normal depending on when the Release Indication is received.
  23. 23. 4. MSC Time-Out Normal Release: If the MSC never received a response on a message (e.g. Identity Request) and there is no radio link time-out or layer 2 time-out, the MSC will send a Clear Command to the BSC. The time-out is depending on the message. When receiving Clear Command, the BSC will send a Channel Release (cause: normal release) and then deactivates the SACCH. Reject (only SDCCH): If the MSC never receives a response on the first message after Establish Indication, the MSC will send a reject message. If the connection was a Location Update it will be a Location Update Reject (cause: network failure) and if the connection was a mobile originating call (CM Service Request) a CM Service Reject (cause: network failure) will be sent. The MSC will then send a Clear Command to the BSC and the call is cleared by Channel Release (cause: normal release). 5. Assignment to TCH Before sending an Assignment Command from the BSC at TCH assignment, the following two criterion have to be fulfilled: a. There must be a TCH channel available, i.e. no congestion b. b. The locating algorithm must have received at least one valid measurement report If either of the criterion is not fulfilled, Assignment Command will not be sent and a Channel Release (cause: abnormal release, unspecified) will be sent to the mobile station and a Clear Request to the MSC. From a subscriber point of view, the most serious dropped calls are those that interrupts an ongoing conversation, i.e. a call dropped on the TCH. If the call is dropped on the SDCCH the user simply re-dials again and hopefully succeeds with the new call setup. From a system point of view, the dropped calls on the SDCCH are more serious. A radio link time-out on the SDCCH will occupy an SDCCH sub-channel for approximately (RLINKUP+RLINKT)/2 seconds and increase the risk for SDCCH congestion.
  24. 24. CONERRCNT is one counter of object type MOTS, which indicates abnormally terminated connections on one time slot and is incremented for TCH and SDCCH when a connection is dropped. The counter CONERRCNT should be used to identify problem device and interference on a certain channel. The counter will identify a TS that have more dropped calls than the average TS.
  25. 25. Notice!!! When a call is abnormally disconnected, that is CLEAR REQUEST is sent to the MSC, a check is made in the function Assignment or Handover if any of the following type of urgency state existed. If more than one type of urgency state are indicated by the locating procedure, the following priority (highest priority first) is used to determine the type of urgency state: 1 Excessive TA 2 Low signal strength in downlink and/or uplink 3 Bad quality in downlink and/or uplink 4 Sudden loss of connection When the type of urgency state is determined, one of the counters for this urgency state is stepped. It should be noticed that at most one counter is incremented for a dropped connection (except for low signal strength where at most two counters are incremented).
  26. 26. COUNTERS COUNTERS DESCRIPTION CDIS_OTH CNDROP - CDISSS - CDISQA - CDISTA SDCCH drop due to other reasons CDISQA CDISQA Total SDCCH Drop due to Bad Quality CDISSS CDISSS Total SDCCH Drop due to Low Signal Strength CDISTA CDISTA Total SDCCH Drop due to Excessive Timing Advance CNDROP CNDROP Total Dropped SDCCH Connections CNRELCONG CNRELCONG Released SDCCH connections due to TCH Congestion CNROCNT CNROCNT Total Accepted Random Accesses CNUCHCNT CNUCHCNT Total Number of Defined SDCCH's TFDISQABL TFDISQABL Total TCH Drop due to Bad Quality Both link TFDISQADL TFDISQADL Total TCH Drop due to Bad Quality Down link TFDISQAUL TFDISQAUL Total TCH Drop due to Bad Quality Up link TFDISSBL TFDISSBL Total TCH Drop due to Low Signal Strength Both link TFDISSDL TFDISSDL Total TCH Drop due to Low Signal Strength Down link TFDISSUL TFDISSUL Total TCH Drop due to Low Signal Strength Up link TFDISTA TFDISTA Total TCH Drop due to Excessive Timing Advance TFMSESTB TFMSESTB Total TCH Connections FR TFNDROP TFNDROP Total Dropped TCH Connections FR TFSUDLOS TFSUDLOS Total Sudden Loss TCH and SDCCH DROP CALL COUNTERS
  27. 27. T_DR-C 100 * TFNDROP / TFMSESTB Dropped TCH Connections of Total TCH Connections FR and HR T_DR-S 100 * TFNDROP / ( TFCASSALL + ( SUMIHOVERSUC - SUMIHOSUCBCL - SUMIHOSUCWCL ) - ( SUMOHOVERSUC - SUMOHOSUCBCL - SUMOHOSUCWCL ) ) Dropped TCH Connections of Total Conn. Terminated in the Cell FR and HR T_DR_BQ_BL 100 * TFDISQABL / TFNDROP Dropped Connections at Bad Quality both links of Total Number of Droppe T_DR_BQ_DL 100 * TFDISQADL / TFNDROP Dropped Connections at Bad Quality downlink of Total Number of Dropped T_DR_BQ_UL 100 * TFDISQAUL / TFNDROP Dropped Connections at Bad Quality uplink of Total Number of Dropped TC T_DR_ERL 60 * TFNDROP / ( PERLEN * ( TFTRALACC / TFNSCAN ) ) Dropped TCH Connection per Erlang FR and HR T_DR_ERLM PERLEN * ( TFTRALACC / TFNSCAN ) / ( TFNDROP ) Erlang Minutes per Dropped TCH Connection FR and HR [min] T_DR_HO 100 * ( SUMOHOVERCNT - SUMOHOVERSUC - SUMOHORTTOCH + SUMEOHOVERCNT - SUMEOHOVERSUC - SUMOHORTTOCH ) / ( TFNDROP ) Lost HOs of Dropped TCH Connections FR and HR T_DR_OTH 100 * ( TFNDROP - ( TFDISSBL + TFDISSDL + TFDISSUL + TFDISQABL + TFDISQADL + TFDISQAUL + TFDISTA + TFSUDLOS) ) / TFNDROP Dropped TCH Connections due to Other Reasons FR T_DR_SS_BL 100 * TFDISSBL / TFNDROP Dropped Connections at Low SS both links of Total Number of Dropped TCH T_DR_SS_DL 100 * TFDISSDL / TFNDROP Dropped Connections at Low SS downlink of Total Number of Dropped TCH C T_DR_SS_UL 100 * TFDISSUL / TFNDROP Dropped Connections at Low SS uplink of Total Number of Dropped TCH Con T_DR_SUD 100 * TFSUDLOS / TFNDROP Suddenly lost Connections of Total Number of Dropped TCH Connections T_DR_TA 100 * TFDISTA / TFNDROP Dropped TCH due to Excessive Timing Adv. FR and HR S_DR-C 100 * CNDROP / CMSESTAB Dropped SDCCH Connections of Total SDCCH Connections S_DR_BQ 100 * ( CDISQA + CDISQASUB ) / CNDROP Dropped SDCCH Connections due to Bad Quality S_DR_ERL 60 * CNDROP * CNSCAN / ( CTRALACC * PERLEN ) Number of Dropped SDCCH Connections per Erlang S_DR_ERLM ( CTRALACC * PERLEN ) / ( CNDROP * CNSCAN ) Erland Minutes per Dropped SDCCH Connection [min] S_DR_HO 100 * ( CCHHOCNT - CCHHOSUC - CCHHOTOCH ) / CNDROP Lost Handovers of Total Number of Dropped SDCCH connections S_DR_OTH 100 * ( CNDROP - ( CDISSS + CDISSSSUB + CDISQA + CDISQASUB + CDISTA ) ) / CNDROP Dropped SDCCH Connections due to Other Reasons S_DR_SS 100 * ( CDISSS + CDISSSSUB ) / CNDROP Dropped SDCCH Connections due to Low SS S_DR_TA 100 * CDISTA / CNDROP Dropped SDCCH Connections due to Excessive Timing Advance TCH and SDCCH DROP CALL FORMULA
  28. 28. Dropped Call Analysis 1. Check dropped calls per cell. Select cells with high dropped call rate. 2. Check reason for dropped calls for selected cells 3. Check ratio of lost handovers to dropped calls. The reason is to determine if the high dropped call rate is related to handover performance. Check also which cell relations that have more dropped calls than the average neighbour relation. 4. Check dropped calls per TS to find faulty devices or interference. Check BTS error log Further activities: . Run CTR/MTR/CER on the cell with high dropped call rate. . Perform drive tests and site surveys. . Check frequency plan, coverage and interference plots . Check output power and cell parameter settings
  29. 29. Dropped Calls on SDCCH Radio Link Layer Description Counter Stepped The following cases will step CNDROP. ∀. Error indication (Sequence Error ) and Channel Release, abnormal release: unspecified. A fault has occurred on the radio link layer (GSM 08.58). This message is sent if the Layer 2 sequence counters on the air interface have failed and can not recover. An example when this happens is when the BER is high and the signal strength is low. This is thus caused by bit errors. ∀. Radio Link Time-Out The probable cause is low signal strength or high interference. ∀. Time-Out on Layer 2: T200 Expiration Probable reason is low signal strength or high interference. ∀. Unexpected Release Indication The reason to unexpected release indication could be that the mobile station gets confused when the user behaves strangely .Abnormal Release unspecified
  30. 30. •Release Indication SAPI 3, abnormal release unspecified The system sends an Establish Request SAPI3 to establish a SAPI 3 link for sending an SMS. If the mobile fails to reply on SABM the BTS sends Release Indication and Error Indication: Timer T200. The system responds to this by a Channel Release, abnormal release: unspecified. This case can be caused by low signal strength or high interference. ∀.Release Indication after no response from mobile station, abnormal release unspecified. The mobile does not respond when it should. One case could be when no Setup message is received from the mobile station. After ten seconds the mobile station is releasing the connection. This will cause a radio link time-out. This is probably a mobile station error. ∀.No Assignment Command, abnormal release, unspecified The probably reason is either that no measurement reports are received from the mobile station or congestion, and no available channels. ∀.Unsuccessful Assignment on TCH, abnormal release unspecified The mobile never establishes itself on TCH and failed to return to SDCCH. After a time-out in the BSC, the call is released by sending Channel Release (cause: abnormal release unspecified) to the mobile station. The probable reason is low signal strength or high interference.
  31. 31. The following cases may occur: 1. Assignment Failure is sent, abnormal release unspecified The assignment to TCH failed but the mobile succeeded to return to the old SDCCH. The system disconnects the call after a time-out and sends Channel Release with cause “abnormal release, unspecified”. 2. Layer 2 time-out on Assignment Command The mobile station does not send acknowledge on Layer 2, probably because of low signal strength or bad quality. The mobile is lost. 3. Mobile fails to return to the old SDCCH, abnormal release unspecified The mobile received the Assignment Command, fails to establish contact with TCH and finally fails to return to the old SDCCH. Probable Reasons -. Low Signal Strength on Down or Uplink The reaon for poor coverage could be too few sites, wrong output power, shadowing, no indoor coverage or network equipment failure. Action: Check coverage plots.Check output power. Perform drive tests. Check BTS error log Solution: Add new sites. Increase output power. Repair faulty equipment. Poor Quality on Down or Uplink Action: Check C/I and C/A plots.Check frequency plan.Perform drive tests. Solution: Change frequency. Use available radio features.
  32. 32. Too High Timing Advance Action: Check if the cell parameter TALIM is < 63. Check if the co-channel cells are over-heard. Solution: Set TALIM to a value close to 63.Tilt antenna/reduce antenna height/output power, etc. for co-channel cells. . Mobile Error Some old mobiles may cause dropped calls if certain radio network features are used. Another reaons is that the MS is damaged and not working properly. Action: Check MS fleet. Solution: Inform operator. . Subscriber Behaviour Poorly educated subscribers could use their handsets incorrectly by not raising antennas, choosing ill-advised locations to attempt calls, etc. Action: Check customer complaints and their MS. . Battery Flaw When a subscriber runs out of battery during a conversation, the call will be registered as dropped call due to low signal strength. Action: Check if MS power regulation is used. Check if DTX uplink is used. . Congestion on TCH The SDCCH is dropped when congestion on TCH. Action: Check TCH congestion ⇒chapter 3.3. Solution:Increase capacity on TCH or using features like Assignment to anothercell, Cell Load Sharing, HCS etc.
  33. 33. Dropped Calls on TCH Probable Reasons Radio Network Related Problem Description . Low Signal Strength on Down or Uplink Normally a call is dropped at the border of large rural cell with insufficient coverage. Bad tunnel coverage cause many dropped calls as well as so called coverage holes. A quick reduction in signal strength can be the result of moving into a garage, elevator or even behind a street corner. Bad indoor coverage will result in dropped calls. Building shadowing could be another reason. Action: Check coverage plots. Perform drive tests & site survey Check output power. Check power balance and link budget Check if omni site. Check antenna configuration & type. Check antenna installation Solution: Add a repeater to increase coverage in for example a tunnel. Change to a better antenna (with higher gain) for the base station. Add a new base station if there are large coverage holes.
  34. 34. Handover Performance Handover is a key function in a GSM network. If the handover performance is poor the subscriber will perceive the quality of the network as bad. Handover performance statistics should preferably be measured on 24 hour data or longer. FORMULA : Total number of internal and external outgoing or incoming Handover Attempts Total number of Handover Attempts per cell relation Handover Attempts of total Handover Decisions The following checks should be performed: 1. Check for handover relations with low traffic (e.g. less than 10% of average number of handovers per relation). 2. Check for unbalanced handover relations (e.g. relations which have not approximate the same number of handovers in either direction. Remark: Unbalanced handovers relation indicate problem if measured on whole days, assuming that the same number of people go in and out of the cell coverage area. 3. Check if the reason for unbalanced handover is congestion.
  35. 35. Probable Reasons ∀. Bad Locating Parameter Setting A bad setting of locating control parameters might result that the locating will seldom rank the cell as a candidate. Action: Check parameter setting. Solution:Correct bad parameter setting. ∀. Unnecessary Neighbouring Cell Relation None or very few handovers might indicate an unnecessary neighbouring cell relation. Action: Check neighbour cell relations ⇒chapter 11. Solution:Check if the relations really should be defined. Remove unnecessary cell relations. ∀. The Base Station is Defined But Not in Service. Action: Check reason for BTS not in service. Solution:Take action to set the base station in service. ∀. HW faults. Action: Check BTS error log [8].
  36. 36. ∀. Missed measurement frequencies in BA-list This will cause no handover decisions to the target cell. Action: Check measurement frequencies list. Solution:Add missed frequencies. ∀. Permitted Network Color Code problem If NCCPERM doesn’t include neighbouring cell’s NCC, there will be no handovers. Action: Check NCCPERM. Solution:Add NCC of neighbours to NCCPERM. ∀. Wrong Use of HCS Action: Check HCS related paremeters Solution:Change to normal value. ∀. Poor inter-MSC/BSC handover performance If the cell is at the border of inter-BSC or inter-MSC, poor inter-MSC/BSC handover performance will cause few or no handover attempts. Action: Check inter-MSC/BSC handover performance
  37. 37. Unsuccessful Handovers There can be two reasons why an attempt is counted as unsuccessful: either the mobile station was lost or the call was reverted to the old cell and channel.Unsuccessful handover may lead to a dropped call.Investigate if there is any difference between incoming and outgoing handovers. This can give further reasons of the problem area. The lost handovers are registered at outgoing handover. Note that also dropped call counters are stepped.
  38. 38. Probable Reasons. Congestion A high congestion might lead to dragged calls (handover performed at a not intended location) and a lot of unsuccessful handovers. Action: Check TCH congestion. Solution: Add more TCH capacity. . Timer Expire After MS is Lost The MS never answers the base station. Action: Check coverage Check interference. . Link Connection or HW Failure Action: Check BTS error log.Perform site visit.Perform link performance measurements. Solution:Repair faulty equipment. . Bad Antenna Installation Action: Perform site survey and check antenna installation .Check antenna cabling. Solution:Adjust antenna installation, antenna type or cabling. Antennas Connected to Wrong Feeder Action: Perform site survey and check antenna installation. Check antenna cabling. Solution:Correct the antenna to the right sector.
  39. 39. . Incorrect Down tilt Action: Perform site survey and check antenna installation. Solution:Correct antenna tilting. . The MS Measures Signal Strength of Another Co- or Adjacent Cell than Presumed. Action: Check frequency plan. Perform drive tests. Solution:Decrease interference. . Wrong Neighbouring Cell is Defined. This can happen when a new site is added and old definitions are not undefined and new relations are not inserted. Action: Check parameter setting.Run Undefined Neighbouring Cell Recording. Solution: Define or remove neighbouring cells.
  40. 40. . Wrong Output Power due to Faulty Transceiver. Action: Check output power setting. Check BTS error log . Wrong Locating Parameter Setting. Action: Check locating parameters Solution:Correct parameter setting. . Poor inter-MSC/BSC handover performance. For outer or external cell, wrong definitions in either MSCs or BSC may be reason for the problem. Action: Check inter-MSC/BSC handover performance. Missing Handover Relations Missing handover relations will act as coverage holes. Suddenly the MS can not perform handover to the best server (i.e. not in the BA-list), instead a handover is performed to the second best candidate. Action: Check existing handover relations. Check with actual site positions and coverage plots if more relations need to be added.Run Undefined Neighbouring Cell Recording. Solution:Add missing handover relations.
  41. 41. . Too Many Neighbours Defined Many defined measurement frequencies defined (>16) will decrease the accuracy of the mobile measurements to locate the best six servers. Many measurement frequencies mean few samples per frequency and problem for mobiles to decode the BSIC. Action: Check number of definitions. Solution: Remove unnecessary definitions. . Too Many Measurement Channels Defined. Action: Check number of measurement channels Solution: Remove unnecessary measurement channels. . Bad Use of Radio Network Features Incorrect use of radio features such as Dynamic Power Control, Locating, Intra-Cell Handover, Frequency Hopping, etc. Action: Check Feature parameter setting. Solution:Correct strange and erroneous parameter setting.
  42. 42. . Delayed Handover Decision A delayed handover decision can be due to congestion in the target cell. Action: Check handover parameters. . Bad Radio Coverage Action: Check coverage plots. . High Interference, Co-channel or Adjacent. The potential handover candidate is disturbed by interference.Outgoing handover due to bad uplink quality may indicate interference fromco-channel another MS. On the border, the quality may be rather bad and the signal strength low.Bad downlink quality may indicate interference from another co-channel base station. Action: Check interference. Solution:Change frequency plan. Handover Reversions(HORTTOCH) A handover reversion is when the MS is going back to the old channel. This happens when the MS fails to establish itself on the new traffic channel but succeeds to return to the old traffic channel. If the mobile does not succeed to return it will be lost.
  43. 43. Probable Reasons . Interference The potential handover candidate is disturbed by interference and thus making it impossible to complete the handover signalling.Outgoing handover due to bad uplink quality may indicate interference from co-channel another MS. On the border, the quality may be rather bad and the signal strength low. Bad downlink quality may indicate interference from another co-channel base station Action: Check if many handovers are performed due to downlink or uplink bad quality. . Lower Output Power on TCH than on the BCCH in the Target Cell. Action: Check output power. Check BSTXPWR and BSPWR defination . Not good neighbouring cell relation defined. Action: Delete the relation if not necessary. Receiver Antenna Problem or RBS HW problems (in candidate cell) Action: Check antenna installation. Check RBS HW and Error log of the target cell
  44. 44. Ping-Pong Handovers Successful Handovers back to old cell within 10 seconds of Total successful handovers. Probable Reasons - Wrong Parameter Setting Action: Check handover hysteresis. Check locating parameters Solution:Correct unfortunate parameter setting. Lower Output Power on TCH than on the BCCH in the Target Cell. Action: Check output power. Coverage Holes Coverage holes might lead to Ping-Pong handovers especially for slow moving mobiles. The problem could be shadowing by high buildings and straight streets with good coverage from another nearby base station. Action: Perform drive tests.Check locating parameters. Solution: Add coverage, i.e. a new micro base station. Change locating parameters such as hysteresis values. . No Best Server Several possible equal signals from two or more base stations might lead to a Ping-Pong effect. This is valid especially for slow moving mobiles for example handheld indoor. Often, this situation also causes bad quality and finally a high ratio of dropped calls. Action:Perform drive tests. Solution: Add new base station to increase coverage.
  45. 45. Urgency Handovers The percentage of urgency handovers is a good indicator or interference and from which direction the problem occurs. The wrong setting of Timing Advance Limit will also cause urgency handovers.Always start to check that the setting of the parameters TALIM, QLIMUL/DL as well as the filter length QLEN is correct. Probable Reasons . Wrong setting of TALIM TALIM is the timing advance limit for doing urgency handover. A lower setting of TALIM will cause many handover decisions due to excessive timing advance. Action: Check TALIM. Solution: Change TALIM to normal value. . Bad Coverage There might be a combination with bad coverage and high interference. Action: Check coverage plots. Solution:Improve coverage. A change of frequency might solve the problem but is a more unreliable method as this change might be forgotten at next new frequency plan introduction. . Uplink Interference This might be an interference from other MSs. Action: Check co-channel sites Check if external interference.Check coverage. . Downlink Interference This might be an interference from other base stations. Action: Check co-channel sites Check if external interference Check coverage.
  46. 46. Intra-Cell Handover Performance An intra-cell handover is normally performed in cells with high signal strength and bad quality. Probable Reasons Reasons to unsuccessful intra-cell handovers or abnormally high amount of intra-cell handovers. . Faulty Parameter Setting Action: Check Intra-Cell Handover parameters. Solution:Correct strange or faulty parameters. . Internal Interference Action: Check if uplink or downlink problem.Check co-channel sites Check Dropped Call Performance Check Handover Performance Solution: Change frequency plan. External Interference A cell with only one frequency disturbed but with several tranceivers will ‘get’ interference on all call connections if frequency hopping is introduced.For each call connection, also the disturbed frequency is used in the hopping sequence.A high number of Intra-Cell Handovers indicates that there is interference.The disturbed link could easily be identified by the formulas for up- and downlink interference. To be sure that this is an external problem, a site visit should be performed and RF measurements should be carried out. Action:Check ratio Intra-Cell Handovers to TCH Connections.Check if uplink or downlink problem.Check if frequency hopping is used.Perform Idle Channel Measurements.Perform site visit and drive test.Perform RF measurements in the disturbed cell. Solution:Identify and remove external interference. A temporary solution might be to change the disturbed frequency.
  47. 47. Interference The type of interference should be identified, Internal interference, co-channel or adjacent channel interference, means that the problem exists within the own network due to a bad frequency plan, bad site location, congestion or too high antenna location. . External External interference means that there is another transmitter or something else acting as a transmitter outside the network such as TV transmission, repeaters microwave links, or other mobile systems like AMPS. To solve this problem the operator either has to change affected frequencies to avoid the disturbance or to negotiate with the owner of the source of the disturbance. . Intermodulation If the interference is intermodulation the signal is reflected in an undesired way. There might be problems with the combination of different transmitters on the same base station or faulty equipment.
  48. 48. IMtool+ calculates all intermodulation products of up to 20 transmitters, up to the fifth order. Calcs that result in negative numbers are not valid. The total number of calculations performed and the total number of valid results within the specified range are listed at the bottom of the page. Transmitter A = 952.00000 Transmitter B = 937.40000 Transmitter C = 950.20000 Transmitter D = 952.60000 Transmitter E = 953.00000 Transmitter F = 955.00000 List IM hits between 890.00000 MHz and 960.00000 MHz Results: a = 952.00000 b = 937.40000 c = 950.20000 d = 952.60000 e = 953.00000 f = 955.00000 -a + 2b = 922.80000 -a + 2c = 948.40000 -a + 2d = 953.20000 -a + 2e = 954.00000 -a + 2f = 958.00000 2a + -c = 953.80000 2a + -d = 951.40000 2a + -e = 951.00000 2a + -f = 949.00000 -2a + 3b = 908.20000
  49. 49. BSIC PLANNING (NCC + BCC )