Accessibility/Retainability

2G TROUBLESHOOTING

DCR (Retainability)
There are two types of call drops given below:
>Call ...
Accessibility/Retainability

2G TROUBLESHOOTING

Drop Call Rate está relacionado con los sigs KPIs:
>TCH Drop Rate
>HO Suc...
Accessibility/Retainability

2G TROUBLESHOOTING

DCR (RETAINABILITY)
Causas:
>Low SS UL
>Low SS DL
>Sudden
>RxQual UL
>RxQ...
Accessibility/Retainability

2G TROUBLESHOOTING
Unsuccessful Handovers

D

Check congestion
performance

Is the cell at in...
Accessibility/Retainability

2G TROUBLESHOOTING

MS Paging response- After receiving the immediate
assignment command, MS ...
Accessibility/Retainability

2G TROUBLESHOOTING

SDCCH Congestion(Accessibility)
SDCCH Activities:
>Mobility Management
–
...
Accessibility/Retainability

2G TROUBLESHOOTING

SDCCH Drops(Accessibility):
RANDOM ACCESS SUCCESS RATE (Accessibility):

...
Accessibility/Retainability

2G TROUBLESHOOTING
Not Approved Random Accesses

Check BSIC allocation

Check frequency plan
...
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Kpi 2g troubleshootin

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Kpi 2g troubleshootin

  1. 1. Accessibility/Retainability 2G TROUBLESHOOTING DCR (Retainability) There are two types of call drops given below: >Call drop over SDCCH: Indicating the call drop occurs in the course during which BSC assigns a SDCCH to an MS but a TCH has not been successfully assigned yet . >Call drop over TCH: Indicating the call drop occurs after BSC assigns a TCH to MS successfully. The Channel currently occupied is of TCH type when BSC sends a clear Request message to MSC. The cause values for sending clear Request are as follows: > Radio Interface Message Failure >O&M Intervention > Equipment Failure >Protocol Error Between BSS and MSC >Preemption Causes of Drop Call: >Coverage >HO >UPLINK /DOWNLINK CAUSED BY ANTENNA & FEEDER SYSTEM >Interference >Transmission Failure Coverage: Discontinuous coverage (Blind area): >Call drop is caused by isolated BTS. As the signal is of weak strength and poor quality at the edge of an isolated BTS, handover to other cells cannot be implemented, and thus call drop occurs. >If BTS lies in the place where the landform is intricate and radio propagation environment is complicated (e.g., a mountainous area), it may cause call drop owing to discontinuous coverage. Poor Indoor Coverage : >In the place where many buildings are located, call drop easily occurs due to high transmission attenuation, low indoor level and great penetrate loss. Beyond Coverage : >Owing to some reasons, the coverage of a serving cell is beyond the defined coverage. For example, the power in cell A is so high that a MS still occupies the signals of cell A after it moves out of the coverage of the adjacent cell B that has been defined by cell A and reaches cell C. >However, cell A has not defined cell C as an adjacent cell yet, so at this time the MS cannot find a proper cell when it tries to perform a handover according to the adjacent cell B provided by cell A, thus call drop occurs. Shortage of coverage : It may be caused by some equipment failure in a cell. For example, the antenna is obstructed or the carrier taking BCCH (power amplifier) gets faulty. Handover: Unreasonable Parameters : >For example, if the level of the handover candidate cell is set to be too low and the handover threshold is set to be too little, some MSs will be handed over to the adjacent cell when the level of the adjacent is a little stronger than that of the serving cell for a time. >But after a while, if the signal of the adjacent cell faint, and it happens no proper cell is available for handover, call drop could occur. See example 6 for call drop resulting from improper settings of handover parameters. Adjacent cell undefined: >If an adjacent cell has not been defined yet, MS will keep communicating in the serving cell until it goes out of its coverage. At this time, call drop shall occur since MS cannot be handed over to a cell with stronger signals. >Existence od adjacent cells with same BSIC and BCCH Frequency . Traffic Congestion : >Unbalance of traffic may cause handover failure due to lack of handover channel available for the destination BTS. When reestablishment of handover channel fails too, call drop occurs. >BTS Clock out of synchronization and frequency offset beyond limits ,which can cause handover failure and call drop . Interference >There are co-channel interference, adjacent-channel interference and inter-modulation interference. When MS receives signals in the serving cell with strong co-channel or adjacent-channel interference, it may aggravate BER and make MS cannot accurately demodulate BSIC of the adjacent cell or BTS cannot correctly receive measurement reports of MS. >The interference threshold is set as co-channel carrier-tointerference ratio C/Iƒ9dB and adjacent-channel carrier-tointerference ratio C/Aƒ9dB. When the interference index is so bad that it exceeds the threshold, conversations in network shall be interfered, thus conversation of poor quality and call drop might occur. Causes UL /DL caused by Antenna & Feeder System >Improper installation of antenna and/or feeder. For example, the Tx antenna between two cells is installed just reversedly, which shall make the uplink signal level is much poorer than the downlink one, thus cause call drop, single pass or difficult connection occurring far from the BTS. >If single polarization antenna is adopted, a cell has two sets of such antennas. If their azimuths are different, call drop might occur. • A directional cell has a main antenna and a diversity antenna, so it is possible that BCCH and SDCCH of this cell come from the two different antennas. Different azimuths will cause different coverage, consequently, although the user can receive BCCH signal, it cannot occupy SDCCH sent by another antenna when originating a call, thus call drop occurs. >As there are Abis interface and A interface link, poor quality transmission and unstable transmission link also may cause call drop. • Observe transmission and board alarms (e.g., FTC failure alarm, A interface PCM out of sync alarm, LAPD link break alarm, power amplifier alarm, HPA alarm, TRX alarm, CUI/FPU alarm). Based on alarm data, analyze whether transmission is intermittent or whether there are faulty boards (e.g., the carrier board is faulty or in poor contact). • Check transmission paths, test BER and check whether E1 connector or grounding of equipment is reasonable, thus decrease call drops by ensuring stable transmission quality. • Observe whether there are too many call drops caused by transmission problem via traffic measurement. a) in TCH performance measurement of traffic measurement observe whether there are too many A interface failures when TCH is occupied. b) In TCH performance measurement observe whether the TCH availability rate is abnormal. c) In TCH performance measurement observe whether there are too many call drops caused by interruption of terrestrial link.
  2. 2. Accessibility/Retainability 2G TROUBLESHOOTING Drop Call Rate está relacionado con los sigs KPIs: >TCH Drop Rate >HO Success Rate – – – TCH Drop Call (Retainability): The formula for measuring TCH Drop Rate (Optimi, Movistar, Ericsson <Ericsson uses more parameters in case of denominator who make reference to HO>) is: = _ _ + + + + + + – – – – – Interference on Uplink/Downlink Excessive TA Incorrect/Not optimum Parameter settings for example power regulation Missing Ncells or Congestion in Ncells MS or MS Battery problems Subscriber Behavior Antenna/Hardware or Transmission faults Incorrect Installations Check site location & TALIM YES NO Check lost handovers Most dropped Call during HO? Perform handover analysis YES NO Check site position YES Remove site or change frequency NO Check BTS Error Log Check locating parameters HW fault? TFNDROP The total number of dropped full-rate TCH in UL subcell. TFNDROPSUB The total number of dropped full-rate TCH in OL subcell. THNDROP The total number of dropped half-rate TCH in UL subcell. TCHNDROPSUB The total number of dropped half-rate in OL subcell. THCASSALL Number of assignement complete messages for all MS power classes in UL subcell, half-rate. THCASSALLSUB Number of assignement complete messages for all MS power classes in OL subcell, half-rate. TFCASSALL Number of assignement complete messages for all MS power classes in UL subcell, full-rate. TFCASSALLSUB Number of assignement complete messages for all MS power classes in OL subcell, full-rate. Bad parameter setting? YES Correct parameter setting Swap & repair HW YES NO Check link quality and Synchronization NO Transmission fault? Check radio network features Perform link investigation YES NO Power regulation used properly? NO Correct power regulation parameters Best server exists? Add site NO YES Check Dropped Calls per TS basis YES Check output power Check MS fleet Perform site survey Power balance? NO Adjust output power DTX used? NO Introduce DTX YES NO Activate frequency hopping YES Missing neighbours? YES Run NCS NO Check dropped call reason Bad quality? YES Perform interference analysis NO NO A Low signal strength Perform drive tests Perform MTR/CTR recordings YES Frequency hopping used? The TCH Drop Call reasons has the following priority order: >Excessive TA – TA > TALIM cell parameter >Low signal strength in downlink and/or uplink – SSUL < LOWSSUL (BSC parameter) – SSDL < LOWSSDL >Bad quality in downlink and/or uplink – RxQualUL> BADQUL (BSC parameter) – RxQualDL> BADQDL >Sudden loss of connection – None of the above >Other – Non radio-related TCH Drop Call Optimization Analyze the following issues that could be possible reasons for poor TCH Drop performance: – Low Signal strength on Uplink/Downlink High timing advance Dragoon site? Dropped Calls on TCH 100 A YES Perform low signal strength analysis Check antenna installation END
  3. 3. Accessibility/Retainability 2G TROUBLESHOOTING DCR (RETAINABILITY) Causas: >Low SS UL >Low SS DL >Sudden >RxQual UL >RxQual DL >Others RxQual UL: Causas: Una celda A radia e interfiere el área de cobertura de una celda B. Los MS conectados a la celda B están interfiriendo a los MS conectados en la celda A. Checar: TA - Time in advance RxQual DL: Causas: Una BTS A radia e interfiere el área de cobertura de una celda B y la BTS A interfiere a los MS conectados a la BTS B. Checar: Cobertura de BTS A y B TA – Timing Advance Asignación de frecuencias HCN Asinación de MAIO HW – Posible radio dañado Low SS UL: Checar: TA relacionado con problema en TMA. Observar gráfica de SS_DL y SS_UL si se mueve un valor de 12dB por lo tanto si es TMA. Low SS DL: Checar: HW - Antena dañada HW - VSWR con valor alto HW - TRX dañado Sudden: Checar: Problema relacionado directamente con HW Others: Checar: Corrimientos en enlace Transcoders de la BSC HOFR (RETAINABILITY) The formula for measuring HOFR (Optimi, Movistar) is: = ATT= HOVERCNT SUCC=HOVERSUC − 100 sum of HO attempts sum of HO Successful Handover in Ericsson BSS system is controlled by what is called as the locating algorithm in the BSC. Locating algorithm i operates on the basis of Measurement Reports (MR) sent in by the MS on SACCH. The inputs that the BSC uses for making a handover decision, from the received MRs from the MS is the DL signal strength, DL quality, and the signal strength of the six best reported neighbours. From the serving BTS, for the same MS the BSC will use UL signal strength, UL quality and TA. Possible reasons for poor HO Performance: >Neighbor Cells impact the following radio network areas: Coverage Interference Incorrect parameter settings Unnecessary or Missing NCELL relations Hardware or Link/Transmission Faults Missing Frequencies in the Measurement list Poor Inter MSC/BSC handover performance TCH Congestion Frecuency plan, specifically for BCCH carriers The values of the hysteresis The filter lengths The BSIC planning The length of the active mode BA list The defined nieghbours The settings of parameters controlling TA and BQ urgency HO The setting of parameters controlling Assignment to other cell The values of the layer change thresholds when more than one Hierarchical layer si used >Handovers for Downlink Quality -The HO is performed because DL conditions on the serving cell are becoming too poor, there the BSC selects the best neighboring cell as the target for the HO Investigation -Too strict configuration of the Handover parameters -Hardware problems : Especially if the rates are high (around 100%) it can be due to the radio parts of the BTS on the downlink (TRX, transmission antenna, feeders ...) -GSM interference : co-channel or adjacent channel -Radio Coverage : HO for UL Quality as well as HO for Level reasons (UL and/or DL) shall be observed on the cell. >Handovers for Uplink Quality- The HO is performed because UL conditions on the serving cell are becoming too poor, there the BSC selects the best neighboring cell as the target for the HO >Investigation -Bad parameters settings -If rates are high, probably hardware problem on the radio uplink transmission part (TRX, antenna, diversity, feeders) -Coverage problems(if also associated to HO for uplink level) -Jamming on the uplink (if confirmed by bad RACH efficiency) -GSM interference : if there’s an equivalent rate of handovers for downlink quality >Handovers for Downlink Level -The HO is performed because DL conditions on the serving cell are becoming too poor, there the BSC selects the best neighboring cell as the target for the HO >Investigation -In normal cases, this should be very rare so if the rates are high it is probably: Bad parameters settings or Hardware problems on the downlink >Handovers for Uplink Level -The HO is performed because UL conditions on the serving cell are becoming too poor, there the BSC selects the best neighboring cell as the target for the HO >Investigation -Insufficient coverage (if associated also with an equivalent rate of uplink quality) -Bad handover parameters settings for the cell -Hardware problems (high rates) if the downlink and the uplink are not balanced -Could come from ping pong handover between two cells (one leaves one of the cells for quality, then comes back for level and so on, for example)
  4. 4. Accessibility/Retainability 2G TROUBLESHOOTING Unsuccessful Handovers D Check congestion performance Is the cell at inter-MSC border? TCH congestion? Add TCH capacity YES YE S Check inter-MSC HO performance NO _ _ Check BTS Error Log NO Check SAE setting SW congestion? HW fault? YE S Swap & repair HW Increase SAE YES NO NO Check link quality Check Neighbouring cell definitions Wrong cells defined? YES Remove incorrect definitions Transmission fault? YES Add missing relation NO NO Too many neighbours? Improve transmission YE S Perform site visit NO Missing neighbours? Remove unnecessary relations YES NO Too many measurement channels? Check antenna installation Antenna connected to wrong feeder? YES YE S Change feeder Review and correct the defined MBCCHNO NO NO Incorrect downtilt? Check Locating parameters YE S Correct tilting NO Strange or corrupt parameter setting? YES Correct parameters Hided antenna? YE S Change antenna position YE S Correct installation NO NO C Paging Success Rate (ACCESSIBILITY) The formula for measuring Paging Success Rate (Optimi, Movistar) is: Bad antenna installation? C NO = + − + + 100 PAGPCHCONG Number of paging messages discarded due to full cell paging queue PAGETOOOLD Number of paging messages discarded due to being too long in the paging queue. At the point when a page is taken from the paging queue, its age is calculated and compared to the BTS parameter AGE-OF-PAGING (the parameter is set to 5seg in Ericsson BSS). If it is too old, it is discarded and PAGETOOLD is incremented. THCASSALL Number of assignement complete messages for all MS power classes in UL subcell, half-rate. THCASSALLSUB Number of assignement complete messages for all MS power classes in OL subcell, half-rate. TFCASSALL Number of assignement complete messages for all MS power classes in UL subcell, full-rate. TFCASSALLSUB Number of assignement complete messages for all MS power classes in OL subcell, full-rate. END Delayed handover YES decision? Check handover parameters ACCESSIBILITY NO Ability for mobiles to set up calls. Check use of radio features Are all radio features used? YES Introduce unused features NO Check coverage Check interference Probable causes related to accessibility: Congestion due to amount of traffic Interference (Quality of network) Hardware or Transmission issues Software file congestion Coverage (no coverage) Perform drive test · Timer Expiry After MS is Lost YES Low SS on cell border? YES Improve coverage NO NO Bad quality? D Call attempts Start from: Successful random accesses To: successful immediate SDCCH assignements Finally: Successful assignement on TCH YES Reduce interference for potential candidate Accessibility includes de following KPIs: Paging Success Rate Location Update Success Rate Random Access Failure SDCCH Time Congestion TCH Assignment Failure Paging process: > Step-1 In response to an incoming call, the MSC initiates the paging process by broadcasting a “paging request” message on the paging sub channel (IMSI or TMSI of the MS and its Paging Group) and starts timer T3113. A “paging message” consists of the mobile identity (IMSI or TMSI) of the MS being paged and its “paging group number”. >Step-2 The BSC receives this page and processes the paging request and schedules it for transmission on the PCH at appropriate time. >Step-3 The MS on its part will analyse the paging messages sent on the paging sub channel corresponding to its paging group. >Step-4 Upon receipt of a page at the MS, the MS responds by transmitting a channel request on the RACH. >Step-5 BSS in response to the received “channel request”, will process it and immediately assign the MS a SDCCH (immediate assignment / assignment reject; done over AGCH). >Step-6
  5. 5. Accessibility/Retainability 2G TROUBLESHOOTING MS Paging response- After receiving the immediate assignment command, MS switches to the assigned “SDCCH” and transmits a “Paging Response”. >Step-7 The establishment of the main signalling link is then initiated (E1) with information field containing the “PAGING RESPONSE” message and the “paging response” is sent to the MSC. >Step-8 Upon receipt of the “Paging Response” MSC stops the timer T3113. If the timer T3113 expires and a “Paging Response” message has not been received, the MSC may repeat the “Paging Request” message and start T3113 all over again. The number of successive paging attempt is a network dependent choice >Paging Capacity for BTS: MFRMS*(CCCH blocks - AGBLK) * X (X=4 when TMSI based paging is used & X=2 when IMSI based paging is used) When the rate of “paging load” at the RBS becomes higher than what the RBS is able to handle (paging capacity of RBS), RBS will start discarding pages (check for high “page discard” stats at the cell level). Paging Queue Length = 14 – (Number of Paging Groups/10) Which means higher the number of paging groups in a cell (that is higher the MFRMS settings), lower will be the Paging Queue length, this will compensate for lower number of Paging Queue available at lower MFRMS. One “Paging Queue” per “Paging Group” is available at the cell level (that means when we decrease the number of paging groups for the cell by reducing MFRMS to lower numbers, we actually reduce the number of available paging queue). >Paging capacity in BSC: Paging Bottleneck at the BSC usually is the number of RP signals that can be sent from the CP to the RPDs in the TRHs. Calculations for number of pages per second: NO _ PAGE= RPSIG NO _ TRH ⋅RPp With “more” TRHs the paging capacity for the BSC decreases. With “more” LACs within a BSC the paging capacity for the BSC increases. Analyze the following issues that could be possible reasons for poor Paging Performance: – Insufficient coverage – High interference – Non-optimum Paging Strategy – Non-optimum Paging parameter settings – Paging Congestion in MSc, BSC or BTS – De-activated or incorrect use of Paging features The following actions can improve Paging Congestion: MSC Paging Congestion Increase SAE BSC/BTS Paging Congestion Check Paging Strategy Increase number of Location Areas (This will increase SDCCH load) Set BCCHTYPE to NCOMB Use TMSI paging requests Ensure IMSI attach/detach is activated (ATT = yes) Decrease T3212/BTDM (This will increase SDCCH load) (2) RPSIG Maximum number of RP signals per second TRXpTRH Average number of TRXs per TRH NO_LA Number of Location Areas NO_TRH Number of TRHs belonging to the BSC NO_TRX Number of TRXs belonging to the BSC NO_CELLS Number of cells RPp Probability that an RP signal is sent to an TRH NO_PAGE Number of pages per second Low Location Updating Success border cell? Yes Reallocate to another LA or MSC Increase CRH No Low CRH Hysterisis? Yes Increase CRH No Low Paging Success Yes Re-dimensioning LA No No TMSI Attach / Detach? Yes Activate Short perodic registration? Yes Check T3212 in BSC & BTDM in MSC No No Wrong periodic location updating setting? Yes Correct T3212 in BSC, BTDM & GTDM in MSC No SDCCH Congestion? Paging congestion? Interference? Yes Increase SDCCH channel Adaptive configuration feature Plan more LA Use TMSI paging Yes Activate No Implicit detach after not periodic registration not use? No END Yes Activate Yes Correct settings for SAE 500 Block MLUAP, MLCAP,MLVAP,MMMLR Yes Increase SDCCH channel Adaptive configuration feature No Automatic deregistration? No TMSI paging not use? Check frequency plan Activate radio feature No SDCCH Congestion? Yes Yes No Software File Congestion? TRXpTRH RPp=1−1− NO _ CELLS  NO _ TRX ⋅NO _ LA     Location Area (LA) dimensioning strategies: >Size of a LA must not exceed the maximum paging capacity for the BTS / BSC >In a rural area, it is easy to find LA border cells. But there’s no reason to have a smaller LA than required >General rule of thumb: 1 LAC per BSC >If a BSC covers a large area with high traffic, consider splitting the LA. This will reduce the paging load in BTS and BSC >In larger cities, SDCCH load will be higher for LA border cells. If difficult is experienced to find LA border cells and the BSC coverage area is small, several BSCs can share one LA >Define LA border at cells with low subscriber density Bad LA Demensioning? No (1) Location Update (ACCESSIBILITY) END Yes Activate automatic de-registration
  6. 6. Accessibility/Retainability 2G TROUBLESHOOTING SDCCH Congestion(Accessibility) SDCCH Activities: >Mobility Management – Normal Location update – Periodic Registration – IMSI Attach / Detach – Connection Management – Call setup – SMS Point to Point – Fax Setup SDCCH Channel Allocation: >Channel Configuration can be done as follow – Combined BCCH/SDCCH on 1 TS (SDCCH/4) – Non-Combined BCCH and SDCCH on 2 TS (SDCCH/8) – Cell Broadcast Channel (CBCH) – The Cell Broadcast service provides the transmission of an SMS from a message-handling centre to all MSs in the serving area of the BTS. – If the Cell Broadcast service is active in a cell, one signaling sub-channel is replaced by one CBCH resulting in a SDCCH/7 SDCCH Channel Allocation Profile (CHAP) >SDCCH Channel Allocation Profile (CHAP) – Channel Allocation is the feature that selects and allocates suitable channels when one or more channels are required – Channel Allocation Profile (CHAP) is the parameter that provides different channel allocation strategies – For SDCCH assignments, CHAP 8 prioritize new assignments as follows: 1) OL/SDCCH 2) UL/SDCCH (if OL/SDCCH is congested) SDCCH Dimensioning Strategy: >GOS Approach: – SDCCH should be dimensioned for better GOS compare to TCH. Typical range is 0.5% - 1%. – Immediate Assignment of TCH with “TCH as last option strategy” is recommended. – It is recommended to add SDCCH when 0.5 Erlang of signaling traffic is carried by TCH. – Half Rate penetration, usage of Extended Range cell and HSCSD should be taken into consideration. >STS Approach: Accurate dimensioning is achieved by using STS cell statistics SDCCH Congestion Optimization: SDCCH Congestion >Analyze the following issues that could be possible reasons for SDCCH Congestion: – SDCCH Dimensioning – Incorrect use of SDCCH capacity features such as Adaptive Configuration of Logical channels – TCH Congestion – Location Areas not optimized – SMS usage and Cell Broadcast channel – SDCCH Availability >The following recommendations will have a positive impact on SDCCH congestion – Try not to use combined BCCH/SDCCH (SDCCH/4) – Use the optional SDCCH capacity features such as the Adaptive Configuration of Logical Channel feature when available. – Use Immediate Assignment on TCH, SDCCH first. – For manual dimensioning, use STS cell statistics – Configure one SDCCH on the BCCH carrier and the others on the hopping layer (Non-BCCH) – Use the Ericsson SDCCH Dimensioning guideline. >The following issues are worth taking into account when analyzing SDCCH Congestion: – Use LAPD STS statistics to evaluate LAPD congestion and optimize the LAPD Concentration factor – Analyze border cells with a large amount of random access attempts resulting in SDCCH congestion. >Use Random Access optimization to solve the SDCCH congestion – Take into account Timeslot priority when deciding where to configure SDC Check HW availabilty Low availability? YES See TCH & SDCCH Availability NO Check site position Location area border? YES YES NO Check and increase CRH Change Location Area Border Check TCH traffic TCH Congestion? YES Add TCH capacity NO Check SMS activity Many SMS messages? YES Redimension SDCCH NO Check periodic registration Too frequent registration? YES Change registration interval timers NO E E Check channel configuration Combined SDCCH? YES Make channel non-combined NO Is cell broadcast used? YES Avoid cell broadcast if possible NO Check traffic trend Short term traffic growth? YES No activity NO Check SDCCH mean holding time Long mean holding time? YES Check HW and number of False Accesses NO Check SDCCH dimensioning Underdimensioned SDCCH? YES Redimension SDCCH Check Adaptive configuration of logical channel and parameters ACSTATE off? NO END YES Switch on
  7. 7. Accessibility/Retainability 2G TROUBLESHOOTING SDCCH Drops(Accessibility): RANDOM ACCESS SUCCESS RATE (Accessibility): Check SDCCH Dropped Per Cell The formula for measuring SDCCH Drop Rate(Ericsson, Optimi) is: _ _ = − − The formula for measuring SDCCH Drop Rate(Ericsson) is: Check drop reason 100 CNDROP The total number of dropped SDCCH channels in a cell CNRELCONG Total number dropped (released connections on SDCCH due to TCH or Transcoder congestion in UL subcell. CNRELCONGSUB The subset d for OL subcell CMSESTAB Total number of successful MS channel establishment on SDCCH. SDCCH Drops are classified into one of the following categories: >SDCCH Drops because of Timing Advance >SDCCH Drops because of Low Signal Strength >SDCCH Drops because of Bad Quality >SDCCH Drops due to Other Reasons SDCCH Drop Optimization >Analyze the following issues that could be possible reasons for SDCCH Drops: – Low signal strength – Interference – Pathloss Imbalance between UL/DL – High Timing Advance – MS error or Subscriber behavior – TCH Congestion – Transmission Congestion (LAPD Concentration) – Hardware or Transmission failures >The following issues are worth taking into account when analyzing SDCCH Drops: SDCCH Drops and TCH Drops often drop because of the same RF reasons, such as for example insufficient coverage – Only use SDCCH power regulation and SDCCH HO when the network has good coverage – BSC and transmission problems can impact SDCCH drops so investigate Transcoders, A-interface and LAPD – SDCCH Drops on the hopping layer are normally showing worse values than SDCCH on the BCCH Bad quality? • • YES • • Check ICM Check frequency Plan Run MRR & FAS Frequency scan NO Low SS? YES • Refer to Low SS analysis flowchart. NO YES High TA? • Check site location & TALIM NO YES TCH Congestion ? • • Check S_DR-S_NTC Increase TCH or using Radio Features, CLS, HCS NO Check BTS error log HW fault? YES • Swap & Repair HW NO = 1− + + 100 RAACCFA Number of Failed Random Accesses. This counter is incremented for a Random access received with too high TA, values that are not used or in case of "software file congestion" (i.e. when the internal storage area in the BSC is full which is a very rare case only occurring at very high loads CNROCNT Number of Failed Random Accesses. This counter is incremented for a Random access received with too high TA, values that are not used or in case of "software file congestion" (i.e. when the internal storage area in the BSC is full which is a very rare case only occurring at very high loads. PDRAC The counter value is incremented when a 44.058 CHANNEL REQUIRED containing 44.018 CHANNEL REQUEST with establishment cause "One Phase Packet Access" or "Single Block Packet Access" is received on RACH. B B Check Link Quality YES • Transmission fault? NO Check MS fleet Perform Drive Test Perform MTR/CTR/MRR Perform site survey Check antenna installation END Perform link investigation The measurements are done per cell for random access channels. For every cell there are counters registering the number of random access attempts received in the BSC. Random Accesses can be distributed into the following types: >MS Originated RA >Call Re-establishment >Emergency Calls >Answer to Paging >All Other Cases – IMSI Attach/Detach – Location Updates – Period Registration >Analyze the following issues that could be possible reasons for high Random Access failure rate: -High interference -Lack of Coverage -High Timing Advance -Handover access burst mistaken as random access burst -Non-optimized parameter settings -Software Congestion (SAE)
  8. 8. Accessibility/Retainability 2G TROUBLESHOOTING Not Approved Random Accesses Check BSIC allocation Check frequency plan Access burst from another co-channel cell YES Change BSIC or frequency plan NO Check cell parameter setting MAXTA too low? Increase MAXTA YES NO Check site location High located site? YES Consider tilting or lowering site NO Check SAE (COFLP, DBTSP:TAP=SAACTION;) Software file congestion? Increase SAE YES NO Check interference Check if unknown access code High noise floor? YES Reduce interference NO SUMEIABSUCC Sum of Successful External Assignment Handovers to Better Cell (Incoming Handover) SUMEIAWSUCC Sum of Successful External Assignment Handovers to Worse Cell (Incoming Handover) SUMIABSUCC Sum of Successful Internal Assignment Handovers to Better Cell (Incoming Handover) SUMIAWSUCC Sum of Successful Internal Assignment Handovers to Worse Cell (Incoming Handover) The TCH assignment success rate measures how often a call set-up, ordered by the MSC, has been successful >The major issues impacting TCH assignment failures are: -TCH congestion -Radio reasons such as coverage and interference -Hardware or transmission problems -Congestion can be defined as follow: -The probability that a call attempt cannot be handled successfully, because of not enough free circuits and other switching equipments -The Congestion (Blocking) Probability represented by B (n, A) is determined by Offered traffic A and the number of available circuits n END TCH Assignment Failure (Accessibility): The formula for measuring TCH Assignment Failure (Ericsson) is: = + + !" " + #$ − %&'" + #$ − %&'" + #$$ ( 100 + #$$ outg(AB+AW)=SUMEOABSUCC+SUMEOAWSUCC+SUMOABSU CC+SUMOAWSUCC inc(AB+AW)=SUMEIABSUCC+SUMIAWSUCC+SUMIABSUCC+S UMIAWSUCC SUMEOABSUCC Sum of Successful External Assignment Handovers to Better Cell (Outgoing Handover) SUMEOAWSUCC Sum of Successful External Assignment Handovers to Worse Cell (Outgoing Handover) SUMOABSUCC Sum of Successful Internal Assignment Handovers to Better Cell (Outgoing Handover) SUMOAWSUCC Sum of Successful Internal Assignment Handovers to Worse Cell (Outgoing Handover) Congestion Improvement >Congestion can be improved by short, medium and long term actions >Medium and Long term: – Expansions: Add capacity to existing cells – Sectorize omni cells – Indoor and capacity microcells – Additional macro sites >Short term: – Optimization of access parameters – Activation and optimization of capacity features Congestion Improvement – Expansions Expansions need to be planned in advance – Before capacity is needed Important for operators to have an expansion procedure in place Expansions can be limited by a variety of issues, for example available spectrum The following presentation take a more detailed look at traffic and capacity management - Overlaid/Underlaid subcells Cell Load Sharing (CLS) AMR Half-rate (AHR) Assignment to Worse Cell Hierarchical Cell Structures (HCS) Analyze the following issues that could be possible reasons for TCH Assignment failures: Low Signal strength or no dominant serving cell Interference Congestion on TCH Incorrect use of Capacity features Faulty Hardware or transmission Incorrect Output power It is worth taking into account the following issues when optimizing TCH Assignment failures: Make sure that problems with network availability are not the cause of congestion Often congestion problems are caused by network design issues such as cells covering a large area Try to make sure that as far as possible calls are made on the best serving cell to reduce the negative impact on Retainability and Voice Quality Radio capacity features such as for example CLS should preferably be only used as short or medium term solutions for congestion. Low TCH Assignment Success Rate Check TCH congestion Congestion on TCH? Add tranceivers or BTS YES Correct parameters NO Check output power YES Low output power? YES Check output power parameters Corrupt parameter setting? NO NO Check BTS Error Log Check Signal Strength of BCCH and TCH NO HW fault? Swap & repair HW YES YES Check coverage plots Adjust TCH output power Perform drive tests Dominant server exists? NO Add BTS YES Check Interference Congestion Improvement – Short Term >The following features can be used to ensure efficient use of installed capacity and to reduce congestion: Low SS for call access? NO Disturbance on SDCCH or target TCH? NO END YES Improve & adjust frequency plan
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