GSM TCH Assignment Failure & SolutionsZte university

Training goalsThis guidebook mainly describes TCH allocation failure and investigation of the problem.

ContentsTCH Assignment Failure & InvestigationRelevant Counters & Assignment success RateTypical Cases

Definition of TCH assignment failureIt refers to the number of TCH call attempts after MS occupies SDCCH;The statistics counted is the number of Assignment Request sent from MSC to BSC. number of busy hour TCH call attempts (excl. HO) It refers to the number of calls when MS fails to occupy TCH after it seizes SDCCH. The statistics gathered is Assignment Failure + Clear Request (radio interface failure). Allocation failure may be caused due to lack of TCHs or problems on lower layers. Number of busy hour TCH assignment failures (excl. HO) MS fails to access into TCH after it sends out request for TCH assignment to BSC.

Definition of TCH assignment failure

BTS

BSC

Assignment Command

MSC

Channel Activation

Channel ActivationAck

Ms

Assignment Request

Assignment Failure2

Assignment Command

SABM

UA

Assignment Complete

Assignment Complete

Assignment Complete

Assignment Failure3

Assignment Failure1

Remarks:Assignment Failure1BSC cant complete MSCs request for TCHAssignment Failure2BTS cant complete MSCs request for TCH

Assignment Failure3MS is not able to occupy TCH assigned by BSC

Main causes of TCH allocation failureTraffic congestion in cellsHardware problemCo-channel or adjacent channel interference Antenna feeder problem Unreasonable setting of parametersTransmission problem at A interface or Abis interface Influence of repeater TCH allocation failure

Problem handling proceduresCheck hardware Check if traffic congestion exists Check if radio parameters areset reasonable Check indicators like BER, idle interference band class Check antenna system TCH assignment failure

ContentsTCH Assignment Failure & InvestigationRelevant Counters & Assignment success RateTypical Cases

Relevant counters_V2C11609 total number of attempts for TCH (excl.HO)C11653 total number of successful TCH assignments (excl.HO)Definition: It represents the total number of attempts to acquire TCHs (seizure attempts) after all SDCCHs in cells are seized. It includes MOC and MTC, and very early assignment of TCH as SDCCH, but handovers are excluded. TCH falls into TCH/F and TCH/H.Definition: It represents the number of received activation response messages after channels are activated for TCH assignment (signaling, speech and data).

Relevant counters_V2C11654 total number of TCH allocation failures (excl.HO)total number of unsuccessful TCH assignments (excl.HO)Definition: It represents the number of negative replies in response to TCH activation request for assignment or the number of activation acknowledge which havent been received (signaling, voice and data).Definition: It represents the number of negative replies in response to TCH assignment command (signaling, speech and data), or the number of unreceived Establish Indication or Assignment Complete.

Relevant counters_V3 & TCH assignment success rate

ContentsTCH Assignment Failure & InvestigationRelevant Counters & Assignment success RateTypical Cases

Low TCH allocation success rate in a cellIndicator statistics at OMCR showed that TCH allocation success rate was low in Cell2 under BTS D1A. Checked radio parameters, but no problem was found. From the dynamic management at OMCR, we found that one TRX in the cell couldn't be seized. We confirmed the TRX has problem. After the TRX board with problem was replaced, TCH allocation success rate increased greatly and problem was solved. 123Problem description Problem analysisAdjustment

Low TCH allocation success rate in a cell

Antenna connection problem TCH allocation success rate was low in Cell1 in an area, which was below 94.0%. From analysis of OMCR performance indicators, we found the problem was caused by the low TCH assignment success rate at air interface. From further analysis, we found the assignment failure rate of TRX3 was high (Maio=2), and DL signal was weak Hardware engineers went to the site and checked, finding that antenna system was connected wrong (the TRX3 of Cell1 was connected to the antenna of Cell2.)123Problem descriptionProblem analysisAdjustment

Antenna connection problem

Antenna connection problem

Change LAPDTCH assignment success rate was low in three cells under a BTS (S111), just around 10%. We made CQT on site and found that call completion rate was very low during idle hours, just one or two out of ten was completed. From hardware checking, no problem was found. Therefore, we confirmed its caused by LAPD timeslot problem. Changed LAPD from 1:4 multiplex to non-multiplex. The TCH assignment success rate improved from 10% to 99%.123Problem descriptionProblem analysisAdjustment

Interference within networkPerformance statistics showed that TCH assignment failure rate was high in Cell2 and Cell3, which led to low TCH allocation success rate. From the planning software, we could see the two cells were right opposite each other, and their overlapped coverage area was large, and co-channel existed, thus TCH assignment failure rate was high After we changed frequency point in Cell3 from 119 to 101, number of TCH assignment failures in the two cells reduced greatly, from 20times/h to 1time/hour.123Problem descriptionProblem analysisAdjustment

Discrepancy in time at BSC and OMCR A customer reported that call completion rate exceeded 100%, almost reached115%. Analysis of basic measurement data showed that Sum of CallProceeding and CallConfirm was 4955, while that measured by customer was 9428. We found that time at BSC was 5mins 30secs behind the time at OMCR.We adjusted time at OMCR and renewed data of OMCR with PMDR, and the data in OMCR and signaling device conformed to each other 123Problem descriptionProblem analysisAdjustment

Questions for thinkingWhich parameters can influence TCH allocation success rate?Please introduce counters and signaling statistical points relating to TCH allocation success rate.