GSM TCH Congestion & SolutionsZTE university

Training goalsTo know the signaling flow of TCH occupation and relevant counters;To know causes of TCH congestion;To know the process of investigating TCH congestion;To know the common methods of handling TCH congestion

ContentsTCH occupation signaling & relevant countersCauses of TCH congestionProblem investigation processCommon methods of solving TCH congestionTypical cases

What is TCH congestion?TCH congestion means MS applies for TCH during the access process; MSC will send Assignment Request signaling to BSC after it confirms MS application for TCH; BSC will search for suitable TCHs; if no TCHs are available, BSC will refuse the request, and record congestion as one time.Congestion often brings inconvenience to subscribers, thus it is the most complained problem . High TCH congestion rate has harmful effect on call establishment rate, handover success rate and call drop rate.

TCH congestion & relevant counter

KPI nameTCH blocking rate Indicator definitionTCH congestion times/TCH call attempts)*100%/ Counter formula(C900060020+C900060031+C900060043+C900060047+C900060022+C900060033+C900060045+C900060049)*100%/ (C900060019+C900060030+C900060042+C900060046+ C900060021+C900060032+C900060044+C900060048)

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TCH congestion & relevant counter

CounterCounter DescriptionC900060019Number of voice TCH/F seizure attempts for assignmentC900060020Number of voice TCH/F seizure failure for assignmentC900060021Number of voice TCH/F seizure attempts for handoverC900060022Number of voice TCH/F seizure failure for handoverC900060030Number of data TCH/F seizure attempts for assignmentC900060031Number of data TCH/F seizure failure for assignmentC900060032Number of data TCH/F seizure attempts for handoverC900060033Number of data TCH/F seizure failure for handover

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TCH congestion & relevant counter

CounterCounter DescriptionC900060042Number of voice TCH/H seizure attempts for assignmentC900060043Number of voice TCH/H seizure failure for assignmentC900060044Number of voice TCH/H seizure attempts for handoverC900060045Number of voice TCH/H seizure failure for handoverC900060046Number of data TCH/H seizure attempts for assignmentC900060047Number of data TCH/H seizure failure for assignmentC900060048Number of data TCH/H seizure attempts for handoverC900060049Number of data TCH/H seizure failure for handover

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Signaling of TCH congestionUpon receiving Assignment Request from MSC, BSC will search for suitable TCHs.If no usable TCHs are available, BSC will send a Assignment Failure message to MSC, and the system records congestion as one time.

ContentsTCH occupation signaling & relevant countersCauses of TCH congestionProblem investigation processCommon methods of solving TCH congestionTypical cases

Causes of TCH congestionTCH channelCongestionHigh dense traffic exceeds BTS capacityLow TCH usability due to hardware problemLarge Traffic burstToo large coverage, isolated island effectUnreasonable setting of radio parametersProblem with adjacent cells

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ContentsTCH occupation signaling & relevant countersCauses of TCH congestionProblem investigation processCommon methods of solving TCH congestionTypical cases

Process of handling TCH congestionA cell with high TCH congestion rateTCH availability is low?Any problem with adjacent cell?Caused by too many handovers?Check radio parametersIsolated-effect exists due to too large coverage?If the BTS reaches its max configuration?Expand the BTS with enough TRXsInvestigate hardware Investigate adjacent cellsOptimize HO parameters to reduce HOReduce coverage and eliminate the effectAdjust parametersLower BTS power, increase down-tilt to abate congestionYesYesYesYesYesYesDue to high traffic density

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Contents TCH occupation signaling & relevant countersCauses of TCH congestionProblem investigation processCommon methods of solving TCH congestionTypical cases

Common methods of solving TCH congestionTrafficcontrolOpen HRControlcoverageSolutions to congestionexpansion

Common methods of traffic controlControl cell selectionControl cell reselectionPBGT HO Macro-micro HO, Traffic HOControl coverage rangeC1CBACBQC2CROCRHTRX static Power class, Down tiltControl Handover

Traffic controlcell selectionCell selectionWhen MS is open or it enters coverage from blind area, it detects the cells signal and stays synchronous with the cell, then it scans BCCH in different cells and select one to reside according to cells priority and the principle C1>1. C1: parameter for cell pathloss judgment principleC1 = RXLEVELDLRXLEV_ACCESS_MINMAX(MS _TXPWR_MAX - P )RXLEVEL: MS receive level averageDLRXLEV_ACCESS_MIN: minimum MS access levelMS_TXPWR_MAXmax BCCH power levelPmax MS output level

Traffic controlcell selectionCell selection priorityIn PHASE2+ regulations, parameters CBA and CBQ to represent the cells priority are added. Through combined settings of the two parameters, we can get different selection priorities, and we can set lower priority for congested cells.

CellBarQualifyCellBarAccessCell selection priorityCell reselection status00NormalNormal01BarredBarred10LowNormal11LowNormal

Traffic controlcell reselectionCell reselectionWhen C2 algorithm is open, MS reselects the cell with max C2 to reside.

whenPT11111C2C1CROTOHPTTwhenPT11111C2C1CROof whichWhen x

Traffic controlhandover based on layers PBGT HOThrough setting PBGTHoLayer and NCellLayer, we can control whether the handover can be carried out among undefined layer, same layer different frequency band, upper layer, and lower layer, thus we can reach flexible control over traffic distribution. Traffic HOThrough setting parameters: layer relation-TrafficHoLayrCtl (same layer, upper layer, lower layer), frequency band TrafficHoFreqCtl and NCellLayer, we can contol the layer and frequency band for target cell of traffic handover, and traffic distribution can be controlled flexibly as well.

Macro-micro HOMacro-micro handover is to handover the MS moving with slow speed from macro cell layer to micro cell layer. The micro cell mentioned here is just a concept in logic. In this example, DCS1800 cell can be regarded as micro cell, and the macro-micro handover can only be carried out to adjacent cells on lower layer. Use MacroMicroHoThs to control the difficulty of handover to lower layers.

For specific parameters, please refer to relevant technical guidebooks.

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Traffic controlcoverage controlControl antenna down-tilt

Static power class

Static power classstatic RF power stepMax real transmitting powerPn0Maximum output power1Maximum output power 2dB2Maximum output power 4dB3Maximum output power 6dB4Maximum output power 8dB5Maximum output power 10dB6Maximum output power 12dB

Half RateFeatures of HR

Application of HRHR application in area with burst trafficHR application at area with dense trafficHR application at areas with lower-end subscribers

Fast expansion of TRX is not necessary;Increase capacity at the cost of speech quality;Flexible setting of cell layers and configuration;Some terminals (MS) can not support HR.

Note radio interference at areas with HR open;Note subscribers sensitivity of speech quality;Note setting of HR threshold;Note rate of terminals supporting HR.

Half Rate

Half RateActivating HR solution achieves fast expansion of wireless network, relieves network intense capacity, effectively solves partial traffics congestion, and can be supplement for urgent expansion solution.Proportion of the MS supporting HR codecUsing HR will lower the speech quality during conversation in worse radio C/I areasHR solution suits for low-value areas with outburst heavy traffics

The ZTE BSC avoids the occurrence of HR fragmentations to reserve continuous and complete timeslot for FR allocation and data services. In addition, the BSC can gather the fragmental HR channels by internal cell handover, which is re-packaging mechanism for ongoing half rate calls.In poor radio environments, the speech quality of the HR decreases.The ZTE BSS monitors the strength of radio signals and speech quality to dynamically switch between the HR and FR for a subscriber. In this way, the network capacity increases with the satisfying service quality.Two HR subscribers share 16 kbps transmission of Abis interface; therefore, the transmission in Abis interface is not affected.HR solution increases the capacity and spectrum efficiency.

Half Rate

Full namePreferred speech version (half)DescriptionWhen implementing the CS channel allocation policy,describe the preferred half-rate speech version. Halfrate version 1 refers to HR, and version 3 for AMRValue RangeNot specify the preferred version, Half-rate version 1, Half-rate version 3

Half Rate

Full nameLow priority of channel select priorityDescriptionWhen implementing the CS channel allocation policy,BSC describes the channel select strategy while TCH allocationValue RangeNo change, Full rate first, Half rate first, Only full rate allowed, Only half rate allowed

Half Rate

Full nameHigh priority of channel select priorityDescriptionWhen implementing the CS channel allocation policy, BSC describes the channel select strategy while TCH allocationValue RangeNo change, Full rate first, Half rate first, Only full rate allowed, Only half rate allowed

Half Rate

Full nameDynamic tsDescriptionYES: Representing dynamic timeslot; NO: Representing fixedly configured timeslotValue RangeYes/No

Flow of network expansion

Gather each cells traffic report of a weekTraffic (actual)>traffic (theoretical)No expansion needFilter out each cells max traffic volume to be the base of expansion calculation.

Look up in Erl B, obtain the cells theoretical busy hour traffic Erl (theoretical)Open certain percent of HR, calculate theoretical traffic to be supported Traffic (actual)>traffic HR (theoretical)Calculate number of TRX needed for expansion (actual need)TRX(actual need)>max number of TRX allowedAdd new BTSCell splitOpen HR for expansionComplete expansion planYesNoNoRate of open HR shall not be too high

ContentsTCH occupation signaling & relevant countersCauses of TCH congestionProblem investigation processCommon methods of solving TCH congestionTypical cases

Typical case 1TCH congestion rate at an overseas BTS was shown higher than usual after it's been swapped with ZTE equipment.Problem descriptionFrom the dynamic data management, we observed that all FR TCHs have been occupied, while a lot of HR TCHs were idle. Problem analysisThrough signaling analysis, we found congestion just occurred on assigning FR TCH. Basically, it was confirmed that the assignment failure was caused by congestion due to lack of FR TCH ;After checking the channel assignment parameters of MSC, BSC and cells, we found the system takes the first speech version assigned by MSC as default; after most TRXs were configured with HR TCH, the channel assignment priority in radio parameters has not been changed accordingly;

Typical case 1Problem analysisSince most TRXs in these cells were configured with static HR TCHs, there were just a few FR TCHs, which led to congestion due to lack of TCH/F;The primary cause of congestion is there is still a certain number of MSs do not support HR.

Problem handlingAdjusted ChanSelectPrio (channel selection priority), changed the default No Select to half Rate First , The problem was solved.

Typical case 2Problem descriptionDuring optimization of a local network, the busy hour congestion rate (incl. HO) of two cells was high (10%), while that (excl. HO) was normal. And number of TCH assignment failure was high, which for the two cells were 89 and 61. Traffic volume was lower than that before optimization.Interference band was normal.Congestion rate was normal before optimization.

Problem analysisSince the congestion rate deteriorated after optimization, and only the rate with HO included went bad, so we could exclude reasons like antenna interference, hardware problem, etc., but focus on HO.Recorded the two cells handover statistical performance of 15mins, we found almost all handovers from a certain cell CGI=**1768 to these two cells failed.

Frequent congestion due to improper parameter settings

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Problem handlingChecked the two cells HO statistics, we found they were co-channel and co-BSIC, and they were adjacent cells to a certain cell at the same time, so almost all handovers from the certain cell to each of these two cells failed inevitably, and the failure just occurred during TCH assignment.

After the two cells BCCH and BSIC were adjusted, HO and congestion returned to normal.

Typical case 2

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Typical case 3Problem descriptionCongestion rate in two cells under a certain BTS increased suddenly during 21002300 pm, and the rate even reached 30%, while there was no congestion during other periods.Number of call attempts and traffic volume were obviously increased when congestion occurred.Other radio indicators were in normal status.

Serious congestion due to traffic burstProblem analysisChecked if high dense traffic existed within coverage of the two cells, its found that there was a high school dormitory building, and traffic burst occurred after school.We checked the two cells' configuration, which has already reached the max allowed.After checking we found HR in the two cells was off, so its suggested that HR be open for cell expansion.

Problem handlingObtained the two cells busy hour traffic of 7 consecutive days from performance report.Found out the number of TCHs supporting traffic from Erl B.From calculation, we found that HR of 40% TCHs had to be open to satisfy the actual traffic need.Opened the two cells dynamic HR, and set the threshold as 60%.When HR was open, the congestion disappeared.

Typical case 3

UserLabelTCH availableTCH trafficTCH congestion rateTCH overflow timesTCH call attemptsSite77_bts12624.7823.516122603Site93_bts22523.4818.174282355Site77_bts12624.2723.395892518Site93_bts22523.1417.954072267Site77_bts12624.8928.97372550Site93_bts22523.7220.895072426Site77_bts13729.730.44 132831Site93_bts23528.420.26 72692Site77_bts14030.120.53 152881Site93_bts23628.140.11 32655

Questions for thinkingWhy congestion occurs when traffic per TCH channel reaches 0.4Erl in cells with single TRX?

What methods can be used to achieve traffic balance?

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