Ericsson2G KPI formulae and their description

STS

Statistics and Traffic Measurement Subsystem (STS) is implemented in the BSC (and MSC). It gives statistics about events in different parts of the system such as cells and equipment. By continuously supervising theresults from STS the operator can obtain a very good overview of the radio network performance which can help to detect problems early.

Different events occurring in an Ericsson GSM network are counted andcollected by a subsystem called Statistics and Traffic Measurement Subsystem (STS).

STS is available in two different versions, STS on APG and STS on SP. STS on SP is available on IOG equipped BSCs and STS on APG is available on APG40 equipped BSCs

The central part in STS is the Measurement Database (MDB) where allmeasurements are collected from different blocks in the Central Processor (CP). The contents of the MDB are written to STS report files defined by the user. These STS files are then fetched from the BSC and processed by OSS or a user defined external tool.

By combining and comparing different counters a general understanding of the radio network behavior can be obtained.

. During a call several counters are affected. The allocation of a Stand-alone Dedicated Control Channel (SDCCH) can be successful or can fail due to congestion or the SDCCH could later drop due to low signal strength. Each event will result in different counters to be stepped.

In the BSC these events can be handovers, call setups, dropped calls, allocation of different channels etc. There are also a number of status counters, reporting the status of equipment within the network such as the current number of occupied channels.

Some of the 2G KPIs are explained next.

The following formula, T_TRAFF, shows the average level of all TCH traffic in a cell (UL+OL) in Erlang or, more accurate, the mean number of allocated TCH channels.

Total (FR+HR) TCH Traffic Level in a Cell:

TFTRALACC Traffic level accumulator for full-rate TCH. Thecorresponding counter for half-rate is THTRALACC

TFNSCAN Number of accumulations of traffic level counter forfull-rate TCH. The corresponding counter for half-rate isTHNSCAN

T_TRAFF

T_AVAIL

The channel availability is very difficult to measure despite counters such as TAVAACC, number of available TCHs. This is due to the fact that TNUCHCNT, number of defined TCHs, depends on whether the number is system defined or operator defined.

System defined means that the number of TCHs is based on the number of allocated frequencies instead of the number of installed TRXs.

Operator defined means that the number of defined TCH channels is calculated as the required number of Basic Physical Channels (BPCs) defined by command (parameter NUMREQBPC) for the cell/channel group minus the number of BPCs used for BCCH and SDCCH in the cell/channel group. This is especially useful when synthesizer hopping is used (more frequencies than hardware).

The equation below can be used to calculate the number of available TCHs of total number of defined TCHs but the result will not be correct if the feature Adaptive configuration of logical channels is used. If Adaptive configuration of logical channels is activated the number of TCHs might change in the cell depending on the SDCCH traffic level. If the number of TCHs are operator defined or if synthesizer hopping is not active the following formula can be used:

TCH Drop.

The total number of terminated calls in a cell is then expressed as:N_CALLS = I_CALLS + Inc(AB+AW) - Outg(AB+AW)(Net Sum of Calls Terminated in Cell)

N_CALLS Number of calls terminated in a cell.I_CALLS Number of initiated calls in a cell, e.g. the sum of thefour CASSALL counters for TCH or CMSESTAB for SDCCH.Inc : Sum of all incoming handovers to a cell from all its neighbors.Outg : Sum of all outgoing handovers from a cell to all its neighbors.AW Number of successful assignments to worse cell, counter HOSUCWCL.AB Number of successful assignments to better cell, counter HOSUCBCL.

T_DR_S

TFNDROP The total number of dropped full-rate TCH in underlaidsubcell. The counter is also available for half-rate andfor overlaid subcells, e.g. THNDROPSUB.TN_DROP = TFNDROP + TFNDROPSUB + THNDROP + THNDROPSUB(Total Number of Drops on TCH)

The formula for drop on SDCCH, drop due to TCH congestion excluded, is:CNDROP : The total number of dropped SDCCH channels in a cell.CNRELCONG : Number of released connection on SDCCH due to TCH and transcoder congestion in underlaid and overlaid sub cell. The subset for overlaid sub cells is CNRELCONGSUB .The two counters are located in CLSDCCH and CLSDCCHO respectively. CNDROP is stepped at the same time. CMSESTAB : Successful MS channel establishments on SDCCHS_DR

There are 6 different stats available in Ericsson system which give the individual percentage contribution of each possible factor that can contribute towards the net SDCCH Drop:

Drop Reason, Low Signal Strength Uplink (%).Drop Reason, Low Signal Strength Downlink (%).Drop Reason, Bad Quality Uplink (%).Drop Reason, Bad Quality Downlink (%).Drop Reason, Excess TA (%).Drop Reason, Other (%).

Call Setup Success Rate.

CSSR = [(CCALLS-CCONGS)/(CCALLS)] * [(CMESTAB-(CNDROP-CNRELCONG))/(CMESTAB)]* (TCASSALL/TASSALL)* 100

CCALLS : Call attempt counter (on SDCCH).CCONGS :Congestion counter for under laid sub cell. Stepped per congested allocation attempt.CMESTAB : Successful MS establishment on SDCCH. This counter is a sum of both overlaid and under laid.CNDROP : The total number of dropped SDCCH channels in a cell.CNRELCONG : Number of released connections on SDCCH due to TCH- or Transcoder (TRA) congestion in both under laid and overlaid sub cell.CSSR

TASSAL : Number of first assignment attempts on TCH for all MS power classes. Successful attempts are counted in the target cell and failed attempts are counted in the serving cell. The serving cell is the cell where the mobile station was tuned to an SDCCH or TCH

TCASSALL : 'Assignment complete' counter (TCASSALL) is incremented for the same conditions as the 'Assignment attempt' counter (TASSALL) if the attempt was successful. The 'Assignment complete' counter is incremented for the target cell. As TCASSALL is fetched by STS from the same object type as TASSALL they are always synchronized.

CSSR = 100 * ([TCASSALL + TFCASSALLSUB + THCASSALL + THCASSALLSUB]/ TASSALL) * (1 - ([RAOREQ + RAAPAG + RACALRE]/CNROCNT) * (CCALLS-CMSESTAB)/CCALLS) * (1 - ([RAOREQ + RAAPAG + RACALRE]/CNROCNT)* (CNDROP - CNRELCONG)/CMSESTAB)

In general,CSSR = TCH Assignment * (1-SD Establishment for TCH Calls) * (1-SD Drops for TCH Calls)

Successful attempts are counted in the target cell and failed attempts are counted in the serving cell.

Ericsson formula for CSSR

CNROCNT : Number of accepted random accessesRAACCFA : Failed random access.RAANPAG : Number of random accesses, answer to paging.The counter will be stepped for cause value 100x xxxx.RACALRE :Call re-establishment.The counter will be stepped for cause value 110x xxxx, Call reestablishment, TCH/F is needed, or originating call and NECI bit not set to 1, or procedures that can be completed with an SDCCH and NECI bit not set to 1.TFCASSALLSUB : Number of assignment complete messages for all MS power classes in underlaid subcell, full-rate.THCASSALLSUB : Number of assignment complete messages for all MS power classes in underlaid subcell, half-rate.

This KPI does not only cover TCH handover but also handovers during assignment and SDCCH handovers.

HOT_SUC = 100 * ( SUMOHOSUCC + SUMEOHSUCC ) / ( SUMOHOATT + SUMEOHATT )HOT_SUC

SUMEOHOSUCC : Sum of Successful External Handovers (Outgoing Handover)SUMOHOSUCC :Sum of Successful Internal Handovers (Outgoing Handover)SUMOHOATT : Sum of Internal Handover Attempts (Outgoing Handover)SUMEOHOATT :Sum of External handover Attempts (Outgoing Handover)

The handovers that are not successful or not revert to the old cell. A high rate of reverted Handovers could indicate that there exists two cells with the same BSIC -BCCH combination, and both of them are defined as neighbors

Other reasons are that the handover is attempted before the uplink in the new cell works, and the old cell is sufficiently good for allowing a reversion. The failure in the new cell can be due to too aggressive settings for HO, or an indication of a faulty RX in the target cell with lower sensitivity.

Handover reversions do occur in any network even at good conditions - normally the rate is around 1%. The reason for this is likely temporary dips, collisions in the air interface etc. The Handover failure is not perceived by thesubscriber, but could lead to a subsequent drop due to penalties in locating and handovers to other cells that are not good candidates

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