ius1.0_call drop analysis

Use case

Tool: ANATOMSpecification Number: 2A Related Request-Nr:

Title: Call Drop Analysis

Author: Alfredo Rocchetti Department: COM MN PG NT NE 5Date: 10.12.2002 Phone: +49 89 636 49197Description-Version: 1.0 Description-State: IUSRelevant Specifications:

Title Call Drop Analysis Date 10.12.2002Number 2A Version IUS 1.0Author Alfredo ROCCHETTI of 32

Table of Contents

1.1 Document History.............................................................................................................................. 4

1.2 Abbreviations..................................................................................................................................... 4

2 OBJECTIVE...............................................................................................................................5

3 REQUESTED KPIS / MEASUREMENTS..................................................................................6

3.1 High number of Call Drops...............................................................................................................63.1.1 Drops per Traffic Erlang..................................................................................................................63.1.2 Number of Dropped Calls................................................................................................................7

3.2 Call Drop typologies identification...................................................................................................93.2.1 Protocol Layers and Timers.............................................................................................................93.2.2 Drops due to S-Counter timeout (L1 failure)..................................................................................103.2.3 Drops due to TTRAU and/or TSYNC timeout (L1 failure).............................................................103.2.4 Drops due to LAPD Sequence Error (L2 failure)...........................................................................123.2.5 Drops due to T200 expiration (L2 failure)......................................................................................123.2.6 Drops due to HO Access failure (L2 failure)..................................................................................133.2.7 Drops due to HO Excessive distance............................................................................................143.2.8 Drops due to expiration of timer for MS capability indication.........................................................143.2.9 Drops due to Intercell HO failure...................................................................................................143.2.10 Drops due to Intracell HO failure...............................................................................................153.2.11 KPI for the calculation of the Drop per cause distribution.........................................................15

3.3 Call Drops because of high interferences in downlink.................................................................16

3.4 Too High RTF Drops figures (when TTRAU timer > T8 and T10 timers).....................................16

3.5 Call Drops because of high interference (or other problem like distance) in uplink (when TTRAU timer < T8 and T10 timers)..............................................................................................................17

3.6 Too high T_MSRFPCI Drops figures (when T_MSRFPCI timer > T8 and T10 timers)................17

4 PROBLEM DETECTION..........................................................................................................18

4.1 Problem description........................................................................................................................ 184.1.1 General definition for call drop.......................................................................................................184.1.2 Too High Erlang Call Drops Figures..............................................................................................184.1.3 Call Drops cause identification and evaluation on base of their distribution..................................184.1.4 Call Drops because of high interferences in downlink...................................................................184.1.5 Too high RTF Drops figures (when TTRAU timer > T8 and T10 timers)........................................194.1.6 Call Drops because of high interference (or distance) in uplink (when TTRAU timer < T8 and T10 timers) 194.1.7 Too high T_MSRFPCI Drops figures (when T_MSRFPCI timer > T8 and T10 timers)..................20

4.2 Problem reporting............................................................................................................................ 204.2.1 Detection of cell with Call Drop problems......................................................................................20

RELATED DOCUMENTATION:......................................................................................................27

ANATOM USER MANUALS....................................................................................................27


MAPINFO PROFESSIONAL USER GUIDE............................................................................274.2.2 Call Drop causes distribution.........................................................................................................284.2.3 Call drops because of high interference in DL...............................................................................294.2.4 Too High RTF Drop figures...........................................................................................................304.2.5 Call drops because of high interference in UL...............................................................................314.2.6 Call drops because of missing terrestrial resources......................................................................32


GENERAL

1.1 Document History

Version Date Change0.1 03.11.2004 DRAFT AFI0.2 15.11.2004 AFI: Editorial corrections and concept modification in chapter

3.4 and chapter 4.1.51.0 10.12.2004 IUS: Editorial corrections

1.2 Abbreviations

ADJCN ADJacent Cell NumberBCCH Broadcast Control CHannelBSC Base Station ControllerBSCN Base Station Controller NumberBSIC Base Station Identity CodeBTS Base Transceiver Station BTSN Base Transceiver Station NumberCD Call DropCDR Call Drop RateCELLGLID Cell Global IDentityHO HandoverKPI Key Performance IndicatorNB NeighborPBGT Power BuDGeTRTF Radio Transcoder Failure


2 Objective

The Call Drop Rate is one of the main indicators of the global network quality.For a successful working of a radio network system, a correct site distribution on the territory is necessary, a correct antenna installation (antenna elevation and tilting, antenna beam width and orientation), a correct frequency planning and finally a correct definition of the parameters ruling the Power Control and the HO procedures.For instance in a frequency limited network (number of available frequencies is relatively low) with aggressive frequency reuse (1 to 1 in some cases), an unequal traffic distribution among the cells can cause a high call drop rate in the neighbor cells around the highly congested cell (that acts against its neighbor as strong interferer).Therefore as long as one among those above mentioned elements is not optimally designed, the number of dropped calls grows above normal in the network under observation.

For this reason the “Call Drop Rate” is assumed in many contracts as one of the important criteria to verify a network during its acceptance phase.

The goal of “Call Drop Analysis” UC is, for existing networks, to detect cells, which have problems with high number of call drops (cells that overcome a defined percentage of call drops during their busy hour time).A second target of this analysis is to identify potential problems in the network analyzing the drop typologies mostly occurring in the cells of the network. This identification is useful to better define the Use Cases that better fit for the further investigations.


3 Requested KPIs / Measurements

In order to analyze the call drop performance some specific KPIs have been created and other have been used that are already part of the standard delivery of ANATOM tool.“Too High Erlang Call Drops Figures in a Cell” KPI generally indicates which are the cells that are overcoming a predefined threshold of call drops per Traffic Erlang.

3.1 High number of Call Drops

Long name: Too High Erlang Call Drops Figures Short name: ErlCallDrop_HIGHDescription: This KPI can show two different results: 0 or 1 with the meaning

0=”Problem does not exist” and 1=”Problem exists”.The problem exists if the number of call drops exceeds a certain percentage of the carried traffic defined in DROPS_ERL.

Formula: anatom_globals.gt (DROPS_ERL, (3600/90)*0.02)Remarks: If the number of DROP per Erlang calculated in DROPS_ERL overcomes

the threshold value defined in the second part of the expression, this KPI will be set to 1.3600 is the number of second in 1 hour, 1ErlH traffic is 3600sec traffic.The reference Mean Holding Time for a call is defined to be 90 seconds.0.02 correspond to 2% threshold set as the max value acceptable for a call drop rate during the observation time.It is recommended to verify this KPI during busy hour time, because the threshold value has its meaning under the worst traffic conditions e.g. at busy hour.

KPIs used in formula:

DROPS_ERL (new)

ANATOM category:

BTS - Call Drop Analysis

3.1.1 Drops per Traffic Erlang


Long name: Number of Drops per Traffic ErlangShort name: DROPS_ERLDescription: This KPI verifies the number of dropped calls with respect to the amount

of traffic in the cellFormula: nvl(NUMBER_DC/TCH_TRAF_CARR_DR, 0)

Remarks: It is recommended to verify this KPI during busy hour time, because the threshold value has its meaning under the heavier traffic conditions e.g. at busy hour.

KPIs used in formula:

NUMBER_DC (new, see next page)TCH_TRAF_CARR_DR (predefined)

ANATOM category:


3.1.2 Number of Dropped Calls

Long name: Number of Dropped Calls

Short name: NUMBER_DCDescription: This KPI sums all the lost calls in one cell

Formula: anatom_globals.value_add (HO_LOSS_INTERCELL, HO_LOSS_INTRACELL) + anatom_globals.value_add ((TCH_LOSS_DIST_F_1 + TCH_LOSS_DMR_F_1 + TCH_LOSS_HOAF_F_1 + TCH_LOSS_MSRFPCI_F_1 + TCH_LOSS_OTHER_F_1 + TCH_LOSS_RLF_F_1 + TCH_LOSS_RTF_F_1 + TCH_LOSS_SER_F_1 + TCH_LOSS_T200_F_1) , (TCH_LOSS_DIST_H_1 + TCH_LOSS_DMR_H_1 + TCH_LOSS_HOAF_H_1 + TCH_LOSS_MSRFPCI_H_1 + TCH_LOSS_OTHER_H_1 + TCH_LOSS_RLF_H_1 + TCH_LOSS_RTF_H_1 + TCH_LOSS_SER_H_1 + TCH_LOSS_T200_H_1) , (TCH_LOSS_DIST_F_2 + TCH_LOSS_DMR_F_2 + TCH_LOSS_HOAF_F_2 + TCH_LOSS_MSRFPCI_F_2 + TCH_LOSS_OTHER_F_2 + TCH_LOSS_RLF_F_2 + TCH_LOSS_RTF_F_2 + TCH_LOSS_SER_F_2 + TCH_LOSS_T200_F_2) , (TCH_LOSS_DIST_H_2 + TCH_LOSS_DMR_H_2 + TCH_LOSS_HOAF_H_2 + TCH_LOSS_MSRFPCI_H_2 + TCH_LOSS_OTHER_H_2 + TCH_LOSS_RLF_H_2 + TCH_LOSS_RTF_H_2 + TCH_LOSS_SER_H_2 + TCH_LOSS_T200_H_2) )

Remarks:HO_LOSS_INTERCELL is based on the UNIHIRLC counter in SBS and is triggered by T8 expiration in case of Intercell BSC controlled HO.HO_LOSS_INTRACELL is based on the UNIHIALC counter in SBS and is triggered by T10 expiration in case of Intracell BSC controlled HO.TCH_LOSS_XXX_YZ are based on NRFLTCH counters [1..36] in SBSNote to SBS counters:

1. NRFLTCH [9,19,27,36] count the dropped calls due to T8 and T10 expiration in case HO is MSC controlled!(see PM:SBS Message Flows at page 74)

2. NRFLTCH [9,19,27,36] do not count the dropped calls due to MSC timer Trr7 expiration (fix value set to 15sec) in case BSC timer T8>Trr7 in case of MSC controlled HO. The error introduced by this is small if BSC clusters are well distinct and MSC controlled HO is only left for the inter-BSC HO (all the other HO should be BSC controlled).

3. The possibility to set to “MSC controlled” all the intercell intra-BSC HOs does not regard the “Direct Retry HO” which stays always BSC controlled when the target cell is intra-BSC.Therefore UNIHIRLC, in case of MSC controlled intercell-HO, counts only the dropped intercell intra-BSC direct retry HOs.

4. UNIHIRLC is triggered as well by DROPPED SDCCH - SDCCH HO (see PM:SBS Message Flows at page 61), therefore in the calculation of the CDR it is necessary to consider the successful SDCCH - SDCCH HO.

KPIs and RAW used in formula:

HO_LOSS_INTERCELL (predefined, raw counter, SBS counter: UNIHIRLC)HO_LOSS_INTRACELL (predefined, raw counter, SBS counter: UNIHIALC)TCH_LOSS_XXX(XXXX)_Y_Z: (predefined, raw counter, SBS

counter: NRFLTCH[1..36])ANATOM BTS - Call Drop Analysis


category:


3.2 Call Drop typologies identification

ANATOM tool provides a set of raw counters, which allow the better determination of the possible reasons behind a high Call Drop rate. On a base of this raw counters the following KPIs have been created and stored under the Category “CallDropAnalysis”:

RLF_DROPS RTF_DROPS SER_DROPS T200_DROPS T3105_HOAF_DROPS TOO_HIGH_DIST_DROPS T_MSRFPCI_DROPS T8_INTERCELL_HO_DROP T10_INTRACELL_HO_DROPS

From the analysis of the drop typologies and their distribution it is possible to state some considerations on the possible reasons behind the dropping; therefore in next chapter 3.2.1 the most relevant Timers and Counters that have an impact on the call dropping measurements are presented. Chapters from 3.2.2. to 3.2.10 describe KPIs related with these timers and counters and give indications on the effects of different Timer settings; each chapter points out the possible suspected reason for an abnormal call dropping measured with the specific KPI.Finally the chapter 3.2.11 resumes and describes in short the KPIs, which describe the distribution of the dropped calls among the possible causes.It is important to underline that the analysis of the above mentioned call drops typologies can be done only under the precondition of a correct knowledge of the L1 to L3 timers and counters configuration in the network.

3.2.1 Protocol Layers and Timers

In the next table the relation among timers and protocol layers is pointed out.

Table 1: Release causes, timers and protocol layer relations

L3 T8: determines the time to receive the HO COMPLETE message. T8 is defined as the time that BSC layer 3 will wait for a HO to complete before releasing the source channel. Therefore a HO Failure message is triggered by missed HOCMP by expiration of this timer.T10 : determines the time to return the ASSIGNMENT COMPLETE message in case of call setup and intra-cell HO.T_MSRFPCI: (hardcoded at 20s) at call set-up and HO; expires if no HO CMP or ASS CMP (first L3 message from MS) is received.

L2 T3105*NY (and T3124 for ms) for detection of HO Access Failure (triggered by missed PHYS INFO) Connection Failure (HO Access Failure)T200*(N200+1) for LAPDM messages in ACK mode (triggered by missed ACK to SABM, Disconnect or I-frames). Error Indication (T200 Expired N200+1 times)SER Unexpected L2 Um message has been received

L1 Connection Failure (Radio Link Failure)RLF (S-counter expired)

Connection Failure Link (Remote Transcoder Failure) (TTRAU at call set-up,TSYNC at call on going, TSYNCDL and TSYNCUL for AMR)


3.2.2 Drops due to S-Counter timeout (L1 failure)

Long name: Radio Link Failure Drops Short name: RLF_DROPSDescription: S-Counter in Layer1 generates a resource release and a call drop when

the Counter expiresFormula: anatom_globals.value_add (TCH_LOSS_RLF_F_1,

TCH_LOSS_RLF_H_1, TCH_LOSS_RLF_F_2, TCH_LOSS_RLF_H_2)Remarks: Radio Link Failure (RLF) DROPS:

It means that the S-Counter (SACCH-Counter) in BTS and/or in MS has expired; when MS or/and BTS did not receive/decode a SACCH frame the counter is decreased of 1 unit and if the counter reaches 0 the signaling resources are released and consequently the associated call is dropped.Layer 1 control.Possible reasons:

interference problem in UL and/or DL MS enters in a no coverage zone (e.g. garage, subway)

KPIs and RAW counters used in formula:

TCH_LOSS_RLF_F_1,TCH_LOSS_RLF_H_1,TCH_LOSS_RLF_F_2,TCH_LOSS_RLF_H_2 (all predefined)

ANATOM category:


3.2.3 Drops due to TTRAU and/or TSYNC timeout (L1 failure)

Long name: Remote Transcoder Failure DropsShort name: RTF_DROPSDescription: One of the TTRAU, TSYNC, (TSYNCUL, TSINCDL for AMR calls) L1

Timers has expired and the call is dropped.TTRAUUsed at connection establishment or at HO. In case of HO the timer stops after reception of HO ACCESS (only after forwarding this msg to BSC, the BTS will receive the idle TRAU framesTSYNC, TSYNCUL,TSYNCDLThe BTSE starts these timers if 3 UL and/or DL TRAU frames have not been correctly received from the BTS and it is reset if a correct frame is received again before expiration time

Formula: anatom_globals.value_add (TCH_LOSS_RTF_F_1, TCH_LOSS_RTF_H_1, TCH_LOSS_RTF_F_2, TCH_LOSS_RTF_H_2)

Remarks: The meaning of this drop counter is strictly related to relative configuration of the timer TTRAU against timers T8 and T10:

1. If TTRAU is bigger than T8 and T10 the dropped calls due to RTF should be strictly due to TSYNC expiration (call drop is not due to failed HO attempt but due to problem on the terrestrial interface).In this case call dropping due to failed HO procedure will be identified by expiration of timer T3105 (HOAF) or timers T8 and T10 (inter and intra HO failure)

2. If TTRAU is smaller than T8 and T10 then the Radio Transcoder Failure (RTF) DROPS will count the dropped HOs, because of missing MS’s answer to HOCMD msg. from BTS: this setting requires attention, because of the interworking with timer T3124 in MS. In other words the timer TTRAU should be shorter than T8


and T10 but it must leave time to the MS to try a fallback to the old cell (MS starts this process after the nth expiration of timer T3124, set by GSM standard at 320ms, and triggered by MS at the moment of sending the HO_ACCESS msg. to the target cell). If MS does not receive the PHYS_INFO within T3124 then a fallback to the old cell should be attempted (therefore TTRAU timer setting shall allow this attempt).

The second TTRAU timer setting is more complicated and risky to generate higher failure figure dropping potentially successful fallback attempts; moreover the information that such configuration gives is already available indirectly from the observation of the fallback counters.Indeed, a high number of fallbacks means that MSs received the HO_CMD from network but they did not manage to reach the target cell with their HO_ACCESS msg. or they did not receive the answer (PHYS_INFO) from the new cell.In the opposite, a relative low number of fallback and a relatively high number of intra and inter cell HO failures indicate most probably that the HO_CMD sent to MS has not been received and timer T8 or T10 has expired; such a situation indicates most probably a high interference in DL (see remarks in T8 and T10 dedicated KPI).

Coming back to the indication given by this RTF drop counter:If TTRAU > T8 and T10 then this drop counter will indicate only the expiration of TSYNC timer (or the equivalent AMR timers in UL and DL), the most probable reason are related to trouble on the terrestrial interface like:

Problem in through connection between a-bis and a-sub resources Cross-connector at PCMS and/or PCMB without appropriate

configuration of the link resources TRAU problem (HW or SW) BSC problem (HW or SW)

If T3124 < TTRAU <T8 or T3124< TTRAU < T10 the drop counter can outline that the TTRAU timer in the target cell has expired and BTS has not received a HO ACCES msg from MS.

Possible reasons:

Received level in UL is too low, because MS tries to jump on a too far away cell (maybe because of imperative HO due to bad quality in DL).

due to a non correct overlapping of adjacent cell, the MS is camped in a traffic island and the neighbors of the

serving cell are quite far away.KPIs and RAW counters used in formula:

TCH_LOSS_RTF_F_1,TCH_LOSS_RTF_H_1,TCH_LOSS_RTF_F_2,TCH_LOSS_RTF_H_2) (all predefined)

ANATOM category:



3.2.4 Drops due to LAPD Sequence Error (L2 failure)

Long name: LAPD Sequence Error DropsShort name: SER_DROPSDescription: This kind of drops is due to unexpected sequences on L2 LAPD messages. In

high interference cases a L2 message of interfering mobile is interpreted as message of the victim MS and the call of the victim MS is dropped.L2 messages of different MS are mixed up (high interferences in the network with limited number of RF for BCCH and no ciphering is active).

Formula: anatom_globals.value_add(TCH_LOSS_SER_F_1, TCH_LOSS_SER_H_1, TCH_LOSS_SER_F_2, TCH_LOSS_SER_H_2)

Remarks: Appearance of these drops is unusual (under normal condition: normal freq. planning and ciphering activated).In network with frequency reuse factor 1 to 1 and no ciphering such kind of a call drop is most likely to appear, (messages of different MSs on same frequencies with different L2 seq. numbers interfere each others).These drop occurrences can be eliminated if ciphering is active.


TCH_LOSS_SER_F_1,TCH_LOSS_SER_H_1,TCH_LOSS_SER_F_2,TCH_LOSS_SER_H_2) (all predefined raw counter)

ANATOM category:


3.2.5 Drops due to T200 expiration (L2 failure)

Long name: Timer T200 expired N200 +1 times

Short name: T200_DROPSDescription: If the acknowledgement from the MS is not received within T200 the

transmission of the layer 2 frame is repeated and T200 restarted. The total number of repetitions is restricted to N200 - a fixed value specified by GSM, which depends on the control channel types:

N200(SDCCH)=23, N200(SACCH)=5, N200(FACCH/FR)=34, N200(FACCH/HR)=29

If T200 expires after the last layer 2 frame repetition the BTS sends an ERROR INDICATION with cause T200 expired N200+1 times (followed by a RELEASE INDICATION) message to the BSC, which releases the associated resources and the call is dropped.Only for those messages that are transmitted in acknowledged mode the timer T200 is used as waiting time for the layer2 acknowledgement to a transmitted message.Messages in acknowledged mode are:

SABM, DISCONNECT and I-frames.

In particular ASSCMD and HOCMD are messages in acknowledge mode.This timer counts all the drops due to this reason.

Formula: anatom_globals.value_add (TCH_LOSS_T200_F_1, TCH_LOSS_T200_H_1, TCH_LOSS_T200_F_2, TCH_LOSS_T200_H_2)

Remarks: Conditions for the triggering of similar ERRIN to the BSC are as following:

1. T200*(N200+1) < T8


2. T200*(N200+1) < TTRAU3. T200*(N200+1) < T_MSRFPCI

These preconditions must be all TRUE in order to have the T200 timer expiration before the expiration of any other timer.In this case if T200 Timer expired N200+1 times, it means that ASS_CMD or HO_CMD has been not received by MS.If any of the above condition is not fulfilled than the drops will be counted in the counter related to the timer, which expires first.T200 drops is a indication of a DL interference problem or the HO has been triggered when MS is already near to lose the coverage of the originating cell (bad coverage or bad neighbour cell overlapping).


TCH_LOSS_T200_F_1,TCH_LOSS_T200_H_1,TCH_LOSS_T200_F_2,TCH_LOSS_T200_H_2) (all predefined)

ANATOM category:


3.2.6 Drops due to HO Access failure (L2 failure)

Long name: HO Access Failure DropsShort name: T3105_HOAF_DROPSDescription: The timer T3105 is used for the repetition of PHYSICAL INFO message during

the HO procedure between non synchronized cells. BTS waits for SABM as answer to the sent PHYSICAL INFO. The timer can expire max NY times then a failure is declared and resources are released.

Formula: anatom_globals.value_add (TCH_LOSS_HOAF_F_1, TCH_LOSS_HOAF_H_1, TCH_LOSS_HOAF_F_2, TCH_LOSS_HOAF_H_2)

Remarks: HO Access Failure DROPS: the expiration of timer T3105 shows the impossibility for the MS to establish a layer 2 connection in the target cell (no UA answer from the new cell to SABM sent by MS to the new cell).Therefore, it is important that the T3105*NY1 is bigger than the sum of timer T3124 (set in MS and fix by standard to 320ms) and the time necessary for the attempt of the layer-2 establishment in the new cell (the SABM message is sent max 5 times and the retransmission interval time is 34 TDMA frames for a total max time of 800ms). Field experiences have shown that he following setting is suitable:

NY1*T3105 >= 2 sec.

In this way it is ensured the possibility to MS to fallback to the old cell in case the layer-2 establishment to the new cell fails.

If this rule is not considered, call drops would be counted even if there is no real call drop but just a reversion to old channel during HO.

Due to high interferer in DL the Source BTS could force the MS to the execution of an imperative HO, but distance to target cell is too high or the target cell is too interfered as well. Then the MS tries to start the fallback procedure but fails because of high interference or because level is too low. This kind of drops could be due to MS in traffic islands.


TCH_LOSS_HOAF_F_1,TCH_LOSS_HOAF_H_1,TCH_LOSS_HOAF_F_2,TCH_LOSS_HOAF_H_2 (all predefined)

ANATOM category:



3.2.7 Drops due to HO Excessive distance

Long name: Max DISTance exceeded_DROPSShort name: TOO_HIGH_DIST_DROPSDescription: Call is dropped because the MS overcame the threshold of the max

distance allowed in that cell set in the database.Formula: anatom_globals.value_add(TCH_LOSS_DIST_F_1,

TCH_LOSS_DIST_H_1, TCH_LOSS_DIST_F_2, TCH_LOSS_DIST_H_2)Remarks: TOO_HIGH_DIST_DROPS: call is dropped because the MS overcame the

threshold of the max distance set in the database for the cell without performing any successful HO towards any other cell.Symptom of coverage problem.


TCH_LOSS_DIST_F_1TCH_LOSS_DIST_H_1TCH_LOSS_DIST_F_2TCH_LOSS_DIST_H_2) (all predefined)

ANATOM category:


3.2.8 Drops due to expiration of timer for MS capability indication

Long name: MS Radio Frequency Power Capability Indication DropsShort name: T_MSRFPCI_DROPSDescription: The process that leads to a successful conclusion of a HO has not been

completed and the MS power capabilities are not know to the new BTS, which releases the allocated resources and the call is dropped.

Formula: anatom_globals.value_add (TCH_LOSS_MSRFPCI_F_1, TCH_LOSS_MSRFPCI_H_1, TCH_LOSS_MSRFPCI_F_2, TCH_LOSS_MSRFPCI_H_2)

Remarks The meaning of the drops due to the expiration of this timer is related to the reciprocal configuration of this timer and the timers T8 or T10.It is HW-coded at 20 second therefore should be an event that never appear. Appearance of this kind of drops can be indication of protocol problem at higher level between BSC and MSC. (If T8 and T10 are set >> than T_MSRFPCI, then the expiration of this timer is practically equivalent to a HO Access Failure - HOAF- (this should manifest in a relative low number of drops due to HOAF and a relatively high number of drops due to T_MSRFPCI timer).If timer T_MSRFPCI is bigger than T8 and T10 then drops due to T_MSRFPCI timer expiration could indicate possible troubles on the signaling between BSC and target BTS or possible BSC congestion.


TCH_LOSS_MSRFPCI_F_1TCH_LOSS_MSRFPCI_H_1TCH_LOSS_MSRFPCI_F_2TCH_LOSS_MSRFPCI_H_2 (all predefined)

ANATOM category:


3.2.9 Drops due to Intercell HO failure

Long name: Intercell BSC controlled HO DropsShort name: T8_INTERCELL_HO_DROPSDescription: As said in the previous chapters the drops counted under this KPI

depends of the setting of timer T8, the expiration of this timer indicates a


sure failure of the HO procedure and the call is dropped.Formula: nvl (HO_LOSS_INTERCELL, 0)Remarks: Attention!

the counter associated to the KPI counts the case of SDCCH to SDCCH HO in case this feature is activated


(HO_LOSS_INTERCELL) (predefined)

ANATOM category:


3.2.10 Drops due to Intracell HO failure

3.2.11

KPI for the calculation of the Drop per cause distributionFor each of the KPI defined in chapters 3.2.2 to 3.2.10 the related percentage on the total number of the dropped call is available under the category “CallDropAnalysis”

The following table shows the name and the relation:

KPI NAME KPI Formula Related to KPIPERC_RLF_DROPS nvl(RLF_DROPS / NUMBER_DC, 0) RLF_DROPSPERC_RTF_DROPS nvl(RTF_DROPS / NUMBER_DC, 0) RTF_DROPSPERC_SER_DROPS nvl(SER_DROPS / NUMBER_DC, 0) SER_DROPSPERC_T10_DROPS nvl(T10_INTRACELL_HO_DROPS /

NUMBER_DC, 0)T10_INTRACELL_HO_DROPS

PERC_T200_DROPS nvl(T200_DROPS / NUMBER_DC, 0) T200_DROPSPERC_T3105_DROPS nvl(T3105_HOAF_DROPS / NUMBER_DC, 0) T3105_HOAF_DROPSPERC_T8_DROPS nvl(T8_INTERCELL_HO_DROPS /

NUMBER_DC, 0)T8_INTERCELL_HO_DROPS

PERC_TOO_HIGH_DIST_DROPS

nvl(TOO_HIGH_DIST_DROPS / NUMBER_DC, 0)

TOO_HIGH_DIST_DROPS

PERC_T_MSRFPCI_DROPS nvl(T_MSRFPCI_DROPS / NUMBER_DC, 0) T_MSRFPCI_DROPS


Long name: Intracell BSC controlled HO DropsShort name: T10_INTRACELL_HO_DROPSDescription: As said in the previous chapters the drops counted under this KPI

depends of the setting of timer T10, the expiration of this timer indicates a sure failure of the HO procedure and the call is dropped.

Formula: nvl(HO_LOSS_INTRACELL, 0)

Remarks:KPIs and RAW counters used in formula:

(HO_LOSS_INTRACELL ) (predefined)

ANATOM category:


3.3 Call Drops because of high interferences in downlink

ANATOM category:


3.4 Too High RTF Drops figures (when TTRAU timer > T8 and T10 timers)

3.5

Call Drops because of high interference (or other problem like distance) in uplink (when TTRAU timer < T8 and T10 timers)


Long name: Intracell BSC controlled HO DropsShort name: CAUSE_HIGH_DL_INTERF_DROPSDescription: A very high number of drops due to T200 timer expiration (L2) indicates

probable problem of high interferences in downlink (ASSCMD/HOCMD messages do not arrive to MS), however a relative high number of drops due to Radio Link Failure (S-COUNTER expired) together with a high number of T200 drops can bring to the same conclusions.This KPI returns value 1 if call drops in the cell are highly probable due to interference in DL.

Formula: anatom_globals.log_or (anatom_globals.gt ( PERC_T200_DROPS, 0.4) , anatom_globals.log_and ( anatom_globals.gt ( anatom_globals.value_add (PERC_T200_DROPS, PERC_RLF_DROPS, 0) , 0.55) , anatom_globals.gt ( PERC_T200_DROPS, 0.25) ) )

Remarks: It is suggested to verify the value of this KPI during “Busy Hour”KPIs and RAW counters used in formula:

PERC_T200_DROPS (new, see CHP.3.2.11)PERC_RLF_DROPS (new, see CHP.3.2.11)

Long name: Intracell BSC controlled HO DropsShort name: HIGH_RTF_DROP_FIGUREDescription: If the timers T8 and T10 are accordingly configured, the expiration of

timers TSYNC, TSYNCDL and TSYNCUL should point out that a call has been dropped during speech phase (not because of missing steps in the signalization call flow) and this is mirrored in the RTF counter.Under this condition this KPI should seldom returns value 1, then the call drops in the cell are highly probable due to problem on the terrestrial side and most probably at TRAU side.It is suggested in this case to proceed with investigation of the HW and SW installed in the involved TRAUs; past experience shown that a reset of the involved TRAU HW can solve the problem at least temporarily.

Formula: anatom_globals.gt (PERC_RTF_DROPS, 0.1)Remarks: If TTRAU timer > T8 and T10 timers, then the timer expiration points out a

interference problemIt is suggested to verify the value of this KPI during “Busy Hour”


PERC_T_MSRFPCI_DROPS (new, see CHP 3.2.11)

ANATOM category:


3.6

Too high T_MSRFPCI Drops figures (when T_MSRFPCI timer > T8 and T10 timers)


Long name: High drop figured due to TTRAU expirationShort name: CAUSE_HIGH_UL_INTERF_DROPSDescription: If T200 and TTRAU are appropriately configured the expiration of TIMER

TTRAU should point out that MS has received the HOCMD or ASSCMD in DL but its answer (HO ACCESS) does not reach the target BTS, in case TTRAU has been set too long the expiration of the Timer T3124 in the MS triggers the escalation for the expiration of S-Counter (=> RLF)If this KPI returns value 1 it is suggested to verify with the dedicate Use Case the interference level in UL.

Formula: anatom_globals.log_or (anatom_globals.log_and (anatom_globals.lt (PERC_T200_DROPS, 0.2) ,anatom_globals.gt (PERC_RTF_DROPS, 0.35)) , anatom_globals.log_and (anatom_globals.lt (PERC_T200_DROPS, 0.2), anatom_globals.gt (anatom_globals.value_add (PERC_RLF_DROPS , PERC_RTF_DROPS,0), 0.45)))

Remarks: Precondition: TTRAU timer < T8 and T10 timersIt is suggested to verify the value of this KPI during “Busy Hour”


PERC_T200_DROPS (new, see CHP 3.2.11)PERC_RLF_DROPS (new, see CHP 3.2.11)PERC_RTF_DROPS (new, see CHP 3.2.11)

ANATOM category:


4

Problem detection

4.1 Problem description

4.1.1 General definition for call dropA call is "dropped" when it is disconnected prematurely during the active holding period.

For tests purposes the definition of active holding period length varies and it is mostly based on the average holding time per user typical for the network under analysis. So, being 90 second the averaged holding time in a network, a test call is "dropped" when it is disconnected prematurely during the 90-second holding period.

Call drops may occur due to congestion, interference, weak signal strength etc.

A failure occurs normally if:

The required signal quality is below the required value for a period longer than a given time interval (immediate loss of signal, coverage problem). Generally speaking, the length of this interval is highly dependent on the application/service provided by the mobile network. A real-time, delay sensitive service, such as voice, can tolerate only short interruptions and after a few seconds the call may be considered to be lost. In non-real time data services a longer interruption may go completely unnoticed since no or only a few packets are transmitted during the time when the quality is too low.

Cell congestion (target cells for HO are congested – no HO is possible and the quality of signal in originating cell causes drops)

Signaling congestion (e.g. due to burst SMS traffic)

Also the procedure to complete the HO during the execution phase of the HO comes into play.During the HO execution phase, a long and sensitive procedure may be prone to errors or delays such that the connection may be lost due to poorer and poorer signaling quality before the signaling procedure to hand the mobile over fails.

4.1.2 Too High Erlang Call Drops Figures

The problem here is to identify those cells that during their busy hour overcome a predefined threshold (generally contractually agreed between the operator and the technology provider).Most common used threshold is 2% drop rate.It is highly important to relate this threshold to the traffic supported in the cell under analysis and to the mean call length, because drops of long calls have different impact on the calculation of the drop call rate than drops of short calls.


Long name: Intracell BSC controlled HO DropsShort name: HIGH_TMSRFPCI_DROP_FIGUREDescription: This timer expires only after call flow procedure on RF has been

completed, then it points out the possibility of a trouble in the terrestrial side. If KPI reports value 1 it is suggested to verify the resource allocation on the A-bis side.

Formula: anatom_globals.gt ( PERC_T_MSRFPCI_DROPS, 0.2)Remarks: Precondition: T_MSRFPCI timer > T8 and T10 timers

It is suggested to verify the value of this KPI during “Busy Hour”KPIs and RAW counters used in formula:

PERC_T_MSRFPCI_DROPS (new, see CHP 3.2.11)

ANATOM category:


4.1.3 Call Drops cause identification and evaluation on base of their distribution

Identification of the main causes related to the call dropping in a network and knowledge of their distribution can give an indication on what steps of further investigation are needed in order to verify the network status and define possible improvement proposals.The KPIs described in chapters 3.2.2 to 3.2.11 have been created with this purpose.

4.1.4 Call Drops because of high interferences in downlink

The distribution of call drops among the possible cause can help in the identification of the possible root cause for the dropping.However the different call dropping types cannot give a univocal answers on the possible cell/network problem without taking in account the network planning and configuration.Indeed if a high number of drops due to T200 timer expiration (L2) indicates clearly a problem (high interferences) in downlink (ASSCMD/HOCMD messages do not arrive to MS), a relative high number of drops due to Radio Link Failure (S-COUNTER expired) alone do not automatically mean a sure problem in uplink (interference and/or signal level problem in UL and/or DL are the main cause that trigger this type of failures, its presence could be related to a particular configuration of other timers like T3105, T8, T10 or TTRAU timers).

1. If T200 > 40 % of all drop causes then interference problem in DL exists2. If 25%< T200< 40% and T200 + RLF >> 55% of all drop causes then interference problem

in DL exists3. (This case has not been considered in the KPI described in chapter 3.3 because it is

involving HO analysis, which are already subject of other dedicated Use Cases).If HO ATT due to Quality DL is high and TTRAU and HOAF call drop are high (see next chapter) then interference problem in DL exists (and MS does not receive the sufficient level to reach the target BTS probably because interference in UL is quite high as well)

4.1.4.1 Threshold used in the KPI definition

KPI Threshold Chapter of KPI description

Comments

PERC_T200_DROPS >40% 3.3PERC_T200_DROPS PERC_T200_DROPS + PERC_RLF _DROPS

>25%>55%

3.3 The two conditions must be verified together.

4.1.5 Too high RTF Drops figures (when TTRAU timer > T8 and T10 timers)

If the timers mentioned in the title are accordingly configured, the expiration of timers TSYNC, TSYNCDL and TSYNCUL should point out that call have been dropped during speech phase (not because of missing steps in the signalization call flow).In this case, if RTF Drop figures reach a relevant level among all the dropping causes in the cell (the UC default is set to 10%), then the cell could have problem at the terrestrial interface.It is suggested in this case to proceed to investigate the SW and HW of the involved equipment especially at TRAU side.



Comments

PERC_RTF_DROPS >10% 3.4 TTRAU > T8 and TTRAU > T10


4.1.6 Call Drops because of high interference (or distance) in uplink (when TTRAU timer < T8 and T10 timers)

If T200 and TTRAU are appropriately configured the expiration of TIMER TTRAU should point out that MS has received the HOCMD or ASSCMD in DL but its answer (HO ACCESS) has not reached the target BTS, in case TTRAU has been set too long, the expiration of the Timer T3124 in the MS triggers the escalation for the expiration of S-Counter (=> RLF):

1. T200 < 20% and TTRAU >35% and HO ATT UL QUALITY >= 15% then interference in UL exists

2. T200 < 20 and TTRAU+RLF >45 % and HO ATT UL QUALITY >= 15% then interference in UL exists

3. T200 < 20% and TTRAU >35% and HO ATT UL LEVEL >= 15% then level problem in UL, cells do not overlap optimally at their border or maybe traffic island problem occurs

4. T200 < 20% and TTRAU+RLF >45 % and HO ATT UL QUALITY >= 15% then level problem in UL, cells do not overlap optimally at their border or maybe traffic island problem occurs

The KPI described in chapter 3.5 in reality checks only the first part of the problem. In other words it identifies only the situation that there are conditions to suspect a problem in UL. The identification if this problem is due to low signal levels (coverage problems or traffic island problem) or to high level of interference in UL is left to further investigation made with the appositely designed UC.



Comments

PERC_T200_DROPSPERC_RTF_DROPS

<20%>35%


PERC_T200_DROPS PERC_RTF_DROPS + PERC_RLF _DROPS

<20%>45%


4.1.7 Too high T_MSRFPCI Drops figures (when T_MSRFPCI timer > T8 and T10 timers)

Under this condition any problem on the radio side should have been detected by the expiration of other timers like T8 or TTRAU and so on. Therefore expiration of this timer could be given only because of restriction on the terrestrial interface (A-bis resources or BSC response time).A verification of the dedicated signaling resource to the cell should be performed.



Comments

PERC_T_MSRFPCI_DROPS >20% 3.6 The timer T_MSRFPCI must be longer than any other mentioned timer.

4.2 Problem reporting

4.2.1 Detection of cell with Call Drop problems


A specific report was defined in ANATOM to detect the described Call Drop problems and to display them to the user.The report is called ”Detection of cells with too high Call Drop rate.arc”.and can be opened through universal report template in ANATOM.

After the report is open the user has only to specify objects and time range to be analyzed and start the report by pushing the button ‘Refresh’. Afterwards the following result will be displayed for the selected objects and time range.

Figure 1: View of the Anatom report defined for the identification of cells with too high Call Drop problem

The report contains one column for each of the KPI used for the detection of the problem and an extra column for the carried TCH traffic.The lines are grouped per SITE name and Sector.

How to read the entries (example describes Figure 2 below ):For the Site “AIRPORT” the problem ‘Too High Call Drop’ was reported 11 times in the selected time range and for the selected Sector 1 the number of problem appearances has been 3.


If the user is now interested in seeing the details of the Site “AIRPORT” he can expand the data grid to SECTOR level.The user will then see details like in the following Figure 2: it is possible (thanks to the filtering setting to 1 in the column ERLCALLDROP_HIGH) to see date and hours when the problem arose and it is possible evaluate – thanks to the column TCH_TRAFFIC_CARRIER – the relevance of the arisen problem.

Figure 2: Details on Site “AIRPORT” in Sector 1

The observation time is on hour base and a range of ten days as shown in Figure 3.

Figure 3: BSC and time selection

4.2.1.1 Used filters and threshold


Object selection

panel

Time selection panel

4.2.1.1.1 Object filtering

To limit the number of objects available for reporting to the statistical relevant ones a pre query filter was defined as follows.

Figure 4: Filter definition based on TCH_TRAFFIC_CARRIED KPI

This means at least at one hour of the specified observation period the sum of carried traffic must be higher or equal to 5 ErlangHour per hour to let the Cell (BTS) appear in the object selection panel of the universal report. The user of the report can customize this filter to his needs by changing the thresholds, changing the filter or adding additional filters.

4.2.1.1.2 Thresholds used for problem detectionFollowing thresholds are used for problem detection and are predefined through KPI definition.

1. Too High number of Call Drops

Threshold 2%

If the Number of call drops per Erlang/hour, on an hour basis, exceeds the value 2% the problem is indicated as existent for this hour. The threshold is calculated on a base of a call Mean Holding Time of 90 seconds as shown in the formula below.

Remark: The user can change the applied thresholds by changing the KPI definition in ANATOM formula editor as shown in Figure 5.

Figure 5: KPI-Formula editor


Applied threshold

Formula editor

4.2.1.2 Additional analysis possibilities with ANATOM

In this chapter additional analysis possibilities with ANATOM tool are described, which are not available as predefined.As ANATOM also offers geographical representation of data the analysis results can be also displayed on a map. From the predefined report supporting the described use case the user can switch to geographical representation by choosing the related ‘sheet’ named ‘geographical’.The pre-selected KPIs, raw counters and CM attributes are applied automatically to this sheet. Also the pre-query filter will be valid for this sheet. By pressing the button refresh the data will be available for display on the map.

Example: The user wants to see for a specific day the cells which showed the problem ‘ High number of call drops’.

He selects all cells and the day of interest through the object and time selection panel.He presses the button ‘refresh’.

The following map is displayed.

Figure 6: KPI geographical representation

Automatically the problem ‘Too high Call Drop Rate’ will be displayed on the map.If the user wants to personalize the representation, a different way can be followed:

Select the feature ‘Create thematic map’ from the map toolbar


Create thematic map

The following dialog box will open

Figure 7: Thematic map

Select Type: Individual and press Next>

The following dialog box will open

Figure 8: Create thematic shield

Select table ‘Layer_THM’ holding the columns displayed in the grid of the report.Select the field of your interest. In the example it is the field ‘ERLCALLDROP_HIGH’.

Press Next>.

The following dialog-box will popup.


Type: individual

Figure 9: Create Thematic “Styles...” customization shield

Figure 10: Create Thematic “Legend...” customization shield

By pushing the button ‘OK’ the following representation will be displayed on the map.


Changing colours for representation

Changing legend

Figure 11: Excessive call drop report on geographical view

Remark: Further possibilities are described in ANATOM user manual and related documents.

Related documentation:

ANATOM user manuals MapInfo Professional User Guide


http://www.mapinfo.com/support/documentation/manuals.cfm#mi_pro

https://ims.icn.siemens.de/livelink/livelink/Open/325317170

4.2.2 Call Drop causes distribution

A specific report was defined in ANATOM to detect the described Call Drop problems and to display them to the user.The report is called ”Call Drop causes distribution.arc”.and can be opened through universal report template in ANATOM.

After the report is opened the user has only to specify objects and time range to be analyzed and start the report by pushing the button ‘Refresh’. Afterwards results will be displayed for the selected objects and time range in a similar way like the screenshot in the next figure.

Figure 12: Call drop distribution report


4.2.3 Call drops because of high interference in DL

A specific report was defined in ANATOM to detect the described Call Drop problems and to display them to the user.The report is called ”HIGH INTERFERENCE IN DL.arc”.and can be opened through universal report template in ANATOM.


Figure 13: High interference in DL report


4.2.4 Too High RTF Drop figures

A specific report was defined in ANATOM to detect the described Call Drop problems and to display them to the user.The report is called ”Too high RTF figures.arc”.and can be opened through universal report template in ANATOM.


Figure 14: Too high RTF drop figures report


4.2.5 Call drops because of high interference in UL

A specific report was defined in ANATOM to detect the described Call Drop problems and to display them to the user.The report is called

”HIGH INTERFERENCE IN UL.arc”.

and can be opened through universal report template in ANATOM.


Figure 15: High interference in UL report


4.2.6 Call drops because of missing terrestrial resources

A specific report was defined in ANATOM to detect the described Call Drop problems and to display them to the user.The report is called

”HIGH_T_MSRFPCI.arc”.

and can be opened through universal report template in ANATOM.


Figure 16: High T_MSRFPCI figures report


ius1.0_call drop analysis

Documents