DCR Analisys

DN01160043Version 1 english

Nokia NetworksCompany Confidential

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Drop Call Rate Formula Analysis in a GSM Network

Document type: Operating ProcedureCreator: Carola BottazziReviewer: Emilia De-RangoApprover: Paolo SandronDate approved: 2001-10-23Function:

Table of contents

1. Purpose ..................................................................................................................................... 32. Scope ........................................................................................................................................ 33. Responsibilities.......................................................................................................................... 34. Drop call rate concept ................................................................................................................ 4

4.1 Period of observation...................................................................................................... 44.2 Area of observation ........................................................................................................ 4

5. DCR Formulas based on Traffic/Handover counters .................................................................. 65.1 Numerator: Failure Counters .......................................................................................... 65.2 Denominator: Seizure/Assignment Counters .................................................................. 85.3 Call Re-establishment impact ......................................................................................... 125.4 "GSM" Formula............................................................................................................... 13

5.4.1 CELL BASIS ..................................................................................................... 135.4.2 AREA BASIS .................................................................................................... 14

5.5 "1139" Formula............................................................................................................... 155.5.1 CELL BASIS ..................................................................................................... 165.5.2 AREA BASIS .................................................................................................... 16

5.6 "1148" Formula............................................................................................................... 185.6.1 CELL BASIS ..................................................................................................... 195.6.2 AREA BASIS .................................................................................................... 19

6. DCR Formulas based on Serlev counters .................................................................................. 216.1 "Serlev" Formula............................................................................................................. 21

6.1.1 NUMERATOR................................................................................................... 226.1.2 DENOMINATOR............................................................................................... 226.1.3 CELL BASIS ..................................................................................................... 226.1.4 AREA BASIS .................................................................................................... 236.1.5 CALL RE-ESTABLISHMENT ............................................................................ 23

6.2 "Conversation" Formula.................................................................................................. 256.2.1 NUMERATOR................................................................................................... 256.2.2 DENOMINATOR............................................................................................... 256.2.3 CELL BASIS ..................................................................................................... 256.2.4 AREA BASIS .................................................................................................... 26

7. ANALYZED Data ....................................................................................................................... 277.1 Measurement counter statistics ...................................................................................... 277.2 BSC Clear Code Observation Measurements................................................................. 28



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7.3 Test bed measurements: ................................................................................................ 288. RESULTS – NORMAL CELLS................................................................................................... 29

8.1 Comparison of different formulas.................................................................................... 298.2 Distribution of failures ..................................................................................................... 338.3 Comments on Serlev Formula ........................................................................................ 388.4 Comments on 1139 Formula .......................................................................................... 408.5 1148 Formula ................................................................................................................. 438.6 Cell-BH........................................................................................................................... 49

9. results – cells with iuo ................................................................................................................ 5110. results – Call Re-establishment.................................................................................................. 5411. Conclusions ............................................................................................................................... 5512. References ................................................................................................................................ 5513. Glossary .................................................................................................................................... 5514. Version history ........................................................................................................................... 5515. Appendices ................................................................................................................................ 56

15.1 Appendix 1. Call Phases................................................................................................. 5615.2 Appendix 2. Tests on Serlev Counters............................................................................ 58



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1. PURPOSE

Dropped call ratio is one of the most important indicators to monitor the networkquality.

Purpose of the document is to provide a proposal of new formulas aimed at gettingnear the subscriber viewpoint as much as possible. Particular attention is thereforegiven to the several phases of the call and their impact both, on the DCR value andsubscriber perception.

All counter families are taken into account for the new formula definitions.

The formulas proposed take into account the main features activated in a GSMnetwork (Queuing, Directed Retry, TCH/SDCCH handovers, AMH, IUO/IFH, Call Re-establishment) and are defined for all types of cells (Normal, Parent, Child).

The formulas use counters that are compatible with S9 BSC SW and with T12 OMCSW version.

2. SCOPE

In this document five DCR formulas are discussed.The first formula measures the Dropped Call Ratio from TCH_channel_activationmessage.The second and the third are new proposals based on Traffic/Handover counters andmeasure the DCR from Connect_ack and Assignment_complete messages,respectively.The fourth is a new proposal based on Serlev counters and tracks failures fromAssignment_complete message up to Disconnect.The fifth and last one formula presented takes into consideration only conversationphase (from Connect_ack up to Disconnect messages) in DCR calculation.

All the formulas were tested and compared using statiastical data: different networkconfigurations (features) were taken into account; different analysis approaches (interm of observation periods and observation areas) were also considered.

Furthermore some tests have been performed in Nokia Test Bed, in order to verifythe trigger points of some counters involved in the formula.

3. RESPONSIBILITIES

Consulting Manager is responsible for all the activities related to Drop Call RateConsultancy.



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4. DROP CALL RATE CONCEPT

The formula of the Drop Call Rate is defined as the ratio between the failures occurred during thecall in the period of observation and the number of calls occurring in the area of observation.

Different formulas can be developed according to the period of observation and the area ofobservation, according to customer needs. The different approaches are described below.

4.1 Period of observation

The signaling chart of a mobile (originated/terminated) call can be divided into several phases(see Appendix 1 for a detailed description of call phases). Particularly, the main phasesconcerning TCH channel type are:

• Phase 3: Setup phase, From Assignment_request to Assignment_complete• Phase 2: Setup phase, from Assignment_complete to Connect_ack• Phase 15: Conversation phase, from Connect_ack to Disconnect• Phase 4: Release phase, from Disconnect to ReleaseIn addition, also Handover phases (phases 9-11 for outgoing handovers, phases 12-14 forincoming handovers) need to be considered, in case of TCH-->TCH handovers occur.

The possibility to distinguish among different phases is essential from a network monitoring pointof view, since each phase can be affected by different problems.Besides, each phase is perceived in a different way by the end user. A failure in phase 15 isperceived as a real drop, whereas a failure in phase 3 is perceived as an access failure. A failurein phase 2 is also perceived as access failure but it can be more critical since the alert might havealready started. A failure in phase 4 is usually not perceived by the end user, as the call is beingreleased anyway.

The period of observation has mainly impact on the numerator of the formula, since all and onlythose failures triggered inside the monitored period of observation have to be taken into account.

4.2 Area of observation

The DCR can be computed on cell basis or on area basis.

In the first case each and every cell is separately monitored.The DCR is intended as percentage of failures with respect to the total assigned TCH amount in acell, (including new calls and incoming handovers), rather than the total amount of calls starting inthe cell. The idea behind this is to have an estimation of failure rate with respect to the traffic loadof the cell.Besides, the TCH assignments due to incoming handovers need to be taken into account (at thedenominator) in order to make the formula suitable for all types of traffic in the cell. The DCR valuefor a cell serving a high-speed-traffic-type (e.g. motorway), where all the traffic enters the cell byhandover procedure, is as reliable as the DCR value of a cell serving a stationary-traffic area (e.g.shopping centre), where the majority of calls start and ends inside the same cell.

On a generic area level it is convenient to compute the failure rate with respect to the calls that arestarted in the area, thus ignoring the incoming handover contribution to the amount of TCHassignments. In this way the estimated DCR is relative to the real number of users (each user isconsidered only ones, according to the area where the conversation was started).



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If the observation area is quite big, the incoming handover contribution to the traffic load of thearea is negligible compared to the total amount of new call assignments: therefore the estimatedDCR is also proportional to the traffic load in the area.

In the following session different formula proposals, based on different counter families, will bepresented and different approaches (in terms of time and area observation periods) will beconsidered.



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5. DCR FORMULAS BASED ON TRAFFIC/HANDOVER COUNTERS

Several formula proposals can be made with Traffic/Handover counters, according to the callphases included in the period of observation (see Appendix 1 for call phases description).

In all these proposals the DCR is computed in a similar way, as ratio between:

• number of counters triggering all the TCH failures inside the considered area and time periodand• number of counters triggering all the TCH allocations: the triggering point changes according

to the time period (call phases) starting point.

5.1 Numerator: Failure Counters

The numerator must contain all the possible TCH failures leading to a dropped call in the relevantarea. For this purpose, not only failures during the setup/conversation/release phase (phases 2-4,15), but also failures that occur during outgoing handovers (phases 9-11) need to be considered.Instead, failures that occur during incoming handovers (phases 12-14) do not have to beconsidered, as they do not always mean a drop (the mobile can reverse to the old channel).

Figure 1 shows all the TCH failure counters together with the relative triggering phases.

113911401013/1014

1029/10301084/10851087/1088104610471048104910501070

1081

Figure 1 Failure counters and relative trigger points (Traffic family)

Considering the whole period where TCH is active, from TCH_channel_activation to Release, thepossible causes (and relative counters) leading to a drop call are:



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• Radio Failures = 001013 + 001014 = a.tch_radio_fail + a.tch_rf_old_ho;

• Transcoder Failures = 001029 + 001030 = a.tch_tr_fail + a.tch_tr_fail_old;

• Failures on Abis interface = 001084 + 001085 = a.tch_abis_fail_call + a.tch_abis_fail_old;

• Failures on A interface = 001087 + 001088 = a.tch_a_if_fail_call + a.tch_a_if_fail_old;

• Other Failures = 001046 + 001047 + 001048 + 001049 + 001050 =a.tch_lapd_fail + a.tch_bts_fail + a.tch_user_act + a.tch_bcsu_reset + a.tch_netw_act;

• Failure on TCH channel activation = 001081 = a.tch_act_fail_call

• Forced Releases = 001070 = a.forced_releases

wherea = p_nbsc_traffic.

Notice that:

• forced released calls due to an incoming pre-emption call are considered as failures since,from the subscriber's viewpoint, a forced release is a dropped call.

The TCH failures occurring only in the setup phase (phases 2-3), from TCH_channel_activation toConnect_ack, can be counted as sum of:

• Radio Failure in setup = 001139 = a.tch_rel_due_radio_fail;• BSS Failure in setup = 001140 = a.tch_rel_due_bss_fail


It should be noticed that counters 1139, 1140 are also triggered in phases (9-11 and 12-14), i.e.when the failures occur during handover, if the HO is started before conversation phase. Thiscauses an overestimation of the number of drops in setup phase, as failures during incominghandovers (phases 12-14) do not always mean drop of the calls.



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5.2 Denominator: Seizure/Assignment Counters

The expression of the denominator is derived by considering all the allocated TCHs that are still inplace at the starting point of the time observation period (call phases) under consideration.Different contributions to the TCH allocation (new calls, incoming handovers) need to be included,according to the observation area under consideration.

Figure 2 shows all the seizure/assignment counters together with the relative triggering points.

10091099

40434056

404440574074

1148

1165

Figure 2 Seizure/Assignment counters and relative trigger points (Traffic/Handover families).

The following type of seizures might need to be considered:

• Normal Seizure = 001009 = a.tch_norm_seiz• FACCH Call Setup = 001099 = a.tch_seiz_due_sdcch_cong• Direct Accessto Super TRX = 001165 = a.succ_tch_seiz_dir_acc


Please note that:• Counter 001009 is also triggered in case of Direct Accessto Super TRX: this needs to be

taken into account in order to avoid counting twice the Direct Accesscontribution.• TCH seizures due to incoming handovers are not taken into consideration since failures on

incoming handovers do not have to be considered at the numerator: therefore the incoming



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handover contribution to the denominator is always considered starting from theHandover_complete.

The following type of assignments might need to be considered:

• Normal Assignment = 001148 = a.ms_tch_succ_seiz_ass_compl• Directed Retry (external, internal intercell, internal intracell) = 004044 + 004057 + 004074 =

b.msc_i_sdcch_tch + b.bsc_i_sdcch_tch + b.cell_sdcch_tch• Incoming Handover (external, internal intercell) = 004043 + 004056 = b.msc_i_tch_tch +

b.bsc_i_tch_tch

wherea = p_nbsc_traffic;b = p_nbsc_ho.

Please notice that:• New call TCH allocation contribution is taken into account either by counter 1009 or by counter

1148 according to the time observation period (call phases) under consideration.• TCH Assignment to Super TRX contribution is taken into account by counters 4057_

bsc_i_sdcch_tch (child cell) and 4074_ cell_sdcch_tch (regular cell).• Direct Accesscontribution is also included in counters 4057_ bsc_i_sdcch_tch (child cell) and

4074_ cell_sdcch_tch (regular cell): this needs to be taken into account since counters 1009and 1165 also trigger TCH allocation for Direct Access.

The detailed procedures of Direct Accessto super TRX and TCH Assignment to Super TRX aredescribed in Figures 3-6.

Intra Dir Acc SUCC004074

cell_sdcch_tch

TCH REQUESTS001010

tch_request

Cell A (target, super TRX)

Direct Access Attempt004077

cell_sdcch_tch_at

TCH SEIZURES FORNEW CALL001009

tch_norm_seiz

TCH REQUESTSFOR NEW CALL001026

tch_call_req

DIR ACC TCH REQ REJ DUE LACK001166tch_req_dir_acc_rej_due_lack

TCH SEIZURES FORDIRECT ACCESS001165tch_succ_seiz_for_dir_acc

Congestion

No Congestion

Direct Access (SDCCH-TCH HO)TO SUPER TRX

Cell A (source, regular TRX)


Normal TCH call request handling

Figure 3: intracell Direct Accessprocedure.



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Direct Access in success004057

bsc_i_sdcch_tch


TCH REQUESTS001010

tch_request

Cell B (target, super TRX)Direct Access in att004060

bsc_i_sdcch_tch_at

Direct Access OUTSUCC 004065bsc_o_sdcch_tch

Direct Access OUT ATT004068bsc_o_sdcch_tch_at

TCH SEIZURES FORNEW CALL001009

tch_norm_seiz

Direct Access (SDCCH-TCH HO)TO SUPER TRX CELL B

TCH REQUESTSFOR NEW CALL001026

tch_call_req

Congestion


Normal TCH call request handling

No Congestion

DIR ACC TCH REQ REJ DUE LACK001166tch_req_dir_acc_rej_due_lack

TCH SEIZURES FORDIRECT ACCESS001165tch_succ_seiz_for_dir_acc

Figure 4: intercell Direct Accessprocedure.

TCH REQUESTS001010

tch_request

TCHREQUESTSFOR NEWCALL001026

tch_call_req

TCH SEIZURES FORHO (incl. DR in)001008

tch_ho_seiz

Intra DR SUCC004074

cell_sdcch_tch


TCH REQUESTS001010

tch_request

Cell A (target, super TRX)

DR (SDCCH-TCH HO)TO SUPER TRX

TCH congestion

DR ATT004077

cell_sdcch_tch_at

Figure 5: intracell TCH Assignment to super TRX procedure.



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TCH REQUESTS001010

tch_request

TCHREQUESTSFOR NEWCALL001026

tch_call_req

TCH SEIZURES FORHO (incl. DR in)001008

tch_ho_seiz

DR IN SUCC004057

bsc_i_sdcch_tch


TCH REQUESTS001010

tch_request

Cell B (target, super TRX)

DR (SDCCH-TCH HO)TO SUPER TRX CELL B

TCH congestion

DR IN ATT004060

bsc_i_sdcch_tch_at

DR OUT SUCC004065

bsc_o_sdcch_tch

DR OUT ATT004868

bsc_o_sdcch_tch_at

TCH REQUESTS REJ DUE LACK001011tch_rej_lack

Figure 6: intercell TCH Assignment to super TRX procedure.



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5.3 Call Re-establishment impact

In case of Call Re-establishment, a new TCH seized is recorded at the denominator (by means ofcounter 1009_ a.tch_norm_seiz) and in the mean time a failure is recorded at the numerator.From the end user point of view, this failure doesn’t lead to a dropped call due to the Call Re-establishment procedure. Nevertheless, as far as concerns the network performance this failure isnot negligible.In this document two different approaches will be considered, according to whether a dropfollowed by Call Re-establishment has to be counted or not.

The following counters (belonging to Resource Access family) trigger the number ofEstablish_indications for Call Re-establishment:

• 003020 = a.successful_estab_indication message on SDCCH;• 003025: successful estab_indication message on TCH (if FACCH is enabled).

Wherea = p_nbsc_res_access.

Cell A

Cell BCell A

Drop:failure counter


Re-establish:1009, 3020/3025

Re-establish:1009, 3020/3025

Re-establish:1009, 3020/3025

Re-establish:1009, 3020/3025



Figure 7: Call Re-establishment procedure (Traffic/Resource availability counters).

Figure 7 shows the Call Re-establishment procedure. The final formula may be corrected bysubtracting successful estab_indication message on SDCCH/TCH both from the numerator andthe denominator: in this way nor the failure, not the seizure for Call Re-establishment isconsidered.

Still, this correction is a bit critical for two reasons:• in case the call is re-established on a different cell, then the failure will be normally counted in

the old cell; instead, the new cell will have one failure less, and one seizure less also: thereforethe failure is "cancelled" in the wrong cell. This problem is not relevant when DCR is computedon area basis.

• the actual number of Call Re-establishment in the cell is lower than successfulestab_indication message on_SDCCH/TCH due to the failures, which may occur from theEstablishment_indication (received from the BTS) to the Assignment_complete (sent to theMSC). No more counters are available to correct that value.



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5.4 "GSM" Formula

The most general DCR formula used in GSM network (thus nominated as "GSM" formula) countsall the TCH failures across the whole period where TCH is active, i.e. all TCH call phases areincluded in DCR computation (see Figure 8).

TCH_ activation

Assignment_cmpl

Connect_ack

Disconnect

Release

GSM

Figure 8: call phases taken into account by "GSM" formula.

The numerator is therefore simply the sum of all the failure counters with triggering period fromTCH_channel_activation to Release (listed above).

At the denominator:• the number of TCH seizures for normal assignment and FACCH call setup is considered (i.e.

trigger point: TCH_channel_activation): 001009 + 001099;• directed retry contribution is considered: 004044 + 004057 + 004074;• Incoming handover contribution is considered ONLY if the DCR is computed on cell basis:

004043 + 004056;• TCH assignment to super TRX contribution is taken into account by Directed Retry counters

(004057 + 004074);• The number of seizures for Direct Accessto Super TRX (001165) has to be subtracted from

the number of normal seizures (001009), since Direct Accessto Super TRX contribution isalready taken into account by Directed Retry counters (004057 + 004074);

• The Call Re-establishment contribution might be considered or not: in the first case counters3020 + 3025 have to be subtracted both from the numerator and the denominator.

5.4.1 CELL BASIS

The DCR formula on cell basis is as follows (Call Re-establishment correction not taken intoaccount):

( a.tch_radio_fail + a.tch_rf_old_ho + a.tch_tr_fail ++ a.tch_tr_fail_old + a.tch_abis_fail_call + a.tch_abis_fail_old ++ a.tch_a_if_fail_call + a.tch_aif_fail_old + a.tch_lapd_fail ++ a.tch_bts_fail + a.tch_user_act + a.tch_bcsu_reset ++ a.tch_netw_act + a.forced_releases + a.tch_act_fail_call )

(a.tch_norm_seiz + a.tch_seiz_due_sdcch_cong + b. msc_i_sdcch_tch + b.bsc_i_sdcch_tch +b.cell_sdcch_tch + b.msc_i_tch_tch + b.bsc_i_tch_tch - a.succ_tch_seiz_dir_acc )

DCRCELL =



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(1013 + 1014 + 1029 + 1030 + 1084 + 1085 + 1087 + 1088 + 1046 + 1047 + 1048 + 1049 + 1050 + 1070 +1081 )

( 1009 + 1099 + 4044 + 4057 + 4074 + 4043 + 4056 - 1165)

DCRCELL =

TCH Failures from TCH_ch_activation to Release

TCH seizured for normalassignment

Directed Retry,Direct Access,TCH assignment contribution

Incoming Handoverscontribution

Direct accesscontribution alsoincluded in 1009

FACCH call setupcontribution

5.4.2 AREA BASIS

The DCR formula on area basis is as follows (Call Re-establishment correction not taken intoaccount):

( a.tch_radio_fail + a.tch_rf_old_ho + a.tch_tr_fail ++ a.tch_tr_fail_old + a.tch_abis_fail_call + a.tch_abis_fail_old ++ a.tch_a_if_fail_call + a.tch_aif_fail_old + a.tch_lapd_fail ++ a.tch_bts_fail + a.tch_user_act + a.tch_bcsu_reset ++ a.tch_netw_act + a.forced_releases + a.tch_act_fail_call )

(a.tch_norm_seiz + a.tch_seiz_due_sdcch_cong + b. msc_i_sdcch_tch + b.bsc_i_sdcch_tch +b.cell_sdcch_tch - a.succ_tch_seiz_dir_acc )

DCRAREA =

(1013 + 1014 + 1029 + 1030 + 1084 + 1085 + 1087 + 1088 + 1046 + 1047 + 1048 + 1049 + 1050 + 1070 +1081 )

( 1009 + 1099 + 4044 + 4057 + 4074 - 1165)

DCRAREA =

TCH Failures from TCH_ch_activation to Release

TCH seizured for normalassignment

Directed Retry,Direct Access,TCH assignment contribution

Direct accesscontribution alsoincluded in 1009




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5.5 "1139" Formula

The first DCR formula proposed in this document counts all the TCH failures in conversation(phase 15) and release (phase 4) phases only, i.e. the DCR is computed over the period fromConnect_ack to Release of the call (see Figure 9).

This formula will be referred as "1139" formula.

TCH_ activation

Assignment_cmpl

Connect_ack

Disconnect

Release

1139

Figure 9: call phases taken into account by "1139" formula.

At the numerator the number of TCH failures in phases 15+4 is obtained by subtracting thenumber of TCH failures in setup phase 2+3 (i.e. from TCH_channel_activation to Connect_ack)from the total amount of TCH failures.

The numerator is therefore equal to: "GSM" formula numerator – (1139 + 1140 + 1081).

At the denominator:

• the number of the number of normal seizures (1009) is lowered by the amount of TCH failuresin phase 2+3 (1139 + 1140 + 1081) since the starting point of time observation period hasmoved from TCH_channel_activation to Connect_ack;

• FACCH call set up contribution is considered (1099);• Directed Retry contribution is considered: 004044 + 004057 + 004074;• Incoming handover contribution is considered ONLY if the DCR is computed on cell basis:

004043 + 004056;• TCH assignment to super TRX contribution is taken into account by Directed Retry counters

(004057 + 004074);• The number of seizures for Direct Accessto Super TRX (001165) has to be subtracted from

the number of normal seizures (001009), since Direct Accessto Super TRX contribution isalready taken into account by Directed Retry counters (004057 + 004074);

• Call Re-establishment contribution might be considered or not: in the first case counters 3020+ 3025 have to be subtracted both from the numerator and the denominator.

Please notice that:• FACCH call setup contribution (1009) at the denominator is considered starting from

TCH_channel_activation: in fact the real number of TCH assignment for FACCH call setup isless due to TCH failures that might occur between TCH_channel_activation andAssignment_complete. No more counters are available to correct this value. Nevertheless this



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inaccuracy is negligible since the number of FACCH call setup seizures is much lower than thetotal number of TCH assignments.

• As previously underlined, counters 1139, 1140 are also triggered in phases (9-11 and 12-14),i.e. when the failures occur during handover, if the HO is started before conversation phase.This causes an overestimation of the number of drops in setup phase, as failures duringincoming handovers (phases 12-14) do not always mean drop of the calls. The impact ofincoming handover failures on "1139" formula will be analysed in session 8: RESULTS -NORMAL CELLS.

5.5.1 CELL BASIS


( a.tch_radio_fail + a.tch_rf_old_ho + a.tch_tr_fail ++ a.tch_tr_fail_old + a.tch_abis_fail_call + a.tch_abis_fail_old ++ a.tch_a_if_fail_call + a.tch_aif_fail_old + a.tch_lapd_fail ++ a.tch_bts_fail + a.tch_user_act + a.tch_bcsu_reset ++ a.tch_netw_act + a.forced_releases - a.tch_rel_due_radio_fail +- a.tch_rel_due_bss_fail )

(a.tch_norm_seiz - a.tch_act_fail_call - a.tch_rel_due_radio_fail - a.tch_rel_due_bss_fail +a.tch_seiz_due_sdcch_cong + b. msc_i_sdcch_tch + b.bsc_i_sdcch_tch + b.cell_sdcch_tch +b..msc_i_tch_tch + b.bsc_i_tch_tch - a.succ_tch_seiz_dir_acc )

DCRCELL =

(1013 + 1014 + 1029 + 1030 + 1084 + 1085 + 1087 + 1088 + 1046 + 1047 + + 1048 + 1049 + 1050 + 1070 - 1139 - 1140)

(1009 - 1081 - 1139 - 1140 + 1099 + 4044 + 4057 + 4074 + 4043 + 4056 - 1165)

DCRCELL =

TCH Failures from TCH_ch_activation to end of call TCH Failures from TCH_ch_activation to conn_ack

TCH assigned for normalassignment Directed Retry,Direct Access,

TCH assignment contributionIncoming Handovers

contributionDirect access contribution also

included in 1009


5.5.2 AREA BASIS


( a.tch_radio_fail + a.tch_rf_old_ho + a.tch_tr_fail ++ a.tch_tr_fail_old + a.tch_abis_fail_call + a.tch_abis_fail_old ++ a.tch_a_if_fail_call + a.tch_aif_fail_old + a.tch_lapd_fail ++ a.tch_bts_fail + a.tch_user_act + a.tch_bcsu_reset ++ a.tch_netw_act + a.forced_releases - a.tch_rel_due_radio_fail +- a.tch_rel_due_bss_fail )

(a.tch_norm_seiz - a.tch_act_fail_call - a.tch_rel_due_radio_fail - a.tch_rel_due_bss_fail +a.tch_seiz_due_sdcch_cong + b. msc_i_sdcch_tch + b.bsc_i_sdcch_tch + b.cell_sdcch_tch -a.succ_tch_seiz_dir_acc )

DCRAREA =



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(1013 + 1014 + 1029 + 1030 + 1084 + 1085 + 1087 + 1088 + 1046 + 1047 + + 1048 + 1049 + 1050 + 1070 - 1139 - 1140)

(1009 - 1081 - 1139 - 1140 + 1099 + 4044 + 4057 + 4074 - 1165)

DCRAREA =

TCH Failures from TCH_ch_activation to end of call TCH Failures from TCH_ch_activation to conn_ack

TCH assigned for normalassignment Directed Retry,Direct Access,

TCH assignment contributionDirect access contribution also

included in 1009




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5.6 "1148" Formula

A second DCR formula proposed in this document counts all the TCH failures in setup phase 2,conversation (phase 15) and release (phase 4) phases, i.e. the DCR is computed over the periodfrom Assignment_complete to Release of the call (see Figure 10).

This formula will be referred as "1148" formula.

TCH_ activation

Assignment_cmpl

Connect_ack

Disconnect

Release

1148

Figure 10: call phases taken into account by "1148" formula.

At the numerator the number of TCH failures in phases 2+15+4 is obtained by subtracting thenumber of TCH failures in setup phase 3 (i.e. from TCH_channel_activation toAssignment_complete) from the total amount of TCH failures.

The number of TCH failures in setup phase 3 (i.e. from TCH_channel_activation toAssignment_complete) is obtained as difference between the number of seizures for normal call(1009 - 1165) and the number of TCH normal assignments (1148).

The numerator is therefore equal to: "GSM" formula numerator – (1009-1165-1148).

At the denominator:

• the number of normal assignments (1148) is considered instead of the number of normalseizures (1009) since the starting point of time observation period has moved fromTCH_channel_activation to Assignment_complete;

• FACCH call set up contribution is considered (1099);• Directed Retry contribution is considered: 004044 + 004057 + 004074;• Incoming handover contribution is considered ONLY if the DCR is computed on cell basis:

004043 + 004056;• TCH assignment to Super TRX contribution is taken into account by Directed Retry counters

(004057 + 004074);• Direct Accessto Super TRX contribution is taken into account by Directed Retry counters

(004057 + 004074);• Call Re-establishment contribution might be considered or not: in the first case counters 3020

+ 3025 have to be subtracted both from the numerator and the denominator.

Please notice that:• FACCH call setup contribution (1009) at the denominator is considered starting from

TCH_channel_activation: in fact the real number of TCH assignment for FACCH call setup is



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less due to TCH failures that might occur between TCH_channel_activation andAssignment_complete. No more counters are available to correct this value. Nevertheless thisinaccuracy is negligible since the number of FACCH call setup seizures is much lower than thetotal number of TCH assignments.

• Since in phase 3 (i.e. from TCH_channel_activation to Assignment_complete) both TCH andSDCCH channels are active, the difference between the number of seizures for normal call(1009 - 1165) and the number of TCH normal assignments (1148) is also affected by thefailures on SDCCH channel. Therefore the estimation of number of TCH failures in setupphase 3 (i.e. from TCH_channel_activation to Assignment_complete) is accurate as far as thefailure on SDCCH in the same phase is negligible compared to TCH failures. No morecounters are available to correct this inaccuracy. The impact of SDCCH failures on "1148"formula will be analyzed in session 8: RESULTS -NORMAL CELLS.

5.6.1 CELL BASIS


( a.tch_radio_fail + a.tch_rf_old_ho + a.tch_tr_fail ++ a.tch_tr_fail_old + a.tch_abis_fail_call + a.tch_abis_fail_old ++ a.tch_a_if_fail_call + a.tch_aif_fail_old + a.tch_lapd_fail ++ a.tch_bts_fail + a.tch_user_act + a.tch_bcsu_reset ++ a.tch_netw_act + a.forced_releases - (a.tch_norm_seiz - a.succ_tch_seiz_dir_acc - a.ms_tch_succ_seiz_ass_compl)

(a.ms_tch_succ_seiz_ass_compl + b.msc_i_sdcch_tch +b.bsc_i_sdcch_tch + b.cell_sdcch_tch +b.msc_i_tch_tch + b.bsc_i_tch_tch )

DCRCELL =

(1013 + 1014 + 1029 + 1030 + 1084 + 1085 + 1087 + 1088 + 1046 + 1047 + + 1048 + 1049 + 1050 + 1070 - - (1009 -1165 - 1148)

1148 + 4044 + 4057 + 4074 + 4043 + 4056)

DCRCELL =

TCH Failures from TCH_ch_activation to end of call TCH Failures from TCH_ch_activation to Ass_cmpl

TCH assigned for normalassignment contribution

Directed Retry, Direct access to super,TCH assignment contribution


5.6.2 AREA BASIS




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( a.tch_radio_fail + a.tch_rf_old_ho + a.tch_tr_fail ++ a.tch_tr_fail_old + a.tch_abis_fail_call + a.tch_abis_fail_old ++ a.tch_a_if_fail_call + a.tch_aif_fail_old + a.tch_lapd_fail ++ a.tch_bts_fail + a.tch_user_act + a.tch_bcsu_reset ++ a.tch_netw_act + a.forced_releases - (a.tch_norm_seiz - a.succ_tch_seiz_dir_acc - a.ms_tch_succ_seiz_ass_compl)

(a.ms_tch_succ_seiz_ass_compl + b.msc_i_sdcch_tch +b.bsc_i_sdcch_tch + b.cell_sdcch_tch)

DCRAREA =

(1013 + 1014 + 1029 + 1030 + 1084 + 1085 + 1087 + 1088 + 1046 + 1047 + + 1048 + 1049 + 1050 + 1070 - - (1009 -1165 - 1148)

1148 + 4044 + 4057 + 4074

DCRAREA =

TCH Failures from TCH_ch_activation to end of call TCH Failures from TCH_ch_activation to Ass_cmpl

TCH assigned for normalassignment contribution

Directed Retry, Direct access to super,TCH assignment contribution



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6. DCR FORMULAS BASED ON SERLEV COUNTERS

6.1 "Serlev" Formula

A third DCR formula proposed in this document is based on a different counter family than theothers, precisely Serlev Counter family. This formula will be referred as "Serlev" formula.

"Serlev" formula counts all the TCH failures in setup phase 2 and conversation (phase 15) i.e. theDCR is computed over the period from Assignment_complete to Disconnect (see Figure 11).

TCH_ activation

Assignment_cmpl

Connect_ack

Disconnect

Release

Serlev

Figure 11: call phases taken into account by "Serlev" formula.

Norm ass Intracell HO

InternalCall re-est

Incoming HO(int/ext)

Incomingcall re.est

Intercell HO(int/ext)

Drop

Normalrelease

Drop

SDCCH -->SDCCHext HO

OutgoingCall re-est

inout

DR (int/ext)

TCH ass(intra/inter)

DA(intra/inter)

57033

57034

57033

57033

57033

57032

57034570364076

5703557037

57036

57032

Figure 12: Serlev counters involved in "Serlev" formula: updating procedure.



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Figure 12 gives an overview of the counters involved in "Serlev" formula. The main peculiarity withrespect to the counters previously considered is that TCH failures do not trigger any failurecounter: normal release counters are updated instead.

6.1.1 NUMERATOR

The number of TCH failures (Numerator) is computed by subtracting the number of normalreleases from the number of assignments.

The assignments to be considered are:

• Normal assignment + Directed Retry (external, internal intercell, internal intracell) = 57033 =tch_new_call_assign;

• Incoming Handover (external, internal intercell) = 57034 = tch_ho_assign;• Call Re-establishment = 57032 = tch_re-est_assign

Please notice that Direct Accessto Super TRX and TCH assignment to super TRX contribution isalso included in counter 57033.

The releases to be considered are:

• Normal call release = 57035 =tch_norm_release;• TCH channel release due to outgoing handovers (external, internal intercell) = 57036 =

tch_ho_release;• Call re-established call release = 57037 = tch_re-est_release.

Please, notice that in case of successful intracell handover both counters 57036(tch_ho_release)and 57034(tch_ho_assign) are incremented, thus compensating each other at the numerator (noextra correction is needed).

6.1.2 DENOMINATOR

The denominator is computed by adding the different contributions to the assignments:

• Normal assignment + Directed Retry (external, internal intercell, internal intracell);• Incoming Handover (external, internal intercell) contribution to be considered only when DCR

is computed on cell basis• Call Re-establishment contribution.

Please, notice that the counter 4073 (cell_tch_tch) belonging to Traffic family has to be subtractedfrom counter 57034(tch_ho_assign) so that only intercell handover contribution is taken at thedenominator

6.1.3 CELL BASIS




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( 57033 (tch_new_call_assign) + 57032 (call_reest_assign) + ( c57034 (tch_ho_assign) - 4073 (cell_tch_tch) ))

( 57033 (tch_new_call_assign) + 57034 (tch_ho_assign) + 57032 (call_reest_assign)

- ( 57035 (tch_norm_release) + 57036 (tch_ho_release) + 57037(tch_re-est_release) )

TCH assignment for call re-establishment

DCRCELL =

Assignment_complete for normal assignment,Directed Retry, Direct Access, TCH assignment

Handover_complete for intra/intercell,internal/external handovers

Call re-establishment releaseOutgoing handovers

6.1.4 AREA BASIS


( 57033 (tch_new_call_assign) + 57032 (call_reest_assign))

( 57033 (tch_new_call_assign) + 57034 (tch_ho_assign) + 57032 (call_reest_assign)

- ( 57035 (tch_norm_release) + 57036 (tch_ho_release) + 57037(tch_re-est_release) )DCRAREA =

6.1.5 CALL RE-ESTABLISHMENT

As already discussed for the Traffic/Handover based formulas, Call Re-establishment procedurehas different impact on DCR formula according to whether a drop before re-establishment isconsidered as drop (as it actually is, as far as the network performance is concerned) or ignored(as perceived by the end user).

Cell A

Cell BCell A

DropDrop

Re-establish:57032

Re-establish:57032

Assignment57033

Assignment57033 Release

57037

Release57037

DropDropAssignment57033

Assignment57033 Re-establish:

57032

Re-establish:57032

Release57037

Release57037

Figure 13: Call Re-establishment procedure (Serlev counters).

Figure 13 describes the Call Re-establishment procedure with Serlev counters.



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The "Serlev" formula presented above counts every drop regardless if the call is going to be re-established.This formula may be corrected by eliminating the counter 57032(tch_call_re-est_assign) both fromthe numerator and the denumerator. For every call re-established, there will be 1 seizure less atthe numerator and thus also 1 drop less. At the denominator the call re-established call is countedonly once (1 assignment).The limitation of this correction is that in case the call is re-established on a different cell, then theold cell will have the drop, whereas the new cell will have 1 assignment less and also 1 drop less,i.e. the failure is "cancelled" in the wrong cell. This problem is not relevant when DCR is computedon area basis.

The DCR formula on cell basis, corrected so that call re-established dropped are not counted, isas follows:

( 57033 (tch_new_call_assign) + ( c57034 (tch_ho_assign) - 4073 (cell_tch_tch) ))

( 57033 (tch_new_call_assign) + 57034 (tch_ho_assign) +

- ( 57035 (tch_norm_release) + 57036 (tch_ho_release) + 57037(tch_re-est_release) )

Assignment_complete for normal assignment,Directed Retry, Direct Access, TCH assignment

Handover_complete for intra/intercell,internal/external handovers

Outgoing handovers

TCH assignment for intercell HO(internal + external)

DCRCELL =

Call re-establishment release

The DCR formula on cell basis, corrected so that call re-established dropped are not counted, isas follows:

57033 (tch_new_call_assign)

( 57033 (tch_new_call_assign) + 57034 (tch_ho_assign)

- ( 57035 (tch_norm_release) + 57036 (tch_ho_release) + 57037(tch_re-est_release) )DCRAREA =



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6.2 "Conversation" Formula

In addition to the formulas already presented, the DCR formula used in Conversation network wasalso taken into consideration during the analysis, for comparison and completeness.

This formula will be referred as "Conversation" formula.

"Conversation" formula is based on Serlev counters and tracks all the failures in conversationphase (phase 15) only, i.e. from Connect_ack up to Disconnect (see Figure 14).

TCH_ activation

Assignment_cmpl

Connect_ack

Disconnect

Release

cegetel

Figure 14: call phases taken into account by "Conversation" formula.

6.2.1 NUMERATOR

At the numerator the failures are counted by means of the counter 57007 (dropped_calls).

6.2.2 DENOMINATOR

At the denominator the number of TCH still in place at the Connect_ack is computed as sum of:

• Normal seizures + FACCH call setup + Direct Accessto super + TCH assignment to super +incoming external handovers = 57015 = convers_started;

• Directed retry contribution = 004044 + 004057 + 004074 = b.msc_i_sdcch_tch +b.bsc_i_sdcch_tch + b.cell_sdcch_tch

• Incoming internal handovers = 4056 = b.bsc_i_tch_tch

wherea = p_nbsc_traffic;b = p_nbsc_ho.

Please notice that Directed Retry and incoming internal handover contribution is taken intoaccount by introducing Traffic/Handover counters in the formula as no Serlev counters areavailable for this target.• Also notice that Direct Accessto super TRX and TCH Assignment to super TRX contributions

are counted twice: this leads to an overestimation of the formula denumerator where the IUOis active. The impact of this on the DCR value will be analyzed and discussed in session 9:RESULTS – CELLS WITH IUO.

6.2.3 CELL BASIS

The DCR formula on cell basis is as follows:



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57015 (convers_started) + 4044 + 4057 + 4074 + 4056

57007 (dropped_calls)

TCH failures oncalls,

int outgoing ho,ext outgoing ho

TCH connect_ack fornorm seiz,

FACCH call setup,Dir Access to super,TCH ass to super,ext incoming ho

Directed Retry,Direct Access,

TCH assignmentcontribution


DCRCELL =

6.2.4 AREA BASIS

The DCR formula on area basis is as follows:

57015 (convers_started) - c57009 (ext_ho_target_success) + 4044 + 4057 + 4074

57007 (dropped_calls)

TCH failures oncalls,

int outgoing ho,ext outgoing ho

TCH connect_ack fornorm seiz,

FACCH call setup,Dir Access to super,TCH ass to super,ext incoming ho

Directed Retry,Direct Access,

TCH assignmentcontribution

DCRAREA =

Succ ext incoming ho (TCH-TCH& SDCCH-SDCCH)



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7. ANALYZED DATA

The formulas presented in the previous paragraphs were tested on live network, so that to verifytheir reliability and to perform a detailed comparison with the formula actually used in GSMnetwork ("GSM" formula).

7.1 Measurement counter statistics

The following sets of data from GSM network were collected by GSM and subsequently analyzedby Nokia:

• SET1: Traffic Measurement Counters, Handover Measurement Counters and Serlev Countersstatistics, collected on hour basis for 66 cells, across a measurement period of 1 day. No IUO,nor call-re-establishment were active in this group of cells;

• SET2: Traffic Measurement Counters, Handover Measurement Counters and Serlev Countersstatistics, collected on hour basis for 117 cells serving a contiguous area, across ameasurement period of 6 days. No IUO, nor call-re-establishment were active in this group ofcells.

• SET3: Traffic Measurement Counters, Handover Measurement Counters and Serlev Countersstatistics, collected on hour basis for 558 cells, across a measurement period of 1 day. NoIUO, nor call-re-establishment were active in this group of cells;

The collected data were grouped and averaged in several ways in order to test the formulas fromseveral viewpoints, according to the operator needs.This also allowed to better check the formulas reliability: obtained results were compared for allanalyzed cases and from that the homogeneous behavior of the formulas was verified.

The performed analysis are listed below:

• SET1:formulas tested on cell basis; 66 cells analyzed on daily basis;• SET1:formulas tested on cell basis; 66 cells analyzed on hour basis;• SET2:formulas tested on cell basis; 177 cells analyzed on daily basis;• SET2:formulas tested on cell basis; 177 cells analyzed on hour basis;• SET2:formulas tested on cell basis; 6 cells with highest traffic load analyzed (separately) on

hour basis;• SET2:formulas tested on cell basis; 177 cells analyzed in 2 busy hours;• SET2:formulas tested on area basis; 177 cells analyzed on daily basis;• SET2:formulas tested on area basis; 177 cells analyzed on hour basis;• SET3:formulas tested on cell basis; 558 cells analyzed on daily basis;• SET3:formulas tested on cell basis; 558 cells analyzed on hour basis;• SET3:formulas tested on cell basis; 12 cells with highest traffic load analyzed (separately) on

hour basis;• SET3:formulas tested on cell basis; 558 cells analyzed in 2 busy hours;

Notice that "cells with highest traffic load" are cells having an average number of assignment perhour of the order of 300-3000.Two busy hours were taken into consideration: from 12:00-->13:00 and from 19:00-->20:00.



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The following features were randomly active in the analyzed cells:

• Queuing• Directed Retry• The main Handover Functionality's (including Ho due to distance)• Advanced Multilayer Handling• Frequency Hopping• SDCCH Handover

IUO (IFH) and Call Re-establishment were active only in some set of cells in order to studyseparately the impact of these two features on the DCR formulas.

7.2 BSC Clear Code Observation Measurements

Where needed, the results from statistics analysis were compared with data fromClear_Code_Observations collected by GSM for the following cells:

• CELL1 8 days, from 9-12 july and from 16-19 july• CELL2 6 days, from 9-10 july and from 16-19 july• CELL3 4 days, from 11-12 july and from 16-17 july• CELL4 5 days, from 30 ago to 03 sept• CELL5 5 days, from 30 ago to 03 sept

This gives a total of 28 days distributed over 5 different cells: although this amount of data is notenough to perform statistical analysis, nevertheless it is enough to point out specific problems orbehaviors concerning failure counters and/or failure distribution among different call phases.

7.3 Test bed measurements:

Additionally, some specific tests were performed in Nokia Test bed in order to verify the triggerpoints of some counters utilized in the "Serlev" formula. The results of these additional tests arepresented and discussed in Appendix2.



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8. RESULTS – NORMAL CELLS

8.1 Comparison of different formulas

Figures 15-21 show the DCR results for the following different formulas as obtained from theanalyzed data.Precisely, "Serlev" formula, "1139" formula and "Conversation" formula are showed in comparisonwith GSM actual formula. The remaining proposal "1148" formula will be discussed later on in thischapter.

Area - Day - DCR

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

1 2 3 4 5 6

form GSMform 1139form serlevform conv

Figure15:DCR formula (on area basis) comparison. The DCR are calculated on day basis (6 days all together) for 177cells SET2)

Area_Day:

0

5000

10000

15000

20000

25000

30000

1 2 3 4 5 6

GSM form

1139 form

serlev form

Conv form

Figure16:DCR formula on area basis: numerator comparison. The DCR are calculated on day basis (6 days alltogether) for 177 cells (SET2)



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DCR formula comparison

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

cell/day

DC

OGSMform

1139 form

serlev form

Conv form

Figure17:DCR formula (on cell basis) comparison. The DCR are calculated on day basis (6 days all together) for 13cells (SET2). The cells have variable traffic load, with a number of call/ho seizures from 2000 to 40000 per day.

Numerator comparison

0

100

200

300

400

500

600

cell/day

form GSM

form 1139

form serlev

form conv

Figure18:DCR formula on cell basis: numerator comparison. The DCR are calculated on day basis (6 days alltogether) for 13 cells (SET2). The cells have variable traffic load, with a number of call/ho seizures from 2000 to40000 per day



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Figure19:DCR formula (on cell basis) comparison. The DCR are calculated on day basis (1 day) for 66 cells (SET1).The cells have variable traffic load, with a number of call/ho seizures from 1000 to 10000 per day

numerators

0

50

100

150

200

250

300

350

400

cell

num

formformformform conv

Figure20:DCR formula on cell basis: numerator comparison. The DCR are calculated on day basis (1 day) for 66 cells(SET1). The cells have variable traffic load, with a number of call/ho seizures from 1000 to 10000 per day



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DCR formula

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

cell DCR

GSM1139serlevconv form

Figure21:DCR formula (on cell basis) comparison. The DCR are calculated on day basis (1 day) for 62 cells (SET3).The cells belong to BSC 33207.

The formulas show similar behaviour in all the graphs, as expected. In particular it can be noticedthat:

• "GSM" formula gives the highest DCR estimation due to the fact that the TCH failures arecounted over the biggest period (from TCH_Channel_activation to Release of the call);

• "Conversation" formula gives the lowest DCR estimation due to the fact that the TCH failureare counted over the shortest period (from Connect_ack to Disconnect);

• The DCR values predicted by "Serlev" and "1139" formulas lies in between the other 2formulas, with "Serlev" formula being on average grater than "1139". In fact "Serlev" formulacounts the TCH failures from Assignment_complete to Disconnect, whereas "1139" formulacounts the TCH failures from Connect_ack to Release. These two periods are partiallyoverlapped and therefore the difference between the two formulas depends on the amount offailures in both setup phase 2 (from Assignment_complete to Connect_ack ) and releasephase 4 (from Disconnect to Release). "Serlev" formula being grater than "1139" means that,on average, the number of failures in setup phase 2 is grater than the number of failures inrelease phase 4.

In general the analyzed data agree with the expected results thus confirming the reliability of thepresented formulas.Please notice that graphs and results relative to the remaining analyzed data agree with theshown ones.



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8.2 Distribution of failures

From the comparison of the previously presented formulas, the distribution of TCH failures amongthe call phases can be obtained.This information allows to separately monitor the different call phases thus immediately pointingout which one is the most critical for a given cell, i.e. where the majority of failures occurs. Theoperator can thus immediately focus any further analysis and action on the phase where the meanproblem is.

Figures 22-23 show the TCH failure distribution obtained from the analyzed statistical data. Eachcolumn corresponds, respectively, to the following averaged data:

• SET1:formulas tested on cell basis; 66 cells analyzed on daily basis (1 day);• SET2:formulas tested on area basis; 117 cells analyzed on daily basis (6 days);• SET2:formulas tested on area basis; 117 cells analyzed on hour basis (6 days);• SET2:formulas tested on cell basis; 117 cells analyzed on daily basis (6 days);• SET2:formulas tested on cell basis; 117 cells analyzed on hour basis (6 days);• SET2:formulas tested on cell basis; 117 cells analyzed in busy hour from 12:00->13:00 (6

days);• SET2:formulas tested on cell basis; 117 cells analyzed in busy hour from 19:00->20:00 (6

days);• SET2:formulas tested on cell basis; BTS 10 (BSC1) analyzed on hour basis (6 days);• SET2:formulas tested on cell basis; BTS 10 (BSC4) analyzed on hour basis (6 days);• SET2:formulas tested on cell basis; BTS 11 (BSC1) analyzed on hour basis (high load);• SET2:formulas tested on cell basis; BTS 12 (BSC1) analyzed on hour basis (high load);• SET2:formulas tested on cell basis; BTS 27 (BSC4) analyzed on hour basis (high load);• SET2:formulas tested on cell basis; BTS 30 (BSC1) analyzed on hour basis (high load);• SET3:formulas tested on cell basis; 558 cells analyzed on daily basis (1 day);• SET3:formulas tested on cell basis; 558 cells analyzed on hour basis (1 day);

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

%

Failure distribution

release phase

conversation phase

setup phase

Figure22:Failure distribution among setup (ph2+3), conversation (ph 15) and release (ph 4) phases as obtained fromstatistical data.



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0

10

20

30

40

50

60

%

Failure distribution in setup phase

Ass_cmpl--> conn_ack

TCH_act --> Ass_cmpl

Figure23:Failure distribution between setup phase 3 (from TCH_activation and Assignment_cmpl and setup phase 2(from Assignment_cmpl to Connect_ack) as obtained from statistical data.

Precisely, Figure 22 shows the failure distribution between setup phase (i.e. phase 2 and 3together, from TCH_Channel_activation to Connect_ack), conversation phase (i.e. phase 15, fromConnect_ack to Disconnect) and release phase (i.e. phase 4, from Disconnect to Release). InFigure 23 the setup phase is further split into phase 3 (from TCH_Channel_activation toAssignment_complete) and phase 2 (from Assignment_complete to Connect_ack).



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Failure distribution

OPI form

TCH_ activation

Assignment_cmpl

Connect_ack

Disconnect

Release

GSM

1139

Serlev

conv

Figure24: Failure distributions among different call phases as can be obtain by comparison of different formulas.

The percentage of failures in the call phases was obtained as follows (also explained in Figure 24):

• Phase 2+3 (num"GSM" formula – num"1139" formula)/num "GSM" formula• Phase 15 num "Conversation" formula /num "GSM" formula• Phase 4 (num"1139" formula – num"Conversation" formula)/num "GSM" formula• Phase 3 (num"Serlev" formula – num"Conversation" formula)/num "GSM"

formula• Phase 2+4 (num"GSM" formula – num"Serlev" formula)/num "GSM" formula

It can be noticed that:

• The distribution of failures among setup, conversation and release phases is quite uniform forall the considered cases: since the different columns correspond to data averaged on differentbasis refer to the data SET presented above, this result gives reliability to the percentagesfound, thus confirming the reliability and usefullness of the formula under analysis;

• The failures really perceived by the end user as "dropped call" are, on average, only 40-45%of the total TCH failures. Therefore "GSM" formula gives a pessimistic estimation of the DCRperceived by the end users. The non-perceived-by-the-end-user failures could be negligibleand could be treated separately with different priorities;

• The 10-15% of failures occur during release phase: considering that these failures are totallynegligible from the network performance point of view (the call is being terminated anyway),the "real" DCR can be considered on average 10-15% less than the calculated one;

• The amount of TCH failures during setup phase takes the 35-40% of the total failures. As faras the end user is concerned they are perceived as access failures and have therefore lowerimpact on the quality of the network. Nevertheless, these failures make the traffic load of thecell lower and need therefore to be reduced as much as it is possible. It is correct to threat



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them separately from the conversation failures as usually they are originated by different typesof problems;

• The distribution of failures before and after Assignment_complete in the setup phase is notperfectly uniform: in some cases phase 2 is more critical, whereas in other cases it is the otherway around and on average 50% is taken from each phase. This result is confirmed by someClear_Code_observation analysis described below.

Figures 25-26 show the TCH failure distribution obtained from the Clear Code Observation data:each column corresponds to a given cell in a given day (or couple of days).The results are in good agreement with the ones obtained by comparing the formula predictions,showing that these percentages reflect the real behavior of the network.

0%10%20%30%40%50%60%70%80%90%

100%

%

Cell11

Cell 1

Cell 1

Cell 2

Cell 2

Cell 3

Cell 3

Cell 4

Cell 4

Cell 5

Failure distribution from CC_Obs

release phase

conversation phase

setup phase

Figure25:Failure distribution among setup (ph2+3), conversation (ph 15) and release (ph 4) phases as obtained fromClear Code Observation data.



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0

10

20

30

40

50

60

70

80

%

Cell1

Cell1

Cell1

Cell 2

Cell2

Cell3

Cell3

Cell4

Cell4

Cell5

Failure distribution in setup phase - CC_Obs

Ass_cmpl-->connect

TCH_act-->Ass_cmpl

Figure26:Failure distribution between setup phase 3 (from TCH_activation and Assignment_cmpl and setup phase 2(from Assignment_cmpl to Connect_ack) as obtained from Clear Code Observation data.

Please notice that:• cell CELL5 presents a very high number of failures in call setup both on TCH and SDCCH this

odd behavior is the reason of the odd failure distribution (compared to the others);• cells CELL1, CELL2 and CELL3 are reported with and without half rate: this explains the

slightly different behavior of the same cell in different days.

In conclusion, the results of this analysis confirm the reliability of the presented formulas; thesimultaneous usage of all the formulas is recommended in order to get the major benefit from anetwork monitor and optimization point of view.



38 (60)

8.3 Comments on Serlev Formula

At the numerator of "Serlev" formula the number of TCH failures occurring inside a givenmeasurement period is calculated as difference between all the normal assignments and all thenormal releases triggered in the same measurement period.As a consequence of this implementation, every call, which is already in place when themeasurement period starts (i.e. the normal assignment was triggered in the previousmeasurement period), will lower the estimated failure number. In the same way, every call, whichis still in place when the measurement period stops (i.e. the normal release will be triggered in thenext measurement period), will increase the estimated failure number.This effect makes the comparison between "Serlev" formula (where the TCH failures are triggereddirectly) and other formulas quite difficult.In fact, some cases were found where "Serlev" formula gives greater DCR than "GSM" formulaand other cases where "Serlev" formula gives lower DCR than "Conversation" formula.The impact of this effect on "Serlev" formula can be estimated as: (total number of seizures inside the measurement period)x(mean holding time)/(measurementperiod in minutes)For a mean holding time of 3 minutes and a measurement period of 1 day, this gives a theoreticalaccuracy in failure prediction of 0.2% with respect to the number of seizures: for 5000 seizures itcorresponds to 10 units.For a measurement period of 1 hour, the same calculation leads to a theoretical accuracy of 5%with respect the number of seizures, which again corresponds (for 200 seizures) to 10 units.Notice that, since the effects of ongoing calls at the starting/ending points of the measurementperiod compensate each other, in general the "Serlev" formula is expected to be much moreaccurate than the above estimations: this is confirmed by the analyzed data, as discussed below.

zone

n° c

ells

n° B

SC

s

cell

type

area

/cel

l bas

is

mea

s. p

erio

d

n° d

ays

num

ser

lev

> n

um G

SM

aver

aged

% d

iffer

ence

ove

r se

izur

es

max

% d

iffer

ence

ov

er s

eizu

res

refe

renc

e %

diff

eren

ce

num

ser

lev

< n

um C

onv

aver

aged

% d

iffer

ence

ove

r se

izur

es

max

% d

iffer

ence

ov

er s

eizu

res

1 66 1 normal cell day 1 20% 0.08% 0.28% 0.20% 0 0 01 117 2 normal area day 6 0 0 0 0.20% 0 0 01 117 2 normal cell day 6 1.50% 0.06% 0.12% 0.20% 0.40% 0.09% 0.20%3 558 11 normal cell day 1 4.50% 0.1 0.5 0.20% 0.3 0.1 0.121 117 2 normal cell hour 6 23.50% 0.60% 2% 5% 18% 0.60% 3%1 117 2 normal cell hour 6 21% 0.60% 2.70% 5% 21% 0.60% 3%1 117 2 normal area hour 6 19% 1.09% 2% 5% 29% 1% 2.50%

Table1: "Serlev"formula numerator comparison with "GSM" and "Conversation" formulas as obtained from differentdata set analysis.

Table1 shows the results from the analysed data; it can be noticed that:

• For measurement periods of 1 day, in only 5% of cases the "Serlev" is greater than "GSM"numerator; for measurement periods of 1 hour this occurs in 15% of cases. In all cases themismatches are much smaller than the theoretical estimation (0.2% for day and 5% for hour).

• for measurement periods of 1 day, in only 0.1% of cases the "Serlev" is lower than"Conversation" numerator; for measurement periods of 1 hour this occurs in 20% of cases. Inall cases the mismatches are much smaller than the theoretical estimation.



39 (60)

In case the traffic is increasing/decreasing across the measurement period, then the Serlevformula will over/underestimate the DCR. Figure 27 shows a case where this is happening: DCRformulas are plotted for the same group of cells (area basis) for different days. When the trafficload is stable the agreement between the formulas is optimum, whereas where the busy/nighthours approach, then the comparison of "Serlev" and other formulas is more critical.

For example, considering the measurement periods from 8:00-->9:00 and from 9:00--->10:00: thenumber of seizures inside the 2 periods is, respectively, equal to 23562 and 39433. The differencebetween the number of calls that are ongoing across the border of period 8:00-->9:00 (consideringa mean holding time of 3 minutes) is equal to (39.433*0.05 – 23562*0.05)= 793. This explains thedifference between "Serlev" and "GSM" numerators, which is equal to 325 units.

Area - hour - numerator

-400-200

0200400

600800

10001200

14001600

hours/days

fGSM

form 1139

form serlev

form convl

Figure 27:DCR formula on area basis: numerator comparison. The DCR are calculated on hour basis (24 hours x 6days all together) for 177 cells (SET2).

For a better comparison on hour basis it is strictly recommended to compare several subsequenthours across a big time range, so that not only one single DCR value, but the whole trend acrossthe time range is taken into account.The effects just described have negligible impact on longer measurement periods (e.g.1 day), asconfirmed from the analyzed data.

In conclusion, "Serlev" formula gives reliable results for measurement periods of 1 day or longer.On hour basis (especially when the traffic load is increasing or decreasing) it is suggested to usethis formula always together with the others taking into account the whole trends.



40 (60)

8.4 Comments on 1139 Formula

As already pointed out in the previous paragraphs, due to the fact that countersTCH_rel_due_radio_fail(1139) and TCH_rel_due_bss_fail(1140) are updated also in phases (12-14) i.e. in case a failure occur on the target channel during handover, "1139" formula cansometimes overestimate the real number of drops occurring before Connection_acknowledged: (afailure on target during handover does not always mean a drop). As result, the formula cansometimes underestimate the real DCR value.

One theoretical way to solve this inaccuracy would be introducing two correction factorsmultiplying the counters TCH_rel_due_radio_fail(1139) and TCH_rel_due_bss_fail(1140) so thatonly the "right portion" of counted failures is taken into account.The numerator of the formula would then become:

(1013 + 1014 + 1029 + 1030 + 1084 + 1085 + 1087 + 1088 + 1046 + 1047 + + 1048 + 1049 + 1050 + 1070

- 1139*A - 1140*B)

NUM1139 =

The amount of TCH failures occurring during an handover in setup phase can be computed for agiven cell by comparing the values of each counter (TCH_rel_due_radio_fail(1139) andTCH_rel_due_bss_fail(1140)) and the total number of TCH failures with the same (Radio/BSS)causes occurring in phases 2+3. The former information is obtained from Statistical TrafficMeasurements; the latter is obtained from Clear Code Observation Measurements for the samecell.

To tune these correction factors for each and every cell would be very time consuming and wouldrequire continuous updating as the network grows. Nevertheless, a reasonable estimation of themaximum inaccuracy of formula "1139" can be computed as described below.

cell

date

1139

sta

t

1139

TC

H

%d

iffe

renc

e

1140

sta

t

1140

TC

H

%d

iffe

enc

e

cell1 09-10 jun 38 38 0 45 45 0.0cell1 11-12 jun 45 45 0 64 59 7.8cell1 16-17 jun 49 49 0 54 51 5.6cell1 18-19 jun 35 35 0 38 36 5.3cell2 09-10 jun 51 51 0 61 56 8.2cell2 16-17 jun 67 67 0 71 67 5.6cell2 18-19 jun 69 69 0 68 66 2.9cell3 11-12 jun 91 91 0 100 96 4.0cell3 18-19 jun 100 100 0 113 106 6.2cell4 30 ago 49 49 0 25 19 24.0cell4 31 ago 41 41 0 13 13 0.0cell5 30 ago 1 1 0 7 7 0.0cell5 31 ago 0 0 0 5 5 0.0

Table3: percentage of TCH failures occurring during an inc/out HO in setup phase (ph 2+3) as obtained from statisticsand Clear Code Observation data.



41 (60)

Table3 shows the percentage of TCH failures occurring during an handover in setup phase withrespect to the TCH failures in phases 2+3. The calculation was performed from Clear CodeObservation data for 5 different cells and a total number of 21 days.

It can be noticed that:

• Counter TCH_rel_due_radio_fail(1139) has never contribution from handover failures;• Counter TCH_rel_due_bss_fail(1140) has an averaged contribution of 6% from handover

failures (both incoming and outgoing): only in one case the impact of handover failures is 24%(i.e. 6 units).

Considering also that:• the above given percentage include both, incoming and outgoing handovers;• no TCH-TCH handovers can occur in phase 3 since the mobile is still on SDCCH channel:

therefore the inaccuracy affects only phase 2

Reasonable "upper values" for the correction factors are: 1 for TCH_rel_due_radio_fail(1139) and0.8 for TCH_rel_due_bss_fail(1140).

zona

n° c

elle

n° B

SC

tipo

celle

area

/cel

l bas

is

mea

s. p

erio

d

n° g

iorn

i

delta

% m

edio

num

113

9MO

D(1

140*

0.8)

delta

abs

med

io fo

rm 1

139M

OD

(114

0*0.

8

1 66 1 normal cell day 1 5.50% 0.021 117 2 normal area day 6 6.70% 0.071 117 2 normal area hour 6 8.70% 0.81 117 2 normal cell day 6 8% 0.041 117 2 normal cell hour 6 14% 0.041 117 2 normal cell hour 6 14% 0.031 117 2 normal cell hour 6 11% 0.043 558 11 normal cell day 1 7.70% 0.04

Table3: comparison between "1139" formula with and without the correction factors.

Figure 28 shows the comparison between "1139" formula with and without the correction factors.The two formulas were compared for all analyzed data (see Table3) and the resulting absoluteshift between the two is around 0.04. This can be taken as maximum inaccuracy of "1139"formula.



42 (60)

1139-1139_MOD formula

0

0.5

1

1.5

2

2.5

3

cell

DC

GSM11391139_MOD

Figure 28:DCR formula (on cell basis) comparison. "1139" form is compared with "1139_MOD" where correctionfactors have been introduced. The DCR are calculated in Busy Hour (from 12:00-->13:00) for 63 high load cells(SET2).

In conclusion, "1139" formula is characterized by an intrinsic inaccuracy that varies on cell andtraffic basis; an upper estimation of this inaccuracy is 9% (-0.04 in DCR absolute value).A part from this shift, this formula is quite reliable and useful to obtain an estimation of failuredistribution in setup phase. Its usage is suggested together with "Serlev" and "1148" formulas,which can be taken as upper reference values.



43 (60)

8.5 1148 Formula

In the numerator of "1148" formula the number of TCH failures occurring afterAssignment_complete is obtained by subtracting the difference TCH_norm_seiz(1009) - asscomplt(1148) from the total number of TCH failures.As already pointed out in the previous paragraphs, the difference between TCH_norm_seiz(1009)- ass complt(1148) also includes the number of SDCCH failure in call setup.This is the reason why sometimes the following relation occurs:

TCH_norm_seiz(1009) - ass complt(1148) > TCH_rel_due_radio_fail(1139) +TCH_rel_due_bss_fail(1140) (1)

Meaning that the cell under analysis is affected by a high number of SDCCH failures in call setup(phase 3). In some cases the SDCCH failures are higher than the total number of TCH failures,thus leading to negative values of the formula.

cell-

id

data

tch

fail

ph3

sd fa

il ph

3

tch

fail

conv

aver

age

seiz

ures

to12182 30 ago 36 3 57 3000to12182 31 ago 21 0 58 3000to12182 01 sett 29 1 60 3000to12182 2 sett 27 2 41 3000to12182 3 sett 32 5 59 3000to02027 30 ago 7 493 4 300to02027 31 ago 6 429 5 300to02027 01 sett 6 220 6 300to02027 2 sett 7 289 7 300to02027 3 sett 10 860 3 300

Table4: TCH and SDCCH failures in setup (ph3) and conversation (ph 15) phases as obtained from Clear CodeObservation data.

Clear code observations were analyzed for a cell characterized by negative values of "1148"formula: the results of the analysis are summarized in Table4. It was found that the cell underanalysis is affected by a huge amount of SDCCH abis failures (dx cause 306 = ass_fail_msg) inphase 3. On the other hand, the number of TCH failures is very low, meaning that this is not acritical cell from TCH point of view. The traffic load of this cell is also quite low; this might berelated to the high amount of failures in call setup.The setup phase of a call before Assignment_complete (phase 3) is very critical since bothSDCCH and TCH channels are in place and the mobile has to move from one to the otherchannel. The standard SDCCH failure formulas do not make a distinction between this phase andthe previous phases were SDCCH is in place. "1148" formula can be therefore used to underlinespecific SDCCH problems in setup phase. This is the main advantage of this formula.



44 (60)

"1148" formula was analyzed and compared with the other formulas under study. Table5 showsthe results concerning relation (1).

zone

n° c

ells

n° B

SC

s

cell

type

area

/cel

l bas

is

mea

s. p

erio

d

n°da

ys

1009

-114

8 <

set

up fa

iav

erag

ed %

di

ffere

nce

over

TC

H to

tal

failu

res

abso

lute

diff

eren

ce

1 66 1 normal cell day 1 23% 26% 201 117 2 normal area day 6 0 0 01 117 2 normal area hour 6 2% 5% 301 117 2 normal cell day 6 10% 13% 101 117 2 normal cell hour 6 5% 42% 41 117 2 normal cell hour 6 6% 33% 3.53 558 11 normal cell day 1 11% 10% 10

Table5: comparison between the difference (TCH_norm_seiz(1009) - ass complt(1148)) and the number of TCHfailures in call setup

It can be noticed that:• Relation (1) is satisfied in a number of cases that varies with the analyzed cells. This is

obvious, as the high SDCCH failures in setup are not a problem that uniformly affects all cells.• When relation (1) is satisfied, the "1148" formula is optimistic by 5 % to 40 %: in these cases, it

is therefore not reliable as DCR formula.

In case when relation (1) is not satisfied, "1148" form shows quite reliable results. Table 6 andFigures 29-34 show the comparison between "1148" and "1139" formulas (only cases whenrelation (1) is not satisfied are considered). The observed positive shift between the two formulasis expected, since "1148" formula counts the TCH failures from Assignment_complete to Release,whereas "1139" formula counts the TCH failures from Connect_ack to Release.

zona

n° c

elle

n° B

SC

tipo

celle

area

/cel

l bas

is

mea

s. p

erio

d

n° g

iorn

i

num

114

8 >

113

9

delta

% m

edio

sui

fail

delta

abs

delta

abs

med

io fo

rm 1

139-

1148

1 66 1 normal cell day 1 77% 8% 5 0.041 117 2 normal area day 6 100% 8.50% 1250 0.141 117 2 normal area hour 6 98% 10% 44 0.181 117 2 normal cell day 6 90% 10% 11 0.093 558 11 normal cell day 1 89% 8% 6 0.07



45 (60)

Table6: comparison between the difference (TCH_norm_seiz(1009) - ass complt(1148)) and the number of TCHfailures in call setup

Area - Day - DCR

0.000.200.400.600.801.001.201.401.601.802.00

1 2 3 4 5 6

formGSM

form 1139

form 1148

form conv

Figure 29: DCR formula (on area basis) comparison. The DCR are calculated on day basis (6 days all together) for 177cells (SET2)

1139-1148 formula

0.00

0.50

1.00

1.50

2.00

2.50

3.00

cell/day

DC

GSM form

1139 form

1148 form

Figure 30: DCR formula (on cell basis) comparison. The DCR are calculated on day basis (6 days all together) for 13cells (SET2). The cells have variable traffic load, with a number of call/ho seizures from 2000 to 40000 per day



46 (60)

DCR formula

0.00

0.20

0.40

0.60

0.80

1.00

1.20

cells

DC

form GSM

form 1139

form 1148

Figure31:DCR formula (on cell basis) comparison. The DCR are calculated on day basis (1 day) for 38 cells (SET1).The cells have variable traffic load, with a number of call/ho seizures from 1000 to 10000 per day

1139-1148 form

0

0.5

1

1.5

2

2.5

3

cell

DC

formGSM

form 1139

form 1148

Figure 32: DCR formula (on cell basis) comparison. The DCR are calculated in busy hour from 12:00-->13:00 for 54high load cells (SET2).



47 (60)

DCR formula

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

cell DCR

GSM11391148

Figure33:DCR formula (on cell basis) comparison. The DCR are calculated on day basis (1 day) for 40 cells (SET3).The cells belong to BSC 33207.

1139-1148 form comparison -

0

0.5

1

1.5

2

2.5

3

cell

DC

formGSM

1139

1148

Figure34:DCR formula (on cell basis) comparison. The DCR are calculated in busy hour from 12:00-->13:00 for 60high load cells (SET2).

Please notice that graphs and results relative to the remaining analyzed data agree with theshown ones.

Notice that:

TCH_norm_seiz(1009) - ass complt(1148) = SDCCH_Failures(ph3) + TCH_Failures(ph3)

Where SDCCH_Failures(ph3) is the total amount of SDCCH failures in phase 3 andTCH_Failures(ph3) is the total amount of TCH failures in phase 3.

Also notice that:



48 (60)

TCH_rel_due_radio_fail(1139) + TCH_rel_due_bss_fail(1140)= TCH_Failures(ph3) +TCH_Failures(ph2) + TCH_Failures:out_ho(ph2) + TCH_Failures_inc_ho(ph2)

Where TCH_Failures(ph2) is the total amount of TCH failures in phase 2 andTCH_Failures:out_ho(ph2) is the total amount of TCH failures during an outgoing handover insetup phase and TCH_Failures_inc_ho(ph2) is the total amount of TCH failures during anincoming handover in setup phase.

Therefore the difference between "1148" and "1139" formula numerators:

Ass_complt(1148) - TCH_norm_seiz(1009) + TCH_rel_due_radio_fail(1139) +TCH_rel_due_bss_fail(1140)

Is equal to:

TCH_Failures(ph2) + TCH_Failures:out_ho(ph2) + (TCH_Failures_inc_ho(ph2) -SDCCH_Failures(ph3)). (2)

When relation (1) is not satisfied, the previous relation is grater than zero.This leads to 2 observations:

• TCH_Failures_inc_ho(ph2) which is the inaccuracy of "1139" form is less than the differencebetween the 2 formulas: the difference between the 2 formulas can be taken as "upper limit"for "1139" formula, meaning that the inaccuracy of "1139" formula can not be grater;

• The two terms TCH_Failures_inc_ho(ph2), and SDCCH_Failures(ph3) subtract each otherin relation (2): since these 2 terms are totally uncorrelated, then on average they compensateeach other and therefore the difference between "1148" and "1139" formula numerators isvery closed to the number of TCH failures in phase2.

The average shift between the two numerator is around 8% (which corresponds to an absoluteshift of 0.07 between the two formulas): this value is very closed to the "upper inaccuracyestimation" previously found for "1139" formula, thus confirming the sensibility of the estimation.



49 (60)

8.6 Cell-BH

The results of different proposals were compared on cell Busy Hour. For this purpose 8 cells fromSET2 were selected. The cells are high traffic cells (i.e. 2000 to 4000 seizures per hour) and aredistributed under two different BSCs. For each cell the Busy Hour was selected and the behaviorwas analyzed over 6 days.Figure 35 shows the comparison between all the presented formulas. As already pointed out, the"Serlev" formula gives often unreliable results due to the rapidly changing traffic load in the cell:again, it is recommended to use this formula over a longer observation period or by looking at thetrend over few subsequent hours.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

20:0

019

:00

19:0

018

:00

19:0

012

:00

10:0

018

:00

12:0

019

:00

19:0

019

:00

18:0

012

:00

19:0

012

:00

16:0

019

:00

12:0

0

GSM form1139 form1148 formserlev formconvl form

Figure 35: DCR formula (on cell basis) comparison. The DCR are calculated in cell Busy Hour for 8 cells (SET2).The busy hours are indicated on the x-axis.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

20:0

019

:00

19:0

018

:00

19:0

012

:00

10:0

018

:00

12:0

019

:00

19:0

019

:00

18:0

012

:00

19:0

012

:00

16:0

019

:00

12:0

0

GSM form1139 form1148 formconv form

Figure 36: DCR formula (on cell basis) comparison. The DCR are calculated in cell Busy Hour for 8 cells (SET2).The busy hours are indicated on the x-axis. For clearness "Serlev" formula has been taken away from the graph.



50 (60)

The results from "GSM", "1139", "1148" and "Conversation" formulas are showed again in Figure36 for clearness: they confirm what already found in the previous sessions.In Figure 37 "1139" formula with the Correction Factors is also introduced, confirming that thepossible estimated shift of "1139" is well inside the difference between "1139" and "1148"formulas.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

20:0

019

:00

19:0

018

:00

19:0

012

:00

10:0

018

:00

12:0

019

:00

19:0

019

:00

18:0

012

:00

19:0

012

:00

16:0

019

:00

12:0

0

GSM form1139 form1148 formconv form1139 mod

Figure 37: DCR formula (on cell basis) comparison. The DCR are calculated in cell Busy Hour for 8 cells (SET2).The busy hours are indicated on the x-axis. "1139" formula with correction factors is also showed.



51 (60)

9. RESULTS – CELLS WITH IUO

The Intelligent Underlay Overlay impact on DCR formulas was verified by analyzing a set of 152cells where the feature was active. Precisely 40 cells are regular cells, 3 cells are child cells andthe remaining 111 cells are normal cells. Traffic, Handover, Serlev measurement countersstatistics were given on hour basis for a period of 6 days.

Figure 38 shows the comparison between all the proposed DCR formulas for the regular cells. Thetrend agrees with the results found for normal cells, showing the reliability of the formulas.

IUO cells - Formula

0

0

0

0

0

0

0

0

0

0

0

cell

GSM

F1139

1148

sserlev convl

Figure38:DCR formula (on cell basis) comparison. The DCR are calculated on daily basis (6 days all together) for 14regular cells characterized by a number of handovers for good C/I ratio grater than 500.

IUO cells - numerator

0

100

200

300

400

500

600

cell

11391148 serlev

Figure39: DCR formula on cell basis: numerators comparison. The DCR are calculated on daily basis (6 days alltogether) for 14 regular cells characterized by a number of handovers for good C/I ratio grater than 500



52 (60)

Figure 40 shows the comparison between the DCR formulas for the child cells. It can be noticedthat "GSM" formula and "1148" formula are totally overlapped. In fact, in child cells there are nonormal seizures and all the TCH are assigned with the handover procedure, therefore the setupphase of the call is carried on in the parent cell.Please notice that "Serlev" formula is not included in this comparison since, in case of child cells, itgives odd-high values. This specific issue needs to be further analyzed separately from thisconsultancy activity.

DCR formula comparison - child cells

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

cells

DC

form GSM

form 1139

form 1148

form conv

Figure40: DCR formula on cell basis comparison. The DCR are calculated on daily basis (6 days all together) for 3child cells.



53 (60)

As already discussed in the theoretical part of this report, "Conversation" formula denominatordouble counts the TCH assignment for Direct Accessand TCH assignments to super reuse. Forthe examined child cells, in particular, there were no cases of TCH assignments to super reuse: itis therefore possible to correct the formula by subtracting the number of TCH seizure for DirectAccess(counter c1165) from the denominator.The effect of this inaccuracy of "Conversation" form on DCR value is shown in Figure 41 and Table7. Although the cases of Direct Accessare not negligible (more than 1000 per day), they are only asmall portion of the total number of assignment, thus leading to an averaged shift of 6.12% onDCR values (which corresponds to an absolute shift of 0.02).

% difference between denominators 6.12% difference between formulas 6.12absolute difference between formulas 0.02

Table 7: Inaccuracy of "Conversation" formula in case of Direct Accessand TCH assignment are active in the network.

DCR formula comparison

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

cells

DCR

form GSM

form 1139

form 1148

form conv

form conv_MOD

Figure41: DCR formula on cell basis comparison. The DCR are calculated on daily basis (6 days all together) for 3child cells. "Conversation" formula shift is also showed.



54 (60)

10. RESULTS – CALL RE-ESTABLISHMENT

The impact of Call Re-establishment on the DCR formulas was separately evaluated. A set of 53cells covering a contiguous area was studied over a period of 6 days.As already described in sessions 5.3 and 6.1.5, Call Re-establishment procedure has differentimpact on DCR formula according to whether a drop before re-establishment is considered asdrop (as it actually is, as far as the network performance is concerned) or ignored (as perceived bythe end user). Particularly, the two approaches lead to different DCR formulas.Figure 42 shows the comparison between "Serlev" formulas when Call Re-establishment isconsidered or not. The averaged absolute shift between DCR values is 0.05 which corresponds to13% of averaged percentage shift.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

cell/day

DC

R

call re-est = no drop

call re-est = drop

Figure42:DCR formula (on cell basis) comparison: Call Re-establishment impact. The DCR are calculated on dailybasis (6 days all together) for 21 cells characterized by a number of TCH_re-establishment_Assign grater than zero.Call Re-establishment = drop means that all the drop are counted regardless if the calls are re-established afterwords.Call Re-establishment = no drop means that only those drops that are not re-established are taken into account in DCRcomputation.



55 (60)

11. CONCLUSIONS

A complete analysis of possible DCR formula proposals has been performed.Each proposal has been studied from the theoretical viewpoint and involved counters have beeninvestigated.All the proposals were then tested in live network, taking into account different networkconfigurations, different observation periods, different observation areas and different types ofcells.Particular attention was given to the comparison with the Formula actually used in GSM network.From the comparison peculiarities and limitations of each proposal were underlined.The final output is that major benefit can be obtained (from a network optimisation point of view)by taking all the proposals simultaneously in place.

12. REFERENCES

[1] S9 Nokia Electronic Documentation (NED) - Chapter 3.1 Functional Description

[2] S9 Nokia Electronic Documentation (NED) - Chapter 2.7 Supplementary manual

13. GLOSSARY

DCR Drop Call Rate

IUO Intelligent Underlay Overlay

IFH Intelligent Frequency Hopping

AMH Advanced Multilayer Handling

14. VERSION HISTORY

Version Date approved Version history1-0 2001-10-23 First version



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15. APPENDICES

15.1 Appendix 1. Call Phases

Figure 43 shows the main phases of a call.Precisely:

• phase 1 covers the starting signaling of the mobile originating and the mobile terminating callestablishment. The dedicated channel can be an SDCCH or TCH.

• phase 2 covers the MM signaling between the mobile subscriber and the MSC. The MMsignaling is transparent for the BSC occurring on an SDCCH or TCH.

• phase 3 covers the mobile subscriber assignment signaling from the SDCCH to the TCH bymeans of the Basic Call Setup.

• phase 4 covers the release on an SDCCH or a TCH.• phase 15 covers MM signaling between the MS and the MSC during conversation on a TCH.

In addition to these, there are other signaling phases:• phase 5 covers the FACCH call setup assignment signaling (from Assignment_request to

Alert): it is the same as phase 3 where FACCH call setup is active. The dedicated channelcan only be a TCH.

• phase 8 covers ciphering signaling (from Chipering_mode_command toChipering_mode_complete): it is a sort of "bracket" inside phase2. The dedicated channel canbe an SDCCH or a TCH.

handover phases:• phase 9 covers the external handover signaling of the source side on an SDCCH and a TCH.• phase 10 covers the internal intra-cell handover signaling of the source side on an SDCCH

and a TCH.• phase 11 covers the internal inter-cell handover signaling for the source side on an SDCCH or

a TCH.• phase 12 covers the external handover signaling for the target side on an SDCCH and a TCH.• phase 13 covers the internal intra-cell handover signaling for the target side on an SDCCH

and a TCH.• phase 14 covers the internal inter-cell handover signaling for the target side on an SDCCH or

a TCH.

There are then phases covering SMS establishment:• phase 6 covers the SMS establishment signaling of a TCH. The dedicated channel can only

be a TCH.• phase 7 covers SDCCH SMS establishment. The dedicated channel can only be an SDCCH.

A complete description of call phases can be found in Ref [1].



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Phase 1

Phase 15

Phase 2

Phase 3

Phase 2

Phase 4

Figure43: main call phases.



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15.2 Appendix 2. Tests on Serlev Counters

Following is the description of tests executed in Nokia test bed in order to verify the correctupdating of Serlev counters. The results show that the tested counters follow the expectedbehavior according to NED documentation (Ref [2]). The test results are summarized in Table 8.

test

n°

5703

2tc

h_re

-est

_ass

ign

5703

3tc

h_ne

w_c

all_

assi

gn

5703

4tc

h_ho

_ass

ign

5703

5tc

h_no

rm_r

elea

se

5703

6tc

h_ho

_rel

ease

5703

7tc

h_re

-est

_rel

ease

1.1 External DR target target1.2 Internal DR target target2.1 External HO source Targe

ttarget source

2.2 Internal Intercell HO source Target

target source

2.3 Intracell HO ok Ok ok ok2.4 Intracell IUO HO ok Ok ok ok3.1 Intracell TCH assignment to super ok ok3.2 Intercell TCH assignment to super target target

4 Intracell Direct Accessto super ok ok5 Stability test ok ok ok ok

Table 8: List of performed tests.

• Test 1.1

Purpose: Verify counter updating during External Directed Retry procedure.

The test has been executed forcing a Directed Retry for MTC. The parameterDISABLEINTERNALHO = Y so that the handover was MSC-controlled. The callwas subsequently released. The counters 57033(tch_new_call_assign) and57035(tch_normal_release) were correctly updated on the target cell.

• Test 1.2

Purpose: Verify counter updating during Internal Intercell Directed Retry procedure.

The test has been executed forcing a Directed Retry for MOC; the call wassubsequently released. The counters 57033(tch_new_call_assign) and57035(tch_normal_release) were correctly updated on the target cell.

• Test 2.1

Purpose: Verify counter updating during External Handover procedure.



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The test has been executed forcing intercell Handover for MOC. The parameterDISABLEINTERNALHO = Y so that the handover was MSC-controlled. The callwas subsequently released. The counters 57033(tch_new_call_assign) and57036(tch_ho_release) were correctly updated on the source cell; counters57034(tch_ho_assign) and 57035 (tch_normal_release) were correctly updated onthe target cell.

• Test 2.2Purpose: Verify counter updating during Internal Intercell Handover procedure.

The test has been executed forcing intercell Handover for MOC. The call wassubsequently released. The counters 57033(tch_new_call_assign) and57036(tch_ho_release) were correctly updated on the source cell; counters57034(tch_ho_assign) and 57035 (tch_normal_release) were correctly updated onthe target cell.

• Test 2.3Purpose: Verify counter updating during Internal Intercell IUO (parent --> child)Handover procedure.

The test has been executed forcing intercell Handover for good C/I ratio (fromparent ---> child). The call was subsequently released. The counters57033(tch_new_call_assign) and 57036(tch_ho_release) were correctly updated onthe source cell (parent); counters 57034(tch_ho_assign) and 57035(tch_normal_release) were correctly updated on the target cell (child).

• Test 2.4

Purpose: Verify counter updating during Intracell Handover procedure.

The test has been executed forcing intracell Handover for MOC and MTC. The callwas subsequently released. The counters 57033(tch_new_call_assign),57036(tch_ho_release), 57034(tch_ho_assign) and 57035 (tch_normal_release)were correctly updated on the cell both for MOC and MTC.

• Test 2.5Purpose: Verify counter updating during Intracell IUO Handover procedure.

The test has been executed forcing an intracell Handover for good C/I ratio fromregular layer to super layer and than an handover back from super layer to regularlayer (for bad C/I ratio). The call was subsequently released. The counters57033(tch_new_call_assign) and 57035 (tch_normal_release) were correctlyupdated on the cell; counters 57036(tch_ho_release), 57034(tch_ho_assign) werecorrectly updated for both intracell handovers.

• Test 3.1

Purpose: Verify counter updating during Intracell TCH Assignment to super TRXprocedure.

The test has been executed forcing an Intracell TCH Assignment to super TRX forMOC and MTC; the call was subsequently released. The counters



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57033(tch_new_call_assign) and 57035(tch_normal_release) were correctlyupdated for both MOC and MTC.

• Test 3.2

Purpose: Verify counter updating during Intercell TCH Assignment to super TRXprocedure.

The test has been executed forcing an Intercell (parent ---> child) TCH Assignmentto super TRX for MOC; the call was subsequently released. The counters57033(tch_new_call_assign) and 57035(tch_normal_release) were correctlyupdated on the target cell (child).

• Test 4

Purpose: Verify counter updating during Intracell Direct Accessto super TRXprocedure.

The test has been executed forcing an Intracell Direct Accessto super TRX forMOC; the call was subsequently released. The counters57033(tch_new_call_assign) and 57035(tch_normal_release) were correctlyupdated.

• Test 5

Purpose: Verify counter updating during a stability test.

The stability test consisted of subsequent ms-ms calls. Intracell handovers werecontinously forced during each call. The total duration of the test was 14 hours.Traffic measurements, Handover measurements and Serlev countersmeasurements were active. The counter statistics were recorded and checked withthe NMS.All the counters were correctly updated and no discrepancies between differentcounter family results were observed.

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