Call Setup Performance

8/12/2019 Call Setup Performance

1/25

Cegetel GSM System

Call Set Up Performance

March 1999


2/25

27/04/99 Cegetel & Motorola Confidential Proprietary

Table of Contents

1 INTRODUCTION.......................................................................................................................... 3

2 DEFINITION OF CALL SET-UP SUCCESS RATE................................................................. 4

2.1 CALCULATION OF CALL SET-UP SUCCESS RATE ........................................................... ........... 42.2 CALL SET-UP FAILURES......................................................... ................................................... 4

2.3 INVESTIGATION OF LOW CALL SET-UP SUCCESS RATE.......................................................... .. 7

3 COMPARISON OF CEGETEL AND MOTOROLA CALL SET-UP SUCCESS RATE

FORMULA ........................................................... ............................................................ ...................... 8

4 A INTERFACE ANALYSIS ....................................................................................................... 10

4.1 CALL SET UP SUCCESS........................................................... ................................................. 10

4.2 MOBILE ORIGINATED CALL SET-UP........................................................... ............................. 10

4.3 MOBILE TERMINATED CALL SET-UP.......................................................... ............................. 11

4.4 PERFORMANCE OF SIGNALLING PROCEDURES ON SDCCH.................................................... 13

5 PERFORMANCE MANAGEMENT STATISTICS ANALYSIS............................................ 15

5.1 NETWORK BUSY HOUR CALL SET-UP PERFORMANCE ................................................... ......... 155.2 BSS BUSY HOUR CALL SET-UP PERFORMANCE ............................................................ ......... 16

5.3 NETWORK WORST CELL BUSY HOUR PERFORMANCE................................................... ......... 18

5.4 NANTES BUSY HOUR CALL SET-UP PERFORMANCE ...................................................... ......... 19

5.5 NANTES WORST CELL BUSY HOUR PERFORMANCE....................................................... ......... 20

6 DATABASE PARAMETERS IMPACTING CALL SET-UP SUCCESS RATE................... 22

6.1 ASSIGN_SUCCESSFUL => 15000 ..................................................... ......................................... 22

6.2 BSSMAP_T10 => 14000......... ............................................................ ...................................... 22

6.3 INTERFER_BANDS,4 => 63 ........................................................ .............................................. 22

6.4 RXLEV_ACCESS_MIN =>8........................ ............................................................ .................... 22

6.5 CONGESTION RELIEF/CALL QUEUING ........................................................ ............................. 23

6.6 DISCONTINUOUS TRANSMISSION (DTX) ................................................... .............................. 23

6.7 POWER CONTROL......................................................... ........................................................... 23

6.8 LOCATION AREA OPTIMISATION ...................................................... ....................................... 23

6.9 CARRIER PRIORITISATION ...................................................... ................................................. 23

6.10 SDCCH PLACEMENT ................................................... ........................................................... 24

APPENDIX A: PERFORMANCE METRIC DEFINITIONS .................................................... .... 25


3/25


4/25


2 Definition Of Call Set-Up Success Rate

2.1 Calculation Of Call Set-Up Success Rate

The Motorola call set-up success rate is defined as the success of SDCCH

accesses which require a TCH (i.e. system accesses for call attempts rather thanSMS or supplementary service attempts) completing a TCH assignment. The keystatistic formula defined to calculate call set-up success rate is:

100%

))req_sms]C[cm_serv_OK_ACC_PRO-ON_SDCCH(SMS_INIT_-

eq_emerg]C[cm_ser_rOK_ACC_PRO+ponse]C[page_resOK_ACC_PRO+

ablish]C[cm_reestOK_ACC_PRO+req_call]C[cm_serv_OK_ACC_PRO(

)CSIGN_HO_SUCONGEST_AS+STOTAL_CALL(

The denominator of the call set-up success rate key statistic calculates the numberof SDCCH accesses that require a TCH i.e. mobile originated call attempts, mobileterminated call attempts, emergency call attempts and call reestablishmentattempts. Since Paging Responses can relate to either mobile terminated call

attempts or mobile terminated SMS attempts, and SMS attempts do not require aTCH, the mobile terminated SMS attempts must be not included in the denominator.The number of mobile terminated SMS attempts is calculated by subtracting themobile originated SMS attempts from the total number of SAPI 3 SMS connectionsestablished on the SDCCH, represented by the statistics counterSMS_INIT_ON_SDCCH.

The numerator of the call set-up success rate metric calculates the number ofcompleted call set-up TCH assignments, including those that were achieved throughthe use of a directed retry handover to a neighbouring cell.

There are currently two outstanding problem reports for the OMC Call Set-UpSuccess rate calculation (key statistic TCH ASSIGN SUCCESS RATE in GSR 3).Problem Report 116079 will correct the name to CALL SET-UP SUCCESS RATE in

GSR 4 while Problem Report 116328 corrects the formula in GSR 4. Both issuescan be worked around in GSR 3 through the use of Customised Key Statistics.

To ensure accurate calculation of call set-up success rate it is essential that all rawstatistics that make up the formula be enabled in all cells under investigation.

The CAT2 tool in the Cell Optimisation Product (COP) which is already installed onthe OMC provides a utility to test which statistics are disabled, identify the cellsimpacted and enable the necessary statistics.

2.2 Call Set-Up Failures

The signalling flow from mobile station to MSC via the BSS for a successful mobileoriginated call is shown in Figure 2-1.

With reference to figure 2-1, the call set-up success rate as defined in section 2.1measures the success of a CM SERVICE REQUEST received by the BSS on theair interface reaching transmission of an ASSIGNMENT COMPLETE to the MSC onthe A interface.

There are many protocol exchanges that must complete for a successful call set-upe.g.

The MSC must confirm the SCCP connection establishment.The MSC connection refusal rate for BSS initiated SCCP CONNECTIONREQUESTS for all connection types (call, location update, SMS,supplementary service, IMSI detach or call re-establishment) can be

calculated using BSS raw statistics.


5/25


Methodology and Definitions Page 5

If enabled, the authentication procedure must complete successfully.This MSC-MS procedure, transparent to the BSS cannot be measured by theBSS statistics. MSC statistics or A-interface analysis must be used.

If enabled, the ciphering procedure must complete successfully.Ciphering failures can be calculated through the BSS raw statisticcipher_mode_fail.

If enabled, the MS IMEI identification procedure must complete successfully.This MSC-MS procedure, transparent to the BSS cannot be measured by theBSS statistics. MSC statistics or A-interface analysis must be used.

If enabled, the TMSI reallocation procedure must complete successfully.This MSC-MS procedure, transparent to the BSS cannot be measured by theBSS statistics. MSC statistics or A-interface analysis must be used.

Throughout the protocol exchanges on the SDCCH, the channel must not

suffer an RF loss.

The SDCCH RF loss rate for all connection types (call, location update, SMS,supplementary service, IMSI detach or CM re-establishment) can becalculated using BSS raw statistics.

The MSC must be able to provide the service requested by the mobile stationin the call control Set Up message.

This MSC-MS procedure, transparent to the BSS cannot be measured byBSS statistics. MSC statistics or A-interface analysis must be used.

The MSC must be able to initiate an assignment request to the BSS.

Failures in this area can only be calculated at the MSC or through A-interfaceanalysis.

The BSS must be able to successfully allocate a TCH resource whenrequested i.e. no TCH blocking must exist. Note that this may be achievedthrough the use of assignment queuing, congestion relief or directed retry.

The call set-up TCH blocking rate can be calculated using BSS raw statistics.

The mobile must successfully complete the assignment to the allocated TCH.The TCH radio assignment success rate can be calculated using BSS rawstatistics, however this calculation includes intra cell handovers as well asMSC initiated assignment requests.

Note that the subscriber may manually terminate the call set-up procedure atany stage.

This can only be measured by the MSC or A interface analysis.


6/25


MOBILE

STATION

BSS MSC

CHANNEL REQUEST

(RACH)

IMMEDIATE ASSIGNMENT

(AGCH)

SABM (CM SERVICE REQUEST)

(SDCCH)

CR [COMPLETE L3 INFO

(CM SERVICE REQUEST)]UA (CM SERVICE REQUEST)

(SDCCH)

CC (AUTHENTICATION REQUEST)I (AUTHENTICATION REQUEST)

(SDCCH)

I (AUTHENTICATION RESPONSE)

(SDCCH) DT1 (AUTHENTICATION RESPONSE)

DT1 (CIPHER MODE COMMAND)I (CIPHERING MODE COMMAND)

(SDCCH)

I (CIPHERING MODE COMPLETE)

(SDCCH) DT1 (CIPHER MODE COMPLETE)

DT1 (IDENTITY REQUEST)I (IDENTITY REQUEST)

(SDCCH)

I (IDENTITY RESPONSE)

(SDCCH) DT1 (IDENTITY RESPONSE)

DT1 (SETUP)

I (SETUP)

(SDCCH)

I (CALL PROCEEDING)

(SDCCH)

DT1 (CALL PROCEEDING)

DT1 (TMSI REALLOCATION COMMAND)I (TMSI REALLOCATION COMMAND)

(SDCCH)

I (TMSI REALLOCATION COMPLETE)

(SDCCH) DT1 (TMSI REALLOCATION COMPLETE)

CALL SET UP

ATTEMPT

Figure 2-1 Successful Call Set-Up Signalling Flow Diagram


7/25


Methodology and Definitions Page 7

MOBILE

STATION

BSS MSC

DT1 (ALERTING)

DT1 (CONNECT)I (CONNECT)

(FACCH)

I (CONNECT ACKNOWLEDGE)

(FACCH) DT1 (CONNECT ACKNOWLEDGE)

DT1 (ASSIGNMENT REQUEST)I (ASSIGNMENT COMMAND)

(SDCCH)

I (ASSIGNMENT COMPLETE)

(FACCH)

DT1 (ASSIGNMENT COMPLETE)

SABM ()

(FACCH)

UA ()

(FACCH)

I (ALERTING)

(FACCH)

CALL SET UP

SUCCESS

Figure 2-1 (continued) Successful Call Set-Up Signalling Flow Diagram

2.3 Investigation Of Low Call Set-Up Success Rate

To investigate cases of low call set-up success rate it is essential to consider thecontribution of each of phases of call set-up highlighted in section 2.2. This was

carried out for the BSSs under investigation in the Cegetel network using A-interface analysis and BSS performance management statistics analysis in sections4 and 5 respectively.


8/25


3 Comparison Of Cegetel And Motorola Call Set-Up SuccessRate Formula

The Motorola call set-up success rate formula is fully defined in section 2 andcompares the number of times that the MS successfully establishes on the TCH

against the number of mobile accesses on the SDCCH that require a TCH to beallocated.

100%

))req_sms]C[cm_serv_OK_ACC_PRO-ON_SDCCH(SMS_INIT_-

eq_emerg]C[cm_ser_rOK_ACC_PRO+ponse]C[page_resOK_ACC_PRO+

ablish]C[cm_reestOK_ACC_PRO+req_call]C[cm_serv_OK_ACC_PRO(

)CSIGN_HO_SUCONGEST_AS+STOTAL_CALL(

The Cegetel formula "Taux dEchecs dEtablissement DAppel" makes an alternativecalculation by measuring the number of successful SDCCH accesses which do notsuffer an SDCCH RF loss and do not fail to access a TCH.

+

++

++

msc_from_req_ma

)calls_total

blkd_ms_to_cmd_ma

msc_from_req_ma(

blkd_ms_to_cmd_ma

1x

req_supp]C[cm_serv_OK_ACC_PRO

_update]C[locationOK_ACC_PRO

_detach]imsi[PROC_ACC_OK

]sms_req_serv_cm[PROC_ACC_OKeq_emerg]C[cm_ser_rOK_ACC_PRO

+ponse]C[page_resOK_ACC_PRO

+ablish]C[cm_reestOK_ACC_PRO

+req_call]C[cm_serv_OK_ACC_PRO

sd_losses_rf

1(1

As a general comparison the Motorola formula is based on success statistics e.g.total_calls, and the Cegetel formula is based on failure statistics e.g. rflosses_sdand ma_cmd_to_ms_blkd.

The Motorola formula gives an accurate perspective of call set-up success asperceived by the subscriber and relates purely to call set-up attempts, excludingSMS, supplementary services and location updates.

The Cegetel formula gives a good measure of the failures by the MS to access thenetwork due to the radio sub system problems e.g. RF losses and TCH blockingand TCH assignment failures. However the Cegetel formula is not based on call set-up system accesses alone and does not cover all possible call set-up failurescenarios (see section 2.2), therefore the formula does not give an accurateperspective of call set-up success as perceived by the subscriber.

The Cegetel formula is based on all 8 type of system accesses by the mobile stationi.e. mobile originated and mobile terminated call attempts, mobile originated andmobile terminated SMS attempts, mobile originated supplementary serviceattempts, call re-establishment attempts, location update attempts and IMSI detachattempts. The initiation of location updates and the success of IMSI detach attemptsare not actually experienced by the subscriber. A high ratio of location updates to

other MS procedures e.g. calls/SMS/SS could give a false view of networkperformance as experienced by the user. There is a strong probability that the RF


9/25


10/25


4 A Interface Analysis

4.1 Call Set Up Success

The overall call set up success rate for each BSS under investigation was

determined from the A interface data by comparing the volume of successfullycompleted TCH assignments (Assignment Complete) against the volume of callattempts (CM Service Request/Paging Response for calls). The results are shownin figure 4-1.

BSS Overall Call Set-UpSuccess Rate

Overall CallAttempts

St. Herblain A 96.75 2184

St. Herblain B 95.82 1962

St. Herblain D 95.76 1886

St. Herblain E 95.91 3449

Figure 4-1 Overall Call Set-Up Success Rate From A Interface Data

The overall call set-up success rate figure was broken down into mobile originatedand mobile terminated call set-up success rates for each BSS, the results areshown in figure 4-2.

BSC MobileOriginated

Call Set-UpSuccess Rate

MobileOriginated Call

Attempts

MobileTerminated

Call Set-UpSuccess Rate

MobileTerminated

Call Attempts

St. Herblain A 96.52 1321 97.10 863

St. Herblain B 95.50 1222 96.35 740

St. Herblain D 95.33 1264 96.62 622

St. Herblain E 94.89 2252 97.83 1197

Figure 4-2 Mobile Originated and Mobile Terminated Call Set-Up Success RateFrom A Interface Data

Each BSS shows a slightly better mobile terminated call set-up success rate thanmobile originated call set-up success rate. This is unusual since it is normallyassumed that mobile originating subscribers are in better areas of system coveragethan mobile terminating subscribers.

4.2 Mobile Originated Call Set-Up

The mobile originated call set-up failures were divided into 7 categories to evaluatethe distribution of failure causes. The 7 call set-up failure categories were defined asfollows:

MSC refusing the establishment of the SCCP connection through a DTAP CMService Reject.


11/25


Performance Page 11

MSC clearing an established SCCP connection with an unprovoked BSSMAPClear Command.

SDCCH RF losses

User initiated clearing during call set-up, either through a DTAP CM ServiceAbort or DTAP IMSI Detach Indication

An MSC generated DTAP Release Complete in response to a MS generatedDTAP Set Up indicating a failure of the MSC to provide the resourcesrequested by the mobile station.

TCH Assignment Failures due to either TCH blocking at the BSS or the failureof the mobile station to access the target TCH channel with subsequentsuccessful re-establishment on the SDCCH channel.

TCH Assignment Failures (Lost MS) due to the failure of the mobile to accessthe target TCH channel and failure to re-establish on the SDCCH channel.

The distribution of causes of mobile originated call set-up failures is shown in figure4-3.

BREAKDOWN OF MOBILE ORIGINATED CALL SET UP FAILURES

FROM A INTERFACE SIGNALLING DATA

0.00

1.00

2.00

3.00

4.00

5.00

6.00

St. Herblain A St. Herblain B St. Herblain D St. Herblain E

BSC

%CALLSE

TUPFAILURES

MSC CM Service Reject MSC Clear Command SDCCH RF Losses User Initiated Clearing

Set Up / Release Complete Losses TCH Assignment Failures TCH Assignment Failures (MS Lost)

Figure 4-3 Breakdown of Mobile Originated Call Set-Up Failures

Figure 4-3 illustrates that call set-up failures can be attributed to the BSS, MSC andmobile station. In each BSS radio interference provides the largest impact on callset-up failures through SDCCH RF losses and TCH assignment failures. Howeverthe MSC contributes between 0.25% and 0.75% of call set-up failures and themobile subscriber contributes between 1% and 2% of call set-up failures by abortingthe call-set up attempt.

4.3 Mobile Terminated Call Set-Up

The mobile terminated call set-up failures were divided into 9 categories to evaluatethe distribution of failure causes. The 9 call set-up failure categories were defined asfollows:


12/25


MSC refusing the establishment of the SCCP connection through an DTAPCM Service Reject.

MSC clearing an established SCCP connection with an unprovoked BSSMAPClear Command.

SDCCH RF losses. MSC or MS initiated clearing during call set-up, through a DTAP Disconnect.

MSC or MS initiated clearing during call set-up, through a DTAP Release.

MS initiated clearing during call set-up, through a DTAP IMSI DetachIndication.

An MS generated DTAP Release Complete in response to a MSC generatedDTAP Set Up indicating a failure of the MS to support the resourcesrequested by the MSC.

TCH Assignment Failures due to either TCH blocking at the BSS or the failureof the mobile station to access the target TCH channel with subsequentsuccessful re-establishment on the SDCCH channel.

TCH Assignment Failures (Lost MS) due to the failure of the mobile to accessthe target TCH channel and failure to re-establish on the SDCCH channel.

The distribution of causes of mobile originated call set-up failures is shown in figure4-4.

BREAKDOWN OF MOBILE TERMINATED CALL SET UP FAILURES

FROM A INTERFACE SIGNALLING DATA

0.00

1.00

2.00

3.00

4.00

5.00

6.00


BSC

CALLSETUPFAILURE%

MSC SCCP Connection Refused IMSI Detach Indication SDCCH RF Loss

Set Up / Release Complete MSC Clear Command DTAP Release

DTAP Disconnect TCH Assignment Failures TCH Assignment Failures (MS Lost)

Figure 4-4 Breakdown of Causes Of Mobile Terminated Call Set-Up Failure

Figure 4-4 illustrates a wide variety of causes of call set-up failure for mobileterminated call attempts. The MSC appears to refuse a higher number of BSSSCCP connection requests in comparison to the mobile analysis in section 4.2.There is a large percentage of failures due to the mobile station or MSC releasingthe connection through a DTAP Disconnect or Release. In order to determine if this

is due to system issues or subscriber behaviour, the causes of these failure cases


13/25


Performance Page 13

need further analysis to establish the root cause. The impact of radio interferenceon call set-up attempts is evident but not as significant as for mobile originated calls.

4.4 Performance Of Signalling Procedures On SDCCH

The success rates of the authentication, ciphering and mobile station IMEI identity

procedures are shown for all call set-ups, mobile originated call set-ups and mobileterminated call set-ups in figures 4-5 to 4-7 respectively.

It is clear from figures 4-5 to 4-7 that there are no performance issues with theauthentication, ciphering or mobile station identity procedures. It is assumed that thefailures that exist are due to radio interference causing loss of the air interfaceconnection during the signalling procedure.

PERFORMANCE OF AUTHENTICATION, CIPHERING AND MS IDENTITY PROCEDURES FOR ALL

CALL ESTABLISHMENTS

95.00

95.50

96.00

96.50

97.00

97.50

98.00

98.50

99.00

99.50

100.00


BSS

SUCCESSRATE(%)

AUTHENTICATON SUCCESS CIPHERING SUCCESS MS IDENTITY SUCCESS

Figure 4-5 Performance Of Authentication, Ciphering and MS IdentityProcedures For All Call Set-Ups

PERFORMANCE OF AUTHENTICATION, CIPHERING AND MS IDENTITY PROCEDURES FOR MOBILE

ORIGINATED CALL ESTABLISHMENT

95.00

95.50

96.00

96.50

97.00

97.50

98.00

98.50

99.00

99.50

100.00


BSS

SUCCESSRATE(%)


Figure 4-6 Performance Of Authentication, Ciphering and MS IdentityProcedures For Mobile Originated Call Set-Up


14/25


PERFORMANCE OF AUTHENTICATION, CIPHERING AND MS IDENTITY PROCEDURES FOR MOBILE

TERMINATED CALL ESTABLISHMENT

95.00

95.50

96.00

96.50

97.00

97.50

98.00

98.50

99.00

99.50

100.00


BSS

SUCCESSRATE(%)


Figure 4-7 Performance Of Authentication, Ciphering and MS IdentityProcedures For Mobile Terminated Call Set-Up


15/25


Performance Page 15

5 Performance Management Statistics Analysis

The following sub- sections analyse the call set-up success performance at Networklevel, BSS level and cell level through BSS Performance Management statistics.The typical call set-up failure causes that can be measured through BSS

Performance Management statistics, call set-up TCH blocking rate, SDCCH RF lossrate and TCH Radio Assignment Failure rate are also shown at each level ofanalysis in an effort to show the contribution of each failure case. It should be notedthat these failure cases are those that are incorporated in the Cegetel metric " TauxdEchecs dEtablissement DAppel".

5.1 Network Busy Hour Call Set-Up Performance

The daily busy hour call set-up performance at a network level covering the 34Motorola BSSs under OMC1 from the 16

thMarch 1999 to the 24

th March 1999 is

shown in figure 5-1.

Network Daily Busy Hour Call Set-Up Success Rate

87

88

89

90

91

92

93

94

95

96

16-Mar-99 17-Mar-99 18-Mar-99 19-Mar-99 20-Mar-99 21-Mar-99 22-Mar-99 23-Mar-99 24-Mar-99

Date

CallSet-UpSuccessRate(%)

0

10000

20000

30000

40000

50000

60000

70000

80000

90000

100000

CallVolume

BH Call Setup Success Rate (%) BH Call Volume

Figure 5-1 Network Daily Busy Hour Call Set-Up Success

Figure 5-1 shows that the network busy hour call set-up success rate has significantvariation from day to day, ranging from a maximum of over 95% to a minimum lessthan 91%.

Figure 5-2 shows the network busy hour TCH blocking rate, SDCCH RF loss rate

and TCH radio assignment success rate, the three BSS related failure causes thatcontribute to call set-up failures.


16/25


Network Daily Busy Hour Call Set-Up Failures

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8


Date

FailureRate(%)

BH Call Setup TCH Blocking Rate (%) BH SDCCH RF Loss Rate (%) BH TCH Radio Assignment Failure Rate (%)

Figure 5-2 Network Daily Busy Hour Call Set-Up Failure Causes

It is clear from figure 5-2 that the network level TCH blocking rate is much lowerthan the SDCCH RF loss and TCH radio assignment success rate which are directlyrelated to the network radio frequency plan and health/installation of radio andantenna hardware. This correlates with the call set-up failure causes found in the Ainterface analysis in section 4. It should be noted that the sum of the three BSSrelated call set-up failure causes shown in figure 5-2 does not account for all callset-up failures shown on figure 5-1. This highlights the significant contribution of callset-up failures attributable to the signalling procedures between the MSC andmobile station and the influence of the MSC and subscriber that were discussed insection 2.2.

5.2 BSS Busy Hour Call Set-Up Performance

The weekly busy hour call set-up performance for each Motorola BSS is shown infigure 5-3.

BSS Busy Hour Call Set-Up Success Rate

Over Period 16th March 1999 to 24th March 1999

0

10

20

30

40

50

60

70

80

90

100

Bsc_

Alen

con_

A_39

28

Bsc_

Alen

con_

B_39

28

Bsc_

Ango

ulem

e_A_

3508

Bsc_

Ango

ulem

e_B_

3508

Bsc_

Arra

don_

B_39

16

Bsc_

Chall

ans_

3942

Bsc_

Chat

eaub

riant

_394

0

Bsc_

Chat

eaur

oux_

3939

Bsc_

Chole

t_39

06

Bsc_

La_R

oche

_Sur

_Yon

_A_3

911

Bsc_

La_R

oche

_Sur

_Yon

_B_3

944

Bsc_

La_R

oche

lle_B

_394

5

Bsc_

Lava

l_B_3

922

Bsc_

Le_M

ans_

A_39

09

Bsc_

Le_M

ans_

B_39

09

Bsc_

Lorie

nt_B

_395

1

Bsc_

Luco

n_39

43

Bsc_

Nante

s_B_

3929

Bsc_

Niort_

3910

Bsc_

Parth

enay

_393

2

Bsc_

Poitie

rs_A_

3912

Bsc_

Poitie

rs_B_

3936

Bsc_

Porn

ichet

_390

5

Bsc_

Porn

ichet_

B_39

52

Bsc_

Roch

elle_

3900

Bsc_

Saint

es_3

901

Bsc_

Saint

es_B

_390

1

Bsc_

St_H

erbla

in_A_

3919

Bsc_

St_H

erbla

in_B_

3923

Bsc_

St_H

erbla

in_C_

3904

Bsc_

St_H

erbla

in_D_

3953

Bsc_

Tour

s_A_

3903

Bsc_

Tour

s_B_

3908

Bsc_

Tour

s_C_

3955

BSS

CallSet-UpSuccess

Rate(%)

0

1000

2000

3000

4000

5000

6000

7000

CallVolume


Figure 5-3 BSS Weekly Busy Hour Call Set-Up Success


17/25


Performance Page 17

Figure 5-3 shows that there are 4 BSSs with significantly lower call set-up successrate, BSC Angouleme B 3508, BSC Chateauroux 3939, BSC La Rochelle B 3945and BSC Saintes 3901.

Figure 5-4 shows the BSS weekly busy hour TCH blocking rate, SDCCH RF lossrate and TCH radio assignment success rate, the three BSS related failure causesthat contribute to call set-up failures.

BSS Busy Hour Call Set-Up Failures


0

1

2

3

4

5

6

7

8

9

Bsc_

Alen

con_

A_39

28

Bsc_

Alen

con_

B_39

28

Bsc_

Ango

ulem

e_A_

3508

Bsc_

Ango

ulem

e_B_

3508

Bsc_

Arra

don_

B_39

16

Bsc_

Chall

ans_

3942

Bsc_

Chate

aubr

iant_3

940

Bsc_

Chate

auro

ux_3

939

Bsc_

Chole

t_390

6

Bsc_

La_R

oche

_Sur

_Yon

_A_3

911

Bsc_

La_R

oche

_Sur

_Yon

_B_3

944

Bsc_

La_R

oche

lle_B

_394

5

Bsc_

Lava

l_B_3

922

Bsc_

Le_M

ans_

A_39

09

Bsc_

Le_M

ans_

B_39

09

Bsc_

Lorie

nt_B_

3951

Bsc_

Luco

n_39

43

Bsc_

Nante

s_B_

3929

Bsc_

Niort_3

910

Bsc_

Parth

enay

_393

2

Bsc_

Poitie

rs_A_

3912

Bsc_

Poitie

rs_B_

3936

Bsc_

Porn

ichet_

3905

Bsc_

Porn

ichet_

B_39

52

Bsc_

Roch

elle_

3900

Bsc_

Saint

es_3

901

Bsc_

Saint

es_B

_390

1

Bsc_

St_H

erbla

in_A_

3919

Bsc_

St_H

erbla

in_B_

3923

Bsc_

St_H

erbla

in_C_

3904

Bsc_

St_H

erbla

in_D_

3953

Bsc_

Tour

s_A_

3903

Bsc_

Tour

s_B_

3908

Bsc_

Tour

s_C_

3955

Failure Rate (%)

BSS


Figure 5-4 BSS Weekly Busy Hour Call Set-Up Failure Causes

The TCH radio assignment success rate for BSC Angouleme A 3508 and BSC

Laval B 3922 produced negative values and are not plotted in figure 5-4.The negative values are due to either to essential raw statistics being disabled insome cells or BTS RSLs out of service at time of statistics collection and requiresfurther investigation.

The negative value for TCH radio assignment success for BSC Angouleme A 3508is due to the raw statistic ma_cmd_to_ms being disabled in 3 cells, La_Couronne-11[z4] (cell ID 208-10-3508-11629), Voeuil_Et_Giget-13[z4] (cell id 208-10-3508-11620) and Voeuil_Et_Giget-13[z4] (cell id 208-10-3508-1620).

The negative value for TCH radio assignment success rate for BSC Laval B 3922 isdue to raw statistic anomalies at both cells at Soulge_Sur_Ouette-4[z2] (cell ids208-10-3922-15309 and 208-10-3922-5309). It is most likely that this is due to a BTSsite reset during the busy hour statistics interval, particularly since both cells at the

same site are affected at the same time.

The low call set-up success rate for La_Rochelle_B_3945 can be attributed to ahigh TCH radio assignment failure rate. This is entirely due to the TCH radioassignment failure rate for a single cell, Rochefort_Sur_Mer-6[z4] (cell id 208-10-3945-11718) which has a success rate of only 50.29%. If this cell is excluded fromthe analysis then the BSS TCH radio assignment success rate drops to 1.79%, amore typical value. It is apparent that a significant hardware fault or heavy RFinterference is causing subscribers to repeatedly attempt and fail to make a call onthis cell, dragging down the whole BSS call set-up performance.

The low call set-up success rate for BSC Angouleme B 3508, BSC Chateauroux3939, BSC La Rochelle B 3945 and BSC Saintes 3901 require further analysis todetermine the root cause.


18/25


5.3 Network Worst Cell Busy Hour Performance

The weekly busy hour call set-up performance for the worst fifteen cells that carry atleast 100 calls per busy hour is shown in figure 5-5.

Busy Hour Call Set-Up Success Rate For Worst 15 Cells Carrying Significant Call Volume


0

10

20

30

40

50

60

70

80

90

100

208-10-3508-

1619

208-10-3508-

21616

208-10-3508-

1604

208-10-3945-

11718

208-10-3508-

21613

208-10-3508-

1642

208-10-3911-

28533

208-10-3901-

1701

208-10-3508-

21619

208-10-3912-

17913

208-10-3508-

11619

208-10-3910-

27905

208-10-3936-

8637

208-10-3944-

8505

208-10-3939-

13601

Cell ID

CallSet-UpSuccessRate(%)

0

50

100

150

200

250

300

350

400

CallVolume


Figure 5-5 Weekly Busy Hour Call Set-Up Success For Worst PerformingCells Carrying Significant Call Volume

Figure 5-5 shows that there are a significant number of cells with very poor call set-up success rate that need to be investigated. Resolution of these cells will help togive a substantial improvement in overall call set-up success rate with their

significant call volume and poor success rate.

Figure 5-6 breaks down the call set-up failures for BSS related causes for the15cells with the worst busy hour call set-up performance.

Busy Hour Call Set-Up Failures For Worst 15 Cells Carrying Significant Call Volume


0

10

20

30

40

50

60

208-10-3508-

1619

208-10-3508-

21616

208-10-3508-

1604

208-10-3945-

11718

208-10-3508-

21613

208-10-3508-

1642

208-10-3911-

28533

208-10-3901-

1701

208-10-3508-

21619

208-10-3912-

17913

208-10-3508-

11619

208-10-3910-

27905

208-10-3936-

8637

208-10-3944-

8505

208-10-3939-

13601

Cell ID

FailureRate(%)


Figure 5-6 Weekly Busy Hour Call Set-Up Success For Worst Performing

Cells Carrying Significant Call Volume


19/25


Performance Page 19

Figure 5-6 provides the root cause for some cells poor call set-up performance, forexample one cell has a high TCH blocking rate and one cell has a high TCHassignment failure rate. The other cells where the root cause cannot be determinedfrom the failure causes displayed in figure 5-6 require further investigation throughcall trace or A-interface analysis.

5.4 Nantes Busy Hour Call Set-Up PerformanceThe daily busy hour call set-up performance at a network level covering 112 cellsidentified by Cegetel in the Nantes region from the 16

th March 1999 to the 24

th

March 1999 is shown in figure 5-7.

Nantes Daily Busy Hour Call Set-Up Success Rate

94.2

94.4

94.6

94.8

95

95.2

95.4

95.6

95.8

96


Date

CallSet-UpSuccessR

ate(%)

0

2000

4000

6000

8000

10000

12000

14000

CallVolume


Figure 5-7 Nantes Daily Busy Hour Call Set-Up Success

Figure 5-7 shows a high and consistent level of call set-up success, with amaximum just below 96% and a minimum just below 95% over the period of study.This compares very closely with the call set-up success rate measured in section4.1 from A interface data for the same BSSs.

Figure 5-8 breaks down the call set up failure causes for the Nantes region at anetwork level.


20/25


Nantes Daily Busy Hour Call Set-Up Failures

0

0.2

0.4

0.6

0.8

1

1.2

1.4

16/03/99 17/03/99 18/03/99 19/03/99 20/03/99 21/03/99 22/03/99 23/03/99 24/03/99

Date

CallSet-UpFailureRate(%)


Figure 5-8 Nantes Daily Busy Hour Call Set-Up Failure Causes

Figure 5-8 shows that failures related to radio interference or radio hardwareperformance contributes to almost 1.5% of call set-up failures in the Nantes region.However as shown in section 4 this is not the only significant cause of failure.Further analysis of call trace and A interface signalling is recommended to evaluatethe other major causes of failure.

5.5 Nantes Worst Cell Busy Hour Performance

The weekly busy hour call set-up performance for the ten worst cells, carrying at

least 100 calls per day, out of the 112 cells identified by Cegetel that cover theNantes region is shown in figure 5-9.

Busy Hour Call Set-Up Success Rate For Worst 10 Cells In Nantes Carrying Significant Call Volume


84

85

86

87

88

89

90

91

92

93

94

95

208-10-

3953-4440

208-10-

3923-4438

208-10-

3929-34449

208-10-

3953-24437

208-10-

3904-4492

208-10-

3929-14400

208-10-

3904-4446

208-10-

3919-34428

208-10-

3953-14440

208-10-

3953-14472

208-10-

3923-14422

Cell ID

CallSet-Up

SuccessRate(%)

0

100

200

300

400

500

600

700

Ca

llVolume


Figure 5-9 Nantes Worst Cell Busy Hour Call Set-Up Success

It is clear that the worst cells in the Nantes region still have a high level of call set-upsuccess. The failure causes for each cell are shown in figure 5-10.


21/25


Performance Page 21

Busy Hour Call Set-Up Failures For Worst 10 Nantes Cells Carrying Significant Call Volume


0

1

2

3

4

5

6

7

208-10-

3953-4440

208-10-

3923-4438

208-10-

3929-34449

208-10-

3953-24437

208-10-

3904-4492

208-10-

3929-14400

208-10-

3904-4446

208-10-

3919-34428

208-10-

3953-14440

208-10-

3953-14472

208-10-

3923-14422

Cell ID

CallFailureRate(%)


Figure 5-10 Nantes Worst Cell Busy Hour Call Set-Up Failure Causes

Figure 5-10 clearly shows that 2 cells suffer significant TCH congestion and 7 cellssuffer radio interference or radio hardware related problems. Resolution of the TCHblocking and radio interference/radio hardware issues on the cells will raise the callset-up performance of the Nantes region to a very high level.


22/25


6 Database Parameters Impacting Call Set-Up Success Rate

The database parameters and features discussed in this section all have a directimpact on the call set-up success rate. The recommended settings for eachdatabase parameter are designed to maximise the call set-up success rate.

6.1 assign_successful => 15000

This BSC call processing timer is used by the SSM software process at the BSCduring an assignment or intra cell handover procedure to guard against notreceiving a response from the RRSM software process at the BTS indicating eithera successful or unsuccessful assignment or intra cell handover. To maximise theassignment/handover success rate this timer should be long enough to allow theMS to receive the assignment command over the air interface, attempt and fail toaccess the target channel and return to the source channel, including all LAPDmlayer 2 repeats at each stage. The timer should not be so long that resources areheld up unnecessarily introducing channel congestion.

To maximise the assignment success rate and hence the call set-up success rate, it

is recommended that this timer should be set to 15000ms.

6.2 bssmap_t10 => 14000

This BTS call processing timer is used by the RRSM software process at the BTSduring an assignment or intra cell handover procedure to guard against notreceiving a response from the mobile indicating either a successful or unsuccessfulassignment or intra cell handover. To maximise the assignment/handover successrate this timer should be long enough to allow the MS to receive the assignmentcommand over the air interface, attempt and fail to access the target channel andreturn to the source channel, including all LAPDm layer 2 repeats at each stage.The timer should not be so long that resources are held up unnecessarilyintroducing channel congestion. The timer should be set in cascade with the SSM

timer assign_successful such that bssmap_t10 is less than assign_successful.To maximise the assignment success rate and hence the call set-up success rate, itis recommended that this timer should be set to 14000ms.

6.3 interfer_bands,4 => 63

This database parameter sets the upper limit of uplink interference for channelallocation. In order to ensure that an SDCCH or TCH resource is allocated for everysystem access, regardless of uplink interference, it is recommended thatinterfer_bands,4 is set to a value of 63, the maximum. If interfer_bands,4 is set to avalue less than 63 and the uplink interference level measured for an individualchannel falls between interfer_bands,4 and 63 then that channel will not beavailable for allocation to a mobile station. Although it is recommended to set this

parameter at the maximum of 63, channels that fall into the final interference bandwill always be allocated last. From GSR4 onwards SDCCHs will always be allocatedregardless of the interference band.

6.4 rxlev_access_min =>8

Rxlev_access_min is a standard GSM parameter broadcast in the SystemInformation on the BCCH and used by the mobile station to calculate the path losscriterion C1 for cell selection and reselection. If rxlev_access_min is set very lowthen mobiles will be allowed to access the network in areas of poor coverage at therisk of dropping calls due to poor signal strength. Setting rxlev_access_min at aslightly higher level will ensure that the mobile station is in an area of reasonableradio coverage before a call is attempted, improving the chances of successful

system access and call establishment. However rxlev_access_min should not beset too high otherwise the network coverage as perceived by the subscriber through


23/25


Configuration Page 23

the mobile station display will be reduced. Additionally in an effort to attract roamingsubscribers it is advantageous to set rxlev_access_min to a lower value thancompetitor networks. If there is an aim to maximise network performance then it isrecommended to set rxlev_access_min to a value of 8.

6.5 Congestion Relief/Call Queuing

If the network suffers light levels of TCH blocking then either call queuing orcongestion relief should be utilised. Call queuing should be deployed first in anattempt to prevent TCH blocking. Congestion relief provides a more aggressivequeuing mechanism, handing over mobiles already active in a call to neighbouringcells that meet a user specified handover margin, lower than the standard handovermargin. If TCH congestion is more widespread and not prevented throughcongestion relief then more radios must be deployed.

For call queuing the MSC must set the queuing indicator in the BSSMAPAssignment Request message and the following settings are recommended toconfigure the BSS to queue a maximum of 10 TCH requests for a maximum of 5seconds.

queue_management_information 10max_q_length_full_rate_channel 10

bssmap_t11 5000

6.6 Discontinuous Transmission (DTX)

In order to minimise radio interference and maximise the call set-up success rate itis recommended that uplink and downlink DTX are both enabled.

6.7 Power Control

In order to minimise radio interference and maximise the call set-up success rate itis recommended that uplink and downlink power control are both enabled. The fastmobile power down and optimised power control (from GSR4) features should beutilised to maximise the impact of power control to reduce interference levels.

6.8 Location Area Optimisation

If a mobile station attempts to make a call in the region of a location area boundarythen the call set-up may not start on the strongest available cell, due to the effect ofthe cell_reselect_hysteresis parameter. This GSM parameter sets a margin toprevent the mobile making multiple ping pong location updates. If the mobile doesmake a call attempt on a cell other than the strongest cell then there is a risk of callset-up failure doe to RF interference. To minimise the interference and maximisethe call set-up success rate, it is recommended that the number of location areasare minimised and the boundaries are located in areas of lower call activity.

6.9 Carrier Prioritisation

From GSR 4 onwards the carrier prioritisation feature should be used to minimiseradio interference. The feature should be configured to allocate BCCH TCHs inpreference to non-BCCH TCHs, since the BCCH radio transmits at full power at alltimes regardless of whether TCHs or SDCCHs on the BCCH carrier are in use.Further careful prioritisation of non-BCCH TCHs in tandem with local cells utilisingthe same frequency allocation can help further reduce interference levels and helpcall set-up success rate.


24/25


6.10 SDCCH Placement

If BCCH frequencies are planned with a less aggressive repeat pattern than non-BCCH frequencies then from GSR 4 onwards it is recommend that the SDCCHplacement feature is used to configure SDCCHs on the BCCH carrier. Thisconfiguration will help minimise SDCCH radio interference and maximise call set-upsuccess rate.


25/25


APPENDIX A: PERFORMANCE METRIC DEFINITIONS

This appendix defines the performance metrics used within the report to benchmarknetwork performance. Each metric is an arithmetic calculation of raw statistics.

Call Set-Up Success Rate

The proportion of mobiles which successfully access a TCH, having requested anappropriate service on accessing the SDCCH.

( TOTAL_ CALLS + CONGEST_ ASSIGN_ HO_ SUC )

( OK_ ACC_ PROC[cm_ serv_ req_ call] + OK_ ACC_ PROC[cm_ reestablish]

+ OK_ ACC_ PROC[page_ response] + OK_ ACC_ PROC[cm_ serv_ req_ sms]

+ OK_ ACC_ PROC[cm_ser_ req_ emerg] - SMS_ INIT_ ON_ SDCCH )

100%

Call Set-Up TCH Blocking Rate

The proportion of all call origination requests for TCH resources which fail due to noavailable TCH resources.

100%MSC_FROM_REQ_MA

MS_BLKDMA_CMD_TO_

SDCCH RF Loss Rate

The proportion of SDCCH activations resulting in a successful mobile access whichsubsequently suffer an RF loss.

RF_ LOSSES_ SD

ALLOC_ SDCCH - CHAN_ REQ_ MS_ FAIL100%

TCH Radio Assignment Failure Rate

The proportion of attempted TCH assignments that result in the mobile failing tosuccessfully accessing the TCH. Assignments for call establishment and intra cellhandover are both included.

100%)MSMA_CMD_TO_

_HOINTRA_CELLSTOTAL_CALL1(

+

Call Setup Performance

Documents

Transcript of Call Setup Performance