Qualit de service et KPI

1. La qualit du Service (QoS) Gnralement, la qualit de service ou Quality of Service (QoS) est la capacit de transfrer dans les bonnes conditions un type de trafic donn, en termes de disponibilit, dbit, et dlai de transition. La qualit de service pour le rseau dtermine le degr de satisfaction de l'utilisateur aux services offerts.Dans le but d'offrir une qualit de service acceptable il faut que certains problmes doivent tre rsolus. Ces problmes sont principalement lis : La couverture

Ce problme ne peut pas tre dtect par le systme mais valu par les plaintes des abonnes et par les mesures radio. Les causes probables de ce problme sont les suivants : Mauvaise configuration du rseau c'est--dire problme li la position des sites, ou les types d'antennes ; Problme d'installation qui peut tre due la perte des puissances dans les cbles ; Problme de maintenance.

La disponibilit du rseau

C'est la probabilit d'obtention d'un nouvel appel. La diminution de taux d'appel aboutis implique que les abonnes ne peuvent pas tablir une communication. Les actions de l'chec d'tablissement d'appel s'expliquent par : Le niveau d'accs minimum dans la cellule ; L'interfrence et la mauvaise couverture radio.

La qualit de voix

L'oprateur agit contre le problme de la mauvaise qualit de communication, par les mesures systme et par les analyseurs de la qualit vocale. Les causes de dgradation de la qualit de la voix sont : La hors couverture ; La mauvaise installation ; La qualit des terminaux.

Les coupures d'appels

La coupure de communication peut tre engendr par : La mauvaise couverture ; Les interfrences.

2. Les canaux de transmissionsLe Canal est un support de transmission Physique ou Logique permettant de vhiculer une information, On distingue deux grandes catgories de canaux : les canaux physiques et les canaux logiques.

a) Les canaux physiques Le multiplexage frquentiel divise en canaux les plages de frquences et le multiplexage temporel partage l'usage d'une voie de transmission etre huit communications diffrentes. La trame, unit temporelle de base du rseau GSM, se divise en huit intervalles temporels d'une dure de 577us et chaque intervalle constitue un canal de communication dans lequel un message lmentaire appel paquet est transmis priodiquement. Ce paquet est un ensemble structur de bits. Les canaux physiques sont caractriss par une paire de Frquence et un Time Slot particulier par Frquence.

La dure dune trame TDMA est 4.6152 ms La largeur des canaux tant de 200 kHz.

La rpartition des canaux physiques selon la mthode daccs F/TDMA

b) Les canaux logiques

Les Canaux Logiques peuvent tre class en deux groupes : les Canaux de Trafic not TCH Traffic Channel) et les Canaux de Signalisations not CCH (Control Channel). Les canaux de trafic : Ils permettent de transmettre la parole 13 kbit/s ou des donnes jusqu' 12 kbit/s. Il est allou aux phases d'tablissement de communications et la transmission de courts messages alphanumriques. Les canaux de signalisation : en rseau GSM, il y'a trois classes de canaux de signalisation: i) les Canaux de mise en Veille ou canaux de diffusion :

Le BCCH : permet de diffuser les informations systmes concernant la cellule Le FCCH : Permet d'asservir le mobile en frquence ; Le SCCH : Permet de faire la synchronisation et l'identification.

ii) Les Canaux de transport communs: Le RACH : Permet la demande d'allouer la ressource ; LAGCH : Est la rponse au RACH, il permet d'allouer la ressource (SDCCH) au MS ; Le PCCH : Permet de rechercher la MS par diffusion de son identit lors d'une rception d'appel, d'une rception des SMS, d'une Authentification.

iii) Les Canaux de transport ddis :

Il y a trois canaux ddis, notamment le SDCCH, le SACCH et le FACCH. Le SDCCH : Permet de faire la signalisation des usagers, il a un SACCH associ. Ce sont des canaux vus comme des TCH, mais ils sont utiliss pour la signalisation ; Le SACCH : permet de faire la transmission des mesures radio lectrique. Il est associ aux autres canaux ; Le FACCH : Se substitue un TCH pour couler rapidement de la signalisation lors d'un HANDOVER.Figure rcapitulative des canaux de transmissions.3. Les indicateurs de performancesAfin de permettre aux oprateurs d'obtenir des informations sur la qualit du service offert par leur rseau et de l'optimiser, des indicateurs de performance appels KPIs (Key Performance Indicators) qui spcifient le fonctionnement radio des cellules ont t galement dfinis. En effet, un KPI est une valeur reprsentative permettant d'valuer la performance de systme. Ces indicateurs permettent la localisation des anomalies de rseau et par suite, l'identification et le diagnostic des causes de ces problmes afin de ragir avec des actions correctives adquates.

2G KPIs en fonction de la nature du service (CS ou PS)3G KPIs en fonction de la phase du service

CSPS

Indicateurs de l'accessibilit et de russite dappel -Call Setup Success Rate-SDCCH Request-SDCCH Congestion Rate-TCH assign Fail rate

Indicateurs d'tablissement TBF

-TBF Requests,-TBFEstablishment Success Rate...

Indicateurs de continuit

-Call Drop Rate-TCH Drop Rate-Handover Success Rate

Indicateurs de sortie TBF

-TBF Normal Release Rate-TBF Acceptable Release Rate-DL TBF Success Rate

Indicateurs de mobilit

-SDCCH Trafic-TCH Available Ressources.

Indicateurs de Dbit

-TBF Drop Rate-UL TBF Drop Rate-DL TBF Drop Rate.

Indicateurs daccessibilit-Call setup Success Rate-RRC Setup Success Rate

Indicateurs de disponibilit-RRC Congestion Rate-RAB Congestion Rate -Call Admission Refused Rate

Indicateurs de continuit-CS Service Drop Rate-AMR Call Drop Rate

Indicateurs de mobilit:

-Handover Success Rate

Indicateurs de traffic-UL Streaming Traffic Volume-DL Streaming Traffic Volume

Call setup success rate

- Call Set up Success Rate is one of the most important Key performance Indicators (KPIs) used by all mobile operators.- there is no standard measurement possible for this parameter. Therefore the different operators can measure it differently1) real cssr is calculated as ratio of the assigned TCHs to the channel requests (1)NB: The other procedures, which can be completed with an SDCCH, like SMS, location updating etc. are not counted because they do not represent the request for the speech call.NumTCHAssig represents the number of successfully assigned TCH (number of ASSIGNMENT COMPLETE messages)NumCHReaqSpeech represents the number of CHANNEL REQUEST messages (see Fig. 2) but related only to request for a mobile originated (MO) or mobile terminated (MT) call.

2) implemented cssr:

* Up to now it is not possible to distinguish between the requests for the speech call and other ones.* One of the possibilities how to solve this problem could be using of simplified formula:

NumCHReq represents total number of CHANNEL REQUEST messagesorNumCHReq = NumCHReqSpeech + NumCHReqNonSpeech,

So the implemeted cssr can be expressed as

Under condition thatNumCHReqNonSpeech/NumCHReqSpeech 20 % NumCHReqNonSpeech 0.2 * NumCHReqSpeech

The implemeted cssr formula can be modified using binomial series as follows:

the absolute error in measurement is calculated as follow

In case that NumCHReqNonSpeech is equal to zero, real cssr = implemented cssr Nevertheless NumCHReqNonSpeech/NumCHReqSpeech is on the range of tenths of percent (10%) which can lead to the big systematic error. Therefor mobile operators only use this formula only if NumCHReqNonSpeech/NumCHReqSpeech 1 % is true

3) Second possibility could be to use in the denominator of the real cssr the number of TCH attempts (NumTCHAttempt) instead of NumCHReqSpeech.

But in this case the result of the calculation will be TCH Assignment Success rate which is different than Call Set up Success Rate.

4)

so back to the first equation, we try to find a way to calculate NumCHReqSpeech.

From this message flow for the call set up in GSM , we can extract the next model (Model for calculation of NumCHReqSpeech)

message flow for the call set up in GSM

Model for calculation of NumCHReqSpeech

ImmAssSuccRate represents Immediate Assignment Success Rate given by formula:

NumEstInd represents the number of ESTABLISH INDICATION messagesSDCCHSuccRate represents SDCCH Success Rate given by formula:

Therefore, from the previous model we can express NumTCHAttempt as follow

then for NumCHReqSpeech we can write

Replacing NumCHReqSpeech in the first equation (real cssr)

This equation is the best approach and is provided for by Siemens, it provides exactly the Call Set up Success Rate in case the Direct TCH Assignment feature is disabled. Possible distortion when this feature is enabled

5)Some of the vendors (for example Siemens) provide the possibility to assign TCH directly. In this case the result of Immediate Assignment procedure is occupation of Fast Associated Control channel (FACCH) instead of SDCCH. On the Siemens side the Direct TCH assignment is used for emergency calls when usually authentication and ciphering are excluded and the call set up is provided up to 1 second instead of standard time frame from 3 to 4 seconds.Therefore, when Direct TCH Assignment feature is enabled the result of Immediate Assignment procedure for some of channel requests can be occupation of FACCH instead of SDDCH. The Direct TCH Assignment is used in order to decrease time needed for call set up in such a way that authentication and ciphering are excluded It means that FACCH is occupied only very short time in comparison to SDCCH occupation which leads to FACCH Success Rate close to 100 %

The model for calculation of NumCHReqSpeech becomes

Model for calculation of NumCHReqSpeech when Direct TCH assignment is enabledFrom this model follows

Or SDCCHSuccRate 1SDCCHDropRateThen

In practice, SDCCHDropRate does not exceed 0.5 %, we can the previous result using

NumCH Re qSpeech:

Replacing it in real cssr equation

It represents exact formula that should be used for calculation of Call Set up Success rate when Direct TCH Assignment feature is enabled

However this quotient cannot be calculatedSo we use the expression of real cssr with DIRECT TCH ASSIGNMENT IS DISABLED

Absolute error

Relative error

The relative error in the worst case will be equal to SDCCHDropRate which is maximum 0.5 %.

SDCCH Congestion Rate

SDCCH congestion rate is one of the important counters that indicate accessibility in circuit service, this counter provides the ratio of failed SDCCH seizures due to busy SDCCH to the total requests for the SDCCH. SDCCH congestion rate indicates the failed requests for the SDCCH for various reasons. SDCCH congestion rate also indicates the status of the SDCCH resource utilization Huawei defines SDCCH congestion rate as the ratio of failed SDCCH seizures due to busy SDCCH to the total request for the SDCCH.

SDCCH congestion rate formula

SDCCH seizures (capture)The request for SDCCH in GSM are mainly made in the following two procedure A. The procedure for requesting the SDCCH for services such as point to point calls, location updating (only SDCCH), call re-establishment or short messages.B. The procedure of SDCCH handovers (including both the intra-BSC and inter-BSC handovers)

Signaling procedure and measurement points

A1: Immediate assignment requestB1: Failed immediate channel assignment due to busy SDCCHs or failed SDCCH configurations

Factors for SDCCH Congestion Rate in GSM Congestion caused by faults on equipment or transmission (the fault on BTS, BSC, and Abis interface cause the SDCCH congestion. The alarm Excessive Loss of E1/T1 Signals in an Hour also cause the SDCCH congestion). Congestion caused by insufficient signaling resources (the heavy traffic and bursts cause the SDCCH congestion. Proper setting of the numbers of SDCCHs and TCHs, and SDCCHs dynamic dynamic conversion function can relieve the congestion). Congestion caused improper data configuration (the SDCCH congestion related to the relevant parameters of the BSC such as SDCCH availability, LAC, and T3101 (Timer used in immediate assignment procedure), and T3112 (the timer used for periodic updating). If these parameters set correctly, the SDCCH congestion can be relieved. In addition, if the assignment procedure is set to Late Assignment, the timer of the SDCCH being occupied increases, which may lead to congestion). Congestion caused by interference (Interference on the Um interface also causes congestion. For example, if the main BCCH in the serving cell and the TCH in neighboring cell share the same TRX frequency and BTS BSIC, the handover access on this TCH may be mistaken as random access. As a result, the SDCCH is abnormally allocated and congestion occurs. The excessive receive sensibility can make the interference signal mistaken as access signal, which lead to congestion.

TCH Call Drop Rate

Definition and formulaTCH one of the most significant KPIs for telecom operators, is related to retainability and defined as the ratio of call drops to successful TCH seizures after the BSC successfully assign TCHs to MSs. A too high TCH call drop rate adversely affect the users experience. The TCH call drop can be measured from the from the following aspects. TCH call drop rate (including handover) TCH call drop rate (excluding handover)TCH call drop rate (including handover) = (Number of TCH Call Drops + Number of TCH Call Drops during very early assignement ) / Number of successful TCH seizures x 100%TCH call drop rate (excluding handover) = Number of call drops on TCH / ( Number of successful TCH seizures + Number of successful incoming internal inter cell handovers + Number of successful incoming external inter cell handovers Number of succssful outgoing internal inter cell handover Number of successful outgoing external inter cell handover ) x 100%Through analysis you can find out that the TCH call drop rate (including handover) is lower than TCH call drop rate (excluding handover). The number of TCH Call Drops during the very early assignement is small, therefore the difference between the numerators of both ratios can be omitted. Including the TCH seizures in the case of handovers, the denominator of the formula for the TCH call drop rate (including handover) is greater than the denominator of the formula for TCH call drop rate (excluding handover). Therefore the result of the formula for TCH call drop rate (including handover) is smaller than the formula for TCH call drop rate (excluding handover).

Signaling Procedure and Measurement Points Immediate assignment procedure Assignment procedure

Intra-BSC handover Procedure Incoming BSC handover procedure

Note: The meaning of measurement points TCH-SUCC: indicates the number of successful TCH seizures during the very early assignment procedure TCH-SUCC-A: indicates the number of successful TCH seizures. TCH-SUCC-B: indicates the number of successful incoming internal inter-cell handovers plus the number of successful internal intra-cell handovers. TCH-SUCC-C: indicates the number of successful incoming external inter-cell handovers

Facts affect the TCH Call drop rateAccording to the user complaints and network optimization experience, the major factors that affect the TCH call drop rate are as follow: Hardware failure (when TRX or a combiner is faulty, seizing the TCH become difficult) Transmission problem (when the transmission quality one the A or Abis interface is poor) Version upgrade ( after upgrade BTS and BSC versions can be incompatible) Parameter setting (the setting of some parameters on the BSC and MSC sides may affect the TCH call drop rate) Intra-network and inter-network interference (interference from scramblers, privately installed antennas, intermodulation interference from BTSs, intra network co-channel and adjacent channel interference) Coverage problem (discontinuous coverage (blind areas), poor indoor coverage, insufficient coverage (signal from antenna is blocked)...) Antenna system problem ( antennas are improperly connected . if a directional cell has a main and diversity antennas, the BCCH and SDCCH of the cell may be transmitted from different antennas and if the two antennas have different pitch angles or azimuths the coverage areas are different. A MS can receive the BCCH signals from one antenna and when a call is made the MS cannot seize the SDCCH transmitted by the other antenna ) Imbalance between uplink and downlink (transmit power of the BTS is high, tower mounted amplifier TMA or BTS amplifier does not work properly ) Repeater problem (uplink and downlink gain is inappropriately set, wide frequency repeater is used and the gain is set to a great value strong interference may occur ..)

Temporary Block Flow TBF establishment success rate