GSM_GPRS_Evaluatioin and optimization

Telecommunications engineering Major :

Mobile Services and networks

End Studies Project Topic :

GSM/GPRS Evaluation and optimization tool Elaborated by :

Fatma HAMDI

Supervisors :

M. Taieb MASMOUDI M. Mohamed Taher MISSAOUI

Work proposed and elaborated within

University year : 2005/2006

FatmaHAMDI, end studies project 2005/2006

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Dedications In the memory of my father To my dear mother To my brothers To my sisters To my friends To Wijden To those who like me I dedicate this work.

✠ Fatma HAMDI

FatmaHAMDI, end studies project 2005/2006

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Abstract

This report represents my end of studies project within the Mobile Network Direction of “Tunisie Télécom”. It was held from February 2006 to June 2006. “Tunisie Télécom” is the historic operator of Tunisia. It provides several services for cellular and fixed network subscribers. The Mobile network Direction is responsible for satisfying subscribers in terms of quality of service. One of the complexities of the GSM and GPRS systems rests in the way they satisfy users and involve better services. As often, Quality of service optimization led to a greater complexity. So the huge success of those networks requires more and more qualified people. A software allowing the live observation of the network performance eases the training of those people. This project is aimed at the design and implementation of such software. It describes the base principles of GSM and GPRS, especially on the radio way, and includes the architectural and functional specification of the application. Moreover, it joins a real case study which aids to more understand our optimization way.

Fatma HAMDI, end studies project 2005/2006

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Acknowledgment I would like to thank all the persons who helped me on this project. I wish to thank particularly : All the employees of Mobile Network Direction of “Tunisie Télécom” for their cordial greeting. Taieb MASMOUDI, Quality Optimization Director, my supervisor at “Tunisie Télécom” who gave me precious advices in planning this project and provided me with technical knowledge and support throughout my training period. Mohamed Taher MISSAOUI, Master assistant at Sup’ Com, my supervisor at Sup’Com for the interest he showed in my work. All my thanks and my great recognition to all the professors of Sup' Com for the formation that they gave us.


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Summary

The evaluation and optimization of networks GSM/GPRS is presented as crucial and dynamic activities for operators. Our end of studies project relates to the development of a tool of analysis and optimization of a network GSM/GPRS and the study of a real case. This Tool, a Post Processing Tool, was specified starting from the study of drive test that provides a lot of data which has to be analysed in a practical and effective way and using Key Performance Indicators extracted from RNO (Radio Network Optimization) that help the optimization process. Its principle is based on the import of relevant measurements and, on their use for a practical visualisation of the results by means of maps and graphs. The collection of GSM/GPRS data makes it possible to evaluate networks in terms of radio coverage, communication quality, interference, etc... After a description of network GSM/GPRS in terms of architecture and functionalities, a presentation of the principal indicators of quality of service GSM/GPRS as well as the various parameters which allow the management of this QoS, we describe the tool starting from a case study. The originality of this work comes from, the development of a methodology of performance evaluation and optimization of network GSM/GPRS. This methodology, also aids to locate the anomalies in the network and propose some recommendations.

Key Words: GSM/GPRS, evaluation, optimization, QoS, indicators, drive test, Key Performance Indicators.


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Table of contents

Abstract.............................................................................................................................…....ii Summary............................................................................................................................…...iv Acronyms...................................................................................................................................x Table of figures......................................................................................................................viii Tables list .................................................................................................................................. x General introduction................................................................................................................ 1 Chapter 1: GSM/GPRS Generality ........................................................................................ 3 1.1. Introduction ......................................................................................................................... 3 1.2. An overview of GSM .......................................................................................................... 3 1.2.1. GSM services ............................................................................................................... 3 1.2.2. Basic overview of the GSM Network .......................................................................... 3 1.2.2.1. The Network Switching Subsystem ...................................................................... 4 1.2.2.2. The Base Station Subsystem ................................................................................. 4 1.2.2.3 The Operating Sub-System .................................................................................... 5 1.2.3. GSM interfaces............................................................................................................. 5 1.2.4. Radio link aspects......................................................................................................... 6 1.2.4.1. Channel structure................................................................................................... 6 1.2.4.2. Speech coding ....................................................................................................... 8 1.2.4.3. Channel coding and modulation............................................................................ 8 1.2.4.4. Discontinuous transmission................................................................................... 9 1.2.4.5. Power control ...................................................................................................... 10 1.2.5. Network aspects ......................................................................................................... 10 1.2.5.1. Radio resources management.............................................................................. 11 1.2.5.2. Mobility management ......................................................................................... 12 1.2.5.2.1. Location updating......................................................................................... 12 1.2.5.2.2. Authentication and security.......................................................................... 13 1.2.5.3. Communication management.............................................................................. 14 1.3. GSM limits ........................................................................................................................ 14 1.4. An Overview of GPRS...................................................................................................... 15 1.4.1. GPRS Services ........................................................................................................... 15 1.4.1.1. PTP (Point-To-Point) Services............................................................................ 15 1.4.1.2. PTM (Point-To-Multipoint) Services.................................................................. 15 1.4.2. Basic overview of the GPRS Network....................................................................... 15 1.4.2.1. The GPRS Base Station Subsystem .................................................................... 16 1.4.2.2. The GPRS Network Switching Subsystem ......................................................... 16 1.4.3. GPRS Interfaces ......................................................................................................... 16 1.4.4. GPRS radio link aspects............................................................................................. 17 1.4.4.1 Logical channels................................................................................................... 17 1.4.4.2 Radio link adaptation: .......................................................................................... 18 1.4.5. Network aspects ......................................................................................................... 18 1.4.5.1. Mobility Management in GPRS.......................................................................... 18 1.4.5.1.1. Accessing the GPRS Network...................................................................... 18 1.4.5.1.2. Mobility Management States ....................................................................... 18


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1.4.5.2. Transmission / Signalling Planes in GPRS ......................................................... 19 1.5 Conclusion.......................................................................................................................... 20 Chapter 2 : GSM/GPRS Network Supervision and Optimization .................................... 21 2.1 Introduction ........................................................................................................................ 21 2.2. QoS evaluation Criteria..................................................................................................... 21 2.3 QoS Supervision Techniques ............................................................................................. 22 2.3.1. Drive Test ................................................................................................................... 22 2.3.1.1 Measurement chain .............................................................................................. 22 2.3.1.2 Drive Test measurement....................................................................................... 22 2.3.1.3 Advantages and drawbacks .................................................................................. 24 2.3.2 RNO parameters .......................................................................................................... 25 2.3.2.1 RNO definition..................................................................................................... 25 2.3.2.1 Key Performance Indicators examples................................................................. 25 2.3.2.2 Advantages and drawbacks .................................................................................. 26 2.3.4 Analysis process .......................................................................................................... 26 2.4. GSM network Parameters ................................................................................................. 27 2.4.1. Parameters types......................................................................................................... 27 2.4.2. Parameters examples .................................................................................................. 27 2.5. Detection problems steps .................................................................................................. 28 2.5.1 Interference.................................................................................................................. 28 2.5.2 Congestion................................................................................................................... 29 2.5.2.1 TCH/PDCH Congestion ....................................................................................... 30 2.5.2.2 SDCCH Congestion ............................................................................................. 30 2.5.3 Coverage problem ....................................................................................................... 31 2.5.4 Call/session drop problem........................................................................................... 32 2.5.5 Call/Session set up failure problem............................................................................. 34 2.6 Conclusion.......................................................................................................................... 34 Chapter 3 : Implementation and validation of the tool ...................................................... 35 3.1 Introduction ........................................................................................................................ 35 3.2. Objectives.......................................................................................................................... 35 3.3. Programming languages.................................................................................................... 35 3.4. Post processing tool structure............................................................................................ 36 3.4.1 Post processing tool architecture................................................................................. 36 3.4.2 Selected data................................................................................................................ 37 3.4.2.1 Drive test .............................................................................................................. 37 3.4.2.2 RNO data.............................................................................................................. 37 3.5. Radio optimization tool description .................................................................................. 37 3.5.1 Authentication ............................................................................................................. 37 3.5.2 Main menu................................................................................................................... 38 3.5.3 Statistics ...................................................................................................................... 41 3.5.3.1 Coverage statistics................................................................................................ 41 3.5.3.2 RXQUAL statistics .............................................................................................. 43 3.5.3.3 T_Adv statistics.................................................................................................... 44 3.5.3.4 Coding schemes statistics..................................................................................... 46 3.5.4 Map view..................................................................................................................... 47 3.5.4.1 Coverage map....................................................................................................... 50 3.5.4.2 Quality of communication map............................................................................ 52 3.5.4.3 Better cell map ..................................................................................................... 53 3.6. Real case study.................................................................................................................. 53 3.6.2 Analysis and optimization area: .................................................................................. 53


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3.6.3 Optimization process................................................................................................... 56 3.6.3.1 Coverage problem ................................................................................................ 56 3.6.3.2 Call set up failure problem ................................................................................... 58 3.6.3.3 Interference problem ............................................................................................ 58 3.6.3.4 Congestion problem ............................................................................................. 60 3.6.3.5 Call drop problem................................................................................................. 61 3.7 Conclusion.......................................................................................................................... 62 Conclusion............................................................................................................................... 63


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Table of figures Figure 1.1 : GSM architecture.................................................................................................... 3 Figure 1.2 : Signalling protocol structure in GSM................................................................... 10 Figure 1.3 : GPRS architecture. ............................................................................................... 15 Figure 1.4 : GPRS State Model................................................................................................ 19 Figure 2.1 : Drive test measurement chain.............................................................................. 22 Figure 2.2 : analysis process. ................................................................................................... 27 Figure 2.3 : Interference analysis and optimization ................................................................. 29 Figure 2.4 : Congestion analysis and optimization. ................................................................. 31 Figure 2.5 : Coverage analysis and optimization. .................................................................... 32 Figure 2.6 : Call drop problem analysis and optimization. ...................................................... 33 Figure 2.7 : Call set up failure problem analysis and optimization.......................................... 34 Figure 3.1: Post processing tool architecture. .......................................................................... 36 Figure 3.2 : Authentication interface........................................................................................ 38 Figure 3.3 : Main menu interface. ............................................................................................ 38 Figure 3.4 : Physical parameters interface. .............................................................................. 39 Figure 3.5 : Logical parameters interface................................................................................. 40 Figure 3.6 : “About” interface.................................................................................................. 40 Figure 3.7 : RXLEV statistics interface. .................................................................................. 41 Figure 3.8 : Open file interface. ............................................................................................... 42 Figure 3.9 : Coverage histogram interface. .............................................................................. 42 Figure 3.10 : RXQUAL statistics interface. ............................................................................. 43 Figure 3.11 : Quality histogram interface. ............................................................................... 44 Figure 3.12 : T_ADV statistics interface. ................................................................................ 45 Figure 3.13 : T_ADV histogram. ............................................................................................. 45 Figure 3.14 : Coding schemes statistics. .................................................................................. 46 Figure 3.15 : GPRS RXQUAL statistics.................................................................................. 47 Figure 3.16 : Starting MapInfo interface.................................................................................. 48 Figure 3.17 : MapInfo analysis interface. ................................................................................ 48 Figure 3.18 : Choosing file interface........................................................................................ 49 Figure 3.19 : Saving MapInfo tables interface. ........................................................................ 49 Figure 3.20 : Importing other files interface. ........................................................................... 50 Figure 3.21 : Analysis kind interface. ...................................................................................... 50 Figure 3.22 : RXLEV thresholds.............................................................................................. 51 Figure 3.23 : Level of received power in GMS/GPRS network. ............................................. 51 Figure 3.24 : RXQUAL thresholds. ......................................................................................... 52 Figure 3.25 : Quality communication map............................................................................... 52 Figure 3.26 : Better cell map.................................................................................................... 53 Figure 3.27 : Analysis and optimization area........................................................................... 54 Figure 3.28 : Optimization area coverage map. ....................................................................... 55 Figure 3.29 : Optimization area Quality map........................................................................... 55 Figure 3.30 : RXLEV problem interface.................................................................................. 56


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Figure 3.31 : Coverage and call set up failure problem optimization interface. ...................... 57 Figure 3.32 : Coverage optimization interface......................................................................... 57 Figure 3.33 : Call set up failure problem optimization interface. ............................................ 58 Figure 3.34 : Quality problem interface. .................................................................................. 59 Figure 3.35 : Interference and congestion optimization interface............................................ 59 Figure 3.36 : Interference optimization interface..................................................................... 60 Figure 3.37 : Congestion optimization interface. ..................................................................... 61 Figure 3.38 : Call drop problem optimization interface........................................................... 61


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Tables list Table 1. 1 : GSM Logical Channels........................................................................................... 7 Table 2. 1 : RXLEV evaluation…………………….................................. ………………….23 Table 2. 2 : Correspondence between RXQUAL and BER ..................................................... 23 Table 3. 1 : coverage thresholds…………………………………...…………………………41 Table 3. 2 : Quality thresholds. ................................................................................................ 43 Table 3. 3 : coverage results..................................................................................................... 51 Table 3. 4 : Quality results. ...................................................................................................... 52


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Acronyms AuC Authentification Center BSS

Base Station Sub-System

BTS

Base Transceiver Station

BSC Base Station Controller BSS

Base Station Subsystem

BCCH Broadcast Control Channel BSIC Base Station Identification Code DTX Discontinuous Transmission EIR

Equipment Identity Register

FDMA Frequency Division Multiple Access GSM Global System for Mobile Communication GPRS General Packet Radio Service GMM GPRS Mobility Management GMSC Gateway Mobile Switching Center GGSN Gateway GPRS Support Node GMM GPRS Mobility Management GPS

Global Positionner System

HLR Home Location Register HSCSD High Speed Circuit Switched Data ISDN Integrated Service Digital Network KPI

Key Performance Indicators

LAPD Link Access Protocol on the D channel LAPDm Link Access Protocol on the Dm channel MS

Mobile Station

MSC Mobile-service Switching Center MAC Medium Access control MM

Mobility Management

NSS

Network Sub System

OMC Operation and Maintenance Center OSS

Operating Sub-System


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PCU Paquets Controler Unit PDP

Packet Data Protocol

PLMN Public Land Mobile Network QoS

Quality of Service

RLC Radio Link Control RNO Radio Network Optimization SGSN Serving GPRS Support Node SNDCP Sub Network Dependent Convergence Protocol. SMS Short Message Service TDMA Time Division Multiple Access TBF Temporary Block Flow VLR Visitor Location Register


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General introduction


GSM is the European standard for cellular communications developed by ETSI (European Telecommunications Standards Institute). Throughout Europe and the rest of the world (including North America), GSM has been widely adopted. It has already been implemented in over 100 countries. The most important service in GSM is voice telephony. Voice is digitally encoded and carried by the GSM network as a digital stream in a circuitswitched mode. GSM offers data services already but they have been constrained by the use of circuit switched data channels over the air interface allowing a maximum bit rate of 14.4 kbit/s. For this reason, the GSM standard has continued its natural evolution to accommodate the requirement for higher bit rates. The HSCSD is one solution that addresses this requirement by allocating more time slots per subscriber and thus better rates. It remains however insufficient for data applications such as Web browsing. Moreover, HSCSD relies on circuitswitching techniques making it unattractive for subscribers who want to be charged based on the volume of the data traffic they actually use rather than on the duration of the connection. In turn, service providers need effective means to share the scarce radio resources between more subscribers. In a circuit switched mode, a channel is allocated to a single user for the duration of the connection. This exclusive access to radio resources is not necessary for data applications with the use of packet switched techniques [1]. GPRS stands out as one major development in the GSM standard that benefits from packet switched techniques to provide mobile subscribers with the much needed high bit rates for data transmissions. It is possible theoretically for GPRS subscribers to use several time slots (packet data channels) simultaneously reaching a bit rate of about 170kbit/s. Volume-based charging is possible because channels are allocated to users only when packets are to be sent or received. Data applications make it possible to balance more efficiently the network resources between users because the provider can use transmission gaps for other subscriber activities.


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With the growth of these services, operators do their utmost to satisfy subscribers’ needs. So, they aim to provide a good quality of service. In this context, we have approached this subject. Our project gives a solution of how operators can optimize their network in terms of QoS. It includes three chapters. In the first part, we will speak about general information about GSM/GPRS architecture and functionalities. The second part is a study of drive test and KPI indicators from “Tunisie Télécom” network and a proposition of some algorithms used in order to overcome some problems that affect the network. In the third part, we will implement the algorithms described in the previous chapter and make a case study that validates our tool.


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GSM/GPRS Generality

Chapter 1: GSM/GPRS Generality

1.1. Introduction It is interesting to deal with general information about GSM/GPRS network because many concepts will be used in the next chapters. In this chapter, we will present GSM and GPRS networks in terms of services, architectures, interfaces, mobility management.

1.2. An overview of GSM 1.2.1. GSM services Today, in addition to circuit-switched voice services, GSM supports the following data services: • Circuit-Switched Data: A dedicated connection is set up for the duration of the call, regardless of whether data is being transferred. The data throughput rate is 9.6 or 14.4 Kbps, depending on the coding scheme supported by the network and terminal device. • SMS: GSM also supports sending and receiving short text messages (approximately 160 characters) known as SMS on a signalling channel. Billions of these messages are sent per month and the numbers are growing. The circuit-switched services provided by the GSM technology are augmented by packet-switched services provided by the GPRS overlay [1].

1.2.2. Basic overview of the GSM Network

Figure 1. 1 : GSM architecture.


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GSM/GPRS Generality

1.2.2.1. The Network Switching Subsystem The NSS is responsible for call control, service control and subscriber mobility management functions. HLR (Home Location Register) The HLR is a database used to store and manage permanent data of subscribers such as service profiles, location information, and activity status. MSC (Mobile Switching Center) The MSC is responsible for telephony switching functions of the network. It also performs authentication to verify the user’s identity and to ensure the confidentiality of the calls. The Authentication Center (AuC) provides the necessary parameters to the MSC to perform the authentication procedure. The AuC is shown as a separate logical entity but is generally integrated with the HLR. The Equipment Identity Register (EIR) is on the other hand a database that contains information about the identity of the mobile equipment. It prevents calls from unauthorized or stolen mobile stations. VLR (Visitor Location Register) The VLR is a database used to store temporary information about the subscribers and is needed by the MSC in order to service visiting subscribers. The MSC and VLR are commonly integrated into one single physical node and the term MSC/VLR is used instead. When a subscriber enters a new MSC area, a copy of all the necessary information is downloaded from the HLR into the VLR. The VLR keeps this information so that calls of the subscriber can be processed without having to interrogate the HLR (which can be in another PLMN) each time. The temporary information is cleared when the mobile station roams out of the service area. GMSC (Gateway Mobile Switching Center) A GMSC is an MSC that serves as a gateway node to external networks, such as ISDN or wireline networks. 1.2.2.2. The Base Station Subsystem The BSS performs radio-related functions. It consists of BTSs (Base Transceiver Stations) and BSCs (Base Station Controllers) [2]. BTS (Base Transceiver Station) The BTS handles the radio interface to the MS. It consists of radio equipment (transceivers and antennas) required to service each cell in the network. BSC (Base Station Controller)


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GSM/GPRS Generality

The BSC provides the control functions and physical links between the MSC and the BTS. A number of BSCs are served by one MSC while several BTSs can be controlled by one BSC.

1.2.2.3 The Operating Sub-System It assures the network management and supervision. We find the Network Management Centre (NMC) which controls and reports network problems, we find also the Operation and Maintenance Center (OMC) that assures the functionalities below: • Alarm Handling, • Fault Management, • Performance Management, • Configuration Management, • Software Version Management, • Network Statistics Data Collection, • Network Status Control.

1.2.3. GSM interfaces •

Um: The air interface is used for exchanges between a MS and a BSS,

•

Abis: This is a BSS internal interface linking the BSC and a BTS, and it has not been standardised. The Abis interface allows control of the radio equipment and radio frequency allocation in the BTS,

•

A: The A interface is between the BSS and the MSC. The A interface manages the allocation of suitable radio resources to the MSs and mobility management,

•

B: The B interface between the MSC and the VLR uses the MAP/B protocol. Most MSCs are associated with a VLR, making the B interface "internal". Whenever the MSC needs access to data regarding a MS located in its area, it interrogates the VLR using the MAP/B protocol over the B interface,

•

C: The C interface is between the HLR and a GMSC. Each call originating outside of GSM (i.e., a MS terminating call from the PSTN) has to go through a Gateway to obtain the routing information required to complete the call, and the MAP/C protocol over the C interface is used for this purpose. Also, the MSC may optionally forward billing information to the HLR after call clearing,


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GSM/GPRS Generality

•

D: The D interface is between the VLR and HLR, and uses the MAP/D protocol to exchange the data related to the location of the MS and to the management of the subscriber,

•

E: The E interface interconnects two MSCs. The E interface exchanges data related to handover between the anchor and relay MSCs using the MAP/E protocol,

•

F: The F interface connects the MSC to the EIR, and uses the MAP/F protocol to verify the status of the IMEI that the MSC has retrieved from the MS,

•

G: The G interface interconnects two VLRs of different MSCs and uses the MAP/G protocol to transfer subscriber information, during e.g. a location update procedure,

•

H: The H interface is between the MSC, and uses the MAP/H protocol to support the transfer of short messages,

•

I: The I interface is the interface between the MSC and the MS. Messages exchanged over the I interface are relayed transparently through the BSS.

1.2.4. Radio link aspects The International Telecommunication Union (ITU), which manages the international allocation of radio spectrum (among other functions) allocated the bands 890-915 MHz for the uplink (mobile station to base station) and 935-960 MHz for the downlink (base station to mobile station) for mobile networks in Europe. Since radio spectrum is a limited resource shared by all users, a method must be devised to divide up the bandwidth among as many users as possible. The method chosen by GSM is a combination of Time and Frequency Division Multiple Access (TDMA/FDMA). The FDMA part involves the division by frequency of the total 25 MHz bandwidth into 124 carrier frequencies of 200 kHz bandwidth. One or more carrier frequencies are then assigned to each base station. Each of these carrier frequencies is then divided in time, using a TDMA scheme, into eight time slots. One time slot is used for transmission by the mobile and one for reception. They are separated in time so that the mobile unit does not receive and transmit at the same time, a fact that simplifies the electronics [3]. 1.2.4.1. Channel structure A total of 156.25 bits is transmitted in 0.577 milliseconds, giving a gross bit rate of 270.833 kbps. There are three other types of burst structure for frame and carrier synchronization and frequency correction. The 26bit training sequence is used for


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GSM/GPRS Generality

equalization. The 8.25 bit guard time allows for some propagation time delay in the arrival of bursts. Each group of eight time slots is called a TDMA frame, which is transmitted every 4.615 ms. TDMA frames are further grouped into multiframes to carry control signals. There are two types of multiframe, containing 26 or 51 TDMA frames. The 26frame multiframe contains 24 Traffic Channels (TCH) and two Slow Associated Control Channels (SACCH) which supervise each call in progress. The SACCH in frame 12 contains eight channels, one for each of the eight connections carried by the TCHs. The SACCH in frame 25 is not currently used, but will carry eight additional SACCH channels when half rate traffic is implemented. A Fast Associated Control Channel (FACCH) works by stealing slots from a traffic channel to transmit power control and handover signalling messages.

The channel

stealing is done by setting one of the control bits in the time slot burst. In addition to the Associated Control Channels, there are several other control channels which (except for the Standalone Dedicated Control Channel) are implemented in time slot 0 of specified TDMA frames in a 51frame multiframe. Type Traffic Channel TCH

Name TCH/F

Sens MS

Function Information transmission at a rate of 22.8 Kb/s TCH/H Information transmission at a rate of 11.4 Kb/s Broadcast Control FCCH Carries information for Channel BCCH frequency correction SCH Carries Information for frame synchronisation BCCH Transmits general information from point to multipoint CBCH Broadcasts short messages to cells that support this service Common Control RACH Request allocation of a Channel CCCH SDCCH PCH Pages MSs AGCH Allocates a SDCCH or directly a TCH Dedicated Control SDCCH Low rate channel used for Channel DCCH signalling, is not associated to a traffic channel SACCH Link supervisor FACCH HO execution Table 1. 1 : GSM Logical Channels.


Network

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GSM/GPRS Generality

1.2.4.2. Speech coding GSM is a digital system, so speech signals, inherently analog, have to be digitized. The method employed by current telephone systems for multiplexing voice lines over high speed trunks and optical fiber lines, is Pulse Coded Modulation (PCM). The output stream from PCM is 64 kbps, too high a rate to be feasible over a radio link. The 64 kbps signal contains much redundancy, although it is simple to implement.

The GSM group studied

several voice coding algorithms on the basis of subjective speech quality and complexity (which is related to cost, processing delay, and power consumption once implemented) before arriving at the choice of a Regular Pulse Excited - Linear Predictive Coder (RPELPC) with a Long Term Predictor loop. Basically, information from previous samples, which does not change very quickly, is used to predict the current sample.

The coefficients of the linear

combination of the previous samples, plus an encoded form of the residual, the difference between the predicted and actual sample, represent the signal.

Speech is divided into 20

millisecond samples, each of which is encoded as 260 bits, giving a total bit rate of 13 kbps. 1.2.4.3. Channel coding and modulation Due to natural or man made electromagnetic interference, the encoded speech or data transmitted over the radio interface must be protected as much as is practical. The GSM system uses convolutional encoding and blocks interleaving to achieve this protection. The exact algorithms used differ for speech and for different data rates. Recall that the speech codec produces a 260 bit block for every 20 ms speech sample. From subjective testing, it was found that some bits of this block were more important for perceived speech quality than others. The bits are thus divided into three classes: Class Ia 50 bits - most sensitive to bit errors, Class Ib 132 bits - moderately sensitive to bit errors, Class II 78 bits - least sensitive to bit errors. Class Ia bits have a 3 bit Cyclic Redundancy Code added for error detection. If an error is detected, the frame is judged too damaged to be comprehensible and it is discarded. It is replaced by a slightly attenuated version of the previous correctly received frame. These 53 bits, together with the 132 Class Ib bits and a 4 bit tail sequence (a total of 189 bits), are input into a 1/2 rate convolutional encoder of constraint length 4. Each input bit is encoded as two output bits, based on a combination of the previous 4 input bits. The convolutional encoder thus outputs 378 bits, to which are added the 78 remaining Class II bits, which are


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GSM/GPRS Generality

unprotected. Thus every 20 ms speech sample is encoded as 456 bits, giving a bit rate of 22.8 kbps. To further protect against the burst errors common to the radio interface, each sample is diagonally interleaved.

The 456 bits output by the convolutional encoder are

divided into 8 blocks of 57 bits, and these blocks are transmitted in eight consecutive timeslot bursts. Since each timeslot burst can carry two 57 bit blocks, each burst carries traffic from two different speech samples. Recall that each timeslot burst is transmitted at a gross bit rate of 270.833 kbps. This digital signal is modulated onto the analog carrier frequency, which has a bandwidth of 200 kHz, using Gaussian filtered Minimum Shift Keying (GMSK). GMSK was selected over other modulation schemes as a compromise between spectral efficiency, complexity of the transmitter, and limited spurious emissions.

The

complexity of the transmitter is related to power consumption, which should be minimized for the mobile station. The spurious radio emissions, outside of the allotted bandwidth, must be strictly controlled so as to limit adjacent channel interference, and allow for the coexistence of GSM and the older analog systems (at least for the time being). 1.2.4.4. Discontinuous transmission Minimizing cochannel interference is a goal of any cellular system, since it allows better service for a given cell size, or the use of smaller cells, thus increasing the overall capacity of the system. Discontinuous transmission (DTX) is a method that takes advantage of the fact that a person speaks less that 40 percent of the time in normal conversation, by turning the transmitter off during silence periods. An added benefit of DTX is that power is conserved at the mobile unit. The most important component of DTX is, of course, Voice Activity Detection. It must distinguish between voice and noise inputs, a task that is not as trivial as it appears, considering background noise.

If a voice signal is misinterpreted as

noise, the transmitter is turned off and a very annoying effect called clipping is heard at the receiving end. If, on the other hand, noise is misinterpreted as a voice signal too often, the efficiency of DTX is dramatically decreased. Another factor to consider is that when the transmitter is turned off, there is a very silent silence heard at the receiving end, due to the digital nature of GSM. To assure the receiver that the connection is not dead, comfort noise is created at the receiving end by trying to match the characteristics of the transmitting end's background noise.


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GSM/GPRS Generality

1.2.4.5. Power control There are five classes of mobile stations defined, according to their peak transmitter power, rated at 20, 8, 5, 2, and 0.8 watts. To minimize cochannel interference and to conserve power, both the mobiles and the Base Transceiver Stations operate at the lowest power level that will maintain an acceptable signal quality. Power levels can be stepped up or down in steps of 2 dB from the peak power for the class down to a minimum of 13 dBm (20 milliwatts). The mobile station measures the signal strength or signal quality (based on the Bit Error Ratio), and passes the information to the Base Station Controller, which ultimately decides if and when the power level should be changed. Power control should be handled carefully, since there is the possibility of instability. This arises from having mobiles in cochannel cells alternatingly increase their power in response to increased cochannel interference caused by the other mobile increasing its power. This in unlikely to occur in practice but it is (or was as of 1991) under study.

1.2.5. Network aspects Ensuring the transmission of voice or data of a given quality over the radio link is only part of the function of a cellular mobile network. A GSM mobile can seamlessly roam nationally and internationally, which requires that registration, authentication, call routing and location updating functions exist and are standardized in GSM networks. In addition, the fact that the geographical area covered by the network is divided into cells necessitates the implementation of a handover mechanism.

Figure 1. 2 : Signalling protocol structure in GSM. The signalling protocol in GSM is structured into three general layers, depending on the interface, as shown in Figure 1.2. Layer 1 is the physical layer, which uses the channel


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GSM/GPRS Generality

structures discussed above over the air interface. Layer 2 is the data link layer. Across the Um interface, the data link layer is a modified version of the LAPD protocol used in ISDN, called LAPDm. Across the A interface, the Message Transfer Part layer 2 of Signalling System Number 7 is used. Layer 3 of the GSM signalling protocol is itself divided into 3 sublayers. •

Radio Resources Management: Controls the setup, maintenance, and termination of radio and fixed channels, including handovers,

•

Mobility Management: Manages the location updating and registration procedures, as well as security and authentication,

•

Connection Management: Handles general call control, and manages Supplementary Services and the Short Message Service.

1.2.5.1. Radio resources management The radio resources management (RR) layer oversees the establishment of a link, both radio and fixed, between the mobile station and the MSC. The main functional components involved are the mobile station, and the Base Station Subsystem, as well as the MSC. The RR layer is concerned with the management of an RR-session, which is the time that a mobile is in dedicated mode, as well as the configuration of radio channels including the allocation of dedicated channels. An RR-session is always initiated by a mobile station through the access procedure, either for an outgoing call, or in response to a paging message. The details of the access and paging procedures, such as when a dedicated channel is actually assigned to the mobile, and the paging sub-channel structure, are handled in the RR layer. In addition, it handles the management of radio features such as power control, discontinuous transmission and reception, and timing advance. In a cellular network, the radio and fixed links required are not permanently allocated for the duration of a call. Handover, or handoff as it is called in North America, is the switching of an on-going call to a different channel or cell. The execution and measurements required for handover form one of basic functions of the RR layer. There are four different types of handover in the GSM system, which involve transferring a call between: •

Channels (time slots) in the same cell : Intra-Cell Handover,

•

Cells (Base Transceiver Stations) under the control of the same Base Station Controller (BSC) : Intra-BSC Handover,


11

GSM/GPRS Generality •

Cells under the control of different BSCs, but belonging to the same Mobile services Switching Center (MSC) : Inter-BSC and Intra-MSC Handover,

•

Cells under the control of different MSCs : Inter-MSC Handover.

The first two types of handover, called internal handovers, involve only one Base Station Controller (BSC). To save signalling bandwidth, they are managed by the BSC without involving the Mobile services Switching Center (MSC), except to notify it at the completion of the handover. The last two types of handover, called external handovers, are handled by the MSCs involved. An important aspect of GSM is that the original MSC, the anchor MSC, remains responsible for most call-related functions, with the exception of subsequent interBSC handovers under the control of the new MSC, called the relay MSC. 1.2.5.2. Mobility management The Mobility Management layer (MM) is built on top of the RR layer, and handles the functions that arise from the mobility of the subscriber, as well as the authentication and security aspects. Location management is concerned with the procedures that enable the system to know the current location of a powered-on mobile station so that incoming call routing can be completed. 1.2.5.2.1. Location updating A powered-on mobile is informed of an incoming call by a paging message sent over the PAGCH channel of a cell. One extreme would be to page every cell in the network for each call, which is obviously a waste of radio bandwidth. The other extreme would be for the mobile to notify the system, via location updating messages, of its current location at the individual cell level. This would require paging messages to be sent to exactly one cell, but would be very wasteful due to the large number of location updating messages. A compromise solution used in GSM is to group cells into location areas. Updating messages are required when moving between location areas, and mobile stations are paged in the cells of their current location area. The location updating procedures, and subsequent call routing, use the MSC and two location registers: the Home Location Register (HLR) and the Visitor Location Register (VLR). When a mobile station is switched on in a new location area, or it moves to a new location area or different operator's PLMN, it must register with the network to indicate its current location. In the normal case, a location update message is sent to the new MSC/VLR, which records the location area information, and then sends the location information to the subscriber's HLR. The information sent to the HLR is normally the SS7 Fatma HAMDI, end studies project 2005/2006

12

GSM/GPRS Generality

address of the new VLR, although it may be a routing number. The reason a routing number is not normally assigned, even though it would reduce signalling, is that there is only a limited number of routing numbers available in the new MSC/VLR and they are allocated on demand for incoming calls. If the subscriber is entitled to service, the HLR sends a subset of the subscriber information, needed for call control, to the new MSC/VLR, and sends a message to the old MSC/VLR to cancel the old registration. For reliability reasons, GSM also has a periodic location updating procedure. If an HLR or MSC/VLR fails, to have each mobile register simultaneously to bring the database up to date would cause overloading. Therefore, the database is updated as location updating events occur. The enabling of periodic updating, and the time period between periodic updates, is controlled by the operator, and is a trade-off between signalling traffic and speed of recovery. If a mobile does not register after the updating time period, it is deregistered. A procedure related to location updating is the IMSI attach and detach. A detach lets the network know that the mobile station is unreachable, and avoids having to needlessly allocate channels and send paging messages. An attach is similar to a location update, and informs the system that the mobile is reachable again. The activation of IMSI attach/detach is up to the operator on an individual cell basis. 1.2.5.2.2. Authentication and security Since the radio medium can be accessed by anyone, authentication of users to prove that they are who they claim to be is a very important element of a mobile network. Authentication involves two functional entities, the SIM card in the mobile, and the Authentication Center (AuC). Each subscriber is given a secret key, one copy of which is stored in the SIM card and the other in the AuC. During authentication, the AuC generates a random number that it sends to the mobile. Both the mobile and the AuC then use the random number, in conjunction with the subscriber's secret key and a ciphering algorithm called A3, to generate a signed response (SRES) that is sent back to the AuC. If the number sent by the mobile is the same as the one calculated by the AuC, the subscriber is authenticated. The same initial random number and subscriber key are also used to compute the ciphering key using an algorithm called A8. This ciphering key, together with the TDMA frame number, use the A5 algorithm to create a 114 bit sequence that is XORed with the 114 bits of a burst (the two 57 bit blocks). Enciphering is an option for the fairly paranoid, since the signal is already coded, interleaved, and transmitted in a TDMA manner, thus providing protection from all but the most persistent and dedicated eavesdroppers. Another level of security is performed on the mobile equipment itself, as opposed to the mobile subscriber. As mentioned


13

GSM/GPRS Generality

earlier, each GSM terminal is identified by a unique International Mobile Equipment Identity (IMEI) number. A list of IMEIs in the network is stored in the Equipment Identity Register (EIR). 1.2.5.3. Communication management The Communication Management layer (CM) is responsible for Call Control (CC), supplementary service management, and short message service management. Unlike routing in the fixed network, where a terminal is semi-permanently wired to a central office, a GSM user can roam nationally and even internationally. The directory number dialled to reach a mobile subscriber is called the Mobile Subscriber ISDN (MSISDN), which is defined by the E.164 numbering plan. This number includes a country code and a National Destination Code which identifies the subscriber's operator. The first few digits of the remaining subscriber number may identify the subscriber's HLR within the home PLMN. An incoming mobile terminating call is directed to the Gateway MSC (GMSC) function. The GMSC is basically a switch which is able to interrogate the subscriber's HLR to obtain routing information, and thus contains a table linking MSISDNs to their corresponding HLR. A simplification is to have a GSMC handle one specific PLMN. It should be noted that the GMSC function is distinct from the MSC function, but is usually implemented in an MSC. The routing information that is returned to the GMSC is the Mobile Station Roaming Number (MSRN), which is also defined by the E.164 numbering plan. MSRNs are related to the geographical numbering plan, and not assigned to subscribers, nor are they visible to subscribers. The most general routing procedure begins with the GMSC querying the called subscriber's HLR for an MSRN. The HLR typically stores only the SS7 address of the subscriber's current VLR, and does not have the MSRN. The HLR must therefore query the subscriber's current VLR, which will temporarily allocate an MSRN from its pool for the call. This MSRN is returned to the HLR and back to the GMSC, which can then route the call to the new MSC. At the new MSC, the IMSI corresponding to the MSRN is looked up, and the mobile is paged in its current location area.

1.3. GSM limits GSM has provided for circuit-switched data services since 1992. However, the maximum data rate is limited to 14.4 kbit/s without timeslot bundling, and most operators support only 9.6 kbit/s. More important is that users must accept duration-based charging even for bursty traffic type services like surfing the Internet. These liabilities are another


14

GSM/GPRS Generality

important issue concerning error detection, failure statistics, and performance measurements is the dearth of long-term experience and an unfortunate lack of trust by many optimization engineers in the GPRS mobile stations that are available today.

1.4. An Overview of GPRS 1.4.1. GPRS Services GPRS Services are defined to fall in one of two categories: PTP (Point-To-Point) and PTM (Point-To-Multipoint) services. 1.4.1.1. PTP (Point-To-Point) Services GPRS will support applications based on IP. Applications based on the Connection Oriented Network Protocols are also defined to be supported. The X.25 protocol was initially mentioned but has been dropped in recent standard developments. 1.4.1.2. PTM (Point-To-Multipoint) Services The PTM services provide the subscribers with the capability to send data to multiple destinations within one single service request. With the exception of PTM-M (Point-ToMultipoint Multicast) services, groups must be defined and members are required to join an ongoing call to become participants. A PTM-G (Point-to-Multipoint Group) call is usually restricted to members located within a specific geographical area. An IP-M (IP-Multicast) call is on the other hand independent of the geographical area of the participants and can be internal to the network or distributed across the internet.

1.4.2. Basic overview of the GPRS Network

Figure 1. 3 : GPRS architecture.


15

GSM/GPRS Generality

1.4.2.1. The GPRS Base Station Subsystem GPRS has minor impact on the existing GSM BSS making it easy to reuse existing component and links without major modifications. This is possible because GPRS uses the same frequency bands and hopping techniques, the same TDMA frame structure, the same radio modulation and burst structure as GSM. A new functional component, called PCU, (Packet Control Unit) was added to the BSS in the GPRS standard to support the handling of data packets. The PCU is placed logically between the BSS and the GPRS NSS. 1.4.2.2. The GPRS Network Switching Subsystem The GPRS NSS can be viewed as an overlay network ensuring the link between mobile users and data networks. GPRS introduces a new functional element to the GSM infrastructure: GSN (GPRS Support Node) which can be either a SGSN (Serving- GSN) or a GGSN (Gateway-GSN). This addition is necessary for the GSM network in order to support packet data services. The network is generally divided into several service areas controlled by separate SGSNs. Only one SGSN serves a MS at a given time provided it is located in its service area. The SGSN is primarily responsible for keeping track of the MSs it serves, and for access control to data services. The GGSN on the other hand provides the interface to external PDNs (Packet Data Networks). The SGSN is connected to the BSS by Frame Relay and to possibly several GGSNs via a GPRS backbone network. The HLR database is updated to contain GPRS subscriber information. Adaptations to an existing MSC/VLR are not required but the GPRS standard suggests some enhancements to coordinate between the SGSN and the MSC/VLR if the optional interface between the two is to be supported [2].

1.4.3. GPRS Interfaces Different network components of the GPRS are connected together by well defined interfaces. Some new interfaces to GSM have been added in GPRS to support packet switched data mainly between GGSNs, SGSNs and other network components. The following interfaces have been defined [3]: •

Um interface between MS and BTS is very similar to GSM and defines the modulation type, error correction/detection technique, power control information etc,

•

A interface between n BTS and BSC defines the channel allocation, power measurement information etc,


16

GSM/GPRS Generality •

Gb interface connects BSCs to SGSN,

•

Gn interface is used between GSNs of same PLMN to exchange user profile when the user moves from one SGSN to another,

•

Gp interface is defined between two GSNs of different PLMN for exchanging the user profile and other signaling information between a SGSN and GGSN of another area,

•

Gf interface is used between SGSN and EIR to query the IMEI information if a MS tries to register with the network,

•

Gr interface between SGSN and HLR is used to get the user profile, the current SGSN address and the PDP address (es) for each user in PLMN,

•

Gc interface between GGSN and HLR is used by GGSN to query user's location and profile to update its location register,

•

Gi interface connects GGSN to external PDN (e.g. X.25 or IP),

•

Gs interface between SGSN and MSC/VLR is used to perform paging request of circuit switched GSM call for combined attachment procedure,

•

Gd interface between SMS-Gateway (SMS-GMSC) and SGSN is used to exchange short message service (SMS) messages.

1.4.4. GPRS radio link aspects 1.4.4.1 Logical channels A PTCH channel can transfer either subscribers data or control ones related to RLC level (acknowledgments). So, it can support: •

PDTCH, Packet Data Transfert Channel, which transmits subscribers data,

•

PACCH, Packet Associated Control Channel, which transmits acknowledgments, power control.

•

PTCCH, Packet Timing Control Channel, transmitting timing advance value for a group of mobiles.

System information and accessing to the network can use GSM logical channel: BCCH, PCH, RACH and AGCH. But, it is possible to use GPRS channels for these functions. There are: •

PBCCH, Packet Broadcast Control Channel, which broadcasts system information.

•

PPCH, Packet Paging Access Channel, to page MSs.

•

PRACH, Packet Random Access Channel, for the access.

•

PAGCH, Packet Random Grant Channel, which transmits allocation messages.


17

GSM/GPRS Generality

1.4.4.2 Radio link adaptation: Four coding schemes were defined for data transmission. If, we have less interference in the network, we can use coding scheme which offers important rates. But, if cells are interfered, we have to more protect data and as a consequence limiting rates [8]. The coding schemes CS1 and CS2 offer more protection. However, CS3 and CS4 involve important rates. In “Tunisie Télécom” network, only CS1 and CS2 are used. The following table summarizes coding parameters.

Coding Scheme Service data rate (kb/s) CS1

9.05

CS2

13.4

CS3

15.6

CS4

21.4

Table 1.2 : Coding schemes parameters.

1.4.5. Network aspects 1.4.5.1. Mobility Management in GPRS 1.4.5.1.1. Accessing the GPRS Network A MS can connect to the GPRS network by requesting a GPRS attach procedure. The outcome is the establishment of a logical link between the MS and a single SGSN and the creation of a mobility management context. The logical link is uniquely defined by the identifier TLLI (Temporary Logical Link Identifier) and is used subsequently in messages exchanged between the MS and SGSN. This identifier is changed when the MS is served by a new SGSN. 1.4.5.1.2. Mobility Management States The MS in GSM can be in one of two states: Idle or Dedicated. A channel allocation is held for the MS exclusively when it is in Dedicated mode due to the nature of circuit switched connections. When the connection is released, the MS returns to Idle mode. A GPRS MS on the other hand can share radio channels with other subscribers connected to the network. For this reason, The MS is defined to have three possible states: Idle, Ready, and Standby. •

Idle State:


18

GSM/GPRS Generality

A MS in the Idle state is not traceable and can only receive PTM-M transmissions such as general broadcast events destined to a specific geographical area. The MS needs to perform the attach procedure in order to connect to the GPRS network and become reachable. •

Ready State:

Data is sent or received in this state. The MS informs the SGSN when it changes cells. The MS may explicitly request (or can be forced by the network) to detach in which case it moves to Idle. A timer monitors the Ready state and upon its expiry, the MS is put on Standby. The timer insures that resources are not wasted by an inactive MS. •

Standby State:

A connected MS which is inactive is put in the Standby state. Moving back to Ready can be triggered by sending data or signalling information from the MS to the SGSN. Upon arrival of data destined to the MS, the SGSN pages the latter and a response to the page moves the MS back to the Ready state. The MS may wish (or can be forced by the network) to terminate the connection by requesting to detach in which case it returns to Idle. A timer is used by the SGSN to monitor the tracking of the MS, and when it expires, the MS is detached and is considered unreachable. GPRS Attach

Idle GPRS

Ready

Detach

PDU Transmission/Reception

GPRS Detach/ Timer Expiry

Standby

Timer Expiry Figure 1. 4 : GPRS State Model. 1.4.5.2. Transmission / Signalling Planes in GPRS A layered protocol structure is adopted for the transmission and signalling planes in GPRS (Figure 1.5). The SNDCP (Sub Network Dependent Convergence Protocol) serves as a mapping of the characteristics of the underlying network such as IP. Mobility management functionality is supported by the GMM (GPRS Mobility Management) and SM (Session Management) layers. The LLC (Logical Link Control) layer provides a logical link between the MS and the SGSN and manages reliable transmission while at the same time supporting


19

GSM/GPRS Generality

point-to-point and point-to-multipoint addressing. The RLC (Radio Link Control), MAC (Medium Access Control), and GSM RF (Radio Frequency) layers control the radio link, the allocation of physical channels and radio frequency. LLC PDUs (Packet Data Units) between the MS and the SGSN are relayed at the BSS. The BSSGP (Base Station System GPRS Protocol) layer handles routing and QoS between the BSS (Base Station System) and the SGSN. The GTP (GPRS Tunnelling Protocol) is the basis for tunnel signalling and user PDUS between the SGSN and GGSN.

Figure 1.5 : GPRS Transmission/Signalling Planes from MS to GGSN.

1.5 Conclusion In this chapter, I have presented an overview of GSM and GPRS and covered most of the key architectural and functional aspects. This overview is really necessary to understand the rest of this project. In reality, these specifications are not sufficient to judge a network performance. That is why, presenting criteria and indicators of quality of service are essential to evaluate the performance of a network. It is obvious that these networks are very complex to deploy but it rests another complexity which is how supervising and optimizing these networks using these criteria.


20

GSM/GPRS Network Supervision and Optimization

Chapter 2 : GSM/GPRS Network Supervision and Optimization

2.1 Introduction With the rapid growth of the GSM networks and the introduction of GPRS, high quality of service is a competitive advantage for a service provider. Quality of service can be characterized by such factors as contiguity of coverage, accessibility to the network, speech quality and number of dropped calls. Service providers must continually strive to improve their quality of service if they want to keep customers. This chapter provides an insight into network performance management and quality of service (QoS) of GSM/GPRS network. It identifies the components of QoS and the available mechanisms to analyze and evaluate them. This part also identifies important key performance indicators (KPIs) that need to be monitored and optimized and cites important data collected by protocols analyser in A interface.

2.2. QoS evaluation Criteria Quality of service is a measure of the reliability and usability of the telecommunications network. There are three quality aspects of a mobile network that are fundamental from the user’s point of view: •

Coverage: It indicates the geographic extent over which the network will reliably provide service. the strength of the signal is measured using test equipment and this can be used to estimate the size of the cell. Poor coverage can be caused by insufficient sites number, bad network configuration (antennas direction, antennas position…), installation and maintenance problems,

•

Accessibility: Consists in the ability of a mobile network to set up and hold calls from mobile-fixed networks and from mobile-mobile networks,

•

Audio Quality: It means the conversation perception for successfully setting up calls


21


over a predetermined period of time. There are many factors which can degrade the speech quality of the end-to-end connection: Propagation conditions, poor coverage, external interference, cochannel interference, terminals quality…

2.3 QoS Supervision Techniques 2.3.1. Drive Test Drive tests allow the mobile network to be tested through the use of a team of people who take the role of users and take the QoS measures to judge the QoS of the network. This test does not apply to the entire network, so it is always a statistical sample. 2.3.1.1 Measurement chain Drive test equipments are: •

MS: a mobile supporting GSM and GPRS equipped with special software. It is called trace mobile,

•

GPS (Global Positioning System): It is a US government satellite system that provides users with location of the measurement point,

•

PC: It is a computer equipped with interface carte RS 232 in order to make the link between the serial output of the MS and the serial port of the PC.

Figure 2. 1 : Drive test measurement chain. 2.3.1.2 Drive Test measurement Drive test tool offers the measurement of many indicators. We can cite: •

Longitude, Latitude (X, Y) of the measurement point,

•

RXLEV: RXLEV gets values between –110dBm and –48dBm. It characterizes the coverage of the network. RXLEV is a GSM unit and is defined as P (dBm) +110 where P


22


represents the received power. We distinguish RXLEVSUB where DTX is enable and RXLEVFULL where we DTX technique is not activated,

RXLEV

Evaluation

RXLEV ≥ -60dBm

Deep Indoor

-72dBm ≤ RXLEV<-60dBm

Indoor

-82dBm ≤ RXLEV<-72dBm

Incar

-94dBm ≤ RXLEV ≤ -82dBm

Outdoor

RXLEV<-89dBm

Poor coverage

Table 2. 1 RXLEV evaluation. •

RXQUAL: Quality of the received signal is a key parameter for evaluating network performance. It indicates the Bit Error Rate (BER). We distinguish RXQUALFULL where the received signal quality is averaged over a time interval containing both periods of voice information and periods of no transmission and RXQUALSUB where the received signal quality is averaged only over the periods when voice information is present,

RXQUAL BER 0

Less than 0.2%

1

From 0.2% to 0.4%

2

From 0.4% to 0.8%

3

From 0.8% to 1.6%

4

From 1.6% to 3.2%

5

From 3.2% to 6.4%

6

From 6.4% to 12.8%

7

Greater than 12.8%

•

Table 2.2 Correspondence between RXQUAL and BER T-ADV : Time Advance gets values between 0 and 63,

•

MS_TXPWR_MAX : MS maximum allowed transmission power,

•

LAC : Location Area Code,

•

Cell_Id : Cell identification number,


23


•

BSIC : It is used to distinguish between two BTSs using the same beacon channel. It is better to allow the same BSIC to two BTSs in the same cluster,

•

TIME : Measurement time,

•

MODE : idle or dedicated,

•

RXFREQ : It is the frequency number of the channel allowed to the MS in reception,

•

BCCHFREQ, BSIC, RXLEV of the six neighbouring cells,

•

BER: Bit Error Rate is the ratio of erroneously received bits to all received bits. It is important to notice that BER is evaluated before channel decoding, i.e. after equaliser. BER is used for defining the RXQUAL value according to Table 2.2,

•

FER: Frame Erasure Rate displays the percentage of lost or bad speech frames. This field is not displayed if the attached mobile does not support frame erasure measurements. The FER range is 0 to 100%,

•

RLC Throughput: GPRS RLC data rate,

•

LLC Throughput: GPRS LLC data rate.

We note that two different modes of measurement are available for this tool: - The "Connected" mode used when a communication has been established and TCH channel has been provided, - In "Idle" mode, when the mobile phone is not connected to the network. Only information from the BCCH channel such as received signal level and the normal procedures of the disconnected mode like cell re-selection are available and network events. 2.3.1.3 Advantages and drawbacks •

Drive Test strengths - Good source of RF data with detailed position information that can be used to identify and resolve radio problems [5], - Scanner data is already available for multiple vendors.

•

Drive Test weaknesses - Can be time-consuming and expensive to perform measurements. Many operator look to minimize drive testing for this reason, - Measurements do not accurately represent the experience of pedestrian and in building users,


24


- Measurements do not give performance information on all subscribers, only on an individual call. As a result, performance is only measured for the network components serving that specific call, - There are many vendors with many different formats, which are usually proprietary, - Large-scale production of handsets for testing may not happen until optimization is well underway.

2.3.2 RNO parameters 2.3.2.1 RNO definition RNO is a tool related to the OMC-R, it uses OMC-R counters to define Key Performance Indicators KPI. RNO enables engineers to quickly identify problem areas and take proactive steps to fix the problem before it impacts customers. Over the thousands of existing GSM and GPRS indicators, you can define your own QoS indicator alert thresholds, create your own QoS reports, and export reports to Microsoft Excel for easy publication and post-processing. 2.3.2.1 Key Performance Indicators examples Call drop rate: It is a measure of the calls dropped in the network. A dropped call can be defined as one that gets terminated on its own after being established. As this indicator gives a quick overview of network quality and revenues lost, this easily makes it one of the most important parameters in network optimization. It is measured against the SACCH frame. If the SACCH frame is not received, then it is considered to be dropped call [9]. TBF Drop rate: This dropping increases the number of TBF for GPRS session needs. So, it increases TBF set up delay. Call success rate: It indicates the ratio of calls that were completed after being generated. Call setup success rate: Rate of calls going until TCH successful assignment that is not interrupted by SDCCH DROP neither by Assignment failures. TBF success rate: Indicates the ratio of TBF that were completed after being generated. Radio call drop rate: Call drop due to Radio link failures (Radio link time-out or Lapdm timer expiry). Handover success rate: It indicates the quality of the mobility management in the radio network.


25


HO call drop rate: Call dropped during intracell, internal intercell and external HO execution. SDCCH assign cong fail rate: Rate of SDCCH unsuccessful seizures during radio link establishment procedure (congestion, radio access problem, BSS Problem). SDCCH drop rate: Rate of dropped SDCCH (SDCCH is established for any transaction call establishment, location updating procedure…). TCH/PDCH Blocking rate (%): TCH/PDCH blocking caused by cell congestion. SDCCH Blocking rate (%): SDCCH blocking caused by cell congestion. Downlink quality HO rate (%): Rate of HO that occurs because of quality degradation in down link. Uplink quality HO rate (%): Rate of HO that occurs because of quality degradation in up link.

2.3.2.2 Advantages and drawbacks •

KPI Strengths - Good source of low-resolution radio data for both uplink performance metrics, and downlink transmit parameters for a single call [5], - Cost to perform measurements is low and no additional hardware is necessary, - Testing can be performed for any handset or user equipment.

•

KPI Weaknesses - Often not available for initial infrastructure release of a new technology, - Specialized knowledge required to run the application and retrieve the results, however, the task itself is relatively straightforward, - No positioning information, however OMC data can be synchronized to drive test data with position information, - Limited capability to monitor multiple calls at the same time (due to system performance issues).

2.3.4 Analysis process After collecting information from the techniques described above, we have to combine the different indicators and analyse them in order to detect problems. After that, we have to do many actions to overcome these problems.


26


Drive test measurements

Key performance indicators

Problems detection and analysis

-

Parameters adjustment Antennas direction change On site intervention Maintenance actions…

Figure 2. 2 : analysis process.

2.4. GSM network Parameters 2.4.1. Parameters types •

Equipment related parameters: System parameters (activation of certain functionalities such as ciphering, power control …), product related parameters (software versions),

•

Engineering parameters: Can be modified by the operators at the OMCs : Numbering (BSC number...), network design (sites numbers …), optimization for system tuning (handover margins, access thresholds …), operation (barred cells…) [6].

2.4.2. Parameters examples Cell reselect offset : Used to favour a cell in the reselection process, RxLev access min : The minimum received signal level at a MS for access to a cell, Cell bar access : Cell Access Barred, Cell reselect hysteresis : Avoid the reselection of cells belonging to different localization zones and reduces the unsuccessful paging rate, Temporary offset: Avoid Ping-Pong cell reselection,


27


MS max power CCCH : The maximum allowed power of the mobile when transmitting on the RACH, RxLev min : The minimum Received signal level at a MS from an adjacent cell for handover into that cell to be permitted, HO margin (n) : Hysteresis allowing meeting a tradeoff between the ping-pong handover rate and the quality of service,

2.5. Detection problems steps 2.5.1 Interference The signal at the receiving antenna can be weak by virtue of interference from other signals. These signals may be from the same network or may be due to man-made objects. However, the major cause of interference in a cellular network is the radio resources in the network. Indeed, high capacity increases interference because of reuse of frequency technique. We distinguish three kinds of interference: •

Co-Channel interference: occurs when radio transmitters from two adjacent cells transmit on the same channel, in the same TDMA timeslot. When this happens, the signal is temporarily distorted. The result will be poor speech quality, drop-outs or even complete call losses in voice calls, GPRS data connections will slow down significantly. Cochannel interference cannot be avoided, since the same channels must be re-used in other cells not far apart. In GSM various techniques have been developed to reduce the problem, for example discontinuous transmission, frequency hopping, power control and adaptive multi-rate coding. However, the problem remains the ultimate limiting factor for network capacity,

•

Adjacent channel interference: This phenomenon is due to the use of two frequencies having adjacent bandwidth by sites not well separated,

•

Co-site interference: occurs when two neighbouring frequencies are used in the same site.

To avoid this problem, many actions can be taken: Operators should choose an adequate cluster for frequency reuse technique, change antenna direction and reduce the BTS transmission power. The diagram below summarizes interference analysis and optimization procedure :


28


Begin

Percentage of poor RXQUAL ≥ threshold

No

No interference problem

Yes Call drop

≥ threshold No No interference problem

Yes Better cell HO ≤ threshold

No No interference problem

Yes HO drop ≥ threshold

No No interference problem

Yes HO interference ≥ threshold

No Yes -

No interference problem

Frequency change Antennas actions Yes ……

End

Figure 2. 3 : Interference analysis and optimization

2.5.2 Congestion A network is congested when the available resources are not sufficient to satisfy the experienced workload traffic.


29


2.5.2.1 TCH/PDCH Congestion •

Causes:

TCH/PDCH availability, missing neighbours, missing assignments in neighbour list, traffic distribution. •

Action:

– Check TCH/PDCH availability. TCH/PDCH time slots may go into sleep mode. Real-time data can show if certain time slots are constantly idle. If this occurs over a long period of time and especially during the busy hour, a BTS restart and retest validation may be required, – Check for cell mean holding time (MHT) and compare it with that of the surrounding cells in the area. Greater MHT may be due to missing or incorrect neighbour cell definitions. Check the radio plan for missing neighbour cell assignments, – Use traffic management (load shedding) techniques that force traffic originating near the cell border to the surrounding cells. This can be achieved with optimum use of capacityefficient features such as directed retry, cell load-sharing (traffic reason handover or changing the handover hysteresis parameters), and handover offset between two neighbour cells, – In a hierarchical cell structure, distribute traffic to lower or higher cell levels as required, using layer threshold and layer threshold hysteresis, – Redistribute traffic among cells within the same layer, using early handover from a congested cell to another cell. This can be accomplished by adjusting handover hysteresis and handover offset [7]. 2.5.2.2 SDCCH Congestion •

Causes:

SDCCH availability, high number of location updates, high level of short message service (SMS) traffic, high number of call set-up bids, •

Action :

– Check historical statistics of SDCCH availability. In some systems, time slots may go into sleep mode. Historical data can show if certain time slots are constantly idle. If this occurs over a long period of time and especially during the busy hour (BH), a base transceiver station (BTS) restart and retest validation may be required, - Check for high number of location updates, call set-ups, and SMS traffic. Increasing the cell reselect hysteresis (CRH) will delay GPRS reselection. It might be wise to expand SDCCH resources, if possible. This can be done at the expense of one TCH, which can be converted to eight SDCCHs. It is advisable to aim for no SDCCH congestion at all times. Fatma HAMDI, end studies project 2005/2006

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Begin

Percentage of poor RXQUAL ≥ threshold

No

No congestion problem

Yes Congestion rate ≥ threshold

No No congestion problem

Yes -

Frequency change New sites ……

End

Figure 2. 4 : Congestion analysis and optimization.

2.5.3 Coverage problem The BTS coverage can stretch 30Km as maximum diameter depending on the traffic in urban or rural areas. Coverage problem appears if waves transmitted by the mobile can not reach the nearest BTS or if waves transmitted by the BTS are not received with sufficient power which can be detectable by the mobile. Sufficient coverage can be caused by antennas special disposition, we can consider the case of an obstruct between the antenna and the mobile (building, mountain…). To resolve this problem, we have to take many actins in concerned sites : •

Adding sites: This solution is preferred in the case of coverage absence or if waves transmitted by BTS antennas can not reach the nearest area with sufficient power,

•

Antennes change :

- Tilting, - Change antennas orientation, - Configuration change: This action aims to rise many antenna parameters such as transmission power (BS_TXPWR_MAX). The diagram below summarizes coverage analysis and optimization procedure :


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Begin

Percentage of poor RXLEV ≥ threshold

No

No coverage problem

Yes Call drop

≥ threshold No No coverage problem

Yes HO RXLEV ≤ threshold

No Yes

-

No coverage problem

Frequency change Antennas actions ……

End

Figure 2. 5 : Coverage analysis and optimization.

2.5.4 Call/session drop problem This problem is caused by: i) BSS Drop: It is due to an interior materiel problem in the BTS. To avoid this kind of problem, we have to check the BTS software or do drive test in the concerned site. ii) Radio drop: The call or session is dropped because of a problem in the radio interface. To identify the exact cause of dropping, we have to check HO causes.

If the cause is quality, dropping is caused by congestion,

If the cause is interference, the cell can be interfered. Thus, we must compare the frequency of the concerned cell to those of its neighbours. Else, we can « tilt » neighbouring sites antennas,

If the cause is RXLEV degradation, dropping is due to cell poor coverage or to a problem in materiel cell components.


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iii) Session drop because of a transmission problem : A transmission problem means that there is a problem in interfaces or transit delay overtaking.

In

this

case,

changing

parameters

is

crucial

(WI_PR,

T_NETWORK_RESPONCE_TIME…).The diagram below summarizes call/sessions drop analysis and optimization procedure : Begin

Call drop

≥ threshold No No call drop problem

Yes BSS drop ≥ threshold

Yes Call drop is due to an interior problem in the BTS

No radio drop

≥ threshold Yes

Call drop is due a radio interface problem

HO Quality ≥ threshold

Yes

The cause is interference

No HO interference ≥ threshold Yes

Yes The cause is interference

No HO RXLEV ≥ threshold

Yes No

The cause is poor coverage

End

Figure 2. 6 : Call drop problem analysis and optimization.


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2.5.5 Call/Session set up failure problem The first thing to check is the coverage. If we have in the concerned site good coverage, we have to see if channels are congested. First, we have to check circuit channels and then packet channels. Session set up failure may also be caused by Gb interface congestion or BSS problem. The diagram below shows how using RXLEV and KPI indicators to see if this problem is due to coverage : Begin

Percentage of poor RXLEV ≥ threshold

No

No congestion problem

Yes Call set up ≥ threshold

No No congestion problem

Yes -

Antennas actions New sites ……

End

Figure 2.7 : Call set up failure problem analysis and optimization.

2.6 Conclusion In this chapter, we have presented the principal indicators of quality of service as well as the various parameters which allow the management of this quality. Then, we have enumerated the various problems which can be encountered in such a network and proposed possible actions which can be carried out to avoid these problems. Our post processing tool will be presented in the following chapter.


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Implementation and validation of the tool

Chapter 3 : Implementation and validation of the tool

3.1 Introduction In this chapter, we will develop a tool that aims to evaluate the QoS in a GSM/GPRS network, detect some problems and eventually propose recommendations for each problem. We deal with a brief presentation of programming languages and after that a description of our tool. Eventually, we will do a case study based on field trial measurements.

3.2. Objectives This tool has to provide a solution for improving GSM/GPRS performance in terms of Quality of service. First, it gives a detailed evaluation of the network and after that it detects problems, locates them and eventually offers some recommendations. The evaluation is obtained through visual analysis: •

Via map: This tool is able to transform drive test files into maps for visual analysis of many QoS indicators such as RXLEV, RXQUAL, Call drop,…

•

Via graph: This tool is able to make histograms that contain statistical information summarizing the data in drive test files. They will be used with RNO parameters for post processing.

3.3. Programming languages To develop our tool, we have used two programming languages Visual Basic and Map Basic. Visual Basic is a highly popular language in the commercial world because it allows for the rapid development of Windows based programs. VB is particularly strong at creating front ends for databases. In Visual Basic, a project is built on a form using controls (also called objects). By interacting with the controls using events, we get the computer to do assigned tasks via instructions we provide in order to get a suitable graphic interface. To get information from the files available, we have used Access where we have created our data base. This database contained RNO indicators and physical and logical parameters. Map Basic is the MapInfo programming language, providing application designers with control


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over many aspects of MapInfo functions from file manipulation to geographic analysis and interface design. Through Map Basic, it is possible to simplify complex or often repeated tasks, thus reducing the training level required for every-day operators. We note also that there are deep links between MapInfo and Map Basic versions.

3.4. Post processing tool structure 3.4.1 Post processing tool architecture This tool is an executable program (‘.exe’), written in Visual Basic and Map Basic. It uses two external applications: •

Microsoft Access database, which stocks data coming from RNO (Radio Network Optimization) center and containing Key Performance Indicators (KPI). It includes also physical and logical parameters for “Tunisie Télécom” network.

•

Geographical software processing tool, MapInfo, for map making display.

The following figure describes the architecture of the tool. On the left are the types of imported files and their origin; on the right are the internal links of the software with different applications. Data from drive tests are converted into excel files. RNO measurement is stored in an Access database. The post-processing tool uses Visual Basic for making statistics and analysing different data and Map BASIC scripts to manage MapInfo software.

Figure 3. 1: Post processing tool architecture.


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3.4.2 Selected data Selected records are those which give the best characterisation of the air interface, and which allow us to judge the QoS. 3.4.2.1 Drive test The first thing to do is to convert drive test files which are not directly accessible into excel files. Therefore, we have used Agilent (E6474A). Agilent is an advanced software solution that enables fast, accurate visualization and analysis of information collected by drive test equipment enabling users to enhance the network performance and improve their operational efficiency. In this project, we have used Agilent only to transform drive test files. The Radio measurements that we have used during our tool: • Total power received by the mobile RXLEVSUB: We have chosen this parameter because “Tunisie Télécom” uses DTX technique, • Radio received quality RXQUALSUB, • BCCH of the serving cell, • Longitude and latitude of each measurement point, • Cell identity, • Uplink and Downlink coding Scheme. The series of measurements GSM were carried out in an urban area. We have used a measurement chain equipped with two terminals GSM used respectively for measurement in dedicated mode (measurements done in TCH channel to estimate the quality of communication) and in idle mode (measurements taken in BCCH channel). 3.4.2.2 RNO data In fact, data collected from the handset often only informs users of the symptoms of the performance, therefore in order to determine the cause of the symptoms, such as identifying the interfering pilots, it is necessary to monitor measurements collected from the RNO. We have selected cell identity, call drop rate, radio drop rate, BSS drop rate, congestion rate, call success rate, call set up rate, HO better cell, HO Quality, HO interference, HO RXLEV.

3.5. Radio optimization tool description 3.5.1 Authentication Fatma HAMDI, end studies project 2005/2006

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Only users who have the suitable password can accede to our tool. If user enterS wrong password, an error message is posted and thus user can try again.

Figure 3. 2 : Authentication interface.

3.5.2 Main menu

Figure 3. 3 : Main menu interface. The main menu contains five fields. The first one is drive test analysis. Here, user can choose the drive test file that he wants to analyse. He can do many statistics. For instance, user can


38


make histograms that show the percentage of good, fair and bad indicators such as RXLEV, RXQUAL, T_ADV. Also, user can represent the recorded information through a map. An example of a map could be the display of the level for the received signal in GSM and GPRS, a second could give the level of Quality. The second field is KPI analysis. Here, user can see the value of many indicators for each cell, in addition, users can focus on areas of data that are of interest, such as high dropped call rates or setup failure rates, by using the key performance indicators obtained by RNO. Another field is optimization which shows recommendations that we have to do if there are problems. This tool gives also a daily list of physical parameters for each cell, for instance tilt, azimuth, LAC, BSC, MSC, BCCH, geographic position… This list can be daily updated.

Figure 3. 4 : Physical parameters interface. Also, users are able to view logical parameters and their values for each cell, for example GSM_Rxlev_access_min, GSM_penalty_time… These parameters can also be daily updated.


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Figure 3. 5 : Logical parameters interface. And finally the field “about” which gives general information about our tool:

Figure 3. 6 : “About” interface.


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3.5.3 Statistics The post-processing tool has a graphic module, which allows us to visualise measurements taken in a graphical way. Various attributes of the drive test files can be counted or combined to give general indications, such as percentage of good RXLEV, fair RXLEV, poor RXLEV… For statistics, we have divided the total course into small courses with low length. The analysis will be made on each one of these small courses and will thus allow a better localization of the problems in space. 3.5.3.1 Coverage statistics We can analyse the coverage in a selected area by using the RXLEV indicators. First, we have to fix the thresholds for deep indoor, indoor, incar and outdoor coverage. “Tunisie Télécom” fixes thresholds as shown in the table below: Min

Max

Deep indoor

-60

Indoor

-72

-60

Incar

-82

-72

Outdoor

-94

-82

Poor coverage

-94

Table 3.1 : coverage thresholds. The window below allows users to make statistical information summarizing the data in drive test files. They can choose any drive test files, fix the suitable thresholds and view histograms summarizing the repartition of RXLEV.

Figure 3.7 : RXLEV statistics interface.


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First, user has to select the drive test file he wants to analyse:

Figure 3. 8 : Open file interface. The following graph shows the percentage of each coverage kind.

Figure 3. 9 : Coverage histogram interface.


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3.5.3.2 RXQUAL statistics In addition to the representation of RXLEV, users are able to view RXQUAL in a graphical way. So, they will have a general idea about the Quality of Service that offers their network. We have used the following thresholds:

Min

Max

Good

0

3

Fair

4

4

Poor

5

7

Table 3.2 : Quality thresholds.

Figure 3. 10 : RXQUAL statistics interface. The following window provides a complete overview of the content of the drive test in terms of RXQUAL indicator. In fact, the statistical distribution of this element allows engineers to assess the overall quality of the radio conditions on the test route.


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Figure 3. 11 : Quality histogram interface. 3.5.3.3 T_Adv statistics Various users of a cellular system are at variable distances from their basic station and endure variable propagation delay τp . In context TDMA, it should be taken care that two mobiles which use two consecutive slots do not send bursts which overlap at the receiver of the BTS. To avoid such a problem, we can manage parameter T_Adv corresponding to time “go and return” of the signal. The distant mobile must advance its emission of each one of its slots of one τp. Also, T_ADV gives idea about extended cell. So, if the percentage of mobiles far from the BTS is bigger than a threshold fixed by each operator, engineers can deploy an extended cell in the concerned area. Therefore, it is interesting to make general statistics summarizing the repartition of T_Adv.


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Figure 3. 12 : T_ADV statistics interface.

Figure 3. 13 : T_ADV histogram.


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3.5.3.4 Coding schemes statistics It is interesting for operators to know the percentage of any coding scheme used in the network. For instance, if the use of CS1 is very important, we can conclude that there is an interference problem and this is assured by checking other parameters such as RXQUAL. The following graph shows a histogram of coding scheme. We note that, the percentage of CS2 (5.4%) is acceptable, so our GPRS area is not interfered.

Figure 3. 14 : Coding schemes statistics. And when checking RXQUAL histogram, we notice that the percentage of poor quality is 0 % and this illustrates coding schemes statistics.


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Figure 3. 15 : GPRS RXQUAL statistics.

3.5.4 Map view The second step in our tool is the creation of maps. Maps are created through MapInfo. This software uses a script language, MapBasic. The post-processing tool generates various recoverable data files under MapInfo, and a MapBasic script file. Once the script is launched under MapInfo, using the different file generated, the requested map, with the appropriate sets of themes for the legend, is created. The Post Processing tool allows the user to choose a raster image (geographical map), which is used in the background. We have chosen Tunisia map for our representation. When user chooses the field called “map”, the window below is posted.


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Figure 3. 16 : Starting MapInfo interface. When user clicks on the first button, a MapInfo application is open and user can run map Basic application by clicking on the second button. To analyse an area, user has to import its drive test files and then save them as MapInfo tables.

Figure 3. 17 : MapInfo analysis interface.


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Figure 3. 18 : Choosing file interface.

Figure 3. 19 : Saving MapInfo tables interface. We note that user can add as many files as he wants to get stronger analysis. For instance if we have problems in the drive test file, we can replace it by other files.


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Figure 3. 20 : Importing other files interface. The Tool proposed a variety of maps to analyse signal coverage, Quality, Better cell analysis in GSM/GPRS systems. A specific setting (Colours, Thresholds) is allocated for each map.

Figure 3.21 : Analysis kind interface. 3.5.4.1 Coverage map The first map shows the GSM received level power signal (RXLEV). For this analysis, we have chosen the same thresholds as indicated in statistics part. For this map, Blue colour is allocated for points where we have deep indoor coverage, cyan colour for indoor coverage, green colour for incar coverage, yellow for outdoor coverage and red colour for poor coverage. We have used the following thresholds


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Figure 3. 22 : RXLEV thresholds. This map helps us to visualise the concerned area. From this map, we can extract the following results: Coverage

Percentage (%)

Deep indoor

32.28

Indoor

27.03

Incar

27.99

Outdoor

11.02

Poor coverage

1.65

Table 3.3 : coverage results. According to these results, we notice that there are areas where we have problem in the coverage. This visualisation is not sufficient to locate problems. So, the other parts in our tool will show how can us do this task.

Figure 3. 23 : Level of received power in GMS/GPRS network.


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3.5.4.2 Quality of communication map

Figure 3. 24 : RXQUAL thresholds. For this map, we have used RXQUALSUB indicator. Three Levels of quality are represented on the map Blue colour corresponds to good quality, cyan colour for fair quality and red colour for poor quality. We have imported many files corresponding to Tunis area.

Figure 3.25 : Quality communication map. From this map, we can get the number of samples for each kind of quality. Coverage

Percentage (%)

Good

88.65

Fair

0.54

Bad

10.8

Table 3.4 : Quality results. According to this display, we remark that bad quality percentage is high. So, an optimization process is needed in this area.


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3.5.4.3 Better cell map

Figure 3. 26 : Better cell map. A colour is allocated for each cell. User will be able to know where the mobile changes cell and this allow him to have an idea about better cell handover in the network. Also, better cell map is a way that shows the partition of cells in this area.

3.6. Real case study In this study, we have worked with drive test files done for GSM, but this work can be applied for GPRS drive test files.

3.6.2 Analysis and optimization area: We have divided the total course into small courses with low length. The analysis will be made on each one of these small courses and will thus allow a better localization of the problems in space. So each drive test file will be analysed closely. In this project, we have focused on a part from “bon lieu sud” area. This area contains 67 cells and its course respects the criteria of an optimization course which are : - Without change envisaged during the day of test, - With high traffic, Fatma HAMDI, end studies project 2005/2006

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- TCH congestion is almost null in the majority of cells, - Interference handover exceeds the threshold, The following map shows this area :

Figure 3. 27 : Analysis and optimization area. According to Figure 3.26, the percentage of Deep Indoor coverage, Indoor coverage, incar coverage, Out Door coverage and poor coverage are respectively 27.74%, 14%, 29.8% and 6.68%. These results incite us to say that this area has coverage problem. According to Figure 3.27, the percentage of good, fair and poor quality are respectively 75%, 1.09%, 23.9%.In fact, these maps are not sufficient to judge the QoS in this area. That is why, we have made statistics and analyse KPI and this helps us to locate exactly problems.


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Figure 3. 28 : Optimization area coverage map.

Figure 3. 29 : Optimization area Quality map.


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3.6.3 Optimization process 3.6.3.1 Coverage problem The optimization process begins from statistics. If the percentage of poor coverage is bigger than a definite value, a message box is appeared and indicated that it could be a coverage problem in the network.

Figure 3. 30 : RXLEV problem interface. In order to troubleshoot the network and to be assured if there is a coverage problem, it is crucial for users to monitor some key Performance Indicator closely. First of all, user has to precise which problem he wants to optimize. Thus, the following window is appeared.


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Figure 3. 31 : Coverage and call set up failure problem optimization interface. By clicking on the first button, we deal coverage optimization. To analyse coverage problem, we have focused on two KPI: Call drop rate and HO RXLEV rate. If call drop rate and HO RXLEV rate are respectively bigger than 0.02 and 0.2, the concerned cell has coverage problem and some recommendations to overcome this problem are proposed. Else, the concerned cell has not coverage problem.

Figure 3. 32 : Coverage optimization interface. Fatma HAMDI, end studies project 2005/2006

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3.6.3.2 Call set up failure problem If RXLEV statistics reveal that it could be a call set up failure problem in the network, we move to KPI analysis. For this problem, we have focused on call set up rate in order to be assured if there is a call set up failure problem. For cells which have this problem, we have proposed some solutions.

Figure 3. 33 : Call set up failure problem optimization interface. 3.6.3.3 Interference problem If there is a quality problem, we have to know its cause: Interference or congestion. For this reason; it is interesting to move to KPI analysis.


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Figure 3. 34 : Quality problem interface. User can choose the problem he wants to analyse by clicking on one of the following button:

Figure 3. 35 : Interference and congestion optimization interface. Fatma HAMDI, end studies project 2005/2006

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For interference, we have focused on four KPI: Call drop rate, better cell HO rate, interference HO, HO drop. If these call drop rate, interference HO rate and HO drop rate are respectively bigger than 0.02, 0.1 and 0.02 and if better cell HO is smaller than 0.2, there is an interference problem. Some solutions on the concerned cell are available in our tool.

Figure 3. 36 : Interference optimization interface. 3.6.3.4 Congestion problem If quality of communication is not good, the network may have a congestion problem. To clarify the cause of quality degradation, user can be interested on congestion by checking congestion rate indicator. If this KPI is bigger than 0.2 than the cell is congested. Our tool checks all the cells in the network and gives recommendations for congested cells.


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Figure 3. 37 : Congestion optimization interface.

3.6.3.5 Call drop problem In order to analyse this problem, we have checked six indicators: Call drop rate, BSS drop, radio drop, HO Quality, HO interference, HO RXLEV. After seeing if the cell has this problem, the tool gives the cause of this problem: Poor coverage or interference.

Figure 3. 38 : Call drop problem optimization interface. Fatma HAMDI, end studies project 2005/2006

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3.7 Conclusion In this chapter, we have made a study, an implementation, a validation of the tool and a real case study of analysis and optimization. In this tool, we have used drive test informations and key performance indicators for an automatic analysis and an easy optimization of a real area. We notice that this application remains open to the improvement because the world of telecommunications and especially the networks mobile are very evolutionary.


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Conclusion

Conclusion

The Telecommunications field are very revolutionary. The operators and the suppliers of service propose not only voice services, but also other services such as WAP, ftp, WEB which exploit the resources of the mobile networks. So, operators and suppliers have to face the degradation of quality of services by optimization. For instance, they have to overcome many problems that come from the introduction of these services ; we can cite coverage, interference, congestion….In this context, we have worked in this project. We have first of all made an overview about architecture and functionalities of the GSM/GPRS network. Second, we have analysed many indicators that are related to the quality of service and proposed some algorithms that help the optimization process. Eventually, we have developed our tool. The stake of this tool was to be able to process by easy way using data obtained on the radio interface during field trial and data extracted from RNO. It allows the visualisation of traces from measurement tools and RNO indicators by the means of maps or graphs. Then, and using these graphs and RNO indicators, this tool analyses an area chosen by the user, detect problems and propose some recommendations From a personal point of view, this project was for me an excellent occasion to supplement my education by an experience in the world of the company. The study was carried out in real areas. The team in this direction works with experienced individuals and the responsibility related to the success conclusion for a complete project enabled me to discover the multiple issues of engineer work.


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Bibliography

[1]: www.gsmworld.com, [2] M. Mouly and M. B. Pautet, the GSM System for Mobile Communications, Palaiseau, France, 1992, [3] www.cs.tu-berlin.de/~jutta/gsm/js-intro.html, [4] www.it.iitb.ac.in/~satwajit/seminar/node13.html, [5] www.actix.com, [6] Cellular networks optimization, TABBANE Sami 2002, [7] http://www.agilent.com/find/wireless, [8] Xavier Lagrange, Philippe Godlewski, Sami Tabbane, « Réseaux GSM des principes à la norme », Hermes edition, Paris, 1995, 1996, 1996, 1999, 2000, [9] ALCATEL Tunisie, "Séminaire de formation Introduction to Radio Fine Tuning B7", 2002. [10] Sami Tabbane, Lagrange Xavier, Godlewski Philippe, "Réseaux GSMDCS", 4éme édition, HERMES Science Publication ,Paris,1999.


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GSM_GPRS_Evaluatioin and optimization

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