Today's phone could be described as a charismatic tool that has the ability to keep human beings captivated for a considerable amount of their precious time. Users remain in the illusory wonderland with free services, while their data becomes the subject to monetizing by a genie called big data. In other words, users pay with their personal data but the price is in a way invisible. Poor means to observe and to assess the consequences of data disclosure causes hindrance for the user to be aware of and to take preventive measures. Mobile operating systems use permission-based access control mechanism to guard system resources and sensors. Depending on the type, apps require explicit consent from the user in order to avail access to those permissions. Nonetheless, it does not put any constraint on access frequency. Granted privileges allow apps to access to users' personal information for indefinite period of time until being revoked explicitly. Available control tools lack monitoring facility which undermines the performance of access control model. It has the ability to create privacy risks and nontransparent handling of personal information for the data subject. This thesis argues that app behavior analysis yields information which has the potential to increase transparency, to enhance privacy protection, to raise awareness regarding consequences of data disclosure, and to assist the user in informed decision making while selecting apps or services. It introduces models and methods, and demonstrates the risks with experiment results. It also takes the risks into account and makes an effort to determine apps' privacy-friendliness based on empirical data from app-behavior analysis. / Today's phone could be described as a charismatic tool that has the ability to keep human beings captivated for a considerable amount of their precious time. Users remain in the illusory wonderland with free services, while their data becomes the subject to monetizing by a genie called big data. In other words, users pay with their personal data but the price is in a way invisible. They face hindrance to be aware of and to take preventive measures because of poor means to observe and to assess consequences of data disclosure. Available control tools lack monitoring properties that do not allow the user to comprehend the magnitude of personal data access. Such circumstances can create privacy risks, erode intervenability of access control mechanism and lead to opaque handling of personal information for the data subject. This thesis argues that app behavior analysis yields information which has the potential to increase transparency, to enhance privacy protection, to raise awareness regarding consequences of data disclosure, and to assist the user in informed decision making while selecting apps or services. It introduces models and methods, and demonstrates the data disclosure risks with experimental results. It also takes the risks into account and makes an effort to determine apps' privacy-friendliness based on empirical data from app-behavior analysis.

This thesis aims to present solutions to improve the performance of SQLite library on mobile systems. In particular, two approaches are presented to add lightweight locking mechanisms to the SQLite library and improve concurrency of the SQLite library on Android operating system. The impact on performance is discussed after each section. Many applications on the Android operating system rely on the SQLite library to store ordered data. However, due to heavy synchronization primitives used by the library, it becomes a performance bottleneck for applications which push large amount of data into the database. Related work in this area also points to SQLite database as one of the factors for limiting performance. With increasing network speeds, the storage system can become a performance bottleneck, as applications download larger amounts of data. The following work in this thesis addresses these issues by providing approaches to increase concurrency and add light-weight locking mechanisms. The factors determining the performance of Application Programming Interfaces provided by SQLite are first gathered from IO traces of common database operations. By analyzing these traces, opportunities for improvements are noticed. An alternative locking mechanism is added to the database file using byte-range locks for fine-grained locking. Its impact on performance is measured using SQLite benchmarks as well as real applications. A multi-threaded benchmark is designed to measure the performance of fine grained locking in multi-threaded applications using common database operations. Recent versions of SQLite use write ahead logs for journaling. We see that writes to this sequential log can occur concurrently, especially in flash drives. By adding a sequencing mechanism for the write ahead log, the writes can proceed simultaneously. The performance of this method is also analyzed using the synthetic benchmarks and multi-threaded benchmarks. By using these mechanisms, the library is observed to gain significant performance for concurrent writes.

Η εισαγωγή των νέων Τεχνολογιών της Πληροφορίας και των Επικοινωνιών (ΤΠΕ) στην εκπαίδευση αποτελεί πλέον γεγονός παγκοσμίως συμβάλλοντας ουσιαστικά στη βελτίωση των διαδικασιών διδασκαλίας , μάθησης και απόκτησης γνώσεων αλλά και δημιουργώντας περιβάλλοντα για την ανάπτυξη ιδιαίτερων δεξιοτήτων . Καθοριστικό ρόλο στην πραγματικότητα αυτή έχει διαδραματίσει η ραγδαία ανάπτυξη του διαδικτύου και οι επιμέρους εφαρμογές του . Τα τελευταία χρόνια η πρόσβαση στον παγκόσμιο ιστό δεν περιορίζεται στους επιτραπέζιους υπολογιστές αλλά πλέον περιλαμβάνει τα κινητά τηλέφωνα , τα PDAs και γενικότερα κάθε είδους κινητή συσκευή . Μάλιστα στις αναπτυσσόμενες χώρες ο αριθμός των χρηστών που πλοηγούνται στον παγκόσμιο ιστό είναι μεγαλύτερος από τον αντίστοιχο αυτών που πλοηγούνται μέσω επιτραπέζιων υπολογιστών . Τα παραπάνω γεγονότα έχουν ως φυσικό επακόλουθο την δημιουργία και συνεχή ανάπτυξη ενός νέου είδους εκπαίδευσης : την κινητή ηλεκτρονική μάθηση (mobile e-learning) , η οποία βασίζεται στην ιδέα της μάθησης μέσω κινητών συσκευών οπουδήποτε οποιαδήποτε στιγμή θα χαρακτηρίσει τις νέες στρατηγικές, τις πρακτικές , τα εργαλεία , τις εφαρμογές , και τους πόρους για να εκπληρώσει την υπόσχεση της πανταχού παρούσας, προσωπικής, και συνδεδεμένης εκμάθησης, Στόχος της διπλωματικής εργασίας είναι να μελετήσει και να συγκρίνει τις διαθέσιμες εφαρμογές , εργαλεία , Συστήματα Διαχείρισης Μάθησης (ΣΔΜ) & Λειτουργικά Συστήματα κινητών συσκευών ώστε να αξιολογήσει τις εκπαιδευτικές δυνατότητες τους και να καθορίσει αν μπορούν να συμβάλλουν στην παροχή αποτελεσματικής κινητής ηλεκτρονικής μάθησης στους χρήστες τους . / The integration of the Information and Communication Technologies (ICT) in the field of education is nowadays a fact worldwide contributing in improving the teaching methods , learning and knowledge gaining as well as creating environments which facilitate the development of special skills . The vast development of the Web and its applications have played a key role establishing the current reality. During the last years the access to the Web , not only from desktop PCs but from mobile devices too , such as mobile phones and PDAs , is a fact. Furthermore , in developing countries the number of users that browse the Web through mobile devices is larger than the number of users that browse the web from desktop PCs . As a natural consequence of the above facts a new kind learning has emerged and constantly been developed ever since : Mobile e-learning , based on the idea of learning with mobile devices anywhere at any time , which will feature new strategies , practices , tools, applications and resources to fulfill the promise of ubiquitous , personal and connected learning . The aim of this thesis is to study and compare the available applications, tools, Learning Management Systems and mobile Operating Systems in order to evaluate their educational potentials and to determine whether they are capable of contributing to providing effective mobile e-learning to their users .

The subject of this thesis is to compare the most widely used mobile operating systems from the perspective of the development, distribution and monetization of applications developed for these systems. Compared systems are Android, iOS, Symbian OS, Windows Phone and BlackBerry OS. The thesis defines a total of 16 criteria against which systems are compared. These criteria are further divided into three categories, namely SDK features, functionality and distribution and monetization. All criteria have assigned weights which are determined by the paired comparison method using the Fuller's triangle, which is part of the Annexes of this work. Practical part of this thesis is focused on determining the values of defined criteria. Each criterion is evaluated separately, including an analysis of results. The overall evaluation is performed in a separate part of the thesis and is composed by categories evaluation and overall evaluation.