Πλοήγηση ανά Συγγραφέα "Rekanos, Ioannis T."
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Τεκμήριο Change Point Detection in Time Series Using Higher-Order Statistics: A Heuristic Approach(2013) Hilas, Constantinos S.; Rekanos, Ioannis T.; Mastorocostas, Paris Ast.Changes in the level of a time series are usually attributed to an intervention that affects its temporal evolution. The resulting time series are referred to as interrupted time series and may be used to identify the events that caused the intervention and to quantify their impact. In the present paper, a heuristic method for level change detection in time series is presented. The method uses higher-order statistics, namely, the skewness and the kurtosis, and can identify both the existence of a change in the level of the time series and the time instance when it has happened. The technique is straightforwardly applicable to the detection of outliers in time series and promises to have several applications. The method is tested with both simulated and real-world data and is compared to other popular change detection techniques.Τεκμήριο Clustering of telecommunications user profiles for fraud detection and security enhancement in large corporate networks: a case study(2015-07-01) Hilas, Constantinos S.; Mastorocostas, Paris A.; Rekanos, Ioannis T.A user’s transactions with modern networks and services produce a vast amount of user related data. The byproduct of every phone call a person makes or every web page one visits is translated into a log record with usage data. By studying these log records, the user’s behavior is revealed and one may come up with clues about user preferences, identify security issues, or discover fraudulent use of the network or the service one provides. Thus, the modeling of network users’ behavior may serve as an invaluable tool for the IT manager. In this paper, many of these issues are discussed and emphasis is given on the construction of appropriate user behavior representation in telecommunications. As an example, the application of two clustering techniques is presented, with the task to identify appropriate user behavior representations (profiles) inside a large organization’s telecommunications network, in order to spot fraudulent usage. Through this study a researcher and/or the organization’s network manager may gain more insight into the problems of user profiling and fraud detection.Τεκμήριο Design of 3-pole PCS-type monoblock filter using an equivalent circuit approach(2006-10-02) Tsitsos, Stelios; Gibson, Andrew A. P.; Davis, Lionel E.; Rekanos, Ioannis T.Computer-aided synthesis techniques of ceramic monoblock filters are presented in this work. These techniques make use of an accurate equivalent circuit as the basic design and tuning tool. This circuit is extracted from first electromagnetic principles and is subsequently modified and optimised in order to meet the desired filter specifications. From the change in the equivalent circuit parameter values, useful information can be derived for the necessary structural changes. This, combined with a structural optimiser, provides a very useful approach for the design of monoblock filters.Τεκμήριο A genetic programming approach to telecommunications fraud detection and classification(2014-03) Hilas, Constantinos S.; Kazarlis, Spyridon A.; Rekanos, Ioannis T.; Mastorocostas, Paris A.Telecommunications fraud has drawn the researchers’ attention due to the huge economic burden on companies and to the interesting aspect of users’ behavior modeling. In the present paper, an application of genetic programming to fraud detection is presented. Genetic programming is used for case classification in order to distinguish between normal and fraudulent activities in a telecommunications network. Implications to appropriate user behavior modeling are, also, discussed. Real world cases of defrauded user accounts are modeled by means of selected usage features and comparisons with other approaches are made.