Papers by Constantinos Hilas
Expert Systems With Applications, Nov 1, 2009
Telecommunications fraud not only burdens telecom provider's accountings but burdens individual u... more Telecommunications fraud not only burdens telecom provider's accountings but burdens individual users as well. The latter are particularly affected in the case of superimposed fraud where the fraudster uses a legitimate user's account in parallel with the user. These cases are usually identified after user complaints for excess billing. However, inside the network of a large firm or organization, superimposed fraud may go undetected for some time. The present paper deals with the detection of fraudulent telecom activity inside large organizations' premises. Focus is given on superimposed fraud detection. The problem is attacked via the construction of an expert system which incorporates both the network administrator's expert knowledge and knowledge derived from the application of data mining techniques on real world data.
This paper proposes the application of Reverse Direction (RD) protocol to enhance performance in ... more This paper proposes the application of Reverse Direction (RD) protocol to enhance performance in future Full Duplex (FD) Wireless Local Area Networks (WLANs). Full duplex communications in WLANs will be possible when a receiving node has frames ready for the transmitting node. This requires the receiver to decode the frame header for each and every frame in the Transmission Opportunity (TXOP) of the transmitter, in order to obtain the source and destination addresses. It is shown that, in FD-capable WLANs, this procedure may lead to suboptimal performance, reducing the benefits of FD communications. A simple and effective method, based on the usage of RD protocol, is proposed in order to enhance efficiency in future FD WLANs. The performance of the proposed solution is evaluated and compared to the standard method in terms of maximum achievable throughput.
Radar absorbing materials (RAM) design for a desired frequency and angle range is presented. We e... more Radar absorbing materials (RAM) design for a desired frequency and angle range is presented. We evaluate the performance of particle swarm optimization (PSO) and differential evolution (DE) regarding their applicability to absorber design. The results show that the DE algorithm outer performs PSO variants.
Journal of Engineering Science and Technology Review, Oct 1, 2016
The Medium Access Control (MAC) layer of modern Wireless Local Area Networks (WLANs) is known to ... more The Medium Access Control (MAC) layer of modern Wireless Local Area Networks (WLANs) is known to be susceptible to a number of misbehaving actions. Malicious users are able to alter MAC-layer parameters or alter packet markings to fraudulently gain better service levels from the network to the disadvantage of well-behaved users. If a user is able to successfully perform such acts then the Quality of Service (QoS) functionality of the WLAN can be dramatically altered. In this paper we consider altering the QoS markings on frames as the topic of investigation. This is a type of misbehavior that is harder to detect. Existing research work exploits game theory to discourage such abusive behaviors. Unlike these proposals we follow a deterministic approach to detect and confront this type of anomaly.
A dynamic fuzzy neural system is proposed, for time series anomaly detection. The model is entitl... more A dynamic fuzzy neural system is proposed, for time series anomaly detection. The model is entitled BFuzzTiD (Block-diagonal Fuzzy Time-series Detector) and consists of fuzzy rules whose consequent parts are three-layer small recurrent neural networks. The hidden layer of each network has blocks of neurons that feed back to each other. BFuzzTid is trained by the Dynamic Resilient Propagation algorithm. The model learns the dynamics of the time series such that it can classify them by detecting the anomaly points. A comparative analysis is conducted with a series of time series anomaly detection models, in order to investigate the capabilities of the proposed detector.
Electronics Letters, Jun 1, 2018
An analytical model is described to estimate the performance of the carrier sense multiple access... more An analytical model is described to estimate the performance of the carrier sense multiple access with collision avoidance (CSMA/CA) protocol during the simultaneous transmit/receive (STR) operation of the upcoming IEEE 802.11ax amendment for WLANs. Furthermore, based on that model, an enhancement to the operation of CSMA/CA to increase its performance during the STR mode is proposed.
Applied Mathematics & Information Sciences, 2015
A user's transactions with modern networks and services produce a vast amount of user related dat... more 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.
2023 46th International Conference on Telecommunications and Signal Processing (TSP)
Contemporary engineering sciences, 2018
In this paper a computational intelligence-based filter for real-time separation the adventitious... more In this paper a computational intelligence-based filter for real-time separation the adventitious discontinuous lung sounds from the vesicular sounds is proposed. The filter uses two Dynamic Fuzzy Neural Networks to perform the task of separation of the lung sounds, obtained from patients with pulmonary pathology. The networks are trained by the Simulated Annealing Dynamic Resilient Propagation algorithm and the resulting filter is applied to three major classes of lung sounds. In order to highlight the learning characteristics and the performance of the proposed separation scheme, extensive experimental analysis is conducted, where a comparison with other filters is given.
A block-diagonal fuzzy neural network for short-term load forecasting is proposed. DBD-FELF consi... more A block-diagonal fuzzy neural network for short-term load forecasting is proposed. DBD-FELF consists of fuzzy rules with consequent parts that are neural networks with internal recurrence. These networks have a hidden layer which consists of pairs of neurons with feedback connections between them. The overall fuzzy model partitions the input space in partially overlapping fuzzy regions, where the recurrent neural networks of the respective rules operate. The partition of the input space and determination of the fuzzy rule base is performed by use of Fuzzy C-Means clustering algorithm and the RENNCOM constrained optimization method is applied for consequent parameter tuning. The electric load time-series of the Greek power system is examined, and hourly-based forecasting for the whole year is performed. The performance of DBD-FELF is tested via extensive experimental analysis and the results are promising, since an average percentage error of 1.18% is attained, along with an average ...
The main task of the dissertation is the modeling of user behavior in a telecommunications networ... more The main task of the dissertation is the modeling of user behavior in a telecommunications network in order to detect fraudulent activities. User behavior characterization is achieved by means of appropriate user profiles. These can be either one- or multi-dimensional structures. One-dimensional user activity representations are actually time-series of a network usage measure, e.g. calls per day or bulk of downloaded data. Multi-dimensional profiles may use more usage characteristics such as calls per day, corresponding duration, cost, hour-of-day, etc. User profiles are analyzed through artificial intelligence, statistical and data mining methods. The dissertation uses both a dofferential and an absolute approach to the problem. Problem heuristics as well as all experimentation and conclusions are based on real worldΗ παρούσα διατριβή ασχολείται με το πρόβλημα της μοντελοποίησης της συμπεριφοράς χρηστών σε τηλεπικοινωνιακά δίκτυα με στόχο τον εντοπισμό περιπτώσεων τηλεπικοινωνιακής...
IEEE Transactions on Magnetics, 2009
The design of dual-band base station antennas under constraints for mobile communications is addr... more The design of dual-band base station antennas under constraints for mobile communications is addressed in this paper. Given the antenna geometry, the method of moments (MoM) is used to compute the antenna characteristics. Two distinct multi-objective evolutionary algorithms are applied in order to find the Pareto front of the feasible solutions that satisfy the design constraints. In the present work, the Multi-Objective Particle Swarm Optimization with fitness sharing (MOPSO-fs) is compared with the Nondominated Sorting Genetic Algorithm-II (NSGA-II) in order to optimize the antenna geometry. Two design cases are presented. The first case is a five-element array operating in GSM1800/UMTS frequency bands. The second base station antenna array consists of six elements operating in UMTS/WLAN (2.4 GHz) frequency bands.
2018 7th International Conference on Modern Circuits and Systems Technologies (MOCAST)
Transmission Opportunity (TXOP) is the key Medium Access Control (MAC) layer mechanism that enabl... more Transmission Opportunity (TXOP) is the key Medium Access Control (MAC) layer mechanism that enables modern Wireless Local Area Networks (WLANs) to efficiently exploit network resources. It is essentially a resource allocation technique that enables a contention winning station with multiple backlogged frames belonging to a specific traffic class to transmit a burst of packets for a pre-defined time interval without having to re-content for medium access. However, while multimedia traffic flows are allowed to benefit from TXOP, best effort applications are precluded from utilizing it. In the recent IEEE 802.11ac amendment an enhanced TXOP version allows multiple applications belonging to different traffic classes (including best effort and background traffic) to share the contention-free period and transmit simultaneously. That feature is available only when the Down-Link Multi-User Multiple-Input Multiple-Output (DL-MU-MIMO) is enabled in a Quality of Service (QoS) Access Point (QAP) of a WLAN that operates in infrastructure mode. In this paper, an alternative mechanism is presented that allows the sharing of the TXOP in a sequential manner and can be applied to stations in ad-hoc WLANs with co-existing Voice over IP (VoIP) and non-multimedia flows. The proposed mechanism is analyzed and the results obtained are encouraging.
2018 7th International Conference on Modern Circuits and Systems Technologies (MOCAST)
Transmission Opportunity (TXOP) is the key Medium Access Control (MAC) layer mechanism that enabl... more Transmission Opportunity (TXOP) is the key Medium Access Control (MAC) layer mechanism that enables modern Wireless Local Area Networks (WLANs) to efficiently exploit network resources. It is essentially a resource allocation technique that enables a contention winning station with multiple backlogged frames belonging to a specific traffic class to transmit a burst of packets for a pre-defined time interval without having to re-content for medium access. However, while multimedia traffic flows are allowed to benefit from TXOP, best effort applications are precluded from utilizing it. In the recent IEEE 802.11ac amendment an enhanced TXOP version allows multiple applications belonging to different traffic classes (including best effort and background traffic) to share the contention-free period and transmit simultaneously. That feature is available only when the Down-Link Multi-User Multiple-Input Multiple-Output (DL-MU-MIMO) is enabled in a Quality of Service (QoS) Access Point (QAP) of a WLAN that operates in infrastructure mode. In this paper, an alternative mechanism is presented that allows the sharing of the TXOP in a sequential manner and can be applied to stations in ad-hoc WLANs with co-existing Voice over IP (VoIP) and non-multimedia flows. The proposed mechanism is analyzed and the results obtained are encouraging.
2018 7th International Conference on Modern Circuits and Systems Technologies (MOCAST)
Transmission Opportunity (TXOP) is the key Medium Access Control (MAC) layer mechanism that enabl... more Transmission Opportunity (TXOP) is the key Medium Access Control (MAC) layer mechanism that enables modern Wireless Local Area Networks (WLANs) to efficiently exploit network resources. It is essentially a resource allocation technique that enables a contention winning station with multiple backlogged frames belonging to a specific traffic class to transmit a burst of packets for a pre-defined time interval without having to re-content for medium access. However, while multimedia traffic flows are allowed to benefit from TXOP, best effort applications are precluded from utilizing it. In the recent IEEE 802.11ac amendment an enhanced TXOP version allows multiple applications belonging to different traffic classes (including best effort and background traffic) to share the contention-free period and transmit simultaneously. That feature is available only when the Down-Link Multi-User Multiple-Input Multiple-Output (DL-MU-MIMO) is enabled in a Quality of Service (QoS) Access Point (QAP) of a WLAN that operates in infrastructure mode. In this paper, an alternative mechanism is presented that allows the sharing of the TXOP in a sequential manner and can be applied to stations in ad-hoc WLANs with co-existing Voice over IP (VoIP) and non-multimedia flows. The proposed mechanism is analyzed and the results obtained are encouraging.
2018 41st International Conference on Telecommunications and Signal Processing (TSP), 2018
Transmission Opportunity (TXOP) sharing is the main Medium Access Control (MAC) layer contributio... more Transmission Opportunity (TXOP) sharing is the main Medium Access Control (MAC) layer contribution of the IEEE 802.11ac amendment. It is essentially an enhanced version of the standard TXOP mechanism initially introduced by the IEEE 802.11e amendment, specifically engineered to improve performance of the Downlink Multi-User Multiple-Input-Multiple-Output (DL MU-MIMO) technology. DL MU-MIMO transmissions with TXOP sharing are performed only by the Access Point (AP) allowing multiple frames from multiple traffic classes to be transmitted simultaneously towards multiple users in the downlink. However, in the reverse path, acknowledgments must be transmitted sequentially. The default acknowledgment scheme selected by the IEEE 802.11ac is the Block Acknowledgments (BlockACK) which is a poll-based approach. In this paper, we investigate the application of an acknowledgment-free scheme, known as No Acknowledgment (NoACK), during DL MU-MIMO transmissions in IEEE 802.11ac Wireless Local Area Networks (WLANs). Its impact on the overall achieved throughput is evaluated through an analytical study.
2018 41st International Conference on Telecommunications and Signal Processing (TSP), 2018
Transmission Opportunity (TXOP) sharing is the main Medium Access Control (MAC) layer contributio... more Transmission Opportunity (TXOP) sharing is the main Medium Access Control (MAC) layer contribution of the IEEE 802.11ac amendment. It is essentially an enhanced version of the standard TXOP mechanism initially introduced by the IEEE 802.11e amendment, specifically engineered to improve performance of the Downlink Multi-User Multiple-Input-Multiple-Output (DL MU-MIMO) technology. DL MU-MIMO transmissions with TXOP sharing are performed only by the Access Point (AP) allowing multiple frames from multiple traffic classes to be transmitted simultaneously towards multiple users in the downlink. However, in the reverse path, acknowledgments must be transmitted sequentially. The default acknowledgment scheme selected by the IEEE 802.11ac is the Block Acknowledgments (BlockACK) which is a poll-based approach. In this paper, we investigate the application of an acknowledgment-free scheme, known as No Acknowledgment (NoACK), during DL MU-MIMO transmissions in IEEE 802.11ac Wireless Local Area Networks (WLANs). Its impact on the overall achieved throughput is evaluated through an analytical study.
2018 41st International Conference on Telecommunications and Signal Processing (TSP), 2018
Transmission Opportunity (TXOP) sharing is the main Medium Access Control (MAC) layer contributio... more Transmission Opportunity (TXOP) sharing is the main Medium Access Control (MAC) layer contribution of the IEEE 802.11ac amendment. It is essentially an enhanced version of the standard TXOP mechanism initially introduced by the IEEE 802.11e amendment, specifically engineered to improve performance of the Downlink Multi-User Multiple-Input-Multiple-Output (DL MU-MIMO) technology. DL MU-MIMO transmissions with TXOP sharing are performed only by the Access Point (AP) allowing multiple frames from multiple traffic classes to be transmitted simultaneously towards multiple users in the downlink. However, in the reverse path, acknowledgments must be transmitted sequentially. The default acknowledgment scheme selected by the IEEE 802.11ac is the Block Acknowledgments (BlockACK) which is a poll-based approach. In this paper, we investigate the application of an acknowledgment-free scheme, known as No Acknowledgment (NoACK), during DL MU-MIMO transmissions in IEEE 802.11ac Wireless Local Area Networks (WLANs). Its impact on the overall achieved throughput is evaluated through an analytical study.
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Papers by Constantinos Hilas