Aggeliki Sgora

University of Piraeus, Le Pirée, Attica, Greece

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Publications (23)8.58 Total impact

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    ABSTRACT: Next generation wireless networks consist of many heterogeneous access technologies that should support various service types with different quality of service (QoS) constraints, as well as user, requirements and provider policies. Therefore, the need for network selection mechanisms that consider multiple factors must be addressed. In this paper, a network selection method is proposed by applying the analytic network process to estimate the weights of the selection criteria, as well as a fuzzy version of technique for order preference by similarity to ideal solution to perform the ranking of network alternatives. The method is applied to a heterogeneous network environment providing different QoS classes and policy characteristics. Each user applies the method to select the most appropriate network, which satisfies his or her requirements in respect of his or her service-level agreement (SLA). Performance evaluation shows that when the user requests only one service, the proposed method performs better compared to the original technique for order preference by similarity to ideal solution, as well as the Fuzzy AHP-ELECTRE method. Moreover, the proposed method can be applied in cases where a user requires multiple services simultaneously on a device. The sensitivity analysis of the proposed method shows that it can be properly adjusted to conform to network environment changes. Copyright © 2014 John Wiley & Sons, Ltd.
    International Journal of Communication Systems 09/2014; · 1.11 Impact Factor
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    ABSTRACT: Wireless Mesh Networks are considered as a promising solution for offering low-cost access to broadband services. However, one of the main challenges in the design of these networks is their vulnerability to security attacks. In this paper, we analyze the fundamental security challenges and constraints of these networks, classify several possible attacks, and survey several intrusion prevention, detection, and response mechanisms found in the literature. Copyright © 2013 John Wiley & Sons, Ltd.
    Security and Communication Networks 09/2013; · 0.43 Impact Factor
  • Janne Seppänen, Martín Varela, Aggeliki Sgora
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    ABSTRACT: Recently, network researchers have taken a great interest in quality of experience (QoE) and in the new aspects it brings in the study of the link between network conditions and user satisfaction. Also, the realization that the information of users’ satisfaction can be directly applied in the network management in a real-time manner has resulted in a fair amount of publications. Although the systems and frameworks presented in these publications tackle the subject of QoE-driven management quite successfully, they often concentrate on certain applications or technologies. We present a generic QoE management framework, which is applicable to a broad range of systems. We also demonstrate an instantiation of this framework as a network access point management system for RTP-based video. This system is not only able to positively affect the perceived quality of the multimedia application considered, but also to reduce over-prioritization and optimize resource usage.
    Journal of Visual Communication and Image Representation 01/2013; · 1.20 Impact Factor
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    ABSTRACT: The Long-Term Evolution (LTE) standard is a promising solution to mobile connectivity. It provides high data rates at a relatively low cost. At the same time, video traffic has so far outpaced other traffic types in growth, making it the dominant traffic type in current wireless networks. In this paper we study the concept of Quality of Experience (QoE) for video traffic in LTE systems. Since video users are more annoyed by unpredictable disruptions than overall reduced video fidelity, we developed an adaptive video coding scheme that aims to improve QoE in LTE networks. In the proposed model users of the same quality class are classified into a number of service levels providing different QoE satisfaction thresholds to their members. The QoE driven adaptation scheme adapts the rate of the transmitted video, so that the QoE experienced by the users remains in satisfactory levels. Two different policies for transmission rate adaptation are studied, namely the adaptive and the coordinated approach. The level of the transmission rate for both policies is regulated by the linear slow start and/or the exponential increase. Simulation results showed that the proposed algorithm for the both transmission rate adaptation policies succeeds in minimizing packet loss and delay in the video transmission, by adjusting the video resolution.
    Wireless Communication Systems (ISWCS 2013), Proceedings of the Tenth International Symposium on; 01/2013
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    ABSTRACT: The LTE standard is a promising solution to wireless connectivity. It promises high data rates at a low cost. At the same time, video traffic has so far outpaced other traffic types in growth, making it now the dominant traffic type in wireless networks. In this paper we study the concept of Quality of Experience for video traffic in LTE systems, Since video users are more annoyed by unpredictable packet loss, rather than reduced video fidelity, we developed an adaptive video coding scheme that aims to improve QoE in LTE networks, by continuously adapting the video fidelity, in order to keep the packet loss at acceptable levels.
    Informatics (PCI), 2012 16th Panhellenic Conference on; 01/2012
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    ABSTRACT: Congestion can degrade the performance of a Wireless Sensor Network (WSN) since it causes energy waste, throughput reduction and increased information loss. To prevent this situation several congestion avoidance mechanisms and schemes may be applied. The aim of this paper is to survey existing congestion avoidance mechanisms to classify them and to discuss their advantages and drawbacks.
    Informatics (PCI), 2012 16th Panhellenic Conference on; 01/2012
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    ABSTRACT: The IP Multimedia Subsystem (IMS) seems to be the technology that will prevail in Next Generation Networks (NGNs), since the interworking environment and the service flexibility that this technology offers to the currently deployed wireless broadband technologies makes it appealing to users, service developers and network operators. In this paper we propose a heterogeneous network model based on the IMS that integrates the Worldwide Interoperability for Microwave Access (WiMAX), Universal Mobile Telecommunications System (UMTS) and Wireless Local Area Network (WLAN) technologies and provides guaranteed QoS. We present the complete signalling flow concerning the authorization, registration, session set up and vertical handoff processes, as well as, an analytic model for cost analysis of the proposed architecture.
    Computer Communications. 01/2011; 34:1077-1099.
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    ABSTRACT: The tremendous growth of wireless technologies has introduced the potential of continuous service adaptation to the users’ needs by giving them the ability to be able to select and access the proper network based on different criteria. Moreover, next generation wireless networks have been designed to provide support for multimedia services, with different traffic characteristics and different Quality of Service (QoS) guarantees. However, the expansion of Wireless Local Area Networks (WLANs) and Worldwide Interoperability for Microwave Access (WiMAX) networks poses new research era in the decision of the access network selection. In this paper, the existing access network selection schemes are classified into three categories: the network-centric, the user-centric and the collaborative schemes, and are analyzed respectively. Moreover, we propose a multicriteria access network selection algorithm applied in a WiMAX–WLAN environment, in order to facilitate the provision of high quality services and at the same time to satisfy different types of user Service Level Agreements (SLAs).
    Pervasive and Mobile Computing. 01/2011; 7:584-594.
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    ABSTRACT: Multihop networks are used in a variety of ways, that include ad-hoc networks, wireless sensor networks, and mesh networks. These networks consist of nodes that connect over a wireless channel, and communicate over multiple links by forwarding each other's messages. The throughput is determined by the contention among the nodes for the wireless resources. Thus routing algorithms, influence the level of contention at each location of the network, and play an important role in the network throughput. Additionally, the sequence of transmission opportunities of each node, which is determined by the scheduling algorithm, should also be carefully designed, since it affects the available resources of each node. In this paper we study the effect of routing and scheduling algorithms on the throughput of multihop networks. We propose a per-flow joint routing/scheduling algorithm that has the property of routing the flows in a way that avoids congested areas with limited availability, and also schedules links, in a way that a) slots are assigned to flows, instead of links or nodes, b) the assignment is fair c) fairness does not lead to underutilization. Simulation results showed that the proposed algorithm increases the average throughput, and the degree of satisfaction of each node, while at the same time it decreases the average delay.
    Proceedings of the 16th IEEE Symposium on Computers and Communications, ISCC 2011, Kerkyra, Corfu, Greece, June 28 - July 1, 2011; 01/2011
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    Aggeliki Sgora, Dimitrios D. Vergados
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    ABSTRACT: The IP Multimedia System (IMS) is a promising solution for converging new technologies with existing networks. The interworking environment and service flexibility that IMS offers to the currently deployed wireless broadband technologies, makes it appealing to users, service developers and network operators. In our previous work a heterogeneous network model based on IMS that integrates the Worldwide Interoperability for Microwave Access (WiMAX), Universal Mobile Telecommunications System (UMTS) and Wireless Local Area Network (WLAN) technologies and provides guaranteed QoS was proposed. In this paper an analytical model is derived in order to evaluate the signalling cost of mobility management during a vertical handoff.
    Informatics, Panhellenic Conference on. 09/2010;
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    ABSTRACT: Ubiquitous service delivery requires the selection of the optimal access network in a heterogeneous wireless environment. However, since the selection of an access network in such an environment depends on several parameters with different relative importance (such as the network and the application characteristics, the user preferences, the service cost), it is a difficult task to be achieved. In this paper, an effective access network selection algorithm for heterogeneous wireless networks is proposed that combines two Multi Attribute Decision Making (MADM) methods, the Analytic Hierarchy Process (AHP) method and the Total Order Preference by Similarity to the Ideal Solution (TOPSIS) method. More specifically, the AHP method is used to determine weights of the criteria and the TOPSIS method is used to obtain the final access network ranking.
    Computers and Communications (ISCC), 2010 IEEE Symposium on; 07/2010
  • Mobile Lightweight Wireless Systems - Second International ICST Conference, MOBILIGHT 2010, Barcelona, Spain, May 10-12, 2010, Revised Selected Papers; 01/2010
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    ABSTRACT: In wireless multihop networks, route selection plays an important role in network performance in terms of throughput capacity, as well as, energy efficiency. In general, the route selection strategy should distribute the traffic throughout the network, in order to avoid congestion. This strategy also has the side-effect of distributing the energy consumption among all nodes, so that the energy starvation of highly used nodes is avoided. However, the optimal route selection strategy is not always the same with respect to throughput and energy efficiency. In this paper we evaluate several route selection strategies in terms of average message transmission durations and network connectivity lifetime. The simulation results show which strategy is more beneficial for each objective, and how a combination of strategies may lead to balanced results.
    GLOBECOM Workshops (GC Wkshps), 2010 IEEE; 01/2010
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    ABSTRACT: A promising solution for wireless environments is the wireless mesh technology that envisages supplementing wired infrastructure with a wireless backbone for providing Internet connectivity to mobile nodes (MNs) or users in residential areas and offices. The IEEE 802.11 TGs has started to work in developing a mesh standard for local area wireless networks. Although a lot of progress has been made and a few new drafts have been released recently, there exist many issues that demand enhanced or even new solutions to 802.11s mesh networking. This paper aims to overview the latest version of the IEEE 802.11s protocol (Draft 2.02), especially the MAC and routing layers, and to point out the challenges that these networks have to overcome in these layers.
    08/2009: pages 263-271;
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    I. Tinnirello, A. Sgora
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    ABSTRACT: In the last years, several strategies for maximizing the throughput performance of IEEE 802.11 networks have been proposed in literature. Specifically, it has been shown that optimizations are possible both at the medium access control (MAC) layer, and at the physical (PHY) layer. In fact, at the MAC layer, it is possible to minimize the channel waste due to collisions and backoff expiration times, by tuning the minimum contention window as a function of the network congestion level. At the PHY layer, it is possible to improve the transmission robustness, by selecting a suitable modulation/coding scheme as a function of the channel quality perceived by the stations. However, the feasibility of these optimizations rely on the availability of MAC/PHY measurements, which are often impracticable or very rough. In this paper, we propose a joint MAC/PHY estimator based on a bi-dimensional extended Kalman filter, devised to separately track the collision probability and the channel error probability suffered by each station. To this purpose, we derive a relationship between the unobservable system state and measurements which are performed in a distributed way by all the competing stations.
    Global Telecommunications Conference, 2008. IEEE GLOBECOM 2008. IEEE; 01/2009
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    A. Sgora, D. Vergados
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    ABSTRACT: Handoff is a key element in wireless cellular networks in order to provide quality of service (QoS) to the users and to support users' mobility. Handoff failure will result in the forced termination of an ongoing call. From the user's point of view, the service of a handoff request is more important, as the forced termination of an ongoing call is more annoying than the blocking of new calls. Therefore, in order to support QoS to the users and to provide ubiquitous coverage, the handoff procedure ought to be further investigated. This paper provides a comprehensive survey of the basic elements, the different types and phases of the handoff procedure. Moreover, particular interest has been given in the horizontal handoff execution phase by discussing and classifying the most recent handoff prioritization schemes into categories based on the concepts that these schemes adopt, e.g. channel reservation, handoff queueing, channel transferred, subrating, genetic and hybrid schemes and in the vertical handoff decision phase by presenting different decision algorithms.
    IEEE Communications Surveys &amp Tutorials 01/2009; · 4.82 Impact Factor
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    ABSTRACT: In wireless multihop networks communication between two-end nodes is carried out by hopping over multiple short wireless links. Traditional CSMA/CA based media access control does not work well in a multihop scenario, since transmitters are often out of reach of other users' sensing range. Therefore TDMA seems to be a more promising solution. Several algorithms may be found in the literature for scheduling TDMA transmissions for these networks. These algorithms try to determine the optimal scheduling, in order to increase the capacity and reduce the delay for a given network topology. In our previous work, a TDMA scheduling algorithm has been proposed that schedules the transmissions in a fair manner and takes into account the communication requirements of the active flows of the network. In this research effort, we compare the performance of the proposed algorithm with several TDMA scheduling algorithms, in terms of frame length, throughput and fairness. Simulation results showed that the proposed algorithm exhibits improved performance compared to the other solutions, not only in terms of fairness, but also in terms of throughput.
    Communications Workshops, 2008. ICC Workshops '08. IEEE International Conference on; 06/2008
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    ABSTRACT: In wireless multihop networks communication between two end-nodes is carried out by hopping over multiple short wireless links. Traditional CSMA/CA based media access control does not work satisfactory in a multihop scenario, since an intended target of a communication may be subject to mutual interference imposed by concurrent transmissions from nodes which cannot directly sense each other, causing unfair throughput allocation. Although TDMA seems to be a more promising solution, careful transmission scheduling is needed in order to achieve error-free communication and fairness. In our previous work, a TDMA scheduling algorithm has been proposed that schedules the transmissions in a fair manner and takes into account the communication requirements of the active flows of the network. In this paper, in order to further improve this scheme, we combine the TDMA scheduling algorithm with two different load-balancing algorithms. Simulation results showed that the proposed load-balancing policies can further improve the performance of the TDMA scheduling algorithm.
    Wireless Pervasive Computing, 2008. ISWPC 2008. 3rd International Symposium on; 06/2008
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    ABSTRACT: Wireless multi-hop networks have recently been conceived as a networking paradigm. However, their deployment has been limited, mainly due to the fact that they lack fairness. In this research effort, we briefly review the state of art in fairness mechanisms in multi-hop networks and we propose an algorithm that tries to differentiate the traffic among the connections in a wireless multi-hop ad-hoc sensor network, so that the fairness is enhanced. This is achieved by an adaptive scheme, which tries to assign a higher priority to the traffic connections that experience delay greatly larger than the average, so that their delay is reduced, and the fairness of the system is improved.
    Proceedings of the 3rd International Conference on Mobile Multimedia Communications, MobiMedia 2007, Nafpaktos, Greece, August 27-29, 2007; 01/2007
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    ABSTRACT: Wireless multihop networks have recently been conceived as a networking paradigm. Several algorithms may be found in the literature for scheduling TDMA transmissions for these networks. These algorithms try to determine the optimal scheduling, in order to increase the capacity and reduce the delay for a given network topology. However, to our best knowledge, no TDMA scheduling algorithms have been developed, that take into consideration the traffic requirements of the active flows of the multihop network. At the same time, the fairness of a network is closely related to the scheduling scheme. In this research effort, we propose an intelligent algorithm that can schedule the transmissions in a fair manner, taking into account the communication requirements of the active flows of the network. Full Text at Springer, may require registration or fee
    International Federation for Information Processing Digital Library; Artificial Intelligence and Innovations 2007: from Theory to Applications;. 01/2007;