Lazaros Merakos

Modern mobile and stationary devices are equipped with multiple network interfaces aiming to provide wireless and wireline connectivity either in a local LAN or the Internet. Multipath TCP (MPTCP) protocol has been developed on top of legacy TCP to allow the simultaneous use of multiple network paths in the communication route between two end-systems. Although the combination of multiple paths is beneficial in case of links with similar network characteristics, MPTCP performance is challenged as heterogeneity among the used paths increases. This work provides an overview of the MPTCP protocol operation, analyzes the state-of-art packet scheduling and congestion control algorithms available in literature, and examines the impact of the various algorithm combinations on MPTCP performance, by conducting an extensive experimental evaluation under diverse path-heterogeneity conditions.

The expanded scope and attack surface of the today’s multi-tier heterogeneous wireless network (MHWN), which constitutes the basis of our daily life activities, dictates a deeper understanding of how distributed Denial of Service (DDoS) attacks can impact its performance. In this work, we assess the performance of large-scale MHWNs that are subject of random DDoS attacks, considering that i) each tier may support a different radio access technology and spectrum band, and ii) end terminals have limited access to available tiers. Depending on the capability of end terminals to perfectly detect DDoS actors (or not), we provide a solid analytical framework for assessing the impact of different user association policies on both the service probability and the user load distribution per tier. Among others, our numerical analysis highlights the importance of striking a good balance between optimal system-wide service probability and fair load distribution across the tiers of the MHWN.

Dynamic Adaptive Streaming over HTTP (DASH) has prevailed as the dominant way of video transmission over the Internet. This technology is based on receiving small sequential video segments from a server. However, one challenge that has not been adequately examined is the obtainment of video segments in a way that serves both the needs of the network and the improvement in the Quality of Experience (QoE) of the users. One effective way to achieve this is to implement and study caching and DASH technologies together. This paper investigates this issue by simulating a network with multiple video servers and a video client. It then implements both the peer-to-many communications in the context of adaptive video streaming and the video server caching algorithm based on proposed criteria that improve the status of the network and/or the user. Specifically, we investigate the scenario of delivering DASH-based content with the help of an intermediate server, apart from a main server, to demonstrate possible caching benefits for different sizes of intermediate storage servers. Extensive experimentation using emulation reveals the interplay and delicate balance between caching and DASH, guiding such network design decisions. A general tendency found is that, as the available buffer size increases, the video playback quality increases to some extent. However, at the same time, this improvement is linked to the random cache selection algorithm.

The Internet of Things (IoT) ecosystem emerges rapidly and paves the way towards collecting huge amounts of data that can feed innovative information-based systems. The connectivity infrastructure that will support the IoT ecosystem encompasses deployments that define the so-called Low-Power Wide Area Networks (LPWANs). LPWANs are an attractive solution for IoT service provisioning, since they incorporate energy-efficient and cost-effective technologies. In this context, a comprehensive study is provided to assist in the currently increasing research and development interest on LPWANs. First, recent cloud-based and open-source approaches for data management are discussed, while the major key wireless technologies for network access are presented. Second, the potential of Long Range (LoRa) modulation and Long Range Wide Area Network (LoRaWAN) protocol, as key LPWAN wireless technologies in unlicensed spectrum bands, is further analyzed. The fundamental principles of these technologies are presented, and a thorough study on the relevant research activities is conducted.

Various aspects of the detection of events in wireless powered communication networks (WPCN) are studied and analyzed under the assumption of highly noisy sensor measurements. In WPCN, networks sensor nodes’ stored energy is a scarce resource and must be treated sparingly. Frequent false alarm detections force superfluous transmissions, thus depleting nodes’ energy storage. This has an adverse effect on the probability of successful transmission of the information message and its delay in case of a true positive detection. In this work, the detection problem is approached using an optimal stopping framework, where the involved likelihoods are highly unstable due to the noisy measurements. A classical AR filter is adopted in order to smooth the posterior likelihoods prior to their usage in the detection phase and its performance is contrasted to that of a novel Beta Particle Filter smoother. The effects of the smoothing filters on the achieved false alarm rate and detection delay are examined using numerical and simulation results. Moreover, the assessment of the detection process takes into account critical WPCN parameters, such as the charging efficiency and the location of the sensors, thus aiding the system design.

The use of a wireless sensor network to monitor an area of interest for possible hazardous events has become a common practice. The difficulty of replacing or recharging sensor batteries dictates the use of energy harvesting as a means to extend the network’s lifetime. To this end, energy beamforming is used in a millimeter wave wireless power sensor network with randomly deployed nodes. A simple protocol is proposed that allows nodes to report their charging conditions in an effort to select efficient energy-beamforming strategies. Analytical expressions for the probability of successful information reception and successful reporting are provided for two benchmark schemes: the random and the circular energy-beamforming scheme. A Markov chain is used for the former to model the energy level of sensor nodes. Simple sector selection strategies are presented and their performance, in terms of delay and failure information delivery, is assessed through simulations.

The proliferation of radio-enabled equipment, including sensors, controllers, actuators, mobile handhelds, etc., into the today's heterogeneous wireless network (HWN) has led to the support of different vertical services on-top of the same physical infrastructure. Such an approach increases the potential for enhanced multiplexing gains due to the joint utilization of multiple network tiers and their resources. However, it also links the availability of new services to the robustness of the HWN infrastructure against malicious attacks. In this paper, we study the impact of two prominent denial of service (DoS) attacks, the jamming and the black hole attack, on network coverage of multi-tier HWNs. In particular, we consider a HWN model of multiple tiers, where the locations of nodes belonging to each tier are modeled by a homogeneous Poisson Point Process (PPP) with known intensity. Each tier consists of regular nodes, jammers (disrupting downlink communications by transmitting at constant power), and black holes (appearing as regular nodes but without forwarding received packets to their legitimate destination). The type of nodes for a given network tier is given by a prescribed probability, whereas jammer and black hole nodes are considered to act independently from each other. We further focus on the challenging scenario where end users can access a limited number of network tiers, while we also allow different tiers to potentially utilize different spectrum bands. Assuming that successful service reception from a WNE belonging to a given network tier requires a minimum received signal quality threshold to be attained, which for analytical tractability we consider to be higher than the unit, we derive exact expressions and performance bounds on the coverage probability of random multi-tier HWNs with joint jamming and black hole attacks depending on the capability of network nodes to detect and avoid association with malicious nodes (i.e. jammers or black holes). Detailed numerical results highlight the usefulness of the proposed analysis, providing valuable insights on system design and parameterization towards enhanced network robustness.