Sliding Window Random Linear Network Coding (SW RLNC) is a promising approach for ensuring resilient communication in next-generation networks. In order to enhance both the coding efficiency and the overall performance of the coding scheme, the optimal configuration of the coding parameters should be addressed. Previous studies suggest that the trade-off between coding efficiency and overall performance in SW RLNC schemes is determined by two critical parameters, namely, the code rate and the coding window size. In this work, we address the joint optimization of the code rate and the coding window size by formulating it as a single-objective Sequential Model-Based Optimization problem and employ Bayesian Optimization (BO) models for efficiently solving it. To demonstrate proof-of-concept, we carry out an extensive evaluation campaign assessing the effectiveness of different BO models for this optimization problem. The results confirm that the optimization framework reliably finds optimal solutions with relatively few evaluations. Finally, we present a practical approach for leveraging the proposed optimization framework in real-world scenarios, allowing efficient adaptation to channel variations without requiring real-time re-optimization.

Sliding Window Random Linear Network Coding (RLNC) offers a clear path towards achieving ultra-high reliability and low latency at the same time. Such requirements are pivotal for a wide range of applications in the future Internet as well as in 5G and beyond networks. While traditional RLNC has been extensively used for some years now, its Sliding Window flavor is rather recent and extremely promising because of its implementation advantages and the high degree of customization. Probably the most essential parameter of Sliding Window RLNC is the coding depth, i.e., the extent of non-coded packets protected by a coded one. In this work, for the first time, we elaborate on properly choosing the coding depth and shed light on the related trade-offs. We, first, show, experimentally, that significant performance gains can be obtained by fine-tuning the coding depth. Then, we propose and validate an analytical framework that allows us to decide the coding depth based on a channel’s reliability profile. Finally, we introduce a dynamic algorithm that, based on our analytical findings, can improve the performance of sliding window RLNC in the presence of bursts of errors.

Wireless communications are subject to the chaotic scattering of wireless waves. However, the recently-proposed software-defined metasurfaces (SDMs) can transform the wireless propagation into a programmable and even fully deterministic phenomenon. Unnatural aerial paths can be created by program-ming multiple SDMs deployed in an environment, to alter the direction, phase, amplitude and polarization of waves impinging upon them. Optimizing this programming is an open challenge as parasitic effects, e.g. parasitic reflection lobes, can appear and deteriorate the quality of the received power delay profile. This paper proposes a novel optimization algorithm based on the ant colony framework, through which the SDM programming options are explored, gradually converging to a significantly improved received power delay profile. The study first presents a novel graph-based model of SDM-enabled environments, which ab-stracts the underlying physics, facilitating their cross-disciplinary understanding. Simulation results showcase the flexibility of the model and optimization algorithm in multiple settings.

Dynamic replication is a wide-spread multi-copy routing approach for efficiently coping with the intermittent connectivity in mobile opportunistic networks. According to it, a node forwards a message replica to an encountered node based on a utility value that captures the latter's fitness for delivering the message to the destination. The popularity of the approach stems from its flexibility to effectively operate in networks with diverse characteristics without requiring special customization. Nonetheless, its drawback is the tendency to produce a high number of replicas that consume limited resources such as energy and storage. To tackle the problem we make the observation that network nodes can be grouped, based on their utility values, into clusters that portray different delivery capabilities. We exploit this finding to transform the basic forwarding strategy, which is to move a packet using nodes of increasing utility, and actually forward it through clusters of increasing delivery capability. The new strategy works in synergy with the basic dynamic replication algorithms and is fully configurable, in the sense that it can be used with virtually any utility function. We also extend our approach to work with two utility functions at the same time, a feature that is especially efficient in mobile networks that exhibit social characteristics. By conducting experiments in a wide set of real-life networks, we empirically show that our method is robust in reducing the overall number of replicas in networks with diverse connectivity characteristics without at the same time hindering delivery efficiency.

Random Linear Network Coding (RLNC) schemes that use a sliding window, also known as Sliding Window RLNC schemes, have proved to be efficient for reliable data transmission over an unreliable link. They combine the concept of the sliding window from (Automatic Repeat reQuest) ARQ protocols with RLNC in order to maximize the data flow and quickly recover from losses through redundant coded packets. We make the observation that, in these protocols, optimizing the data flow rate and the coding operation are two distinct objectives that cannot be met only by fine-tuning the sliding window size. To tackle the problem, we propose rapidARQ, a sliding window RLNC scheme with feedback that uses two distinct windows; the sliding window, which is used to optimize the data flow based on the link's bandwidth-delay product, and the coding window, which is used exclusively in the coding process. We experimentally show that rapidARQ outperforms other state-of-the-art sliding window RLNC schemes and achieves superior throughput-delay performance that better fits in the context of Ultra-Reliable Low-Latency Communication (URLLC). More interestingly, it does so while significantly reducing the coding complexity at the same time. The superior performance of rapidARQ is more prominent in channels with large bandwidth-delay products, a fact that renders its utility to current and future networks more essential.

Mutli-copy routing is an efficient and wide-spread approach for coping with the intermittent connectivity of mobile opportunistic networks. One popular multi-copy approach is known as “dynamic” replication; each node creates replicas on a contact basis using a utility that determines a node's fitness for delivering a message to its destination. This scheme is highly flexible, configurable with different utility functions and able to operate efficiently in networks with diverse characteristics. Nonetheless, its drawback is the tendency to produce a high number of replicas that consume limited resources such as energy and storage. Our approach to tackle this problem relies on the observation that, based on their utility values, the network nodes can be grouped into clusters that portray different delivery capabilities. Then, to avoid unnecessary replication, we exploit this finding to replicate a packet to nodes that belong to clusters with increasing delivery capability instead of replicating it to nodes with increasing utility. The new method works in synergy with the basic dynamic replication algorithms and is fully configurable, in the sense that it can be used with virtually any utility function. By conducting experiments in a diverse set of real-life networks, we empirically show that the method effectively reduces the overall number of replicas without hindering delivery efficiency.

Network coding has been successfully used in the past for efficient broadcasting in wireless multi-hop networks. Two coding approaches are suitable for mobile networks; random linear network coding (RLNC) and XOR-based coding. In this paper, we focus on the problem of multiple source broadcasting in mobile ad hoc networks. We make the observation that RLNC provides increased resilience to packet losses compared with XOR-based coding. We develop an analytical model that justifies our intuition. However, the model also reveals that combining RLNC with probabilistic forwarding, which is the approach taken in the literature, may significantly impact RLNC's performance. Therefore, we take the novel approach to combine RLNC with a deterministic broadcasting algorithm in order to prune transmissions. More specifically, we propose a connected dominating set-based algorithm that works in synergy with RLNC on the “packet generation level.” Since managing packet generations is a key issue in RLNC, we propose a distributed scheme, which is also suitable for mobile environments and does not compromise the coding efficiency. We show that the proposed algorithm outperforms XOR-based as well as RLNC-based schemes even when global knowledge is used for managing packet generations.

Privacy preservation in opportunistic networks, such as disruption and delay tolerant networks, constitutes a very challenging area of research. The wireless channel is vulnerable to malicious nodes that can eavesdrop data exchanges. Moreover, all nodes in an opportunistic network can act as routers and thus, gain access to sensitive information while forwarding data. Node anonymity and data protection can be achieved using encryption. However, cryptography-based mechanisms are complex to handle and computationally expensive for the participating (mobile) nodes. We propose SimBet-BF, a privacy-preserving routing algorithm for opportunistic networks. The proposed algorithm builds atop the SimBet algorithm and uses Bloom filters so as to represent routing as well as other sensitive information included in data packets. SimBet-BF provides anonymous communication and avoids expensive cryptographic operations, while the functionality of the SimBet algorithm is not significantly affected. In fact, we show that the required security level can be achieved with a negligible routing performance trade-off.

It has been observed that opportunistic networks exhibit a highly unbalanced traffic load distribution, mainly because of the heterogeneity in mobility and the greedy routing decisions, leading to packet drops due to storage constraints. The existing strategies rely either on fairness techniques or on diverting traffic to alternative routes in order to control congestion. The result is a dilemma between performance and fairness. In this work, we introduce a congestion control mechanism that provides a tunable trade-off between efficiency and fairness. We rely on the social preferences of the nodes for dynamically tuning the aforementioned trade-off. Our simulations show that the proposed algorithm achieves high delivery ratio, combined with low end-to-end delay and routing cost, without sacrificing fairness under high traffic load.

One of the dominant approaches in coping with the intermittent connectivity of opportunistic networks is packet replication. However, the need for a distributed operation forces the nodes carrying a message copy to make replication decisions without taking into account the replication state of other nodes. This strategy can lead to the creation of an excessive number of replicas thus exhausting the limited node resources such as energy and storage capacity. In this paper, we propose a simple yet efficient method which, without incurring any additional cost, allows nodes to share information about the replication process in order to avoid unnecessary replication. Our approach significantly reduces the routing cost without sacrificing delivery rate. Furthermore, it is generic in the sense that it can be implemented regardless of the utility metric used for making the replication decision. We validate the performance gains of our algorithm through analysis as well as extensive simulations.

Multiple Description coding (MD) has been proved to be very efficient for video multicast over random networks when combined with network coding. However, in the case where heterogeneous receivers are present in the network, the impact of the method is bounded by the fact that the solution to the inherent rate allocation problem is based on a compromise between low and high bandwidth receivers. To tackle this problem, we propose a novel video multicast scheme that enhances the combination of MD and network coding with a mechanism that allows all intermediate nodes to actively participate in MD coding by adjusting the size of the video descriptions. In this way, it is possible to tune the rate of the transmitted video stream in order to meet the needs of each receiver. The proposed method significantly improves the quality of the video delivered to all the receivers in the network, regardless of their heterogeneity.

We propose a novel priority-based approach that enables the optimal control of the transmission power and the use of the available network resources of a multihop Direct Sequence Code Division Multiple Access (DS-CDMA) Wireless Visual Sensor Network (WVSN). TheWVSN nodes can either monitor different scenes (source nodes) or retransmit videos of other nodes (relay nodes). Moreover, in real environments the source nodes monitor different scenes that may be of dissimilar importance. Hence a higher end-to-end quality is demanded for those nodes that are assigned a higher priority. Overall, each node has different power and resource requirements, and therefore a global optimization approach is required. For the purpose of enhancing the delivered video quality of the source nodes with respect to their priorities, we define and suggest the use of priority-based optimization criteria. Experimental results that assess the proposed approach are provided and conclusions are drawn.

Broadcasting is a key component of several networking mechanisms in mobile ad hoc networks. However, its traditional implementation suffers from resource consumption caused by message redundancy. Several efficient algorithms have been proposed to tackle this problem. Among them, the class of algorithms that construct on-demand a connected dominating set, known as dominant pruning, yields an improved performance. Dominant pruning algorithms capitalise on neighbourhood information and the previous hop of the broadcast message in order to minimise the set of nodes that forward a message. The proposed algorithm makes the observation that all the nodes, visited by a broadcast message, may be exploited in order to minimise the cost of broadcasting. The analysis of the proposed algorithm as well as extensive simulation results confirm the rationale behind the proposed approach and prove the performance gains.

In this paper, a distributed on-demand routing protocol for Low Earth Orbit (LEO) satellite systems, named multiservice on-demand routing (MOR), is proposed and evaluated. The proposed protocol adjusts the routing procedure to the QoS requirements of different traffic classes. The performance of the MOR protocol is compared to the unique proposal for traffic class dependent routing in the literature and the good characteristics of the proposed scheme are corroborated by ample simulation experiments, where significant gains in performance are witnessed.

In this paper, we propose a lightweight distributed routing algorithm, called Distributed Load-Aware Routing (DLAR). The proposed protocol adopts a distributed approach to handle the complexity of the satellite system and at the same time provides a hop-by-hop mechanism for splitting traffic load in order to alleviate the problem of congestion that occurs near polar regions. The performance of DLAR is assessed through extensive simulations and compared to the performance of centralized routing schemes proposed so far in the literature. Simulation results document and confirm the positive characteristics of the proposed protocol.

In this paper, a location-assisted on-demand routing (LAOR) protocol for low earth orbit (LEO) satellite systems is proposed and evaluated. This protocol can be viewed as a variant of the well-known ad-hoc on-demand distance vector (AODV) routing protocol, tailored to the requirements imposed by the characteristics of LEO satellite systems' topology. The LAOR protocol is assessed for different link-cost metrics and compared to centralized routing protocols proposed in the literature thus far. Simulation studies further document and confirm the positive characteristics of the proposed protocol.

We address the problem of efficiently distributing query messages among peers in a wireless ad hoc network. We assume that peers are organized in classes. Each peer possesses a local database and can answer queries posed by other peers. Each peer can also pose queries to all the peers belonging to a certain class and/or within a certain range of distance in the network. Contrary to traditional p2p lookup queries, we are interested in collecting answers from as many peers as possible. To efficiently serve this purpose, we take advantage of routing and application layer specifics (e.g., class information, network distance) to avoid flooding and at the same time preserve compatibility with traditional routing and transport mechanisms.

In this paper we propose a handover method for multiservice non-geo satellite systems. The proposed algorithm can be used for satellite handover as well as for cell handover when a fixed amount of resources is allocated to each cell. Specifically, we consider the case in which the satellite system provides services to users of different QoS requirements. The first type of users require the minimization and, if possible, the elimination of the probability of forcing a connection in progress to termination during a handover. On the other hand, the users of the second category do not have any specific handover requirements. The new method is evaluated through extensive simulations and is proved to meet the requirements of the first type of users while at the same time maximizes the system utilization for both types of users.

On demand routing protocols that exploit local caches have received a lot of attention lately in wireless ad-hoc networking. In this paper, we specifically address cache management, an issue that has been a main source of criticism for the applicability of such protocols. In particular, we tackle the problem of accessing the cache content efficiently. To this end, we propose summarizing the cache content so that we achieve efficient lookups. This not only saves both the restrictive resources of the wireless devices such as computational power and energy but also improves the overall protocol performance. We use Bloom filters as summaries. Our experimental results using the ns simulator show that both resource savings and performance improvements are attained when such filters are integrated within the DSR protocol which is one the most widely used instance of an on demand protocol.

Wireless mobile networks are characterized by stochastic topology variations. Sudden movements of mobile hosts in and out of each other's range impose limitations on the system performance. Routing protocols have been proposed for producing stable routes. All these protocols address the issue of route stability rather than connection stability. This paper proposes a new protocol for enhancing stability of connections rather than routes, without increasing the related overhead. The new protocol is based on the employment of a number of disjoint paths in each route discovery phase. Its performance is evaluated by simulations in the Java programming language that present adaptability to increased mobility.

This paper deals with the performance evaluation of various path selection strategies that are suitable for non-GEO satellite systems. The modified Dijkstra (1959) algorithm is used for different link cost functions. These various cost functions result to different performance in terms of system utilization, rerouting failure probability and delay parameters. In particular four metrics have been examined for the establishment of paths. The first two are based on the propagation delay and the traffic at the network whereas the other two are based on new, special factors that characterize the operation of inter-satellite links (ISLs). The scenarios are tested through extended real-time simulations and fruitful conclusions are derived at the end of the study.