Maurice Khabbaz

Disruption-Tolerant Networking: A Comprehensive Survey on Recent Developments and Persisting Challenges

Nowadays, wireless networks are witnessing several deployments in various extreme environments where they suffer from different levels of link disruptions depending on the severity of the operating conditions. In all cases, their operation requirements are differently altered and their performance is negatively affected rendering them heterogeneous by nature. In the open literature, these networks are known as Intermittently Connected Networks (ICNs). The existing Internet protocols fail to operate properly in the context of ICNs, thus raising a variety of new challenging problems that are attracting the attention of the networking research community. Delay-/Disruption-Tolerant Networking emerged as a highly active area of research where networking experts compete in addressing the various ICN problems. Over time, unicast routing, one of the architectural key components common to all ICNs, became an almost independent field of research in which significant efforts continue to be invested. In contrast, network architectural designs, scheduling and forwarding issues dating from the early days of Inter-Planetary Networks (IPNs) have received relatively little attention and accumulate numerous pending challenges. Moreover, the gap caused by the lack of accurate ICN mathematical models is still large irrespective of some of the appreciated seminal works in this direction. This paper sheds the light over the latest advancements in each of the above-mentioned research sectors and highlight pending open issues in each of them.

Probabilistic Bundle Relaying Schemes in Two-Hop Vehicular Delay Tolerant Networks

One class of Vehicular Delay-Tolerant Networks consists of two node types: stationary and mobile. Stationary nodes deployed along roadsides cannot directly communicate as they are considerably distant. Mobile nodes mounted over vehicles opportunistically entering the range of a stationary source serve as relays that carry bundles to the destination. In this letter, we introduce a novel relaying scheme that probabilistically determines a vehicles suitability to carry bundles. Hence, bundles are released to a present vehicle if and only if that latter contributes in minimizing the mean transit delay. Extensive simulations were performed to gauge the merit of the proposed scheme.

Modeling and Delay Analysis of Intermittently Connected Roadside Communication Networks

Vehicular networks outline a challenging terrestrial application of the emerging delay-tolerant networking (DTN) paradigm where wireless links experience frequent disruptions. Thus, continuous end-to-end paths are unguaranteed. Under such conditions, mobile vehicles present opportunistic relaying capabilities that promote network connectivity, particularly between stationary and isolated roadside units. In this context, we investigate a challenging information-delivery-delay minimization problem. Information is encapsulated into bundles buffered at the source, and vehicles opportunistically transport them to the destination. Consequently, bundles undergo both queueing and transit delays. We propose a probabilistic bundle release scheme (PBRS) under which a roadside unit performs typical Internet-like forwarding where a single bundle is only released to an arriving relatively high-speed vehicle. This ensures a minimized bundle transit. In contrast, under a greedy bundle release scheme (GBRS), a bundle is released to any arriving vehicle, regardless of its speed. Two queueing models are developed to characterize a roadside unit and evaluate its performance under both schemes. A simulation framework is set up to validate these models. Results indicate the inefficiency of the typical Internet packet-like release mechanism as it incurs excessive bundle queueing delays. A bulk bundle release (BBR) extension is proposed as an effective solution. We show that GBRS-BBR outperforms PBRS-BBR.

Vehicular networking: A survey on spectrum access technologies and persisting challenges

Advanced wireless technologies are, nowadays, being exploited as means for intelligent transportation management and on-the-road driving assistance. However, the recent synergic efforts in both industry and academia are symptomatic of a paradigm shift in Intelligent Transportation Systems. Todays vehicles, being equipped with computerized modules and wireless devices, characterize themselves by a revolutionary smart personality. In particular they can carry and distribute information, they inter-communicate and are capable of communicating with other stationary units deployed along roadways. Most importantly, they can sense. Ultimately, the transportation infrastructure embraces versatile wireless communication systems with the objective of provisioning Wireless Access in Vehicular Environments (WAVE). In order to catalyze the realization of this objective, the U.S. Federal Communications Commission (FCC) has allocated 75 MHz of spectrum for Dedicated Short Range Communications (DSRC). The Institute of Electrical and Electronics Engineers (IEEE) has specifically tailored the 802.11p standard to regulate access to this spectrum. The IEEE 1609.4 protocol allows for multi-channel access and is currently being standardized. Alternatively, researchers have engaged in the design of scheduling policies for the purpose of increasing the spectrum access efficiency and optimizing the performance of vehicular networks in terms of several classical metrics. The recently emerging spectrum scarcity problem has led numerous investigations on the potential exploitation of cognitive radios as well as the feasibility of Vehicular Dynamic Spectrum Access (VDSA) schemes with the objective of: a) enhancing spectrum utilization and b) improving the networks throughput and response time. This paper sheds the light over the latest advancements in each of the above-mentioned research sectors and highlights pending open issues in each of them.

Delay-Aware Data Delivery in Vehicular Intermittently Connected Networks

The open literature encloses numerous studies on the efficiency of retransmission mechanisms used in typical data communication networks for the purpose of recovering from packet transmission errors or losses. This paper revolves around the design and analysis of a Delay-Aware Data Delivery (DADD) scheme for Vehicular Intermittently Connected Networks (VICNs). At the heart of DADD is a novel mechanism that allows a source stationary roadside unit (SRU) to carry out necessary bundle retransmissions to high speed vehicles newly entering its communication range. In turn, these vehicles will guarantee delay-minimal delivery of the retransmitted bundles to the destination SRU. A mathematical model is developed to characterize the operation of the source SRU under DADD as well as to evaluate the resulting bundle delivery delay. To verify the validity and the accuracy of the proposed model, extensive simulations are conducted where the performance of DADD is compared to that of two other existing schemes. Results show that DADD outperforms these two schemes by 14.28% to 36.84%.

Modeling and Analysis of DSA-Based Vehicle-to-Infrastructure Communication Systems

This paper presents an in-depth investigation on the feasibility of dynamic spectrum access (DSA) in vehicular environments. We present a comprehensive description of the DSA-based vehicle-to-infrastructure (V2I) communication as it takes place in the context of a scenario where spectral resources are limited. Founded on top of this description is a queueing model whose primary objectives are to capture and characterize the dynamics of this type of communication system and assess its performance in terms of several classical metrics. Simplicity and tractability distinguish the proposed model herein from existing models in the literature. Extensive simulations and numerical analysis are conducted for the purpose of validating the proposed model, evaluating the performance of DSA-based communication, and highlighting its limitations.

Energy harvesting in vehicular networks: a contemporary survey

Vehicular networks have recently been witnessing an upsurge of interest in energy consumption control. Precisely, in the majority of vehicular networking scenarios, roadside units are deployed along roadways in rural areas as well as on the sides of long highways where a direct connection to the electric grid is merely available. In such situations, these roadside units will be equipped with rechargeable batteries with maintenance requiring costly human intervention. Thus far, the literature offers several proposals of efficient operation schemes for roadside units aiming at optimizing their energy consumption, hence, elongating their duration of availability and participation in the network. Energy harvesting presents itself as an appealing alternative to power/recharge nodal batteries in wireless networks. This article starts by presenting a concise general overview of energy harvesting sources, techniques, and applications. Second, it investigates the feasibility of energy harvesting in vehicular networks, specifically the different challenges confronting its applicability in vehicular environments. Finally, pending related open research problems and directions are presented.

Which Vehicle To Select

In wireless networking, an Opportunistic Bundle Release Mechanism (OBRM) is a data bundle forwarding mechanism characterized by its ability to operate over Intermittently Connected Networks (ICNs) where end-to-end paths are not continuously available. Known for their intrinsic connectivity intermittence, vehicular networks constitute an ideal recreation ground for OBRMs. This letter, proposes the Optimal Vehicle Selection OBRM (OVS-OBRM) with the objective of minimizing the average bundle delivery delay.

Modeling and Delay Analysis of a Retransmission-Based Bundle Delivery Scheme for Intermittent Roadside Communication Networks

This paper proposes a novel bundle delivery scheme (BDS) aimed at achieving a delay-minimal bundle delivery in the context of an intermittent roadside network. The realization of this objective is challenging whenever network information is completely unavailable. The concept of virtual space (VS) presents itself as an efficient solution that allows the source to perform necessary data bundle retransmissions to a subset of arriving vehicles. In turn, these vehicles will secure earlier delivery of the retransmitted bundles to the destination. A thorough empirical performance evaluation of the BDS shows that this scheme exhibits a delay improvement of 22.6%-40% relative to other existing schemes.

A Probabilistic Bundle Relay Strategy in Two-Hop Vehicular Delay Tolerant Networks

A persisting major challenge in Vehicular Delay- Tolerant Networks (VDTNs) is the delay minimization of data delivery when communicating nodes are stationary, arbitrarily deployed along roadsides and considerably apart that they cannot establish direct communication between each other. A source opportunistically releases bundles of data to cooperating vehicles passing by, hoping that they will successfully deliver them to the intended destination. Several complex strategies that tackle this problem have been proposed in the open literature. Nevertheless, these strategies often implicitly assume complete network knowledge. In this paper, we propose a rather simple Probabilistic Bundle Relay Strategy (PBRS) that relaxes the availability of complete network information. A queuing model is formulated to represent VDTN stationary sources where PBRS is deployed. We introduce the bundle release probability parameter which expresses the likelihood that a bundle is released by the source to a vehicle passing by. The proposed model is studied analytically and theoretical expressions of its characteristic parameters are all derived. In particular, we compute the time it takes to release a head-of-line bundle (referred to as the bundles service time). Moreover, the model is validated through a simulation study that gauges its merit. The simulation results show that even with partial network knowledge the proposed queueing model can guarantee acceptable bundle delivery delay.