Protocols developed during the last years for Wireless Sensor Networks (WSNs) are mainly focused on energy efficiency and autonomous mechanisms (e.g. self-organization, self-configuration, etc.). ...Nevertheless, with new WSN applications, new QoS requirements appear, such as time constraints. Real-time applications require the packets to be delivered before a known time bound which depends on the application requirements. We particularly focus on applications which consist in alarms sent to the sink node. We propose Real-Time X-layer Protocol (RTXP), a real-time communication protocol. RTXP is a MAC and routing real-time communication protocol that is not centralized, but instead relies only on local information. To the best of our knowledge, it is the first real-time protocol for WSNs using an opportunistic routing scheme in order to increase the packet delivery ratio. In this paper we describe the protocol mechanisms. We give theoretical bounds on the end-to-end delay and the capacity of the protocol. Intensive simulation results confirm the theoretical predictions and allow to compare RTXP with a real-time scheduled solution. RTXP is also simulated under harsh radio channel, in this case, the radio link introduces probabilistic behavior. Nevertheless, we show that RTXP performs better than a non-deterministic solution. It thus advocates for the usefulness of designing real-time (deterministic) protocols even for highly unreliable networks such as WSNs.
Duty-cycling prolongs the lifetime of battery-powered wireless sensor networks (WSNs). However, it incurs an additional delay because the nodes may be asleep. Energy constraint is not the only ...constraint in WSNs. Many applications have real-time constraints, which means the sink has to be informed before a deadline when an event occurs. Moreover, wireless links among low-power radios are highly unreliable. This poses major challenges for researchers who want to design protocols for real-time applications. In this paper, a novel forwarding scheme based on distributed wakeup scheduling is proposed which can guarantee bounded delay on the messages that are delivered, and can have higher delivery ratios for ultra-low duty-cycle WSNs under unreliable links. The proposed wakeup scheduling algorithm schedules the wakeup time of each node according to the hop number and expected delivery ratio to the sink. We model the forwarding scheme and analyze its properties. Simulation results are in line with the mathematical model and show that the proposed algorithm has good performances in terms of delivery ratio, end-to-end delay and energy efficiency.
Le souci des morts est le propre de notre commune humanité, par-delà l'infini répertoire des pratiques funéraires qui l'expriment et le traduisent. Mais les relations entre les vivants et les morts, ...le commerce qui s’établit de part et d’autre de la tombe, les échanges matériels et immatériels dont elle est le pivot, sont pour les historiens et les archéologues de formidables révélateurs des structures (sociales, économiques, symboliques) de la société qu’ils étudient. En tenant son LXVIIIe congrès à Jérusalem, ville-sanctuaire et ville-cimetière où tant d’hommes et de femmes sont venus ou ont espéré mourir au Moyen Âge, la Société des historiens médiévistes de l’Enseignement supérieur public a souhaité interroger les relations entre les vivants et les morts en un lieu placé au cœur des croyances et des attentes eschatologiques des trois grandes traditions monothéistes. Attentifs à la diversité des mondes chrétien, juif et musulman qui se croisent dans l’histoire de Jérusalem, les auteurs ont mobilisé l’ensemble des traces laissées par les gestes, les pratiques et les croyances des hommes et des femmes du Moyen Âge à l’égard de leurs morts : de l’étude archéologique des sépultures à leurs représentations, de l’épigraphie funéraire à la mémoire archivistique des défunts, du commentaire des écritures saintes au récit des morts illustres ou anonymes. Ainsi sont mis en lumière la prise en charge des défunts, l’espace qui leur est assigné en propre ou en partage dans la société de leur temps, l’économie des échanges matériels et mémoriels entre morts et vivants, les croyances enfin qui investissent les multiples temporalités de leurs relations de part et d’autre de la tombe. Le Moyen Âge n’a cessé de bruisser du commerce des vivants et des morts : ce volume donne à entendre cette conversation ininterrompue.
L’article s’intéresse aux Arméniens professant, à la différence de leurs compatriotes, la foi chalcédonienne. Un tableau aussi précis que possible de l’implantation géographique des communautés, qui ...varie suivant les siècles et les vicissitudes politiques et militaires, est d’abord dressé. L’auteur évoque ensuite deux cas bien documentés (ce qui est rare, étant donné le caractère très lacunaire des sources) d’adoption, par des groupes d’Arméniens, de la foi chalcédonienne, avant de s’attarder sur les aspects culturels et artistiques. Enfin, l’attitude des Arméniens restés « grégoriens » à l’égard des Arméniens chalcédoniens, qui oscille entre tolérance et persécution, est étudiée.
This article is devoted to Armenians advocating Chalcedonian creed, contrary to their kinsmen. A very minute picture of the geographical distribution of these communities, varying through time and ...political or military troubles, is drawn as an introduction. Then the author provides two very well-documented examples–notably an extremely rare instance given the incomplete nature of such sources–of conversion by Armenian groups to the Chalcedonian creed, before examining the related cultural and artistic aspects. Lastly, a study if done of the attitude of the still ‘Gregorian’ Armenians toward the Chalcedonian Armenians, wavering between tolerance and persecution.
Geographic routing has received increasing attention in the context of Wireless Sensor Networks since it frees the network from the energy-demanding task of building and maintaining a structure. It ...requires however each node to know its position, which may be a prohibitive assumption for many applications. To this end, some prior work has focused on inferring a node’s location from a set of location-aware anchor nodes.
In this work, we free ourselves from positioning techniques and anchor nodes altogether, and introduce and analyze the concept of virtual coordinates. These coordinates are chosen randomly when a node is switched on, and are updated each time the node relays a packet. As this process goes on, the virtual coordinates of the nodes converge to a near-optimal state. When using a greedy geographic approach on top of these coordinates, we show that the number of hops to reach the destination exceeds the shortest path by a few percent only. Moreover, our approach guarantees delivery even when nodes appear/disappear in the network, and under realistic transmission models.
We analytically prove the correctness of our protocol. Moreover, extensive simulations are used to show that our position-free solution outperforms existing geographic protocols – such as Greedy-Face-Greedy (GFG) or Greedy Perimeter Stateless Routing (GPSR) – in terms of energy-efficiency, path length and robustness.
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