Experimental Performance Evaluation and Frame Aggregation Enhancement in IEEE 802.11n WLANs

The IEEE 802.11n standard promises to extend today’s most popular WLAN standard by significantly increasing reach, reliability, and throughput. Ratified on September 2009, this standard defines many new physical and medium access control (MAC) layer enhancements. These enhancements aim to provide a data transmission rate of up to 600 Mbps. Since June 2007, 802.11n products are available on the enterprise market based on the draft 2.0. In this paper we investigate the effect of most of the proposed 802.11n MAC and physical layer features on the adhoc networks performance. We have performed several experiments in real conditions. The experimental results demonstrated the effectiveness of 802.11n enhancement. We have also examined the interoperability and fairness of 802.11n. The frame aggregation mechanism of 802.11n MAC layer can improve the efficiency of channel utilization by reducing the protocol overheads. We focused on the effect of frame aggregation on the support of voice and video applications in wireless networks. We also propose a new frame aggregation scheduler that considers specific QoS requirements for multimedia applications. We dynamically adjust the aggregated frame size based on frame's access category defined in 802.11e standard

Graph-based methods for Significant Concept Selection

It is well known in information retrieval area that one important issue is the gap between the query and document vocabularies. Concept-based representation of both the document and the query is one of the most effective approaches that lowers the effect of text mismatch and allows the selection of relevant documents that deal with the shared semantics hidden behind both. However, identifying the best representative concepts from texts is still challenging. In this paper, we propose a graph-based method to select the most significant concepts to be integrated into a conceptual indexing system. More specifically, we build the graph whose nodes represented concepts and weighted edges represent semantic distances. The importance of concepts are computed using centrality algorithms that levrage between structural and contextual importance. We experimentally evaluated our method of concept selection using the standard ImageClef2009 medical data set. Results showed that our approach significantly improves the retrieval effectiveness in comparison to state-of-the-art retrieval models

Adaptation de la technique W-CDMA à Diff-Serv

Les télécommunications ont subi en l'espace de deux décennies des évolutions et bouleversements profonds. Dans le panorama des systèmes de télécommunications, les réseaux mobiles occupent, notamment depuis la fin des années 1980, une place chaque jour plus grande en termes de recherche, d'investissement, de revenus, d'abonnés... Dans le paysage des télécommunications, le principal facteur de cette révolution est la mondialisation de l'économie, catalysée par les réseaux de télécommunications dont l'Internet est à ce jour le représentant le plus significatif et la croissance exponentielle de la demande qui pousse les opérateurs à développer leurs systèmes et techniques employés et à introduire des méthodes nouvelles et efficaces pour l'optimisation des ressources radio: la qualité de services. Nous essayons d'étudier avec plus de précision le facteur « Qualité de Services » dans les réseaux cellulaires sans fil qui est un thème nouveau qui pose un certain nombre de problèmes. D'abord, dans ce genre de réseaux, il est nécessaire de proposer des solutions simples et robustes par mesure de scalabilité. Ensuite, la mobilité introduit une notion d'incertitude dans le réseau qu 'il est important de prendre en compte. Enfin, vu la rareté de la ressource physique, il est important de l'optimiser au maximum. Nous présentons un mécanisme très simple permettant de mettre de la qualité de service « Diff-Serv » dans un réseau de type CDMA. Nous montrons l'avantage d'un tel mécanisme par rapport à d'autres solutions plus "dures". Nous étudions enfin les limites de cette méthode quant au facteur d'échelle (scalabilité), aux contraintes temporelles, au multiplexage et enfin à l'adaptation aux applications actuelles telles que le Web (trafic asymétrique) et la téléphonie sur IP

The Development of the Dementia Screening Battery-100: Instrument Presentation, Reliability, and Construct Validity

Background/Aim: The screening of dementia in non-Westerners has so far relied on translations and adaptations of reputed instruments. Other efforts focused on developing culturally appropriate tests or tests in touch with new developments in the field. This study presents the rationale behind the construction of a new dementia screening test: the Dementia Screening Battery-100 (DSB-100). Methods: The DSB-100 was administered to 46 demented individuals and 159 healthy matched controls. All demented participants met the DSM-IV criteria for dementia. The healthy controls showed no cognitive impairment and were independent in activities of daily living. The DSB-100 was administered as part of a larger neuropsychological assessment to collect additional indices on the severity of patients' dementia, depression, and frontal dysfunctions. The same information was used for comparisons with DSB-100 scores. Results: Multiple regression analysis suggested that age and education, but not the variable sex, are essential in predicting cognitive performance. Construct validation yielded 4 factors, namely attention-visuospatial factors, memory, language, and executive functions. The results showed that the DSB-100 has a high interrater reliability and an acceptable overall internal homogeneity. Conclusion: These results validate the DSB-100 and suggest its appropriateness for dementia screening in Tunisian elderly and possibly elderly people from other cultures with modifications to some subscales.Scopu

Handling Nodes Mobility and Failure During Bootstrapping in Randomly Deployed Ring-based Wireless Sensor Networks

Distributed Hash Table (DHT)-based protocols are new approaches proposed to Wireless Sensor Networks (WSN). Their main advantage resides on the easy integration of DHT-based WSN into the Internet Of Things. However, these protocols face multiple challenges in their bootstrapping phase, especially at the case of randomly deployed WSN. We presented in a recent work a bootstrapping protocol for use in DHT-based protocols having the structure of ring in static WSN. However, in most cases, there are some nodes that may fail or move in the network. Bootstrapping should take into account such situations in order to achieve better performance. In this paper, we propose a distributed local recovery method for use in our bootstrapping protocol that allows it to handle efficiently nodes failure and mobility. Simulation results have shown that our proposed approach is able to ensure the local recovery in a timely and energy-efficient manner