Magnesium and Aluminium alloys Dissimilar Joining by Friction Stir Welding

Abstract Multi-material lightweight structures are gaining a great deal of attention in several industries, in particular where a trade-off between reduced weight, improved performances, and cost compression is required. Magnesium alloys, such as the zinc-rare earth elements ZE41A alloy, fulfill the first two requirements; however, they are susceptible to corrosion and relatively expensive. Lightweight structures hybridization, for instance combining Magnesium alloys and Aluminium alloys, is currently under consideration as a potential solution to this problem. Nevertheless, dissimilar joining of Magnesium and Aluminium alloys is challenging due to the significant differences in physical properties, as well as to the precipitation of brittle intermetallic compounds, such as Al 12 Mg 17 and Al 3 Mg 2 . In this study, the dissimilar joining of Magnesium and Aluminium alloys by friction stir welding process is discussed. In particular, 4 mm thick plates of ZE41A Mg alloy and AA2024-T3 Al alloy were welded in the butt joint configuration. The feasibility of the process was assessed by means of microstructure and mechanical analysis. The formation of brittle intermetallic compounds was investigated as well

Scalability and Reliability Aware SDN Controller Placement Strategies

International audienceThe decoupling of control and data planes in Software-Defined Networking (SDN) brings benefits in terms of logically centralized control and application programming. But, the single point of management in physically centralized SDN architectures is a potential point of failure and a bottleneck that compromises network reliability and performance. Such centralized designs may also face scalability challenges especially in networks with a large number of hosts (e.g. IoT-like networks). To avoid such concerns, SDN control architectures are usually designed as physically distributed systems. This raises practical challenges about the best approach to decentralizing the control plane while maintaining the logically centralized network view. In particular, determining the number of controllers and locating them in the network is a hard task that should be addressed appropriately. This paper proposes two novel strategies that cover different aspects of the controller placement problem with respect to performance and reliability criteria. These strategies use two types of heuristics that are compared and assessed on large-scale topologies to provide operators with guidelines on how to find their optimal controller placement that meets their specific needs

Adaptive State Consistency for Distributed ONOS Controllers

International audienceLogically-centralized but physically-distributed SDN controllers are mainly used in large-scale SDN networks for scalability, performance and reliability reasons. These controllers host various applications that have different requirements in terms of performance, availability and consistency. Current SDN controller platform designs employ conventional strong consistency models so that the SDN applications running on top of the distributed controllers can benefit from strong consistency guarantees for network state updates. However, in large-scale deployments, ensuring strong consistency is usually achieved at the cost of generating performance overheads and limiting system availability. That makes weaker optimistic consistency models such as the eventual consistency model more attractive for SDN controller platform applications with high-availability and scalability requirements. In this paper, we argue that the use of the standard eventual consistency models, though a necessity for efficient scalability in modern SDN systems, provides no bounds on the state inconsistencies tolerated by the SDN applications. To remedy that, we propose an adaptive consistency model for the distributed ONOS controllers following the notion of continuous and compulsory (per-controller) eventual consistency, where network application states adapt their eventual consistency level dynamically at run-time based on the observed state inconsistencies under changing network conditions. When compared to the ONOS approach to static eventual consistency, our approach proved efficient in minimizing state synchronization overheads while taking into account application state consistency SLAs and without compromising the application requirements of high-availability, in the context of large-scale SDN networks

Adaptive State Consistency for Distributed ONOS Controllers

International audienceLogically-centralized but physically-distributed SDN controllers are mainly used in large-scale SDN networks for scalability, performance and reliability reasons. These controllers host various applications that have different requirements in terms of performance, availability and consistency. Current SDN controller platform designs employ conventional strong consistency models so that the SDN applications running on top of the distributed controllers can benefit from strong consistency guarantees for network state updates. However, in large-scale deployments, ensuring strong consistency is usually achieved at the cost of generating performance overheads and limiting system availability. That makes weaker optimistic consistency models such as the eventual consistency model more attractive for SDN controller platform applications with high-availability and scalability requirements. In this paper, we argue that the use of the standard eventual consistency models, though a necessity for efficient scalability in modern SDN systems, provides no bounds on the state inconsistencies tolerated by the SDN applications. To remedy that, we propose an adaptive consistency model for the distributed ONOS controllers following the notion of continuous and compulsory (per-controller) eventual consistency, where network application states adapt their eventual consistency level dynamically at run-time based on the observed state inconsistencies under changing network conditions. When compared to the ONOS approach to static eventual consistency, our approach proved efficient in minimizing state synchronization overheads while taking into account application state consistency SLAs and without compromising the application requirements of high-availability, in the context of large-scale SDN networks

Distributed SDN Control: Survey, Taxonomy and Challenges

International audienceAs opposed to the decentralized control logic underpinning the devising of the Internet as a complex bundle of box-centric protocols and vertically-integrated solutions, the SDN paradigm advocates the separation of the control logic from hardware and its centralization in software-based controllers. These key tenets offer new opportunities to introduce innovative applications and incorporate automatic and adaptive control aspects, thereby easing network management and guaranteeing the user’s QoE. Despite the excitement, SDN adoption raises many challenges including the scalability and reliability issues of centralized designs that can be addressed with the physical decentralization of the control plane. However, such physically distributed, but logically centralized systems bring an additional set of challenges. This paper presents a survey on SDN with a special focus on the distributed SDN control. Besides reviewing the SDN concept and studying the SDN architecture as compared to the classical one, the main contribution of this survey is a detailed analysis of state-of-the-art distributed SDN controller platforms which assesses their advantages and drawbacks and classifies them in novel ways (physical and logical classifications) in order to provide useful guidelines for SDN research and deployment initiatives. A thorough discussion on the major challenges of distributed SDN control is also provided along with some insights into emerging and future trends in that area

Pilot-aided channel estimation and performance of ASTC-MIMO-OFDM system in a correlated Rayleigh frequency-selective channel

“This material is presented to ensure timely dissemination of scholarly and technical work. Copyright and all rights therein are retained by authors or by other copyright holders. All persons copying this information are expected to adhere to the terms and constraints invoked by each author's copyright. In most cases, these works may not be reposted without the explicit permission of the copyright holder." “Copyright IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.”Algebraic Space-Time Codes (ASTC) for MIMO systems are based on quaternion algebras. Thanks to their algebraic construction, the ASTC codes are full-rank, full-rate and have the non-vanishing determinant property. These codes have been proposed for MIMO flat fading channels in order to increase the spectral efficiency and to maximize the coding gain. The purpose of this work is to analyze the performance of the ASTC in a frequency selective Rayleigh channel. To deal with the frequency selectivity, we use the OFDM modulation. Methods for channel estimation using pilot symbol have been proposed. BER performances of ASTC-MIMO-OFDM system, under several propagation conditions, have been evaluated

Validation de la version française de l’échelle abrégée d’appréciation psychiatrique étendue avec ancrage, BPRS-E(A) [Validation of the French version of the expanded Brief Psychiatric Rating Scale with anchor BPRS-E(A)].

International audienceThe Brief Psychiatric Rating Scale was initially developed as a rapid method to assess symptom change in psychiatric inpatients of various diagnoses. The original version was expanded to an 18-item version and thereafter to a 24-item version to increase sensitivity to a broader range of psychotic and affective symptoms. The latest version of the expanded 24- item BPRS provides probe questions and detailed anchor points for the ratings for each item. Studies have shown the expanded and anchored 24-item BPRS to be a sensitive and effective measure of psychiatric symptoms with good interrater reliability that can be maintained over time. To our knowledge, there are eight published papers including factor analyses of the BPRS-E(A). While many similarities are evident between these studies, inconsistencies are apparent that may have been due to sample size, characteristics and / or methodological differences in the factor analysis computation. Among these studies, six provided a four-factor solution. There was no French version of this scale available. After its translation into French and back translation, we investigated the validity of the French BPRS-E(A) version. We carried out a component analysis on the data of 111 participants of various diagnoses, mostly hospitalised for a first psychotic episode, yielding to a three-factor solution (positive symptoms--disorganisation; depression-anxiety and negative symptoms). A good internal consistency and interrater reliability were found. These results confirm the psychometric value of the BPRS-E(A) in its French version. We compared those findings to earlier reports; similarities and differences are discussed