Understanding differences in MAC performance

The suitability and performance of medium access protocols in vehicular environments is already being investigated over a long period of time. Carrier Sense Multiple Access (CSMA) has been shown to perform sufficiently well in most situations and being able to support safety and efficiency vehicular applications. Recently, Self-organizing Time-Division Multiple Access (STDMA) is being considered as an alternative and has been shown to coordinate the channel slightly better under certain situations. However, when comparing both protocols the precise details of radio and network conditions and parametrization of the protocols are decisive on which protocol takes a slight lead. Consequently, scenarios can be constructed quite easily in which one protocol is superior over the other one. The focus of this work is thus not to absolutely compare both protocols, but rather to understand the strengths and weaknesses of both protocols in certain situations. In particular, we consider i) to which degree hidden nodes influence the coordination ability, ii) how an extended carrier sensing range is beneficial and iii) how temporary fading influences the performance of both MAC protocols. Our results show that while an extended carrier sensing range is only beneficial for CSMA, the existence and severity of fading is far less detrimental for STDMA than for CSMA

An Experimental Testbed and Methodology for Characterizing IEEE 802.11 Network Cards

It has been observed that IEEE 802.11 commercial cards produced by different vendors show a different behavior in terms of perceived throughput or access delay. Performance differences are evident both when the cards contend alone to the channel, and when heterogeneous cards contend together. Since the performance disaligment does not disappear by averaging the environmental factors (such as propagation conditions, laptop models, traffic generators, etc), it is evident that the well known throughput-fairness property of the DCF protocol is not guaranteed in actual networks. In this paper we propose a methodological approach devised to experimentally characterize the IEEE 802.11 commercial cards thus understanding and predicting their performances in different network scenarios. We set up some specific experiments using a custom test equipment, able to classify the card behavior not only in terms of figures which are evident to the user perspective (such as the throughput), but also in terms of low-level channel access operations and delays. Our approach is able to detect potential hardware limits or not-standard MAC implementations, which severely affect the contending card performance

Theoretical investigaion of the performance of alternative aviation fuels in an aero-enginve combustion chamber

When considering alternative fuels for aviation, factors such as the overall efficiency of the combustion process and the levels of emissions emitted to the atmosphere, need to be critically evaluated. The physical and chemical properties of a fuel influence the combustion efficiency and emissions and therefore need to be considered. The energy content of a biofuel, which is influenced negatively by the presence of oxygen in the molecular structure (i.e. oxygenated chemical compounds), is relatively low when compared with that of conventional jet fuel. This means that the overall efficiency of the process will be different. In this paper two possible scenarios have been investigated in order to assess the potential to directly replace conventional jet fuel with Methyl Buthanoate - MB (a short chain FAME representing biofuel) and a synthetic jet fuel (FT fuel) using Computational Fluid Dynamics (CFD) modelling in a typical Modern Air-Spray Combustor (MAC). In addition the impact of fuel blending on the combustion performance has been investigated. Computational Fluid Dynamics (CFD) has been verified and validated over past decades to be a powerful design tool in industries where experimental work can be costly, hazardous and time consuming, to support the design and development process. With recent developments in processor speeds and solver improvements, CFD has been successfully validated and used as a tool for optimizing combustor technology. Combustion of each fuel is calculated using a mixture fraction/pdf approach and the turbulence-chemistry interaction has been modelled using the Laminar Flamelet approach. Detailed chemical reaction mechanisms, developed and validated recently by the authors for aviation fuel including kerosene, synthetic fuel and bio-aviation fuel have been employed in the CFD modelling. A detailed comparison of kerosene with alternative fuel performance has been made.

Portability, compatibility and reuse of MAC protocols across different IoT radio platforms

To cope with the diversity of Internet of Things (loT) requirements, a large number of Medium Access Control (MAC) protocols have been proposed in scientific literature, many of which are designed for specific application domains. However, for most of these MAC protocols, no multi-platform software implementation is available. In fact, the path from conceptual MAC protocol proposed in theoretical papers, towards an actual working implementation is rife with pitfalls. (i) A first problem is the timing bugs, frequently encountered in MAC implementations. (ii) Furthermore, once implemented, many MAC protocols are strongly optimized for specific hardware, thereby limiting the potential of software reuse or modifications. (iii) Finally, in real-life conditions, the performance of the MAC protocol varies strongly depending on the actual underlying radio chip. As a result, the same MAC protocol implementation acts differently per platform, resulting in unpredictable/asymmetrical behavior when multiple platforms are combined in the same network. This paper describes in detail the challenges related to multi-platform MAC development, and experimentally quantifies how the above issues impact the MAC protocol performance when running MAC protocols on multiple radio chips. Finally, an overall methodology is proposed to avoid the previously mentioned cross-platform compatibility issues. (C) 2018 Elsevier B.V. All rights reserved

Enhanced Web-Based Otitis Study Case vs Simple Paper-Case: Impact on Medical Student Objec-tive Structured Clinical Exam (OSCE) Performance

Background: Distance education methods have taken on greater importance as medical student education has moved off campus into the community. What the best methods are for conveying information to students at distant sites has not been determined. Objective: To determine if students at distant community sites who received an otitis media study case by e-mail that was enhanced with a referral to a web-based otitis study case, performed better on otitis OSCE stations than students who received the same case not enhanced with visuals or referrals to a web-based otitis case. Design/Methods: Students were randomized by community site to receive either the enhanced (E) or simple otitis study case (S). Students were e-mailed an otitis media study case during the 5th week of the rotation. Those randomized to the E-case received a case that started with a case scenario followed by a "Task" that instructed them to go to this web address: http://www.aap.org/otitismedia/www/vc/ear/index.cfm (American Academy of Pediatrics Otitis Web Site). They were then to select "Case 1" which was a continuation of the case scenario present on their e-mail. A list of learning objectives was also printed on the e-mail. Students receiving the S-case viewed the same case scenario and objectives, but were not instructed to go to the web page. All students rotated through two OSCE otitis stations. In the first station they interviewed a simulated patient(OSCE-SP) and counseled her on the management of her 12 month old with otitis. Within that station they viewed a video of a pneumoscopic exam of two ears, one ear with otitis and the other ear normal. At the 2nd otitis station the student presented the case to a faculty and was asked a series of questions about otitis media(OSCE-PR). Scores on the two stations were compared by group. Results: There were 198 students who took the OSCE. 178 (90%) responded to a survey that indicated they had opened and read the e-mailed case. There were 87 students in the E-group and 91 in the S-group. The mean ±s.d. OSCE-SP station score for the E-group was 72.6 ±12.0 vs 75.4 ±9.8 for the S-group, p=0.09. For the otitis presentation station the scores for the E-group and S-group were respectively, 82.9 ±9.6 and 83.7 ±9.4, p=0.55. Conclusions: These data suggest that enhanced visual distance education cases may offer no distinct advantage over simple paper-type case study guide

The long and winding road: Routine creation and replication in multi-site organizations

Prior research on organizational routines in the ‘capabilities’ literature has either studied how new routines are created during an exploratory process of variation and selection or how existing routines are replicated during a phase of exploitation. Few studies have analyzed the life cycle of new routine creation and replication as an integrated process. In an in-depth case study of England’s Highways Agency, this paper shows that the creation and replication of a new routine across multiple sites involves four sequential steps: envisioning, experimenting, entrenching and enacting. We contribute to the capabilities research in two ways: first, by showing how different organizational levels, capabilities and logics (cognitive and behavioural) shape the development of new routines; and second, by identifying how distinct evolutionary cycles of variation and selective retention occur during each step in the process. In contrast with prior research on replication as an exact copy of a template or existing routine, our study focuses on the replication of an entirely new routine (based on novel principles) that is adapted to fit local operational conditions during its large-scale replication across multiple sites. We draw upon insights from adjacent ‘practice research’ and suggest how capabilities and practice studies may complement each other in future research on the evolution of routines

A Construct-Modeling Approach to Develop a Learning Progression of how Students Understand the Structure of Matter

This paper builds on the current literature base about learning progressions in science to address the question, “What is the nature of the learning progression in the content domain of the structure of matter?” We introduce a learning progression in response to that question and illustrate a methodology, the Construct Modeling (Wilson, 2005) approach, for investigating the progression through a developmentally based iterative process. This study puts forth a progression of how students understand the structure of matter by empirically inter-relating constructs of different levels of sophistication using a sample of 1,087 middle grade students from a large diverse public school district in the western part of the United States. The study also shows that student thinking can be more complex than hypothesized as in the case of our discovery of a substructure of understanding in a single construct within the larger progression. Data were analyzed using a multidimensional Rasch model. Implications for teaching and learning are discussed—we suggest that the teacher’s choice of instructional approach needs to be fashioned in terms of a model, grounded in evidence, of the paths through which learning might best proceed, working toward the desired targets by a pedagogy which also cultivates students’ development as effective learners. This research sheds light on the need for assessment methods to be used as guides for formative work and as tools to ensure the learning goals have been achieved at the end of the learning period. The development and investigation of a learning progression of how students understand the structure of matter using the Construct Modeling approach makes an important contribution to the research on learning progressions and serves as a guide to the planning and implementation in the teaching of this topic. # 2017 Wiley Periodicals, Inc. J Res Sci Teach 54: 1024–1048, 201

Not All Wireless Sensor Networks Are Created Equal: A Comparative Study On Tunnels

Wireless sensor networks (WSNs) are envisioned for a number of application scenarios. Nevertheless, the few in-the-field experiences typically focus on the features of a specific system, and rarely report about the characteristics of the target environment, especially w.r.t. the behavior and performance of low-power wireless communication. The TRITon project, funded by our local administration, aims to improve safety and reduce maintenance costs of road tunnels, using a WSN-based control infrastructure. The access to real tunnels within TRITon gives us the opportunity to experimentally assess the peculiarities of this environment, hitherto not investigated in the WSN field. We report about three deployments: i) an operational road tunnel, enabling us to assess the impact of vehicular traffic; ii) a non-operational tunnel, providing insights into analogous scenarios (e.g., underground mines) without vehicles; iii) a vineyard, serving as a baseline representative of the existing literature. Our setup, replicated in each deployment, uses mainstream WSN hardware, and popular MAC and routing protocols. We analyze and compare the deployments w.r.t. reliability, stability, and asymmetry of links, the accuracy of link quality estimators, and the impact of these aspects on MAC and routing layers. Our analysis shows that a number of criteria commonly used in the design of WSN protocols do not hold in tunnels. Therefore, our results are useful for designing networking solutions operating efficiently in similar environments