The Role of Satellite Systems in Future Aeronautical Communications

Materials scienc

A cross-layer approach to enhance QoS for multimedia applications over satellite

The need for on-demand QoS support for communications over satellite is of primary importance for distributed multimedia applications. This is particularly true for the return link which is often a bottleneck due to the large set of end-users accessing a very limited uplink resource. Facing this need, Demand Assignment Multiple Access (DAMA) is a classical technique that allows satellite operators to offer various types of services, while managing the resources of the satellite system efficiently. Tackling the quality degradation and delay accumulation issues that can result from the use of these techniques, this paper proposes an instantiation of the Application Layer Framing (ALF) approach, using a cross-layer interpreter(xQoS-Interpreter). The information provided by this interpreter is used to manage the resource provided to a terminal by the satellite system in order to improve the quality of multimedia presentations from the end users point of view. Several experiments are carried out for different loads on the return link. Their impact on QoS is measured through different application as well as network level metrics

Chapter The Role of Satellite Systems in Future Aeronautical Communications

Materials scienc

On the Impact of Link Layer Retransmissions on TCP for Aeronautical Communications

In this article, we evaluate the impact of link layer retransmissions on the performance of TCP in the context of aeronautical communications.We present the architecture of aeronautical networks, which is manly driven by an important channel access delay, and the various retransmission strategies that can be implemented at both link and transport layers. We consider a worst case scenario to illustrate the benefits provided by the ARQ scheme at the link layer in terms of transmission delay.We evaluate the trade-off between allowing a fast data transmission and a low usage of satellite capacity by adjusting link layer parameters

Detection of Extracellular Phosphatase Activity of Heterotrophic Prokaryotes at the Single‐Cell Level by Flow Cytometry

Monitoring cell activity using substrates, which turn fluorescent due to biological activity, allows observing the presence and dynamics of sub‐populations, and provides a very valuable insight in ecological studies. The phosphatase substrate ELF97 phosphate (ELF‐P) is a useful tool to detect and quantify phosphatase activity (PA) of microorganisms at the single‐cell level. Most of the studies dealing with PA at the single‐cell level focus on autotrophic cells and only few concern heterotrophic prokaryotes (referred as bacteria in the text). While flow cytometry is a promising tool to assess the single‐cell analysis, only microscopy tools have been used until now to measure the ELF labeling associated with bacteria expressing PA. Therefore, we have developed a new protocol that enables the detection of ELF alcohol (ELFA), the product of ELF‐P hydrolysis, making possible the specific identification of bacteria showing PA using flow cytometry

Analysis of Phosphatase Activity from Aquatic Heterotrophic Bacteria at the Single Cell Level by Flow Cytometry: Example of a Development Achieved in the Regional Flow Cytometry Platform for Microbiology (Precym) Hosted by the Oceanology Center of Marseille

Marine microbes play a very important role in biogeochemical cycles. Because of their ­biodiversity, the diversity of their metabolisms, and their ­physiological heterogeneity (live/dead, active/inactive cells), it is necessary to study them at the single cell level. Flow cytometry has become a technique of choice to reach this goal, providing a fast, reliable, and multiparametric analysis of individual cells. However, the cost of such a technique makes flow cytometry out of reach for small institutes or laboratory units. It also requires a qualified and well-trained operator. To make this technique available to all the microbiologists of the “Provence Alpes Côte d’Azur” Region, a Regional Flow Cytometry Platform (PRECYM), hosted by the Oceanology Centre of Marseille, was created by a consortium of six laboratories. Its goal is to provide researchers with the possibility to consider new approaches and to guarantee optimal technical, economic, and scientific exploitations of these instruments. An example of such a technical development is described in this chapter. The ELF97 phosphatase substrate has been shown to be a performing tool to assess the phosphorus status of phytoplankton populations. Recently, this technique has been successfully applied to marine heterotrophic bacteria in culture samples in which ELF-labeling was observed by microscopy. In this study, we describe a new protocol for ­ELF-labeling of heterotrophic bacteria that allow detection by flow cytometry. ELF-labeled samples were stored in liquid nitrogen for up to 4 months before analysis without any significant loss of total or ELF-labeled cell abundance and of relative ELF ­fluorescence intensity. This method enables studying ectoenzymatic alkaline phosphatase activity of heterotrophic bacteria at the single cell level in environments even with low cell abundance. It has been successfully applied in both fresh and marine samples. This approach is set to improve our understanding of the physiological response of heterotrophic bacteria to phosphorus limitation

Phospholipid synthesis rates in the eastern subtropical South Pacific Ocean

Membrane lipid molecules are a major component of planktonic organisms and this is particularly true of the microbial picoplankton that dominate the open ocean; with their high surface-area to volume ratios, the synthesis of membrane lipids places a major demand on their overall cell metabolism. Specifically, the synthesis of cell membrane phospholipids creates a demand for the nutrient phosphorus, and we sought to refine our understanding of the role of phospholipids in the upper ocean phosphorus cycle. We measured the rates of phospholipid synthesis in a transect of the eastern subtropical South Pacific from Easter Island to Concepcion, Chile as part of the BIOSOPE program. Our approach combined standard phosphorus radiotracer incubations and lipid extraction methods. We found that phospholipid synthesis rates varied from less than 1 to greater than 200 pmol P L−1 h−1, and that phospholipid synthesis contributed between less than 5% to greater than 22% of the total PO43− incorporation rate. Changes in the percentage that phospholipid synthesis contributed to total PO43− uptake were strongly correlated with the ratio of primary production to bacterial production, which supported our hypothesis that heterotrophic bacteria were the primary agents of phospholipid synthesis. The spatial variation in phospholipid synthesis rates underscored the importance of heterotrophic bacteria in the phosphorus cycle of the eastern subtropical South Pacific, particularly the hyperoligotrophic South Pacific subtropical gyre