IEEE 802.11b Ad Hoc Networks: Performance Measurements

In this paper we investigate the performance of IEEE 802.11b ad hoc networks by means of an experimental study. An extensive literature, based on simulation studies, there exists on the performance of IEEE 802.11 ad hoc networks. Our analysis reveals several aspects that are usually neglected in previous simulation studies. Firstly, since different transmission rates are used for control and data frames, different transmission ranges and carrier-sensing ranges may exist at the same time in the network. In addition, the transmission ranges are in practice much shorter than usually assumed in simulation analysis, not constant but highly variable (even in the same session) and depends on several factors. Finally, the results presented in this paper indicate that for correctly understanding the behavior of an 802.11b network operating in ad hoc mode, several different ranges must be considered. In addition to the transmission range, the physical carrier sensing range is very important. The transmission range is highly dependent on the data rate and is up to 100 m, while the physical carrier sensing range is almost independent from the data rate and is approximately 200 m. Furthermore, even though stations are outside from their respective physical carrier sensing range, they may still interfere if their distance is lower than 350 m.Giuseppe Anastasi received the Laurea degree in Electronics Engineering and the Ph.D. degree in Computer Engineering both from the University of Pisa, Italy, in 1990 and 1995, respectively. He is currently an associate professor of Computer Engineering at the Department of Information Engineering of the University of Pisa. His research interests include architectures and protocols for mobile computing, energy management, QoS in mobile networks, and ad hoc networks. He was a co-editor of the book Advanced Lectures in Networking (LNCS 2497, Springer, 2002), and published more than 50 papers, both in international journals and conference proceedings, in the area of computer networking. He served in the TPC of several international conferences including IFIP Networking 2002 and IEEE PerCom 2003. He is a member of the IEEE Computer Society.Eleonora Borgia received the Laurea degree in Computer Engineering from the University of Pisa, Italy, in 2002. She is currently working toward her Ph.D. degree at the IIT Institute of the Italian National Research Council (CNR). Her research interests are in the area of the wireless and mobile networks with particular attention to MAC protocols and routing algorithms for ad hoc networks.Marco Conti received the Laurea degree in Computer Science from the University of Pisa, Italy, in 1987. In 1987 he joined the Italian National Research Council (CNR). He is currently a senior researcher at CNR-IIT. His research interests include Internet architecture and protocols, wireless networks and ad hoc networking, mobile computing, and QoS in packet switching networks. He co-authored the book Metropolitan Area Networks (Springer, London, 1997), and published in journal and conference proceedings more than 100 research papers related to design, modeling, and performance evaluation of computer-network architectures and protocols. He served as the technical program committee chair of the IFIP-TC6 conferences Networking 2002 and PWC 2003, and technical program committee co-chair of ACM WoWMoM 2002. He is serving as technical program committee co-chair of the IEEE Symposium on a World of Wireless, Mobile and Multimedia Networks (WoWMoM 2005). He served as guest editor for the Cluster Computing Journal (special issue on Mobile Ad Hoc Networking), IEEE Transactions on Computers (special issue on Quality of Service issues in Internet Web Services), and ACM/Kluwer Mobile Networks & Applications Journal (special issue on Mobile Ad hoc Networks). He is member of IFIP WGs 6.2, 6.3 and 6.8.Enrico Gregori received the Laurea in electronic engineering from the University of Pisa in 1980. He joined CNUCE, an institute of the Italian National Research Council (CNR) in 1981. He is currently a CNR research director. In 1986 he held a visiting position in the IBM research center in Zurich working on network software engineering and on heterogeneous networking. He has contributed to several national and international projects on computer networking. He has authored more than 100 papers in the area of computer networks and has published in international journals and conference proceedings and is co-author of the book Metropolitan Area Networks (Springer, London, 1997). He was the General Chair of the IFIP TC6 conferences: Networking2002 and PWC2003 (Personal Wireless Communications). He served as guest editor for the Networking2002 journal special issues on: Performance Evaluation, Cluster Computing and ACM/Kluwer Wireless Networks Journals. He is a member of the board of directors of the Create-Net association, an association with several Universities and research centres that is fostering research on networking at European level. He is on the editorial board of the Cluster Computing, of the Computer Networks and of the Wireless Networks Journals. His current research interests include: Wireless access to Internet, Wireless LANs, Quality of service in packet-switching networks, Energy saving protocols, Evolution of TCP/IP protocols.     

WCA: A Weighted Clustering Algorithm for Mobile Ad Hoc Networks

In this paper, we propose an on-demand distributed clustering algorithm for multi-hop packet radio networks. These types of networks, also known as ad hoc networks, are dynamic in nature due to the mobility of nodes. The association and dissociation of nodes to and from clusters perturb the stability of the network topology, and hence a reconfiguration of the system is often unavoidable. However, it is vital to keep the topology stable as long as possible. The clusterheads, form a dominant set in the network, determine the topology and its stability. The proposed weight-based distributed clustering algorithm takes into consideration the ideal degree, transmission power, mobility, and battery power of mobile nodes. The time required to identify the clusterheads depends on the diameter of the underlying graph. We try to keep the number of nodes in a cluster around a pre-defined threshold to facilitate the optimal operation of the medium access control (MAC) protocol. The non-periodic procedure for clusterhead election is invoked on-demand, and is aimed to reduce the computation and communication costs. The clusterheads, operating in dual power mode, connects the clusters which help in routing messages from a node to any other node. We observe a trade-off between the uniformity of the load handled by the clusterheads and the connectivity of the network. Simulation experiments are conducted to evaluate the performance of our algorithm in terms of the number of clusterheads, reaffiliation frequency, and dominant set updates. Results show that our algorithm performs better than existing ones and is also tunable to different kinds of network conditions.     

Energy-Efficient Multicasting of Session Traffic in Bandwidth- and Transceiver-Limited Wireless Networks

In this paper, we address the impact of resource limitations on the operation and performance of the broadcasting and multicasting schemes developed for infrastructureless wireless networks in our earlier studies. These schemes, which provide energy-efficient operation for source-initiated session traffic, were previously studied without fully accounting for such limitations. We discuss the node-based nature of the all-wireless medium, and demonstrate that improved performance can be obtained when such properties are exploited by networking algorithms. Our broadcast and multicast algorithms involve the joint choice of transmitter power and tree construction, and thus depart from the conventional approach that makes design choices at each layer separately. We indicate how the impact of limited frequency resources can be addressed. Alternative schemes are developed for frequency assignment, and their performance is compared under different levels of traffic load, while also incorporating the impact of limited transceiver resources. The performance results include the comparison of our algorithms to alternative link-based algorithms for broadcasting and multicasting.     

Divisible Load Scheduling in Systems with Limited Memory

In this work we consider scheduling divisible loads on a distributed computing system with limited available memory. The communication delays and heterogeneity of the system are taken into account. The problem studied consists in finding such a distribution of the load that the communication and computation time is the shortest possible. A new robust method is proposed to solve the problem of finding optimal distribution of computations on star network, and networks in which binomial trees can be embedded (meshes, hypercubes, multistage interconnections). We demonstrate that in many cases memory limitations do not restrict efficiency of parallel processing as much as computation and communication speeds.     

GPS-free Positioning in Mobile Ad Hoc Networks

We consider the problem of node positioning in ad hoc networks. We propose a distributed, infrastructure-free positioning algorithm that does not rely on GPS (Global Positioning System). Instead, the algorithm uses the distances between the nodes to build a relative coordinate system in which the node positions are computed in two dimensions. Despite the distance measurement errors and the motion of the nodes, the algorithm provides sufficient location information and accuracy to support basic network functions. Examples of applications where this algorithm can be used include Location Aided Routing [10] and Geodesic Packet Forwarding [2]. Another example are sensor networks, where mobility is less of a problem. The main contribution of this work is to define and compute relative positions of the nodes in an ad hoc network without using GPS. We further explain how the proposed approach can be applied to wide area ad hoc networks.     

Evaluation of Task Assignment Policies for Supercomputing Servers: The Case for Load Unbalancing and Fairness

While the MPP is still the most common architecture in supercomputer centers today, a simpler and cheaper machine configuration is appearing at many supercomputing sites. This alternative setup may be described simply as a collection of multiprocessors or a distributed server system. This collection of multiprocessors is fed by a single common stream of jobs, where each job is dispatched to exactly one of the multiprocessor machines for processing.The biggest question which arises in such distributed server systems is what is a good rule for assigning jobs to host machines: i.e. what is a good task assignment policy. Many task assignment policies have been proposed, but not systematically evaluated under supercomputing workloads.In this paper we start by comparing existing task assignment policies using a trace-driven simulation under supercomputing workloads. We validate our experiments by providing analytical proofs of the performance of each of these policies. These proofs also help provide much intuition. We find that while the performance of supercomputing servers varies widely with the task assignment policy, none of the above task assignment policies perform as well as we would like.We observe that all policies proposed thus far aim to balance load among the hosts. We propose a policy which purposely unbalances load among the hosts, yet, counter-to-intuition, is also fair in that it achieves the same expected slowdown for all jobs – thus no jobs are biased against. We evaluate this policy again using both trace-driven simulation and analysis. We find that the performance of the load unbalancing policy is significantly better than the best of those policies which balance load.     

QoS Support by the HiperLAN/2 MAC Protocol: A Performance Evaluation

HiperLAN/2 (HIgh PErformance Radio Local Area Network) is a new standard from ETSI (European Telecommunications Standards Institute) for high-speed wireless LANs, interconnecting portable devices to each other and to broadband core networks, based on different networking technologies such as IP, ATM, IEEE 1394, and others. This paper introduces the basic features of the HiperLAN/2 MAC protocol. It presents performance evaluation results, specifically related to the mechanisms provided by HiperLAN/2 to manage bandwidth resource requests and granting. These results are assessed in terms of their flexibility and efficiency in supporting delay sensitive traffic, such as voice and Web data traffic, which are expected to be transported by broadband wireless LANs.     

Algorithms in nature: the convergence of systems biology and computational thinking

Computer science and biology have enjoyed a long and fruitful relationship for decades. Biologists rely on computational methods to analyze and integrate large data sets, while several computational methods were inspired by the high‐level design principles of biological systems. Recently, these two directions have been converging. In this review, we argue that thinking computationally about biological processes may lead to more accurate models, which in turn can be used to improve the design of algorithms. We discuss the similar mechanisms and requirements shared by computational and biological processes and then present several recent studies that apply this joint analysis strategy to problems related to coordination, network analysis, and tracking and vision. We also discuss additional biological processes that can be studied in a similar manner and link them to potential computational problems. With the rapid accumulation of data detailing the inner workings of biological systems, we expect this direction of coupling biological and computational studies to greatly expand in the future.     

Making parks make a difference: poor alignment of policy,planning and management with protected-area impact,and ways forward

Policy and practice around protected areas are poorly aligned with the basic purpose of protection, which is to make a difference. The difference made by protected areas is their impact, defined in program evaluation as the outcomes arising from protection relative to the counterfactual of no protection or a different form of protection. Although impact evaluation of programs is well established in fields such as medicine, education and development aid, it is rare in nature conservation. We show that the present weak alignment with impact of policy targets and operational objectives for protected areas involves a great risk: targets and objectives can be achieved while making little difference to the conservation of biodiversity. We also review potential ways of increasing the difference made by protected areas, finding a poor evidence base for the use of planning and management ‘levers’ to better achieve impact. We propose a dual strategy for making protected areas more effective in their basic role of saving nature, outlining ways of developing targets and objectives focused on impact while also improving the evidence for effective planning and management.     

绿色基础设施研究进展

综述了绿色基础设施的起源发展,总结了推动其概念形成的发展脉络,分别是人居环境视角、生态保护视角和绿色技术视角。提出了绿色基础设施在空间、功能、要素上的内涵,阐述了它与生态系统服务的外延关系。通过文献研究,综述了绿色基础设施在气候变化、人体健康、空气质量、雨洪管理、公众认知和社区参与、评价研究等领域的国际研究进展。结合我国绿色基础设施的研究现状和问题进行评述,并对未来发展提出展望。     

A Web-Based Data Architecture for Problem-Solving Environments: Application of Distributed Authoring and Versioning to the Extensible Computational Chemistry Environment

Next-generation problem-solving environments (PSEs) promise significant advances over those now available. They will span scientific disciplines and incorporate collaboration capabilities. They will host feature-detection and other agents, allow data mining and pedigree tracking, and provide access from a wide range of devices. Fundamental changes in PSE architecture are required to realize these and other PSE goals. This paper focuses specifically on issues related to data management and recommends an approach based on open, metadata-driven repositories with loosely defined, dynamic schemas. Benefits of this approach are discussed, and the redesign of the Extensible Computational Chemistry Environment's (Ecce) data storage architecture to use such a repository is described, based on the distributed authoring and versioning (DAV) standard. The suitability of DAV for scientific data, the mapping of the Ecce schema to DAV, and promising initial results are presented.     

Fertiliser application positively affects plants performance but reduces seed viability in seashore mallow (Kosteletzkya pentacarpos): implication for biomass production and species conservation

Kosteletzkya pentacarpos seashore mallow, is recognised as an important plant species for several industrial applications, especially in North America and Asia. In Europe, seashore mallow is highly threatened and subject of conservation actions (e.g. reintroduction). In order to define appropriate multipurpose cultivation protocols, suitable for biomass production and for conservation purposes, we investigated the effect of varying levels of soil salt content and nutrient availability on seashore mallow. As expected, seashore mallow had the best performance in terms of growth rate, flowering and fruit production when fertiliser was applied, while salt had only limited effects. Interestingly, seeds produced by fertilised plants showed a lower germination performance and higher mortality than seeds developed from non‐fertilised plants. Our results highlight a trade‐off between parental plant growth and seed performance, the former being enhanced and the latter being reduced by fertiliser application. The causes of this trade‐off are related to a transgenerational maternal effect of fertiliser application that has important implications for seashore mallow cultivation. Biomass production benefits from fertiliser application but if the quality of seeds and the fitness of the next generation are major objectives (e.g. seed production and species conservation), fertiliser application is not recommended.     

DeconvTest: Simulation framework for quantifying errors and selecting optimal parameters of image deconvolution

Deconvolution is an essential step of image processing that aims to compensate for the image blur caused by the microscope's point spread function. With many existing deconvolution methods, it is challenging to choose the method and its parameters most appropriate for particular image data at hand. To facilitate this task, we developed DeconvTest: an open‐source Python‐based framework for generating synthetic microscopy images, deconvolving them with different algorithms, and quantifying reconstruction errors. In contrast to existing software, DeconvTest combines all components required to analyze deconvolution performance in a systematic, high‐throughput and quantitative manner. We demonstrate the power of the framework by using it to identify the optimal deconvolution settings for synthetic and real image data. Based on this, we provide a guideline for (a) choosing optimal values of deconvolution parameters for image data at hand and (b) optimizing imaging conditions for best results in combination with subsequent image deconvolution.     

Electrochemical Capacitors: Performance Metrics and Evaluation by Testing and Analysis

Electrochemical capacitors (i.e., supercapacitors) as energy storage technologies have attracted a lot of attention because of the increasing demand for efficient high-power delivery. Over the past decades, various advanced electrode materials and cell design have been developed to improve the performance of electrochemical capacitors. Hybrid capacitors and pseudocapacitors achieve much higher energy density due to their fast surface redox reactions. However, although all of them can be classified as electrochemical capacitors, different charge storage mechanisms alter their electrochemical characteristics. All the speculation and potential uncertainties, in the absence of standard performance metrics and test method, make it rather challenging to reliably evaluate the performance of various electrochemical systems, which hinder the development of advanced energy storage technologies and make the comparison between different energy storage systems (batteries, electrochemical capacitors, and hybrid devices) not so meaningful. In this regard, the uncertainties related to the performance metrics and evaluation of electrochemical capacitors are summarized and the ways in which the uncertainties can be minimized for both electric double layer capacitors (EDLCs) and hybrid supercapacitors is analyzed. It is hoped this effort will provide an accurate assessment method for determining the device properties to a high level of confidence and promote the standardization process of performance evaluation for the electrochemical energy storage systems.     

ELOVL2 controls the level of n-6 28:5 and 30:5 fatty acids in testis, a prerequisite for male fertility and sperm maturation in mice

ELOVL2 is a member of the mammalian microsomal ELOVL fatty acid enzyme family, involved in the elongation of very long-chain fatty acids including PUFAs required for various cellular functions in mammals. Here, we used ELOVL2-ablated (Elovl2(-/-)) mice to show that the PUFAs with 24-30 carbon atoms of the ω-6 family in testis are indispensable for normal sperm formation and fertility in male mice. The lack of Elovl2 was associated with a complete arrest of spermatogenesis, with seminiferous tubules displaying only spermatogonia and primary spermatocytes without further germinal cells. Furthermore, based on acyl-CoA profiling, heterozygous Elovl2(+/-) male mice exhibited haploinsufficiency, with reduced levels of C28:5 and C30:5n-6 PUFAs, which gave rise to impaired formation and function of haploid spermatides. These new insights reveal a novel mechanism involving ELOVL2-derived PUFAs in mammals and previously unrecognized roles for C28 and C30 n-6 PUFAs in male fertility. In accordance with the function suggested for ELOVL2, the Elovl2(-/-) mice show distorted levels of serum C20 and C22 PUFAs from both the n-3 and the n-6 series. However, dietary supplementation with C22:6n-3 could not restore male fertility to Elovl2(+/-) mice, suggesting that the changes in n-6 fatty acid composition seen in the testis of the Elovl2(+/-) mice, cannot be compensated by increased C22:6n-3 content.     

Macaque Pterygoid Muscles: Internal Architecture, Fiber length, and Cross-Sectional Area

Models of mastication require knowledge of fiber lengths and physiological cross-sectional area (PCS), a proxy for muscle force. I dissected 36 medial pterygoid and 36 lateral pterygoid muscles from 30 adult females of 3 macaque species (Macaca fascicularis, M. mulatta, M. nemestrina) using gross and chemical techniques and calculated PCS. These macaques have mechanically similar dietary niches and exhibit no significant difference in muscle architecture or fiber length. Fiber length does not scale with body size (mass) for either total pterygoid muscle or for medial pterygoid muscle mass. However, fiber length scales weakly with lateral pterygoid muscle mass. In each case, differences in PCS among species result from differences in muscle mass not fiber length. Medial pterygoid PCS scales isometrically with body size; larger animals have greater force production capabilities. Medial and lateral pterygoid PCS scale positively allometrically with facial size; individuals with more prognathic faces and taller mandibular corpora have greater PCS, and hence force, values. This positive allometry counters the less efficient positioning of masticatory muscles in longer-faced macaques. PCS is only weakly correlated with bone proxies previously used to estimate muscle force. Thus, predictions of muscle force from bone parameters will entail large margins of error and should be used with caution.     

Tahitala's Revenge: Monumental Architecture and the Great Canoe,Lomipeau

In discussing ways that monumental architecture symbolises political power, I suggest that vehicles are a unique category of monument construction. Their ability to travel, and to be used for politically momentous tasks, increases the prestige of the rulers who make use of them. The Lomipeau, a large double‐hulled canoe used by Tongan rulers in the sixteenth century, exemplifies the concept of a monumental vehicle. To demonstrate the concept, I examine the origins of the canoe, the archaeology of sites associated with its exploits, and its use in ceremonial voyages of national importance. Identifying vehicles as a distinctive class of monumental architecture in future studies will help us to comprehend the complex variety of ways leaders express power across cultures.     

Macaque Masseter Muscle: Internal Architecture,Fiber Length and Cross-Sectional Area

Models of mastication require knowledge of fiber lengths and physiological cross-sectional area (PCS): a proxy for muscle force. Yet only a small number of macaques of various species, ages, and sexes inform the previous standards for masseter muscle architecture. I dissected 36 masseters from 30 adult females of 3 macaque species—Macaca fascicularis, M. mulatta, M. nemestrina—using gross and chemical techniques and calculated PCS. These macaques have mechanically similar dietary niches and exhibit no significant difference in masseter architecture or fiber length. Intramuscular tendons effectively compartmentalize macaque masseters from medial to lateral. Fiber lengths vary by muscle subsection but are relatively conservative among species. Fiber length does not scale with body size (mass) or masseter muscle mass. However, PCS scales isometrically with body size; larger animals have greater force production capabilities. PCS scales positively allometrically with facial size; animals with more prognathic faces and taller mandibular corpora have greater PCS, and hence force, values. This positive allometry counters the less efficient positioning of masseter muscles in longer-faced animals. In each case, differences in PCS among species result from differences in muscle mass not fiber length. Masseter PCS is only weakly correlated with bone proxies previously used to estimate muscle force. Thus predictions of muscle force from bone parameters will entail large margins of errors and should be used with caution.