Network Topology (network + topology)

Distribution by Scientific Domains

Selected Abstracts

Optimization of Network Topologies for Service Providers in the Telecommunications Market

Dieter B. Pressmar
Following the deregulation of communications services, a growing number of commercial providers are offering global voice and data communications services via rented infrastructures. These service providers are faced with the challenge to determine cost-effective network-topologies, considering both the variety of contractual arrangements with national and international network providers, and the communication profiles of their customers. This paper introduces a planning model for solving the optimization problem outlined above. The model is evaluated on the basis of mixed-integer optimization. Practical deployment of this approach is discussed with respect to the run-time characteristics of the MIP solvers. Subsequently, a genetic algorithm is applied to the model and the results are compared to those of the mixed-integer optimization. [source]

Utilization of Self-Sorting Processes To Generate Dynamic Combinatorial Libraries with New Network Topologies

Isabelle Saur Dr.
Abstract The synthesis of water-soluble, organometallic macrocycles is described. They were obtained by self-assembly in reactions of the half-sandwich complexes [{Ru(C6H5Me)Cl2}2], [{Ru(p -cymene)Cl2}2], [{Rh(Cp)Cl2}2], and [{Ir(Cp*)Cl2}2] with the ligand 5-dimethylaminomethyl-3-hydroxy-2-methyl-4-(1H)-pyridone in buffered aqueous solution at pH 8. The structure of the Ru,(p -cymene) complex was determined by single-crystal X-ray crystallography. Upon mixing, these complexes undergo scrambling reactions to give dynamic combinatorial libraries. In combination with structurally related complexes based on amino-methylated 3-hydroxy-2-(1H)-pyridone ligands, an exchange of metal fragments but no mixing of ligands was observed. This self-sorting behavior was used to construct dynamic combinatorial libraries of macrocycles, in which two four-component sub-libraries are connected by two common building blocks. This type of network topology influences the adaptive behavior of the library as demonstrated in selection experiments with lithium ions as the target. [source]

Network Topology of a Hybrid Organic Zinc Phosphate with Bimodal Porosity and Hydrogen Adsorption,

Shu-Hao Huang
Füll,s auf! Ein nanoporöses hybrides Zinkphosphat (siehe Bild; ,[Zn2PO4]-Säulen in Dunkelblau) mit in die Struktur integrierten organischen Liganden (Amin: grau; Arylcarboxylat: cyan) weist eine ungewöhnliche bimodale Porosität und Wasserstoffgas-Speicherfähigkeit auf. [source]

What makes biochemical networks tick?

FEBS JOURNAL, Issue 19 2004
A graphical tool for the identification of oscillophores
In view of the increasing number of reported concentration oscillations in living cells, methods are needed that can identify the causes of these oscillations. These causes always derive from the influences that concentrations have on reaction rates. The influences reach over many molecular reaction steps and are defined by the detailed molecular topology of the network. So-called ,autoinfluence paths', which quantify the influence of one molecular species upon itself through a particular path through the network, can have positive or negative values. The former bring a tendency towards instability. In this molecular context a new graphical approach is presented that enables the classification of network topologies into oscillophoretic and nonoscillophoretic, i.e. into ones that can and ones that cannot induce concentration oscillations. The network topologies are formulated in terms of a set of uni-molecular and bi-molecular reactions, organized into branched cycles of directed reactions, and presented as graphs. Subgraphs of the network topologies are then classified as negative ones (which can) and positive ones (which cannot) give rise to oscillations. A subgraph is oscillophoretic (negative) when it contains more positive than negative autoinfluence paths. Whether the former generates oscillations depends on the values of the other subgraphs, which again depend on the kinetic parameters. An example shows how this can be established. By following the rules of our new approach, various oscillatory kinetic models can be constructed and analyzed, starting from the classified simplest topologies and then working towards desirable complications. Realistic biochemical examples are analyzed with the new method, illustrating two new main classes of oscillophore topologies. [source]

Performance analysis and interoperability issue of consolidation algorithms in point-to-multipoint ABR services

Naris Rangsinoppamas
Abstract In this paper we propose a performance analysis of the consolidation algorithms in point-to-multipoint ABR services in ATM network. We also present a new efficient consolidation algorithm, which overcomes some drawbacks of the previous works. The performance of the previous works and our proposed consolidation algorithms are analysed mathematically. General equations for calculating the response time and allowed cell rate of the source are derived. The derived equations show relatively good agreement with simulation results and can be applied for using in various network topologies. In addition, the interoperability issue in multicast available bit rate (ABR) is investigated. It is found that the branch points implemented with different consolidation algorithms can work interoperably. It is concluded from the simulation results that the performance of the point-to-multipoint ABR services is dominated by the most upper stream branch point (the nearest one to the source), therefore, in order to avoid the consolidation noise and get a faster response in the network, the most upper stream branch point has to be implemented with a high accuracy consolidation algorithm while the lower stream branch point(s) should be implemented with a fast response consolidation algorithm with a less degree of complexity. Copyright © 2002 John Wiley & Sons, Ltd. [source]

Efficient probabilistic reasoning in BNs with mutual exclusion and context-specific independence

Carmel Domshlak
Prior work has shown that context-specific independence (CSI) in Bayes networks can be exploited to speed up belief updating. We examine how networks with variables exhibiting mutual exclusion (e.g., "selector variables"), as well as CSI, can be efficiently updated. In particular, directed-path singly connected and polytree networks that have an additional common selector variable can be updated in linear time (given null and general conjunctive evidence, respectively), where quadratic time would be needed without the mutual exclusion requirement. The above results have direct applications, as such network topologies can be used in predicting the ramifications of user selection in some multimedia data browsing systems. © 2004 Wiley Periodicals, Inc. Int J Int Syst 19: 703,725, 2004. [source]

A tutorial on using genetic algorithms for the design of network topology

Bassam Al-Bassam
The design of network topology is an important part of network design, since network topology is directly associated with network operational behavior, capacity, reliability, and cost. This paper is a tutorial paper concerned with illustrating how the optimization capabilities of genetic algorithms can be used to design suitable network topologies considering basic topology problems. Simple genetic algorithms have been developed for the topology problem of mesh networks, considering single node and single link failure tolerance. The algorithms are based on criteria of two important measures: minimizing the length of communication links; and minimizing traffic flow through these links for given traffic loads. The first measure contributes to minimizing the cost of cabling, while the second measure contributes to minimizing the cost of link capacity. The work provides a useful approach and tools to network students and professionals concerned with the topology design of backbone networks. The developed software is made available on the Internet.,Copyright © 2006 John Wiley & Sons, Ltd. [source]

Efficient visualization of large routing topologies

Siew Cheong Au
As the Internet grows in size and complexity, network managers face a significant challenge in trying to understand the behaviour of routing protocols in large networks. In this paper, we present a tool called VLNT (visualizing large network topologies), which helps network managers to analyse complex routing topologies. A key contribution of our system is a novel hybrid layout algorithm, which significantly reduces the computation time required to layout large network topologies in comparison to conventional layout approaches. In addition our algorithm includes a novel termination criterion that avoids unnecessary iterations when optimizing the network layout. We demonstrate how the visualization features of VLNT can be used to analyse and improve BGP routing topologies, and provide examples using real-life routing data.,Copyright © 2004 John Wiley & Sons, Ltd. [source]

Study of the influence of bias and matching networks on the distortion and memory of FET-based power amplifiers

Jon Santiago
Abstract A study of the influence of bias and matching networks on the distortion and memory in FET-based power amplifiers has been carried out at the device and amplifier circuit levels. The study includes simulated and experimental results that allow us to identify effects produced by the introduction of particular biasing and matching networks in the power amplifier design. The influence of the bias point, as well as of biasing and matching network topologies on the nonlinear, short- and long-term memory behavior has been studied by means of simulations and measurements, using different power amplifier prototypes. © 2008 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2008. [source]

Resource management model and performance evaluation for satellite communications

Axel Jahn
Abstract Efficient resource management is mandatory to achieve maximum system capacity for next generation communications systems. Resource management deals with the available spectral band, time, power, and space for a transmission signal. It includes (i) the frequency planning, (ii) the selection of transmit power, and (iii) the assignment of the channels and access nodes to the users. The paper presents a generalized notation as well as graph algorithms for resource management problems. Impairment graphs can be used for frequency planning, whereas flow graphs are suitable for channel access problems. To evaluate the performance of the resource management, service criteria (such as blocking or the carrier to interference ratio C/I) or efficiency criteria (bandwidth requirements) can be derived from the graphs. The resource management techniques are applied to satellite networks with non-geostationary orbits yielding time-variant network topologies. As a simple example, the channel assignment and capacity optimization of the EuroSky Way system are shown. Furthermore, a comparison of fixed, dynamic and hybrid channel allocation schemes (FCA, DCA, HCA) for a typical MEO satellite scenario is given. Satellite diversity and its impact on bandwidth requirement and transmission quality is also examined. Finally, it is shown how spread spectrum systems can be investigated with the presented tools. Copyright © 2001 John Wiley & Sons, Ltd. [source]

Adversarial models for priority-based networks,

C. Àlvarez
Abstract In this article, we propose several variations of the adversarial queueing model and address stability issues of networks and protocols in those proposed models. The first such variation is the priority model, which is directed at static network topologies and takes into account the case in which packets can have different priorities. Those priorities are assigned by an adversary at injection time. A second variation, the variable priority model, is an extension of the priority model in which the adversary may dynamically change the priority of packets at each time step. Two more variations, namely the failure model and the reliable model, are proposed to cope with dynamic networks. In the failure and reliable models the adversary controls, under different constraints, the failures that the links of the topology might suffer. Concerning stability of networks in the proposed adversarial models, we show that the set of universally stable networks in the adversarial model remains the same in the priority, variable priority, failure, and reliable models. From the point of view of protocols (or queueing policies), we show that several protocols that are universally stable in the adversarial queueing model remain so in the priority, failure, and reliable models. However, we show that the longest-in-system (LIS) protocol, which is universally stable in the adversarial queueing model, is not universally stable in any of the other models we propose. Moreover, we show that no queueing policy is universally stable in the variable priority model. Finally, we analyze the problem of deciding stability of a given network under a fixed protocol. We provide a characterization of the networks that are stable under first-in-first-out (FIFO) and LIS in the failure model (and therefore in the reliable and priority models). This characterization allows us to show that the stability problem under FIFO and LIS in the failure model can be solved in polynomial time. © 2004 Wiley Periodicals, Inc. NETWORKS, Vol. 45(1), 23,35 2005 [source]

Gene networks and liar paradoxes

BIOESSAYS, Issue 10 2009
Mark Isalan
Abstract Network motifs are small patterns of connections, found over-represented in gene regulatory networks. An example is the negative feedback loop (e.g. factor A represses itself). This opposes its own state so that when ,on' it tends towards ,off' , and vice versa. Here, we argue that such self-opposition, if considered dimensionlessly, is analogous to the liar paradox: ,This statement is false'. When ,true' it implies ,false' , and vice versa. Such logical constructs have provided philosophical consternation for over 2000,years. Extending the analogy, other network topologies give strikingly varying outputs over different dimensions. For example, the motif ,A activates B and A. B inhibits A' can give switches or oscillators with time only, or can lead to Turing-type patterns with both space and time (spots, stripes or waves). It is argued here that the dimensionless form reduces to a variant of ,The following statement is true. The preceding statement is false'. Thus, merely having a static topological description of a gene network can lead to a liar paradox. Network diagrams are only snapshots of dynamic biological processes and apparent paradoxes can reveal important biological mechanisms that are far from paradoxical when considered explicitly in time and space. [source]

Guest-Dependent Flexible Coordination Networks with Fluorinated Ligands

Kayoko Kasai Dr.
Abstract Guest-dependent flexible coordination networks are formed from 1,4-bis(4-pyridylmethyl)tetrafluorobenzene (bpf), 4,4,-bis(4-pyridylmethyl)octafluorobiphenyl (bpfb), 2,6-bis(4-pyridylmethyl)hexafluoronaphthalene (2,6-bpfn), and 2,7-bis(4-pyridylmethyl)hexafluoronaphthalene (2,7-bpfn) with Cd(NO3)2 in the presence of various organic compounds. The reaction of bpf affords one-dimensional cyclic chains, two-dimensional rhombus grid sheets, and three-dimensional diamond frameworks with threefold interpenetration. The reaction of bpfb mainly affords two-dimensional rhombus grid sheets with twofold parallel interpenetration. The reaction of 2,6-bpfn affords a one-dimensional ladder and two-dimensional rhombus grid, twisted grid, and herringbone sheets. The reaction of 2,7-bpfn affords two-dimensional rhombus grid sheets and grid sheets with dumbbell-shaped cavities. This diversity of network topologies is induced by interactions between the guest molecules and the flexible ligand frameworks. [source]

A peer-to-peer decentralized strategy for resource management in computational Grids

Antonella Di Stefano
Abstract This paper presents a peer-to-peer (P2P) approach for the management, in a computational Grid, of those resources that are featured by numerical quantity and thus characterized by a coefficient of utilization, such as percentage of CPU time, disk space, memory space, etc. The proposed approach exploits spatial computing concepts and models a Grid by means of a flat P2P architecture consisting of nodes connected by an overlay network; such a network topology, together with the quantity of resource available in each node, forms a three-dimensional surface, where valleys correspond to nodes with a large quantity of available resource. In this scenario, this paper proposes an algorithm for resource discovery that is based on navigating such a surface, in search of the deepest valley (global minimum, that is, the best node). The algorithm, which aims at fairly distributing among nodes the quantity of leased resource, is based on some heuristics that mimic the laws of kinematics. Experimental results show the effectiveness of the algorithm. Copyright © 2006 John Wiley & Sons, Ltd. [source]

DSTRP: A new algorithm for high impedance fault detection in compensated neutral grounded M.V. power systems

F. Ruz
The detection algorithms depend on the network topology, the neutral grounding of the secondary winding in H.V./M.V. transformers being one of the main factors to consider. In compensated distribution systems, we need complex detection algorithms based on network parameters estimation. The neutral voltage is the most sensitive magnitude to detect an unusual situation but it is not a selective parameter. In this paper we present the new DSTRP that calculates the influence of each feeder asymmetry in neutral voltage. We also analyze the measurement errors influence that limits the sensitivity threshold of this algorithm. [source]

Producing Supramolecular Functional Materials Based on Fiber Network Reconstruction

Shaokun Tang
Abstract Here, the creation of new supramolecular functional materials based on the reconstruction of three-dimensional interconnecting self-organized nanofiber networks by a surfactant is reported. The system under investigation is N -lauroyl- L -glutamic acid di- n -butylamide in propylene glycol. The architecture of networks is implemented in terms of surfactants, e.g. sorbitan monolaurate. The elastic performance of the soft functional material is either weakened or strengthened (up to 300% for the current system) by reconstructing the topology of a fiber network. A topology transition of gel fiber network from spherulite-like to comb-like to spherulite-like is performed with the introduction of this surfactant. The Span 20 molecules are selectively adsorbed on the side surfaces of the crystalline fibers and promote the nucleation of side branches, giving rise to the transformation of the network architecture from spherulite-like topology to comb-like topology. At high surfactant concentrations, the occurrence of micelles may provide an increasing number of nucleation centers for spherulitic growth, leading to the reformation of spherulite-like topology. An analysis on fiber network topology supports and verifies a perfect agreement between the topological behavior and the rheological behavior of the functional materials. The approach identified in this study opens up a completely new avenue in designing and producing self-supporting supramolecular functional materials with designated macroscopic properties. [source]

Topology and Dependency Tests in Spatial and Network Autoregressive Models

Steven Farber
Social network analysis has been identified as a promising direction for further applications of spatial statistical and econometric models. The type of network analysis envisioned is formally identical to the analysis of geographical systems, in that both involve the measurement of dependence between observations connected by edges that constitute a system. An important item, which has not been investigated in this context, is the potential relationship between the topology properties of networks (or network descriptions of geographical systems) and the properties of spatial models and tests. The objective of this article is to investigate, within a simulation setting, the ability of spatial dependency tests to identify a spatial/network autoregressive model when two network topology measures, namely degree distribution and clustering, are controlled. Drawing on a large data set of synthetically controlled social networks, the impact of network topology on dependency tests is investigated under a hierarchy of topology factors, sample size, and autocorrelation strength. In addition, topology factors are related to known properties of empirical systems. El análisis de redes sociales ha sido y es una dirección prometedora en el avance de las aplicaciones de modelos econométricos y de estadística espacial. El tipo de análisis de redes que proponemos es idéntico al análisis de sistemas geográficos, ya que ambos miden la dependencia entre observaciones conectadas que conforman un sistema. Un punto importante que no ha sido investigado en este contexto es la potencial relación entre las propiedades topológicas de redes (o descripción de redes de sistemas geográficos) y las propiedades de los modelos y pruebas (tests) espaciales. El objetivo de este artículo es investigar (dentro del marco de simulaciones Monte Carlo), la capacidad que poseen las pruebas de dependencia espacial para identificar un modelo autorregresivo espacial/de redes, en los casos en los que dos medidas topológicas de redes (grado de distribución y transitividad) son controlados. Haciendo uso de una base de datos de redes sociales controladas sintéticamente, este artículo evalúa el impacto de la topología de redes en las pruebas de dependencia espacial. Dicho impacto es evaluado con respecto a variaciones en los factores topológicos, el tamaño de muestra, y los niveles de autocorrelación espacial. Adicionalmente, los factores topológicos son relacionados a propiedades conocidas de varios sistemas empíricos. [source]

NETCAP: a capacity planning tool for practical content distribution network designs

Sami J. Habib
Abstract This paper describes a capacity planning tool NETCAP, which is a prototype software program for automatically planning and integrating application-specific content-distribution networks (CDNs). The CDN integration problem consists of two problems: data management system design problem and network topology design problem. The data management system design problem comprises of the server placement and file allocation problems, where the network topology design problem involves determining the network topology with network technology considerations. The CDN integration problem has been formulated as an optimization problem; where the objective function is to optimize a network topology that satisfies both the servers' access requirements and clients' communications. An evolutionary technique is used in NETCAP to search the design space. The experimental results for a CDN integration problem described here demonstrate the effectiveness of NETCAP in finding good CDN designs from a large design space in a few minutes. Copyright © 2006 John Wiley & Sons, Ltd. [source]

Power aware scalable multicast routing protocol for MANETs

R. Manoharan
Abstract Multicasting is an effective way to provide group communication. In mobile ad hoc networks (MANETs), multicasting can support a wide variety of applications that are characterized by a close degree of collaboration. Since MANETs exhibit severe resource constraints such as battery power, limited bandwidth, dynamic network topology and lack of centralized administration, multicasting in MANETs become complex. The existing multicast routing protocols concentrate more on quality of service parameters like end-to-end delay, jitter, bandwidth and power. They do not stress on the scalability factor of the multicast. In this paper, we address the problem of multicast scalability and propose an efficient scalable multicast routing protocol called ,Power Aware Scalable Multicast Routing Protocol (PASMRP)' for MANETs. PASMRP uses the concept of class of service with three priority levels and local re-routing to provide scalability. The protocol also ensures fair utilization of the resources among the nodes through re-routing and hence the lifetime of the network is increased. The protocol has been simulated and the results show that PASMRP has better scalability and enhanced lifetime than the existing multicast routing protocols. Copyright © 2005 John Wiley & Sons, Ltd. [source]

An efficient MAC protocol for multi-channel mobile ad hoc networks based on location information

Yu-Chee Tseng
Abstract This paper considers the channel assignment problem in a multi-channel MANET environment. We propose a scheme called GRID, by which a mobile host can easily determine which channel to use based on its current location. In fact, following the GSM style, our GRID spends no communication cost to allocate channels to mobile hosts since channel assignment is purely determined by hosts' physical locations. We show that this can improve the channel reuse ratio. We then propose a multi-channel MAC protocol, which integrates GRID. Our protocol is characterized by the following features: (i) it follows an ,on-demand' style to access the medium and thus a mobile host will occupy a channel only when necessary, (ii) the number of channels required is independent of the network topology, and (iii) no form of clock synchronization is required. On the other hand, most existing protocols assign channels to a host statically even if it has no intention to transmit [IEEE/ACM Trans. Networks 1995; 3(4):441,449; 1993; 1(6): 668,677; IEEE J. Selected Areas Commun. 1999; 17(8):1345,1352], require a number of channels which is a function of the maximum connectivity [IEEE/ACM Trans. Networks 1995; 3(4):441,449; 1993; 1(6): 668,677; Proceedings of IEEE MILCOM'97, November 1997; IEEE J. Selected Areas Commun. 1999; 17(8):1345,1352], or necessitate a clock synchronization among all hosts in the MANET [IEEE J. Selected Areas Commun. 1999; 17(8):1345,1352; Proceedings of IEEE INFOCOM'99, October 1999]. Through simulations, we demonstrate the advantages of our protocol. Copyright © 2005 John Wiley & Sons, Ltd. [source]

An analytical simulator for deploying IP telephony

K. Salah
Deploying IP telephony or voice over IP (VoIP) is a major and challenging task. This paper describes an analytical design and planning simulator to assess the readiness of existing IP networks for the deployment of VoIP. The analytical simulator utilizes techniques used for network flows and queuing network analysis to compute two key performance bounds for VoIP: delay and bandwidth. The simulator is GUI-based and has an interface with drag-and-drop features to easily construct any generic network topology. The simulator has an engine that automates and implements the analytical techniques. The engine determines the number of VoIP calls that can be sustained by the constructed network while satisfying VoIP QoS requirements and leaving adequate capacity for future growth. As a case study, the paper illustrates how the simulator can be utilized to assess the readiness to deploy VoIP for a typical network of a small enterprise. We have made the analytical simulator publicly available in order to improve and ease the process of VoIP deployment. Copyright © 2008 John Wiley & Sons, Ltd. [source]

A novel steady-state genetic algorithm approach to the reliability optimization design problem of computer networks

A. M. Mutawa
This paper introduces the development and implementation of a new methodology for optimizing reliability measures of a computer communication network within specified constraints. A genetic algorithm approach with specialized encoding, crossover, and mutation operators to design a layout topology optimizing source-terminal computer communication network reliability is presented. In this work, we apply crossover at the gene level in conjunction with the regular chromosome-level crossover operators that are usually applied on chromosomes or at boundaries of nodes. This approach provides us with a much better population mixture, and hence faster convergence and better reliability. Applying regular crossover and mutation operators on the population may generate infeasible chromosomes representing a network connection. This complicates fitness and cost calculations, since reliability and cost can only be calculated on links that actually exist. In this paper, a special crossover and mutation operator is applied in a way that will always ensure production of a feasible connected network topology. This results in a simplification of fitness calculations and produces a better population mixture that gives higher reliability rates at shorter convergence times. Copyright © 2008 John Wiley & Sons, Ltd. [source]

A tutorial on using genetic algorithms for the design of network topology

Bassam Al-Bassam
The design of network topology is an important part of network design, since network topology is directly associated with network operational behavior, capacity, reliability, and cost. This paper is a tutorial paper concerned with illustrating how the optimization capabilities of genetic algorithms can be used to design suitable network topologies considering basic topology problems. Simple genetic algorithms have been developed for the topology problem of mesh networks, considering single node and single link failure tolerance. The algorithms are based on criteria of two important measures: minimizing the length of communication links; and minimizing traffic flow through these links for given traffic loads. The first measure contributes to minimizing the cost of cabling, while the second measure contributes to minimizing the cost of link capacity. The work provides a useful approach and tools to network students and professionals concerned with the topology design of backbone networks. The developed software is made available on the Internet.,Copyright © 2006 John Wiley & Sons, Ltd. [source]

An OPNET-based simulation approach for deploying VoIP

K. Salah
These days a massive deployment of VoIP is taking place over IP networks. VoIP deployment is a challenging task for network researchers and engineers. This paper presents a detailed simulation approach for deploying VoIP successfully. The simulation uses the OPNET network simulator. Recently OPNET has gained a considerable popularity in both academia and industry, but there is no formal or known approach or methodology as to how OPNET can be used to assess the support and readiness of an existing network in deploying VoIP. Our approach and work presented in this paper predict, prior to the purchase and deployment of VoIP equipment, the number of VoIP calls that can be sustained by an existing network while satisfying QoS requirements of all network services and leaving adequate capacity for future growth. As a case study, we apply the simulation approach on a typical network of a small enterprise. The paper presents a detailed description of simulation models for network topology and elements using OPNET. The paper describes modeling and representation of background and VoIP traffic, as well as various simulation configurations. Moreover, the paper discusses many design and engineering issues pertaining to the deployment of VoIP. These issues include characteristics of VoIP traffic and QoS requirements, VoIP flow and call distribution, defining future growth capacity, and measurement and impact of background traffic.,Copyright © 2006 John Wiley & Sons, Ltd. [source]

Mobility prediction and routing in ad hoc wireless networks

William Su
By exploiting non-random behaviors for the mobility patterns that mobile users exhibit, we can predict the future state of network topology and perform route reconstruction proactively in a timely manner. Moreover, by using the predicted information on the network topology, we can eliminate transmissions of control packets otherwise needed to reconstruct the route and thus reduce overhead. In this paper, we propose various schemes to improve routing protocol performances by using mobility prediction. We then evaluate the effectiveness of using mobility prediction via simulation. Copyright © 2001 John Wiley & Sons, Ltd. [source]

A feedback suppression algorithm for reliable satellite multicast based on spatial,temporal prediction of the satellite channel

Markos P. Anastasopoulos
Abstract The major problem limiting the scalability of large-scale multicast satellite networks is feedback implosion that arises whenever a large number of users simultaneously transmit feedback messages (FBMs) through the network, thus occupying a significant portion of satellite system resources. In satellite networks operating above 10,GHz, attenuation due to rain constitutes the dominant fading mechanism deteriorating the quality of service. In this paper, a novel scheme for providing large-scale reliable multicast services through a star-based geostationary satellite topology is presented incorporating accurate channel modeling of the propagation phenomena. The new protocol is based on the selection of an area representative that provides quick FBMs aiming at suppressing FBMs originating from the rest of the network users. The scheme provides a timely reaction to changes of either the channel conditions or the network topology by properly updating the selection of representatives. Through appropriate simulations, comparisons and examples it is demonstrated that the new approach suppresses FBMs very effectively. Copyright © 2009 John Wiley & Sons, Ltd. [source]

Multimedia satellite communications experiments to the international space station

Carlo Matarasso
Abstract In the present concept of the International Space Station ISS, it is planned to provide the communication services between the European Columbus module of the ISS and the ground equipment via the TDRS (tracking and data relay satellite) network provided by the American NASA (National Aeronautics and Space Agency). Especially because of its low uplink data rate, an unacceptable limitation of the communication services with Columbus is to be expected. In order to investigate possible improvements to this situation, the MEDIS proposal studies the feasibility of a 150 Mbps full duplex communication system to the Columbus Module on via two MEO satellites. A mixed topology with optical inter satellite links and Ka-band up/down-links shall be employed. Also possible liaisons with the Artemis mission will be considered. The MEDIS project is a collaboration of Astrium GmbH, which is project manager, Bosch SatCom, GMD-Fokus and the German Aerospace Center (DLR). The preparation phase of the project will investigate the concept of the communications infrastructure and the experiments that could be realized when the MEOs are launched. This stage has been completed in February 2001. An additional study phase followed, which will be completed with a presentation in July 2002. Briefly the following aspects have been studied by DLR in the preparation phase: the overall concept of experimental communication services for Columbus, the system requirements and network topology, the relevant protocol and hardware architectures, useful satellite constellations and link scenarios. Two phases could follow the preparation phase, in phase one the satellite will be launched and the link will be tested. In phase two the satellites will be connected to the ISS. Copyright © 2002 John Wiley & Sons, Ltd. [source]

Parallel Algorithms for Dynamic Shortest Path Problems

Ismail Chabini
The development of intelligent transportation systems (ITS) and the resulting need for the solution of a variety of dynamic traffic network models and management problems require faster-than-real-time computation of shortest path problems in dynamic networks. Recently, a sequential algorithm was developed to compute shortest paths in discrete time dynamic networks from all nodes and all departure times to one destination node. The algorithm is known as algorithm DOT and has an optimal worst-case running-time complexity. This implies that no algorithm with a better worst-case computational complexity can be discovered. Consequently, in order to derive algorithms to solve all-to-one shortest path problems in dynamic networks, one would need to explore avenues other than the design of sequential solution algorithms only. The use of commercially-available high-performance computing platforms to develop parallel implementations of sequential algorithms is an example of such avenue. This paper reports on the design, implementation, and computational testing of parallel dynamic shortest path algorithms. We develop two shared-memory and two message-passing dynamic shortest path algorithm implementations, which are derived from algorithm DOT using the following parallelization strategies: decomposition by destination and decomposition by transportation network topology. The algorithms are coded using two types of parallel computing environments: a message-passing environment based on the parallel virtual machine (PVM) library and a multi-threading environment based on the SUN Microsystems Multi-Threads (MT) library. We also develop a time-based parallel version of algorithm DOT for the case of minimum time paths in FIFO networks, and a theoretical parallelization of algorithm DOT on an ,ideal' theoretical parallel machine. Performances of the implementations are analyzed and evaluated using large transportation networks, and two types of parallel computing platforms: a distributed network of Unix workstations and a SUN shared-memory machine containing eight processors. Satisfactory speed-ups in the running time of sequential algorithms are achieved, in particular for shared-memory machines. Numerical results indicate that shared-memory computers constitute the most appropriate type of parallel computing platforms for the computation of dynamic shortest paths for real-time ITS applications. [source]

Design of irrigation water supply systems using the Q,C feasibility domain concept: I. Introduction and theory,

Gideon Sinai
irrigation; alimentation en eau; qualité de l'eau; systèmes d'irrigation; analyse de la contamination des réseaux; débit de l'eau Abstract The Q,C Feasibility Domain (QCFD) was defined and proposed as a tool for design of multiquality irrigation water supply systems. It determines all feasible combinations of water discharge and water quality, and can be represented by a point, a line, or an area in a diagram of water discharge versus solute flow rate (a Q,J diagram). The shape of the QCFD is the result of dilution of two or more flows from sources of different water quality. (assuming conservative substances) Several types of QCFDs were analyzed at sources, inner nodes of a network, and of consumer outlets. The effect of water discharge constraints (due to flow limitations in the network) on the QCFDs was formulated and analyzed. Computation of QCFDs of dilution junctions by vector addition of their inflows was described. The method was extended numerically to nonlinear mixing due to dependence of water salinity. Use of this method enables computation of QCFDs for inner nodes in networks, including dilution junctions. The effect of network topology and flow direction was discussed. Application and demonstration will follow in the next paper in this series. Copyright © 2008 John Wiley & Sons, Ltd. Le domaine de faisabilité Q,C (QCFD) a été défini et proposé comme un outil pour la conception des systèmes d'alimentation en eau d'irrigation de qualités multiples. Il détermine toutes les combinaisons faisables de débit et de qualité de l'eau, et peut être représenté par un point, une ligne, ou un secteur dans un diagramme débit-concentration (un diagramme de Q,J). La forme du QCFD est le résultat de la dilution de deux écoulements ou plus provenant de sources de qualité différente (en supposant la conservation des quantités). Plusieurs types de QCFD ont été analysés aux sources, n,uds, et sorties du réseau. L'effet des contraintes de débit (dues aux limitations dans le réseau) sur le QCFD a été formulé et analysé. On décrit le calcul de QCFD aux jonctions par l'addition des vecteurs d'apports. La méthode a été étendues numériquement aux mélanges non linéaires du fait de la liaison avec la salinité de l'eau. L'utilisation de cette méthode permet le calcul de QCFD aux n,uds intérieurs des réseaux, y compris les jonctions de dilution. L'effet de la topologie de réseau et du sens d'écoulement a été discuté. L'application et la démonstration suivront dans le prochain papier de cette série. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Mathematically Assessing the Consequences of Food Terrorism Scenarios

Y. Liu
ABSTRACT:, We derive mathematical expressions for the mean number of casualties resulting from a deliberate release of a biological or chemical agent into a food supply chain. Our analysis first computes the amount of contaminated food as a function of the network topology and vessel sizes in the food processing plant. A probabilistic analysis, in which each potential consumer of contaminated food has his own random purchase time, infectious dose, and incubation period, determines the number of people who consume enough tainted food to get infected or poisoned before the attack is detected and food consumption is halted. These simple formulas can be used by the U.S. government and the food industry to develop a rough-cut prioritization of the threats from food terrorism, which would be a 1st step toward the allocation of appropriate prevention and mitigation resources. [source]