TOPNET: a tool for the visual simulation of communication networks

TOPNET is a software package for the simulation of communication networks that is being developed with the aim of exploiting as much as possible the visual aspects of a simulation experiment. The communication network topology and architecture are described by drawings, the system dynamics are represented with a class of timed Petri nets, the simulation experiment is controlled through menus and buttons activated with a mouse, and results are graphically displayed on the screen. With the help of an example, the main characteristics of the user interface and the internal architecture of TOPNET are described     

A method for soil environmental quality evaluation for management and planning in urban areas

Soil represents a complex medium, which makes it difficult to evaluate its quality. In the past, soil quality evaluation was biased towards agricultural production rather than for purposes related to the broad range of functions and services that it performs. Soil function and soil quality in the urban environment differ due to the different needs and roles of soil within the diversity of urban land uses. The quality of urban soil should be evaluated to support public services for good environmental quality management. Planners should also adjust their decisions towards more sustainable urban design. Simple and applicable soil quality evaluation methods accompanied by an operations toolkit that could be used by laypeople are needed.This paper discusses soil functions, soil quality indicators, pedotransfer functions, and urban soil quality. It presents an urban soil quality evaluation method for different land uses within one particular evaluation system. The calculation of three one-value measures of soil quality are introduced: index of soil quality (expresses soil quality/suitability for a particular land use), soil environmental quality index (environmental value of soil) in terms of performing the crucial ecological functions of soil, and land use change index (land use planning impact assessment on soil resources). The use of the method is described in two procedures: urban soil quality control and soil evaluation for urban planning.     

European soils overfertilized with phosphorus: Part 1. Basic properties

Soils overfertilized with phosphorus (P) are widespread in the European Union and there is consensus among soil scientists to better explore their potential to release phosphate. In this work we report the principal physical and chemical properties of twelve overfertilized benchmark soils of contrasting agricultural areas in Italy, Germany, Great Britain, and Spain. The criterion used to consider them as overfertilized was that the available P amount, measured by the regional soil P test, was at least twice as large as the accepted critical level for an average crop. The soils could be usefully divided into three groups, calcareous, slightly acidic and acidic based upon their basic chemical properties and reactions and proportion of the major P fractions (NaOH- plus citrate-bicarbonate-, citrate-bicarbonate-dithionite-, and HCl-extractable P). Six extraction procedures commonly used to evaluate potentially plant-available P (Olsen, anion-exchange resin (resin-), anion plus cation-exchange resin (resin±), Ca acetatelactate (CAL), water, and Fe oxide-impregnated paper strips (strip) were compared. The extractable P values by each method were correlated but the amount of P extracted varied and differed in the order water-P< Olsen-P< CAL-P< strip-P< resin(±)-P相似文献   

Quantitative PIXE analysis of impurities and their influence on the electrical properties of metal-free phthalocyanine

The impurity content of commercial metal-free phthalocyanine (H₂Pc) powder has been determined by proton-induced X-ray emission (PIXE), and high concentrations of some elements, including highly conducting iodine, were found. The effects on the impurity concentrations of heating the H₂Pc to 400° C for 1, 2, 3 and 4 h were investigated. This purification procedure leads to smaller although not negligible impurity content. Some electrical properties of H₂Pc were also measured as a function of purification time and it is shown that they are, as expected, greatly dependent on the impurity content.On leave from CEGEP André-Laurendeau, Montréal, Québec H8N 2J4, Canada.On leave from John Abbott College, Ste Anne De Bellevue, Québec H9X 3L9, Canada.     

Multichannel Local Area Network Protocols

Multiple-access protocols suitable for packet switching local area networks that use several parallel broadcast channels are presented and analyzed by evaluating their throughput and delay characteristics. The local network architecture is first described, then the network performances are evaluated considering a variety of multiple-access schemes that are derived from CSMA and CSMA-CD protocols, by extending the original single channel schemes to a multiple channel system. It is shown that significant performance improvements are achievable with the multiple channel option. Further advantages obtained with the proposed network architecture are a higher reliability and fault tolerance, and a modular network design that allows a gradual system growth following users' demands.     

Performance models of handover protocols and buffering policies inmobile wireless ATM networks

Due to the connection-oriented nature of the asynchronous transfer mode (ATM), one of the challenges in mobile wireless ATM (WATM) systems is the management of terminal handovers. When ATM connections are reestablished to follow terminals moving between areas covered by distinct base stations, seamless handover protocols are necessary to guarantee that ATM cells are delivered to terminals in the correct order, with cell loss rate and delay that satisfy the contracted quality of service (QoS). A promising approach to meet QoS requirements is based on the use of handover buffers at the (destination) base station, where transmitted cells are stored while the connection is being reestablished. Up to date, only simulation and experimental results are available to determine the performance of such protocols and buffering schemes. This paper presents the first attempt to develop an analytical modeling approach to estimate the performance of handover protocols making use of handover buffers at the base station. By incorporating several approximations, the proposed models allow designers to simultaneously take into account numerous system parameters, including handover buffer size, sustainable and peak cell rates of the ATM connection, terminal offered load, and time needed to reestablish the ATM connection. Analytical performance predictions are shown to closely match results of detailed simulation experiments, thus demonstrating the suitability of the proposed modeling framework for the selection of the most adequate solution to handle handover and provide the QoS required by end users     

A simulation study of delay in multichannel CSMA/CD protocols

Simulation is used to estimate the performance of media access control (MAC) protocols derived from carrier-sense multiple access with collision detection (CSMA/CD), and operating in local area networks comprising several parallel broadcast channels. The influence of possible protocol and system alternatives on the network performance is discussed, based on results of the packet delay average, variance, mean square, coefficient of variation, and histogram, as well as the packet rejection probability due to lack of buffer space. The delay incurred by multipacket messages is estimated, comparing the single channel to the multichannel option. Numerical results indicate that the multichannel option provides reductions in both the packet delay average and variance, even when stations are only able to simultaneously receive from a subset of channels     

Slot reuse in MAC protocols for MAN's

Most of the medium-access control (MAC) sublayer protocols recently proposed for application in very high-speed local and metropolitan area networks (LANs and MANs) are based on a slotted transmission scheme. Slotting guarantees very good throughput efficiencies, but further gains are possible if slots can be freed after reaching their destination, thus being available for repeated use as they propagate in the network. The authors describe a simulation-based quantitative analysis of the performance gains obtained by introducing slot reuse in distributed-queue dual-bus (DQDB) and cyclic reservation multiaccess (CRMA) MANs. CRMA-II, the latest evolution of CRMA, naturally incorporates slot reuse in the MAC protocol operations and is considered in the study for comparison purposes. In the case of the standard DQDB protocol, some existing proposals are considered. In the case of CRMA, both a previous IBM proposal and a novel approach, leading to very good performances, are studied     

Generalized stochastic Petri nets: a definition at the net leveland its implications

The class of Petri nets obtained by eliminating timing from generalized stochastic Petri net (GSPN) models while preserving the qualitative behavior is identified. Structural results for those nets are derived, obtaining the first structural analysis of Petri nets with priority and inhibitor arcs. A revision of the GSPN definition based on the structural properties of the models is presented. It is shown that for a (wide) class of nets, the definition of firing probabilities of conflicting immediate transitions does not require the information on reachable markings. Identification of the class of models for which the net-level specification is possible is also based on the structural analysis results. The procedure for the model specification is illustrated by means of an example. It is also shown that a net-level specification of the model associated with efficient structural analysis techniques can have a substantial impact on model analysis