403 research outputs found
Écriture, sciences et numérique
No full textTable ronde animée par Éric Guichard (enssib, Ens-Ulm et CIPh, philosophe de l’internet). Avec :
- Patrick Flandrin (Ens-Lyon, Cnrs et Académie des sciences, physicien).
- Henri Desbois (université de Nanterre, géographe).
- Pierre Borgnat (Ens-Lyon et Cnrs, physicien)
Retrieving dynamic origin-destination matrices from Bluetooth data
No full textThe Bluetooth technology is being increasingly used, among the Automated Vehicle Identification Systems, to retrieve important information about urban networks. Because the movement of Bluetooth-equipped vehicles can be monitored, throughout the network of Bluetooth sensors, this technology represents an effective means to acquire accurate time dependant Origin Destination information. In order to obtain reliable estimations, however, a number of issues need to be addressed, through data filtering and correction techniques. Some of the main challenges inherent to Bluetooth data are, first, that Bluetooth sensors may fail to detect all of the nearby Bluetooth-enabled vehicles. As a consequence, the exact journey for some vehicles may become a latent pattern that will need to be estimated. Second, sensors that are in close proximity to each other may have overlapping detection areas, thus making the task of retrieving the correct travelled path even more challenging.\ud
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The aim of this paper is twofold: to give an overview of the issues inherent to the Bluetooth technology, through the analysis of the data available from the Bluetooth sensors in Brisbane; and to propose a method for retrieving the itineraries of the individual Bluetooth vehicles. We argue that estimating these latent itineraries, accurately, is a crucial step toward the retrieval of accurate dynamic Origin Destination Matrices
On the role of flows and sessions in Internet traffic modeling: an explorative toy-model
No full textIn this work we present a simple toy-model that
is able to explain certain empirical observations reported in a
set of previous papers by Hohn et al. [1]–[3] about the wavelet
spectrum of real traffic traces. Therein, the authors found that
the wavelet spectrum is substantially invariant to flow scrambling
and truncation. Such finding suggested that super-flow structures
above the transport layer — i.e., sessions — can be ignored for
modeling the packet arrival process. Based on the proposed toy model,
we offer an interpretation framework that goes in the
opposite direction, indicating that sessions, not transport-layer
flows, should be taken as the main structural entities in simplified
on/off models
A primal-dual algorithm for link dependent origin destination matrix estimation
No full textOrigin-Destination Matrix (ODM) estimation is a classical problem in transport engineering aiming to recover flows from every Origin to every Destination from measured traffic counts and a priori model information. Taking advantage of probe trajectories, whose capture is made possible by new measurement technologies, the present contribution extends the concept of ODM to that of Link dependent ODM (LODM). LODM also contains the flow distribution on links making specification of assignment models, e.g., by means of routing matrices, unnecessary. An original formulation of LODM estimation, from traffic counts and probe trajectories is presented as an optimisation problem, where the functional to be minimised consists of five convex functions, each modelling a constraint or property of the transport problem: consistency with traffic counts, consistency with sampled probe trajectories, consistency with traffic conservation (Kirchhoff’s law), similarity of flows having similar origins and destinations, and positivity of traffic flows. A proximal primal-dual algorithm is devised to minimise the designed functional, as the corresponding objective functions are not necessarily differentiable. A case study, on a simulated network and traffic, validates the feasibility of the procedure and details its benefits for the estimation of an LODM matching real-network constraints and observations
Retrieving trip information from a discrete detectors network : The case of Brisbane Bluetooth detectors
No full textAutomatic Vehicle Identification Systems are being increasingly used as a new source of travel information. As in the last decades these systems relied on expensive new technologies, few of them were scattered along a networks making thus Travel-Time and Average Speed estimation their main objectives. However, as their price dropped, the opportunity of building dense AVI networks arose, as in Brisbane where more than 250 Bluetooth detectors are now installed. As a consequence this technology represents an effective means to acquire accurate time dependant Origin Destination information. In order to obtain reliable estimations, however, a number of issues need to be addressed. Some of these problems stem from the structure of a network made out of isolated detectors itself while others are inherent of Bluetooth technology (overlapping detection area, missing detections,\...).\ud
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The aim of this paper is threefold: First, after having presented the level of details that can be reached with a network of isolated detectors we present how we modelled Brisbane's network, keeping only the information valuable for the retrieval of trip information. Second, we give an overview of the issues inherent to the Bluetooth technology and we propose a method for retrieving the itineraries of the individual Bluetooth vehicles. Last, through a comparison with Brisbane Transport Strategic Model results, we highlight the opportunities and the limits of Bluetooth detectors networks.\ud
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The aim of this paper is twofold. We first give a comprehensive overview of the aforementioned issues. Further, we propose a methodology that can be followed, in order to cleanse, correct and aggregate Bluetooth data. We postulate that the methods introduced by this paper are the first crucial steps that need to be followed in order to compute accurate Origin-Destination matrices in urban road networks
Mapping the use of Vélo'v
No full textJean-Baptiste Rouquier and Pierre Borgnat, "Cartographie des pratiques du Vélo’v : le regard de physiciens et d’informaticiens"*, Revue Sciences/Lettres [Online], 2 | 2014, 17 http://rsl.revues.org/487 October 2012 (accessed 7 March 2014). From the study of the data of Vélo'v, the Bicycle Sharing System of Lyon and Villeurbanne, several issues are discussed about how researchers from computer science and physics, and not from geography or cartography, were lead to draw maps representing thes..
Harmonic Analysis on directed graphs and applications : From Fourier analysis to wavelets
No full textLa recherche menée dans cette thèse a pour but de développer une analyse harmonique pour des fonctions définies sur les sommets d'un graphe orienté. À l'ère du déluge de données, de nombreuses données sont sous forme de graphes et données sur ce graphe. Afin d'analyser d'exploiter ces données de graphes, nous avons besoin de développer des méthodes mathématiques et numériquement efficientes. Ce développement a conduit à l'émergence d'un nouveau cadre théorique appelé le traitement de signal sur graphe dont le but est d'étendre les concepts fondamentaux du traitement de signal classique aux graphes. Inspirées par l'aspect multi échelle des graphes et données sur graphes, de nombreux constructions multi-échelles ont été proposé. Néanmoins, elles s'appliquent uniquement dans le cadre non orienté. L'extension d'une analyse harmonique sur graphe orienté bien que naturelle, s'avère complexe. Nous proposons donc une analyse harmonique en utilisant l'opérateur de marche aléatoire comme point de départ de notre cadre. Premièrement, nous proposons des bases de type Fourier formées des vecteurs propres de l'opérateur de marche aléatoire. De ces bases de Fourier, nous en déterminons une notion fréquentielle en analysant la variation de ses vecteurs propres. La détermination d'une analyse fréquentielle à partir de la base des vecteurs de l'opérateur de marche aléatoire nous amène aux constructions multi-échelles sur graphes orientés. Plus particulièrement, nous proposons une construction en trames d'ondelettes ainsi qu'une construction d'ondelettes décimées sur graphes orientés. Nous illustrons notre analyse harmonique par divers exemples afin d'en montrer l'efficience et la pertinence.The research conducted in this thesis aims to develop a harmonic analysis for functions defined on the vertices of an oriented graph. In the era of data deluge, much data is in the form of graphs and data on this graph. In order to analyze and exploit this graph data, we need to develop mathematical and numerically efficient methods. This development has led to the emergence of a new theoretical framework called signal processing on graphs, which aims to extend the fundamental concepts of conventional signal processing to graphs. Inspired by the multi-scale aspect of graphs and graph data, many multi-scale constructions have been proposed. However, they apply only to the non-directed framework. The extension of a harmonic analysis on an oriented graph, although natural, is complex. We, therefore, propose a harmonic analysis using the random walk operator as the starting point for our framework. First, we propose Fourier-type bases formed by the eigenvectors of the random walk operator. From these Fourier bases, we determine a frequency notion by analyzing the variation of its eigenvectors. The determination of a frequency analysis from the basis of the vectors of the random walk operator leads us to multi-scale constructions on oriented graphs. More specifically, we propose a wavelet frame construction as well as a decimated wavelet construction on directed graphs. We illustrate our harmonic analysis with various examples to show its efficiency and relevance
Revisiting an old friend: On the observability of the relation between Long Range Dependence and Heavy Tail
No full textInternational audienceTaqqu's Theorem plays a fundamental role in Internet traffic modeling, for two reasons: First, its theoretical formulation matches closely and in a meaningful manner some of the key network mechanisms controlling traffic characteristics; Second, it offers a plau- sible explanation for the origin of the long range dependence property in relation with the heavy tail nature of the traffic components. Numerous attempts have since been made to observe its predictions empirically, either from real Internet traffic data or from numerical simulations based on popular traffic models, yet rarely has this resulted in a satisfactory quantitative agreement. This raised in the literature a number of comments and questions, ranging from the adequacy of the theorem to real world data to the relevance of the statistical tools involved in practical analyses. The present contribution aims at studying under which conditions this fundamental theorem can be actually seen at work on real or simulated data. To do so, numerical simulations based on standard traffic models are analyzed in a wavelet framework. The key time scales involved are derived, enabling a discussion of the origin and nature of the difficulties encountered in attempts to empirically observe Taqqu's Theorem
Estimating link-dependent origin-destination matrices from sample trajectories and traffic counts
No full textIn transport networks, Origin-Destination matrices (ODM) are classically estimated from road traffic counts whereas recent technologies grant also access to sample car trajectories. One example is the deployment in cities of Bluetooth scanners that measure the trajectories of Bluetooth equipped cars. Exploiting such sample trajectory information, the classical ODM estimation problem is here extended into a link-dependent ODM (LODM) one. This much larger size estimation problem is formulated here in a variational form as an inverse problem. We develop a convex optimization resolution algorithm that incorporates network constraints. We study the result of the proposed algorithm on simulated network traffic
On Sampling Methods for Linear Scale-Invariant Systems
No full text4 pagesInternational audienceWe study a class of self-similar processes that are not stationary, nor have stationary increments. They are called Euler-Cauchy (EC) processes and are built as output of linear scale-invariant parametric systems. This article study several discretization methods of EC processes which are not bandlimited processes: direct sampling, bilinear transformation and approximation on fractional B-splines. For the three different methods, we obtain theoretical formulae and compute numerical realizations and properties
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