Amélioration des performances des systèmes à haut débit par une méthode de réduction de la diaphonie dans les câbles multiconducteurs de télécommunications

dsl (Digital Subscriber Line) technology provides low cost access to the consumers and sources of information for any one subscribing to classical analogical telephony. Indeed xdsl services enable high-speed digital transmission on conventional telephone lines. For sixteen years that the internet has risen, a great number of offers proposing increasing bit rate have been developed. Considering this increasing demand of high bit rate access, telecommunication operators perpetually work on finding solutions to optimize the copper lines channel. In that way, we present here a new method of crosstalk reduction in a multiconductor telecommunication cable. Interests and limitations of this method are presented and some simulations on a frequency band extending to 30 MHz will picture those theoretical results.     

Sur un modèle précis de la permittivité effective statique de la ligne microruban suspendue uniforme applicable à la technique des LECF

An accurate modelling of static effective permittivity ?_eff of a uniform suspended microstrip line is presented. This modelling is built on the variational method of moments (Vm). But this method requires high times computing for line of arbitrary shape, the width w of the uniforme line is reduced by the thickness h₁ of the air gap. The stratified microstrip patch antennas with an air gap, of arbitrary shape present this type of situation in the analysis with the method of the Finished Coupled Elementary Lines (F.c.e.l). A formal model proposed in this work, is free of integration and derivation, and is connecting the physical parameters (such as the relative permittivities ?_ri of the mediums i = 1,2…) with those of the geometrical structure (u = w/h₁ and m — h₂/h₁ where h₂ is the thickness of dielectric). The new model is valid for the full 1 ≤ ?_r ≤20, 0.5 ≤m≤ 1.5, 10^?2 ≤ult; +∞ ranges. The model is successfully compared with mesured data. This work give an accurate evaluation of ?_eff then the frequency dependence of the effective permittivity can be easily determined.     

Antennes en réseau à bas niveau de lobes secondaires par commutation de la loi d’alimentation. Optimisation par l’algorithme génétique

The paper presents an approach of synthesis from a new thinned microstrip antennas array with binary feeding law. The operating principle of this type of antennas array consists in establishing a number of elements supplied at the center of the array as well as the feeding of the extreme elements. The problem of synthesis with constraints is dealt with only by the research of the binary feeding law of the array eccentric elements, thus allowing to reduce to the maximum the sidelobes level. To solve this type of problem, the genetic algorithm is proposed.     

Conception des systèmes de protection des réseaux d’énergie à basse tension contre les impulsions électromagnétiques à haute altitude et la foudre

An exoatmospheric nuclear burst creates, by interactions photonsmatters, an electromagnetic field (electromagnetic pulse- Emp ). Its coupling with links,particularly overhead power ac supply lines of ground stations, induced a parasitic electrical pulse fast and intense, susceptible to perturb, damage or destroy down stream electrical or electronic equipments. Otherwise, the lightning can also generates, on the same links, electrical pulses with some différents characteristics. After a quick recall of the problem and specifications, the design methodology of protective devices against this pulse created by nemp and lightning, for ac power lines, is given. This method implement computed aided design (cad)and specific experimental ways. The protection efficiency is evaluated and validated, exemples of achievements described.     

Quelques résultats théoriques et expérimentaux sur la modulation par déplacement de fréquence à deux états et à phase continue d’indice 0,5 (MSK)

Minimumshift keying modulation seems to be very attractive for digital transmission systems in particular because an only small degradation can be expected on non linear channels. In the first part of this paper, some general features of the signal are recalled. After a theoretical method for the calculation of the bit error rate in presence of inter symbol interference is described; the results are compared with experimental values, obtained with a modulator demodulator build incnet laboratories in Lannion.     

L’imagerie médicale: du 2D au 3D, les problèmes particuliers posés par la reconstruction tomographique en tomographie par émission de positons

1917: Radon shows that a function can, be recontructed from its projections. For 60 years, the implementation of this theory to analogue data is a failure because the measurement noise is drastically amplified. 1970: Hounsifeld applies Radon’s theory to digital data and builds the first x-rayct scanner. His approach is generalized to many imaging modalities (nuclear medicine,mri). 2000: 2D x-ray digital detectors make it possible to develop volumetric x-rayct and Positron Emission Tomography (PET) is recognized as a major diagnostic tool in Oncology. For these purposes, fully 3D reconstruction algorithms are developed. This paper explains the technological and clinical interest of a fully 3D approach and the need to digitally process data to optimally use information. We focus on the relationship between the nature of data, the detector technology and available reconstruction methods.     

Une caractérisation non gaussienne et à longue mémoire du trafic Internet et de ses anomalies: validation expérimentale et application à la détection d’attaque de DDoS/Non Gaussian Long Memory Model for Internet Traffic: Experimental Validation and Application to DDoS Detection

Being now a mainstream communication, Internet is subject to many kinds of anomalies (failures, flash-crowds, attacks). In order to compare the statistics of normal traffic with traffic with anomalies, we collect both regular and anomalous traffic. The traffic is collected on the Renater network by the Metrosec project and we produce both Denial of Service (DoS) attacks with real attack softwares (TFN2k, TRIN00) aimed at various services (ICMP, SYN, UDP, TCP), and flash-crowd anomalies. We propose a multiresolution, non-Gaussian model with long memory and the corresponding estimators. It models, jointly at all aggregation levels, normal traffic, and also traffic containing anomalies. We show that the model enables to detect the anomalies in the traffic and distinguish between flash-crowd and Dos types of anomaly.