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Concept# Couche physique

Résumé

vignette|Exemple d'interconnexions en niveau de couche physique
Dans le domaine des réseaux informatiques, la couche physique est la première couche du modèle OSI (de l'anglais Open Systems Interconnection, « Interconnexion de systèmes ouverts »).
Principes
La couche physique est chargée de la transmission effective des signaux électriques, radiofréquences ou optiques entre les interlocuteurs.
Son service est généralement limité à l'émission et la réception d'un bit ou d'un train de bits continu (notamment pour les supports synchrones comme la fibre optique).
Cette couche est chargée de la conversion entre bits et signaux électriques ou optiques.
Elle est en pratique toujours réalisée par un circuit électronique spécifique.
Le service de cette couche est approximativement défini par :

- la norme ISO 7498-1 ;
- précisée par ISO 10022 ;
- précisée par la recommandation X211 du CCITT.

Source officielle

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EE-206: Méthodes de mesure

Ce cours vise à transférer les concepts théoriques et les savoir-faire nécessaires à la réalisation de mesures de bonne qualité. Les contenus méthodologiques et technologiques seront exposés sous forme ex-cathedra et les savoir-faire seront entrainés lors des travaux pratiques.

MICRO-413: Advanced additive manufacturing technologies

Advanced 3D forming techniques for high throughput and high resolution (nanometric) for large scale production. Digital manufacturing of functional layers, microsystems and smart systems.

ME-201: Continuum mechanics

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Réseau informatique

thumb|upright|Connecteurs RJ-45 servant à la connexion des réseaux informatiques via Ethernet.
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Un réseau informatique ( ou DCN) est un ensemble d'équipements reliés entre eux pour échan

Ethernet

thumb|200px|Connecteur RJ45 pour Ethernet.
Ethernet est un protocole de communication utilisé pour les réseaux informatiques, exploitant la commutation de paquets. Il réalise les fonctions de la couc

Wi-Fi

thumb|Exemple d'application du Wi-Fi : impression sans fil.
Le (ou la) Wi-Fi, aussi orthographié wifi, est un ensemble de protocoles de communication sans fil régis par les

We consider a cross-layer design of wireless ad-hoc networks. Traditional networking approaches optimize separately each of the three layers: physical layer, medium access and routing. This may lead to largely suboptimal network designs. In this work, we propose a jointly optimal design of the three layers, and we show a significant performance improvement over the conventional approach. In the first part of this thesis, our goal is to select appropriate performance metrics for the joint optimization problem. To that respect, we analyze several existing rate-maximization performance metrics for wireless ad-hoc networks: maximizing the sum of rates, max-min fairness and proportional fairness. We first show with several examples that it is not clear if and how max-min fairness can be defined on each one of the examples. We give a formal proof that the max-min fair rate allocation exists on a large class of sets of feasible rates, among which are the feasible-rate sets of all known ad-hoc networking examples. We also give a centralized algorithm to compute the max-min fair allocation whenever it exists. Next, we compare the three metrics for ad-hoc scenarios in terms of efficiency and fairness. We prove that, similar to wired networking,maximizing the sum of rates leads to gross unfairness and starvation of all but the flows with the best channel conditions. We also prove that, contrary to wired networking, max-min fairness yields all flows having the same rate, thus causing large inefficiencies. These findings offer theoretical explanations to the inefficiency and unfairness phenomena previously observed in the contexts of 802.11 and UWB networks. Finally, we show that proportional fairness achieves a good trade-off between efficiency and fairness and is a good candidate for a rate-based performance metric in wireless ad-hoc settings. Having shown that the proportional fairness is an appropriate optimization objective for our problem, in the second part of the thesis we consider a joint optimization of rates, transmission powers, medium access (scheduling) and routing, where the goal of the optimization is to achieve proportional fairness. We first analyze networks built on physical layers that have a rate which is a linear function of SNR at the receiver (such as UWB or low-gain CDMA systems). We find that the optimal solution is characterized by the following principles: (1)Whenever a node transmits, it has to transmit with the maximum power; otherwise it has to remain silent (0 - PMAX power control). (2) Whenever data is being sent over a link, it is optimal to have an exclusion region around the destination, in which all nodes remain silent during transmission, whereas nodes outside of this region can transmit in parallel, regardless of the interference they produce at the destination. (3) When a source transmits, it adapts its transmission rate according to the level of interference at the destination due to sources transmitting in parallel. (4) The optimal size of this exclusion region depends only on the transmission power of the source of the link, and not on the length of the link nor on positions of nodes in its vicinity. As for the routing, we restrict ourselves to a subset of routes where on each successive hop we decrease the distance toward the destination. We also show that (5) relaying along a minimum energy and loss route is better than using longer hops or sending directly, which is not obvious since we optimize rate and not power consumption. Finally (6), the design of the optimal MAC protocol is independent of the choice of the routing protocol. We present a theoretical proof of optimality of 0 - PMAX power control, and the remaining findings we show numerically on a large number of random network topologies. Next, we consider narrow-band networks, where rate function is a strictly concave function of SNR. There, previous findings do not always hold. We show that in some cases, the size of the exclusion region and the optimal routing depend on transmission powers, and that the optimal MAC design depends on the choice of routing. Nevertheless, as we show with the example of 802.11 networks, a significant improvement over the existing 802.11 MAC can be achieved even with simpler, suboptimal strategies. Although this result is shown by simulations on a simplified model, it still gives further directions on how to improve the performance of RTS/CTS based protocols.

This thesis evaluates the potential of Ultra Wideband Impulse Radio for wireless sensor network applications. Wireless sensor networks are collections of small electronic devices composed of one or more sensors to acquire information on their environment, an energy source (typically a battery), a microcontroller to control the measurements, process the information and communicate with its peers, and a radio transceiver to enable these communications. They are used to regularly collect information within their deployment area, often for very long periods of time (up to several years). The large number of devices often considered, as well as the long deployment durations, makes any manual intervention complex and costly. Therefore, these networks must self-configure, and automatically adapt to changes in their electromagnetic environment (channel variations, interferers) and network topology modifications: some nodes may run out of energy, or suffer from a hardware failure. Ultra Wideband Impulse Radio is a novel wireless technology that, thanks to its extremely large bandwidth, is more robust to frequency dependent propagation effects. Its impulsional nature makes it robust to multipath fading, as the short duration of the pulses leads most multipath components to arrive isolated. This technology should also enable high precision ranging through time of flight measurements, and operate at ultra low power levels. The main challenge is to design a system that reaches the same or higher degree of energy savings as existing narrowband systems considering all the protocol layers. As these radios are not yet widely available, the first part of this thesis presents Maximum Pulse Amplitude Estimation, a novel approach to symbol-level modeling of UWB-IR systems that enabled us to implement the first network simulator of devices compatible with the UWB physical layer of the IEEE 802.15.4A standard for wireless sensor networks. In the second part of this thesis, WideMac, a novel ultra low power MAC protocol specifically designed for UWB-IR devices is presented. It uses asynchronous duty cycling of the radio transceiver to minimize the power consumption, combined with periodic beacon emissions so that devices can learn each other's wake-up patterns and exchange packets. After an analytical study of the protocol, the network simulation tool presented in the first part of the thesis is used to evaluate the performance of WideMac in a medical body area network application. It is compared to two narrowband and an FM-UWB solutions. The protocol stack parameters are optimized for each solution, and it is observed that WideMac combined to UWB-IR is a credible technology for such applications. Similar simulations, considering this time a static multi-hop network are performed. It is found that WideMac and UWB-IR perform as well as a mature and highly optimized narrowband solution (based on the WiseMAC ULP MAC protocol), despite the lack of clear channel assessment functionality on the UWB radio. The last part of this thesis studies analytically a dual mode MAC protocol named WideMac-High Availability. It combines the Ultra Low PowerWideMac with the higher performance Aloha protocol, so that ultra low power consumption and hence long deployment times can be combined with high performance low latency communications when required by the application. The potential of this scheme is quantified, and it is proposed to adapt it to narrowband radio transceivers by combining WiseMAC and CSMA under the name WiseMAC-HA.

The focus of this thesis is on the study of decentralized wireless multi-hop networks. We are particularly interested in establishing bounds on the traffic-carrying capabilities of wireless ad hoc networks and conditions on the scalability of such networks with node mobility. This theoretical investigation brings forward challenges on the design of such networks. This leads to a second part of this thesis that considers the feasibility and the design of physical layer architectures and schemes for decentralized wireless multi-hop networks. In the first part of this thesis, bounds on the capacity of wireless ad hoc networks with two types of non-uniform traffic patterns are established. We focus on the impact of traffic patterns where local communications predominate and show the improvement in terms of per user-capacity over ad hoc networks with unbounded average communication distances. We then study the capacity of hybrid wireless networks, where long-distance relaying is performed by a fixed overlay network of base-stations. We investigate the scaling of capacity versus the number of nodes and the density of base-stations in the area of the network. It is shown that the gain in performance is mainly due to the reduction in the mean number of hops from source to destination. Then, we investigate the impact of mobility on the ad hoc network capacity. We propose a set of necessary and sufficient conditions under which the long-term averaged throughput in an ad hoc network can remain constant as the number of nodes increases. The main idea is to use a connectivity graph that does not represent the actual physical network, but rather the available communication resources. This graph also allows to translate the problem of maximizing the throughput in ad hoc networks to the multi-commodity flow problem and directly apply related results. In contrast to these macroscopic studies, in the second part we focus on a microscopic analysis of ad hoc wireless networks. We are interested in characterizing the performance of decentralized multiple-access and retransmission schemes for multi-hop wireless networks with the goal of drawing conclusions on cross-layer design. We investigate different transmission strategies in order to assess the tradeoff between spatial density of communications and the range of each transmission. We present tools for characterizing the spatial throughput as a function of topological parameters (e.g node population density) and system parameters (propagation, bandwith etc). The results of this work also show that coding and retransmissions provide means of reliable communication coupled with a completely decentralized multiple-access strategy. Finally, an efficient protocol for the delay-limited fading Automatic Retransmission reQuest (ARQ) single relay channel is considered for cooperative communications. The proposed protocol exploits two kinds of diversity: (i) space diversity available through the cooperative (relay) terminal, which retransmits the source's signals, (ii) ARQ diversity obtained by leveraging the retransmission delay to enhance the reliability. The performance characterization is in terms of the achievable diversity, multiplexing gain and delay tradeoff for a high signal-to-noise ratio (SNR) regime. Then, by letting the source's power level vary over the retransmission rounds, we show the benefits of power control on the diversity.

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