On the Relationship between Transmission Power and Capacity of an Underwater Acoustic Communication Channel

The underwater acoustic channel is characterized by a path loss that depends not only on the transmission distance, but also on the signal frequency. As a consequence, transmission bandwidth depends on the transmission distance, a feature that distinguishes an underwater acoustic system from a terrestrial radio system. The exact relationship between power, transmission band, distance and capacity for the Gaussian noise scenario is a complicated one. This work provides a closed-form approximate model for 1) power consumption, 2) band-edge frequency and 3) bandwidth as functions of distance and capacity required for a data link. This approximate model is obtained by numerical evaluation of analytical results which takes into account physical models of acoustic propagation loss and ambient noise. The closed-form approximations may become useful tools in the design and analysis of underwater acoustic networks.Comment: 6 pages, 9 Figures, Awaiting acceptance to IEEE Oceans 08 (Conference), Kobe, Japa

Broadcasting in Time-Division Duplexing: A Random Linear Network Coding Approach

We study random linear network coding for broadcasting in time division duplexing channels. We assume a packet erasure channel with nodes that cannot transmit and receive information simultaneously. The sender transmits coded data packets back-to-back before stopping to wait for the receivers to acknowledge the number of degrees of freedom, if any, that are required to decode correctly the information. We study the mean time to complete the transmission of a block of packets to all receivers. We also present a bound on the number of stops to wait for acknowledgement in order to complete transmission with probability at least $1-\epsilon$, for any $\epsilon>0$. We present analysis and numerical results showing that our scheme outperforms optimal scheduling policies for broadcast, in terms of the mean completion time. We provide a simple heuristic to compute the number of coded packets to be sent before stopping that achieves close to optimal performance with the advantage of a considerable reduction in the search time.Comment: 6 pages, 5 figures, Submitted to Workshop on Network Coding, Theory, and Applications (NetCod 2009

Whether and Where to Code in the Wireless Relay Channel

The throughput benefits of random linear network codes have been studied extensively for wirelined and wireless erasure networks. It is often assumed that all nodes within a network perform coding operations. In energy-constrained systems, however, coding subgraphs should be chosen to control the number of coding nodes while maintaining throughput. In this paper, we explore the strategic use of network coding in the wireless packet erasure relay channel according to both throughput and energy metrics. In the relay channel, a single source communicates to a single sink through the aid of a half-duplex relay. The fluid flow model is used to describe the case where both the source and the relay are coding, and Markov chain models are proposed to describe packet evolution if only the source or only the relay is coding. In addition to transmission energy, we take into account coding and reception energies. We show that coding at the relay alone while operating in a rateless fashion is neither throughput nor energy efficient. Given a set of system parameters, our analysis determines the optimal amount of time the relay should participate in the transmission, and where coding should be performed.Comment: 11 pages, 12 figures, to be published in the IEEE JSAC Special Issue on Theories and Methods for Advanced Wireless Relay

Random Linear Network Coding For Time Division Duplexing: Energy Analysis

We study the energy performance of random linear network coding for time division duplexing channels. We assume a packet erasure channel with nodes that cannot transmit and receive information simultaneously. The sender transmits coded data packets back-to-back before stopping to wait for the receiver to acknowledge the number of degrees of freedom, if any, that are required to decode correctly the information. Our analysis shows that, in terms of mean energy consumed, there is an optimal number of coded data packets to send before stopping to listen. This number depends on the energy needed to transmit each coded packet and the acknowledgment (ACK), probabilities of packet and ACK erasure, and the number of degrees of freedom that the receiver requires to decode the data. We show that its energy performance is superior to that of a full-duplex system. We also study the performance of our scheme when the number of coded packets is chosen to minimize the mean time to complete transmission as in [1]. Energy performance under this optimization criterion is found to be close to optimal, thus providing a good trade-off between energy and time required to complete transmissions.Comment: 5 pages, 6 figures, Accepted to ICC 200

Random Linear Network Coding For Time Division Duplexing: When To Stop Talking And Start Listening

A new random linear network coding scheme for reliable communications for time division duplexing channels is proposed. The setup assumes a packet erasure channel and that nodes cannot transmit and receive information simultaneously. The sender transmits coded data packets back-to-back before stopping to wait for the receiver to acknowledge (ACK) the number of degrees of freedom, if any, that are required to decode correctly the information. We provide an analysis of this problem to show that there is an optimal number of coded data packets, in terms of mean completion time, to be sent before stopping to listen. This number depends on the latency, probabilities of packet erasure and ACK erasure, and the number of degrees of freedom that the receiver requires to decode the data. This scheme is optimal in terms of the mean time to complete the transmission of a fixed number of data packets. We show that its performance is very close to that of a full duplex system, while transmitting a different number of coded packets can cause large degradation in performance, especially if latency is high. Also, we study the throughput performance of our scheme and compare it to existing half-duplex Go-back-N and Selective Repeat ARQ schemes. Numerical results, obtained for different latencies, show that our scheme has similar performance to the Selective Repeat in most cases and considerable performance gain when latency and packet error probability is high.Comment: 9 pages, 9 figures, Submitted to INFOCOM'0

Manejos de realidad y sus condiciones de (im)posibilidad : tres vías hacia la sociedad del conocimiento

En este texto planteo diversas modos de caracterizar la sociedad del conocimiento, entendida como una hipótesis, un punto de partida, que puede ayudarnos a entender la realidad social contemporánea tras la crisis de la modernidad que es visible a partir de la segunda mitad del siglo XX. Para ello, coloco en el centro de la discusión el concepto manejos de realidad, definido como todas esas prácticas semiótico-materiales por las que se intenta influir de alguna manera sobre el mundo que nos rodea, centrándome, a nivel epistemológico, en aquellos manejos de realidad de gran envergadura: las prácticas científico-políticas. De estas reflexiones iniciales extraigo tres caminos concretos que nos pueden llevar a la materialización (o no) de la hipótesis sociedad del conocimiento, ninguno de ellos excluyente, al contrario, solapándose en muchos puntos: las vía sociológico-moderna, la articulatorio-actancial y la genealógica.The "knowledge society" hypothesis is a useful heuristic for understanding contemporary society after the crisis of modernity that arose in the second half of the 20th century. I shall examine the ways in which reality can be handled (reality handlings), that is, all those materialsemiotic practices through which we try to influence the world that surrounds us. I shall focus especially on the epistemological problems of current scientific and political practices. I conclude with three specific, but not mutually exclusive avenues that might (or might not) help us fully articulate the idea of the "knowledge society": the modern-sociological, the actantial-articulative, and the genealogical

Hacer sociología a través de la teoría del actor-red: de la cartografía impresionista a la suciedad de las mediaciones

En este artículo intento proponer un esbozo de ciertos problemas que me he encontrado en el transcurso de mi investigación, dentro de la disciplina de la sociología, cuando he intentado seguir algunas de las principales tramas del complejo entramado de la teoría del actor-red (ANT), así como de los subterfugios que planteo para bordearlos. Dos son los problemas y dos los posibles subterfugios para abordarlos. Por un lado, me encuentro con el problema de la envergadura y fidelidad de las descripciones que exige la ANT, muy localizadas y minuciosas, que choca con los requerimientos de la teoría sociológica que busca regularidades y abstracciones. El subterfugio que planteo es el de la "cartografía impresionista". Por otro lado, me topo con la dificultad de la irreversibilidad de las mediaciones y las higienizantes representaciones que lleva a cabo en ocasiones la ANT. Para combatir esto, utilizo el subterfugio de adoptar la premisa de la "inevitable suciedad de las mediaciones"In this paper I try to outline the existence of certain problems I have found during my research work, within sociology discipline, when it comes to follow some of the main threads of the complex fabric that constitutes the Actor-Network Theory (ANT). In the same way, I suggest some possible subterfuges to go around those problems. Two are the problems and two are the subterfuges as well destined to tackle them. On the one hand, I face the problem of the magnitude and fidelity that ANT's descriptions demand, thoroughly detailed and local, which clashes with the requirements of sociological theory that seeks abstractions and regularities. The subterfuge I propose is the one called "impressionist cartography". On the other hand, I bump into the difficulty of the irreversibility of mediations and the sanitized representations carried out, sometimes, by ANT. In order to fight this, I use the subterfuge oriented to adopt the premise of the "inevitable dirtiness of mediations

El entramado experto en la construcción de identidades en la contemporaneidad

In this article I explore how the expert knowledge participates in the construction of identities in contemporary societies. After alluding to social figures such as the (pre)modern intellectual along with other contemporary actors, I describe the current social context as marked by the political rationalities of advanced liberalism, ethico-politics and the emergence and consolidation of an expert culture. From there, three consequences can be drawn: firstly, the most important knowledge agent in contemporary societies is collective, the network of experts; secondly, those networks of experts have a great capacity to produce transformations thanks to their grade of permeation throughout social fabric; thirdly, the network of experts acts as a social reality production agent similar to a dispositif that answers to a historical matter of urgency – the necessity of constructing meaning in the contemporaneity – but without a particular willing that guides its strategic orientation.En este artículo exploro cómo el saber experto participa en la construcción de identidades en la contemporaneidad. Partiendo de un breve desarrollo histórico en el que conecto figuras como las del intelectual (pre)moderno con figuraciones del saber experto contemporáneo, llego a la descripción de una sociedad marcada por las racionalidades políticas del liberalismo avanzado, la etopolítica y la emergencia y consolidación de una cultura experta. De aquí se derivan tres conclusiones: primero, el principal agente del saber en la contemporaneidad es colectivo, el entrado experto; segundo, estos entramados expertos gozan de una gran capacidad de mediación gracias a su grado de penetración en todos los flancos del tejido social; tercero, el entramado experto actúa como agente constructor de realidad social equiparándose a un dispositivo que responde a una urgencia histórica —la necesidad de construir sentido en la contemporaneidad— pero sin una particular volición que guie su orientación estratégica