4,114 research outputs found
Motivi letterari nei libri per l’infanzia di Paolo Di Paolo
No full textThe article, organised in the form of a discussion, aims to analyse the themes and motifs of Paolo Di Paolo’s works that are addressed to young audiences. The author has recently published a large number of volumes directed just at younger readers. When beholding the titles that constitute the author’s bibliography, the reader is struck by Di Paolo’s predisposition to transform literary classics: as much in Giacomo il signor bambino as in the edition of the Divina Commedia, the author’s goal is to try to reach his new readers. Therefore, Di Paolo proves to be a prolific author of the genre; in fact, in his bibliography, volumes of fairy tales with a classic slant, such as La mucca volante, are listed as well.L’articolo contiene un’analisi di temi e di motivi delle opere di Paolo Di Paolo dirette al pubblico più piccolo. L’autore negli ultimi anni ha pubblicato un cospicuo numero di volumi indirizzati, infatti, proprio ai lettori più giovani. Ciò che stupisce scorgendo i titoli che costituiscono la bibliografia dell’autore, è la predisposizione a trasformare i classici della letteratura: tanto in Giacomo il signor bambino quanto nell’edizione della Divina Commedia l’obiettivo è cercare di raggiungere i lettori più giovani. Di Paolo si dimostra, quindi, un autore prolifico del genere, tant’è che nell’elenco non mancano volumi fiabeschi dal taglio classico come La mucca volante. Analizzare le opere giovanili dello scrittore costituisce un fatto inedito
MAC and Routing in Wireless Ad Hoc and Sensor Networks: a Cross-Layer Approach
No full text"Respecting a network architecture yields better guarantees of reliability, longevity, and modularity, but much better performance can be potentially achieved through wisely chosen violations to that architecture."
In a nutshell, this is the message of a recent paper (see [1] in Chapter 1) outlining pros, cons, consequences and risks of cross-layer design, a currently widely adopted paradigm for wireless networks. The increasing attention and momentum that cross-layer design has recently gained is explained by its potential advantages, namely the network performance improvements that can be achieved, especially under stringent constraints in terms of hardware and computational power. A short definition of cross-layer design identifies this technique as a means of performing information exchange among different layers in the classic ISO/OSI protocol stack model, and of harvesting the potential design opportunities and performance improvements that follow. However, by breaking the modular structure of the ISO/OSI stack, one may encounter two orders of problems: first, unwanted interactions may be introduced; second, the generality of the architecture is lost. While a careful design phase can overcome the first problem, the second one requires stronger efforts. In fact, any cross-layer design is inherently specific to the type of network and scenario it is applied to, and limits the performance improvements to that specific type. Due to this loss of generality, the same protocol hardly offers the same results as applied to different types of networks.
In this Thesis, we will show two relevant examples of successful cross-layer design applied to two very different kinds of wireless networks. The first example deals with ad hoc networks with multiple antennas and MIMO communications. Due to the specific scenario, it can be assumed that nodes have high throughput needs and can accept to, e.g., spend more energy in performing the processing required by MIMO signaling in order to achieve greater communication speed. The analysis of this scenario is focused on the design of a novel PHY-aware MAC protocol for MIMO ad hoc networks and on the analysis and optimization of its performance.
A completely different point of view is required instead to handle wireless sensor networks (WSNs), the second type of wireless network considered in this Thesis. Peculiar to WSNs are the usually low communication speed, processing capabilities and energy supplies. Among others, these constraints do not allow complicated signal processing or the storage of a large amount of information. In turn this requires to limit the buffer of the nodes (the sensors hence have only a limited packet queue) and also to design protocols whose "state" can be summarized and efficiently held in the limited memory of the sensors. In the Thesis, we will provide an in-depth analysis of a geographic MAC/routing protocol for WSNs, and build upon it to yield a complete solution for channel access and packet forwarding. Part of this study is the design of an algorithm to route packets around connectivity holes, where geographic protocols alone fail.
In the appendix, the same cross-layer design concepts are applied to wireless underwater networks, a particular instance of WSNs where communications take place over long delay, low rate acoustic channels, and incur strongly frequency-dependent channel effects. All results (analysis, simulations, comparisons with other solutions) show that cross-layer design is in fact very effective, and offers valuable opportunities to leverage specific features that can lead to performance improvements in each kind of wireless network
Protocol design issues in underwater acoustic networks
No full textIn this paper we discuss issues related to the design of underwater acoustic network protocols which are tailored around, and leverage on, the differences between underwater acoustics and terrestrial radio. These differences span physical propagation and energy consumption, and influence the design of medium access control, routing and topology management.
By first reviewing a simple model for underwater sound propagation and hardware energy consumption, we introduce a set of solutions which explicitly account for, or make use of, the longer propagation delays of acoustic waves in the water with respect to radio waves in the air, and the different ratio between transmit and receive energy consumption offered by underwater transducers. These protocols deal with the problems of efficiently scheduling transmissions in a fixed 3D deployment, of optimizing the use of energy by choosing the best mechanisms for topology management, and of choosing the best hop length over a multihop path.
We then review some more realistic underwater sound propagation behaviors, and detail their consequences on the simulation of MAC protocols for underwater networks. Finally, we briefly discuss the currently available underwater communications hardware (including both commercial and research modems) and comment on which paradigms are currently realizable, with special regard to those requiring the adaptability of transmit power and frequency
Exploiting the Bandwidth-Distance Relationship in Underwater Acoustic Networks
No full textIn this paper we study the relationship between effective use of the available bandwidth, energy consumption, and transmission delay in a Underwater Wireless Acoustic Sensor Network (UWASN). We compare different solutions to transport data to a sink node, namely multihop transmissions through multiple relays and the use of direct relay-to-sink links that require more power but reach the sink in one hop. We also address the effects of different error control policies. Our analysis shows that the energy consumption can be traded off for delay by choosing different policies and by varying some key parameters in each policy
The effect of different attenuation models on the performance of routing in shallow-water networks
No full textSeveral models can be used to determine the attenuation incurred by sound waves as they travel under water. A trade-off between accuracy and complexity is observed in this respect: the most accurate results are typically yielded by some form of numerical solution to the sound propagation equations, but at the price of high complexity; conversely, simple link budget equations are typically valid only as a first-order approximation, but are much simpler to evaluate. When such different models are applied to network simulations, both the accuracy and the complexity of the chosen model can have a big impact on the simulation time and on the significance of the outcomes. In this paper, we present a comparison among different models of increasing computational complexity for simulating the transmission loss of underwater acoustic channels, when applied to the simulation of multi-hop underwater acoustic networks. All models have been integrated in the DESERT Underwater framework, which is based on the ns2/MIRACLE network simulator. Our results show that the model and its parameters have in fact a big impact on network simulation results in different network topologies, which is consistent with the findings reported by some other papers that recently appeared in the open literature. Our results also show that in some instances simple propagation models provide a useful approximation if their parameters are properly chosen
Coastal patrol and surveillance networks using AUVs and delay-tolerant networking2012 Oceans - Yeosu
No full textIn this paper, we consider a coastal surveillance scenario, where Autonomous Underwater Vehicles (AUVs) patrol an area of interest and inspect surface ships or underwater assets passing through the area. A shore-based control center monitors the AUVs by means of delay-tolerant networking techniques. In particular, as the AUVs carry out their patrolling task, they may get in contact with one another and have a chance to exchange data about the inspected assets (identity, route followed, movement speed, etc.). Given that the area to be patrolled is usually quite large, these contacts are erratic and time-limited: this makes the AUVs and the sink a Delay-Tolerant Network (DTN). To make the communication between AUVs more effective during a contact, we propose a DTN protocol which splits the estimated contact duration between the nodes involve and enhance this protocol using an Automatic Repeat reQuest (ARQ) technique based on selective repeat for error control. Moreover, the structure of the signaling packets exchanged prior to data transmission is designed to help estimate the contact duration and thereby optimize the subsequent data packet exchange. Simulation results demonstrate that the proposed protocol outperforms other ARQ-based DTN routing protocols
On the Impact of Channel Estimation Errors on MAC Protocols for MIMO Ad Hoc Networks
No full textIn this paper, we evaluate the performance of a Medium Access Control (MAC) protocol for MIMO ad hoc networks under imperfect channel estimation. To this end, we also present an analysis of channel estimation errors using correlator-based and Minimum Mean-Square Error (MMSE) channel estimators. Unlike similar works, we specifically focus on a scenario where the presence of several simultaneous, symbol-asynchronous signals makes the problem more complicated than in traditional channel estimation. In particular, we show that there is direct dependence of the channel estimation error on the instantaneous channel matrix. The model we propose in this paper makes it possible to quickly evaluate the performance of channel estimation schemes as a function of the system parameters. In this light, we include the effect of channel estimation errors in an ad hoc networking protocol simulator and thoroughly evaluate their impact. Our results show that there exists a significant interplay between the performance of MAC protocols for MIMO networks and the accuracy of channel estimation. Moreover, we show that interesting tradeoffs arise between MAC- and physical level-related parameters
Modeling the throughput of 1-persistent CSMA in underwater networks
No full textThe aim of this paper is to present a model for the throughput of the 1-persistent CSMA protocol in underwater networks, where the typically large propagation delay with respect to the packet transmission time requires to take into account the spatial distribution of the nodes. Our model is developed based on the analysis carried out in [1] for the non-persistent CSMA protocol. Our results show that the 1-persistent CSMA model developed by Tobagi and Kleinrock is still valid as an approximation, with a few small adjustments, even though it considers an equal propagation delay for all pairs of nodes in the network. The proposed model is validated against simulation results based on the network simulator OMNeT++
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