1,721,007 research outputs found
Use of QUIC Protocol for Efficient Data Transmission Over Satellite in Emergency Scenario
No full textArchitectural Analysis and Performance Evaluation of Integrated Access-Backhaul Non-Terrestrial Networks
No full textDynamic resource allocation based on a TCP-MAC cross-layer approach for DVB-RCS satellite networks
No full textSatellite communication networks can provide multimedia broadband services to fixed and mobile users in several scenarios where terrestrial networks are not present or where they need to be complemented. Moreover, satellite links can be useful to bypass crowded terrestrial networks, thus helping in reducing congestion. In such a scenario, characterized by a high propagation delay and a radio channel affected by losses, standard bandwidth allocation schemes prove to be inefficient when Transmission Control Protocol (TCP) is running. Hence, we propose a novel resource allocation scheme based on the cross-layer interaction between TCP and Medium Access Control (MAC) layers. The interest is here in achieving an efficient transfer of files by means of the FTP application protocol. Our scheme permits to avoid network congestion, to reduce the average file transfer time and to achieve a fair sharing of resources among competing flows. The allocation technique has been applied to an Interactive Satellite Network (ISN) based on the DVB-RCS standard with a group of fixed satellite terminals, which communicate with a Network Control Center (NCC) through a geostationary bent-pipe satellite, according to a classical star topology. Results are obtained through a network simulator
Enhancing transport layer in satellite systems: design and development of an emulation platform
Full text linkL’affermazione di Internet come strumento di comunicazione globale ha portato al progressivo
congestionamento delle reti di telecomunicazione. Inoltre, la diffusione di nuove classi di servizi,
tra cui la telemedicina, il supporto alle emergenze, la divulgazione a larga scala di informazioni,
richiede una connettività a banda larga e ininterrotta. Per tale ragione, le reti d’accesso “senza
filo”, o wireless, stanno guadagnando una crescente importanza grazie alla loro intrinseca
flessibilità di impiego (bassi costi di installazione e possibilità di raggiungere ogni tipo di area).
In tale contesto, l’utilizzo di sistemi satellitari rappresentano una soluzione estremamente
efficiente offrendo elevate capacità di trasmissione e larghe coperture. In particolare, un sistema
satellitare è in grado di garantire un accesso a larga banda ad Internet nelle area scarsamente
popolate in cui il dispiegamento di infrastrutture terrestri non è economicamente vantaggioso, o
in aree in cui il dispiegamento di infrastrutture terrestri rimane impraticabile (per esempio,
oceani, aree desertiche, zone montuose). In aggiunta, i sistemi satellitari se integrati con sistemi
wireless terrestri (WiFi, UMTS) possono favorire l’estensione dell’area di accesso a Internet per
terminali mobili.
La maggior parte dei sistemi satellitari a larga banda si basano su satelliti collocati in orbite
geostazionarie (GEO) a circa 36000 km dalla superficie terrestre. In questo scenario di
comunicazione, i protocolli TCP/IP si trovano ad affrontare problematiche nuove che ne
riducono drasticamente le prestazioni. In particolare, le prestazioni del “Transmission Control
Protocol” (TCP), sono particolarmente sensibili ad alcuni fattori caratterizzanti i sistemi
satellitari geostazionari: elevati ritardi di propagazione, perdite di pacchetti non dovuti a
congestione (BER≠0), presenza di collegamenti asimmetrici, ecc.
L’analisi e la progettazione di uno strato di trasporto ottimizzato per sistemi che includono un
segmento satellitare è stato l’oggetto del programma di dottorato dell’autore, nell’ambito del
“XIX° ciclo di dottorato in Sistemi e Tecnologie per lo Spazio” presso l’università di Roma “Tor
Vergata”.
Questa tesi ha una duplice finalità: presentare tutti i risultati ottenuti dall’autore nell’ambito della
sua attività di ricerca, e sulla base dell’esperienza accumulata, proporre il progetto di una
piattaforma di emulazione ottimizzata per valutare le prestazioni di protocolli TCP/IP in
ambiente satellitare. La trattazione è organizzata in sei capitoli. Il Capitolo 1 tratta i concetti di
base riguardanti i sistemi satellitari: architetture, servizi, standard, sicurezza. Il Capitolo 2
riassume i principali meccanismi del TCP, essenziali per comprendere i temi trattati nei capitoli
successivi. Nel Capitolo 3 vengono approfondite le principali limitazioni del TCP in ambienti
satellitari, nonché le soluzioni proposte per migliorarne le prestazioni, e viene presentata una
vasta gamma di risultati ottenuti per via analitica e tramite simulazioni e sperimentazioni. Il
Capitolo 4 sintetizza i principali risultati ottenuti delle attività svolte dall’autore e finalizzate
all’ottimizzazione dello strato di trasporto per applicazioni Internet in ambiente satellitare. Nel
Capitolo 5 sono descritte le principali caratteristiche della piattaforma di emulazione, denominata
“Satellite Network Emulation Platform” (SNEP), progettata dall’autore allo scopo di riprodurre
un ambiente satellitare conforme allo standard DVB-RCS. Infine, le conclusioni sono sintetizzate
nel Capitolo 6.The rapid growth of Internet applications (i.e., email, file transfer, remote access, web browsing,
e-learning, e-banking, video-conferencing, etc.) is causing progressive congestion of
telecommunication networks. Moreover, new classes of services including telemedicine,
information dissemination, emergency support, disaster response and public information, may
require broadband and untethered connectivity. Therefore, wireless access is gaining growing
importance due to its intrinsic flexibility and ubiquity.
In such a context, the use of satellite systems represents an attractive option, since they can offer
high transmission capacity and large coverage. In fact, a satellite system can guarantee
broadband access to the global network in sparsely populated areas where deploying a terrestrial
infrastructure remains unappealing or in regions where the deployment of terrestrial facilities
remains impractical. Moreover, satellites can be integrated to terrestrial wireless (WiFi, UMTS,
etc.) for enhancing mobile access to the Internet by both pedestrians and vehicles while it is often
the only option for aeronautical and maritime users.
Most of broadband satellite systems are deployed at traditional geostationary orbit (GEO). In this
scenario, IP-based transfers must face some peculiar challenges. In particular, performance of
Internet applications, being based on the “Transmission Control Protocol” (TCP) at the transport
layer, is strongly affected by several factors introduced by satellite systems (i.e., large latency,
losses not due to congestion , high link asymmetry, etc.). Such factors make standard TCP
inefficient over satellite links, resulting in poor performance.
The analysis and the design of an optimized transport layer for system including a satellite
segment has been the object of the author's Ph.D. program in the frame of the 19th cycle of the
Ph.D. course of “Space System and Technologies” at the University of Rome “Tor Vergata”.
This thesis has a twofold aim: present all the achievements coming from the author's research
activity and, based on the collected experience, propose the design of a emulation platform
optimized to test TCP/IP protocols over a likely satellite environment. The work is organized in
six chapters. Chapter 1 deals with basic notions concerning satellite systems. In particular,
network architectures, services, standard and security aspects are briefly discussed. Chapter 2
summarizes main mechanisms regulating the standard TCP protocol. It aims at highlighting the
main TCP protocol characteristics, essential to understand the subsequent chapters which focus
on issues of TCP in satellite-based environments. Chapter 3 offers a comprehensive analysis of
TCP performance over satellite links. Along with the identification of both the main limitation
and the proposed solutions to improve TCP performance, a vast gamut of analytical, simulation
and experimentation results, coming from the author's research activity, are presented. Chapter 4
collects all the outcomes of the author's activity, focusing on the enhancement of the transport
layer for Satellite-based Internet applications. Chapter 5 describes the main characteristics of an
emulation platform, named “Satellite Network Emulation Platform” (SNEP), designed by author
to reproduce the variable characteristics of a satellite network (DVB-RCS like) through an active
emulation approach. Finally, conclusions are drawn in Chapter 6
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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