1,720,960 research outputs found
A DTN Approach to Satellite Communications
No full textSatellite communications pose serious challenges to transport layer performance, mainly because of long propagation delays (especially in geosynchronous systems) and the possible presence of random errors on the satellite link. Solutions that cope with these impairments usually rely upon either the adoption of enhanced versions of transport protocols, or the insertion of intermediate agents, like PEPs (Performance Enhancing Proxy). An alternative approach is to adopt the DTN (Delay/Disruption Tolerant Networking) architecture based on the introduction of the new “bundle” layer in the protocol stack. The aim of the paper is to discuss the advantages and disadvantages of the DTN approach compared to the more conventional solutions mentioned. DTN performance is assessed by considering both fully connected networks and networks where continuous connectivity cannot be guaranteed during data transfer and/or at start-up. Performance is evaluated by using the TATPA testbed (Testbed for Advanced Transport Protocols and Architectures), which is based on a cluster of Linux PCs running the DTNperf application, developed to this end by the authors and now included in the official DTN package
DTN meets smartphones: future prospects and tests
No full textAlthough first conceived for the Interplanetary Internet, DTN (Delay-/Disruption-Tolerant Networking) architecture has successively enlarged its application scope to embrace all “challenged networks”, i.e. networks where the TCP design key assumptions of short RTT, no disruptions, and continuous end-to-end path availability are challenged. The distinctive features of DTN architecture seem ideal for the connectivity problems of modern smartphones. These are both PCs and cell phones and are characterized by a wide range of different communications technologies, including Bluetooth, WiFi, and advanced 3G radio interfaces, like UMTS HSPA (High-Speed Packet Access). The aim of this paper is to show that it is both feasible and useful to adopt DTN architecture on these advanced devices, in addition to the usual TCP/IP protocol stack. To this end, a series of tests is presented and discussed in the paper, considering various communication interfaces, 3G in particular. Tests were greatly facilitated by the introduction onto the market of some smartphones running Linux based operating systems, like Maemo, which allowed the authors the direct use of “official” DTN code (i.e. the DTN2 reference implementation and the DTNperf_2 evaluation tool)
TCP, PEP and DTN Performance on Disruptive Satellite Channels
No full textFixed GEO satellite communications are impaired by long RTTs and the possible presence of packet losses on the satellite radio channel. Moreover, when the satellite receiver is mobile, short and long disruptions due to line of sight obstructions can cause further performance deterioration. In this paper, we present a preliminary assessment of disruption impact on performance comparing different approaches, such as end-to-end TCP, PEP based on TCP splitting and DTN. Performance is evaluated on a Linux testbed, by emulating the satellite link interruptions caused by tunnels on a real railway line. Results show that good performance can be achieved by all of the approaches considered provided that on the satellite channel an efficient TCP variant, like Hybla, is adopted; however, DTN offers best performance. DTN superiority emerges when the disrupted satellite channel is the last leg of a congested path, thanks to the DTN custody transfer mechanism and store and forward capability
DTNperf_2: a Performance Evaluation tool for Delay/Disruption Tolerant Networking
No full textDelay/Disruption Tolerant Networking (DTN) has recently been proposed as a very effective solution to cope with the problems posed by challenged networks (long delays, disruptions, absence of end-to-end connectivity, etc.). In order to assess DTN performance in such scenarios, it is essential to be able to rely on specific evaluation tools. In this paper we present DTNperf_2, which is a client-server evaluation tool designed to assess goodput and to provide logs in DTN bundle layer architectures. The aim is to facilitate the adoption of this tool by the DTN research community. DTNperf_2 features, design and usage are discussed in detail. Moreover, an example of use is given, to demonstrate its capabilities. DTNperf_2 is included in the official DTN suite released by DTNRG
A DTN Approach to Satellite Communications
No full textSatellite communications pose serious challenges to transport layer performance, mainly because of long propagation delays (especially in geosynchronous systems) and the possible presence of random errors on the satellite link. Solutions that cope with these impairments usually rely upon either the adoption of enhanced versions of transport protocols, or the insertion of intermediate agents, like PEPs (Performance Enhancing Proxy). An alternative approach is to adopt DTN (Delay/Disruption Tolerant Network) architecture, based on the introduction of the new “bundle” layer in the protocol stack. The aim of the paper is to discuss the advantages and disadvantages of the DTN approach compared to more conventional solutions. DTN performance is assessed by using a testbed based on a cluster of Linux PCs running the DTNperf application, which was developed to this end by the authors and now included in the official DTN package. Preliminary results are presented and discussed in the paper
Analysis of TCP and DTN Retransmission Algorithms in Presence of Channel Disruptions
No full text“Challenged Networks” often violate the TCP design key assumption of continuous path availability from the source to the sink node. These networks are the preferred target of Delay/Disruption Tolerant Networking (DTN), which aims at providing a more robust network architecture. Against disruptions, however, even standard TCP offers a certain level of robustness, through its many retransmission algorithms. The aim of the paper is to present a comparative analysis of the TCP and DTN Bundle protocol retransmission algorithms triggered by channel disruptions. This analysis provides the reader with a comprehensive view of the many mechanisms involved and a first assessment of their impact on some performance metrics. Although DTN put on top of TCP clearly offers higher resilience against long disruptions, TCP itself can provide a certain level of robustness, provided that the many retransmission parameters are properly set. The analysis is completed by some numerical evaluations on a GEO satellite channel obtained by means of a Linux testbed
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
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