1,720,984 research outputs found
Experimental results on the support of TCP over 802.11b: an insight into fairness issues
No full textGreat attention has been dedicated, in the recent
years, to the WLAN standards that are opening the market
to the short range and high data rate wireless services in
the local and hot spot areas. Technically speaking, the main
strength of the most quoted standard, the IEEE 802.11, is the
fully distributed nature of the access scheme, that provides
cheap and easy-to-install components, able to operate in the
unlicensed spectrum, still guaranteeing broadband capabilities.
The aim of this paper is to deeply investigate traffic issues
in 802.11b networks by emphasizing the interaction between
WLAN link layer parameters or Access Point buffer provisioning
with uplink/downlink TCP fairness. The novel aspect is that
this investigation is fully made in an experimental environment.
A great portion of flows that are exchanged in a WLAN are
TCP-based (e.g. FTP flows). We prove, with real experiments,
that TCP suffers of some inequalities that derive to unfair
bandwidth sharing between uplink and downlink. Our extensive
experimental analysis shows the main effects of these inequalities
on the TCP behavior and highlights some performance anomalies
that are difficult to be measured via simulations
Bottleneck Detection In UMTS Via TCP Passive Monitoring: A Real Case
No full textIn this work we address the problem of inferring the presence of a bottleneck from passive measurement in the UMTS core network. The study is based on one month of packet traces collected in the core network of mobilkom austria AG & Co KG, the leading mobile telecommunications provider in Austria, EU. During the measurement period a bottleneck link in the UMTS core network was revealed and removed, therefore the traces enable the accurate analysis and comparison of the traffic behavior in the two network conditions. The proposed approach exploits statistics of estimated TCP performance parameters (e.g. RTT, re-transmissions) in order to build a set of bottleneck indicators. We show that such statistics are volatile due to the presence of few top users, but this effect can be counteracted with a simple filtering method. Results show that the frequency of re-transmission events is a powerful indicator for the specific type of bottleneck under study, and it can be used to provide early warning about future occurrences of similar events. This application is particularly important for operational UMTS networks nowadays, since the traffic volumes and composition is still under evolution
PARAMETRI GENETICI DELLA MUNGIBILITÀ RILEVATA CON LATTOFLUSSOMETRI NELLE DUE RAZZE BOVINE: BRUNA E FRISONA
No full textObjective of the thesis was the investigation of genetic parameters of milkability in Italian Holstein Friesian and Italian Brown Swiss. A total of 2,886 records of Italian Holstein Friesian and 37,511 records of Italian Brown Swiss were collected using a milkometer. All records included milk yield, SCS, and measures of milk flow as: ascending time, time of plateau, decreasing time, total milking time, and main milking time. To estimate variance components, different multiple trait animal models were used specifically for each breed data. Heritabilities ranged between 0.06 and 0.41 for the Italian Holstein Friesian, and between 0.05 and 0.40 for the Italian Brown Swiss. Heritability and genetic correlation values showed that flow traits collected with milkometers can be genetically selected and that they would contribute to the genetic selection of milkability through the definition of the desired shape of milk flow specific for each one of the two breeds
Large-Scale RTT Measurements from an Operational UMTS/GPRS Network
No full textIn this paper we present some observations about TCP RTT as captured in the live traffic of an operational GPRS/UMTS network. RTT samples are extracted from traces collected by passive monitoring the Gn interfaces of one of the major providers in Austria, EU. We compare results for GPRS and UMTS, and expose some methodological issues involved in performing this type of analysis, for instance the potential bias introduced by heavy users. We also explore the correlation of RTT with time-of-day. The primary motivation for this research is to gain a better understanding of the dominant causes that shape the behaviour and the distribution of RTT across the TCP connections that populate a GPRS/UMTS network. The results presented here are useful for the scientific community, for instance to set more realistic simulation scenarios for other research activities. In addition, RTT measurements as obtained by large-scale passive monitoring can be used in combination with other indicators to build a summary indicator of the performances experienced by the TCP users at the scale of the whole network, i.e. a global RTT-based KPI (key performance indicator)
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
Topology Characterization and Performance Analysis of IEEE 802.15.4 Multi-Sink Wireless Sensor Networks
No full textTopology characterization of wireless sensor
networks is a key issue for network planning, management
and control as well as for maximizing the network performance
and for optimizing protocol design. In this paper we
analyze the structure of the topologies formed by means of
the IEEE 802.15.4 association procedure, i.e., trees rooted
at sink nodes, and we verify how such structures affect
the network performance. Specific attention is dedicated
to understand how the network lifetime is affected by the
type of topology formed during the network set-up. A
comparison of topologies having a single sink and multisink
ones is provided. The determination of performance
in case of multi-sink scenarios is extremely important;
nevertheless, to the best of our knowledge, this problem is
tackled here for the first time as for IEEE 802.15.4
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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