1,720,969 research outputs found
Valutazione e Miglioramento delle Prestazioni del TCP/UDP in Reti Wireless Ad-Hoc Multi-Hop e Multi-Canale
No full textThe most widespread WLAN standard, IEEE 802.11, in the DCF mode, uses channelized bandwidth, where MAC operation takes place within a given channel. Same channel re-use makes it possible to enhance coverage efficiency. Multi-channel protocols can be exploited in WLANs in order to make use of additional bandwidth, or to decouple different transmissions, hoping to reduce collisions in distributed access modes
Analytic Modeling of ad hoc 802.11 networks with hidden nodes applied to a split channel solution for performance improvement
No full textSeveral simulation studies have shown that the performance of IEEE 802.11 DCF in an ad hoc scenario strongly depends on the coverage and interference radii. We state and solve an analytical model for an 802.11 DCF ad hoc network, with an interference radius larger than the coverage radius. The model is developed for the study of a split channel solution, where RTS/CTS signaling is conveyed via a separate, orthogonal channel with respect to data and ACK frames. By exploiting the model we can optimize the bandwidth split of the control and data channels. Further, we compare single channel, split channel and multichannel solutions, thus highlighting that the simple split channel achieves most of the performance advantage potentially offered by a multi-channel 802.11 DCF. © Springer-Verlag Berlin Heidelberg 2007
Scheduling algorithms in optical packet switches with input wavelength conversion
No full textThe objective of this study is to propose a new Optical Packet Switching architecture in which the wavelength converters, needed to solve output packet contentions, are shared per input line; according to this sharing strategy the packets arriving at a given input fiber shares a converter pool that can be accessed when wavelength conversions are required. In the paper, we propose analytical and simulation models able to evaluate the performances of the proposed architecture when control algorithms with different complexity, are adopted. Under a unicast traffic scenario, the obtained performances are compared to the ones of the architecture in which the wavelength converters are shared per output line. The carried out comparison shows that, with respect to the architecture with wavelength converters shared per output line, the proposed architecture allows for a 30% saving of wavelength converters when a simple control algorithm is adopted; the saving can reach 50% if an optimized control algorithm is used. In the paper, we also define a heuristic algorithm able to reach in low computation cost, performance near to the one of the optimum algorithm. (c) 2005 Elsevier B.V. All rights reserved
Autonomous optimized interference management of CDMA cellular access with multichannel
No full textWe analyze the resource allocation problem for
CDMA in a multicell wireless access, by defining an optimization
problem for the radio resource assignment, with a twofold aim:
first, we try to balance selfish single cell performance with
generated interference to surrounding cells; second, we require
each Base Station to assign its resources to mobile stations within
the cell, by relying only on local data and measurements so that
no explicit signalling is required among different cells.
A specific contribution of this work is to explore the potential
of a multichannel approach to the autonomous cell assignment
problem, so as to dynamically decouple stations that are too much
coupled by interference
Advantages of hybrid input/output wavelength conversion in optical packet switches
No full textIn this paper, we propose a new Optical Packet Switching Architecture referred to as Shared Per Input/ Output Line ( SPIOL) in which Wavelength Converters, used to solve output packet contentions, are shared per input and per output line. In this architecture, packets needing conversion can be wavelength translated by converters placed on output lines or by converters placed on input lines. We introduce and discuss the analytical and simulation models we used to evaluate the performance of the architecture in question when a simple control algorithm, referred to as Random Algorithm, is adopted. The proposed architecture performance is compared to that of two other architectures referred to as Shared Per Input Line ( SPIL) and Shared per Output Line ( SPOL), which share converters per output line only and per input line only, respectively. The comparison is carried out under symmetric and asymmetric traffic scenarios. The obtained results show that in some cases the SPIOL architecture allows for a saving in the order of 20% and 30% with respect to the SPIL and SPOL architectures, respectively
On the unfairness among TCP flows in IEEE 802.11 multi-hop ad-hoc networks
No full textThe problem of unfairness among competing flows in TCP multihop wireless networks is well known, and has often been traced to the misbehaviour of dynamic routing protocols in case of link failure. We show, through simulations, that the competition between TCP flows in a multihop wireless network leads to an unfair outcome, independently of the routing protocol involved. We focus on the impact of hidden node collisions on the performance of multi-hop TCP flows, and trace the source of the problem to collisions between RTS frames belonging to the starved flow and long data frames sent by the active flow. Such collisions lead to a rapid increase of the MAC contention window at the TCP sender of the starved flow; this, in conjunction with TCP's congestion control algorithm, allows the running flow to hold the channel for a long time, until the situation is reversed. We also show that a simple multi-channel MAC can ease the competition among flows. © 2008 IEEE
A modular cross-layer scheduling and resource allocation architecture for OFDMA system
No full textPacket scheduling and radio resource allocation in an OFDMA system pursue conflicting goals: the latter aims at achieving short-term efficiency, while scheduling aims at guaranteeing fairness among flows in the long term. We propose a scheduler-allocator architecture that successfully manages to integrate both goals in a loose cross-layer strategy. On each time frame the scheduling module selects a list of packets eligible for transmission, with the goal of achieving long-term fairness; the list is then passed to the radio resource allocator, which finds the best allocation given the current channel state. We evaluate the proposed scheme in a single cell scenario. Results point out that our scheme is able to guarantee throughput fairness among flows, while achieving an efficient allocation of radio resources. © 2006 IEEE
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
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