1,720,972 research outputs found
Adversarial Obstruction of Millimeter Wave Links
Full text linkWe use a stochastic geometry approach to study an adversarial attack to the physical layer of millimeter wave communications, which is extremely vulnerable to blockage due to obstructions. Previous investigations have applied stochastic geometry to the study of millimeter wave communication scenarios with randomly located obstructions, but in this paper we investigate what happens if some of them are actually due to a malicious attacker. It turns out that, with just few strategically positioned obstructions, an adversary can significantly hinder the millimeter wave link operation and cause extremely high outage probabilities. As expected, scenarios with multiple reflections are more robust against this kind of attack, since they can exploit the diversity of reflected paths, despite their lower quality. Conversely, millimeter wave communications without (or with limited) multipath diversity are shown to be extremely fragile. We also investigate the impact on blockage of different parameters of the obstructions, including their number, shape, and size. Finally, we elaborate the applications of our findings to identify countermeasures against this attack
Mobile Networks on the Move: Optimizing Moving Base Stations Dynamics in Urban Scenarios
No full textBase station densification is one of the key approaches for delivering high capacity in radio access networks. However, current static deployments are often impractical and financially unsustainable, as they increase both capital and operational expenditures of the network. An alternative paradigm is the moving base stations (MBSs) approach, by which part of base stations are installed on vehicles. However, to the best of our knowledge, it is still unclear if and up to which point MBSs allow decreasing the number of static base stations (BSs) deployed in urban settings. In this work, we start tackling this issue by proposing a modeling approach for a first-order evaluation of potential infrastructure savings enabled by the MBSs paradigm. Starting from a set of stochastic geometry results, and a traffic demand profile over time, we formulate an optimization problem for the derivation of the optimal combination of moving and static BSs which minimizes the overall amount of BSs deployed, while guaranteeing a target mean QoS for users. Initial results on a two-district scenario with measurement-based network traffic profiles suggest that substantial infrastructure savings are achievable. We show that these results are robust against different values of user density
The Non-Saturated Multiserver Job Queuing Model with Two Job Classes: A Matrix Geometric Analysis
No full textDatacenters comprise large quantities of processors, memory, and input/output modules. These resources are shared among requests (jobs) submitted by datacenter users. Jobs differ in their frequency of arrivals, demand for resources, and execution times. Resource sharing generates contention, especially in heavily loaded systems, that must therefore implement effective scheduling policies for incoming jobs. The First-In First-Out (FIFO) policy is often used for batch jobs, but may produce under-utilization of resources, in terms of wasted servers. This is due to the fact that a job that requires many resources can block jobs arriving later that could be served because they require fewer resources. The mathematical construct often used to study this problem is the Multiserver Job Queuing Model (MJQM), where servers represent resources which are requested and used by jobs in different quantities. Unfortunately, very few explicit results are known for the MJQM, especially at realistic system loads (i.e., before saturation). In this paper, we propose the first exact analytical model of the non-saturated MJQM in case of two classes of customers with exponentially distributed service times and an arbitrary number of identical servers. Our analysis is based on the matrix geometric method. Our results provide insight into datacenter dynamics, thus supporting the design of more complex schedulers, capable of improving performance and energy consumption within large datacenters
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
Modeling Service Mixes in Access Links: Product Form and Oscillations
No full textWe consider an access link of a data network loaded with data flows generated by streaming and elastic services requested by fixed or mobile end users, and subjected to an admission control (AC) algorithm. For the performance analysis of such link we develop a new queuing model and we show that, with the considered AC, the queuing model admits a product form expression for the joint limiting probability distribution of the numbers of active services of the different types. Numerical results show unexpected oscillating behaviors for several performance metrics, and provide interesting insight into the link performance
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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