20,504 research outputs found
Algorithms for Constructing Overlay Networks For Live Streaming
We present a polynomial time approximation algorithm for constructing an
overlay multicast network for streaming live media events over the Internet.
The class of overlay networks constructed by our algorithm include networks
used by Akamai Technologies to deliver live media events to a global audience
with high fidelity. We construct networks consisting of three stages of nodes.
The nodes in the first stage are the entry points that act as sources for the
live streams. Each source forwards each of its streams to one or more nodes in
the second stage that are called reflectors. A reflector can split an incoming
stream into multiple identical outgoing streams, which are then sent on to
nodes in the third and final stage that act as sinks and are located in edge
networks near end-users. As the packets in a stream travel from one stage to
the next, some of them may be lost. A sink combines the packets from multiple
instances of the same stream (by reordering packets and discarding duplicates)
to form a single instance of the stream with minimal loss. Our primary
contribution is an algorithm that constructs an overlay network that provably
satisfies capacity and reliability constraints to within a constant factor of
optimal, and minimizes cost to within a logarithmic factor of optimal. Further
in the common case where only the transmission costs are minimized, we show
that our algorithm produces a solution that has cost within a factor of 2 of
optimal. We also implement our algorithm and evaluate it on realistic traces
derived from Akamai's live streaming network. Our empirical results show that
our algorithm can be used to efficiently construct large-scale overlay networks
in practice with near-optimal cost
Shape-changing solar sails for novel mission applications
In order to increase the range of potential mission applications of solar sail technology, this paper introduces the
concepts of shape change and continuously variable optical properties to large gossamer spacecraft. Merging the two
concepts leads to the idea of solar sails as multi-functional platforms that can have potential benefits over conventional
solar sails by delivering additional key mission functions such as power collection, sensing and communications. To
this aim, the paper investigates the static deflection of a thin inelastic circular sail film with a variable surface reflectivity
distribution. The sail film is modelled as a single surface framed by a rigid supporting hoop structure. When changing
the reflectivity coefficient across the sail surface, the forces acting on the sail can be controlled without changing the
incidence angle relative to the Sun. In addition, by assigning an appropriate reflectivity function across the sail, the
load distribution due to solar radiation pressure can also be manipulated to control the billowing of the film. By an
appropriate choice of reflectivity across the sail, specific geometries can be generated, such as a parabolic reflector,
thus enabling a multi-functional sail. This novel concept of optical reconfiguration can potentially extend solar sail
mission applications
A formal foundation for ontology alignment interaction models
Ontology alignment foundations are hard to find in the literature. The abstract nature of the topic and the diverse means of practice makes it difficult to capture it in a universal formal foundation. We argue that such a lack of formality hinders further development and convergence of practices, and in particular, prevents us from achieving greater levels of automation. In this article we present a formal foundation for ontology alignment that is based on interaction models between heterogeneous agents on the Semantic Web. We use the mathematical notion of information flow in a distributed system to ground our three hypotheses of enabling semantic interoperability and we use a motivating example throughout the article: how to progressively align two ontologies of research quality assessment through meaning coordination. We conclude the article with the presentation---in an executable specification language---of such an ontology-alignment interaction model
SUNNY-CP and the MiniZinc Challenge
In Constraint Programming (CP) a portfolio solver combines a variety of
different constraint solvers for solving a given problem. This fairly recent
approach enables to significantly boost the performance of single solvers,
especially when multicore architectures are exploited. In this work we give a
brief overview of the portfolio solver sunny-cp, and we discuss its performance
in the MiniZinc Challenge---the annual international competition for CP
solvers---where it won two gold medals in 2015 and 2016. Under consideration in
Theory and Practice of Logic Programming (TPLP)Comment: Under consideration in Theory and Practice of Logic Programming
(TPLP
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Development of a distributed knowledge-based system
This paper describes the development of a distributed knowledge-based system. A software system, namely Distributed Algorithmic and Rule-based Blackboard System (DARBS), was developed from its predecessor ARBS, which lacked the distributed computing feature. ARBS has been used in solving a number of engineering problems [1-3]. DARBS now utilises client/server technology. It consists of a centralised database server, called the 'Blackboard' and a number of Knowledge Source Clients (experts). It distributes the workload to a number of clients which are rule-based or other AI systems with specific knowledge in various areas. DARBS is being applied to automatic interpretation of non-destructive evaluation (NDE) data and control of plasma deposition processes
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