142,466 research outputs found
Optimisation of temporal networks under uncertainty
A wide variety of decision problems in operations research are defined on temporal networks,
that is, workflows of time-consuming tasks whose processing order is constrained by precedence
relations. For example, temporal networks are used to formalise the management of projects,
the execution of computer applications, the design of digital circuits and the scheduling of
production processes. Optimisation problems arise in temporal networks when a decision maker
wishes to determine a temporal arrangement of the tasks and/or a resource assignment that
optimises some network characteristic such as the network’s makespan (i.e., the time required
to complete all tasks) or its net present value.
Optimisation problems in temporal networks have been investigated intensively for more than
fifty years. To date, the majority of contributions focus on deterministic formulations where all
problem parameters are known. This is surprising since parameters such as the task durations,
the network structure, the availability of resources and the cash flows are typically unknown
at the time the decision problem arises. The tacit understanding in the literature is that the
decision maker replaces these uncertain parameters with their most likely or expected values
to obtain a deterministic optimisation problem. It is well-documented in theory and practise
that this approach can lead to severely suboptimal decisions.
The objective of this thesis is to investigate solution techniques for optimisation problems in
temporal networks that explicitly account for parameter uncertainty. Apart from theoretical
and computational challenges, a key difficulty is that the decision maker may not be aware
of the precise nature of the uncertainty. We therefore study several formulations, each of
which requires different information about the probability distribution of the uncertain problem
parameters. We discuss models that maximise the network’s net present value and problems
that minimise the network’s makespan. Throughout the thesis, emphasis is placed on tractable
techniques that scale to industrial-size problems
The Dynamics of Internet Traffic: Self-Similarity, Self-Organization, and Complex Phenomena
The Internet is the most complex system ever created in human history.
Therefore, its dynamics and traffic unsurprisingly take on a rich variety of
complex dynamics, self-organization, and other phenomena that have been
researched for years. This paper is a review of the complex dynamics of
Internet traffic. Departing from normal treatises, we will take a view from
both the network engineering and physics perspectives showing the strengths and
weaknesses as well as insights of both. In addition, many less covered
phenomena such as traffic oscillations, large-scale effects of worm traffic,
and comparisons of the Internet and biological models will be covered.Comment: 63 pages, 7 figures, 7 tables, submitted to Advances in Complex
System
Recommended from our members
Constant depth microfluidic networks based on a generalised Murray’s law for Newtonian and power-law fluids
This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.Microfluidic bifurcating networks of rectangular cross-sectional channels are designed
using a novel biomimetic rule, based on Murray’s law. Murray’s principle is extended to
consider the flow of power-law fluids in planar geometries (i.e. of constant depth rectangular
cross-section) typical of lab-on-a-chip applications. The proposed design offers the ability to
control precisely the shear-stress distributions and to predict the flow resistance along the network.
We use an in-house code to perform computational fluid dynamics simulations in order
to assess the extent of the validity of the proposed design for Newtonian, shear-thinning and
shear-thickening fluids under different flow conditions
FAST TCP: Motivation, Architecture, Algorithms, Performance
We describe FAST TCP, a new TCP congestion control algorithm for high-speed long-latency networks, from design to implementation. We highlight the approach taken by FAST TCP to address the four difficulties which the current TCP implementation has at large windows. We describe the architecture and summarize some of the algorithms implemented in our prototype. We characterize its equilibrium and stability properties. We evaluate it experimentally in terms of throughput, fairness, stability, and responsiveness
Two betweenness centrality measures based on Randomized Shortest Paths
This paper introduces two new closely related betweenness centrality measures
based on the Randomized Shortest Paths (RSP) framework, which fill a gap
between traditional network centrality measures based on shortest paths and
more recent methods considering random walks or current flows. The framework
defines Boltzmann probability distributions over paths of the network which
focus on the shortest paths, but also take into account longer paths depending
on an inverse temperature parameter. RSP's have previously proven to be useful
in defining distance measures on networks. In this work we study their utility
in quantifying the importance of the nodes of a network. The proposed RSP
betweenness centralities combine, in an optimal way, the ideas of using the
shortest and purely random paths for analysing the roles of network nodes,
avoiding issues involving these two paradigms. We present the derivations of
these measures and how they can be computed in an efficient way. In addition,
we show with real world examples the potential of the RSP betweenness
centralities in identifying interesting nodes of a network that more
traditional methods might fail to notice.Comment: Minor updates; published in Scientific Report
Graph-based Semi-Supervised & Active Learning for Edge Flows
We present a graph-based semi-supervised learning (SSL) method for learning
edge flows defined on a graph. Specifically, given flow measurements on a
subset of edges, we want to predict the flows on the remaining edges. To this
end, we develop a computational framework that imposes certain constraints on
the overall flows, such as (approximate) flow conservation. These constraints
render our approach different from classical graph-based SSL for vertex labels,
which posits that tightly connected nodes share similar labels and leverages
the graph structure accordingly to extrapolate from a few vertex labels to the
unlabeled vertices. We derive bounds for our method's reconstruction error and
demonstrate its strong performance on synthetic and real-world flow networks
from transportation, physical infrastructure, and the Web. Furthermore, we
provide two active learning algorithms for selecting informative edges on which
to measure flow, which has applications for optimal sensor deployment. The
first strategy selects edges to minimize the reconstruction error bound and
works well on flows that are approximately divergence-free. The second approach
clusters the graph and selects bottleneck edges that cross cluster-boundaries,
which works well on flows with global trends
Mapping the Curricular Structure and Contents of Network Science Courses
As network science has matured as an established field of research, there are
already a number of courses on this topic developed and offered at various
higher education institutions, often at postgraduate levels. In those courses,
instructors adopted different approaches with different focus areas and
curricular designs. We collected information about 30 existing network science
courses from various online sources, and analyzed the contents of their syllabi
or course schedules. The topics and their curricular sequences were extracted
from the course syllabi/schedules and represented as a directed weighted graph,
which we call the topic network. Community detection in the topic network
revealed seven topic clusters, which matched reasonably with the concept list
previously generated by students and educators through the Network Literacy
initiative. The minimum spanning tree of the topic network revealed typical
flows of curricular contents, starting with examples of networks, moving onto
random networks and small-world networks, then branching off to various
subtopics from there. These results illustrate the current state of consensus
formation (including variations and disagreements) among the network science
community on what should be taught about networks and how, which may also be
informative for K--12 education and informal education.Comment: 17 pages, 11 figures, 2 tables; to appear in Cramer, C. et al.
(eds.), Network Science in Education -- Tools and Techniques for Transforming
Teaching and Learning (Springer, 2017, in press
- …