8 research outputs found
A Fast Exact Algorithm for the Optimum Cooperation Problem
Given a graph G=(V,E) with real edge weights, the optimum cooperation problem consists in determining a partition of the graph that maximizes the sum of weights of the edges having nodes in the same partition plus the number of resulting partitions. The problem is also known in the literature as the optimum attack problem in networks. It occurs as a subproblem in the separation of partition inequalities. Furthermore, a relevant physics application exists. Solution algorithms known in the literature require at least |V|-1 minimum cut computations in a corresponding network. In this work, we present a fast exact algorithm for the optimum cooperation problem. By graph-theoretic considerations and appropriately designed heuristics, we considerably reduce the number of minimum cut computations that are necessary in practice. We show the effectiveness of our method by comparing the performance of our algorithm with that of the fastest previously known method on instances coming from the physics application
A Fast Exact Algorithm for the Optimum Cooperation Problem
Given a graph G=(V,E) with real edge weights, the optimum cooperation problem consists in determining a partition of the graph that maximizes the sum of weights of the edges having nodes in the same partition plus the number of resulting partitions. The problem is also known in the literature as the optimum attack problem in networks. It occurs as a subproblem in the separation of partition inequalities. Furthermore, a relevant physics application exists. Solution algorithms known in the literature require at least |V|-1 minimum cut computations in a corresponding network. In this work, we present a fast exact algorithm for the optimum cooperation problem. By graph-theoretic considerations and appropriately designed heuristics, we considerably reduce the number of minimum cut computations that are necessary in practice. We show the effectiveness of our method by comparing the performance of our algorithm with that of the fastest previously known method on instances coming from the physics application
A Fast Exact Algorithm for the Problem of Optimum Cooperation and Structure of Its Solutions
Given a graph with real edge weights, the optimum cooperation problem consists in determining a partition of the graph that maximizes the sum of weights of the edges with nodes in the same class plus the number of the classes of the partition. The problem is also known in the literature as the optimum attack problem in networks. Furthermore, a relevant physics application exists. In this work, we present a fast exact algorithm for the optimum cooperation problem. Algorithms known in the literature require n-1 minimum cut computations in a corresponding network, where n is the number of nodes in the graph. By theoretical considerations and appropriately designed heuristics, we considerably reduce the numbers of minimum cut computations that are necessary in practice. We show the effectiveness of our method by presenting results on instances coming from the physics application. Furthermore, we analyze the structure of the optimal solutions
Regenerator placement in optical networks
Cataloged from PDF version of article.Increase in the number of users and resources consumed by modern applications
results in an explosive growth in the traffic on the Internet. Optical networks
with higher bandwidths offer faster and more reliable transmission of data and
allows transmission of more data. Fiber optical cables have these advantages over
the traditional copper wires. So it is expected that optical networks will have a
wide application area.
However, there are some physical impairments and optical layer constraints
in optical networks. One of these is signal degradation which limits the range of
optical signals. Signals are degraded during transmission and below a threshold
the signals become useless. In order to prevent this, regenerators which are
capable of re-amplifying optical signals are used. Since regeneration is a costly
process, it is important to decrease the number of regenerators used in an optical
network.
To increase the reliability of the network, two edge-disjoint paths between
each terminal on the network are to be constructed. So the second path could
be used in case of a failure in transmitting data on an edge of the first path.
Considering these requirements, selecting the nodes on which regenerators are to
be placed is an important decision.
In this thesis, we discuss the problem of placing signal regenerators on optical
networks with restoration. An integer linear program is formulated for this problem.
Due to the huge size and other problems of the formulation, it is impractical
to use it on large networks. For this reason, a fast heuristic algorithm is proposed
to solve this problem. Three methods are proposed to check the feasibility when
a fixed set of regenerators are placed on specific nodes. Additionally, a branch
and bound algorithm which employs the proposed heuristic is developed to find the optimal solution of our problem. Performance of both the heuristics and
the branch and bound method are evaluated in terms of number of regenerators
placed and solution times of the algorithms.Özkök, OnurM.S
Exact algorithms for network design problems using graph orientations
Gegenstand dieser Dissertation sind exakte Lösungsverfahren für topologische Netzwerkdesignprobleme.
Diese kombinatorischen Optimierungsprobleme tauchen in unterschiedlichen
realen Anwendungen auf, wie z.B. in der Telekommunikation und
der Energiewirtschaft. Die grundlegende Problemstellung dabei ist die Planung bzw.
der Ausbau von Netzwerken, die Kunden durch physikalische Leitungen miteinander
verbinden. Im Allgemeinen lassen sich solche Probleme graphentheoretisch wie folgt
beschreiben: Gegeben eine Menge von Knoten (Kunden, Straßenkreuzungen, Router
u.s.w.), eine Menge von Kanten (potenzielle Verbindungsmöglichkeiten) und eine
Kostenfunktion auf den Kanten und/oder Knoten. Zu bestimmen ist eine Teilmenge
von Knoten und Kanten, so dass die Kostensumme der gewählten Elemente minimiert
wird und dabei Nebenbedingungen wie Zusammenhang, Ausfallsicherheit,
Kardinalität o.ä. erfüllt werden. In dieser Dissertation behandeln wir zwei spezielle
Klassen von topologischen Netzwerkdesignproblemen, nämlich das k-Cardinality
Tree Problem (KCT) und das {0,1,2}-Survivable Netzwerkdesignproblem ({0,1,2}-
SND) mit Knotenzusammenhang. Diese Probleme sind im Allgemeinen NP-schwer,
d.h. nach derzeitigem Stand der Forschung kann es für solche Probleme keine Algorithmen
geben die eine optimale Lösung berechnen und dabei für jede mögliche
Instanz eine effiziente (d.h. polynomielle) Laufzeit garantieren.
Die oben genannten Probleme lassen sich als ganzzahlige lineare Programme
(ILPs) formulieren, d.h. als Systeme aus linearen Ungleichungen, ganzzahligen Variablen
und einer linearen Zielfunktion. Solche Modelle lassen sich mit Methoden
der sogenannten mathematischen Programmierung lösen. Dass die entsprechenden
Lösungsverfahren im Allgemeinen sehr zeitaufwendig sein können, war ein oft genutztes
Argument für die Entwicklung von (Meta-)Heuristiken um schnell eine Lösung
zu erhalten, wenn auch auf Kosten der Optimalität. In dieser Dissertation zeigen
wir, dass es, unter Ausnutzung gewisser graphentheoretischer Eigenschaften der
zulässigen Lösungen, durchaus möglich ist große anwendungsnahe Probleminstanzen
der von uns betrachteten Probleme beweisbar optimal und praktisch-effizient
zu lösen. Basierend auf Orientierungseigenschaften der optimalen Lösungen, formulieren
wir neue, beweisbar stärkere ILPs und lösen diese anschließend mit Hilfe
maßgeschneiderter Branch-and-Cut Algorithmen. Durch umfangreiche polyedrische
Analysen können wir beweisen, dass diese Modelle einerseits formal stärkere Beschreibungen
der Lösungsräume liefern als bisher bekannte Modelle und andererseits
für Branch-and-Cut Verfahren viele praktische Vorteile besitzen. Im Kontext des
{0,1,2}-SND geben wir zum ersten Mal eine Orientierungseigenschaft zweiknotenzusammenhängender Graphen an, die zu einer beweisbar stärkeren ILP-Formulierung führt und lösen damit ein in der Literatur seit langem offenes Problem. Unsere
experimentellen Ergebnisse für beide Problemklassen zeigen, dass während noch
vor kurzem nur Instanzen mit weniger als 200 Knoten in annehmbarer Zeit berechnet
werden konnten unsere Algorithmen das optimale Lösen von Instanzen mit
mehreren tausend Knoten erlauben. Insbesondere für das KCT Problem ist unser
exaktes Verfahren oft sogar schneller als moderne Metaheuristiken, die i.d.R. keine
optimale Lösungen finden.The subject of this thesis are exact solution strategies for topological network design
problems. These combinatorial optimization problems arise in various real-world
scenarios, as, e.g., in the telecommunication and energy industries. The prime task
thereby is to plan or extend networks, physically connecting customers. In general
we can describe such problems graph-theoretically as follows: Given a set of nodes
(customers, street crossings, routers, etc.), a set of edges (potential connections, e.g.,
cables), and a cost function on the edges and/or nodes. We ask for a subset of nodes
and edges, such that the sum of the costs of the selected elements is minimized while
satisfying side-conditions as, e.g., connectivity, reliability, or cardinality. In this
thesis we concentrate on two special classes of topological network design problems:
the k-cardinality tree problem (KCT) and the f0,1,2g-survivable network design
problem (f0,1,2g-SND) with node-connectivity constraints. These problems are in
general NP-hard, i.e., according to the current knowledge, it is very unlikely that
optimal solutions can be found efficiently (i.e., in polynomial time) for all possible
problem instances.
The above problems can be formulated as integer linear programs (ILPs), i.e.,
as systems of linear inequalities, integral variables, and a linear objective function.
Such models can be solved using methods of mathematical programming. Generally,
the corresponding solutions methods can be very time-consuming. This was
often used as an argument for developing (meta-)heuristics to obtain solutions fast,
although at the cost of their optimality. However, in this thesis we show that, exploiting
certain graph-theoretic properties of the feasible solutions, we are able to
solve large real-world problem instances to provable optimality efficiently in practice.
Based on orientation properties of optimal solutions we formulate new, provably
stronger ILPs and solve them via specially tailored branch-and-cut algorithms.
Our extensive polyhedral analyses show that these models give tighter descriptions
of the solution spaces and also offer certain algorithmic advantages in practice. In
the context of f0,1,2g-SND we are able to present the first orientation property
of 2-node-connected graphs which leads to a provably stronger ILP formulation,
thereby answering a long standing open research question. Until recently, both our
problem classes allowed optimal solutions only for instances with roughly up to 200
nodes. Our experimental results show that our new approaches allow instances with
thousands of nodes. Especially for the KCT problem, our exact method is often
even faster than state-of-the-art metaheuristics, which usually do not find optimal
solutions
Hub & regenerator location and survivable network design
Ankara : The Department of Industrial Engineering and the Institute of Engineering and Science of Bilkent University, 2010.Thesis (Ph. D.) -- Bilkent University, 2010.Includes bibliographical references leaves 180-184.With the vast development of the Internet, telecommunication networks are employed
in numerous different outlets. In addition to voice transmission, which is
a traditional utilization, telecommunication networks are now used for transmission
of different types of data. As the amount of data transmitted through the
network increases, issues such as the survivability and the capacity of the network
become more imperative. In this dissertation, we deal with both design and routing
problems in telecommunications networks. Our first problem is a two level
survivable network design problem. The topmost layer of this network consists of
a backbone component where the access equipments that enable the communication
of the local access networks are interconnected. The second layer connects
the users on the local access network to the access equipments, and consequently
to the backbone network. To achieve a survivable network, one that stays operational
even under minor breakdowns, the backbone network is assumed to be
2-edge connected while local access networks are to have the star connectivity.
Within the literature, such a network is referred to as a 2-edge connected/star
network. Since the survivability requirements of networks may change based on
the purposes they are utilized for, a variation of this problem in which local access
networks are also required to be survivable is also analyzed. The survivability of
the local access networks is ensured by providing two connections for every component
of the local access networks to the backbone network. This architecture
is known as dual homing in the literature. In this dissertation, the polyhedral
analysis of the two versions of the two level survivable network design problem is
presented; separation problems are analyzed; and branch-and-cut algorithms are
developed to find exact solutions.
The increased traffic on the telecommunications networks requires the use of high capacity components. Optical networks, composed of fiber optical cables,
offer solutions with their higher bandwidths and higher transmission speeds. This
makes the optical networks a good alternative to handle the rapid increase in the
data traffic. However, due to signal degradation which makes signal regeneration
necessary introduces the regenerator placement problem as signal regeneration is
a costly process in optical networks. In the regenerator placement problem, we
study a location and routing problem together on the backbone component of a
given telecommunications network. Survivability is also considered in this problem
simultaneously. Exact solution methodologies are developed for this problem:
mathematical models and some valid inequalities are proposed; separation problems
for the valid inequalities are analyzed and a branch-and-cut algorithm is
devised.Özkök, OnurPh.D
On the separation of partition inequalities
Given a graph ¢¤£¦¥¨§�©� � with nonnegative ����� weights for each � edge, a partition inequality is of the ������� form. ��� Here denotes the multicut defined � by of. Partition inequalities arise as valid inequalities for optimization problems related � to-connectivity. In this paper, we will show that, if ¢ decomposes into by 1 and 2-node cutsets, then the separation problem for the partition inequalities on ¢ can be solved by mean of a polynomial time combinatorial algorithm provided that such an algorithm exists for ¢���§������� § ¢ � where ¢���§������� § ¢ � are graphs related to ¢���§�������§� ¢ �. W
On some algorithmic aspects of hypergraphic matroids
International audienceHypergraphics matroids were studied first by Lorea [18] and later by Frank et al [8]. They can be seen as generalizations of graphic matroids. Here we show that several algorithms developed for the graphic case can be extended to hypergraphic matroids. We treat the following: the separation problem for the associated polytope, testing independence, separation of partition inequalities, computing the rank of a set, computing the strength, computing the arboricity and network reinforcement