Claudio Sterle

A metaheuristic for a two echelon location-routing problem

In this paper we consider the design problem of a two-echelon freight distribution system. The aim is to define the structure of a system optimizing the location and the number of two different kinds of facilities, the size of two different vehicle fleets and the related routes on each echelon. The problem has been modeled as a two-echelon location-routing problem (2E-LRP). A tabu-search heuristic efficiently combining the composing subproblems is presented. Results on small, medium and large size instances are reported.

A Cut and Branch Approach for the Capacitated p-Median Problem Based on Fenchel Cutting Planes

The capacitated p-median problem (CPMP) consists of finding p nodes (the median nodes) minimizing the total distance to the other nodes of the graph, with the constraint that the total demand of the nodes assigned to each median does not exceed its given capacity. In this paper we propose a cutting plane algorithm, based on Fenchel cuts, which allows us to considerably reduce the integrality gap of hard CPMP instances. The formulation strengthened with Fenchel cuts is solved by a commercial MIP solver. Computational results show that this approach is effective in solving hard instances or considerably reducing their integrality gap.

An exact dynamic programming algorithm for large-scale unconstrained two-dimensional guillotine cutting problems

Abstract In the unconstrained two-dimensional cutting problems (U2DCP) small rectangular objects have to be extracted from a large rectangular sheet, with no limits on the number of small objects. The exact U2DCP solving approaches present in literature show some limits in tackling very large size instances, due to the high memory requirements. In this work we propose five improvements, three original and two derived from the literature, in order to overcome these limits and to reduce the computational burden of the knapsack function based U2DCP solving approaches. These improvements, based on proofed theoretical results, allow to reduce the search space and to avoid redundant solutions without loss of the feasible ones. The presented improvements, together with several computational refinements, are integrated in a new dynamic programming algorithm, which modifies the one by Russo et al. (2013 [16] ). The proposed algorithm has been experienced on test instances present in literature and compared with the best U2DCP solving approaches. The obtained results show that it significantly outperforms them and it determines the optimal solution of unsolved very large size instances.

Optimization Models in a Smart Tool for the Railway Infrastructure Protection

In this paper we describe a smart tool, developed for the European project METRIP (MEthodological Tool for Railway Infrastructure Protection) based on optimal covering integer programming models to be used in designing the security system for a Railway Infrastructure. Two models are presented and tested on a railway station scheme. The results highlight the role that the optimization models can fulfill in the design of an effective security system.

A unified solving approach for two and three dimensional coverage problems in sensor networks

The problem of designing a wired or a wireless sensor network to cover, monitor and/or control a region of interest has been widely treated in literature. This problem is referred to in literature as the sensor placement problem (SPP) and in the most general case it consists in determining the number and the location of one or more kind of sensors with the aim of covering all the region of interest or a significant part of it. In this paper we propose a unified and stepwise solving approach for two and three dimensional coverage problems to be used in omni-directional and directional sensor networks. The proposed approach is based on schematizing the region of interest and the sensor potential locations by a grid of points and representing the sensor coverage area by a circle or by a circular sector. On this basis, the SPP is reduced to an optimal coverage problem and can be formulated by integer linear programming (ILP) models. We will resume the main ILP models used in our approach, highlighting, for each of them, the specific target to be achieved and the design constraints taken into account. The paper concludes with an application of the proposed approach to a real test case and a discussion of the obtained results.

Optimal Location of Security Devices

The design of a security system, in terms of number and position of the security devices composing it, is one of the main issue tackled in METRIP project (MEthodological Tool for Railway Infrastructure Protection). It is a complex problem where a very large set of configurations has to be explored in order to determine the most efficient one, which guarantees the highest protection level. Indeed a good placement of the devices has to satisfy two main targets. On one side it has to guarantee the highest security level, i.e. it has to be able to control the widest achievable area. On the other side, it has to be economically sustainable, i.e. it has to be realized with acceptable costs. In literature this problem is generally referred to as sensor placement problem, widely treated by integer linear programming models and combinatorial optimization methods. In this chapter we will present the main covering models present in literature and adopted in METRIP project for the placement of devices preventing the malicious intrusions in railway assets, with particular reference to the intrusions in a railway station. The applicability of these models will be proved using two test cases which represent two typical railway asset schemes.

Joint content-resource allocation in software defined virtual CDNs

A software defined virtual CDN (SDvCDN) is a virtual cache network deployed fully in software over a programmable cloud network infrastructure that can be elastically consumed and optimized using global information about network conditions and service requirements. We formulate the joint content-resource allocation problem in SDvCDNs as a minimum cost mixed-cast flow problem with resource activation decisions. Our solution jointly optimizes the placement and routing of content objects along with the allocation of the required virtual storage and transport resources, is applicable to arbitrary network topologies, and captures activation and operational costs, content popularity, unicast and multicast delivery, as well as capacity and latency constraints. We present preliminary results for the general mixed integer linear programming solution, as well as conditions for polynomial solvability. The value of our approach goes beyond the optimization of SDvCDNs, as it provides an efficient methodology to jointly address placement (facility location), routing (network flow) and resource allocation (network design/embedding) problems.

Optimal location of flow intercepting facilities to improve security in urban areas

The problem of the optimal location of control facilities in an urban area for improving security in case of large events or critical situations is presented. The paper focuses on the prevention of threats by the control and/or the interdiction of people and vehicle flows entering an area (restricted zone). In general, limited available resources do not allow monitoring all the access points of the area. So it is fundamental to identify the most suitable points where control facilities should be located. Two flow intercepting facility location models will be presented: the first one is devoted to identify the best location of a limited number of control facilities to maximise the security level; the second one is aimed at computing the minimum number of control facilities able to guarantee a prescribed level of security. Results obtained on test cases representative of different urban structures are presented.

Multi-commodity location-routing

We propose a new ILP formulation for the multi-commodity location-routing problem.The new formulation models the location decisions by a flow intercepting approach.The usage of the proposed formulation for the design of a single-tier freight distribution system is discussed.A branch-and-cut algorithm is proposed and experienced on test networks with different topologies. Research on the location-routing problem (LRP) is very active, producing a good number of effective exact and approximated solution approaches. It is noteworthy that most of the contributions present in the literature address the single-commodity LRP, whereas the multi-commodity case has been scarcely investigated. Yet, this issue assumes an important role in many LRP applications, particularly in the context of designing single-tier freight distribution City Logistics systems. To fill this gap, we define a new multi-commodity LRP, proposing an original integer linear programming model for it. The proposed formulation takes into account the multi-commodity feature of the problem, modeling the strategic location and the tactical routing decisions using the flow intercepting approach. We therefore name this problem the flow intercepting facility location-routing problem. It is solved by a branch-and-cut algorithm which exploits cuts derived and adapted from literature. The proposed method is successfully experienced and validated on test instances reproducing different network topologies and problem settings.

Optimizing Investment Decisions for Railway Systems Protection

As demonstrated by recent events, railway systems are often the target of terrorist bombings and attacks. To preserve public safety and essential economic functions, railroad networks should be made as secure and resilient as possible. However, railway protection investments may involve significant and often unaffordable capital expenditure. Given the limited resources available for protection efforts, it is essential that a strategic approach to the planning of security investments is adopted. This chapter presents a mathematical model for identifying the optimal allocation of protective resources among the components of a railway network. The aim is to minimize the impact on passenger flow of worst-case disruptions which might affect both railway stations and tracks. The proposed model is tested on an Italian railroad network to demonstrate how the model results can be used to inform policy making and protection investment decisions.