Browsing by Subject "Hierarchical network design"

Now showing 1 - 4 of 4

Open Access
Hierarchical multimodal hub location problem with time-definite deliveries
(Elsevier, 2012) Alumur, S. A.; Yaman, H.; Kara, B. Y.
Hierarchical multimodal hub location problem is a cost-minimizing hub covering problem where two types of hubs and hub links, accounting for ground and air transportation, are to be established, while ensuring time-definite deliveries. We propose a mixed-integer programming formulation and perform a comprehensive sensitivity analysis on the Turkish network. We show that the locations of airport hubs are less sensitive to the cost parameters compared to the locations of ground hubs and it is possible to improve the service quality at not much additional cost in the resulting multimodal networks. Our methodology provides the means for a detailed trade-off analysis.
Open Access
Hub location problem for air-ground transportation sistems with time restrictions
(2006) Elmastaş, Seda
In this thesis, we study the problem of designing a service network for cargo delivery sector. We analyzed the structure of cargo delivery firms in Turkey and identified the features of the network. Generally, in the literature only one type of vehicle is considered when dispatching cargo. However, our analysis showed that in some cases both planes and trucks are used for a better service quality. Therefore, we seek a design in which all cargo between origin and destinations is delivered with minimum cost using trucks or planes within a given time bound. We call the problem “Time Constrained Hierarchical Hub Location Problem (TCHH)” and propose a model for it. The model includes some non-linear constraints. After linearizations, the TCHH is solved with data taken from cargo delivery firms. The computational results are reported and comparison with the current structure of a cargo delivery firm is given.
Open Access
Routing and scheduling decisions in the hierarchical hub location problem
(Elsevier, 2014) Dükkancı, Okan; Kara, Bahar Y.
Hubs are facilities that consolidate and disseminate flow in many-to-many distribution systems. The hub location problem considers decisions that include the locations of hubs in a network and also the allocations of the demand (non-hub) nodes to these hubs. We propose a hierarchical multimodal hub network. Based on this network, we define a hub covering problem with a service time bound. The hierarchical network consists of three layers. We consider two different structures: ring-star-star (RSS) and ring-ring-star (RRS). The multimodal network has three different types of vehicles in each layer: airplanes, large trucks and small trucks. For the proposed problems (RSS and RRS), we present and strengthen two mathematical models with some variable fixing rules and valid inequalities. We conduct the computational analysis over the Turkish network and the CAB data sets.
Open Access
Survivability in hierarchical telecommunications networks under dual homing
(Institute for Operations Research and the Management Sciences (I N F O R M S), 2014) Karaşan, O. E.; Mahjoub, A. R.; Özkök, O.; Yaman, H.
The motivation behind this study is the essential need for survivability in the telecommunications networks. An optical signal should find its destination even if the network experiences an occasional fiber cut. We consider the design of a two-level survivable telecommunications network. Terminals compiling the access layer communicate through hubs forming the backbone layer. To hedge against single link failures in the network, we require the backbone subgraph to be two-edge connected and the terminal nodes to connect to the backbone layer in a dual-homed fashion, i.e., at two distinct hubs. The underlying design problem partitions a given set of nodes into hubs and terminals, chooses a set of connections between the hubs such that the resulting backbone network is two-edge connected, and for each terminal chooses two hubs to provide the dual-homing backbone access. All of these decisions are jointly made based on some cost considerations. We give alternative formulations using cut inequalities, compare these formulations, provide a polyhedral analysis of the smallsized formulation, describe valid inequalities, study the associated separation problems, and design variable fixing rules. All of these findings are then utilized in devising an efficient branch-and-cut algorithm to solve this network design problem.