Browsing by Author "Rios, M"

Now showing 1 - 6 of 6
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    Allocating servers to facilities, when demand is elastic to travel and waiting times
    (EDP SCIENCES S A, 2005) Marianov, V; Rios, M; Barros, FJ
    Public inoculation centers are examples of facilities providing service to customers whose demand is elastic to travel and waiting time. That is, people will not travel too far, or stay in line for too long to obtain the service. The goal, when planning such services, is to maximize the demand they attract, by locating centers and staffing them so as to reduce customers' travel time and time spent in queue. In the case of inoculation centers, the goal is to maximize the people that travel to the centers and stay in line until inoculated. We propose a procedure for the allocation of multiple servers to centers, so that this goal is achieved. An integer programming model is formulated. Since demand is elastic, a supply-demand equilibrium equation must be explicitly included in the optimization model, which then becomes nonlinear. As there are no exact procedures to solve such problems, we propose a heuristic procedure, based on Heuristic Concentration, which finds a good solution to this problem. Numerical examples are presented.
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    Finding locations for public service centres that compete with private centres: Effects of congestion
    (SPRINGER, 2004) Marianov, V; Rios, M; Taborga, P
    We propose a locational model for public service centres when they compete with private centres. Customers may be captured by the centre offering shorter distance or lower waiting times. Both types of centres provide paid services to high-income customers, and public centres also provide subsidised services to low-income customers. Equilibrium must be found in public centres between low-income population coverage and high-income population capture. We analyse the effects of waiting times on the design of the public service and apply the model to data for Santiago, Chile.
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    Ladybird (Coleoptera : Coccinellidae) dispersal in experimental fragmented alfalfa landscapes
    (CZECH ACAD SCI, INST ENTOMOLOGY, 2005) Grez, AA; Zaviezo, T; Rios, M
    Habitat fragmentation may affect the dispersal behaviour of individuals across the landscape. If there is a high boundary contrast between the remaining fragments and the matrix, individuals should tend to stay inside the fragments, but the configuration of the landscape, i.e. the level of fragmentation and isolation distance between fragments, may modulate this. To test these ideas, we made several mark-recapture experiments with the ladybird Eriopis connexa (Germ.) in manipulated alfalfa model landscapes (30 x 30 m). Specifically we evaluated (i) ladybird movement and permanence in alfalfa and bare ground areas, (ii) how they move across fragment/matrix boundaries, (iii) how their movement between fragments within a landscape is affected by fragmentation level and isolation distance, and (iv) how their permanence in the landscape is affected by fragmentation Ievel and isolation distance. The fragmented alfalfa landscapes had a constant level of habitat loss (84%), but varied in the level of habitat fragmentation (4 or 16 fragments), and isolation distance between fragments (2 or 6 m). We also sampled aphid abundance in the different landscapes through time. We found that ladybirds stay longer and displace less in alfalfa than on bare ground, and that they move less from the alfalfa fragments to the neighbouring bare ground matrix than vice versa. At the landscape level, ladybirds had a higher inter-fragment movement when fragments were closer, they tended to remain in less fragmented landscapes, regardless of their isolation distance. Aphid abundance increased in time, but it was similar in all landscapes. Ladybird movement within fragments, in the matrix and in the boundary between both habitats explains why ladybirds concentrate within alfalfa fragments. However, their dispersal movements at the landscape level do not reflect the higher abundance sometimes found for several species of ladybirds in more fragmented landscapes. At this larger scale, other mechanisms may have a stronger influence in determining the abundance of ladybirds.
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    Multiple path routing algorithm for IP networks
    (ELSEVIER SCIENCE BV, 2005) Rios, M; Marianov, V; Avagliano, A
    Internet routing protocols, such as Open Shortest Path First (OSPF), compute a shortest path tree from each node to other nodes in the network, using link-state information. Such protocols do not consider the queueing situation at a given node. An alternate path of higher cost may be more convenient to use than the optimum path when a long queue is present at the node. This paper proposes a new Multiple Path Routing Algorithm (MPRA), which uses dynamic shortest path tree mechanisms, load balancing among alternative paths to destination, and path feasibility analysis to avoid network loops. A better-cost model is also introduced. Computer simulations show that MPRA measured parameters (throughput, packet delay, etc.) improve over OSPF by amounts close to 30% for unbalanced networks, and close to 20% for balanced networks. (c) 2004 Elsevier B.V. All rights reserved.
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    Survivable capacitated network design problem: new formulation and Lagrangean relaxation
    (PALGRAVE MACMILLAN LTD, 2000) Rios, M; Marianov, V; Gutierrez, M
    This work is focused on the analysis of the survivable capacitated network design problem. This problem can be stated as follows: Given a supply network with point-to-point traffic demands, specific survivability requirements, a set of available capacity ranges and their corresponding discrete costs for each are, find minimum cost capacity expansions such that these demands can be met even if a network component fails. Solving this problem consists of selecting the links and their capacity, as well as the routings for each demand in every failure situation. This type of problem can be shown to be NP-hard. A new linear mixed-integer mathematical programming formulation is presented. An effective solution procedure based on Lagrangean relaxation is developed. Comparison heuristics and improvement heuristics are also described. Computational results using these procedures on different sizes of randomly generated networks are reported.