This thesis studies how structure and energetics influence complex food web dynamics. In Chapter 1, I approach the question by studying a simple food web model that can be modified to include different structural and energetic features. In Chapter 2, I study stability and food web properties in assembled webs that are structured by body-size restrictions and a generalist-specialist tradeoff. The results of both chapters suggest that structure and energetics must be considered to understand food web dynamics. In Chapter 1, I find that food web structure can be modified by weak and strong energetic flows and stabilize dynamics through asynchrony. In Chapter 2, I find that food web assembly with different assemblages of generalists or specialists leads to structures that have corresponding differences in the stability and properties of food webs. Hence, my thesis reexamines the relationship of complexity and dynamics from a topological and energetic framework.

This dissertation considers the random access process in the Medium Access Control (MAC) of communications system. New MAC models are developed to improve the performance of random access based systems. The first contribution is the introduction of a general multichannel random access model with a variable radix. This model is general and can be applied to many existing MAC protocols that utilize random access. It is shown that using the standard Binary Exponential Backoff (BEB) to resolve collisions is not always the best choice. By adjusting the radix, contention efficiency can be improved significantly. The analytical results obtained are confirmed by simulation. The second contribution is the investigation of the variable radix backoff strategy with the contention-based bandwidth request (BW-REQ) mechanism in IEEE 802.16 systems. An analytical model of the BW-REQ procedure is presented which includes a variable radix in the backoff process. Analytical results are presented which show that the variable radix can easily be adjusted to the number of users and the available resources to enhance the efficiency of the Random Access Channel in the uplink subframe. Simulations results are presented to confirm the theory. The third contribution is the development of a reliable Quality of Service (QoS) mechanism for random access systems. The available resources are quantitatively categorized to provide differential services to two classes of users. The model is extended to employ a variable radix strategy. Results show that this strategy can be used in combination with differential services to provide an efficient QoS technique for random access. The fourth contribution is an optimized packet-based finite state Markov chain (FSMC) model for the physical channel. This model employs an equal average fade range duration (AFRD) strategy to partition the signal-to-noise ratio (SNR). The Nakagami-m fading channel model is used as it can span a wide range of fading conditions. The accuracy of the analytical results is confirmed by simulation. A cross-layer Markov model encompassing the FSMC model and a general multichannel random access model is introduced. Finally, a simulation toolbox using object oriented programming is presented. It was used to accurately simulate the models developed in this dissertation. This toolbox is general and can be used for a wide range of MAC models. / Graduate

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Graduation date: 1967

Thesis (Ph. D.)--Michigan State University. Fisheries and Wildlife, Ecology, Evolutionary Biology, and Behavior, 2008. / Title from PDF t.p. (viewed on July 2, 2009) Includes bibliographical references (p. 143-159). Also issued in print.

Thesis (M.Sc.). / Written for the Dept. of Natural Resource Sciences. Title from title page of PDF (viewed 2009/06/29). Includes bibliographical references.

Thesis (Ph. D.)--University of Hawaii at Manoa, 2004. / Includes bibliographical references.

Thesis (Ph.D.) - University of Queensland, 2005. / Includes bibliography.

Cette thèse sur travaux composée de quatre articles s’intéresse aux chaînes logistiques à deux échelons comportant un fournisseur en situation de monopole et de N clients dont les demandes pour un même produit sont corrélées. Les trois premiers articles étudient l’impact simultané des coopérations verticale et horizontale sur la performance globale. L’article 1 propose des extensions du modèle d’optimisation des stocks de Zhu et Thonemann (2004) en complétant notamment la coopération verticale entre le fournisseur et ses clients par une alliance horizontale et un échange d’informations entre clients. L’article 2 propose une modélisation multi-agents individu-centrée afin d’étudier l’impact sur la performance et la stabilité de la chaîne, de la diversité des comportements des clients face au risque et de leurs règles d’interaction dans le cadre d’une possible distorsion de l’information échangée. Nous proposons ensuite deux articles à visée plus applicative. L’article 3 s’intéresse à l’industrie pharmaceutique et étudie l’influence de la coopération entre grossistes-répartiteurs socialement responsables et leur laboratoire fournisseur pour réduire les surstocks et les gaspillages. L’article 4 consiste à comparer un pilotage centralisé à un pilotage décentralisé des stocks suite à un choc important de demande d’un produit alimentaire périssable. Les résultats de simulation font ressortir des conditions d’équilibre ainsi que des recommandations sur le pilotage global de ce type de chaînes. Plus généralement, cette thèse a permis de montrer l’intérêt d’une approche connexionniste de chaînes logistiques complexes avec agents hétérogènes s’échangeant de l’information. / AThis thesis is composed by two papers focusing on two-level supply chains with a monopoly supplier and N clients whose demands for the same products are correlated. The first paper studies the simultaneous impact of vertical and horizontal cooperation on the overall performance and the stability of the chain. It proposes an individual-centered multi-agents approach for studying the impact on performance of different kinds of customer behaviors associated with overstock risk and their interaction rules under possible distortion of the exchanged information. The second paper deals with pharmaceutical distribution and studies the influence of the cooperation between wholesale distributors and a unique supplier to reduce overstocks and wastes. More generally, this thesis has shown the relevance of a connectionist approach of complex supply chains with heterogeneous agents exchanging information.