Browsing by Subject "Networks"
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Item Open Access 3D forest fire propagation simulation(IEEE, 2008-05) Köse, Kıvanç; Grammalidis, N.; Yılmaz, E.; Çetin, A. EnisThe increase in the number of forest fires in the last few years dispatch governments to take precautions. Besides prevention, early intervention is also very important in fire fighting. If the firefighters know where the fire will be in some time, it would be easier for them to stop the fire. Therefore a big need for simulating the fire behavior exists. In this paper we are proposing a system which can simulate the propagation of fire in time. Also this system can visualize the propagation of fire in any 3D-GIS environment, that accepts KMZ as a file format. Besides, any user demanded data can be visualized on the map of the system. This gives the chance of fire planning to firefighters. The system can visualize its results on 3D screens in 3D. Therefore, a better understanding of the terrain can be obtained. ©2008 IEEE.Item Open Access Accurate positioning in ultra-wideband systems(Institute of Electrical and Electronics Engineers, 2011) Soganci, H.; Gezici, SinanAccurate positioning systems can be realized via ultra-wideband signals due to their high time resolution. In this article, position estimation is studied for UWB systems. After a brief introduction to UWB signals and their positioning applications, two-step positioning systems are investigated from a UWB perspective. It is observed that time-based positioning is well suited for UWB systems. Then time-based UWB ranging is studied in detail, and the main challenges, theoretical limits, and range estimation algorithms are presented. Performance of some practical time-based ranging algorithms is investigated and compared against the maximum likelihood estimator and the theoretical limits. The trade-off between complexity and accuracy is observed.Item Open Access Autonomous multiple teams establishment for mobile sensor networks by SVMs within a potential field(2012) Nazlibilek, S.In this work, a new method and algorithm for autonomous teams establishment with mobile sensor network units by SVMs based on task allocations within a potential field is proposed. The sensor network deployed into the environment using the algorithm is composed of robot units with sensing capability of magnetic anomaly of the earth. A new algorithm is developed for task assignment. It is based on the optimization of weights between robots and tasks. The weights are composed of skill ratings of the robots and priorities of the tasks. Multiple teams of mobile units are established in a local area based on these mission vectors. A mission vector is the genetic and gained background information of the mobile units. The genetic background is the inherent structure of their knowledge base in a vector form but it can be dynamically updated with the information gained later on by experience. The mission is performed in a magnetic anomaly environment. The initial values of the mission vectors are loaded by the task assignment algorithm. The mission vectors are updated at the beginning of each sampling period of the motion. Then the teams of robots are created by the support vector machines. A linear optimal hyperplane is calculated by the use of SVM algorithm during training period. Then the robots are classified as teams by use of SVM mechanism embedded in the robots. The support vector machines are implemented in the robots by ordinary op-amps and basic logical gates. Team establishment is tested by simulations and a practical test-bed. Both simulations and the actual operation of the system prove that the system functions satisfactorily. © 2012 Elsevier Ltd. All rights reserved.Item Open Access Coincidence of Myerson allocation rule with Shapley value(Bilkent University, 2003) Kapan, TümerThis thesis studies the coincidence of the Myerson allocation rule in the context of networks with the Shapley value in the context of transferable utility games. We start with a value function defined on networks and derive a transferable utility game from that. We show that without any restrictions on the value function, Myerson allocation rule may not lead to the same payoff vector as the Shapley value of the derived TU game for any network. Under the assumption of monotonicity of the value function, we show the existence of such coincidence and examine the relation of the set of networks satisfying this coincidence to the set of pairwise stable and strongly stable networks. Next, we propose a new stability notion and examine the coincidence of the two vectors under this stability notion. Finally an alternative allocation rule is introduced whose payoff vector coincide with the Shapley value of the derived transferable utility game on the set of efficient networks which coincides with the set of strongly stable networks under this allocation rule.Item Open Access Contests over joint production on networks(Wiley, 2020) Doğan, Serhat; Keskin, K.; Sağlam, ÇağrıWe consider a network of heterogeneous agents where each edge represents a two‐player contest between the respective nodes. In these bilateral contests, agents compete over an endogenous prize jointly produced using their own contest efforts. We provide a necessary and sufficient condition for the existence of Nash equilibrium and characterize the equilibrium total effort for every agent. Our model has insightful results regarding the network type, that is, depending on whether the network is bipartite or nonbipartite. Finally, considering the sum of all expected utilities as an efficiency notion, we investigate the optimal network structure.Item Open Access Diffusion approximation in overloaded switching queueing models(Springer, 2002) Anisimov, V. V.The asymptotic behavior of a queueing process in overloaded state-dependent queueing models (systems and networks) of a switching structure is investigated. A new approach to study fluid and diffusion approximation type theorems (without reflection) in transient and quasi-stationary regimes is suggested. The approach is based on functional limit theorems of averaging principle and diffusion approximation types for so-called Switching processes. Some classes of state-dependent Markov and non-Markov overloaded queueing systems and networks with different types of calls, batch arrival and service, unreliable servers, networks (MSM,Q/MSM,Q/1/∞)r switched by a semi-Markov environment and state-dependent polling systems are considered.Item Open Access Distributed k-Core view materialization and maintenance for large dynamic graphs(Institute of Electrical and Electronics Engineers, 2014-10) Aksu, H.; Canim, M.; Chang, Yuan-Chi; Korpeoglu, I.; Ulusoy, O.In graph theory, k-core is a key metric used to identify subgraphs of high cohesion, also known as the ‘dense’ regions of a graph. As the real world graphs such as social network graphs grow in size, the contents get richer and the topologies change dynamically, we are challenged not only to materialize k-core subgraphs for one time but also to maintain them in order to keep up with continuous updates. Adding to the challenge is that real world data sets are outgrowing the capacity of a single server and its main memory. These challenges inspired us to propose a new set of distributed algorithms for k-core view construction and maintenance on a horizontally scaling storage and computing platform. Our algorithms execute against the partitioned graph data in parallel and take advantage of k-core properties to aggressively prune unnecessary computation. Experimental evaluation results demonstrated orders of magnitude speedup and advantages of maintaining k-core incrementally and in batch windows over complete reconstruction. Our algorithms thus enable practitioners to create and maintain many k-core views on different topics in rich social network content simultaneously.Item Open Access Document replication strategies for geographically distributed web search engines(Elsevier Ltd., 2013) Kayaaslan, E.; Cambazoglu, B. B.; Aykanat, CevdetLarge-scale web search engines are composed of multiple data centers that are geographically distant to each other. Typically, a user query is processed in a data center that is geographically close to the origin of the query, over a replica of the entire web index. Compared to a centralized, single-center search engine, this architecture offers lower query response times as the network latencies between the users and data centers are reduced. However, it does not scale well with increasing index sizes and query traffic volumes because queries are evaluated on the entire web index, which has to be replicated and maintained in all data centers. As a remedy to this scalability problem, we propose a document replication framework in which documents are selectively replicated on data centers based on regional user interests. Within this framework, we propose three different document replication strategies, each optimizing a different objective: reducing the potential search quality loss, the average query response time, or the total query workload of the search system. For all three strategies, we consider two alternative types of capacity constraints on index sizes of data centers. Moreover, we investigate the performance impact of query forwarding and result caching. We evaluate our strategies via detailed simulations, using a large query log and a document collection obtained from the Yahoo! web search engine. (C) 2012 Elsevier Ltd. All rights reserved.Item Open Access Equilibrium refinements for the network formation game(Springer, 2019-01) İlkılıç, Rahmi; İkizler, HüseyinThis paper examines a normal form game of network formation due to Myerson (Game theory: analysis of conflict, Harvard University Press, Cambridge, 1991). All players simultaneously announce the links they wish to form. A link is created if and only if there is mutual consent for its formation. The empty network is always a Nash equilibrium of this game. We define a refinement of Nash equilibria that we call trial perfect. We show that the set of networks which can be supported by a pure strategy trial perfect equilibrium coincides with the set of pairwise-Nash equilibrium networks, for games with link-responsive payoff functions.Item Open Access Essays in collective decision making(Bilkent University, 2014-10) Derya, Ayşe MutluFour different problems in collective decision making are studied, all of which are either formulated directly in a game-theoretical context or are concerned with neighboring research areas. The rst two problems fall into the realm of cooperative game theory. In the first one, a decomposition of transferable utility games is introduced. Based on that decomposition, the structure of the set of all transferable utility games is analyzed. Using the decomposition and the notion of minimal balanced collections, a set of necessary and sufficient conditions for a transferable utility game to have a singleton core is given. Then, core selective allocation rules that, when confronted with a change in total cost, not only distribute the initial cost in the same manner as before, but also treat the remainder in a consistent way are studied. Core selective rules which own a particular kind of additivity that turns out to be relevant in this context are also characterized. In the second problem, different notions of merge proofness for allocation rules pertaining to transferable utility games are introduced. Relations between these merge proofness notions are studied, and some impossibility as well as possibility results for allocation rules are established, which are also extended to allocation correspondences. The third problem deals with networks. A characterization of the Myerson value with two axioms is provided. The first axiom considers a situation where there is a change in the value function at a network g along with all networks containing g. At such a situation, the axiom requires that this change is to be divided equally between all the players in g who are not isolated. The second axiom requires that if the value function assigns zero to each network, then each player gets zero payo at each network. Modifying the rst axiom, along a characterization of the Myerson value, a characterization of the position value is also provided. Finally, the fourth problem is concerned with social choice theory which deals with collective decision making in a society. A characterization of the Borda rule for a given set of alternatives with a variable number of voters is studied on the domain of weak preferences, where indi erences between alternatives are allowed at agents' preferences. A new property, which we refer to as degree equality, is introduced. A social choice rule satis es degree equality if and only if, for any two pro les of two nite sets of voters, equality between the sums of the degrees of every alternative under these two pro les implies that the same alternatives get chosen at both of them. The Borda rule is characterized by the conjunction of faithfulness, reinforcement, and degree equality on the domain of weak preferences.Item Open Access Essays on bargaining theory(Bilkent University, 2018-05) Özcan Tok, ElifBargaining refers to a situation where two or more agents try to decide over how to divide a surplus generated by the economic transactions among these agents. There are two major approaches to bargaining problems: cooperative and non-cooperative approach. The former one focuses on the axioms that a bargaining outcome should satisfy and it is initiated by Nash (1950). The latter one attempts to specify the bargaining procedure and it is pioneered by Stahl (1972) and Rubinstein (1982). This dissertation consists of five essays. The first three essays employ the non-cooperative bargaining approach; the remaining ones employ the cooperative bargaining approach. In the first essay, we study an infinite horizon bargaining game on a network, where the network is endogenously formed. Two specifications of the cost structure regarding the link formation is investigated: zero cost and non-zero cost. The equilibrium of the game is obtained for both specifications. Lastly, we focus on efficiency issue and characterize the efficient networks. In the second essay, an infinite horizon bargaining game between buyers and sellers on a two-sided supply chain network is analyzed where the valuations of the buyers are heterogeneous. We prove that the valuations of the buyers and the network structure have an impact on the equilibrium outcome. In the third essay, we investigate the emergence of reference points in a two-player, infinite horizon, alternating offers bargaining game. The preferences of players preferences exhibit reference-dependence, and their current offers have the potential to in uence future reference points of each other. However, this in uence is limited in that it expires in a finite number of periods. We describe a subgame perfect equilibrium that involves an immediate agreement. We study the in uence of expiration length and reference points on equilibrium strategies and outcomes. In the fourth essay, we study the salience of the reference points in determining the anchors and aspirations in a bargaining problem by introducing two parameters which capture these effects. In the cooperative bargaining literature, the disagreement point or the reference point is employed as an anchor while the ideal (or utopia) point or the tempered aspirations point as an aspiration. In this essay, a bargaining problem with a reference point is studied incorporating these two parameters and hence a family of bargaining solutions is obtained. Consequently, several characterizations for each individual member of this family is proposed. In the fifth essay, we introduce the iterated egalitarian compromise solution for two-person bargaining problems. It is defined by using two well-known solutions to bargaining problems, the egalitarian solution and the equal-loss solution, in an iterative fashion. While neither of these two solutions satisfy midpoint domination {an appealing normative property{ we show that the iterated egalitarian compromise solution does so. To sum up, this dissertation contributes to the diversified fields and practices of bargaining theory.Item Open Access Network dependent altruism and economic growth(Bilkent University, 2016-07) Çetin, SefaneThis thesis studies reference dependent agents in a static network where the reference point is the average behaviour in one's neighbourhood. It shows that the economy grows at a constant rate on the balanced growth path while income inequality and the speed of convergence depend on the specific network structure. For particular networks, initial inequalities remain in the long-run. However, it is also possible that depending on the location of agents, a lagged agent may surpass the ones who had higher initial human capital than her and as a result, a society that has too disperse initial distribution may admit an equilibrium in which the wealth distribution converges towards equality in the long-run. In short, in an economy at which the production depends on human capital, income inequalities can be explained by taking the network structure of the economy into account.Item Open Access PSAR: Power-source-aware routing in ZigBee networks(2012) Tekkalmaz, M.; Korpeoglu I.ZigBee is a recent wireless networking technology built on IEEE 802.15.4 standard and designed especially for low-data rate and low-duty cycle applications such as home and building automation and sensor networks. One of the primary goals of ZigBee is low power consumption and therefore long-living networks. Despite this goal, current network formation and routing protocols described in the ZigBee specification do not fully address power consumption issues. In this work, we propose a distributed routing algorithm to reduce power consumption of battery-powered devices by routing the communication through mains-powered devices whenever possible and consequently increasing the overall network lifetime. The proposed algorithm works on tree topologies supported by ZigBee and requires only minor modifications to the current specification. Our ns-2 simulation results showed that the algorithm is able to reduce the power consumption of battery-powered devices significantly with minimal communication overhead. © Springer Science+Business Media, LLC 2012.Item Open Access A scheme for handling connectionless services in connection-oriented networks(The Institute of Electronics, Information and Communication Engineers, 1997-10-25) Abdelati, M.; Arikan, E.In this work we propose a pricing scheme which serves as an instrument for managing connectionless services in connection-oriented communication networks. The scheme is able to allocate network bandwidth in a Pareto-optimal way that maximizes the total surplus. The key idea is to decompose the service provision procedure among three separate parties whose interactions are governed by a set of competitive pricing mechanisms.