Browsing by Subject "Network analysis (Planning)"

Now showing 1 - 8 of 8

Open Access
A dynamic DRR scheduling algorithm for flow level QOS assurances for elastic traffic
(2006) Kurugöl, Sıla
Best effort service, used to transport the Internet traffic today, does not provide any QoS assurances. Intserv, DiffServ and recently proposed Proportional Diff- Serv architectures have been introduced to provide QoS. In these architectures, some applications with more stringent QoS requirement such as real time traffic are prioritized, while elastic flows share the remaining bandwidth. As opposed to the well studied differential treatment of delay and/or loss sensitive traffic to satisfy QoS constraints, our aim is satisfy QoS requirements of elastic traffic at the flow level. We intend to maintain different average rate levels for different classes of elastic traffic. For differential treatment of elastic flows, a dynamic variant of Deficit Round Robin Scheduler (DRR) is used as oppose to a FIFO queue. In this scheduling algorithm, all classes are served in a round robin fashion in proportion to their weights at each round. The main difference of our scheduler from the original DRR scheduler is that, we update the weights, which are called quantums of the scheduler at each round in response to the feedback from the network, which is in terms of the rate of phantom connection sharing capacity fairly with the other flows in the same queue. According to the rate measured in the last time interval, the controller updates the weights in proportion with the bandwidth requirements of each class to satisfy their QoS requirements, while the remaining bandwidth will be used by the best effort traffic. In order to find an optimal policy for the controller a simulation-based learning algorithm is performed using a processor sharing model of TCP, then the resultant policies are applied to a more realistic scenario to solve Dynamic DRR scheduling problem through ns-2 simulations.
Open Access
Dynamic threshold-based algorithms for communication networks
(2009) Toksöz, Mehmet Altan
A need to use dynamic thresholds arises in various communication networking scenarios under varying traffic conditions. In this thesis, we propose novel dynamic threshold-based algorithms for two different networking problems, namely the problem of burst assembly in Optical Burst Switching (OBS) networks and of bandwidth reservation in connection-oriented networks. Regarding the first problem, we present dynamic threshold-based burst assembly algorithms that attempt to minimize the average burst assembly delay due to burstification process while taking the burst rate constraints into consideration. Using synthetic and real traffic traces, we show that the proposed algorithms perform significantly better than the conventional timer-based schemes. In the second problem, we propose a model-free adaptive hysteresis algorithm for dynamic bandwidth reservation in a connection-oriented network subject to update frequency constraints. The simulation results in various traffic scenarios show that the proposed technique considerably outperforms the existing schemes without requiring any prior traffic information.
Open Access
Exact solution methodologies for the p-center problem under single and multiple allocation strategies
(2013) Çalık, Hatice
The p-center problem is a relatively well known facility location problem that involves locating p identical facilities on a network to minimize the maximum distance between demand nodes and their closest facilities. The focus of the problem is on the minimization of the worst case service time. This sort of objective is more meaningful than total cost objectives for problems with a time sensitive service structure. A majority of applications arises in emergency service locations such as determining optimal locations of ambulances, fire stations and police stations where the human life is at stake. There is also an increased interest in p-center location and related location covering problems in the contexts of terror fighting, natural disasters and human-caused disasters. The p-center problem is NP-hard even if the network is planar with unit vertex weights, unit edge lengths and with the maximum vertex degree of 3. If the locations of the facilities are restricted to the vertices of the network, the problem is called the vertex restricted p-center problem; if the facilities can be placed anywhere on the network, it is called the absolute p-center problem. The p-center problem with capacity restrictions on the facilities is referred to as the capacitated p-center problem and in this problem, the demand nodes can be assigned to facilities with single or multiple allocation strategies. In this thesis, the capacitated p-center problem under the multiple allocation strategy is studied for the first time in the literature. The main focus of this thesis is a modelling and algorithmic perspective in the exact solution of absolute, vertex restricted and capacitated p-center problems. The existing literature is enhanced by the development of mathematical formulations that can solve typical dimensions through the use of off the-shelf commercial solvers. By using the structural properties of the proposed formulations, exact algorithms are developed. In order to increase the efficiency of the proposed formulations and algorithms in solving higher dimensional problems, new lower and upper bounds are provided and these bounds are utilized during the experimental studies. The dimensions of problems solved in this thesis are the highest reported in the literature.
Open Access
Implementation of new and classical set covering based algorithms for solving the absolute p-center problem
(2011) Saç, Yiğit
The p-center problem is a model of locating p facilities on a network in order to minimize the maximum coverage distance between each vertex and its closest facility. The main application areas of p-center problem are emergency service locations such as fire and police stations, hospitals and ambulance services. If the p facilities can be located anywhere on a network including vertices and interior points of edges, the resulting problem is referred to as the absolute p-center problem and if they are restricted to vertex locations, it is referred to as the vertex-restricted problem. The absolute p-center problem is considerably more complicated to solve than the vertex-restricted version. In the literature, most of the computational analysis and new algorithm developments are performed through the vertex restricted case of the p-center problem. The absolute p-center problem has received much less attention in the literature. In this thesis, our focus is on the absolute p-center problem based on an algorithm for the p-center problem proposed by Tansel (2009). Our work is the first one to solve large instances up to 900 vertices on the absolute p-center problem. The algorithm focuses on solving the p-center problem with a finite series of minimum set covering problems, but the set covering problems used in the algorithm are constructed differently compared to the ones traditionally used in the literature. The proposed algorithm is applicable for both absolute and vertex-restricted p-center problems with weighted and unweighted cases.
Open Access
Network design problems in wavelength division multiplexing optical networks
(2001) Erdoğan, Güneş
In this study, we analyze the network design problem arising in Wavelength Division Multiplexing (WDM) networks where tra c is static, wavelength interchanging is allowed and the location and number of the wavelength interchangers are to be determined. Given a topology and tra c data, we try to nd the ber and wavelength interchanger con guration with the minimum cost, that can establish all given connections. We present di erent formulations of the problem and some valid inequalities. Finally, we propose a heuristic method of generating feasible solutions, apply the method on three di erent topologies with varying tra c data, and present the results. The method is based on the idea of partitioning the problem into two; routing problem and wavelength assignment and interchanger location problem. Our results prove to be close to the lower bounds we generate, and indicate that the ber cost performance of the case where all nodes are wavelength interchangers can be attained using a relatively small number of wavelength interchangers.
Open Access
Shortest path problem with re-routing en-route
(2008) Karakaya, Banu
In this study, we examine the shortest path problem under the possibility of “re-routing” when an arc that is being traversed is blocked due to reasons such as road and weather conditions, congestion, accidents etc. If an incident occurs along the arc being traversed, the vehicle either waits until all effects of the incident are cleared and then follows the same path thereafter, or returns to the starting node of that arc and follows an escape route to the destination node, the latter course of action is called as “re-routing”. Also, we consider that this arc is not visited again throughout the travel along the network when an incident occurs and the alternative of not following this arc after the event is chosen. We propose a labeling algorithm to solve this specific problem. Then, a real case problem is analyzed by the proposed algorithm and several numerical studies are conducted in order to assess the sensitivity of the probability and travel time parameters.
Open Access
Storage and access schemes for aggregate query processing on road networks
(2009) Demir, Engin
A well-known example of spatial networks is road networks, which form an integral part of many geographic information system applications, such as locationbased services, intelligent traveling systems, vehicle telematics, and locationaware advertising. In practice, road network data is too large to fit into the volatile memory. A considerable portion of the data must be stored on the secondary storage since several spatial and non-spatial attributes as well as points-ofinterest data are associated with junctions and links. In network query processing, the spatial coherency that exists in accessing data leads to a temporal coherency; in this way, connected junctions are accessed almost concurrently. Taking this fact into consideration, it seems reasonable to place the data associated with connected junctions in the same disk pages so that the data can be fetched to the memory with fewer disk accesses. We show that the state-of-the-art clustering graph model for allocation of data to disk pages is not able to correctly capture the disk access cost of successor retrieval operations. We propose clustering models based on hypergraph partitioning, which correctly encapsulate the spatial and temporal coherency in query processing via the utilization of query logs in order to minimize the number of disk accesses during aggregate query processing. We introduce the link-based storage scheme for road networks as an alternative to the widely used junction-based storage scheme. We present GetUnevaluated-Successors (GUS) as a new successor retrieval operation for network queries, where the candidate successors to be retrieved are pruned during processing a query. We investigate two different instances of GUS operation: the Get-unProcessed-Successors operation typically arises in Dijkstra’s single source shortest path algorithm, and the Get-unVisited-Successors operation typically arises in the incremental network expansion framework. The simulation resultsshow that our storage and access schemes utilizing the proposed clustering hypergraph models are quite effective in reducing the number of disk accesses during aggregate query processing.
Open Access
Topology design in communication networks
(2003) Kepek, İlktuğ Çağatay
In this thesis, we study the topology design problem in communication networks. It is the problem of a Virtual Private Network(VPN) provider. Given a set of customer nodes and a set of commodities, we aim to locate links between customer nodes and route the commodities over these links. The cost to be minimized is the sum of location and routing costs. The problem has capacity, degree and delay constraints. An important characteristic of the problem is that the commodities cannot be split, therefore they must be routed over single paths. We present an integer programming formulation of the problem and introduce two sets of valid inequalities. The problem has two parts: locating links and routing commodities. We first analyze the commodity routing problem and propose an efficient heurisric for it. Finally we propose a heuristic method of generating good feasible solutions to our problem. The final heuristic is a Tabu Search which uses the first heuristic proposed for routing problem as a subroutine. Our results prove to be closer to the lower bounds we generate than previously proposed heuristics.