Browsing by Subject "Computer networks."

Now showing 1 - 20 of 22

Open Access
Adaptive source routing and route generation for multicomputers
(1995) Aydoğan, Yücel
Scalable multicomputers are based upon interconnection networks that typically provide multiple communication routes between any given pair of processor nodes. In such networks, the selection of the routes is an important problem because of its impact on the communication performance. We propose the adaptive source routing (ASR) scheme which combines adaptive routing and source routing into one which has the advantages of both schemes. In ASR, the degree of adaptivity of each packet is determined at the source processor. Every packet can be routed in a fully adaptive or partially adaptive or nonadaptive manner, all within the same network at the same time. The ASR scheme permits any network topology to be used provided that deadlock constraints are satisfied. We evaluate and compare performance of the adaptive source routing and non-adaptive randomized routing by simulations. Also we propose an algorithm to generate adaptive routes for all pairs of processors in any multistage interconnection network. Adaptive routes are stored in a route table in each processor’s memory and provide high bandwidth and reliable interprocessor communication. We evaluate the performance of the algorithm on IBM SP2 networks in terms of obtained bandwidth, time to fill in the route tables, and efficiency exploited by the parallel execution of the algorithm.
Open Access
Analysis and mitigation of interference in multi-radio multi-channel wireless mesh networks
(2013) Uluçınar, Alper Rifat
Wireless mesh networking, which is basically forming a backbone network of mesh routers using wireless links, is becoming increasingly popular for a broad range of applications from last-mile broadband access to disaster networking or P2P communications, because of its easy deployment, self-forming, self-configuration, and self-healing properties. The multi-hop nature of wireless mesh networks (WMNs) aggravates inter-flow interference and causes intra-flow interference and severely limits the network capacity. One technique to mitigate interference and increase network capacity is to equip the mesh routers with multiple radios and use multiple channels. The radios of a mesh router can then simultaneously send or receive packets on different wireless channels. However, careful and intelligent radio resource planning, including flow-radio and channel assignment, is necessary to efficiently make use of multiple radios and channels. This first requires analyzing and modeling the nature of co-channel and adjacent channel interference in a WMN. Through real-world experiments and observations made in an indoor multihop multi-radio 802.11b/g mesh networking testbed we established, BilMesh, we first analyze and model the nature of co-channel and adjacent channel interference. We conduct extensive experiments on this testbed to understand the effects of using multi-radio, multi-channel relay nodes in terms of network and application layer performance metrics. We also report our results on using overlapping in addition to orthogonal channels for the radios of the mesh routers. We then turn our attention to modeling and quantifying adjacent channel interference. Extending BilMesh with IEEE 802.15.4 nodes, we propose computational methods to quantify interference between channels of a wireless communication standard and between channels of two different standards (such as Wi-Fi and ZigBee). Majority of the studies in the literature on channel assignment consider only orthogonal channels for the radios of a multi-radio WMN. Having developed quantitative models of interference, next we propose two optimization models, which use overlapping channels, for the joint flow-radio and channel assignment problems in WMNs. Then we propose efficient centralized and distributed heuristic algorithms for coupling flows and assigning channels to the radios of a WMN. The proposed centralized and distributed schemes make use of overlapping channels to increase spectrum utilization. Using solid interference and capacity metrics, we evaluate the performances of the proposed schemes via extensive simulation experiments, and we observe that our schemes can achieve substantial improvement over single-channel and random flow-radio and channel assignment schemes.
Open Access
Application-specific heterogeneous network-on-chip design
(2011) Demirbaş, Dilek
With increasing communication demands of processors and memory cores in Systems-on-Chips (SoCs), application-specific and scalable Network-on-Chips (NoCs) are emerged to interconnect processing cores and subsystems in Multiprocessor System-on-Chips (MPSoCs). The challenge of application-specific NoC design is to find the right balance among different trade-offs such as communication latency, power consumption, and chip area. This thesis introduces a novel heterogeneous NoC design approach where biologically inspired evolutionary algorithm and 2-dimensional rectangle packing algorithm are used to place the processing elements with various properties into a constrained NoC area according to the tasks generated by Task Graph for Free (TGFF). TGFF is one of the pseudo-random task graph generators used for scheduling and allocation. Based on a given task graph, we minimize the maximum execution time in a Heterogeneous Chip-Multiprocessor. We specifi- cally emphasize on the communication cost as it is a big overhead in a multi-core architecture. Experimental results show that our approach improves total communication latency up to 27% with modest power consumption.
Open Access
Channel assignment and routing for multi-radio wireless mesh networks
(2008) Özdemiray, Ahmet Murat
Wireless Mesh Network is a promising technology since it extends the range of wireless coverage by multi-hop transmission between routers. However, in multihop networks the total throughput decreases with increasing number of nodes and hops. To increase the total throughput, some mesh routers are equipped with multiple radios to use the available bandwidth of multiple non-overlapping channels. However, channel assignment should be done carefully to effectively use this available bandwidth. Moreover, the optimal channel assignment algorithm is NP-hard. In this thesis, we propose a joint channel assignment and routing solution to effectively use the available bandwidth for multi-radio wireless mesh networks with given network topology and traffic profile. Initially, we predict the final routes of the flows and estimate the loads on the links using these path predictions and given traffic profile. Then three different heuristics determine the assignment order of the links. Then the least busy channel among the available channels is assigned to the link. Finally, our routing algorithm routes the flows such that the selected path is the least busy path among the alternatives. We evaluated our channel assignment and routing algorithm using ns-2 simulator which supports multiple channels and multiple radios per node and we compared our results with single channel WMNs, and different algorithms for multi-radio multi-channel WMNs. The results show that our joint algorithm successfully achieves up to 5 times more throughput than single channel WMN with using just 2 radios and 3 channels. Our algorithms also out-performs other compared channel assignment algorithms for multi-radio multi-channel WMNs.
Open Access
Channel assignment and routing for multi-radio wireless mesh networks
(2008) Gülten, Sıtkı
In this study, we analyze the channel assignment and routing problem for multi-radio wireless mesh networks. We assume that each router has more than one radio, the system operates in a time-slotted mode, and channel assignments are static. In particular, within a time slot the channel assignments for radio connections have to obey the interference constraint. The union of all the connections established throughout the time horizon should result in a strongly connected network where each node can communicate with every other node within the given maximum hop-distance or the diameter value. The objective is to minimize the number of time slots used while respecting the interference and diameter restriction. An integer linear program is proposed as an exact methodology to solve the problem for small scale networks. For larger network sizes, three type of heuristic approaches are developed. In order to evaluate the quality of the heuristic solutions, the lower bound of the model is strengthened through the use of valid inequalities and lagrangian relaxation. The subgradient algorithm is used in lagrangian relaxation models to find optimal solutions or lower bounds. The heuristics are tested on a large set of varying network topology instances. The computational experiments illustrate that improvement heuristic based on local search is the most suitable approximation technique.
Open Access
A collaborative system for providing routes between locations
(2008) Uluğ, Kerem Ali
Many systems, such as in-car GPS devices and airline company web sites, provide route information between locations. Although such systems are used widely and can provide route information successfully, users of these systems cannot contribute to the data entry process. In these systems, data is entered by the administrators and these systems cannot take advantage of the route expertise of their users. In this work, we present a collaborative system, which provides routes between locations upon user queries. The data in the system is entered by the users of the system. We present a model which is containing locations, links between locations and relationships between locations (containment, neighborhood and intersection) in order to store the data. For the route finding purpose, we present a customized version of the A* search algorithm. This customized version, named A*CD (A* for Collaborative Data), uses heuristics for estimating the cost remaining to the target location while processing the nodes. A*CD can also provide alternative routes, exclude certain link types in the searches according to user preferences and handle the problems associated with multiple stop transportation lines. As the cost models, we use duration and financial cost. We also present the intuitive connections concept. Even if a route does not exist between the selected locations, the system can provide a route with missing links. The gap(s) between the disconnected locations are filled by the help of the relationships between locations. In order to evaluate the performance of the A*CD algorithm, we present automated tests. These tests show that the costs of the routes that are provided by the A*CD algorithm are close to the actual shortest routes. In order to demonstrate the intuitive connections concept, we also present manual test queries.
Open Access
Comparative evaluation of spectrum allocation policies for dynamic flexgrid optical networks
(2013) Yümer, Ramazan
A novel class-based first-fit spectrum allocation policy is proposed for dynamic Flexgrid optical networks. The effectiveness of the proposed policy is compared against the first-fit policy for single-hop and multi-hop scenarios. Event-based simulation technique is used for testing the spectrum allocation policies under both Fixed Routing and Fixed Alternate Routing algorithms with two shortest paths. Throughput is shown to be consistently improved under the proposed policy with gains of up to 15 % in certain scenarios.
Open Access
Discrete location models for content distribution
(2005) Bektaş, Tolga
The advances in information and computer technology has tremendously eased the way to reach electronic information. This, however, also brought forth many problems regarding the distribution of electronic content. This is especially true in the Internet, where there is a phenomenal growth of demand for any kind of electronic information, placing a high burden on the underlying infrastructure. In this dissertation, we study problems arising in distribution of electronic content. The first problem studied here is related to Content Distribution Networks (CDNs), which have emerged as a new technology to overcome the problems arising on the Internet due to the fast growth of the web-related traffic, such as slow response times and heavy server loads. They aim at increasing the effectiveness of the network by locating identical or partial copies of the origin server(s) throughout the network, which are referred to as proxy servers. In order for such structures to run efficiently, the CDN must be designed such that system resource are properly managed. To this purpose, we develop integer programming models for the problem of designing CDNs and investigate exact and heuristic algorithms for their solution. The second problem considered in this dissertation is Video Placement and Routing, which is related to the so-called Video-on-Demand (VoD) services. Such services are used to deliver programs to the users on request and find many applications in education, entertainment and business. Although bearing similarities with the CDN phenomena, VoD services have special characteristics with respect to the structure of the network and the type of content distributed. We study the problem of Video Placement and Routing for such networks and offer an optimization based solution algorithm for the associated integer programming model. The third problem studied here is the problem of allocating databases in distributed computing systems. In this context, we specifically focus on the well-known multidimensional Knapsack Problem (mKP). The mKP arises as a subproblem in solving the database location problem. We concentrate on the well known cover inequalities that are known to be important for the solution of the mKP. We then propose a novel separation procedure to identify violated cover inequalities and utilize this procedure in a branch-and-cut framework devised for the solution of the mKP.
Open Access
Distributed channel aware link scheduling for CSMA based wireless networks with time-varying channels and delay sensitive applications
(2011) Erkan, Bahadır
In wireless networks, interference between neighboring links is an important issue. The link scheduling algorithm controls the interference between neighboring links such that no adjacent links can be concurrently active. Distributed throughput optimum algorithms for the link scheduling problem have been proposed in the literature. However, the maximum packet delays of these distributed throughput optimum algorithms can become arbitrarily large, which significantly degrades the performances of delay sensitive applications such as “Skype”. In this thesis, we propose two distributed link scheduling algorithms: a full opportunistic algorithm and a delay based adaptive algorithm. The proposed algorithms, while maintaining throughput optimality, increase the average delay performance of the previously proposed throughput optimum scheduling algorithms by 20% under the fading radio channel. We propose a new metric “Effective Goodput”, which measures the rate of packets that are successfully received before their respective playout times for delay sensitive applications. The delay based distributed adaptive scheduling algorithm proposed in the thesis increases the “Effective Goodput” by nearly 100% compared with the throughput optimum scheduling algorithms proposed in the literature.
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
Dynamic wavelength allocation in IP/WDM metro access networks
(2008) Yetginer, Emre
Increasing demand for bandwidth and proliferation of packet based traffic have been causing architectural changes in the communications infrastructure. In this evolution, metro networks face both the capacity and dynamic adaptability constraints. The increase in the access and backbone speeds result in high bandwidth requirements, whereas the popularity of wireless access and limited number of customers in metro area necessitates traffic adaptability. Traditional architecture which has been optimized for carrying circuit-switched connections, is far from meeting these requirements. Recently, several architectures have been proposed for future metro access networks. Nearly all of these solutions support dynamic allocation of bandwidth to follow fluctuations in the traffic demand. However, reconfiguration policies that can be used in this process have not been fully explored yet. In this thesis, dynamic wavelength allocation (DWA) policies for IP/WDM metro access networks with reconfiguration delays are considered. Reconfiguration actions incur a cost since a portion of the capacity becomes idle in the reconfiguration period due to the signalling latencies and tuning times of optical transceivers. Exact formulation of the DWA problem is developed as a Markov Decision Process (MDP) and a new cost function is proposed to attain both throughput efficiency and fairness. For larger problems, a heuristic approach based on first passage probabilities is developed. The performance of the method is evaluated under both stationary and non-stationary traffic conditions. The effects of relevant network and traffic parameters, such as delay and flow size are also discussed. Finally, performance bounds for the DWA methods are derived.
Open Access
Fiber optical network design problems : case for Turkey
(2013) Yazar, Başak
The problems within scope of this thesis are based on an application arising from one of the largest Internet service providers operating in Turkey. There are mainly two different problems: the green field design and copper field re-design. In the green field design problem, the aim is to design a least cost fiber optical network from scratch that will provide high bandwidth Internet access from a given central station to a set of aggregated demand nodes. Such an access can be provided either directly by installing fibers or indirectly by utilizing passive splitters. Insertion loss, bandwidth level and distance limitations should simultaneously be considered in order to provide a least cost design to enable the required service level. On the other hand, in the re-design of the copper field application, the aim is to improve the current service level by augmenting the network through fiber optical wires. Copper rings in the existing infrastructure are augmented with cabinets and direct fiber links from cabinets to demand nodes provide the required coverage to distant nodes. Mathematical models are constructed for both problem specifications. Extensive computational results based on real data from Kartal (45 points) and Bakırköy (74 points) districts in Istanbul show that the proposed models are viable exact solution methodologies for moderate dimensions.
Open Access
GridRoute : a multi-layered grid based routing protocol for delay tolerant mobile networks
(2012) Köksal, Emin Yiğit
This work proposes a new routing protocol for delay-tolerant mobile networks (DTMNs) called GridRoute. The proposed protocol can be adopted considering network requirements such as low message delay or low resource usage. GridRoute is a probabilistic routing protocol that takes advantage of mobility and location information of nodes. It uses a multi-layered grid for contact probability maximization. It requires almost no memory storage of contact or location probabilities for intelligent routing decisions. GridRoute also minimizes the number of redundant messages throughout the network with feasible delay on message delivery, and provides some security advantages like identity secrecy. Our simulation results show that GridRoute outperforms existing routing protocols in terms of memory requirement. It also achieves high delivery ratio, reasonable end-to-end delay and significantly lower message overhead.
Open Access
Joint link
(2009) Kaybal, Fazlı
In this thesis, we study the joint scheduling and routing problem in spatial reuse Time Division Multiple Access (STDMA) based multi-channel/multiradio/multi-rate wireless mesh networks (WMNs). The main objective of the joint scheduling and routing problem addressed in thesis is to reduce the number of required TDMA time slots to deliver all packets to their destinations. Since the optimum solution to the problem is NP-hard, we propose a greedy iterative solution methodology. The problem is formulated as an integer linear program (ILP) under the physical interference model. We consider two versions of the problem in order to investigate the factors affecting the capacity of WMNs. In the first one, we perform scheduling and routing when the number of channels and number of radios are varied for multi-rate WMNs where nodes are equipped with omni-directional antennas. This analysis is done for both single-class (best-effort traffic) and two-class (best-effort and delay sensitive classes) traffic models. We then extend this analysis by adding the power control scheme which allows transmitters to change the transmitting powers slot-by-slot. Finally, joint scheduling and routing problem is extended for WMNs where nodes are equipped with multiple sectored antennas. We show that the network performance is improved with more radio resources, e.g., using multiple orthogonal channels, multiple radios per node, transmit power control scheme, and directional antennas in terms of delay and total dissipated energy. The network throughput when using 3 channels and 3 radios is increased by up to 67.2% compared to single channel WMNs and the total dissipated energy is reduced by up to 45.5% with transmit power control scheme. Finally, when directional antennas with 6 sectors are used at both transmitters and receivers, the network throughput increases by up to 72.6% compared to omni-directional antenna case.
Open Access
Joint routing, gateway selection, scheduling and power management optimization in wireless mesh networks
(2011) Uzunlar, Onur
The third generation (3G) wireless communications technology delivers user traffic in a single step to the wired network via base station; therefore it requires all base stations to be connected to the wired network. On the other hand, in the fourth generation (4G) communication systems, it is planned to have the base stations set up so that they can deliver each other’s traffic to a small number of base stations equipped with wired connections. In order to improve system resiliency against failures, a mesh structure is preferred. The most important issue in Wireless Mesh Networks (WMN) is that the signals that are simultaneously transmitted on the same frequency channel can interfere with each other to become incomprehensible at the receiver end. It is possible to operate the links at different times or at different frequencies, but this also lowers capacity usage. In this thesis, we tackle the planning problems of WMN, using 802.16 (Wi-MAX) protocol, such as deploying a given number of gateway nodes along with operational problems such as routing, management of power used by nodes and scheduling while maximizing the minimum service level provided. The WMN under consideration has identical routers with fixed locations and the demand of each router is known. In order to be able to apply our results to real systems, we work with optimization models based on realistic assumptions such as physical interference and single path routing. We propose heuristic methods to obtain optimal or near optimal solutions in reasonable time. The models are applied to some cities in Istanbul and Ankara provinces.
Open Access
Multi-ring SDH network design over optical mesh networks
(2002) Tan, Tuba Akıncılar
The evolution of networks in telecommunications has brought on the importance of design techniques to obtain survivable and cost-effective transportation networks. In this thesis, we study Synchronous Digital Hierarchy (SDH) ring design problem with an interconnected multi-ring architecture overlaid over an optical mesh network. We decouple the problem into two sub-problems: the first problem is the SDH ring selection, and the second problem is the mapping of these rings onto the physical mesh topology. In this structure, the logical topology consists of SDH Add/Drop Multiplexers (ADMs) and Digital Cross-Connects (DXCs), and the physical topology consists of Optical Cross-Connects (OXCs). The ring selection problem is to choose the rings that give minimum inter-ring traffic in the network. Since inter-ring traffic increases the network cost and complexity, we aim to minimize the inter-ring traffic. We propose a greedy heuristic algorithm for this problem that finds a solution subject to the constraint that the number of nodes on each ring is limited. Numerical results on the ring design problem are presented for different topologies. Once the logical topology is obtained, resilient mapping of SDH rings onto the mesh physical topology is formulated as a Mixed Integer Linear Programming (MILP) problem. In order to guarantee proper operation of SDH ring protection against all single failures, each link on an SDH ring must be mapped onto a lightpath which is link and node disjoint from all other lightpaths comprising the same ring. The objective of this mapping is to minimize the total fiber cost in the network. We also apply a post-processing algorithm to eliminate redundant rings. The postprocessing algorithm is very useful to reduce the cost. We evaluate the performance of our design algorithm for different networks.
Open Access
Parallel processing for progressive refinement radiosity
(1993) Çapın, Tolga K.
Progressive refinement radiosity is an increasingly popular method for realistic image synthesis of non-existing environments. The method successfully approximates the light distribution in an environment, however it requires excessive amount of computation. In this thesis, the progressive refinement method is investigated for parallelization on ring and hypercube-connected multicomputers. Two different approaches for parallelization, based on synchronous parallelism witli static task assignment, are proposed, in order to achieve better coherence in parallel light distributions and obtain good performance on simple topologies. Efficient global circulation schemes are proposed in order to decrease the total volume of communication by asymptotical factors. The first scheme for parallelization is a modification of the sequential algorithm in that several patches shoot their energy at a time, while the second scheme is based on the parallelism level of one shooting patch at a time. The proposed parallel algorithms are evaluated theoretically and implemented for ring and hypercube-connected topologies on Intel’s iPSC72 multicomputer. Load balance quality of the proposed schemes are evaluated experimentally
Open Access
Performance evaluation of telecommunication networks based on multi-regime fluid queues
(2009) Kankaya, Hüseyin Emre
Today’s state-of-the-art search engines utilize the inverted index data structure for fast text retrieval on large document collections. To parallelize the retrieval process, the inverted index should be distributed among multiple index servers. Generally the distribution of the inverted index is done in either a term-based or a document-based fashion. The performances of both schemes depend on the total number of disk accesses and the total volume of communication in the system. The classical approach for both distributions is to use the Central Broker Query Evaluation Scheme (CB) for parallel text retrieval. It is known that in this approach the central broker is heavily loaded and becomes a bottleneck. Recently, an alternative query evaluation technique, named Pipelined Query Evaluation Scheme (PPL), has been proposed to alleviate this problem by performing the merge operation on the index servers. In this study, we analyze the scalability and relative performances of the CB and PPL under various query loads to report the benefits and drawbacks of each method.
Open Access
Robust network design under polyhedral traffic uncertainty
(2007) Altın, Ayşegül
In this thesis, we study the design of networks robust to changes in demand estimates. We consider the case where the set of feasible demands is defined by an arbitrary polyhedron. Our motivation is to determine link capacity or routing configurations, which remain feasible for any realization in the corresponding demand polyhedron. We consider three well-known problems under polyhedral demand uncertainty all of which are posed as semi-infinite mixed integer programming problems. We develop explicit, compact formulations for all three problems as well as alternative formulations and exact solution methods. The first problem arises in the Virtual Private Network (VPN) design field. We present compact linear mixed-integer programming formulations for the problem with the classical hose traffic model and for a new, less conservative, robust variant relying on accessible traffic statistics. Although we can solve these formulations for medium-to-large instances in reasonable times using off-the-shelf MIP solvers, we develop a combined branch-and-price and cutting plane algorithm to handle larger instances. We also provide an extensive discussion of our numerical results. Next, we study the Open Shortest Path First (OSPF) routing enhanced with traffic engineering tools under general demand uncertainty with the motivation to discuss if OSPF could be made comparable to the general unconstrained routing (MPLS) when it is provided with a less restrictive operating environment. To the best of our knowledge, these two routing mechanisms are compared for the first time under such a general setting. We provide compact formulations for both routing types and show that MPLS routing for polyhedral demands can be computed in polynomial time. Moreover, we present a specialized branchand-price algorithm strengthened with the inclusion of cuts as an exact solution tool. Subsequently, we compare the new and more flexible OSPF routing with MPLS as well as the traditional OSPF on several network instances. We observe that the management tools we use in OSPF make it significantly better than the generic OSPF. Moreover, we show that OSPF performance can get closer to that of MPLS in some cases. Finally, we consider the Network Loading Problem (NLP) under a polyhedral uncertainty description of traffic demands. After giving a compact multicommodity formulation of the problem, we prove an unexpected decomposition property obtained from projecting out the flow variables, considerably simplifying the resulting polyhedral analysis and computations by doing away with metric inequalities, an attendant feature of most successful algorithms on NLP. Under the hose model of feasible demands, we study the polyhedral aspects of NLP, used as the basis of an efficient branch-and-cut algorithm supported by a simple heuristic for generating upper bounds. We provide the results of extensive computational experiments on well-known network design instances.
Open Access
Routing in delay tolerant networks with periodic connections
(2010) Mergenci, Cem
In delay tolerant networks (DTNs), the network may not be fully connected at any instance of time, but connections occurring between nodes at di erent times make the network connected through the entire time continuum. In such a case, traditional routing methods fail to operate as there are no contemporaneous end-to-end paths between sources and destinations. This study examines the routing in DTNs where connections arise in a periodic nature. Various levels of periodicity are analyzed to meet requirements of di erent network models. We propose various routing algorithms for periodic connections. Our proposed methods can nd routes that can guarantee earliest delivery and minimum hop count. We evaluate our routing schemes via extensive simulation experiments and also compare them to some other popular routing approaches proposed for delay tolerant networks. Our evaluations show the feasibility and e ectivenes of our schemes as alternative routing methods for delay tolerant networks.