The processes studied by nuclear engineers generally include coupled physics phenomena (Thermal-Hydraulics, Neutronics, Material Mechanics, etc.) and modeling such multiphysics processes numerically can be computationally intensive. A way to reduce the computational burden is to use spatial meshes that are optimally suited for a specific solution; such meshes are obtained through a process known as Adaptive Mesh Refinement (AMR). AMR can be especially useful for modeling multiphysics phenomena by allowing each solution component to be computed on an independent mesh (Multimesh AMR). Using AMR on time dependent problems requires the spatial mesh to change in time as the solution changes in time. Current algorithms presented in the literature address this concern by adapting the spatial mesh at every time step, which can be inefficient. This Thesis proposes an algorithm for saving computational resources by using a spatially adapted mesh for multiple time steps, and only adapting the spatial mesh when the solution has changed significantly. This Thesis explores the mechanisms used to determine when and where to spatially adapt for time dependent, coupled physics problems. The algorithm is implemented using the Deal.ii fiinite element library [1, 2], in 2D and 3D, and is tested on a coupled neutronics and heat conduction problem in 2D. The algorithm is shown to perform better than a uniformly refined static mesh and, in some cases, a mesh that is spatially adapted at every time step.

The need to compactly represent 3D data is motivated by the ever-increasing size of these data. Furthermore, for large data sets it is useful to randomly access and process a small part of the data. In this thesis we propose two methods of compactly representing 3D data while allowing random access. The first is the multiresolution sphere-packing tree (MSP-tree). The MSP-tree is a multiresolution 3D hierarchy on regular grids based on sphere-packing arrangements. The grids of the MSP-tree compactly represent underlying point-sampled data by using more efficient grids than existing methods while maintaining high granularity and a hierarchical structure that allows random access. The second is distance-ranked random-accessible mesh compression (DR-RAMC). DR-RAMC is a lossless simplicial mesh compressor that allows random access and decompression of the mesh data based on a spatial region-of-interest. DR-RAMC encodes connectivity based on relative proximity of vertices to each other and organizes both this proximity data and vertex coordinates using a k-d tree. DR-RAMC is insensitive to a variety of topological mesh problems (e.g. holes, handles, non-orientability) and can compress simplicial meshes of any dimension embedded in spaces of any dimension. Testing of DR-RAMC shows competitive compression rates for triangle meshes and first-ever random accessible compression rates for tetrahedral meshes. / Thesis (Ph.D, Computing) -- Queen's University, 2012-07-17 15:28:39.406

Infrastructure wireless mesh networks (IWMNs) provide inexpensive deployment, flexible extension of wireless infrastructure, and easy access to the Internet. With multiple radios at each node, a capacity per node improves by transmitting over these radios simultaneously using orthogonal channels. However, without properly addressing the problem of channel assignment and routing for those nodes that form wireless infrastructures, the resulting network throughput and reliability are unlikely to meet the requirements of those highly demanding, media distribution applications. On a particular channel, poor resource allocation at a given access point/gateway of the underlying IWMN can amplify the problem even further. Motivated by these problems, we develop, based on the theory of network coding, a set of alternative solutions that addresses the above issues. We first introduce a sub-optimal solution to the joint problem of network coding, channel assignment and link scheduling for throughput optimization in the multi-channel multi-radio IWMN. We mathematically formulate the problem as a linear program, taking into account opportunistic overhearing, among other constraints. Based on this formulation, we develop a sub-optimal, auction-based algorithm for network throughput optimization. Simulation results reveal the effectiveness of our algorithm in exploiting multiple radios and channels while coping with fairness issues arising from auctions. The proposed solution also shows promising gains over traditional routing solutions. Our experimental results on an 802.11 testbed further confirm these results. The second part of this thesis then presents three AP/gateway-oriented solutions that address the link-level issues related to radio resource allocation at a particular AP/gateway node of the underlying IWMN, which operates on a given channel serving a set of wireless clients. Since the last-hop wireless link is normally a bottleneck of the IWMN, the key idea underlying all the proposed solutions is to use a version of network coding at the bottlenecked AP/gateway. We use Markov chains and the probability theory to derive several performance measures related to media distribution for both uplink and downlink applications. Via extensive simulations, we show the promising delay and reliability gains of the network-coding based schemes over the traditional schemes without network coding.

Design and management decisions require an accurate prediction of the performance of the network. Network performance estimation techniques require accurate network traffic models. In this thesis we are concerned with the modelling of network traffic for the wireless mesh network (WMN) environment. Queueing theory has been used in the past to model the WMN environment and we found in this study that queueing theory was used in two main methods to model WMNs. The first method is to consider each node in the network in terms of the number of hops it is away from the gateway. Each node is then considered as a queueing station and the parameters for the station is derived from the number of hops each node is away from the gateway. These topologies can be very limiting in terms of the number of physical topologies they can model due to the fact that their parameters are only dependent on the number of hop-counts each node is away from the gateway. The second method is to consider a fixed topology with no gateways. This method simplifies analysis but once again is very limiting. In this dissertation we propose a queueing based network traffic model that uses a connection matrix to define the topology of the network. We then derive the parameters for our model from the connection matrix. The connection matrix allows us to model a wider variety of topologies without modifying our model. We verify our model by comparing results from our model to results from a discrete event simulator and we validate our model by comparing results from our model to results from models previously proposed by other authors. By comparing results from our model to results of other models we show that our model is indeed capable of modelling a wider variety of topologies. / Thesis (MIng (Computer and Electronic Engineering))--North-West University, Potchefstroom Campus, 2013.

With the increase of mobile and smart device usage, the interest in dynamically forming networks is rising. One such type of network isWireless Mesh Networks (WMNs). WMNs are multi-hop networks, with a decentralised nature that can dynamically form into mesh topologies. Network Coding (NC) is a method that is used to increase the efficiency of networks by encoding and decoding data on packet level by means of an XOR operation. NC works well with WMNs because it can exploit WMNs broadcast and opportunistic listening properties. When implementing NC on WMNs the issue of security has to be taken into consideration. Dong et al. identified various security threats for intra-flow NC in WMNs. Intra-flow NC combines packets within individual flows, where the information is divided into different flows called generations, to optimize the decoding process. They identified threats for each component of intra-flow NC for WMNs. These components include forwarding node selection, data packet forwarding and acknowledgement delivery. These threats respectively for each component are wormhole attacks and link quality falsification, packet pollution and packet dropping and acknowledgementdropping, injection and delay. We identified that most security schemes focus on packet pollution attacks in NC, but not on any other threats. Packet dropping is also a major threat in networks that is not addressed. Both packet pollution and packet dropping are threats identified for the data forwarding component of WMNs. The Delayed Authentication with Random Transformations (DART) security scheme addresses packet pollution in intra-flow NC systems. The scheme is based on time asymmetry and checksums. The DART scheme only addresses packet pollution and not any of the other identified threats. The DART scheme was selected to be enhanced to also address packet dropping. To enhance the DART scheme we added additional information to the DART scheme’s checksum packets to detect malicious packet dropping nodes in the network. The information added to the checksum packet took the form of a HealthMatrix, which indicates how many packets a node has received and verified. The new scheme, called the Packet Dropping Detection (PDD) scheme collects the additional information from the checksum packets at the receiver node. The receiver sends the collected information to the source node which then uses the information to identify the malicious nodes in the network. These nodes are then removed from the network. The results show that this new scheme causes a small decrease in throughput – about 2%. The identification of malicious nodes can be used as a diagnostic tool and faulty nodes can be repaired or removed form the network. The advantage to detect malicious packet dropping nodes far outweighs this decrease in throughput. In this dissertation we investigate the effects of packet pollution and packet dropping on NC networks inWMNs. We also enhance an already existing scheme (DART) to add additional packet dropping detection security to it without a great loss in throughput. / Thesis (MIng (Computer and Electronic Engineering))--North-West University, Potchefstroom Campus, 2013.

Design and management decisions require an accurate prediction of the performance of the network. Network performance estimation techniques require accurate network traffic models. In this thesis we are concerned with the modelling of network traffic for the wireless mesh network (WMN) environment. Queueing theory has been used in the past to model the WMN environment and we found in this study that queueing theory was used in two main methods to model WMNs. The first method is to consider each node in the network in terms of the number of hops it is away from the gateway. Each node is then considered as a queueing station and the parameters for the station is derived from the number of hops each node is away from the gateway. These topologies can be very limiting in terms of the number of physical topologies they can model due to the fact that their parameters are only dependent on the number of hop-counts each node is away from the gateway. The second method is to consider a fixed topology with no gateways. This method simplifies analysis but once again is very limiting. In this dissertation we propose a queueing based network traffic model that uses a connection matrix to define the topology of the network. We then derive the parameters for our model from the connection matrix. The connection matrix allows us to model a wider variety of topologies without modifying our model. We verify our model by comparing results from our model to results from a discrete event simulator and we validate our model by comparing results from our model to results from models previously proposed by other authors. By comparing results from our model to results of other models we show that our model is indeed capable of modelling a wider variety of topologies. / Thesis (MIng (Computer and Electronic Engineering))--North-West University, Potchefstroom Campus, 2013.

With the increase of mobile and smart device usage, the interest in dynamically forming networks is rising. One such type of network isWireless Mesh Networks (WMNs). WMNs are multi-hop networks, with a decentralised nature that can dynamically form into mesh topologies. Network Coding (NC) is a method that is used to increase the efficiency of networks by encoding and decoding data on packet level by means of an XOR operation. NC works well with WMNs because it can exploit WMNs broadcast and opportunistic listening properties. When implementing NC on WMNs the issue of security has to be taken into consideration. Dong et al. identified various security threats for intra-flow NC in WMNs. Intra-flow NC combines packets within individual flows, where the information is divided into different flows called generations, to optimize the decoding process. They identified threats for each component of intra-flow NC for WMNs. These components include forwarding node selection, data packet forwarding and acknowledgement delivery. These threats respectively for each component are wormhole attacks and link quality falsification, packet pollution and packet dropping and acknowledgementdropping, injection and delay. We identified that most security schemes focus on packet pollution attacks in NC, but not on any other threats. Packet dropping is also a major threat in networks that is not addressed. Both packet pollution and packet dropping are threats identified for the data forwarding component of WMNs. The Delayed Authentication with Random Transformations (DART) security scheme addresses packet pollution in intra-flow NC systems. The scheme is based on time asymmetry and checksums. The DART scheme only addresses packet pollution and not any of the other identified threats. The DART scheme was selected to be enhanced to also address packet dropping. To enhance the DART scheme we added additional information to the DART scheme’s checksum packets to detect malicious packet dropping nodes in the network. The information added to the checksum packet took the form of a HealthMatrix, which indicates how many packets a node has received and verified. The new scheme, called the Packet Dropping Detection (PDD) scheme collects the additional information from the checksum packets at the receiver node. The receiver sends the collected information to the source node which then uses the information to identify the malicious nodes in the network. These nodes are then removed from the network. The results show that this new scheme causes a small decrease in throughput – about 2%. The identification of malicious nodes can be used as a diagnostic tool and faulty nodes can be repaired or removed form the network. The advantage to detect malicious packet dropping nodes far outweighs this decrease in throughput. In this dissertation we investigate the effects of packet pollution and packet dropping on NC networks inWMNs. We also enhance an already existing scheme (DART) to add additional packet dropping detection security to it without a great loss in throughput. / Thesis (MIng (Computer and Electronic Engineering))--North-West University, Potchefstroom Campus, 2013.

Recent advances in methods to increase network utilization have lead to the introduction of a relatively new method called Network Coding. Network Coding is a method that can reduce local congestion in a network by combining information sent over the network. It is commonly researched in the information theory field after it was first introduced by Ahlswede et al in 2000. Network Coding was proven in 2003, by Koetter & Medard to be the only way to achieve the throughput capacity defined by the Min cut Max flow theorem of Shannon. It was applied deterministically in wired networks and randomly in wireless networks. Random Network Coding however requires a lot of overhead and may cause possible delays in the network. We found that there is an open question as to determine where in a wireless network, Network Coding can be implemented. In this thesis we propose to find opportunities for the implementation of Network Coding, by searching for known deterministic Network Coding topologies in larger Networks. Because a known topology is used, we will then also know how Network Coding should be implemented. This method of finding opportunities for the implementation of Network Coding using topology can be combined with a routing algorithm to improve the utilization of a wireless network. We implemented our method on three different topologies and searched 1000 random networks for the presence of these topologies. We found that these topologies occurred frequently enough to make our method a viable method of finding opportunities for the implementation of Network Coding. / Thesis (M.Ing. (Computer and Electronical Engineering))--North-West University, Potchefstroom Campus, 2009.

This thesis is concerned with the channel assignment (CA) problem in multi-channel multi-interface wireless mesh networks (M2WNs). First, for M2WNs with general topologies, we rigorously demonstrate using the combinatorial principle of inclusion/exclusion that the CA solution space can be quantified, indicating that its cardinality is greatly influenced by the number of radio interfaces installed on each router. Based on this analysis, a novel scheme is developed to construct a new reduced search space, represented by a lattice structure, that is searched more efficiently for a CA solution. The elements in the reduced lattice-based space, labeled Solution Structures (SS), represent groupings of feasible CA solutions satisfying the radio constraints at each node. Two algorithms are presented for searching the lattice structure. The first is a greedy algorithm that finds a good SS in polynomial time, while the second provides a user-controlled depthfirst search for the optimal SS. The obtained SS is used to construct an unconstrained weighted graph coloring problem which is then solved to satisfy the soft interference constraints. For the special class of full M2WNs (fM2WNs), we show that an optimal CA solution can only be achieved with a certain number of channels; we denote this number as the characteristic channel number and derive upper and lower bounds for that number as a function of the number of radios per router. Furthermore, exact values for the required channels for minimum interference are obtained when certain relations between the number of routers and the radio interfaces in a given fM2WN are satisfied. These bounds are then employed to develop closed-form expressions for the minimum channel interference that achieves the maximum throughput for uniform traffic on all communication links. Accordingly, a polynomial-time algorithm to find a near-optimal solution for the channel assignment problem in fM2WN is developed. Experimental results confirm the obtained theoretical results and demonstrate the performance of the proposed schemes.

The communication range between a civilian Unmanned Aerial Vehicle (UAV) and a Ground Control Station (GCS) is affected by the government regulations that determine the use of frequency bands and constrain the amount of power in those frequencies. The application of multiple UAVs in search and rescue operations for example demands a reliable, long-range inter-UAV communication. The inter-UAV communication is the ability of UAVs to exchange data among themselves, thus forming a network in the air. This ability could be used to extend the range of communication by using a decentralized routing technique in the network. To provide this ability to a fleet of autonomous dirigible UAVs being developed at the University of Ottawa, a new communication framework was introduced and implemented. Providing a true mesh networking based on a novel routing protocol, the framework combines long-range radios at 900 MHz Industrial, Scientific and Medical (ISM) band with the software integrated into the electronics platform of each dirigible. With one radio module per dirigible the implemented software provides core functionalities to each UAV, such as exchanging flight control commands, telemetry data, and photos with any other UAV in a decentralized network or with the GCS. We made use of the advanced networking tools of the radio modules to build capabilities into the software for route tracing, traffic prioritization, and minimizing self-interference. Initial test results showed that without acknowledgements, packets can be received in the wrong order and cause errors in the transmission of photos. In addition, a transmission in a presence of a third broadcasting node slows down by 4-6 times. Based on these results our software was improved to control to flow of transmit data making the fragmentation, packetization, and reassembly of photos more reliable. Currently, using radios with half-wavelength dipole antennas we can achieve a one-hop communication range of up to 5 km with the radio frequency line-of-sight (RF LOS). This can be extended further by adding as many radio nodes as needed to act as intermediate hops.