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ItemDistributed MIMO Interference Cancellation for Interfering Wireless Networks: Protocol and Initial Simulation(Georgia Institute of Technology, 2013-02) Cortes-Pena, Luis Miguel ; Blough, Douglas M.In this report, the problem of interference in dense wireless network deployments is addressed. Two example scenarios are: 1) overlapping basic service sets (OBSSes) in wireless LAN deployments, and 2) interference among close-by femtocells. The proposed approach is to exploit the interference cancellation and spatial multiplexing capabilities of multiple-input multiple- output (MIMO) links to mitigate interference and improve the performance of such networks. Both semi-distributed and fully distributed protocols for 802.11-based wireless networks standard are presented and evaluated. The philosophy of the approach is to minimize modifications to existing protocols, particularly within client-side devices. Thus, modifications are primarily made at the access points (APs). The semi-distributed protocol was fully implemented within the 802.11 package of ns-3 to evaluate the approach. Simulation results with two APs, and with either one client per AP or two clients per AP, show that within 5 seconds of network operation, our protocol increases the goodput on the downlink by about 50%, as compared against a standard 802.11n implementation.
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ItemFast and Accurate Link Discovery Integrated with Reliable Multicast in 802.11(Georgia Institute of Technology, 2013) Lertpratchya, Daniel ; Blough, Douglas M. ; Riley, George F.Abstract—Maintaining accurate neighbor information in wireless networks is an important operation upon which many higher layer protocols rely. However, this operation is not supported in the IEEE 802.11 MAC layer, forcing applications that need it to each include their own neighborhood mechanism, creating redundancies and inefficiencies and failing to capitalize on potential synergies with other MAC layer operations. In this work, we propose to integrate link discovery and neighborhood maintenance with a reliable multicast extension to the IEEE 802.11 MAC.We show through simulations that our protocol adapts to neighborhood changes faster than traditional neighborhood maintenance mechanisms, thereby allowing MAC-layer multicast operations to achieve higher delivery rates. We also demonstrate that our protocol can quickly and reliably distinguish between unidirectional and bidirectional links. Traditional mechanisms assume links are bidirectional based on one-way reception of a short “hello” packet, which results in significant problems with higher-layer operations such as routing because of many unidirectional links being classified as bidirectional.
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ItemExploring the Design Space of Greedy Link Scheduling Algorithms for Wireless Multihop Networks(Georgia Institute of Technology, 2011) Lertpratchya, Daniel ; Blough, Douglas M.It is known that using a spatial TDMA (STDMA) access scheme can increase the capacity of a wireless network over CSMA/CD access scheme. Modern wireless devices are capable of transmitting at different data rates depending on the current network condition. However, little attention has been paid to how best is to use the multiple data rates capability. In this report, we focus on greedy link scheduling algorithms that work with variable rates, where devices can transmit at lower data rates to accommodate lower quality links. We propose criteria that can be used in the scheduling algorithms and investigate performances of different scheduling algorithms that employ these different criteria. We use the more realistic physical interference model, where packet reception rate depends on signal-to-interference-plus-noise ratio. Our investigation shows that by using the variable rate approach, we can increase the overall capacity of the network over traditional single-threshold-based algorithms.
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ItemDetection of Conflicts and Inconsistencies in Taxonomy-based Authorization Policies(Georgia Institute of Technology, 2011) Mohan, Apurva ; Blough, Douglas M. ; Kurc, Tahsin ; Post, Andrew ; Saltz, JoelThe values of data elements stored in biomedical databases often draw from biomedical ontologies. Authorization rules can be defined on these ontologies to control access to sensitive and private data elements in such databases. Authorization rules may be specified by different authorities at different times for various purposes, and as such policy rules may conflict with each other, inadvertently allowing access to sensitive information. Detecting policy conflicts is nontrivial because it involves identification of applicable rules and detecting conflicts among them dynamically during execution of data access requests. It also requires dynamically verifying conformance with required policies and logging relevant information about decisions for audit. Another problem in biomedical data protection is inference attacks, in which a user who has legitimate access to some data elements is able to infer information related to other data elements. This type of inadvertent data disclosure should be prevented by ensuring policy consistency; that is, data elements which can lead to inference about other data elements should be protected by the same level of authorization policies as the other data elements. We propose two strategies; one for detecting policy consistencies to avoid potential inference attacks and the other for detecting policy conflicts. We have implemented these algorithms in Java language and evaluated their execution times experimentally.
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ItemFast, Lightweight Virtual Machine Checkpointing(Georgia Institute of Technology, 2010) Sun, Michael H. ; Blough, Douglas M.Virtual machine checkpoints provide a clean encapsulation of the full state of an executing system. Due to the large nature of state involved, the process of VM checkpoints can be slow and costly. We describe the implementation of a fast and lightweight mechanism of VM checkpointing for the Xen virtualization machine monitor that utilizes copy-on-write techniques to reduce the VM’s downtime and performance overhead incurred by other forms of VM checkpointing.