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Recent Submissions

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    Big-step semantics
    (University of Waterloo, 2009-03-06) Esmaeilsakzali, Shahram; Day, Nancy A.; Atlee, Joanne M.; Niu, Jianwei
    With the popularity of model-driven methodologies, and the abundance of modelling languages, a major question for a requirements engineer is: which language is suitable for modelling a system under study? We address this question from a semantic point-of-view for big-step modelling languages (BSMLs). BSMLs are a popular class of behavioural modelling languages in which a model can respond to an environmental input by executing multiple, possibly concurrent, transitions. We deconstruct the semantics of a large class of BSMLs into high-level, orthogonal semantic aspects and discuss the relative advantages and disadvantages of the semantic options for each of these aspects to allow a requirements engineer to compare and choose the right BSML. We accompany our presentation with many modelling examples that illustrate the differences between a set of relevant semantic options.
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    A heuristic for fair correlation-aware resource placement
    (University of Waterloo, 2009-01-20) Boutaba, Raouf; Karsten, Martin; Young, Maxwell
    The configuration of network resources greatly impacts the communication overhead for data intensive tasks and constitutes a critical problem in the design and maintenance of networks. To address the issue of resource placement, we analyze the implement a heuristic for solving a known NP-complete graph optimization problem called MAXIMUM SIZE BOUNDED CAPACITY CUT. Experimental results for our heuristic demonstrate promising performance on both synthetic and real world data. Next our heuristic is used as a sub-routine to solve another known NP-complete problem called MIN-MAX MULTIWAY CUT whose traits we adapt to yield a resource placement scheme that exploits correlations between network resources. Our experimental results show that the resulting placement scheme achieves a significant savings in communication overhead.
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    Fixed-parameter tractability and improved approximations for segment minimization
    (University of Waterloo, 2009-01-20) Biedl, Therese; Durocher, Stephane; Hoos, Holger H.; Luan, Shuang; Saia, Jared; Young, Maxwell
    The segment minimization problem consists of finding the smallest set of integer matrices that sum to a given intensity matrix, such that each summand has only one non-zero value, and the non-zeroes in each row are consecutive. This has direct applications in intensity-modulated radiation therapy, an effective form of cancer treatment. We show here that for a single row, this problem is fixed-parameter tractable in the largest value of the intensity matrix. We use this to develop approximation algorithms for the full problem. One of these improves the approximation factor from the previous best of log 2 h + 1 to 3/2 . (log3 h + 1), where h is the largest entry in the intensity matrix; another improves the approximation factor from 2. (log D+1) to 24/13 . (log D+1), where D is the largest difference between consecutive elements of a row of the intensity matrix. Experimentation with these algorithms show that they outperform other approximation algorithms on 75% of the 172 test cases we considered, which include both real world and synthetic data.
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    Capacity provisioning a valiant load-balanced network
    (University of Waterloo, 2009-01-20) Curtis, Andrew R.; Lopez-Ortiz, Alejandro
    Valiant load balancing (VLB), also called two-stage load balancing, is gaining popularity as a routing scheme that can serve arbitrary traffic matrices. To date, VLB network design is well understood on a logical full-mesh topology, where VLB is optimal even when nodes or links can fail. In this paper, we address the design and capacity provisioning of arbitrary VLB network topologies. First, we introduce an algorithm to determine if VLB can serve all traffic matrices when a fixed number of arbitrary links fail, and we show how to find a mincost expansion of the network - via link upgrades or installs or both - so that it is resilient to these failures. Additionally, we propose a method to design a new VLB network under the fixed-charge network design cost model. Finally, we prove that VLB is no longer optimal on unrestricted topologies, and can require more capacity than shortest path routing to serve all traffic matrices on some topologies. These results rely on a novel theorem that characterizes the capacity VLB requires of links crossing each cut, i.e., a partition, of the network's nodes.
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    WiFi overcast: Enabling true mobility for realtime applications in the enterprise
    (University of Waterloo, 2009-01-27) Ahmed, Nabeel; Ismail, Usman
    Enterprises are increasingly deploying wireless local area connections (WLANs) to provide mobile access to users in corporate offices. However, existing enterprise WLANs are far from being truly mobile. In particular, they do not adequately support continuous mobility, where users access the network on-the-go. Furthermore, WLANs that do provide continuous mobility support require client modifications, making them hard to deploy in practice 20. In addition, with the growing interest in realtime applications such as voice and video, users are increasingly placing additional (QoS) demands on the network, which for inadequately designed WLANs, does not scale to large numbers of user 10. In this paper, we propose Overcast, a novel WLAN architecture that targets scenarios demanding continuous mobility and real-time support for 802.11 clients. Overcast does not require client modifications and supports all 802.11 standards. Though Overcast borrows some features from prior WLAN designs, it improves on them by incorporating a novel RF mapping framework (proposed in 3) for accurate online detection of RF interference. We describe the architecture of Overcast in detail and discuss our current efforts in realizing such a system on off-the-shelf commodity hardware. We also describe an example application of Overcast to highlight its usefulness in supporting realtime applications in continuously mobile user environments.