Conditional limit theorems for regulated fractional Brownian motion

We consider a stationary fluid queue with fractional Brownian motion input. Conditional on the workload at time zero being greater than a large value $b$, we provide the limiting distribution for the amount of time that the workload process spends above level $b$ over the busy cycle straddling the origin, as $b\to\infty$. Our results can be interpreted as showing that long delays occur in large clumps of size of order $b^{2-1/H}$. The conditional limit result involves a finer scaling of the queueing process than fluid analysis, thereby departing from previous related literature.Comment: Published in at http://dx.doi.org/10.1214/09-AAP605 the Annals of Applied Probability (http://www.imstat.org/aap/) by the Institute of Mathematical Statistics (http://www.imstat.org

How wireless queues benefit from motion: an analysis of the continuum between zero and infinite mobility

This paper considers the time evolution of a queue that is embedded in a Poisson point process of moving wireless interferers. The queue is driven by an external arrival process and is subject to a time-varying service process that is a function of the SINR that it sees. Static configurations of interferers result in an infinite queue workload with positive probability. In contrast, a generic stability condition is established for the queue in the case where interferers possess any non-zero mobility that results in displacements that are both independent across interferers and oblivious to interferer positions. The proof leverages the mixing property of the Poisson point process. The effect of an increase in mobility on queueing metrics is also studied. Convex ordering tools are used to establish that faster moving interferers result in a queue workload that is smaller for the increasing-convex stochastic order. As a corollary, mean workload and mean delay decrease as network mobility increases. This stochastic ordering as a function of mobility is explained by establishing positive correlations between SINR level-crossing events at different time points, and by determining the autocorrelation function for interference and observing that it decreases with increasing mobility. System behaviour is empirically analyzed using discrete-event simulation and the performance of various mobility models is evaluated using heavy-traffic approximations.Comment: Preliminary version appeared in WiOPT 2020. New version with revision

Dynamic contact centers with impatient customers and retrials

[no abstract

An Investigation into Possible Attacks on HTML5 IndexedDB and their Prevention

This thesis presents an analysis of, and enhanced security model for IndexedDB, the persistent HTML5 browser-based data store. In versions of HTML prior to HTML5, web sites used cookies to track user preferences locally. Cookies are however limited both in file size and number, and must also be added to every HTTP request, which increases web traffic unnecessarily. Web functionality has however increased significantly since cookies were introduced by Netscape in 1994. Consequently, web developers require additional capabilities to keep up with the evolution of the World Wide Web and growth in eCommerce. The response to this requirement was the IndexedDB API, which became an official W3C recommendation in January 2015. The IndexedDB API includes an Object Store, indices, and cursors and so gives HTML5 - compliant browsers a transactional database capability. Furthermore, once downloaded, IndexedDB data stores do not require network connectivity. This permits mobile web- based applications to work without a data connection. Such IndexedDB data stores will be used to store customer data, they will inevitably become targets for attackers. This thesis firstly argues that the design of IndexedDB makes it unavoidably insecure. That is, every implementation is vulnerable to attacks such as Cross Site Scripting, and even data that has been deleted from databases may be stolen using appropriate software tools. This is demonstrated experimentally on both mobile and desktop browsers. IndexedDB is however capable of high performance even when compared to servers running optimized local databases. This is demonstrated through the development of a formal performance model. The performance predictions for IndexedDB were tested experimentally, and the results showed high conformance over a range of usage scenarios. This implies that IndexedDB is potentially a useful HTML5 API if the security issues can be addressed. In the final component of this thesis, we propose and implement enhancements that correct the security weaknesses identified in IndexedDB. The enhancements use multifactor authentication, and so are resistant to Cross Site Scripting attacks. This enhancement is then demonstrated experimentally, showing that HTML5 IndexedDB may be used securely both online and offline. This implies that secure, standards compliant browser based applications with persistent local data stores may both feasible and efficient