Stability conditions for a decentralised medium access algorithm: single- and multi-hop networks

We consider a decentralised multi-access algorithm, motivated primarily by the control of transmissions in a wireless network. For a finite single-hop network with arbitrary interference constraints we prove stochastic stability under the natural conditions. For infinite and finite single-hop networks, we obtain broad rate-stability conditions. We also consider symmetric finite multi-hop networks and show that the natural condition is sufficient for stochastic stability

Stability conditions for a discrete-time decentralised medium access algorithm

We consider a stochastic queueing system modelling the behaviour of a wireless network with nodes employing a discrete-time version of the standard decentralised medium access algorithm. The system is {\em unsaturated} -- each node receives an exogenous flow of packets at the rate $\lambda$ packets per time slot. Each packet takes one slot to transmit, but neighboring nodes cannot transmit simultaneously. The algorithm we study is {\em standard} in that: a node with empty queue does {\em not} compete for medium access; the access procedure by a node does {\em not} depend on its queue length, as long as it is non-zero. Two system topologies are considered, with nodes arranged in a circle and in a line. We prove that, for either topology, the system is stochastically stable under condition $\lambda < 2/5$. This result is intuitive for the circle topology as the throughput each node receives in a saturated system (with infinite queues) is equal to the so called {\em parking constant}, which is larger than $2/5$. (The latter fact, however, does not help to prove our result.) The result is not intuitive at all for the line topology as in a saturated system some nodes receive a throughput lower than $2/5$.Comment: 22 page

When queueing is better than push and shove

We address the scheduling problem of reordering an existing queue into its efficient order through trade. To that end, we consider individually rational and balanced budget direct and indirect mechanisms. We show that this class of mechanisms allows us to form efficient queues provided that existing property rights for the service are small enough to enable trade between the agents. In particular, we show on the one hand that no queue under a fully deterministic service schedule such as first-come, first-serve can be dissolved efficiently and meet our requirements. If, on the other hand, the alternative is service anarchy (ie. a random queue), every existing queue can be transformed into an efficient order

Mutual benefits of two multicriteria analysis methodologies: A case study for batch plant design

This paper presents a MultiObjective Genetic Algorithm (MOGA) optimization framework for batch plant design. For this purpose, two approaches are implemented and compared with respect to three criteria, i.e., investment cost, equipment number and a flexibility indicator based on work in process (the so-called WIP) computed by use of a discrete-event simulation model. The first approach involves a genetic algorithm in order to generate acceptable solutions, from which the best ones are chosen by using a Pareto Sort algorithm. The second approach combines the previous Genetic Algorithm with a multicriteria analysis methodology, i.e., the Electre method in order to find the best solutions. The performances of the two procedures are studied for a large-size problem and a comparison between the procedures is then made

The importance in the papers' impact of the number of pages and of co-authors - an empirical estimation with data from top ranking economic journals

On a regular basis the scientific output of academic people has to be evaluated, e.g. to decide tenure. A very important item in this evaluation is the published papers’ quality that tends to be approximated by its impact in the literature. As the measure of this impact requires a long-term analysis, thus it is used as its estimator the journal average impact where each paper is published. But some papers have a single author while others have several and some papers have one or two pages while others have more than fifty. In this work I validate the conjecture that these two variables have a significant and positive effect in papers’ future impact, i.e. in papers quality. Nonetheless, I quantify that this two variables jointly considered merely explain 2.8% of papers’ impact variability.Scientific Skill, Co-authorship, Papers’ impact

Evaluating a virtual learning environment in the context of its community of practice

The evaluation of virtual learning environments (VLEs) and similar applications has, to date, largely consisted of checklists of system features, phenomenological studies or measures of specific forms of educational efficacy. Although these approaches offer some value, they are unable to capture the complex and holistic nature of a group of individuals using a common system to support the wide range of activities that make up a course or programme of study over time. This paper employs Wenger's theories of 'communities of practice' to provide a formal structure for looking at how a VLE supports a pre-existing course community. Wenger proposes a Learning Architecture Framework for a learning community of practice, which the authors have taken to provide an evaluation framework. This approach is complementary to both the holistic and complex natures of course environments, in that particular VLE affordances are less important than the activities of the course community in respect of the system. Thus, the VLE's efficacy in its context of use is the prime area of investigation rather than a reductionist analysis of its tools and components. An example of this approach in use is presented, evaluating the VLE that supports the undergraduate medical course at the University of Edinburgh. The paper provides a theoretical grounding, derives an evaluation instrument, analyses the efficacy and validity of the instrument in practice and draws conclusions as to how and where it may best be used

A Robust and Efficient Three-Layered Dialogue Component for a Speech-to-Speech Translation System

We present the dialogue component of the speech-to-speech translation system VERBMOBIL. In contrast to conventional dialogue systems it mediates the dialogue while processing maximally 50% of the dialogue in depth. Special requirements like robustness and efficiency lead to a 3-layered hybrid architecture for the dialogue module, using statistics, an automaton and a planner. A dialogue memory is constructed incrementally.Comment: Postscript file, compressed and uuencoded, 15 pages, to appear in Proceedings of EACL-95, Dublin