Rate stability and output rates in queueing networks with shared resources

Motivated by a variety of applications in information and communication systems, we consider queueing networks in which the service rate at each of the individual nodes depends on the state of the entire system. The asymptotic behaviour of this type of networks is fundamentally different from classical queueing networks, where the service rate at each node is usually assumed to be independent of the state of the other nodes. We study the per-node rate stability and output rates for a general class of feed-forward queueing networks with a general capacity allocation function. More specifically, we derive necessary conditions of per-node rate stability, and give bounds for the per-node output rate and asymptotic growth rates, under mild assumptions on the allocation function. For a set of parallel nodes, we further prove the convergence of the output rates and give a sharp characterization of the per-node rate stability. The results provide new intuition and fundamental insight in the stability and throughput behavior of queueing networks with shared resources

Heavy traffic analysis of roving server networks

This paper studies the heavy-traffic (HT) behaviour of queueing networks with a single roving server. External customers arrive at the queues according to independent renewal processes and after completing service, a customer either leaves the system or is routed to another queue. This type of customer routing in queueing networks arises very naturally in many application areas (in production systems, computer- and communication networks, maintenance, etc.). In these networks, the single most important characteristic of the system performance is oftentimes the path time, i.e. the total time spent in the system by an arbitrary customer traversing a specific path. The current paper presents the first HT asymptotic for the path-time distribution in queueing networks with a roving server under general renewal arrivals. In particular, we provide a strong conjecture for the system's behaviour under HT extending the conjecture of Coffman et al. [E.G. Coffman Jr., A.A. Puhalskii, M.I. Reiman 1995 and 1998] to the roving server setting of the current paper. By combining this result with novel light-traffic asymptotics we derive an approximation of the mean path-time for arbitrary values of the load and renewal arrivals. This approximation is not only highly accurate for a wide range of parameter settings, but is also exact in various limiting cases

On the relative importance of specific and non-specific approaches to oral microbial adhesion

In this paper, it is suggested that specificity and non-specificity in (oral) microbial adhesion are different expressions for the same phenomena. It is argued that the same basic, physico-chemical forces are responsible for so-called 'non-specific' and 'specific' binding and that from a physico-chemical point of view the distinction between the two is an artificial one. Non-specific interactions arise from Van der Waals and electrostatic forces and hydrogen bonding, and originate from the entire cell. A specific bond consists of a combination of the same type of Van der Waals and electrostatic forces and hydrogen bonding, now originating from highly localized chemical groups, which together form a stereo-chemical combination. The absence or presence of specific receptor sites on microbial cell surfaces must therefore be reflected in the overall, non-specific surface properties of cells as well. This point is illustrated by showing that glucanbinding lectins on mutans streptococcal strains may determine the pH dependence of the zeta potentials of these cells. When studying microbial adhesion, a non-specific approach may be better suited to explain adhesion to inert substrata, whereas a specific approach may be preferred in case of adhesion to adsorbed protein films. Adhesion is, however, not as important in plaque formation in the human oral cavity as is retention, because low shear force periods. during which adhesion presumably occurs, are followed by high shear force periods, during which adhering cells must withstand these detachment forces. Evidence is provided that such detachment will be through cohesive failure in the pellicle mass, the properties of which are conditioned by the overall, non-specific substratum properties. Therefore, in vivo plaque formation may be more readily explained by a non-specific approach.</p

Demand-point constrained EMS vehicle allocation problems for regions with both urban and rural areas

Governments deal with increasing health care demand and costs, while budgets are tightened. At the same time, ambulance providers are expected to deliver high-quality service at affordable cost. Maximum reliability and minimal availability models guarantee a minimal performance level at each demand point, in contrast to the majority of facility location and allocation methods that guarantee a minimal

Impact of solid surface hydrophobicity and micrococcal nuclease production on Staphylococcus aureus Newman biofilms

Staphylococcus aureus is commonly associated with biofilm-related infections and contributes to the large financial loss that accompany nosocomial infections. The micrococcal nuclease Nuc1 enzyme limits biofilm formation via cleavage of eDNA, a structural component of the biofilm matrix. Solid surface hydrophobicity influences bacterial adhesion forces and may as well influence eDNA production. Therefore, it is hypothesized that the impact of Nuc1 activity is dependent on surface characteristics of solid surfaces. For this reason, this study investigated the influence of solid surface hydrophobicity on S. aureus Newman biofilms where Nuc1 is constitutively produced. To this end, biofilms of both a wild-type and a nuc1 knockout mutant strain, grown on glass, salinized glass and Pluronic F-127-coated silanized glass were analysed. Results indicated that biofilms can grow in the presence of Nuc1 activity. Also, Nuc1 and solid surface hydrophobicity significantly affected the biofilm 3D-architecture. In particular, biofilm densities of the wild-type strain on hydrophilic surfaces appeared higher than of the mutant nuc1 knockout strain. Since virulence is related to bacterial cell densities, this suggests that the virulence of S. aureus Newman biofilms is increased by its nuclease production in particular on a hydrophilic surface

Value Function Discovery in Markov Decision Processes with Evolutionary Algorithms

In this paper we introduce a novel method for discovery of value functions for Markov Decision Processes (MDPs). This method, which we call Value Function Discovery (VFD), is based on ideas from the Evolutionary Algorithm field. VFD’s key feature is that it discovers descriptions of value functions that are algebraic in nature. This feature is unique, because the descriptions include the model parameters of the MDP. The algebraic expression of the value function discovered by VFD can be used in several scenarios, e.g., conversion to a policy (with one-step policy improvement) or control of systems with time-varying parameters. The work in this paper is a first step towards exploring potential usage scenarios of discovered value functions. We give a detailed description of VFD and illustrate its application on an example MDP. For this MDP we let VFD discover an algebraic description of a value function that closely resembles the optimal value function. The discovered value function is then used to obtain a policy, which we compare numerically to the optimal policy of the MDP. The resulting policy shows near-optimal performance on a wide range of model parameters. Finally, we identify and discuss future application scenarios of discovered value functions

Throughput modeling of the IEEE MAC for sensor networks

In this paper we provide a model for analyzing the saturation throughput of the ieee 802.15.4 mac protocol, which is the de-facto standard for wireless sensor networks, ensuring fair access to the channel. To this end, we introduce the concept of a natural layer, which reflects the time that a sensor node typically has to wait prior to sending a packet. The model is simple and provides new insight how the throughput depends on the protocol parameters and the number of nodes in the network. Validation experiments with simulations demonstrate that the model is highly accurate for a wide range of parameter settings of the mac protocol, and applicable to both large and small networks. As a byproduct, we discuss fundamental differences in the protocol stack and corresponding throughput models of the popular 802.11 standard

Rate Stability and Output Rates in Queueing Networks with Shared Resources

Abstract Motivated by a variety of applications in information and communication systems, we consider queueing networks in which the service rate at each of the individual nodes depends on the state of the entire system. The asymptotic behaviour of this type of networks is fundamentally different from classical queueing networks, where the service rate at each node is usually assumed to be independent of the state of the other nodes. We study the per-node rate stability and output rates for a class networks with a general capacity allocation function. More specifically, we derive necessary conditions of per-node rate stability, and give bounds for the per-node output rate and asymptotic growth rates, under mild assumptions on the allocation function. For a set of parallel nodes, we further prove the convergence of the output rates for most parameters and give a sharp characterization of the per-node rate stability. The results provide new intuition and fundamental insight in the stability and throughput behavior of queueing networks with shared resources

Influence of glutaraldehyde fixation of cells adherent to solid substrata on their detachment during exposure to shear stress

In order to determine the response of fixed and nonfixed cells adherent to a solid substratum to shear stress, human fibroblasts were allowed to adhere and spread on either hydrophilic glass or hydrophobic Fluoroethylene-propylene (FEP-Teflon) and fixed with glutaraldehyde. Then, the cells were exposed to an incrementally loaded shear stress in a parallel plate flow chamber up to shear stresses of about 500 dynes/cm2, followed by exposure to a liquid-air interface passage. The cellular detachment was compared with the one of nonfixed cells. In case of fixed cells, 50% of the adhering cells detached from FEP-Teflon at a shear stress of 350 dynes/cm2, whereas 50% of the adhering, nonfixed cells detached already at a shear stress of 20 dynes/cm2. No fixed cells detached from glass for shear stresses up to at least 500 dynes/cm2. More than 50% of the nonfixed cells were detached from glass at a shear stress of 350 dynes/cm2. Furthermore, the shape and morphology of fixed cells did not change during the incrementally loaded flow, in contrast to the ones of nonfixed cells, which clearly rounded up prior to detachment.</p

Het Nieuwe Telen: Gerbera - Teeltseizoen: 2010-2011

In het tweede jaar van Het Nieuwe Telen Gerbera is een energie besparing gerealiseerd van 50% ten opzichte van de gedefinieerde referentie. De productie ruim 10% boven de prognose en zeker vergelijkbaar met de praktijk. In vergelijking met het eerste jaar is er meer assimilatie belichting gebruikt, maar minder warmte nodig geweest. Uit de twee jaar durende proef blijkt dat geforceerde ventilatie bijdraagt aan energie besparing in de Gerbera teelt omdat niet met een minimumbuis geteeld hoeft te worden. Voor de gewenst uitrusting in de praktijk zijn uit dit onderzoek leerpunten te halen om de luchtbehandelingskast te optimaliseren. Abstrac