Priority tandem queueing model with admission control

A two-stage multi-server tandem queue with two types of processed customers is analyzed. The input is described by the Marked Markovian Arrival Process (MMAP). The first stage has an infinite number of servers while the second stage has a finite number of servers. The service time at the both stages has an exponential distribution. Priority customers are always admitted to the system. Non-priority customers are admitted to the system only if the number of busy servers at the second stage does not exceed some pre-assigned threshold. Queueing system’s behavior is described in terms of the multi-dimensional asymptotically quasi-Toeplitz continuous time Markov chain. It allows to exploit a numerically stable algorithm for calculation of the stationary distribution of the queueing system. The loss probability at the both stages of the tandem is computed. An economic criterion of the system operation is optimized with respect to the threshold. The effect of control on the main performance measures of the system is numerically demonstrated.     

A multiserver MAP/PH/<Emphasis Type="Italic">N</Emphasis> system with controlled broadcasting by unreliable servers

A multiserver queuing system with an unlimited buffer is considered. The customer arrival is described by the Markov arrival process. The service time has a phase-type distribution. The service may occur with errors. The service strategy is as follows. If the number of busy servers is higher than a certain threshold value at the moment of the customer arrival, then the customer is copied, and the copies are serviced by all the free servers. If the number of busy servers is not higher than this threshold at the moment of the customer arrival, then the customer is serviced by one server. Stationary distributions of the number of customers and the residence time within the system are obtained. The threshold’s influence on the main parameters of the system’s productivity is numerically investigated.     

Two-phase queuing system with a threshold strategy of retrial calls

A two-phase queuing system (QS) has been considered; its first phase is represented by a single-linear system with retrial calls, and the second phase is represented by a multilinear (multiple) unbuffered QS. Queries arrive to the system in the group Markov flow. The system has two operational modes that differ by the strategy of the retrial attempts. Depending on the number of retrial calls, either a decentralized or centralized strategy of retrials is used. A stationary distribution of the system’s state probabilities at the embedded epochs and arbitrary time moments has been found, and formulae for the main characteristics of the system’s productivity have been obtained. Numerical examples are presented.     

The MAP/PH/N multi-server queuing system with broadcasting service discipline and server heating

A multiserver queuing system with an infinite buffer is considered. The incoming customer is described by a Markovian input flow. The heating time of the servers and the time of the customer service have a phase type distribution. The heating of all the free servers starts at the customer’s arrival moment, and the customer receives the service by all these servers after the heating’s termination. The steady state distribution of the customers and their sojourn time in a system are found. The basic performance measures of the system are calculated. The results of the numerical experiments show the potential usability of the proposed service discipline in comparison with the classical one.     

Markov queueing system with finite buffer and negative customers affecting the queue end

A multiserver queueing system with finite buffer, Markov input flow, and Markov (general) service process of all customers on servers with the number of process states and intensities of inter-phase transitions depending on the number of customers in the system is considered. A Markov flow of negative customers arrives to the system; one negative customer “kills” one positive customer at the end of the queue. A recurrent algorithm for computing stationary probabilities of system states is obtained; and a method for calculating stationary distribution of waiting time before starting service of a positive customer is proposed.     

Stealth modification versus nuisance attacks in the LOCKSS peer-to-peer digital preservation system

The LOCKSS system is a world-wide peer-to-peer system for the preservation of academic journals and other archival information published on the Web. The system is deployed at over 200 libraries around the world and currently preserving titles of publishers representing more than 2000 academic titles. It consists of a large number of independent, low-cost, persistent Web caches that cooperate to detect and repair damage to their content by voting in “opinion polls.” Based on our experience working with the LOCKSS system, in this paper, we identify and characterize a fundamental tradeoff in the system’s ability to defend against adversaries with competing goals. In particular, the techniques used to defend against adversaries attempting stealth modification of content being preserved in the system are at odds with those used to identify nuisance adversaries simply trying to disrupt system procedures. We show that with a simple change to the design of the system’s preservation protocol we are able to create moderately strong defense against both of these competing adversaries simultaneously.     

Preventive maintenance scheduling for repairable system with deterioration

Maintenance as an important part in manufacturing system can keep equipment in good condition. Many maintenance policies help to decrease the unexpected failures and reduce high operational cost such as conventional preventive maintenance. But these conventional preventive maintenance policies have the same time interval T that may easily neglect system’s reliability, because the system deteriorates with increased usage and age. Hence, this study has developed a reliability-centred sequential preventive maintenance model for monitored repairable deteriorating system. It is supposed that system’s reliability could be monitored continuously and perfectly, whenever it reaches the threshold R, the imperfect repair must be performed to restore the system. In this model, system’s failure rate function and operational cost are both considered by the effect of system’s corresponding condition, which helps to decide the optimal reliability threshold R and preventive maintenance cycle number. Finally, through case study, the simulation results show that the improved sequential preventive maintenance policy is more practical and efficient.     

Configuration of overloaded servers with dynamic routing

We consider overload of servers in a network with dynamic routing of messages. The system consists of k servers and ℓ independent Poisson input flows. Messages from each flow are directed to m servers, and each message is directed to a server that is the least loaded at the moment of its arrival. In such a system, configuration of overloaded servers depends on the intensity of input flows. A similar effect was considered in [1] for a system with another geometry.     

Analysis of the multi-server Markov queuing system with unlimited buffer and negative customers

Consideration was given to the multi-server queuing system with unlimited buffer, Markov input flow, and Markov (general) process of servicing all customers on servers with the number of process states and intensities of the inter-phase passage depending on the number of customers in the system. Additionally, a Markov flow of negative customers arrives to the system, the arriving negative customer killing the last queued positive customer. A recurrent algorithm to calculate the stationary probabilities of system states was obtained, and a method of calculation of the stationary distribution of the waiting time before starting servicing of a positive customer was proposed.     

Dynamics of a three-module vibration-driven system with non-symmetric Coulomb’s dry friction

In the present paper, a three-module vibration-driven system moving on a rough horizontal plane is modeled to investigate the relation among the system’s steady-state motion, external Coulomb’s dry friction force and internal excitations. Each module of the system represents a vibration-driven system composed of a rigid body and a movable internal mass. Major attention is focused on the primary resonance situation that the excitation frequency is close to the first-order natural frequency of the system. In the case that the external friction is low, the internal excitation is weak and the stick–slip motion is negligible, both methods of averaging and modal superposition are employed to study the steady-state motion of the system. Through a set of algebraic equations, an approximate value of the system’s average steady-state velocity is obtained. Several numerical examples are calculated to verify the validity of the analytical results both qualitatively and quantitatively. It is seen that big quantitative errors will appear if stick–slip motions occur. Then, two mechanisms for the possible stick–slip motions are put forward, which explain the errors on the average steady-state velocity. Numerical simulations verify our analysis on the stick–slip effects and their mechanisms. Finally, to maximize the average steady-state velocity of the system, optimal control problem is studied. It is shown that, in addition to modifying the friction coefficients, the improvement of the system’s efficiency can be provided by changing the initial phase shifts among the three internal excitations.     

Emergence of Cooperative Internet Server Sharing Among Internet Search Agents Caught in the n-Person Prisoner’s Dilemma Game

Information on the Internet can be collected by autonomous agents that send out queries to the servers that may have the information sought. From a single agents perspective, sending out as many queries as possible maximizes the chances of finding the information sought. However, if every agent does the same, the servers will be overloaded. The first major contribution of this paper is proving mathematically that the agents situated in such environments play the n-Person Prisoners Dilemma Game. The second is mathematically deriving the notion of effectiveness of cooperation among the agents in such environments and then presenting the optimal interval for the number of information sites for a given number of information-seeking agents. When the optimal interval is satisfied, cooperation among agents is effective, meaning that resources (e.g., servers) are optimally shared. Experimental results suggest that agents can better share available servers through the kinship-based cooperation without explicitly knowing about the entire environment. This paper also identifies difficulties of promoting cooperation in such environments and presents possible solutions. The long-term goal of this research is to elucidate the understanding of massively distributed multiagent environments such as the Internet and to identify valuable design principles of software agents in similar environments.     

Classical necessary optimality conditions in discrete optimal control problems with nonlocal conditions

We consider an optimal control problem in which the system’s state is described by a system of difference equations with nonlocal (two-point) conditions; this problem includes, as particular cases, the initial value problem (Cauchy problem) and different types of boundary value problems. It is assumed that the admissible controls take values from an open set. The first and second functional variations are calculated; these variations are used to express first and second order necessary optimality conditions in the classical sense for discrete optimal control problems.     

Help desk center operating model as a two-phase queueing system

We consider a two-phase queueing system with a Markovian arrival flow as an operating model for a help desk center. The first phase is a multiserver system with a finite buffer and impatient customers. After getting service in the first phase, a customer either enters the second phase with an infinite buffer or quits the system. Service times at the first and second stages have phase-type distributions with different parameters. We obtain an existence condition for a stationary regime of the system. An algorithm for computing stationary probabilities and basic performance characteristics of the system is presented. Laplace-Stieltjes transforms for the distributions of sojourn and waiting times in the first and second phases are found. Results of numerical experiments are presented. Optimization problem for the system operation is solved numerically.     

Adaptation and user expertise modelling in AthosMail

This article describes the User Model component of AthosMail, a speech-based interactive e-mail application developed in the context of the EU project DUMAS. The focus is on the system’s adaptive capabilities and user expertise modelling, exemplified through the User Model parameters dealing with initiative and explicitness of the system responses. The purpose of the conducted research was to investigate how the users could interact with a system in a more natural way, and the two aspects that mainly influence the system’s interaction capabilities, and thus the naturalness of the dialogue as a whole, are considered to be the dialogue control and the amount of information provided to the user. The User Model produces recommendations of the system’s appropriate reaction depending on the user’s observed competence level, monitored and computed on the basis of the user’s interaction with the system. The article also discusses methods for the evaluation of adaptive user models and presents results from the AthosMail evaluation.The research was done while the author was affiliated with the University of Art and Design Helsinki as the scientific coordinator of the DUMAS project.     

Selection of modeling coordinates for forward dynamic multibody simulations

Modeling mechanical systems in a manner that allows the models to be simulated quickly is vital in many fields, such as real-time simulation and control. Modeling these systems using their symbolic equations, rather than the more widely-used numerical methods, generally produces faster solution times. However, the number, complexity, and computational efficiency of these equations is highly dependent upon which coordinate set was used to model the system. Most coordinate selection methods established thus far are based on the assumption that minimizing the number of modeling coordinates will produce models with faster simulation times. This paper will show that this technique is not always valid and proposes a new technique of selecting a system’s coordinates based on a series of heuristics. A large part of these heuristics will be established by closely analyzing a specific technique used to formulate a system’s equations, and the effect each step of this formulation process will have on the complexity of the final system equations.     

A Two-Phase BMAP|G|1|N → PH|1|M – 1 System with Blocking

The stationary probability distribution of a two-phase queueing system with a finite or an infinite buffer for the first phase and a finite buffer for the second phase is derived. The input flow of the system is a batch Markov arrival process. Both phases have single-servers. The service time distribution is arbitrary for the first phase and of phase-type for the second phase. If the buffer of the second phase is full at the instant of completion of service at the first phase, the first server is blocked until the buffer is freed.     

Retrial queuing system with Markovian arrival flow and phase-type service time distribution

We consider a multi-server queuing system with retrial customers to model a call center. The flow of customers is described by a Markovian arrival process (MAP). The servers are identical and independent of each other. A customer’s service time has a phase-type distribution (PH). If all servers are busy during the customer arrival epoch, the customer moves to the buffer with a probability that depends on the number of customers in the system, leaves the system forever, or goes into an orbit of infinite size. A customer in the orbit tries his (her) luck in an exponentially distributed arbitrary time. During a waiting period in the buffer, customers can be impatient and may leave the system forever or go into orbit. A special method for reducing the dimension of the system state space is used. The ergodicity condition is derived in an analytically tractable form. The stationary distribution of the system states and the main performance measures are calculated. The problem of optimal design is solved numerically. The numerical results show the importance of considering the MAP arrival process and PH service process in the performance evaluation and capacity planning of call centers.     

Goal-driven risk assessment in requirements engineering

Risk analysis is traditionally considered a critical activity for the whole software system’s lifecycle. Risks are identified by considering technical aspects (e.g., failures of the system, unavailability of services, etc.) and handled by suitable countermeasures through a refined design. This, however, introduces the problem of reconsidering system requirements. In this paper, we propose a goal-oriented approach for analyzing risks during the requirements analysis phase. Risks are analyzed along with stakeholder interests, and then countermeasures are identified and introduced as part of the system’s requirements. This work extends the Tropos goal modeling formal framework proposing new concepts, qualitative reasoning techniques, and methodological procedures. The approach is based on a conceptual framework composed of three main layers: assets, events, and treatments. We use “loan origination process” case study to illustrate the proposal, and we present and discuss experimental results obtained from the case study.     

An interleaving crescent broadcasting protocol for near video-on-demand services

This work presents a novel interleaving crescent broadcasting protocol for near video-on-demand service. The interleaving crescent broadcasting protocol is a trade-off among the subscriber’s access latency, maximum buffer requirement, needed subscriber’s bandwidth, and maximum disk I/O transfer rate. A longer subscriber’s access latency may cause a subscriber to leave. A lower maximum buffer requirement, a lower needed subscriber’s bandwidth, and a lower maximum disk I/O transfer rate reduce subscribers’ costs. The interleaving crescent broadcasting protocol not only makes access latency shorter, but also lowers the overall system’s cost. We prove the correctness of the interleaving crescent protocol; provide mathematical analyses to demonstrate its efficiency.