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Abstract In many actual issues, such as call center and cloud service computing center, retrial systems have been widely applied to derive the performance measures. This study takes up a multi-server retrial system with partial servers synchronous vacation and customer geometric loss. An arriving customer finding no servers available either enters a retrial group according to first-in-first-serve discipline or balks. In the retrial group, only the first customer can retry for his service. A customer from the retrial group finding all servers unavailable decides either balks or reenters the retrial group. Partial servers may be on vacation simultaneously when they become idle at the moment of service completion and system conditions satisfy the administrative policy. This retrial system is analyzed as a quasi-birth-death process, and some general descriptors are developed. Optimization problems, including single-objective and bi-objective, are constructed to help the manager’s decision-making. Numerical results are displayed tabularly and graphically. A potential application in which the proposed model can be applied is provided. [ABSTRACT FROM AUTHOR]

Motivated by some real stochastic service systems, we study a feedback retrial queue with customer balking and unreliable servers (servers subject to breakdowns). A customer will either join a retrial orbit or balk if all servers are busy upon his arrival. After a service completion, the customer either leave the system or enter the retrial group again for additional service. The customer who is interrupted by the server failure will be served by another server if available; otherwise, he will be contacted later and treated as leaving the system. We use a quasi-birth-and-death process to analyze this system with almost all possible uncertain factors and derive major performance measures of the system. We also develop a cost function determining the optimal parameter settings of the system under the stability condition. Probabilistic Global Search Lausanne approach is employed in solving the optimization issue. The effects of parameters on performance measures of the system are examined numerically. Finally, we discuss the application of the model in the telephone medical consulting service system. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the availability of two different series system configurations with imperfect coverage, where an unreliable server is responsible for maintaining the system. The primary and standby components are included in both series system configurations. The time-to-failure of the primary and standby components are exponentially distributed. When the server is repairing, it is subject to random breakdown. The time-to-breakdown of the server is assumed to have exponential distribution. The standby components will substitute for the primary components whenever the primary components fail. The recovery from standby to active is imperfect. The coverage factors for primary component failure and for standby component failure are assumed to be the same as c. The repair time of the failed components and the repair time of the breakdown server are generally distributed. Further, we implement a practical model with two different repairable system configurations. By supplementary variable technique, we derive the steady-state availability of these two different series system configurations. Finally, we compare the cost/benefit ratio between two configurations and the availability in the different pairs of the component's repair time and server's repair time distributions. [ABSTRACT FROM AUTHOR]

This investigation is concerned with an M/G/1 machine interference problem with imperfect switchover of standbys, in which an unreliable repairman maintains a group of machines. An unreliable repairman means that the repairman is typically subject to unpredictable breakdowns. The time between two consecutive breakdowns follows an exponential distribution, and recovery time of the unreliable repairman follows a general distribution. The lifetime of operating/standby machines and the repair time of failed machines are exponentially and generally distributed, respectively. Using the method of supplementary variable, the stationary probability distribution is obtained. We develop some performance measures and system reliability indices. Furthermore, the cost effectiveness maximization is also discussed. Finally, a cost model is proposed to find the optimal numbers of operating and standby substations, which minimize the average cost per unit time. [ABSTRACT FROM AUTHOR]

We studied the inferences of an availability system with reboot delay and standby switching failures in which the system consisted of two operating units and one warm standby. The system was studied under the assumption that the time-to-failure and the time-to-repair were assumed to follow an exponential and a general distribution. The reboot times are assumed to be exponentially distributed with parameter β. We constructed a consistent and asymptotically normal estimator of availability for such a repairable system. Based on this estimator, interval estimation and testing hypothesis were developed by using log it transformation. To implement the simulation inference for the system availability, we adopted two repair-time distributions—namely, lognormal and Weibull; and three types of Weibull distributions—characterized by their shape parameters—were considered. Finally, appropriate tables and figures of all simulation results have been included. [ABSTRACT FROM AUTHOR]

We consider the distributions of operating characteristics of an M[x]/G/1 queue under vacation policies, where the first customer of each busy period receives an exceptional service. When all the customers are served in the system exhaustively, the server deactivates and operates one of two vacation policies: (1) multiple vacation policy and (2) single vacation policy. We develop the performance measures for both systems. Finally, some numerical illustrations are also given. These two vacation models have potential applications in day-to-day life, such as post offices, banks, hospitals, etc. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to study the steady-state availability of a repairable system with standby switching failure. The repairable system configuration includes the primary and standby components, where an unreliable server is responsible to repair or monitor the failed ones. The time-to-failure and time-to-repair of the components follow exponential and general distribution, respectively. The server subjects to active breakdown when it is repairing. The time-to-breakdown of the server is also assumed to be exponentially distributed. When the primary components fail, the standby components replace the primary components successfully with probability 1− q . The repair time of the failed components and the repair time of the breakdown server are generally distributed. Further, we frame a practical model with three different repairable system configurations. We use supplementary variable method and integro-differential equations to obtain the steady-state availability of these three different repairable system configurations. Finally, we compare the cost/benefit ratio among the three configurations given the distribution parameters, and to the cost of the primary and standby components. [ABSTRACT FROM AUTHOR]

In this paper, we present a detailed analysis of a multi-server retrial queue with Bernoulli feedback, where the servers are subject to starting failures. Upon completion of a service, a customer would decide either to leave the system with probabilitypor to join the retrial orbit again for another service with complementary probability 1−p. We analyse this queueing system as a quasi-birth–death process. Specifically, the equilibrium condition of the system is given for the existence of the steady-state analysis. Applying the matrix-geometric method, the formulae for computing the rate matrix and stationary probabilities are obtained. We further develop the matrix-form expressions for various system performance measures. A cost model is constructed to determine the optimal number of servers, the optimal mean service rate and the optimal mean repair rate subject to the stability condition. Finally, we give a practical example to illustrate the potential applicability of this model. [ABSTRACT FROM PUBLISHER]

This paper considers the optimal management problem of a finite-capacity M/M/1/K queueing system with-policy in which the server may break down while working. When the number of customers reaches its capacityK, no further arriving customers are allowed to enter the system. Customers are allowed to system with probabilitypor the customers are still unable to enter the system with probability 1 − pas the queue length decreases to a certain threshold valueF. By applying the birth-and-death process, some important performance measures are derived. A cost model, developed to determine the optimal continuous and discrete control parameters for the-policy at a minimum cost, and sensitivity analysis are also studied. [ABSTRACT FROM PUBLISHER]

We consider a machine repair problem with queue-dependent heterogeneous repairmen. The stationary probability distribution of the number of failed machines in the system is derived. A profit model is developed to determine the optimal values of threshold to assign/add one more repairman for increasing the total service rate where the service rate is assumed adjustable. The probabilistic global search Lausanne (PGSL) method is employed to find an initial trial solution. Based on this initial solution, a direct search method is applied to obtain the optimal values of the threshold and, then, the Quasi-Newton method is implemented to adjust the corresponding service rates. Some numerical experiments are performed to justify the efficiency of the optimum approach. [ABSTRACT FROM AUTHOR]

This paper deals with the ( p , N )-policy M/G/1 queue with an unreliable server and single vacation. Immediately after all of the customers in the system are served, the server takes single vacation. As soon as N customers are accumulated in the queue, the server is activated for services with probability p or deactivated with probability (1 − p ). When the server returns from vacation and the system size exceeds N , the server begins serving the waiting customers. If the number of customers waiting in the queue is less than N when the server returns from vacation, he waits in the system until the system size reaches or exceeds N . It is assumed that the server is subject to break down according to a Poisson process and the repair time obeys a general distribution. This paper derived the system size distribution for the system described above at a stationary point of time. Various system characteristics were also developed. We then constructed a total expected cost function per unit time and applied the Tabu search method to find the minimum cost. Some numerical results are also given for illustrative purposes. [ABSTRACT FROM AUTHOR]

In this paper, we examine a repairable system with imperfect coverage, reboot delay and one repair facility. The failure times and repair times of failed units are assumed to be exponential and general distributions, respectively. As a unit fails, it can be immediately detected, located and replaced with a coverage probability c by a standby, if one is available. We use a recursive method and the supplementary variable technique to develop the steady-state probabilities of down units at an arbitrary epoch. Then, an efficient algorithm is constructed to compute the steady-state availability. We adopt two repair time distributions, namely, exponential and gamma, to illustrate the method. Finally, we also perform a sensitivity analysis of the steady-state availability with respect to the system parameters for various repair time distributions. [ABSTRACT FROM AUTHOR]

Abstract: This paper considers a machine repair problem with M operating machines and S standbys, in which R repairmen are responsible for supervising these machines and operate a (V, R) vacation policy. With such policy, if the number of the failed machines is reduced to R − V (R > V) (there exists V idle repairmen) at a service completion, these V idle servers will together take a synchronous vacation (or leave for other secondary job). Upon returning from the vacation, they do not take a vacation again and remain idle until the first arriving failed machine arrives. The steady-state probabilities are solved in terms of matrix forms and the system performance measures are obtained. Algorithmic procedures are provided to deal with the optimization problem of discrete/continuous decision variables while maintaining a minimum specified level of system availability. [Copyright &y& Elsevier]

Abstract: This paper deals with the steady state behavior of an M/G/1 queue with general retrial time and Bernoulli vacation schedule for an unreliable server, which consist of a breakdown period and delay period. While the server is working with primary customers, it may breakdown at any instant and server will be down for short interval of time. Further concept of the delay time is also introduced. The primary customer finding the server busy, down or vacation enters a group of unsatisfied customer called orbit. After the completion of a service, the server either goes for a vacation of random length with probability p or may continue serve for the next unit, if any with probability (1− p). For this model, we first derive the system size distribution at a departure epoch. Secondly, we derive the probability generating function of joint distributions of state of the server and queue size, which is one of chief objectives of the paper. Finally, we obtain some important performance measures and reliability indices of this model. [Copyright &y& Elsevier]

In this paper, we study a repairableK-out-of-(M+W) retrial system withMidentical primary components,Wstandby components and one repair facility. The time-to-failure and time-to-repair of the primary and standby components are assumed to be exponential and general distributions, respectively. The failed components are immediately for repair if the server is idle, otherwise the failed machines would enter an orbit. It is assumed that the retrial times are exponentially distributed. We present a recursive method using the supplementary variable technique and treating the supplementary variable as the remaining repair time to obtain the steady-state probabilities of down components at arbitrary epoch. Then, a unified and efficient algorithm is developed to compute the steady-state availability. The method is illustrated analytically for the exponential repair time distribution. Sensitivity analysis of the steady-state availability with respect to system parameters for a variety of repair time distributions is also investigated. [ABSTRACT FROM AUTHOR]

In this study, we consider an M/M/cretrial queue with Bernoulli vacation under a single vacation policy. When an arrived customer finds a free server, the customer receives the service immediately; otherwise the customer would enter into an orbit. After the server completes the service, the server may go on a vacation or become idle (waiting for the next arriving, retrying customer). The retrial system is analysed as a quasi-birth-and-death process. The sufficient and necessary condition of system equilibrium is obtained. The formulae for computing the rate matrix and stationary probabilities are derived. The explicit close forms for system performance measures are developed. A cost model is constructed to determine the optimal values of the number of servers, service rate, and vacation rate for minimizing the total expected cost per unit time. Numerical examples are given to demonstrate this optimization approach. The effects of various parameters in the cost model on system performance are investigated. [ABSTRACT FROM AUTHOR]

Abstract: This paper considers an infinite buffer queueing system in which servers follow a multi-threshold vacation policy. With such a policy, at a service completion instant, if the number of customers in the system is less than a prefixed threshold value, part of servers together take a single vacation (or leave for a random amount of time doing other secondary job). At the vacation completion instant, they return to the system for serving the customers. Some practical production and inventory systems or call centers could be modeled as this Markovian queue with a multi-threshold vacation policy. Using the Markovian process model, we obtain the exact closed-form expression of rate matrix and the stationary distribution of the number of customers in the system. A cost model is developed to search the joint optimal values of the thresholds of vacation policy and service rate of each server, which minimizes the long-term average cost. Some numerical results are presented to illustrate the optimization procedures. [Copyright &y& Elsevier]

This paper studies a Geo/G/1 queue in which the server operates multiple vacations and may break down while working. After all messages have been served in the system, the server leaves for a vacation. Upon returning from the vacation, the server activates immediately to serve if there are some messages waiting in the queue. Otherwise, the server takes another vacation. This queueing system has potential application in the inbound email contact center. By using the generating function technique, we derive some important system characteristics, such as system size distributions of server on vacation/busy/broken-down, the expected lengths of server vacation period and complete period, and message's waiting time in the queue. In particular, a cost model is developed to determine the optimal parameter values of different vacation distributions that minimize the cost, and some numerical examples are presented for illustrative purpose. [ABSTRACT FROM AUTHOR]

This article proposes a procedure to construct membership functions of the system characteristics of a repairable system with one primary machine, one standby machine and an unreliable service station. Failure and repair times of both the operating and standby machines are assumed to follow fuzzified exponential distributions. In addition, the failure and repair times of the service station also follow fuzzified exponential distributions. The α-cut approach is used to extract a family of conventional crisp intervals for the desired system characteristics from the fuzzy repairable system, determined with a set of parametric nonlinear programs using their membership functions. A numerical example successfully illustrates the practicality of the proposed approach. Because system characteristics are governed by the membership functions, more information is provided for use by designers and practitioners. The successful extension of the parameter spaces to fuzzy environments permits the repairable system to have wider practical applications. [ABSTRACT FROM PUBLISHER]

This article examines an M[x]/G/1 queueing system with an unreliable server and a repair, in which the server operates a randomised vacation policy with multiple available vacations. Upon the system being found to be empty, the server immediately takes a vacation. If there is at least one customer found waiting in the queue upon returning from a vacation, the server will be activated for service. Otherwise, if no customers are waiting for service at the end of a vacation, the server either remains idle with probability p or leaves for another vacation with probability 1 − p. When one or more customers arrive when the server is idle, the server immediately starts providing service for the arrivals. It is possible that an unpredictable breakdown may occur in the server, in which case a repair time is requested. For such a system, we derive the distributions of several important system characteristics, such as the system size distribution at a random epoch and at a departure epoch, the system size distribution at the busy period initiation epoch, and the distribution of the idle and busy periods. We perform a numerical analysis for changes in the system characteristics, along with changes in specific values of the system parameters. A cost effectiveness maximisation model is constructed to show the benefits of such a queueing system. [ABSTRACT FROM AUTHOR]

Abstract: This paper deals with the steady state behaviour of an M x /G/1 queue with general retrial time and Bernoulli vacation schedule for an unreliable server, which consists of a breakdown period and delay period. Here we assume that customers arrive according to compound Poisson processes. While the server is working with primary customers, it may breakdown at any instant and server will be down for short interval of time. Further concept of the delay time is also introduced. The primary customer finding the server busy, down or vacation are queued in the orbit in accordance with FCFS (first come first served) retrial policy. After the completion of a service, the server either goes for a vacation of random length with probability p or may continue to serve for the next customer, if any with probability (1− p). We carry out an extensive analysis of this model. Finally, we obtain some important performance measures and reliability indices of this model. [Copyright &y& Elsevier]

Abstract: This paper analyzes a discrete-time Geo/Geo/1 queueing system with the server subject to breakdowns and repairs, in which two different possible types of the server breakdowns are considered. In Type 1, the server may break down only when the system is busy, while in Type 2, the server can break down even if the system is idle. The server lifetimes are assumed to be geometrical and the server repair times are also geometric distributions. We model this system by the level-dependent quasi-birth–death (QBD) process and develop computation algorithms of the stationary distribution of the number of customers in the system using the matrix analytic method. The search algorithm for parameter optimization based on a cost model is developed and performed herein. [Copyright &y& Elsevier]

We consider an infinite-capacity M/M/c retrial queue with second optional service (SOS) channel. An arriving customer finds a free server would enter the service (namely, the first essential service, denoted by FES) immediately; otherwise, the customer enters into an orbit and starts generating requests for service in an exponentially distributed time interval until he finds a free server and begins receiving service. After the completion of FES, only some of them receive SOS. The retrial system is modelled by a quasi-birth-and-death process and some system performance measures are derived. The useful formulae for computing the rate matrix and stationary probabilities are derived by means of a matrix-analytic approach. A cost model is derived to determine the optimal values of the number of servers and the two different service rates simultaneously at the minimal total expected cost per unit time. Illustrative numerical examples demonstrate the optimisation approach as well as the effect of various parameters on system performance measures. [ABSTRACT FROM AUTHOR]

Abstract: This paper examines an M[x]/G/1 queueing system with a randomized vacation policy and at most J vacations. Whenever the system is empty, the server immediately takes a vacation. If there is at least one customer found waiting in the queue upon returning from a vacation, the server will be immediately activated for service. Otherwise, if no customers are waiting for service at the end of a vacation, the server either remains idle with probability p or leaves for another vacation with probability 1− p. This pattern continues until the number of vacations taken reaches J. If the system is empty by the end of the Jth vacation, the server becomes idle in the system. Whenever one or more customers arrive at server idle state, the server immediately starts providing service for the arrivals. Assume that the server may meet an unpredictable breakdown according to a Poisson process and the repair time has a general distribution. For such a system, we derive the distributions of important system characteristics, such as system size distribution at a random epoch and at a departure epoch, system size distribution at busy period initiation epoch, the distributions of idle period, busy period, etc. Finally, a cost model is developed to determine the joint suitable parameters (p ∗, J ∗) at a minimum cost, and some numerical examples are presented for illustrative purpose. [Copyright &y& Elsevier]

This paper considers an M/M/r queueing system with infinite capacity, in which the number of working servers changes depending on the queue length. The steady-state probability distributions and the expected number of customers in the system are derived, which are used to construct a cost function. In order to minimize the expected cost of the system, we use the genetic algorithm to find the best thresholds of queue length in activating servers and their corresponding service rate. Some illustrative examples are provided to demonstrate how the process of this algorithm works for the optimal management policy of the multi-server queueing system. [ABSTRACT FROM AUTHOR]

Abstract: This paper examines a discrete-time Geo/G/1 queue, where the server may take at most J −1 vacations after the essential vacation. In this system, messages arrive according to Bernoulli process and receive corresponding service immediately if the server is available upon arrival. When the server is busy or on vacation, arriving messages have to wait in the queue. After the messages in the queue are served exhaustively, the server leaves for the essential vacation. At the end of essential vacation, the server activates immediately to serve if there are messages waiting in the queue. Alternatively, the server may take another vacation with probability p or go into idle state with probability (1− p) until the next message arrives. Such pattern continues until the number of vacations taken reaches J. This queueing system has potential applications in the packet-switched networks. By applying the generating function technique, some important performance measures are derived, which may be useful for network and software system engineers. A cost model, developed to determine the optimum values of p and J at a minimum cost, is also studied. [ABSTRACT FROM AUTHOR]

Abstract: This paper considers an infinite-capacity M/M/c queueing system with modified Bernoulli vacation under a single vacation policy. At each service completion of a server, the server may go for a vacation or may continue to serve the next customer, if any in the queue. The system is analyzed as a quasi-birth-and-death (QBD) process and the necessary and sufficient condition of system equilibrium is obtained. The explicit closed-form of the rate matrix is derived and the useful formula for computing stationary probabilities is developed by using matrix analytic approach. System performance measures are explicitly developed in terms of computable forms. A cost model is derived to determine the optimal values of the number of servers, service rate and vacation rate simultaneously at the minimum total expected cost per unit time. Illustrative numerical examples demonstrate the optimization approach as well as the effect of various parameters on system performance measures. [ABSTRACT FROM AUTHOR]

This article deals with the availability behavior of a repairable system in which standby switched to primary is subject to breakdowns. The time-to-failure of the four primary and two standby units are assumed to be exponentially and generally distributed. In addtion, the repair time of service station follow four common distributions: exponential (EXP), Gamma (G), Uniform (U), and Mixture (M). We use a recursive method, and the supplementary variable technique to develop the steady-state availability, Av. The estimator [image omitted] is strongly consistent and asymptotically normal. The interval estimations of Av are constructed by five bootstrap approaches: standard bootstrap confidence interval (SB), the percentile bootstrap confidence interval (PB), the bias-corrected percentile bootstrap confidence interval (BCPB), the bias-corrected and accelerated confidence interval (BCa), and bootstrap pivot confidence interval (BP). Finally, some simulation computations are conducted in order to describe the performances of [image omitted] on various interval estimation by calculating the coverage percentage and the average length of intervals. [ABSTRACT FROM AUTHOR]

This article considers the optimal management problem of a finite capacity M/H2/1 queueing system, in which the unreliable server operates the F-policy. When the number of customers in the system reaches its capacity K ( [image omitted]), no further arriving customers are allowed to enter the system until a certain number of customers who are already in the system have been served so that the number of customers in the system decreases to a predetermined threshold F ( [image omitted]). At that time, the server needs to take an exponential startup time and allow customers to enter the system. The F-policy investigates the most common issue of controlling arrival to a queueing system and a startup time is required before allowing customers to enter the system. This system is useful in many potential applications for wireless communication networks, transportation service systems, and production systems. Some important performance measures are derived. A cost model, developed to determine the optimal capacity K, the optimal threshold F, the optimal setup rate, and the optimal repair rate at a minimum cost, is also studied. [ABSTRACT FROM AUTHOR]

We consider an M/M/c retrial queue with geometric loss and feedback. An arriving customer finding a free server enters into service immediately; otherwise the customer either enters into an orbit to try again after a random amount of time or leave the system without service. After the completion of service, he decides either to join the retrial orbit or to leave the system. The retrial system is modelled by a quasi-birth-and-death process, and some system performance measures are derived. The useful formulae for computing the rate matrix and stationary probabilities are derived by means of matrix-analytical approach. A cost model is derived to determine the optimal values of the number of servers and service rate simultaneously at the minimal total expected cost per unit time. Illustrative numerical examples demonstrate the optimization approach as well as the effect of various parameters on system performance measures. [ABSTRACT FROM AUTHOR]

Abstract: We consider an infinite capacity M/M/c queueing system with unreliable servers, in which the customers may balk (do not enter) and renege (leave the queue after entering). The system is analyzed as a quasi-birth-and-death (QBD) process and the necessary and sufficient condition of system equilibrium is obtained. System performance measures are explicitly derived in terms of computable forms. The useful formulae for computing the rate matrix and stationary probabilities are derived by means of a matrix analytical approach. A cost model is derived to determine the optimal values of the number of servers, service rate and repair rate simultaneously at the minimal total expected cost per unit time. The parameter optimization is illustrated numerically by the Quasi-Newton method. [ABSTRACT FROM AUTHOR]

This paper analyses a discrete-time Geo/Geo/1 queueing system, where all arriving customers demand the first essential service and some of them may further demand an additional optional service. The interarrival times, the service times of the essential service and the second optional service for arrivals are assumed to be random variables with Geometric distributions. Such a model has potential applications in computer or digital communication network systems. We model this system by the level-dependent and independent quasi-birth-death processes and develop efficient computation algorithms of the stationary distribution of the number of customers in the system using matrix analytic method. A cost model is derived to determine the optimal values of the two different service rates simultaneously at the minimal total expected cost per unit time. Illustrative numerical examples demonstrate the optimization approach. [ABSTRACT FROM AUTHOR]

Abstract: This paper examines an queueing system with a randomized vacation policy and at most J vacations. Whenever the system is empty, the server immediately takes a vacation. If there is at least one customer found waiting in the queue upon returning from a vacation, the server will be immediately activated for service. Otherwise, if no customers are waiting for service at the end of a vacation, the server either remains idle with probability p or leaves for another vacation with probability . This pattern continues until the number of vacations taken reaches J. If the system is empty by the end of the Jth vacation, the server is dormant idly in the system. If there is one or more customers arrive at server idle state, the server immediately starts his services for the arrivals. For such a system, we derive the distributions of important characteristics, such as system size distribution at a random epoch and at a departure epoch, system size distribution at busy period initiation epoch, idle period and busy period, etc. Finally, a cost model is developed to determine the joint suitable parameters at a minimum cost, and some numerical examples are presented for illustrative purpose. [Copyright &y& Elsevier]

Abstract: We consider an M/M/c queue with c =2, in which the number of working servers can be adjusted one at a time at arrival epochs or at service completion epochs depending on the number of customers in the system. Analytic closed-form solutions of the infinite capacity M/M/2 queueing system operating under the triadic (0, Q, N, M) policy are derived. The total expected cost function per unit time is developed, to obtain the optimal operating (0, Q, N, M) policy and the optimal service rate, at minimum cost. Some illustrative examples are provided and the genetic algorithm is employed to search for the optimal management policy of the multi-server queueing system. [Copyright &y& Elsevier]

This article examines an M[x]/G/1 queueing system with a randomized vacation policy and at most J consecutive vacations. Whenever the system is empty, the server immediately takes a vacation. Upon returning from a vacation, the server will be immediately activated for service if there is at least one customer waiting in the queue. Otherwise, if no customers are waiting for service at the end of a vacation, the server either remains idle with probability p or leaves for another vacation with probability 1 - p. This pattern continues until the number of vacations taken reaches J. If the system is empty at the end of the Jth vacation, the server becomes idle in the system until at least one customer arrives at the queue. Assume that the server may suffer an unpredictable breakdown and the repair may be delayed. For such a system, we derive the distributions of some important system characteristics, such as the system size distribution at a random epoch and at a departure epoch, the system size distribution at the initiation epoch of a busy period, the distributions of idle and busy periods, and reliability indices. Finally, a cost model is developed to determine the joint parameters (p, J) that correspond to minimum cost, and some numerical examples are presented for illustrative purpose. [ABSTRACT FROM AUTHOR]

Abstract: We study the statistical inferences of an availability system with imperfect coverage. The system consists of two active components and one warm standby. The time-to-failure and time-to-repair of the components are assumed to follow an exponential and a general distribution respectively. The coverage factors for an active-component failure and for a standby-component failure are assumed to be the same. We construct a consistent and asymptotically normal estimator of availability for such repairable system. Based on this estimator, interval estimation and testing hypothesis are performed. To implement the simulation inference for the system availability, we adopt two repair-time distributions, namely, lognormal and Weibull and three types of Weibull distributions characterized by their shape parameters are considered. Finally, all simulation results are displayed in appropriate tables and curves for highlighting the performance of the statistical inference procedures. [Copyright &y& Elsevier]

Abstract: This paper studies the operating characteristics of an M[x]/G/1 queueing system with N-policy and at most J vacations. The server takes at most J vacations repeatedly until at least N customers returning from a vacation are waiting in the queue. If no customer arrives by the end of the Jth vacation, the server becomes idle in the system until the number of arrivals in the queue reaches N. We derive the system size distribution at a random epoch and departure epoch, as well as various system characteristics. [Copyright &y& Elsevier]

Abstract: This paper considers a batch arrival retrial queue with general retrial times under a modified vacation policy. Any arriving batch finding the server busy or on vacation enters an orbit. Otherwise, one customer from the arriving batch obtains the service immediately while the rest join the orbit. If the orbit is empty, the server takes at most vacations repeatedly until at least one customer appears in the orbit upon returning from a vacation. If the orbit is nonempty when the server returns from a vacation, the server waits idly for customers from the orbit or new arrivals. On the other hand, if no customers are recorded in the orbit at the end of the th vacation, the server waits for new customers to arrive. This retrial system has potential applications in packet-switched networks. By applying the supplementary variable technique, some important system characteristics are derived. The results presented in the paper may be useful for network system designers and software system engineers. [Copyright &y& Elsevier]

Abstract: This paper considers a finite buffer M/M/c queueing system in which servers are unreliable and follow a (d, c) vacation policy. With such a policy, at a service completion instant, if the number of customers is reduced to c − d (c > d), the d idle servers together take a vacation (or leave for a random amount of time doing other secondary job). When these d servers return from a vacation and if still no more than c − d customers are in the system, they will leave for another vacation and so on, until they find at least c − d +1 customers are in the system at a vacation completion instant, and then they return to serve the queue. This study is motivated by the fact that some practical production and inventory systems or call centers can be modeled as this finite-buffer Markovian queue with unreliable servers and (d, c) vacation policy. Using the Markovian process model, we obtain the stationary distribution of the number of customers in the system numerically. Some cost relationships among several related systems are used to develop a finite search algorithm for the optimal policy (d, c) which maximizes the long-term average profit. Numerical results are presented to illustrate the usefulness of such a algorithm for examining the effects of system parameters on the optimal policy and its associated average profit. [Copyright &y& Elsevier]

Abstract: This paper deals with an with an additional second phase of optional service and unreliable server, which consist of a breakdown period and a delay period under -policy. While the server is working with any phase of service, it may break down at any instant and the service channel will fail for a short interval of time. Further concept of the delay time is also introduced. If no customer arrives during the breakdown period, the server becomes idle in the system until the queue size builds up to a threshold value . As soon as the queue size becomes at least , the server immediately begins to serve the first phase of regular service to all the waiting customers. After the completion of which, only some of them receive the second phase of the optional service. We derive the queue size distribution at a random epoch and departure epoch as well as various system performance measures. Finally we derive a simple procedure to obtain optimal stationary policy under a suitable linear cost structure. [Copyright &y& Elsevier]

Abstract: This paper investigates a batch arrival retrial queue with general retrial times, where the server is subject to starting failures and provides two phases of heterogeneous service to all customers under Bernoulli vacation schedules. Any arriving batch finding the server busy, breakdown or on vacation enters an orbit. Otherwise one customer from the arriving batch enters a service immediately while the rest join the orbit. After the completion of two phases of service, the server either goes for a vacation with probability p or may wait for serving the next customer with probability (1− p). We construct the mathematical model and derive the steady-state distribution of the server state and the number of customers in the system/orbit. Such a model has potential application in transfer model of e-mail system. [Copyright &y& Elsevier]

Abstract: This paper discusses a discrete-time Geo/G/1 queue, in which the server operates a random threshold policy, namely 〈p, N〉 policy, at the end of each service period. After all the messages are served in the queue exhaustively, the server is immediately deactivated until N messages are accumulated in the queue. If the number of messages in the queue is accumulated to N, the server is activated for services with probability p and deactivated with probability (1− p). Using the generating functions technique, the system state evolution is analyzed. The generating functions of the system size distributions in various states are obtained. Some system characteristics of interest are derived. The long-run average cost function per unit time is analytically developed to determine the joint optimal values of p and N at a minimum cost. [Copyright &y& Elsevier]

Abstract: We consider an M/M/R queue with vacations, in which the server works with different service rates rather than completely terminates service during his vacation period. Service times during vacation period, service times during service period and vacation times are all exponentially distributed. Neuts’ matrix–geometric approach is utilized to develop the computable explicit formula for the probability distributions of queue length and other system characteristics. A cost model is derived to determine the optimal values of the number of servers and the working vacation rate simultaneously, in order to minimize the total expected cost per unit time. Under the optimal operating conditions, numerical results are provided in which several system characteristics are calculated based on assumed numerical values given to the system parameters. [Copyright &y& Elsevier]

This article we study the statistical inferences of an availability system with imperfect coverage. The time-to-failure and time-to-repair of the active and standby components are assumed to be exponential and general distribution, respectively. Assume that the coverage factor is the same for an active-component failure as that for a standby-component failure. Firstly, we propose a consistent and asymptotically normal (CAN) estimator of availability for such repairable system. Based on the CAN estimator of the system availability, interval estimation and testing (hypothesis) are performed. To implement the simulation inference for the system availability, we adopt two repair-time distributions, such as lognormal and Weibull distribution, in which three types of Weibull distribution are considered according to the shape parameter β. The component holds the decreasing repair rate (DRR), constant repair rate (CRR), and increasing repair rate (IRR) if β < 1, β = 1, and β > 1, respectively. Finally, all simulation results are displayed by appropriate tables and curves for understanding performance of the statistical inference procedures presented in this article. [ABSTRACT FROM AUTHOR]

We consider the control policy of an M/G/1 queueing system with a startup and unreliable server, in which the length of the vacation period is controlled either by the number of arrivals during the idle period, or by a timer. After all the customers have been served in the queue, the server immediately takes a vacation and operates an NT vacation policy: the server reactivates as soon as the number of arrivals in the queue reaches a predetermined threshold N or when the waiting time of the leading customer reaches T units. In such a variant vacation system, the steady-state probabilities cannot be obtained explicitly. Thus, the maximum entropy principle is used to derive the approximate formulas for the steady-state probability distributions of the queue length. A comparitive analysis of two approximation approaches, using the first and the second moments of system size, is studied. Both solutions are compared with the exact results under several service time distributions with specific parameter values. Our numerical investigations demonstrate that the use of the second moment of system size for the available information is, in general, sufficient to obtain more accurate estimations than that of the first moment. [ABSTRACT FROM AUTHOR]

Abstract: This paper proposes a procedure to construct the membership functions of the system characteristics of a redundant repairable system with two primary units and one standby in which the coverage factor is the same for an operating unit failure as that for a standby unit failure. Times to failure and times to repair of the operating and standby units are assumed to follow fuzzified exponential distributions. The α-cut approach is used to extract from the fuzzy repairable system a family of conventional crisp intervals for the desired system characteristics, determined with a set of parametric nonlinear programs using their membership functions. A numerical example is solved successfully to illustrate the practicality of the proposed approach. Because the system characteristics are governed by the membership functions, more information is provided for use by management, and because the redundant system is extended to the fuzzy environment, general repairable systems are represented more accurately and the analytic results are more useful for designers and practitioners. [Copyright &y& Elsevier]

Abstract: We consider the queueing system, in which the server operates policy and a single vacation. As soon as the system becomes empty the server leaves for a vacation of random length . When he returns from the vacation and the system size is greater than or equal to a threshold value , he starts to serve the waiting customers. If he finds fewer customers than . he waits in the system until the system size reaches or exceeds . The server is subject to breakdowns according to a Poisson process and his repair time obeys an arbitrary distribution. We use maximum entropy principle to derive the approximate formulas for the steady-state probability distributions of the queue length. We perform a comparative analysis between the approximate results with established exact results for various batch size, vacation time, service time and repair time distributions. We demonstrate that the maximum entropy approach is efficient enough for practical purpose and is a feasible method for approximating the solution of complex queueing systems. [Copyright &y& Elsevier]

Abstract: System characteristics of a two-unit repairable system are studied from a Bayesian viewpoint with different types of priors assumed for unknown parameters, in which the service station is unreliable. Times to failure and times to repair of the operating units are assumed to follow exponential distributions. In addition, failure times and repair times of the service station also follow exponential distributions. When times to failure and times to repair of operating units, failure times and repair times of the service station are with uncertain parameters, a Bayesian approach is adopted to evaluate system characteristics. Monte Carlo simulation is used to derive the posterior distribution for the mean time to system failure and the steady-state availability. Some numerical experiments are performed to illustrate the results derived in this paper. [Copyright &y& Elsevier]

Abstract: This paper models a manufacturing system consisting of M operating machines and S spare machines under the supervision of a group of technicians in a repair facility. Machines fail according to a Poisson process, and the repair (service) process of a failed machine may require more than one phase. In each phase, service times are assumed to be exponentially distributed but may be interrupted when the repair facility encounters unpredictable breakdowns. Two models of manufacturing systems are considered. In the first model, technicians repair failed machines at different rates in each phase. In the second model, a two-phase service system with differing numbers of technicians is considered. Profit functions are developed for both models and optimized by a suitable allocation of the number of machines, spares, and technicians in the system. Finally, a sensitivity analysis (see Cao [X.R. Cao, Realization Probabilities: The Dynamics of Queuing Systems, Springer-Verlag: London, 1994; X.R. Cao, The relations among potentials, perturbation analysis, and Markov decision processes, Discrete Event Dynam. Syst.: Theory Applicat. 8 (1998) 71–87]) is performed to provide an approach that quantifies the impact of changes in the parameters on the profit models. [Copyright &y& Elsevier]