Exact analysis of a discrete material three-station one-buffer merge system with unreliable machines

In this paper, a model of a discrete material flow line consisting of three unreliable machines and one buffer of limited capacity is analysed. A similar system, but with continuous flow of material was examined by Helber and Mehrtens (2001) and Tan (2001). In our system it is assumed that the buffer has two immediate preceding machines, performing the same operations and one immediate succeeding machine that receives material from the buffer. For the case where the buffer reaches its own capacity, one of the two preceding machines has priority over the other to dispose its processed part into the buffer. Processing times are assumed to be deterministic and identical for all machines and are taken as the time unit. Geometrically distributed operation dependent failures at the machines are assumed. All possible transition equations for the examined model are derived and a recursive algorithm that generates the transition matrix for any value N of the storage level is developed. Once the transition matrix is known the performance measures of the model under consideration can be easily evaluated. This model may be used as a building block in a decomposition method to evaluate large production systems with split/merge operations (for example, flow lines with quality inspections and rework loops).     

Enhanced reliable reactive routing (ER3) protocol for multimedia applications in 3D wireless sensor networks

Sensor networks designed especially for the multimedia applications require high data rate and better Quality of Service (QoS). Offering a reliable and energy efficient routing technique in a harsh and complex three-dimensional (3-D) environment for multimedia applications is a challenging job. Geo-routing and geometric routing have been efficient routing schemes for two-dimensional (2-D), but are unable to work properly for 3-D sensor networks. In order to enhance the resilience to link the dynamics in the 3-D sensor network, in this research an Enhanced Reliable Reactive Routing (ER3) is proposed. ER3 is an advancement to the existing reactive routing schemes, to provide energy efficient and reliable routing of data packets in the complex 3-D sensor networks for multimedia applications. The major attraction of ER3 is its backoff scheme, which occurs in the route discovery phase. In backoff scheme robust pilot paths formed between the source and destination are calculated to enable cooperative forwarding of the data packets. The data packets in ER3 are forwarded greedily to the destination from the source and doesn’t require any prior location information of the nodes. The encompassing simulations suggest that the ER3 outperforms the existing routing protocols on the basis of energy efficiency, low latency and high packet delivery ratio.

     

Measuring vulnerabilities and their exploitation cycle

In a world ruled by chaotic causality, Heisenberg's uncertainty principle is only a natural limitation. Analysts only have their personal logic, experience and intuition to depend on in order to make judgments regarding the safety of a system. However, today's analysts are getting bombarded with large amounts of data coming from all kinds of security-related products, such as vulnerability scanners, anti-viruses, firewalls etc, causing information overload and data congestion. Thus, the question remains: How can analysts make a correct judgment regarding the vulnerabilities from which a system is suffering, especially when all the ammunition he/she possesses can not deal with such a complex, ever-changing environment? To this end, we believe that structuring knowledge/information regarding a specific domain in an object-oriented hierarchy tree, and providing a formal model to reason and construct possible scenarios of attacks, will provide an analyst with the necessary ammunition.     

Markovian analysis of production lines with Coxian-2 service times

This paper is concerned with the analysis of reliable production lines. The service times at each station of the line are assumed to follow the Coxian-2 distribution. Raw material arrives at the first station of the line which is assumed that is never empty. Buffers of non-identical capacities are allowed between successive stations. The structure of the transition matrices of these specific type of production lines is examined and a recursive algorithm is developed for generating them, for any number of stations K . This method allows one to obtain the exact solution of a sparse linear system by the use of the Gauss–Seidel method. From the solution of these systems the throughput rate of the production lines is calculated. However, this algorithm is not computationally efficient as it is restricted by the size of the problem. The main contribution of this paper is the study of the transition matrices of production lines with Coxian service times.     

Optimal buffer allocation in short μ-balanced unreliable production lines

In this work, we investigate the optimal buffer allocation in short μ-balanced production lines consisting of machines that are subject to breakdown. Repair times and times to failure are assumed exponential, whereas service times are allowed to follow the Erlang-k distribution (with k=1, 2, 4 and 8). By an improved enumeration procedure and applying the evaluative algorithm of Heavey et al. (European Journal of Operational Research 1993;68:69–89) for the calculation of throughput, we have examined in a systematic way several systems with 3, 4, 5, 6 and 7 stations and with a different total number of buffer slots. We have been able to give answers to some critical questions. These include the effect of the distribution of the service and repair times, the availability of the stations and the repair rates on the optimal buffer allocation and the throughput of the lines.     

Performance evaluation of a push–pull merge system with multiple suppliers,an intermediate buffer and a distribution centre with parallel machines/channels

This paper examines a push–pull merge system with external demand. Multiple reliable non-identical suppliers feed a buffer that is located immediately upstream a distribution centre (DC) with parallel identical reliable machines. The DC performs another operation on the items stored in the preceding buffer and the finished products are stored in another buffer (the finished products buffer) immediately downstream the DC. Customers arrive to the system according to a Poisson process with given intensity λ and remove a finished product from the buffer of finished products. The size of a customer demand is equal to one. Both suppliers and the identical machines at DC have exponential service rates. The considered system is modelled as a continuous-time Markov process with discrete states. An algorithm that generates the transition matrix for any value of the parameters of the system is developed and all possible transition equations are derived and solved analytically. Once the transition matrix is known the performance measures of the model under consideration can be easily evaluated.     

Security Through Deception

ABSTRACT

For each layer of information security there is a number of techniques and tools that can be used to ensure information superiority. Indeed some experts would argue that you cannot have the former without the latter. In today's technological & interconnected world, however, information superiority is very hard to achieve and almost impossible to maintain. This paper will argue that the art of deception is a reliable and cost effective technique that can assure the security of an infrastructure. The paper will conclude by presenting a technical solution of the above statement.     

On the workload and ‘phaseload’ allocation problems of short reliable production lines with finite buffers

Serial flow or production lines are modeled as tandem queueing networks and formulated as continuous-time Markov chains to investigate how to maximize throughput or minimize the average work-in-process (WIP) when the total service time and the total number of service phases among the stations are fixed (these are the workload and ‘phaseload’ allocation problems, respectively). This paper examines both the effect of the kind of service time distribution on the optimal workload allocation in order to maximize throughput or minimize the average WIP of perfectly reliable production lines.

The new approach of this work is the differentiation of the number of service phases of the service time distribution which is assumed to be of phase type at all stations of the flow line and the placement of storage space (buffers) between any two successive stations in order to examine the effect of these factors to the form of the optimal workload vector t and the optimal service phase vector ph.     

Performance evaluation of a two‐echelon supply chain with stochastic demand,lost sales,and Coxian‐2 phase replenishment times

This paper deals with a two‐echelon supply chain comprising a retailer and manufacturer. The retailer faces Poisson demand and follows a (S, s) continuous review inventory policy. The manufacturer produces and ships the retailer's orders with random delay that follows the Coxian‐2 distribution. Assuming lost sales at the retailer and infinite capacity at the manufacturer, we try to explore the performance of the supply chain system. The system is modeled as a continuous‐time Markov process with discrete space. The structure of the transition matrices of these specific systems is categorized as block‐partitioned, and a computational algorithm generates the matrices for different values of system characteristics. The proposed algorithm allows the calculation of performance measures—fill rate, cycle times, average inventory (work in progress [WIP])—from the derivation of the steady‐state probabilities. Moreover, expressions for the holding costs and shortage costs are derived.