Modelling and Simulation of Queuing Models through the concept of Petri Nets

In recent years Petri Nets has been in demand due to its visual depiction. Petri Nets are used as an effective method for portraying synchronization, a concurrency between different system activities. In queuing models Petri networks are used to represent distributed modeling of the system and thus evaluate their performance. By specifying suitable stochastic Petri Nets models, the authors concentrate on representing multi-class queuing systems of various queuing disciplines. The key idea is to define SPN models that simulate a given queue discipline 's behavior with some acceptable random choice. Authors have find system queuing with both a single server and multiple servers with load-dependent service rate. Petri networks in the queuing model have enhanced scalability by combining queuing and modeling power expressiveness of 'petri networks.' Examples of application of SPN models to performance evaluation of multiprocessor systems demonstrate the utility and effectiveness of this modeling method. In this paper, authors have made use of Stochastic Petri nets in queuing models to evaluate the performance of the system

Steady-state analysis of a multiclass MAP/PH/c queue with acyclic PH retrials

A multiclass c-server retrial queueing system in which customers arrive according to a class-dependent Markovian arrival process (MAP) is considered. Service and retrial times follow class-dependent phase-type (PH) distributions with the further assumption that PH distributions of retrial times are acyclic. A necessary and sufficient condition for ergodicity is obtained from criteria based on drifts. The infinite state space of the model is truncated with an appropriately chosen Lyapunov function. The truncated model is described as a multidimensional Markov chain, and a Kronecker representation of its generator matrix is numerically analyzed. © Applied Probability Trust 2016

Modelagem de um Ataque Cibernético em um ambiente IEC 61850 usando Redes de Petri Coloridas.

No padrão IEC 61850, define-se a transmissão dos pacotes de mensagens Sampled Values (SV) no processamento da comunicação, que são valores amostrados de medidas elétricas, e estas mensagens são realmente importantes e críticas na automação de sistemas de potência. Com relação à análise de segurança das redes de comunicação, uma ferramenta muito útil são as Redes de Petri Coloridas (CPNs), porque elas modelam processos assíncronos e concorrentes, o que caracteriza o funcionamento das redes de comunicação, além de permitir a análise de atrasos em sistemas temporizados. Assim, com base neste contexto e sabendo-se que o valor correto das medidas dos valores amostrados (Sampled Values) é realmente crítico na transmissão de mensagens no contexto IEC 61850, para que atue corretamente a automação dos sistemas elétricos de potência, modela-se em três cenários IEC 61850 um ataque cibernético usando as CPNs, que visa modificar os dados a serem transmitidos antes de serem empacotados como uma mensagem SV, o que é realmente crítico para a operação do sistema. Além disso, modela-se uma classificação para os pacotes de mensagens enviados, para que seja possível determinar quais pacotes foram atacados pelo ataque cibernético, ou quais pacotes não foram corretamente enviados ao seu destino final. Os resultados correspondem ao ataque cibernético modelado, mostrando a eficácia do método de modelagem proposto. Por fim, são sugeridas algumas técnicas de mitigação deste tipo de ataque cibernético

Metodologia de Modelagem CPN Aplicada a Análise de Desempenho de Sistemas de Comunicação baseados na Norma IEC 61850.

O desempenho de sistemas de comunicação baseados na norma IEC 61850 tornou-se questão importante no Sistema Elétrico de Potência. Uma vez que os sistemas de automação dos sistemas de potência possuem tempo crítico de atuação, é necessário que os parâmetros de eficiência das redes de comunicação, como exemplo atrasos de mensagens, sejam examinados cuidadosamente. Neste contexto, este trabalho de dissertação apresenta uma metodologia de desenvolvimento de modelos CPN a fim de avaliar o comportamento temporal de sistemas de comunicação baseados na norma IEC 61850. Para isto, são caracterizados dois cenários IEC 61850 encontrados na literatura, um tratando das mensagens Sampled Value e outro tratando das mensagens GOOSE. Em seguida, utilizando o software CPN Tools, são criados modelos baseados nas arquiteturas dos cenários, utilizando métodos já conceituados no tema modelagem por Redes de Petri Coloridas, a fim de avaliar o desempenho destas arquiteturas. De forma a alcançar o objetivo principal desta dissertação, que é definir este formalismo como uma nova ferramenta de avaliação dos sistemas de comunicação IEC 61850, são comparados os resultados obtidos no sistema real com o ambiente modelado. Com a comparação, fica evidente que os sistemas modelados possuem comportamentos semelhantes aos sistemas reais, e a modelagem CPN pode, perfeitamente, ser empregada neste contexto de análise de desempenho IEC 61850

MACHS: Mitigating the Achilles Heel of the Cloud through High Availability and Performance-aware Solutions

Cloud computing is continuously growing as a business model for hosting information and communication technology applications. However, many concerns arise regarding the quality of service (QoS) offered by the cloud. One major challenge is the high availability (HA) of cloud-based applications. The key to achieving availability requirements is to develop an approach that is immune to cloud failures while minimizing the service level agreement (SLA) violations. To this end, this thesis addresses the HA of cloud-based applications from different perspectives. First, the thesis proposes a component’s HA-ware scheduler (CHASE) to manage the deployments of carrier-grade cloud applications while maximizing their HA and satisfying the QoS requirements. Second, a Stochastic Petri Net (SPN) model is proposed to capture the stochastic characteristics of cloud services and quantify the expected availability offered by an application deployment. The SPN model is then associated with an extensible policy-driven cloud scoring system that integrates other cloud challenges (i.e. green and cost concerns) with HA objectives. The proposed HA-aware solutions are extended to include a live virtual machine migration model that provides a trade-off between the migration time and the downtime while maintaining HA objective. Furthermore, the thesis proposes a generic input template for cloud simulators, GITS, to facilitate the creation of cloud scenarios while ensuring reusability, simplicity, and portability. Finally, an availability-aware CloudSim extension, ACE, is proposed. ACE extends CloudSim simulator with failure injection, computational paths, repair, failover, load balancing, and other availability-based modules

Colored stochastic Petri nets for modelling and analysis of mulitclass retrial systems

International audienceMost retrial models assume that customers and servers are homogeneous. However, multiclass (or heterogeneous) retrial systems arise in various practical areas such as telecommunications and cellular mobile networks. Multiclass models are far more difficult for mathematical analysis than single class ones. So, explicit results are available only in few special cases. Actually, so far multiclass retrial systems have been analyzed only by means of queueing theory and almost all studies consider models with several customer’s classes and a service station consisting in one single server or multiple homogeneous (identic) servers and an infinite population size. In this paper, we propose an approach for modelling and analyzing finite-source retrial systems with several customer’s classes and server’s classes using the Colored Generalized Stochastic Petri Nets (CGSPNs). This high-level mathematical model is appropriate for describing and analyzing the performance of systems exhibiting concurrency and synchronization, possibly with heterogeneous components. Using a high-level formalism makes the description of the system easier, while preserving the possibility of obtaining exact performance results. We show how the main steady-state performance indices can be derived and we analyze the behaviour of heterogeneous retrial systems under two service disciplines. The numerical results are graphically displayed to illustrate the effect of system parameters and service discipline on the mean response time