Vasilis P. Koutras

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Nowadays voice over Internet protocol (VoIP) is becoming an evolutionary technology in telecommunications. In this paper the study is focused on the resources that are allocated for VoIP calls. Resource allocation in a VoIP system and resource degradation when new demands for resources arrive at the system are modeled. To counteract resource degradation and(More)
Software rejuvenation is a preventive maintenance technique to prevent failures in continuously running systems that experience software aging. In this paper, rejuvenation is modeled in a redundant computer system via a semi-Markov process in order to counteract software aging. The system can be switched into the standby unit automatically or manually in(More)
In this paper, a software rejuvenation model is presented in which two different rejuvenation actions are considered, perfect and minimal. The concept of a failed rejuvenation action which leads the system to failure is also introduced. The presented model is studied under a Continuous Time Markov Chain (CTMC) framework and a maximum likelihood estimator of(More)
In order to achieve high availability the idea of cluster systems has been commonly used. In this paper a two node cluster system is considered in which software and hardware failures occur. Hardware failures are permanent and hence they are counteracted by a repair or replacement. On the other side, software failures require a reboot. A reboot though is a(More)
In the present study a client-server system is considered, which experiences resource degradation as clients' requests increase. Software rejuvenation is performed in order to counteract resource exhaustion. Two different levels of rejuvenation actions are implemented, perfect, and minimal. Moreover the concept of a failed rejuvenation is introduced to(More)
In this paper, a two-level software rejuvenation model is introduced. The innovative aspect consists in combining partial, full and failed rejuvenation with the concept of triggering rejuvenation even if the software did not reach the failure probable state. In the later case, either partial or full rejuvenation actions can be performed. The aim is to(More)