F. Postiglione

A time-discrete extended gamma process for time-dependent degradation phenomena

The non-stationary Gamma process is a widely used mathematical model to describe degradation phenomena whose growth rate at time t depends only on the current age of the item and not on the accumulated damage up to t. Nevertheless, the Gamma process is not a proper choice when there is empirical evidence that the variance-to-mean ratio of the process varies with time, because the Gamma process implies a constant variance-to-mean ratio. This paper proposes a generalization of the non-stationary Gamma process, which can be viewed as a time discretization of the extended Gamma process and allows one to describe time-dependent degradation phenomena whose variance varies with time t, not necessarily in proportion to the mean. A way to approximate the exact distribution of the degradation growth over a given time interval is given and a test for assessing whether the assumption of the Gamma process can be rejected or not is discussed. Finally, the proposed model is applied to a real dataset consisting of the sliding wear data of four metal alloy specimens.

A random-effects model for long-term degradation analysis of solid oxide fuel cells

Solid oxide fuel cells (SOFCs) are electrochemical devices converting the chemical energy into electricity with high efficiency and low pollutant emissions. Tough very promising, this technology is still in a developing phase, and degradation at the cell/stack level with operating time is still an issue of major concern. Methods to directly observe degradation modes and to measure their evolution over time are difficult to implement, and indirect performance indicators are adopted, typically related to voltage measurements in long-term tests. In order to describe long-term degradation tests, three components of the voltage measurements should be modelled: the smooth decay of voltage over time for each single unit; the variability of voltage decay among units; and the high-frequency small fluctuations of voltage due to experimental noise and lack of fit. In this paper, we propose an empirical random-effects regression model of polynomial type enabling to evaluate separately these three types of variability. Point and interval estimates are also derived for some performance measures, such as the mean voltage, the prediction of cell voltage, the reliability function and the cell-to-cell variability in SOFC stacks. Finally, the proposed methodology is applied to two real case-studies of long-term degradation tests of SOFC stacks.

Reliability and survivability methodologies for next generation networks

This paper aims to review some reliability and availability methodologies suitable to characterize telecommunication networks behavior, in particular, in the presence random failures of network elements. This approach is becoming more and more relevant to assess the Quality of Service offered by a telecom operator to its subscribers, also known as performability. In order to clarify this approaches, we present some examples related to next generation networks based on the IP Multimedia Subsystem. Some notions about the emerging requirements of network survivability are given.

Semi-Markov models for performance evaluation of failure-prone IP multimedia subsystem core networks

Next generation telecommunication core networks are typically based on the Third Generation Partnership Project Internet protocol (IP) multimedia subsystem (IMS). Their planning and deployment must take into account the occurrence of random failures causing performance degradations, in order to assess and maintain a high level of quality of service. In particular, IMS signalling servers can be modelled as repairable multi-state elements where states correspond to different performance levels. This article provides an evaluation of IMS signalling network performance in long runs in terms of two metrics adopted in the practice, such as the number of call set-up sessions that the network can manage at the same time and the call set-up delay. A semi-Markov model has been adopted for the IMS servers, which allows as well for non-exponential probability distributions of sojourn times, as often observed in real networks. Furthermore, a redundancy optimization problem is solved in an IMS-based realistic scenario, to the aim of minimizing the deployment cost of a telecommunication network with a given availability requirement.

A Bayesian approach for non-homogeneous gamma degradation processes

ABSTRACT A Bayesian approach based on the Markov Chain Monte Carlo technique is proposed for the non-homogeneous gamma process with power-law shape function. Vague and informative priors, formalized on some quantities having a “physical” meaning, are provided. Point and interval estimation of process parameters and some functions thereof are developed, as well as prediction on some observable quantities that are useful in defining the maintenance strategy is proposed. Some useful approximations are derived for the conditional and unconditional mean and median of the residual life to reduce computational time. Finally, the proposed approach is applied to a real dataset.

Rejection properties of stochastic-resonance-based detectors of weak harmonic signals.

In [Phys. Rev. E 57, 6470 (1998)]] a thorough characterization in terms of receiver operating characteristics of stochastic-resonance detectors of weak harmonic signals of known frequency in additive Gaussian noise was given. It was shown that strobed sign-counting based strategies can be used to achieve a nice trade-off between performance and cost, by comparison with noncoherent correlators. Here we discuss the more realistic case where besides the sought signal (whose frequency is assumed known) further unwanted spectrally nearby signals with comparable amplitude are present. Rejection properties are discussed in terms of suitably defined false-alarm and false-dismissal probabilities for various values of interfering signal(s) strength and spectral separation.

Robust gravitational wave burst detection and source localization in a network of interferometers using cross-Wigner spectra

We discuss a fast cross-Wigner transform based technique for detecting gravitational wave bursts, and estimating the direction of arrival, using a network of (three) non co-located interferometric detectors. The performances of the detector as a function of signal strength and source location, and the accuracy of the direction of arrival estimation are investigated by numerical simulations.

Secure Mobile IPv6 for B3G Networks

The rapid spread of new radio access technologies, the ever increasing requirements for security, as well as the required capability for the final users to experience continuous connectivity and uninterrupted services of IP applications as they move about from one access network to another, has stimulated the technical and scientific community to investigate future evolution scenarios for 3G networks, generically referred to as Beyond-3G or 4G. Key issues are certainly security provision for applications exchanging data in diverse wireless networks and seamless mobility (handoff) between different coverage domains and, possibly, access technologies. Many proposals are based on the use of the mobile IPv6 protocol. In this paper we analyse the security threats emerging from some MIPv6 mechanisms for mobility management, and we propose a solution to secure them, under the assumption that both communicating end users are attached to a MIPv6-enabled IP multimedia subsystem network

MIPv6 binding authentication for b3g networks

In last years, the introduction of new wireless communication systems has stimulated the technical and scientific community to investigate future evolution scenarios for 3G networks, generically referred to as Beyond-3G or 4G. In order to guarantee high end-to-end quality of service and security, Beyond-3G networks will cope with some issues such as session security in wireless environments and seamless mobility among different coverage domains and, possibly, access technologies. In this paper we analyze the security threats emerging from some mechanisms for mobility management, and we propose a solution to improve the security level under the assumption that both communicating terminals are attached to a 3GPP IP Multimedia Subsystem that supports both the Session Initiation Protocol and the Mobile IP version 6 protocol.

Software defined storage: Availability modeling and sensitivity analysis

Software Defined Storage (SDS) systems are crucial elements in cloud environments, where huge amount of data managed by content and network providers (multimedia content, social data, gaming data etc.) are typically strongly unstructured. Such data, referred to as objects, are handled along with their metadata allowing a web-based addressing as in the REST paradigm. According to the Software Defined concepts, SDS systems rely on decoupling the software plane (namely the set of management services), from the hardware plane (typically common, inexpensive and vendor unlocked hardware). SWIFT (a part of the OpenStack cloud project) represents a valuable example of an SDS system which, in the present work, we intend to characterize in terms of availability. Through such a characterization the minimum cost configuration can be selected, guaranteeing the so-called “five nines” availability requirement. The availability analysis is here performed by modeling the highlevel architecture of SWIFT as a Stochastic Reward Net (SRN) model. Furthermore, a sensitivity analysis is performed to assess the system robustness with respect to variations of some model parameters.