# Automated Methodology for Dependability Evaluation of Wireless Visual Sensor Networks
## 1 Introduction  
Wireless Sensor Networks (WSNs) are nowadays a trend as a support technology for many types of monitoring applications. Recently, applications in the field of smart cities, street lighting, domotics, traffic and pedestrian control, living assistance, parking assistance, waste collection and surveillance activities are becoming common, since they present data volumes and computational demands perfectly suitable for WSNs. Some of these applications may require the addition of visual information to the data set [1]. For example, a street lighting application triggered by motion sensors can be wrongly activated upon the presence of birds; a traffic light control system can better manage its time intervals with an image of how many people or cars are occupying the sidewalk or the road; an intelligent transportation system can easily use images to detect a car accident and then call the proper authorities in addition to the obvious usage of visual data, like surveillance, face detection, intrusion detection, etc. All of these problems can be supported by the use of adequate Wireless Visual Sensor Networks (WSVNs) [2; 3; 4]. Many applications in different monitoring and control domains are said to be safety related. In such kind of systems, a system failure may put people in danger, lead to environmental damages or result in economic losses. Therefore, it is mandatory to evaluate the system dependability in those situations in order to assess its successful operational behavior through the time, although there are relevant challenges to be handled for such assessment. Actually, dependability is a generic concept including attributes as availability, reliability, safety, integrity and maintainability [5], being commonly addressed by its quantitative attributes (reliability and availability) [6; 7; 8; 9]. According to [PERSON] et al. [5], reliability can be defined as the probability of a system to perform its purpose adequately (failure-free) for the intended period of time, under the operating conditions found in a specified environment. On the other hand, availability is the probability of finding the system in a correct operating state at some time into the future, or, in another way, it is the amount of time that a system is actually operating, as the percentage of the total time that it should be operating. One should note that reliability is associated with the system behavior until a failure occurs, while availability is associated with the system behavior even if failures occur. In this case, repair actions are required to take the system from a failed state to an operational state. In order to evaluate dependability, different approaches can be used, commonly categorized as _analytical, simulation_ and _experimental_, each one presenting a different level of accuracy, cost, duration and comprehensiveness. Some authors also define the _formal_ category [10], but it is assumed herein that it is equivalent to the analytical approach, since the tools to model systems in a formal way can also be described analytically. While experimental approaches require the existence of a real system to perform the evaluation at run-time, analytical and simulation approaches are easier to implement because they are based on models. However, the accuracy of their results depend on the accuracy of the values assigned to the model parameters and the considered hypothesis. Moreover, the use of simulation for dependability evaluation brings an important concern: for an accurate estimation of dependability measures, frequent observations of the system-failure event are necessary, which, by definition are rare events. This results in a substantial increase of the simulation time, which could lead to impractical values [8; 10; 11; 12]. Therefore, considering this complex scenario for dependability evaluation, in this paper, we will focus on analytical modeling, since it is generally cheaper, faster and can provide an acceptable level of accuracy.