# Automated Methodology for Dependability Evaluation of Wireless Visual Sensor Networks
## 3 Problem Formulation and Background  
One of the main goals of this paper is to develop an integrated analytical model that reflects the characteristics of the whole application and that allows for computing its dependability. This model is usually an abstraction of the system behavior, being composed by a network of components characterized by their failure distributions and maintenance policies [50].  
System dependability, roughly speaking, can be seen as a service that may or may not be correctly provided at a given instant of time, usually associated to the achievement of some metric related to the application. In this work, the imposed requirement is that the WVSN application needs to monitor a minimum percentage of the area of interest. The achievement of this goal depends on the components that are operating at any instant and the interaction among them.  
WVSN components are basically its nodes. Similarly to [PERSON] et al. [14], in this paper, we assume three types of nodes: sink, visual and scalar. The sink node is a base station, the common destination of all information collected by nodes in the network. Visual nodes are sensor nodes equipped with cameras that gather images from the monitored area. Scalar nodes are sensor nodes unable to collect visual information, but able to re-transmit it, working as relaying nodes.  
A node can fail if at least one of its hardware components fails, i.e., processor, memory, radio, battery and sensor units (e.g., camera). However, for modelling purposes, we partitioned node components in two categories: battery and generic hardware (processor, memory, radio and sensors). This partition is made because these categories present different behaviors, so we propose different models for them. We consider that a node failure is permanent, requiring a replacement or repair action to come back to the operational state. A hardware failure can be a broken electronic component, while we consider a battery failure as equivalent to its full discharge. A hardware repair can be a component replacement or a component fixing. A battery repair can be a replacement or a recharging.  
Nodes interact between them through message exchanges. This interaction can fail if, obviously, nodes fail or if the communication links fail. We consider that a link failure is transient, meaning that the failed link will reestablish its connection after a while, without an external intervention. A link failure could be a radio interference, an occupied channel or a data collision. Once a communication failure occurs, the connection from a node to the sink node could be reestablished by the re-routing or self-healing features of routing protocols.  
The availability evaluation of a WVSN is focused on area coverage and involves all the previous network elements. If the hardware of a visual node fails, it will be incapable of collecting visual information. On the other hand, if a link or an intermediate node that composes a path to the sink node fails, then the visual information of one or more nodes will not be delivered to the sink node. In any case, failures will not allow the sink node to receive all of the visual information related to that minimum percentage. Thus, the WVSN application requirements are not achieved and a visual coverage failure (VCF, hereafter referred as coverage failure) occurs. That way, in order to evaluate the availability of a WVSN, it is necessary to define and to model each element that can fail, besides the relationships among them.  
The next subsections present the proposed dependability models for nodes and links, as well as components directly related to them. First, it will be shown how to compute the coverage area, which allows for identifying the occurrence of a coverage failure, and finally it will be shown how to integrate all those models in the evaluation process.