# A Survey on Monitoring Quality Assessment for Wireless Visual Sensor Networks
## 5 Reviewing Dependable Quality  
Roughly speaking, dependable quality of an application consists of estimating a system's dependability through quantitative attributes (e.g., reliability and availability), characterizing the successful operational behavior of the application over time. This quality category allows the identification of weak points in the application, focusing on the mitigation of them. Additionally, it allows to adjust the duration of the application execution, providing metrics for comparison among different applications and finding improved network configurations. We believe that it is quite reasonable to address the quality of WVSN regarding dependability through reliability and availability parameters, which are very similar quantitative attributes. Reliability is the ability of a system or component to continuously perform its required functions under stated conditions for a specified period of time, without interruption. In another perspective, it is the probability that a system or component will not fail [21; 90; 91]. Differently, availability is the probability of a system or component to be operating as expected at any given instant in time when some stated conditions are guaranteed, with the total considered time including operating and repair time [90]. In this scenario, dependable quality is affected by the network elements that can lead to some application failure, i.e., the sink node will be unable to receive the whole required visual information. In the case of a WVSN application, when a hardware failure occurs in a \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Work** & **Metric** & **Coverage** & **Occlusion** \\ \hline
[6] & Distance range & Area & Modelled \\ \hline
[17; 67; 85] & Distance range & Area & – \\ \hline
[37] & Mean square error & Target & Modelled \\ \hline
[39; 40; 42; 43; 46; 47] & Quadratic error & Target & – \\ \hline
[44; 45; 68] & Probabilistic distance & Target & – \\ \hline
[69] & Bounding box & Target & Modelled \\ \hline
[70; 71] & Bounding box & Target & – \\ \hline
[72] & Bounding box; Region of interest & Target & – \\ \hline
[73] & Distance; Target speed; Velocity; & Target & – \\  & Moving direction & & \\ \hline
[48; 49] & Perspective distortion; Entropy & Area & Modelled \\ \hline
[31; 75; 76] & Weighted angle of view & Target & – \\ \hline
[74] & Weighted angle of view; Coverage & Target & – \\  & correlation & & \\ \hline
[86] & Importance index & Area & – \\ \hline
[87] & Angle of view; Distance range & Target & – \\ \hline
[88; 89] & Angle of view; Distance range & Area & – \\ \hline
[63] & Angle of view; Distance range & Area & Modelled \\ \hline
[77; 78] & Peak signal-to-noise ratio & Not specified & – \\ \hline
[79] & Region of interest & Target & – \\ \hline
[80; 81] & Ellipse fit root mean square error & Target & Modelled \\ \hline
[82] & Occlusion rate & Target & Modelled \\ \hline
[83; 84] & Frame rate; Pixels on target & Target & – \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of the performed content quality reviewing. visual node, visual information will not be able to be collect by that node. Similarly, when a failure occurs in a link or in an intermediate node within the path to the sink node, then the delivering of visual information to the sink node will also be affected. Whatever the case, when a failure occur visual information is missed, leading to a failure condition that can be perceived as a quality depletion. Several metrics have been proposed related to aspects that can affect reliability and availability. If the nodes are battery-powered, the energy consumption becomes a major problem, which can be directly affected by the used routing protocols used. The applied radio power can generate more stable links, although draining more energy. Hardware redundancy (spare nodes) and coverage redundancy can work as a backup system and extend the network lifetime. Dependable quality can be assessed using well defined methodologies to perform availability and reliability assessments in WVSN.