# Failure Mode and Effect Analysis for Cyber-Physical Systems
## 6 Failure Mode and Effect Analysis Application
### Risk Priority Number Calculation  
After all failure modes are identified, with their causes and effects, we calculate the RPN for each failure mode. Since we based our RPN calculation on the possible economic impact of the failure mode, we will be always considering the worst consequences for each case. Starting with the \"Wrong control message injection\" failure mode, which has the worst possible consequences, we assigned 10 to all its RPN factors. Its train collision and train derailment possible consequences can result in several F and SI, high infrastructure damage, severe environmental contamination, and considerable delays. The \"Message Dropping\" and the \"Signal Jamming\" failure modes have the same consequences since their both preceded by a variation of a DoS attack. We assigned 3 to the SF, 2 to IF and DF, and 1 to the EF. These two attacks train full stop and emergency braking consequences may cause light damage to the track and bogies and low delays. To the \"Communication Delay\" failure mode factors, we assigned 1 to the all four factors. Despite being a MitM attack, the replay attack intentions are to provoke data overflow on the target, thus from our economic impact perspective, this failure mode has the minimum RPN. The values displayed in Table 12 are a direct result of consulting the Portuguese railway accident reports [19], where we could find data relative to deaths, serious and light injuries, train, and railway damage, delays, and environmental pollution. Accordingly, all the costs for all possible failure mode consequences were estimated and summed, originating a total cost equivalent for each factor. Then, we provide two RPN calculations, the original and our proposal, to grant an easy comparison between both methods. After gathering all the values for the RPN formula, we proceed with the RPN calculation:  
* Wrong control message injection: \[10\times 0.5+10\times 0.35+10\times 0.05+10\times 0.10=10\] \begin{table}
\begin{tabular}{l l l} \hline \hline
**Failure Mode** & **Failure Cause** & **Failure Effect** \\ \hline \multirow{6}{*}{Wrong Control Messages injection (Packet Spoofing)} & \multirow{3}{*}{Message Spoofing Attack} & Unexpected abrupt braking \\  & & Train location loss \\  & & Train speed control loss \\  & & Train full stop \\  & & Train collision \\  & & Train derailment \\ \hline Message Dropping & \multirow{3}{*}{Message Dropping Attack} & Train full stop \\  & & Emergency braking; \\  & & Change to conventional operation \\ \hline Signal Jamming & \multirow{3}{*}{Jamming Attack} & Train full stop \\  & & Emergency braking; \\  & & Change to conventional operation \\ \hline Communication Delay (Extensive packet duplication and forwarding) & \multirow{3}{*}{Replay Attack} & Train control performance breakdown \\  & & Change to conventional operation \\ \hline \hline \end{tabular}
\end{table} 
Table 11: Failure Modes, Causes and Effects. \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline
**Failure Mode** & **Social** & **Infrastr** & **Environ** & **Delay** & **RPN** \\ \cline{2-6}  & **0.5** & **0.35** & **0.05** & **0.1** & **Original** & **Our Approach** \\ \hline Wrong control message injection & 10 & 10 & 10 & 10 & 10,000 & 10 \\ \hline Message dropping & 3 & 2 & 1 & 2 & 12 & 2.45 \\ \hline Signal jamming & 3 & 2 & 1 & 2 & 12 & 2.45 \\ \hline Communication Delay & 1 & 1 & 1 & 1 & 1 & 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 12: Risk Priority Number values. * Message Dropping: \[3\times 0.5+2\times 0.35+1\times 0.05+2\times 0.10=2.45\] (3)
* Signal Jamming: \[3\times 0.5+2\times 0.35+1\times 0.05+2\times 0.10=2.45\] (4)
* Communication Delay: \[1\times 0.5+1\times 0.35+1\times 0.05+1\times 0.10=1\] (5)  
Then, with all the RPN ranks gathered, we conclude that the \"Wrong control message injection\" failure mode is the one with the highest rank, thus the most dangerous and the first to take into consideration on the next step.