# Failure Mode and Effect Analysis for Cyber-Physical Systems
## 1 Introduction  
Most modern engineering systems have close interaction between cyber and physical components, leading to a new paradigm approach named Cyber-Physical Systems (CPS). CPS are networked systems composed of physical and software components integrated through networking, computation, and monitoring. They are usually misidentified for the Internet of Things (IoT) systems, although CPS emphasizes real-time control and monitoring features [1].  
With the integration of CPS in critical infrastructures, the risk assessment of those systems requires important cautions to avoid social, environmental, and economic costs. Various authors deal with this issue, for example, [PERSON] et al. [1] reviewed different methods for CPS risk assessment considering safety and security concerns. [PERSON] et al. [2] proposed a new risk assessment method based on real-time risk calculation considering CPS run-time conditions. [PERSON] et al. [3] suggested a game-theoretic framework for CPS security risks assessment and systems' failures defense. CPS risk is often addressed with risk assessment methods without a risk prevention step. Regarding this concern, and that most Communications-Based Train Control (CBTC) security problems arise from cyber-attacks' effects, we will analyze CBTC from a risk prevention perspective.  
Failure Mode and Effect Analysis (FMEA) is an engineering method designed to define, identify, and present solutions for system failures, problems, or errors. FMEA identifies necessary decisions to prevent individual system failures [4], and establish the risk priorities of failure modes through the Risk Priority Number (RPN) [5]. To quantify each failure mode's risk, this method takes into consideration the severity, occurrence, and detectability of each failure mode. Each one of these parameters is estimated in predefined categories. Then, calculating the RPN to prioritize the identified vulnerabilities, through the product of the three parameters mentioned previously. Even though FMEA is a renowned risk assessment method, its methodology presents various drawbacks, mostly when considering CPS risk assessment requirements. For example, FMEA's has a critical dependence on the study team evaluation experience, only covers single failures, and by not weighting the parameters under analysis, and it does not consider the influence of each parameter. Finally, the RPN formula is not focused on the economic impact of the systems' failures. Further ahead, these RPN limitations will be properly exposed and explained.  
In this paper, we first demonstrate how the FMEA method behaves in assessing the CBTC system's risk. As the FMEA includes no risk prevention step in its methodology, we introduce a risk prevention analysis for software faults prevention. Regarding FMEA risk score calculation method, we propose an innovative RPN calculation, taking into consideration the failure modes' economic impact, which is calculated by estimating social, infrastructure, delay, and environmental costs.  
The main contributions of this work are the following:  
* Provide a CBTC risk prevention analysis.
* Propose a novel approach to the FMEA's RPN estimation, concerning CPSs' risk analysis importance.
* Provide a CBTC risk analysis through our version of the FMEA RPN calculation.  
The rest of this paper is organized as follows: In Section 2, we provide the state-of-the-art review on CPS risk assessment. In Section 3, we explain FMEA, and propose an extension in Section 4. Next, in Section 5, we introduce the CBTC, describing its main components, and performing a risk prevention analysis to the CBTC system, regarding current approaches. In Section 6, we perform the FMEA risk analysis to the CBTC system. Finally, in Section 7, we conclude our paper and propose future work.