# Wireless Communication Technologies for Safe Cooperative Cyber Physical Systems
## 2 Technical Background
### Co-CPSs Safety Definitions  
A wide variety of technical approaches and methods have been used or proposed to analyse system safety, hazards and risk over several decades. The concept of risk management is addressed by ISO 31000 [29] standard that provides a generic framework for assessing and managing risk across various industries. The aim is to obtain an understanding of the risk to inform decisions regarding whether risk is tolerable with respect to some criteria, to differentiate risk associated with different options/decisions, and to determine if (and which) risk treatment options should be implemented to control or modify risk. Barrier management is a safety philosophy widely used in the oil and gas industry [30]. The idea is to control risk by putting measures in place to prevent undesirable incidents from occurring and limit their effects if they occur. Barriers intended to reduce the likelihood of undesirable incidents are called preventive barriers, whereas barriers implemented to avoid escalation and reduce effects of incidents are called mitigating barriers. Systems-Theoretic Accident Model and Processes (STAMP) is a recent accident model, first introduced by [31], based on systems theory focusing on enforcing behavioural safety constraints rather than preventing failures. STAMP is able to assess complex sociotechnical systems by thinking of safety as a control problem rather than a reliability one. Failure Modes, Effects, and Criticality Analysis (FMECA) (a variant of FMEA adding the assessment of criticality) originated from the U.S. Military and was first described in a Military procedure MIL-P-1629A [32] and later used by NASA in the Apollo program. An FMECA involves reviewing components, sub-systems and assemblies to identify failure modes, causes and effects. The approach is described in [33]. Other significant approaches are the Fault Tree Analysis (FTA) and the Event Tree Analysis (ETA).  
Looking at the industrial context, several standards are applied. The IEC61508 [34] is an international standard of rules applied in industry. It defines functional safety as part of the overall safety relating to the EUC (Equipment Under Control) and the EUC control system which depends on the correct functioning of the E/E/PE safety-related systems, other technology safety-relatedsystems and external risk reduction facilities. Central to the standard are the concepts of risk and safety functions. The risk is a function of the frequency (or likelihood) of the hazardous event and the event consequence severity. The risk is reduced to a tolerable level by applying safety functions that may consist of E/E/PES and/or other technologies. IEC 61508 defines safety integrity level (SIL) as a discrete level (one out of possible four), corresponding to a range of safety integrity values, where SIL 4 is the highest level of safety integrity and SIL 1 is the lowest. The standard has its origins in the process control industry. It covers the complete safety life cycle, and may need interpretation to develop sector-specific standards. In fact, the standard lies at the root of a number of specific domains e.g., IEC26262 for automotive, EN50128 for railway applications, IEC60601 for medical devices, etc.