# Strategies and Methods for the Fault-Tolerant Function Development of Multi-Domain Systems +
## 3 State of the Art
### Function Development
Obviously, some product faults can be detected in these verifications, which may be carried out quite early in a product development process. Faults which appear during the product verification or later during the product operation frequently emerge because of certain conditions. The emergence of faults is the main topic of the subsequent section. While the V-model is an appropriate representation of the overall process and the systematic development (structuring, integration, and testing) of a system, it hardly answers how to assign functions to a certain domain. A set of criteria including cost, weight, time, reliability, accuracy, endurance, performance, and fault-tolerance itself might help in finding the optimal domain to realise the function or react to a possible or actual fault [7]. A central challenge for function development is integration in the MBSE frameworks which are currently present in industrial companies. For nearly two decades, researchers have sought to integrate function information in product models in product lifecycle management (PLM) systems. An early proposition was made by [PERSON] et al. [53] who integrated function in their core product model for PLM. Several researchers have sought to apply systems modelling language (SysML) for this integration (e.g., [PERSON] et al. [54]). Current research proposes to use the so-called Integrated Function Modelling (IFM) framework in connection with structural modelling using an executable graph-based modelling language [52]. A focus on fault and failure in technical systems is evident in research works which concentrate on the analysis aspect of function development and propose the application of the function failure identification and propagation (FFIP) method [55]. On this basis, conceptual design methods for the system architecture phase of systems engineering are conducted which apply functional models for studying early system architecture trade-offs [56]. This method is based on analysis of the functions of a system, their primary allocation to components, and analysis of potential function flow paths. Finally, the probabilities of certain flows are determined in a similar way to application of well-known probabilistic risk assessment (PRA) techniques. This and similar methods, such as the companion flow state logic (FSL) method [57] or the uncoupled failure flow state reasoned (UFFSR) [58], can be helpful for function development but do not support the synthesis aspect of this endeavour. The strategic and methodical approaches which may contribute to the fault-tolerant function development of multi-domain systems are summarised in Table 3; it is important to note that they are usually accompanied by respective computer tools. As mentioned above, faults are inevitable in complex technical systems and these faults may appear during the development and operation of these technical systems. The causes of the emergence of these faults will be discussed in the next section. Figure 4: Essential elements of multi-domain system development processes.