FFFS. Together with FFFIS (see), project specifications and functional interfaces, logical and physical, within ERTMS. They describe the functions, physical characteristics and properties of the modules that make up the system and the interfaces connected to them. The description is made in terms of logical and electrical interfaces, algorithms, protocols, etc. The specifications describe the algorithms through which the variable signals and data are processed and indicate how the corresponding signals managed by the system are generated. In the context of the implementation and configuration of European standards, they represent the necessary level of specification to guarantee an interface that respects the functional, logical, technical and overall constraints required; in interfacing between modules, they can guarantee interchangeability as well as interoperability
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