This part of IEC 61281 is a generic specification for fibre optic communication subsystems (FOCSs). The parameters defined herein form a specifiable minimum set of specifications that are common to all fibre optic subsystems. Additional parameters may be needed depending on the particular application and technology. Those additional parameters will be specified in the relevant documents, as appropriate. Each specified parameter may be measured using one of the test procedures. The use of these parameters for system design is given in design guides.