IEC 61281-1:2017 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 can be used depending on the particular application and technology. Those additional parameters will be specified in the relevant documents, as appropriate. Each specified parameter is measured using one of the test procedures. The use of these parameters for system design is given in design guides. This second edition cancels and replaces the first edition published in 1999. This edition constitutes a technical revision. This edition includes the following significant technical change with respect to the previous edition: addition of new definitions.Keywords: fibre optic communication subsystems (FOCSs)