This part of IEC 61757 specifies terminology, characteristic performance parameters, related test and calculation methods, and specific test equipment for interrogation units used in distributed fibre optic acoustic sensing and vibration measurement systems. This document refers to Rayleigh backscatter and phase detection method by phase-sensitive coherent optical time-domain reflectometry (Õ-OTDR) only. Quasi-static and low frequency operation modes are not covered by this document. Generic specifications for fibre optic sensors are defined in IEC 61757.