This part of IEC 61757 defines detailed specifications for distributed strain measurements with a fibre optic sensor, also known as fibre optic distributed strain sensing. It is applicable to distributed strain sensing systems (DSS) based on spontaneous or stimulated Brillouin scattering in the optical fibre sensor (strain sensitive element), that is, to sensors capable of measuring absolute strain. This document specifies the most important DSS performance parameters and defines the procedures for their determination.