This Recommendation describes a single-mode optical fibre which has the zero-dispersion wavelength around 1300 nm wavelength which is loss-minimized and cut-off shifted single-mode optical fibre at a wavelength around 1550 nm and which is optimized for use in the 1530-1625 nm region. This very low loss cut-off shifted fibre (CSF) can be used for long-distance digital transmission applications such as long-haul terrestrial line systems anfd submarine cable systems using optical amplifiers. The geometrical@ optical (attenuation@ cut-off wavelength@ chromatic dispersion and polarization mode dispersion etc.)@ transmission and mechanical characteristics of this CSF are described below. Some provisions are made to support transmission at higher wavelengths up to 1625 nm. The geometrical@ optical@ transmission and mechanical parameters are described below in three categories of attributes: ? fibre attributes are those attributes that are retained throughout cabling and installation; ? cable attributes that are recommended for cables as they are delivered; ? link attributes that are characteristics of concatenated cables@ describing estimation methods of system interface parameters based on measurements@ modelling@ or other considerations. Information for link attributes and system design are given in Appendix I. This Recommendation@ and the different performance categories found in the tables of clause 7@ is intended to support the following related system Recommendations: ?[b-ITU-T G.957]; ? [b-ITU-T G.691]; ? [b-ITU-T G.692]; ? [b-ITU-T G.959.1]; ? [b-ITU-T G.973]; ? [b-ITU-T G.977]; The meaning of the terms used in this Recommendation@ and the guidelines to be followed in the measurements to verify the various characteristics@ are given in [ITU-T G.650.1] and [ITU-T G.650.2]. The characteristic of this fibre@ including the definitions of the relevant parameters@ their test methods and relevant values@ will be refined as studies and experience progress.