This Recommendation covers optical interface parameter specifications for equipments and systems supporting the Synchronous Digital Hierarchy (SDH) defined in ITU-T Rec. G.707/Y.1322 and operating on single-mode optical fibres conforming to ITU-T Recs G.652@ G.653 and G.654. The purpose of this Recommendation is to provide specifications for the optical interfaces of SDH equipment@ described in ITU-T Rec. G.783@ to achieve the possibility of transverse (multivendor) compatibility on elementary cable sections@ i.e.@ the possibility of mixing various manufacturers' equipments within a single optical section. However@ the specifications in this Recommendation are also intended to be in accordance with ITU-T Rec. G.955 which provides the possibility to achieve longitudinal compatibility for equipment of comparable hierarchical level and application. This Recommendation is based on the use of one fibre per direction. Any other optical arrangements may require different specifications and are for further study.