This document describes a technical compatibility standard specification for a digital transmission interface. For optical applications it addresses very short reach@ short reach@ intermediate reach@ and long reach optical parameters at specific reference points (located close to the transmitter and receiver) in a manner consistent with ITU-T Recommendations G.957@ G.691@ and G.693. Except for OVTG@ OC-1@ and OC-24@ all technical specifications are included in those ITU-T Recommendations. For electrical applications@ T1.102-1993 (R1999) is referenced.