1 Scope 11 2 References 11 3 Terms and definitions 12 4 Abbreviations 14 5 Conventions 17 6 Optical transport network interface structure 18 6.1 Basic signal structure 19 6.1.1 OCh substructure 19 6.1.2 Full functionality OTM n.m (n ≥ 1) structure 19 6.1.3 Reduced functionality OTM nr.m and OTM 0.m