The present document specifies a library of basic Synchronization Distribution (SD) building blocks, referred to as ""atomic functions"" and a set of rules by which they are combined in order to describe a digital transmission equipment. The library defined in the present document forms part of the set of libraries defined in EN 300 417 series. The library comprises the functional building blocks needed to completely specify the generic functional structure of the European digital transmission hierarchy. Equipment that is compliant with the present document should be describable as an interconnection of a subset of these functional blocks contained within t