"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 the present document. The interconnection of these blocks should obey the combination rules given in EN 300 417. The generic functionality is described in EN 300 417-1-1 [6]. The present document assumes that there are only two types of Synchronization Supply Units (SSUs)@ transit and local@ as currently defined in CCITT Recommendation G.812 [17]. However@ STC TM3 has approved in September 1996 a new SSU with enhanced characteristics. The inclusion of such an SSU in the present document is for further study. The present document does not specify the atomic functions that are specific to SSU and Primary Reference Clock (PRC); the Synchronization Status Message (SSM) selection algorithm specified in the present document applies only to SEC's."