"This ETS 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 this ETS forms part of the set of libraries defined in ETS 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 this ETS should be describable as an interconnection of a subset of these functional blocks contained within this ETS. The interconnection of these blocks should obey the combination rules given in ETS 300 417. The generic functionality is described in ETS 300 417-1-1 [1]. This ETS assumes that there are only two types of Synchronization Supply Units (SSUs)@ transit and local@ as currently defined in ITU-T Recommendation G.812 [18]. However@ STC TM3 has approved in September 1996 a new SSU with enhanced characteristics. The inclusion of such an SSU in this ETS is for further study. This ETS 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."