"This Recommendation specifies a library of basic synchronization distribution 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's synchronization functionality. The library defined in this Recommendation forms part of the set of libraries defined furthermore in Recommendations [ITU-T G.783]@ [ITU-T G.705]@ [ITU-T G.798]@ [ITU-T G.8021] and [ITU-T G.8121]. The library comprises the functional building blocks needed to completely specify the generic functional structure of the ITU-T digital transmission hierarchy. Equipment that is compliant with these Recommendations should be describable as an interconnection of a subset of these functional blocks contained within these Recommendations. The interconnection of these blocks should obey the combination rules given in [ITU-T G.806]. The generic functionality is described in [ITU-T G.806]. The specifications in this Recommendation are the superset of functionality of three regional standards bodies. When different processing within a single atomic function is required to support the specific regional processing@ this is identified by means of options I@ II and III processing. ""Option I"" applies to networks optimized for the 2048 kbit/s hierarchy. ""Option II"" applies to networks optimized for the 1544 kbit/s hierarchy that includes the rates 1544 kbit/s@ 6312 kbit/s and 44 736 kbit/s. ""Option III"" applies to networks optimized for the 1544 kbit/s hierarchy that includes the rates 1544 kbit/s@ 6312 kbit/s@ 33 064 kbit/s@ 44 736 kbit/s and 97 728 kbit/s. As a general rule@ this Recommendation does not specify the atomic functions that are specific to PRC ([ITU-T G.811]) and SSU ([ITU-T G.812]) clock equipment; the SSM selection algorithm specified in this Recommendation is only generally applicable to SEC ([ITU-T G.813] and [ITU-T G.8262]) clock equipment. However@ strictly for option II networks@ there are some specific topologies listed containing both SEC and SSU clock equipment@ for which the application of the SSM selection algorithm is described. The general case of using the SSM selection algorithm simultaneously in SSU and SEC clock equipment is for further study for all options. Full integration of the ePRTC@ PRTC@ and eEEC is for further study."