This Recommendation is a merged, revised version of Recommendations G.781, G.782 and G.783 approved under the WTSC Resolution No. 1 procedure in January, 1994. This Recommendation defines the interfaces and functions to be supported by SDH equipment. The description is generic and no particular physical partitioning of functions is implied. The input/output information flows associated with the functional blocks serve for defining the functions of the blocks and are considered to be conceptual, not physical. Not every atomic function defined in this Recommendation is required for every application. Different subsets of atomic functions may be assembled in different ways according to the combination rules given in this Recommendation to provide a variety of different capabilities. Network operators and equipment suppliers may choose which functions must be implemented for each application.