"The present document specifies a library of basic building blocks and a set of rules by which they may be combined in order to describe a digital transmission equipment. The library comprises the functional building blocks needed to specify completely the generic functional structure of the European digital transmission hierarchy. In order to be compliant with the present document@ equipment needs to be describable as an interconnection of a subset of these functional blocks contained within the present document. The interconnections of these blocks should obey the combination rules given. The present document specifies both the components and the methodology that should be used in order to specify SDH equipment; it does not specify an individual SDH equipment as such. The specification method is based on functional decomposition of the equipment into atomic@ compound and major compound functions. The equipment is then described by its Equipment Functional Specification (EFS) which lists the constituent atomic and compound functions@ their interconnection@ and any overall performance objectives (e.g. transfer delay@ availability@ etc.). The concept is illustrated in figure 1. The internal structure of the implementation of this functionality (equipment design) need not be identical to the structure of the functional model@ as long as all the details of the externally observable behaviour comply with the EFS. The equipment functionality is consistent with the SDH multiplexing structure given in ETS 300 147 [1]. Equipment developed prior to the production of the present document may not comply in all details with the present document. Equipment which is normally stated to be compliant with the present document may not fulfil all the requirements in the case that it is interworking with old equipment that is not compliant with the present document. The structure of the present document envisages the addition of new layers@ atomic functions and (major) compound functions. For example@ Asynchronous Transfer Mode (ATM)@ Integrated Services Digital Network (ISDN) and n x 64 kbit/s layers could be added as new complete parts while corresponding adaptation functions are added to existing layers. ""SDH interworking"" functionality can be added as new layers in part 5. This approach allows for short development times for standards describing new functionality."