This part of IEC 61158 is one of a series produced to facilitate the interconnection of automation system components. It is related to other standards in the set as defined by the three-layer fieldbus Reference Model, which is based in part on the Basic Reference Model for Open Systems Interconnection. Both Reference Models subdivide the area of standardization for interconnection into a series of layers of specification, each of manageable size. The Application Service is provided by the Application Protocol making use of the services available from the Data Link or other immediately lower layer. This part of the IEC 61158 series defines the Application Service characteristics that any immediately higher-level protocols may exploit. The contents of the corrigendum of July 2004 have been included in this copy.