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 Data Link Protocol provides the Data Link Service by making use of the services available from the Physical Layer. The contents of the corrigendum of July 2004 have been included in this copy.