"This part of IEC 61158 provides the common elements for basic time-critical messaging communications between devices in an automation environment. The term ""time-critical"" in this context means the prioritized full-duplex collision-free time-deterministic communication@ of which one or more specified actions are required to be completed with some defined level of certainty. Failure to complete specified actions within the required time risks the failure of the applications requesting the actions@ with attendant risk to equipment@ plant@ and possibly human life. This standard defines in an abstract way the externally visible service provided by the Type 21 data-link layer in terms of: a) the primitive actions and events of the service; b) the parameters associated with each primitive action and event@ and the form that they take; and c) the interrelationships between these actions and events@ and their valid sequences. The purpose of this standard is to define the services provided to: ?The Type 21 application layer at the boundary between the application and DLLs of the fieldbus reference model; ?Systems management at the boundary between the DLL and the systems management of the fieldbus reference model. Specifications The principal objective of this standard is to specify the characteristics of conceptual DLL services suitable for time-critical communications@ and to supplement the OSI Basic Reference Model in guiding the development of data link protocols for time-critical communications. A secondary objective is to provide migration paths from previously existing industrial communications protocols. This standard may be used as the basis for formal data link programming interfaces. Nevertheless@ it is not a formal programming interface@ and any such interface will need to address implementation issues not covered by this standard@ including: a) The sizes and octet ordering of various multi-octet service parameters; b) The correlation of paired primitives for request and confirm@ or indication and response. Conformance This standard does not specify individual implementations or products@ nor do they constrain the implementations of data-link entities within industrial automation systems. There is no conformance of equipment to this data-link layer service definition standard. Instead@ conformance is achieved through implementation of the corresponding data-link protocol that fulfils the Type 21 DLL services defined in this standard."