1.1 Overview This standard provides common elements for basic time-critical and non-time-critical messaging communications between devices in an automation environment and material specific to Type 18 fieldbus. The term “time-critical” is used to represent the presence of a time-window, within which one or more specified actions are required to be completed with some defined level of certainty. Failure to complete specified actions within the time window risks 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 18 fieldbus 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 which they take; and .c) the interrelationship between these actions and events, and their valid sequences This standard defines the services provided to • the Type 18 fieldbus application layer at the boundary between the application and datalink layers of the fieldbus reference model, and • system management at the boundary between the data-link layer and systems .management of the fieldbus reference model 1.2 Specifications The principal objective of this standard is to specify the characteristics of conceptual data-link layer services suitable for time-critical communications, and thus 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 specification may be used as the basis for formal DL-Programming-Interfaces Nevertheless, it is not a formal programming interface, and any such interface will need to :address implementation issues not covered by this specification, including a) the sizes and octet ordering of various multi-octet service parameters, and .b) the correlation of paired request and confirm, or indication and response, primitives 1.3 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 fulfills the Type 18 data-link layer services defined in this standard