240 1.1 General 241 This part of IEC 61158 provides common elements for basic time-critical messaging 242 communications between devices in an automation environment. The term time-critical is 243 used to represent the presence of a time-window, within which one or more specified actions 244 are required to be completed with some defined level of certainty. Failure to complete 245 specified actions within the time window risks failure of the applications requesting the 246 actions, with attendant risk to equipment, plant and possibly human life. 247 This standard defines in an abstract way the externally visible service provided by the Type 2 248 fieldbus data-link layer in terms of: 249 a) the primitive actions and events of the service; 250 b) the parameters associated with each primitive action and event, and the form which they 251 take; and 252 c) the interrelationship between these actions and events, and their valid sequences. 253 The purpose of this standard is to define the services provided to: 254 " the Type 2 fieldbus application layer at the boundary between the application and data-link 255 layers of the fieldbus reference model; 256 " systems management at the boundary between the data-link layer and systems 257 management of the fieldbus reference model. 258 Type 2 DL-service provides both a connected and a connectionless subset of those services 259 specified in ISO/IEC 8886. 260 1.2 Specifications 261 The principal objective of this standard is to specify the characteristics of conceptual data-link 262 layer services suitable for time-critical communications and thus supplement the OSI Basic 263 Reference Model in guiding the development of data-link protocols for time-critical 264 communications. A secondary objective is to provide migration paths from previously-existing 265 industrial communications protocols. 266 This specification may be used as the basis for formal DL-Programming-Interfaces. 267 Nevertheless, it is not a formal programming interface, and any such interface will need to 268 address implementation issues not covered by this specification, including: 269 a) the sizes and octet ordering of various multi-octet service parameters; 270 b) the correlation of paired request and confirm, or indication and response, primitives. 271 1.3 Conformance 272 This standard does not specify individual implementations or products, nor does it constrain 273 the implementations of data-link entities within industrial automation systems. IEC CDV 61158-3-2 © IEC:2021 20009 274 There is no conformance of equipment to this data-link layer service definition standard. 275 Instead, conformance is achieved through implementation of the corresponding data-link 276 protocol that fulfills the Type 2 data-link layer services defined in this standard.