1.1 General 282 The fieldbus application layer (FAL) provides user programs with a means to access the 283 fieldbus communication environment. In this respect, the FAL can be viewed as a window 284 between corresponding application programs. 285 This part of IEC 61158 provides common elements for basic time-critical and non-time-critical 286 messaging communications between application programs in an automation environment and 287 material specific to Type 2 fieldbus. The term time-critical is used to represent the presence 288 of a time-window, within which one or more specified actions are required to be completed 289 with some defined level of certainty. Failure to complete specified actions within the time 290 window risks failure of the applications requesting the actions, with attendant risk to 291 equipment, plant and possibly human life. 292 This International Standard defines in an abstract way the externally visible service provided 293 by the Type 2 fieldbus application layer in terms of: 294 a) an abstract model for defining application resources (objects) capable of being 295 manipulated by users via the use of the FAL service, 296 b) the primitive actions and events of the service; 297 c) the parameters associated with each primitive action and event, and the form which they 298 take; and 299 d) the interrelationship between these actions and events, and their valid sequences. 300 The purpose of this document is to define the services provided to: 301 a) the FAL user at the boundary between the user and the application layer of the fieldbus 302 reference model, and 303 b) Systems Management at the boundary between the application layer and Systems 304 Management of the fieldbus reference model. 305 This document specifies the structure and services of the Type 2 fieldbus application layer, in 306 conformance with the OSI Basic Reference Model (ISO/IEC 7498-1) and the OSI application 307 layer structure (ISO/IEC 9545). 308 FAL services and protocols are provided by FAL application-entities (AE) contained within the 309 application processes. The FAL AE is composed of a set of object-oriented application service 310 elements (ASEs) and a layer management entity (LME) that manages the AE. The ASEs 311 provide communication services that operate on a set of related application process object 312 (APO) classes. One of the FAL ASEs is a management ASE that provides a common set of 313 services for the management of the instances of FAL classes. 314 Although these services specify, from the perspective of applications, how request and 315 responses are issued and delivered, they do not include a specification of what the requesting 316 and responding applications are to do with them. That is, the behavioral aspects of the 317 applications are not specified; only a definition of what requests and responses they can 318 send/receive is specified. This permits greater flexibility to the FAL users in standardizing 319 such object behavior. In addition to these services, some supporting services are also defined 320 in this document to provide access to the FAL to control certain aspects of its operation. 321 1.2 Specifications 322 The principal objective of this document is to specify the characteristics of conceptual 323 application layer services suitable for time-critical communications, and thus supplement the 324 OSI Basic Reference Model in guiding the development of application layer protocols for time325 critical communications. 326 A secondary objective is to provide migration paths from previously-existing industrial 327 communications protocols. It is this latter objective which gives rise to the diversity of services 328 standardized as the various Types of IEC 61158, and the corresponding protocols 329 standardized in subparts of IEC 61158-6. 330 This specification may be used as the basis for formal application programming interfaces. 331 Nevertheless, it is not a formal programming interface, and any such interface will need to 332 address implementation issues not covered by this specification, including 333 a) the sizes and octet ordering of various multi-octet service parameters, and 334 b) the correlation of paired request and confirm, or indication and response, primitives. 335 1.3 Conformance 336 This document does not specify individual implementations or products, nor does it constrain 337 the implementations of application layer entities within industrial automation systems. 338 There is no conformance of equipment to this application layer service definition standard. 339 Instead, conformance is achieved through implementation of conforming application layer 340 protocols that fulfill the Type 2 application layer services as defined in this document.