656 1.1 General 657 The Fieldbus Application Layer (FAL) provides user programs with a means to access the 658 fieldbus communication environment. In this respect, the FAL can be viewed as a window 659 between corresponding application programs. 660 This part of IEC 61158 provides common elements for basic time-critical and non-time-critical 661 messaging communications between application programs in an automation environment and 662 material specific to Type 2 fieldbus. The term time-critical is used to represent the presence 663 of a time-window, within which one or more specified actions are required to be completed 664 with some defined level of certainty. Failure to complete specified actions within the time 665 window risks failure of the applications requesting the actions, with attendant risk to 666 equipment, plant and possibly human life. 667 This International Standard specifies interactions between remote applications and defines 668 the externally visible behavior provided by the Type 2 fieldbus application layer in terms of 669 a) the formal abstract syntax defining the application layer protocol data units conveyed 670 between communicating application entities; 671 b) the transfer syntax defining encoding rules that are applied to the application layer 672 protocol data units; 673 c) the application context state machine defining the application service behavior visible 674 between communicating application entities; 675 d) the application relationship state machines defining the communication behavior visible 676 between communicating application entities. 677 The purpose of this document is to define the protocol provided to 678 a) define the wire-representation of the service primitives defined in IEC 61158-5-2, and 679 b) define the externally visible behavior associated with their transfer. 680 This document specifies the protocol of the Type 2 fieldbus application layer, in conformance 681 with the OSI Basic Reference Model (ISO/IEC 7498-1) and the OSI application layer structure 682 (ISO/IEC 9545). 683 1.2 Specifications 684 The principal objective of this document is to specify the syntax and behavior of the 685 application layer protocol that conveys the application layer services defined in IEC 61158-5-2. 686 A secondary objective is to provide migration paths from previously-existing industrial 687 communications protocols. 20018 IEC CDV 61158-6-2 © IEC:2021 688 1.3 Conformance 689 This document does not specify individual implementations or products, nor does it constrain 690 the implementations of application layer entities within industrial automation systems. 691 Conformance is achieved through implementation of this application layer protocol 692 specification.