1.1 General This standard is one of a series produced to facilitate the interconnection of automation system components. It is related to other standards in the set as defined by the three-layer fieldbus reference model described in IEC/TR 61158-1:2010. This standard contains material specific to the Type 21 communication protocol. 1.2 Overview The Fieldbus Application Layer (FAL) provides user programs with a means to access the fieldbus communication environment. In this respect, the FAL can be viewed as a window between corresponding application programs. This standard provides common elements for basic time-critical and non-time-critical messaging communications between application programs in an automation environment, as well as material specific to Type 21. The term “time-critical?? is used to represent the presence of a time-window, within which one or more specified actions must 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 interactions between remote applications. It also defines the externally visible behavior provided by the Type 21 application layer in terms of: the formal abstract syntax defining the application layer protocol data units (APDUs) conveyed between communicating application entities; the transfer syntax defining encoding rules that are applied to the APDUs; the application context state machine defining the application service behavior visible between communicating application entities; the application relationship state machines defining the communication behavior visible between communicating application entities. The purpose of this standard is to: describe the wire-representation of the service primitives defined in IEC 61158-5-21:2010; describe the externally visible behavior associated with their transfer. This standard defines the protocol of the Type 21 application layer in conformance with the OSI Basic Reference Model (ISO/IEC 7498) and the OSI application layer structure (ISO/IEC 9545). 1.3 Specifications The principal objective of this standard is to specify the syntax and behavior of the application layer protocol that conveys the Type 21 application layer services. A secondary objective is to provide migration paths from previously existing industrial communications protocols. 1.4 Conformance This standard does not restrict individual implementations or products, nor does it constrain the implementations of application layer entities in industrial automation systems. Conformance is achieved through implementation of this application layer protocol specification.