1.1 General The data-link layer provides basic time-critical messaging communications between devices in an automation environment. This protocol provides communication opportunities to all participating data-link entities a) in a synchronously-starting cyclic manner, according to a pre-established schedule, and b) in a cyclic or acyclic asynchronous manner, as requested each cycle by each of those data-link entities. Thus this protocol can be characterized as one which provides cyclic and acyclic access asynchronously but with a synchronous restart of each cycle. 1.2 Specifications This part of IEC 61158 specifies a) procedures for the timely transfer of data and control information from one data-link user entity to a peer user entity, and among the data-link entities forming the distributed datalink service provider; b) procedures for giving communications opportunities to all participating DL-entities, sequentially and in a cyclic manner for deterministic and synchronized transfer at cyclic intervals up to one millisecond; c) procedures for giving communication opportunities available for time-critical data transmission together with non-time-critical data transmission without prejudice to the time-critical data transmission; d) procedures for giving cyclic and acyclic communication opportunities for time-critical data transmission with prioritized access; e) procedures for giving communication opportunities based on standard ISO/ IEC 8802-3 medium access control, with provisions for nodes to be added or removed during normal operation; f) the structure of the fieldbus DLPDUs used for the transfer of data and control information by the protocol of this standard, and their representation as physical interface data units. 1.3 Procedures The procedures are defined in terms of a) the interactions between peer DL-entities (DLEs) through the exchange of fieldbus DLPDUs; b) the interactions between a DL-service (DLS) provider and a DLS-user in the same system through the exchange of DLS primitives; c) the interactions between a DLS-provider and a Ph-service provider in the same system through the exchange of Ph-service primitives. 1.4 Applicability These procedures are applicable to instances of communication between systems which support time-critical communications services within the data-link layer of the OSI or fieldbus reference models, and which require the ability to interconnect in an open systems interconnection environment. Profiles provide a simple multi-attribute means of summarizing an implementation's capabilities, and thus its applicability to various time-critical communications needs. 1.5 Conformance This part of IEC 61158 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.