1.1 General The DLL provides basic time-critical data communications between devices in an automated environment. Type 21 provides priority-based cyclic and acyclic data communication using an internal collision-free, full-duplex dual-port Ethernet switch technology. For wide application in various automation applications, Type 21 does not restrict the cyclic/acyclic scheduling policy in the DLL. 1.2 Specifications This standard describes: 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 data link service provider; procedures for giving communication opportunities based on standard ISO/IEC 8802-3 MAC, with provisions for nodes to be added or removed during normal operation; structure of the fieldbus data link protocol data units (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: the interactions between peer data link entities (DLEs) through the exchange of fieldbus DLPDUs; the interactions between a data link service (DLS) provider and a DLS-user in the same system through the exchange of DLS primitives; the interactions between a DLS-provider and a physical layer 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 that support time-critical communications services in the data link layer of the OSI or fieldbus reference models, and that 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-deterministic communications needs. 1.5 Conformance This standard also specifies conformance requirements for systems implementing these procedures. This standard does not contain tests to demonstrate compliance with such requirements.