762 1.1 General 763 The data-link layer provides basic time-critical messaging communications between devices in 764 an automation environment. 765 This protocol provides communication opportunities to all participating data-link entities, 766 sequentially and in a cyclic synchronous manner. Foreground scheduled access is available 767 for time-critical activities together with background unscheduled access for less critical 768 activities. 769 Deterministic and synchronized transfers can be provided at cyclic intervals up to 1 ms and 770 device separations of 25 km. This performance is adjustable dynamically and on-line by re771 configuring the parameters of the local link whilst normal operation continues. By similar 772 means, DL connections and new devices may be added or removed during normal operation. 773 This protocol provides means to maintain clock synchronization across an extended link with 774 a precision better than 10 µs. 775 This protocol optimizes each access opportunity by concatenating multiple DLSDUs and 776 associated DLPCI into a single DLPDU, thereby improving data transfer efficiency for data777 link entities that actively source multiple streams of data. 778 The maximum system size is an unlimited number of links of 99 nodes, each with 255 DLSAPaddresses. Each link has a maximum of 224 779 related peer and publisher DLCEPs. 780 1.2 Specifications 781 This document specifies 782 a) procedures for the timely transfer of data and control information from one data-link user 783 entity to a peer user entity, and among the data-link entities forming the distributed data784 link service provider; 785 b) the structure of the fieldbus DLPDUs used for the transfer of data and control information 786 by the protocol of this document, and their representation as physical interface data units. 787 1.3 Procedures 788 The procedures are defined in terms of 789 a) the interactions between peer DL-entities (DLEs) through the exchange of fieldbus 790 DLPDUs; 791 b) the interactions between a DL-service (DLS) provider and a DLS-user in the same system 792 through the exchange of DLS primitives; 793 c) the interactions between a DLS-provider and a Ph-service provider in the same system 794 through the exchange of Ph-service primitives. 795 1.4 Applicability 796 These procedures are applicable to instances of communication between systems which 797 support time-critical communications services within the data-link layer of the OSI or fieldbus 798 reference models, and which require the ability to interconnect in an open systems 799 interconnection environment. 800 Profiles provide a simple multi-attribute means of summarizing capabilities of an 801 implementation, and thus its applicability to various time-critical communications needs. 802 1.5 Conformance 803 This document also specifies conformance requirements for systems implementing these 804 procedures. This document does not contain tests to demonstrate compliance with such 805 requirements.