The data-link layer provides basic time-critical messaging communications between devices in an automation environment.

This part of IEC 61158 specifies a main protocol with the following characteristics.
• This protocol provides communication opportunities to all participating data-link entities, sequentially and in a cyclic synchronous manner. Foreground scheduled access is available for time-critical activities together with background unscheduled access for less critical activities.
• Deterministic and synchronized transfers can be provided at cyclic intervals up to 1 ms and device separations of 25 km. This performance is adjustable dynamically and on-line by reconfiguring the parameters of the local link whilst normal operation continues. By similar means, DL connections and new devices can be added or removed during normal operation.
• This protocol provides means to maintain clock synchronization across an extended link with a precision better than 10 μs.
• This protocol optimizes each access opportunity by concatenating multiple DLSDUs and associated DLPCI into a single DLPDU, thereby improving data transfer efficiency for datalink entities that actively source multiple streams of data.
• The maximum system size is an unlimited number of links of 99 nodes, each with 255 DLSAP-addresses. Each link has a maximum of 224 related peer and publisher DLCEPs.

This document specifies additional lower layers protocols or implementations of additional lower layers protocols for use in combination with ISO/IEC/IEEE 8802-3.

This document specifies a set of corresponding objects providing a consistent management interface to the lower layers.