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 document describes: 
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 data 
link service provider; 
b) procedures for giving communication opportunities based on ISO/IEC/IEEE 8802-3 MAC, 
with provisions for nodes to be added or removed during normal operation; 
c) structure of the fieldbus data link protocol data units (DLPDUs) used for the transfer of 
data and control information by the protocol of this document, and their representation as 
physical interface data units. 
1.3 Procedures 
The procedures are defined in terms of: 
a) the interactions between peer data link entities (DLEs) through the exchange of fieldbus 
DLPDUs; 
b) the interactions between a data link 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 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 document also specifies conformance requirements for systems implementing these 
procedures. This document does not contain tests to demonstrate compliance with such 
requirements.dition 2.0 2019-04 INTERNATIONAL STANDARD Industrial communication networks – Fieldbus specifications – Part 4-21: Data-link layer protocol specification – Type 21 elements IEC 61158-4-21:2019-04(en) THIS PUBLICATION IS COPYRIGHT PROTECTED Copyright ？ 2019 IEC, Geneva, Switzerland All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form