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, or 
b) in an acyclic manner, as requested 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.dition 2.0 2019-04 INTERNATIONAL STANDARD colour inside Industrial communication networks – Fieldbus specifications – Part 4-24: Data-link layer protocol specification – Type 24 elements IEC 61158-4-24:2019-04(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 utiliz