IEC 61158-3-21:2010 defines in an abstract way the externally visible service provided by the Type 21 fieldbus data-link layer in terms of the primitive actions and events of the service; the parameters associated with each primitive action and event, and the form which they take and the interrelationship between these actions and events, and their valid sequences. It cancels and replaces the IEC/PAS 62573 published in 2008. This bilingual version published in 2012-01 corresponds to the English version published in 2010-08.