This Recommendation | International Standard defines the OS1 Data Link Service in terms of a) the primitive actions and events of the service; b) the parameters associated with each primitive action and event, and the form that they take; and c) the interrelationship between, and the valid sequences of these actions and events.
ITU-T X.212 FRENCH-1995 history
1995ITU-T X.212 FRENCH-1995 INFORMATION TECHNOLOGY - OPEN SYSTEMS INTERCONNECTION - DATA LINK SERVICE DEFINITION