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. The principal objective of this Recommendation | International Standard is to specify the characteristics of a conceptual Data Link Service and thus, supplement the OS1 Reference Model in guiding the development of Data Link Protocols. This Recommendation | International Standard does not specify individual implementation or products, nor does it constrain the implementation of Data Link entities and interfaces within an information processing system. There is no conformance of equipment to this Data Link Service Definition Recommendation | International Standard. Instead, conformance is achieved through implementation of conforming Data Link Protocols that fulfil the Data Link Service defined in this Recommendation | International Standard.