(1) The rules that govern control of the physical connection for sending and receiving data between two or more locations in a network. Examples of data link protocols include (a) asynchronous, (b) binary synchronous communications (BSC), (c) Ethernet, (d) synchronous data link control (SDLC), (e) token-ring network, and (f) X.25. See also communications line.
(2) In SNA, a set of rules for data communication over a data link in terms of a transmission code, a transmission mode, and control and recovery procedures.