The notions of finite state automata, state transition graphs and tables and the set of regular languages being accepted (generated) by such automata are well known. But for some reason these notions have not been rigorously applied in the definition of data communication line control procedures. It is the objective of this paper to do so and to show the naturalness of this approach. We claim that we thereby arrive at a complete, precise and unambiguous definition. Others have attempted this before us. They have, however, not used the descriptional tool of finite state automata. Any one or all of these references thus form the basis on which we will compete and we shall use essentially the line control procedures which these documents set out to define.