The tier automation is presented as a model for communication protocols. Several advantages of the model are cited, among which are universality of representation and manipulability. A scheme for using the tier automation to model specific distributed architectures and their protocols is described. The scheme is then used on a sample protocol and a transmission session with the sample protocol as exhibited.

If we regard reversibility (in the intuitive sense) as the ability to return to a previous state, automata possess varying types and degrees of reversibility.Several properties of automata are defined, each describing a type of connectivity or reversibility.Their relative strengths are considered, as well as whether they are preserved under generalized homomorphisms.It is shown that in the case of finite automata five of these properties are equivalent.The reverse, Rev(A), of an automaton A is defined and its relationship to A is studied.

Arbitrary finite automata are decomposed into their major substructures, the primaries. Several characterizations of homomorphisms, endomorphisms, isomorphisms, and automorphisms of arbitrary finite automata are presented via reduction to the primaries of the automata. Various characterizations of these transition-preserving functions on singly generated automata are presented and are used as a basis for the reduction. Estimates on the number of functions of each type are given.