Michele Boreale¹, Rocco De Nicola², Rosario Pugliese²,
¹ Dipartimento di Scienze dell'Informazione, Universita' di Roma ``La Sapienza''
² Dipartimento di Sistemi e Informatica, Universita' di Firenze

Abstract
We propose a general approach to define behavioural preorders over process terms by considering the pre-congruences induced by three basic observables. These observables provide information about the initial communication capabilities of processes and about their possibility of engaging in an infinite internal chattering. We show that some of the observables-based pre-congruences do correspond to behavioral preorders long studied in the literature. The coincidence proofs shed light on the differences between the must preorder of De Nicola and Hennessy and the fair/should preorder of Cleaveland and Natarajan and of Brinksma, Rensink and Vogler, and on the role played in their definition by tests for internal chattering.