Conformance Relations and Test Derivation

It seems that finite state machines (FSM's) and (finite) labeled transition systems (LTS's) are competing descriptive models for system specifications in various areas, including communication protocols. Much work on the derivation of tests from a given system specification has been done separately for these two models. In this paper, we attempt to make a comparative study of existing approaches of test derivation for these two similar, but still distinct models, and we show that borrowing ideas from one type of model for the realm of the other might be useful. In order to formally compare the two models of FSM's and LTS's, we use a formal framework for conformance testing based on conformance relations and a set of possible observations. We show how the conformance relations can be combined with explicit fault models for the tested implementations. It is shown that, within the FSM framework, a hierarchy of conformance relations used for the comparison between the specification and the tested implementation, reflects the hierarchy of classes of models (completely specified and deterministic, deterministic, and non-deterministic). In particular, the reduction relation between non-deterministic FSM's can be used to derive other useful relations for the other classes of models. A review of test derivation methods for FSM models shows that guaranteed fault coverage within a predefined fault domain may be obtained. The second part of the paper shows that the methods for test suite derivation from FSM specifications can be applied for testing various conformance relations in respect to LTS specifications. The idea is to define, for a given LTS specification and conformance relation, a corresponding FSM specification such that the application of the FSM test suite (based on trace conformance) is equivalent to the verification of the given conformance relation in respect to the LTS specification. The feasibility of this approach is presented for the so-called failure trace equivalence between the LTS specification and the implementation. A corresponding canonical FSM tester can also be defined. Keyword Codes: C.2.2; D.2; D.2.5