A stepwise refinement method of communications service specifications is proposed to generate communications software that can conform to any network architecture. This method uses a two-layered language; one layer is a service specification description language (STR), and the other layer is a supplementary specification description language for implementing STR description on a communications system (STR/D). STR specifies terminal behaviors that can be recognized from a perspective outside of the communications systems. With STR, a communications service is defined by a set of rules that can be described without detailed knowledge of communications systems or communications network architectures. Each STR rule describes a global state transition of terminals. Supplementary specifications, such as terminal control and network control, are needed to implement communications services specified by STR rules. These supplementary specifications are described by STR/D rules. Communications services, such as UPT (Universal Personal Telecommunication), are standardized so that they can be provided on a given functional model consisting of functional entities. Specifications for each functional entity in a network are obtained from the two kinds of initially described specifications mentioned above. The obtained specifications are described by STR(L) and STR/D(L) rules, which specify local specifications of a functional entity. These specifications for functional entities are then transformed into software specifications, and finally communications software is generated from these software specifications. This stepwise refinement method makes it possible to generate communications software that can conform to any functional model from service specifications.

A protocol completion method is proposed to transform protocols synthesized from service specifications into error-free protocols. Communication service specifications described by message sequence charts can be synthesized into protocols. The synthesized protocols may include latent exceptional behaviors that are beyond the given service specifications. Therefore, even if the service specifications themselves are verified, these exceptional behaviors may produce protocol errors such as deadlock states or unspecified reception. Error-free protocols can be obtained from error-free service specifications by synthesizing and then completing the synthesized protocols. By taking account of each service specification through protocol completion, every exceptional behavior can be detected in the protocol entities including erroneous exceptional behaviors. This function can also be applied to resolution of feature interactions. The proposed method is applied to the synthesis of the X.227 protocol from its partial service specifications.

Until now, in a communication system which deals with multiple processes, system behavior has been described by a fixed number of processes. The state reachability problem for specified processes was generally deliberated within a pre-defined number of processes, and was analyzed by essentially searching for all possible behaviors. However, in a system whose number of processes is arbitrary, a given state which is not reachable in some situations which consists of a small number of processes might be reachable in another situation which consists of a larger number of processes. This article discusses the above problem, assuming that the behavior of a system is described by an arbitrary number of processes. After discussing the relationship between our model and the Petri net model, we clarify the properties between the set of reachable states and the number of processes involved in the system, and show an algorithm to obtain a sufficient number of processes for resolving the reachability problem.

A requirement specification acquisition method combined with hypothesis-based reasoning and model reasoning is proposed for obtaining service specifications from the ambiguous and/or incomplete requirement specifications of communications services. Errors at an early stage of software development cost more to debug than those at a later stage. Specification acquisition is the most upstream development process. Nevertheless, the system support for specification acquisition is delayed compared with other development phases.' Users do not always have precise requirements. It is therefore inevitable that user requirements contain ambiguities, insufficiencies and even contradictions. Considering this, it is indispensable to support a specification completion method that derives service specifications from such problem requirements. This paper proposes a combined method to obtain consistent and complete specifications from such problem requirements. Communications service specifications can be described by specifying terminal behaviors which can be recognized from outside the communications system(s). Such specifications are described by a rule-based language. Requirement specifications usually have components that are ambiguous, incomplete, or even contradictory. They appear as rule description and/or missing rules. From such requirements, service specifications are obtained by using hypothesis-based reasoning on input requirements and existing service specifications. When existing specifications cannot be used to obtain complementary specifications, a communications service model is used to propose new rules. The proposed methods are implemented as a part of a communications software development system. The system enables non-experts in communications systems to define their own service specifications.