Requirements specification and design of a simplified telephone network by functional documentation

Dr. Jan Bredereke


We tackle certain kinds of feature interactions in telephone switching systems by employing a more modular specification structure. First, we propose a more suitable structure for modelling the users' interface, and we relate it to the architecture of the current standard for the Intelligent Network (IN). We then extend our view to the requirements on the switching system's behaviour in general. We argue that the requirements for the Plain Old Telephone Service (POTS) should be separated into different concerns, and that this should lead to a redefinition of the interfaces at which new services may be added. We employ the Functional Documentation approach [PaMa95, vSPM93, vS92a] and investigate how it can be extended to cover not only one but a sequence of development contracts. Our approach is related to the refinement approach for software development, but with some modifications: we use a directed acyclic graph of properties, not a linear sequence; we distinguish formally between fundamental and changeable properties; and we order the refinement steps by the likeliness by which they must be taken back. In the appendix, we provide a case study that applies our ideas.