Diagnosis of Multiple Faults in a Nationwide Communications Network

The Network Diagnostic System (NDS) is an ARBY based expert system for fault isolation in a nationwide communications network (COMNET). Due to both the structure and function of the network, failures in COMNET are often multiple component failures (either dependent or independent) or intermittent failures. The maintenance procedure for isolating and correcting faults in COMNET exploits multiple types of knowledge, including the topological structure of COMNET, geographic organization, and frequency of failure information. Using ARBY, fault isolation in NDS is represented as a heuristic search through a space of hypotheses. The available diagnostic tests impose a refinement hierarchy on the space of hypotheses, enabling the exploitation of hierarchical search. Back links to more general hypotheses at higher levels in the refinement hierarchy are introduced to ensure the isolation and repair of multiple and intermittent failures. The NDS currently performs at the level of an intermediate COMNET diagnostician. 1. Introduction This paper describes the development of an expert system for diagnosing multiple faults in a nationwide communications network (COMNET). Our effort focused on understanding the human effort required to implement an expert system and on identifying missing features in the existing technology. In the process of implementing NDS it was necessary to extend existing expert systems technology dealing with multiple and intermittent failures. The domain was chosen on the basis of prior domain experience with trouble shooting electronic equipment, the availability of a domain expert, and by expected benefits from the eventual operational system. Criteria considered during the scoping of the project were that it require less than one person year of effort, that it be a non-trivial application, and that it avoid excessive uninteresting detail. 2. The Problem: Fault Isolation in COMNET COMNET is a geographically distributed communications network supporting a variety of application systems. The application software runs on a central computing facility and is accessed remotely by users throughout North America via interactive terminals. The communication path from each user terminal to the central computer is a circuit (called a 'virtual wire') composed of both digital and analog electronic devices. The field repairable units, i.e. the components which can fail and be repaired, include telecommunication processors, statistical multiplexors, modems, telephone circuits, frequency divisional multiplexors, RS-232 cables and computer terminals (see fig. 1.). The capabilities which had to be included in the NDS are discussed in the remainder of this section. The diagnostic process draws upon several different types …