Analyses to Determine Operational Availability of the VSCS
Michael J. Furlong, Ronald Bishop · Air Traffic Control Quarterly · 1994
This article presents the results of hardware and software availability analyses performed on the Harris voice switching and control system (VSCS). The VSCS is required to meet an inherent system availability of 0.9999999, or 0.9(7). The Federal Aviation Administration has a similar goal for the operational availability. For this article, the VSCS operational availability is determined by combining the inherent system availability and the system’s software availability. The inherent availability analysis shows that the VSCS is performing at an order of magnitude above the specified 0.9(7) requirement. Therefore, achievement of the operational availability will be driven by the availability of the software. Software reliability analyses utilizing defect discovery estimation models based on the Rayleigh distribution were used to predict the latent defect content in the VSCS software. These predictions were then used in projecting the growth of software availability, and thus the operational availability. Also provided is a discussion on reducing the risks of latent defects affecting the operational availability through a test effort that focuses on the system’s fault tolerance. © 1995 John Wiley & Sons, Inc.