To develop a better understanding of the expected performance of apopulation of systems, and to establish a basis for developing downtimeobjectives that better define and control expected service performance,the authors develop a parallel system equipment availability model. Themodel is used to obtain the steady-state downtime distribution over aspecified time period using common channel signaling (CCS) networkaccess links as the reference architecture. It is shown how the downtimedistribution can be qualified by modeling a CCS network access segmentas a system with two independent, restorable units operating in aload-sharing, parallel mode. It is shown that a steady-state systemdowntime distribution over a specified time period exists and has a meanequal to the system average downtime objective. Using these results, itis also shown that systems with a small downtime objective achievesteady state rapidly. The proposed availability model shows that withthe current average downtime objective of two minutes per year, manyaccess segments (98.35%) will experience zero downtime during one yearof steady-state operation, while a few (1%) may have downtime exceedingone hour or more
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