Dependent Failures in Data Communication Systems

Although almost all published analyses of the reliability of communication systems assume failures of different lines are independent, such failures are actually highly correlated. A description of why this is true is given. This is followed by an approximate theoretical analysis of some measurement data. Both approaches yield the inescapable conclusion that assuming failures of different communications lines serving one location are independent can lead to dangerously inaccurate reliability predictions. The significance of these results for the designer of highly reliable systems involving data communications is discussed.