What Happened to My Dial Tone?
A study of FCC service disruption reports
As a first step in trying to increase the dependability of computer systems, we carry out an analysis of the
failure modes of the Public Switched Telephone Network (PSTN), a highly dependable system that shares many
similarities with traditional computer systems. In order to identify the primary failure modes of the PSTN,
we analyzed six months of PSTN outage reports collected by the FCC. We identified three metrics for characterizing
the severity of the outages and use them to evaluate the impact of seven factors on the availability of the PSTN.
Our analysis demonstrates that human error is the primary cause of outages, regardless of the metric used. This
conclusion has significant implications for the design of dependable computer systems, as human error has traditionally
been ignored in fault-tolerant computer design.