1991 US telephone network Signaling System 7 outages
A series of major telephone outages hit several US cities over the course of a few weeks, blocking millions of calls, including some emergency calls.
The damage was public. The root cause was preventable.
A series of major telephone outages hit several US cities over the course of a few weeks, blocking millions of calls, including some emergency calls.
What the software actually got wrong
A three-character typing error in a software update to the signaling system that routes calls between telephone switches caused switches to crash and reboot whenever they received certain messages from other switches, and the fault spread as the update was rolled out to more switches nationwide.
A three-character typing error in a software update to...
A three-character typing error in a software update to the signaling system that routes calls between telephone switches caused switches to crash and reboot whenever they received certain messages from other switches
Requs AI Edge Case flags this exact pattern at the requirements and architecture stage — before a single line of code implementing it exists — so the assumption behind it gets challenged while it is still cheap to fix.
The fault spread as the update was rolled out...
the fault spread as the update was rolled out to more switches nationwide
Requs AI Software FMEA traces this failure mode back to the system-level hazard it feeds, tagging it against the Common Defect Enumeration so it surfaces in review instead of in the field.
Beyond code coverage and "shall" testing
Root causes like this one rarely show up in code coverage or requirements-compliance testing, because nobody wrote a requirement anticipating the specific edge case that broke. Requs AI Edge Case and Requs AI Software FMEA are built to surface exactly this class of overlooked failure mode — before the software is written.
Surfaces this before code exists
Identifies edge cases like this one at requirements and architecture time, using the Common Defect Enumeration to catalog failure patterns seen across hundreds of real-world software failures — including this one.
Connects the failure mode to the hazard
Traces this class of root cause directly to the system-level hazard it can produce, so a defect pattern like this one gets flagged during design review instead of after it ships.
Find the overlooked root causes before they ship.
Schedule a demonstration, or explore how Requs AI Edge Case and Software FMEA use the Common Defect Enumeration.