All Circuits Are Busy Now: The 1990 AT&T Long Distance Network Collapse (1995)
10 hours ago
- On January 15, 1990, a software bug caused a cascading failure in AT&T's long-distance network, blocking over 50 million calls for nine hours.
- The failure resulted in $60 million in lost revenue and significant disruption to businesses relying on the phone network.
- The root cause was a one-line software defect introduced during a December 1989 upgrade, which caused switches to reset in a cascading manner.
- The defect was a break statement in C code that caused improper message handling, overwriting communication data.
- The incident highlights the difficulty of detecting time-dependent software bugs and the need for robust programming practices and fault-tolerant design.