WASHINGTON -- Eight months after a software mistake snarled service for 5 million customers in the metropolitan Washington area, Bell Atlantic Corp. and the company that supplied the flawed program, DSC Communications Corp., said yesterday that they had taken substantial steps to ensure against recurrences.
The tiny software error was equivalent to a mistyped word hidden among five copies of "Moby Dick."
Telecommunications executives had feared that the flaw was in a new international standard, a set of computer rules being followed worldwide, that might have led to network breakdowns in many countries.
But Bell Atlantic, parent of Chesapeake & Potomac Telephone Co. of Maryland, and DSC found that the set of rules had to be changed in only minor ways.
The cause of the breakdown was a "three-bit bug," a tiny computer coding error among 4 million lines of computer code devised by DSC Communications of Plano, Texas.
The error led to the mismanagement of computer messages within the system, akin to overstuffing a mailbox, and resulted in the nine-hour breakdown.
The network disruption on June 26, 1991, in which local callers -- including many in Maryland -- were unable to complete connections, was part of a wider pattern that also affected Pacific Bell.
In all, six incidents occurred within three weeks in late June and early July. All ultimately were traced to DSC's tiny mistake.
John W. Seazholtz, a Bell Atlantic vice president, acknowledged that Bell Atlantic officials had failed to appreciate the danger when there was a similar accident involving American Telephone & Telegraph Co.'s network a year and half before last summer's outage.
Steps planned by Bell Atlantic include the construction of more telephone lines and other facilities to help reduce traffic.