Cause of phone snarls eludes lab scrutiny


DSC Communications Corp., the Plano, Texas-based company whose hardware has been linked to a half- dozen phone outages over the past week, said yesterday that it has duplicated in the lab a "critical problem" that led to the breakdowns.

But it still doesn't know what caused the breakdowns to occur in the first place.

Without admitting that DSC equipment was to blame for the recent spate of outages, DSC said investigation has shown that a "trigger event" causes networks built around DSC equipment to overload.

According to DSC, this critical problem "propagates the congestion to incoming links and to other network elements, ultimately resulting in service disruptions."

In the past week, a half-dozen service disruptions have been linked to DSC-made "signal transfer points," STPs, which are supercomputers that sort and route calls across local phone networks.

Th disruptions include the June 26 outage in the Baltimore area, which left up to 5 million customers of Chesapeake & Potomac Telephone Co. without service for most of the day. Similar outages have occurred since then in western Pennsylvania, Los Angeles and, most recently, San Francisco.

In all cases, DSC equipment flooded local networks with "maintenance messages" -- electronic messages computers send themselves to confirm that components are working properly -- leading to a backup of traffic and, ultimately, service disruptions.

Tuesday, DSC said it had come up with a software patch that will prevent its STPs from flooding networks with computerized messages during an overload.

One person familiar with the patch likened it to a garbage can with a trap door -- when the can becomes full, the bottom automatically opens up so it can't become flooded.

The DSC-engineered software patch was given to Bell Atlantic Corp. last week. Since then, the patch has been installed at all Bell Atlantic sites where DSC equipment has experienced problems, said Tricia Rimo, a Bell Atlantic spokeswoman.

She said Bell Atlantic eventually plans to install the patch at all sites where DSC equipment is used.

A team of 200 telecommunications experts from DSC, Bell Atlantic, Pacific Bell, Northern Telecom, American Telephone & Telegraph Co. and Bellcore, the research arm of the seven Bells, has been working around-the-clock since the first outage to try to find cause of the problem.

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