About 5% of AT&T Corp.'s asynchronous transfer mode (ATM) network users were affected by an intermittent problem with the telecommunications company's ATM network switches last week.

The incident occurred between 1:30 and 5:30 p.m. on Feb. 20, according to AT&T spokesman Dave Johnson. He said that all but four of the ATM switches affected had been restored by 4 p.m. that same day and added that only one switch was still having problems by 4:30 p.m.

The ATM system was operating normally by 5:30 p.m., Johnson said. Citing "competitive reasons," he declined to disclose the total number of ATM switches that were down.

Dale McHenry, AT&T's vice president of data services, said in an interview Feb. 22 that AT&T was conducting a root-cause analysis to pinpoint the origin of the problem.

One likely trigger event, according to McHenry, was a problem on a trunk leading to one of AT&T's large ATM switches. "That trunk went up and down several times," he said, noting that such an event could trigger similar behavior in other ATM switches.

McHenry said technicians were able to stop propagation of the problem from one switch to another within 60 to 90 minutes.

Net Traffic the Culprit?

Jeff Moore, an analyst at Current Analysis Inc. in Sterling, Va., said most network switch problems occur during software changes. He said AT&T's last major outage involved its frame-relay network, which went down for 26 hours in April 1998 during a software upgrade.

But McHenry stressed that it was neither a software nor a hardware problem in this case and said the likely culprit was an aberration in network traffic. He said his team plans to re-create the problem in AT&T's lab to determine how to avoid it in the future.

The ATM switches on the network were made by Murray Hill, N.J.-based Lucent Technologies Inc., McHenry said.

Although AT&T's frame-relay system wasn't directly involved in the recent ATM outage, many of AT&T's permanent virtual circuits over frame relay are aggregated into ATM switches, according to Lisa Pierce, an analyst at Giga Information Group Inc. in Cambridge, Mass.

McHenry acknowledged that the ATM problems may have affected some frame-relay services, but he didn't elaborate.

» posted by ITworld staff

Computerworld

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