A nationwide outage on the ATM network of Qwest Communications International Corp. (NYSE: Q) has thrown so-called "legacy" ATM switches into the spotlight, once again.

The outage occurred between Wednesday and Thursday of this week, affecting OC12 (622 Mbit/s) and OC3 (155 Mbit/s) links on Qwest's high-speed ATM network. Those links are reportedly based on GX 550 ATM core switches from Lucent Technologies Inc. (NYSE: LU). And members of the North American Network Operators' Group (NANOG) mailing list were abuzz with complaints on Thursday. "I'm betting on a bungled software upgrade or something similar," wrote one.

Neither Lucent nor Qwest returned calls about the outage at press time.

But one network engineer at a large West Coast facility said his network was out all night between April 4 and 5, and he thinks the Lucent switches may be to blame. "With those 550 ATM switches, if one acts up it sends a ripple effect through the rest of the network," he says. Outages tied to the switches have been reported before. The engineer says Lucent's had problems with the inter-switch PNNI (private network-to-network interface) code that coordinates the flow of traffic between switches. Qwest's been adjusting the code, he says, and the carrier told him they'd replaced it on the Lucent switches a week or so ago to fix problems.

The outage reportedly did not affect Qwest's IP network at all, since that portion of the net uses packet over Sonet (POS) for high-speed networking, not ATM.

Follow-up discussions on NANOG suggest that Qwest's older, pure ATM gear is being eyed for revision and replacement (see ATM: Over and Out?). This may open the door for several startups, including Équipe Communications Corp. and WaveSmith Networks Inc., that are marketing switches that could replace such gear and cope better with quality-of-service issues in packet-based nets.

- Mary Jander, Senior Editor, Light Reading http://www.lightreading.com