By JOHN FONTANA

Joe Barrett's job is not unlike those of other network architects. His primary responsibility is to keep his network humming at peak performance.

What is different about the challenge facing Barrett, vice president of operations at America Online, is that he has more than 15 million users and moves 42 million e-mails and 309 million instant messages per day.

Not only does that tax a network's resolve, it taxes an IT manager's patience. If Barrett's network gets out of balance, the roar it creates is deafening. Some of the infamous outages suffered by AOL over the past few years have riled users from coast to coast.

But Barrett, who oversees all of AOL's production network, has been able to keep the roar to a low rumble for the leading Internet access and Web portal company.

His most recent move to appease the masses was installing Layer 4 switches. The load-balancing features inherent to Layer 4, which peers inside an IP packet and routes based on port number and other information, enables Barrett to create a stable foundation to carry his tremendous load.

The load is generated by Web-based applications, such as e-mail or stock quote tickers, that define AOL. Since it takes anywhere from five to 40 servers to host a single AOL application and those applications typically hang off a single switch, Barrett figured the smarter the switch, the better the service.

"In an application such as the AOL.com home page, it is an extremely busy page that can't be served off one server," Barrett said. "The Layer 4 capabilities allow us to direct traffic to the best-responding server so we can maintain good response time."

Barrett recently installed ServerIron Layer 4 switches from Foundry Networks Inc. to front-end not only AOL.com, but the company's popular Digital City sites and ICQ, an instant communication and chat service. They also handle the load for AOL's online games.

AOL also may use the technology to support corporate services that will result from its acquisition of Netscape and relationship with Sun Microsystems.

"I can't really talk about where the business is headed, but it is certainly possible to support those types of services from a technical point of view," Barrett said.

Barrett previously used a DNS router to distribute the load between his servers. "DNS is basically a round-robin--everybody gets in line and takes the next server, but that does not ensure that all the servers are about equally busy," Barrett said.

Knowing server loads are balanced is important since Barrett pushes all his connections to about 70 percent saturation.

To handle that, Barrett also deployed a number of Foundry's BigIron switches to provide Gigabit Ethernet uplinks to his Cisco routers, his busiest aggregation points.

"I deal with orders of magnitude more traffic than any other corporation. I'm sensitive to load and not overtaxing what gear can do," Barrett said.

And by balancing the stress level of the gear, Barrett hopes to reduce the stress levels of his customers.