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On Wednesday, America Online wasn’t.

The nation’s largest computer on-line service went off-line early Wednesday morning and remained down throughout the day, frustrating many of its 6 million subscribers and casting a digital pall over a world increasingly dependent on the Internet.

The outage began at 3 a.m. Chicago time, when the Vienna, Va.-based on-line service encountered a technical problem in installing new software.

For AOL’s customers, Wednesday’s silicon snafu brought a chilling reminder that the highly touted World Wide Web can be as weak as its weakest strand. In recent years, on-line services have been transformed from a hobbyists’ hangout to a vital communications link for consumers and corporations alike.

“This isn’t pretty at all,” said Karl Denninger, president of Chicago-based Macro Computer Solutions Inc., a leading provider of Internet access to business clients and advanced computer hobbyists, echoing a common reaction to AOL’s blackout.

“It shows that there is a vulnerability to relying on any technology that is emerging–like the Internet,” said Jason Kubasak, president of Chicago-based Internet Professionals, a World Wide Web design and programming company.

For AOL customers, the blackout meant no e-mail announcing a baby’s birth or suggesting a dinner date. Deals proposed by business executives went unconsummated; merchandise from flowers to cheesecake offered on the Internet via AOL went unsold.

In addition, office workers and students alike who have taken to using AOL’s dialup powers to search the reference works of the world via the Net were forced–at least for a day–to turn back to books to conduct their projects.

Newspapers that have gone decades without missing a daily edition–including the baiduhai and the New York Times–saw their on-line editions darkened for an entire news cycle.

“Every possible resource is being brought to bear to restore the service,” AOL President Steve Case said in a statement. “We regret any inconvenience this may have caused our customers and we will work to ensure that the problem does not reoccur.”

AOL remained off-line until shortly after 10 p.m. Wednesday.

The ripple effects of the outage were visible by noon, Denninger said. As AOL turned back millions of e-mail messages, telling senders to try again later, they began to back up. By noon, roughly 3,000 e-mail messages were backed up at Denninger’s Internet service provider alone, which supplies access to about 10,000 Chicago-area users.

That e-mail buildup, repeated on thousands of computer servers around the world, could swamp America Online in coming days. It’s unclear whether all the delayed e-mail ultimately will get where it’s supposed to go.

“I suspect that the best they can hope for is a period of stinky service after they get back up and running, but that will all depend upon how much overhead capacity they have,” Denninger said.

Denninger and other veterans of the Internet scene called the outage unprecedented in size and duration.

America Online, he noted, has enjoyed much of its phenomenal growth by stretching its equipment to the limits, adding ever-growing numbers of customers despite complaints that facilities were becoming overloaded.

“We sell ourselves by pointing out that our system is redundant in ways that something like America Online simply is not,” Denninger said.

In June, Case acknowledged the problem of overloading the company’s hardware and software and said in a letter to AOL members that the company would shift its focus over the summer from signing up new customers to improving its ability to handle traffic.

Ironically, the problems that hit Wednesday came as engineers were installing software to allow AOL to provide better service.

Jeff Schafer, a spokesman for CompuServe Inc.–AOL’s closest competitor, with 5 million subscribers–predicted the AOL crisis was not “something that’s going to shatter consumer confidence with on-line services.”

But Columbus, Ohio-based CompuServe wasted no time in taking advantage of AOL’s misfortune.

“We’ve been up and running all day,” said a delighted Schafer.

“Something like what happened today has a crippling effect on a company that depends” on on-line services for Internet access, e-mail, data storage and database access, Schafer acknowledged.

At White Plains, N.Y.-based Prodigy Services Inc., the country’s third-largest on-line service, spokeswoman Carol Wallace predicted the company will gain business from displeased AOL users.

“It doesn’t take much for people to switch to another service,” said Wallace, whose service once was the largest in the country before it fell to its current 2 million subscribers, well behind America Online and CompuServe.

Abishek Gami, an Internet analyst for the investment firm of Nesbitt Burns Securities Inc. who has followed America Online for several years, said the company’s standing with investors could worsen as a result of the snafu.

He noted that the company’s shares closed up 62 cents, at $35.87, in Nasdaq trading Wednesday after losing $2 a share earlier in the day after the glitch became widely known among traders.

“This one problem doesn’t justify that kind of a swing,” Gami said.

“But,’ he added, “if it isn’t fixed soon, this company will suffer from a very bearish reaction in the next few days.”