Telecommunications giant MCI WorldCom Inc. has been having Internet problems since Thursday that have frustrated customers and confounded the company's engineers.

The company's problems concern its frame-relay network, one of several kinds of networks providing high-speed Internet access and e-mail service to businesses.

"Less than 15 percent of our network has been affected," said spokeswoman Linda Laughlin, adding that the problem is causing network congestion and slow response for users on the Northeast seacoast and in Chicago.

By yesterday, major customers such as the Chicago Board of Trade had been able to get back online through the frame-relay network, Laughlin said, but she could not predict when the problems would be completely resolved.

Laughlin said the problem has the attention of the highest officials of the Clinton, Miss.-based company, including chief executive Bernard Ebbers.

Sean Donelan, a senior network architect for St. Louis-based Data Research Associates, a company that provides Internet service to 3,000 public libraries worldwide, said MCI's troubles have been "taking down customers in 19 states and two Canadian provinces," with the network "cycling through various states of complete collapse, partial outages, circuits bouncing and being generally unusable."

Like many businesses, DRA does not depend exclusively on one network, so not all of its customers have been affected.