In the year and a half since terrorist attacks in New York and at the Pentagon crippled communications networks along the East Coast, telecommunications companies have invested heavily to fortify their facilities.

Verizon Communications Inc. was hit the hardest in 2001 when iron girders and rubble from the World Trade Center towers collapsed onto its network facility in Lower Manhattan, severing service over more than 200,000 telephone lines and 3 million data circuits. Tens of millions of dollars' worth of equipment was covered in soot and debris. A water-main break flooded the facility's basement. AT&T Corp., meanwhile, had equipment in the basement of the towers that was destroyed by the buildings' collapse.

Both companies have rebuilt their damaged facilities and, like other telecommunications firms, have spent the past 18 months focusing on ways to minimize disruptions in the future.

All over the country, telecommunications companies have added fiber-optic lines, increased their ability to reroute traffic and beefed up their security in response to lessons learned in the September 2001 attacks. The federal government is also participating in the effort by encouraging the industry to take additional steps to protect its networks.

"One thing we learned from 9/11 is that redundancy works, but we have to do more of it," said Mark Marchand, a Verizon spokesman.

Verizon, the dominant local phone provider from Maine to Virginia, was able to get the New York Stock Exchange and other heavy telecommunications users up and running within a week of the disaster by rebuilding networks and rerouting calls to other parts of Verizon's network.

Since the attacks, Verizon has spent $1.4 billion installing new fiber-optic cables and equipment in Lower Manhattan. Long-distance giant AT&T has made similar efforts to bolster its facilities, and like Verizon, has redistributed equipment that had been based in Lower Manhattan elsewhere in the city in an effort to protect itself from a single event wiping out operations.

Qwest Communications International Inc, the largest local telephone company in 14 western states, may have a 100-year history of recovering from natural disasters such as blizzards, floods and tornadoes, but the prospect of man-made threats forced the company to rethink its emergency planning, according to Pamela J. Stegora Axberg, senior vice president for national network services.

One of the first things the company did was to evaluate its buildings for potential threats from car bombs and other attacks. Qwest then worked with local municipalities to erect barricades and take other precautions. The company has adjusted regular security at its buildings based on what national security alerts are issued by the Department of Homeland Security.

Jeffrey M. Goldthorp, chief of network technology at the Federal Communications Commission, has been working with the nation's leading telecommunications companies for the past year. He was reluctant to discuss specifics but did point to one unnamed company that he said recently moved a huge database to a hardened underground shelter. The database will be a key resource in case the network, or any section of it, needs to be rebuilt.

The FCC also recently orchestrated a series of "mutual aid" contracts between companies that allow them to work together immediately after a disaster without having to negotiate costs or other legal issues.

Stegora Axberg noted that the industry has always put a premium on building reliability into its networks. Terrorism is just another added consideration. "There is always more that we can keep doing. I don't know if it is something you are ever done with," Stegora Axberg said.

Qwest Communications International's heavily secured data facility at Dulles features video surveillance systems and bulletproof windows.