When you need a high-speed Internet connection for your home computer, you don't build it from scratch. You don't purchase your own networking equipment, piece it together, and plug it into the nearest Internet backbone. You simply lease, for a small fee, a live connection from your local phone or cable company. They've already built a working Internet infrastructure.

Generally, when a business sets up a high-speed fiber-optic network, a way of transporting voice and data from place to place, it does much the same thing. It simply leases bandwidth on so-called lit fiber, live fiber lines operated by a local telecom.

Yet some businesses take a different route. Early this decade, after its merger with NationsBank, Bank of America knew it would soon outgrow the network connecting its U.S. data centers. The asynchronous transfer mode network offered only 155 Mbps of bandwidth, running at so-called "OC-3" speeds. It could have signed a new lease on an OC-48 service, jumping up to 2.5 Gbps, but in the end, it decided to build a fiber-optic network of its own.

In each of the 12 cities housing its data centers, rather than tap into lit fiber lines, Bank of America signed a long-term contract for several strands of dark fiber—fiber cables just sitting under the streets, unused. It then connected its own equipment and, in essence, constructed a network . This required a bit more time and expertise. But it gave the company more control over its network services, and thanks to the low cost of dark fiber and equipment, it even saved money. Though the new network is more than 16 times faster than the old service, it's less expensive.

In the late nineties, with the economy flying high, telecoms spent billions of dollars installing fiber-optic lines beneath our city streets. But once the bubble burst, the market for live fiber tanked, and today there are literally thousands of miles of fiber just waiting to be turned on. As a result, prices in many areas are wonderfully low. Bank of America was able to step in and lease its dark fiber for a pittance.

"At the time, the U.S. telecom-service environment was spiraling downhill," says Larry Schaeffer, senior vice president of network services. "We saw OC-3–based services going from $60 per mile down to $20, and even in light of those price decreases, we were able to justify our new network." And Bank of America isn't the only one. Ford and Gannett Company, the publisher of USA Today, have also built their own networks from dark fiber.

Gary Gunnerson, an IT architect at Gannett, estimates that in choosing dark fiber over lit fiber the company cut costs by almost 30 percent. This he attributes not only to the low price of dark fiber, but also to low prices on the optical equipment needed to drive it. Last year, the IEEE standards body ratified the CWDM (coarse wavelength division multiplexing) specification, a low-cost way of driving high-bandwidth optical networks. The result is that prices have dipped on all sorts of optical gear.

Cutting the telecom out of the equation frees companies to make their own maintenance decisions, such as when to schedule outages. And they needn't worry that other companies have access to their data. "If you're going to put together a dark-fiber network, it's often because you need a certain level of security," says Stan Hubbard, a senior analyst with Heavy Reading, a New York-based research firm.

Of course, dark fiber isn't for everyone. You need an adventurous IT staff with plenty of experience in the world of fiber optics. "It's a lot of work to pull this sort of thing together," says Gannett's Gunnerson. Your fiber provider will make sure the cables are in working order, but you're responsible for all other maintenance.

Dark fiber isn't always the inexpensive proposition. You may find that a lit-fiber service is the cheaper option. In building a new fiber-optic network between its Southampton and Brooklyn campuses a little over a year and a half ago, Long Island University chose a lit service from Lightpath. "At the time, dark fiber was just too expensive," says George Baroudi, LIU's chief information officer. "But prices are coming down, and we're considering it for our new data-recovery center."

As a homeowner, you'd never build your own Internet connection. But in running a business, you just might build your own fiber-optic network.