FLAGSTAFF, Ariz. - When vandals sliced a fiber-optic cable in the Arizona desert last month, they did more than time-warp thousands of people back to an era before computers, credit cards or even phones. They exposed a glaring vulnerability in the nation's Internet infrastructure: no backup systems in many places.

Because Internet service is largely unregulated by the federal government and the states, decisions about network reliability are left to the service providers. Industry analysts say these companies generally do not build alternative routes, or redundancies, unless they believe it is worthwhile financially.

The result: While most major metropolitan areas in the U.S. have backup systems, some smaller cities and many rural areas do not.

"The more rural the location, the more likely that there's only one road in and out of that location," said Sean Donelan, a former infrastructure security manager in the U.S. Homeland Security Department who now works for a cybersecurity firm. "If someone manages to cut that fiber, you'll generally see a one- or two- or three-day outage."

Despite its own warnings about such vulnerabilities two decades ago, the federal government has taken no steps to require Internet companies to have backup systems, even as it has provided billions of dollars in subsidies to expand broadband Internet into unserved areas.

"Our first responsibility is to make sure that people actually have service," said Agriculture Secretary Tom Vilsack, co-chairman of President Barack Obama's newly created Broadband Opportunity Council.

In northern Arizona last month, tens of thousands of residents were without Internet service - some for as long as 15 hours - after vandals cut through an underground bundle of fiber-optic cables owned by CenturyLink. ATMs went down, stores couldn't process credit cards, college students in Flagstaff had to put their research on hold, and even 911 emergency service was lost.

Earlier this month, several thousand people lost Internet and phone service for half a day when an electric company crew accidentally cut a fiber-optic line in northern New Mexico.

When an underwater fiber-optic cable became wrapped around a big rock and broke in 2013, some residents of Washington state's San Juan Islands were without Internet and telephone service for 10 days.

Among them was aerospace consultant Mike Loucks, who said he was shocked to find out his home phone, cellphone and Internet service did not work independently of each other. All went down because they relied on the same cable. He ended up taking a ferry to the mainland to dial in to conference calls from his car outside a McDonald's.

"When I figured out what all had been routed to this cable, it's a single-point failure thing," he said. "That's pretty dumb. Why don't you guys have a backup cable?"

He was so frustrated that he switched Internet providers.

CenturyLink, the broadband provider in the Arizona and Washington outages, declined to make officials available for an interview about its Internet infrastructure. But spokeswoman Linda Johnson said in an email that the company acts quickly to restore service and "is constantly investing in its local network and strives to deliver new services and build redundancy where possible."

After the San Juan Islands outage, CenturyLink spent $500,000 to install a microwave system that now backs up the underwater cable. A microwave system is wireless technology that relies on a series of above-ground antennas or towers to transmit data. It's more often used in rural areas.

Companies have been deploying more than 10 million miles of fiber annually in the U.S., increasing the risk of damage from backhoes, trench-diggers and shovels, according to an analysis by a network reliability committee of the Alliance for Telecommunications Industry Solutions. The number of outages on high-capacity fiber-optic lines in the U.S. more than doubled from 221 in 2010 to 487 last year, according to the Federal Communications Commission.

Fiber-optic cables form the spine of the Internet. A fiber bundle contains dozens of tiny glass fibers - each about the width of a human hair - that use light waves to transmit data. The fibers often are buried along existing rights of way for highways, railroads or pipelines. It is common for a telecommunications company to install the cables and then lease space on them to others.

That saves money for everyone involved. But it also means outages can affect a wide variety of services.

Backup options for Internet

lines can be costly, complicated

By DAVID A LIEB

Associated Press

Hunter Newby describes himself as a real estate entrepreneur, even if he's not marketing houses or land. Instead, he's selling space on a new fiber-optic transmission line to Internet providers, telecommunications companies and anyone wanting high-speed data.

Newby is chief executive of Allied Fiber, a New York firm that is nearing completion of a broadband route from Miami to Atlanta. His long-range vision is to build a new fiber-optic loop around the entire U.S. that is physically distinct from existing Internet routes but connects to them.

Newby's network is just one of the potential solutions to what experts describe as a lack of redundancy that makes the nation's high-speed Internet highway vulnerable to outages if a fiber-optic cable gets cut or a key connection site gets damaged. That is particularly true in rural areas and smaller cities.

How many broadband lines have backup systems is unclear because the industry has been largely unregulated and Internet providers will not divulge details about their networks.

Newby said the Internet "has numerous single points of failure all over the place." His goal, he said, "is to create physical diversity" around those spots.

One of those weak points was exposed last month when vandals sliced a fiber-optic cable in the Arizona desert between Phoenix and Flagstaff, leaving some people and businesses without Internet service for as long as 15 hours. There was no backup system.

Industry analysts say there are several ways around such problems:

- Companies can build a second fiber-optic transmission line between cities, allowing Internet traffic to be rerouted when one line goes down. But that could double the cost of extending service into a particular area.

- Networks can be designed with switches that reroute traffic along other existing routes. In one hypothetical scenario, Internet traffic from Phoenix could follow a detour to Los Angeles and Las Vegas to get to Flagstaff. But those alternative paths would need enough capacity to handle the suddenly enlarged load.

- A satellite or microwave system can be installed to serve as a backup to a fiber-optic cable between two places. But the service could be slower and a relatively expensive insurance policy.

"There almost always are options. The question is whether the other routes are economically viable," said Bill Woodcock, executive director of the Packet Clearing House, a San Francisco nonprofit that supports Internet infrastructure.

Newby said his customers typically already have access to another fiber-optic route. Some are choosing his line as a new primary path, others as a backup plan.

"How they build protection in their network architecture is up to them," he said.