FLAGSTAFF, Ariz. (AP) — 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 service providers. Industry analysts say these companies generally do not build alternative routes, or redundancies, unless they believe it is worthwhile financially.

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The result: While most major U.S. metropolitan areas 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 up to 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 and even 911 service was lost.

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.

“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?”

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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.”

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. has more than doubled in recent years, from 221 in 2010 to 487 last year, the Federal Communications Commission says.

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.

As early as 1995, however, the U.S. Commerce Department’s National Institute of Standards and Technology warned that the “power of optical fiber technology is diminishing the number of geographic transmission routes,” concentrating the flow of information and “resulting in an increase in network vulnerability.”

Since 2009, the U.S. Agriculture and Commerce departments have provided about $10 billion in grants and loans to expand broadband Internet access. The departments said recipients were encouraged but not required to build redundancies into their projects.

The FCC says about half the rural U.S. lacks access to high-speed Internet service. It plans to distribute about $20 billion over the next five years to support rural broadband, but it does not require recipients to build network backup systems against outages.

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Lieb reported from Jefferson City, Missouri.

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Follow David A. Lieb at https://twitter.com/DavidLieb and Felicia Fonseca at https://twitter.com/FonsecaAP .