Why a massive cell phone outage hit the Southeast

cell phone outage
A massive cell phone outage that hit the Southeast was likely caused by a cut to AT&T's fiber-optic cable in Kentucky. This images from Downdetector.com shows the outage.

If you live in the Southeast and couldn't place a call last night, you're not alone.

Cell phone customers of all four major carriers had no service Tuesday afternoon and into the evening in parts of Kentucky, Tennessee, Alabama and Georgia. Service was restored Tuesday night.

Outages of that size are rare, and the fact that Verizon (VZ), AT&T (T), Sprint (S) and T-Mobile (TMUS) all went down makes the event less commonplace. Telephone networks typically have redundancies in place that limit the impact of massive outages.

How could such a widespread outage happen, affecting all four nationwide cell phone carriers? It could have been as simple as a cut cable.

Buddy Rogers, spokesman for the Kentucky Division of Emergency Management, told CNNMoney that a fiber-optic cable belonging to AT&T was cut along the Kentucky-Tennessee border Tuesday.

He wasn't yet sure how or why the line was cut -- it could have been vandalism or a raccoon. Kentucky's Commonwealth Office of Technology is investigating.

Wireless companies are a notoriously secretive bunch, so none commented directly on the cause. But Sprint, in a statement, said the issue appeared to be "caused by a local exchange provider." AT&T said it was a "hardware related issue," but did not confirm that its cable was cut.

A cut in AT&T's fiber cable adds up as the likeliest root cause of the outage, given the companies' statements. (AT&T is the local exchange provider for the region, and fiber counts as "hardware.")

But how could a single cut to a fiber cable bring down service on Verizon, Sprint and T-Mobile too?

To find an explanation let's journey through the foggy mists of time, all the way back to 1984, when Ma Bell (AT&T) broke up. The "Baby Bells" became responsible for maintaining the local telephone service infrastructure.

Years later, when cell phone towers started going up, the wireless companies had to connect them to the telephone system operated by the Baby Bells. Eventually, those Baby Bells started combining to form massive telecom giants again -- two of which are today's AT&T and Verizon.

So even though Sprint and T-Mobile are responsible for carrying the signal from your phone to their cell towers, they're still largely dependent on AT&T and Verizon to carry cell phone calls over the landline and fiberoptic infrastructure in many regions. And Verizon and AT&T are dependent on one another (and other, smaller companies) when they're operating in regions in which they don't control the fiber and landlines.

What's unusual about this event is that fiber cuts typically don't result in massive outage. A single hoodlum (or squirrel) shouldn't be able to disrupt service to an entire region with a single, well placed cut.

"Fiber is usually deployed with resiliency effects put in place, so a cut is not a disaster," said Ken Rehbehn, wireless analyst for 451 Research. "It makes you wonder: Was there a chain of events leading to this?"

Though the root cause of the problem is still unknown, the good news is that it was fixed within hours. It appears to have begun around 3 p.m., and Verizon reported on Twitter that the issue was fixed as of 8 p.m. Tuesday evening.

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