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High-speed wireless data networks are vulnerable to a simple jamming technique that could block service across much of a city, according to research findings provided to a federal agency last week.

The high-bandwidth mobile network technology LTE (long-term evolution) is rapidly spreading around the world. But researchers show that just one cheap, battery-operated transmitter aimed at tiny portions of the LTE signal could knock out a large LTE base station serving thousands of people. “Picture a jammer that fits in a small briefcase that takes out miles of LTE signals—whether commercial or public safety,” says Jeff Reed, director of the wireless research group at...