The San Francisco Bay Area Rapid Transit system (BART) shut down subterranean cellular phone service on August 11, stifling protests that had been set to take place on its train platforms that day. Demonstrators had planned to stop trains from running in response to the fatal shooting of an unarmed passenger by the BART police on July 3. But without the ability to coordinate their efforts via cellphones, acts of civil disobedience never crystallized.

In the days since, a media firestorm has ensued over a perceived violation of civil liberties by transit system officials. A hacker group has targeted BART, and the Federal Communications Commission has begun an investigation. All that aside, though: How exactly did BART disable cellular service, and could it happen elsewhere?

On Friday afternoon, BART officials said that they had switched off the transit system's base stations, disabling its underground wireless network. (Officials initially claimed they had asked wireless service providers, such as Verizon and AT&T, to disable service, and later acknowledged that they had cut service themselves). As a result, for approximately three hours, commuters riding BART trains were unable to surf the web, use their phones or dial 911.

BART's wireless service is provided by a company called WiFi Rail. According to its web site, the WiFi Rail network utilizes a backbone of fiber-optic cables that run throughout the underground transit system. These connect a network of wireless access points, routers and switches. Because BART privately owns and operates this underground network, BART officials have the power to switch it off.

The situation is very different above ground, where cellphone service emanates from towers operated directly by Verizon, AT&T and the other wireless service providers. Control of that service doesn't pass through a middleman, as it does underground in the Bay Area.

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