Summary：Passengers on two Shenzhen subway lines cried foul yesterday after authorities abruptly shut down 3G service to determine what has been causing trains to unexpectedly stop between stations.

Passengers on two Shenzhen subway lines cried foul yesterday after authorities abruptly shut down 3G service to determine what has been causing trains to unexpectedly stop between stations.

Shenzhen Metro, which had been at a loss to explain the sudden train failures on the Huangzhong and Shekou lines, said it cut off the wireless signal for 24 hours in order to test a theory that interference from mobile wireless networks was to blame.

Almost every subway on the mainland uses the 2.4-gigahertz frequency for its wireless train operating system, the same frequency used by consumer electronic devices.

The outage infuriated passengers accustomed to fiddling with their wireless devices during lengthy commutes.

"Suddenly my iPhone returned to [the] Stone Age, with [the] capability only to make phone calls and send text messages," said Lai Mingjuan, who has an hour-long trip between her downtown office and home in the suburbs. "Without an internet connection I don't know what to do with my hands."

The 3G suspension added to public concern that Shenzhen Metro would seek to permanently end mobile data services on the trains. The Sina Weibo microblogging service was buzzing with passenger complaints and opposition to any long-term disruption.

The Shenzhen Metro company issued a brief statement on its own Weibo account at noon, saying that the suspension had been approved by the city government. It apologised for the inconvenience to passengers.

The transport company blames the emergence of portable Wi-fi routers, which allow consumers to create mobile wireless networks using a single 3G SIM card. It believes interference from the devices triggers the trains' emergency brakes.

"The problem for Shenzhen Metro is a new problem caused by new technology - it's not our fault," Shenzhen Metro spokesman Wang Yuzhu said.

Telecommunications authorities initially resisted the transport company's calls to shut down the 3G service, suggesting that the metro had attempted to save money by not buying a dedicated wireless band.

But some experts predict similar problems will occur in other cities as the portable routers become more popular.

"Portable routers appeared on the [Chinese] market last year," Wang said. "More than 80 per cent of them were sold and used in Shenzhen. It becomes a problem as more and more people use them."

A Beijing-based railway communications expert said there were many solutions but all were expensive.
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