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Pocket Wi-Fi hotspots paralyse Chinese metro lines

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Shenzhen Metro is blaming customer Wi-Fi for disruptions to its service. The subway system for the city of Shenzhen in Guangdong province, China, depends on the unlicensed 2.4GHz band to link up its signalling systems.

Following network failures in October, and a trial blocking of 3G signals earlier this month, the Shenzhen tube operating company wrote to China's regulator asking for permission to block the signal. Caijing magazine reports that permission has now been refused, leaving Metro bosses at a loss on how to resolve the issue - which has seen two lines of the network repeatedly shut down and threatens other systems around China.

Customer Mi-Fi devices create Wi-Fi hotspots that are backhauled over China Mobile's 3G network, and they're very popular, particularly in Shenzhen - which, the South China Morning Post tells us, accounts for 80 per cent of sales. That's the legit kit, which only nudges the 100mW legal cap, but engineers trying to keep the network running reckon black-market devices are kicking out three times that amount. They add that once eight of either kind come into range then the Metro's signalling system stops.

2.4GHz is reserved, globally, for unlicensed ISM (Industrial, Scientific and Medical) use, largely because it was considered worthless as it gets absorbed by water and because the band is rife with interference from microwave ovens. However, radio is a lot cleverer these days, and Wi-Fi is squeezing every cent out of the spectrum while Bluetooth dances around it, and numerous door locks, remote controls and other consumer devices fill any gaps which remain.

Originally it was the unlicensed nature of the band which made it so popular, but these days it is also the low cost of the kit. International standardisation means a Wi-Fi router, Bluetooth headset, or just a radio chip, can be sold anywhere - providing massive economies of scale.

There's also the freedom from regulatory process. Set up a link at 5.8GHz and (in the UK) you'll have to fill in forms and register each transmitter, but do the same thing at 2.4GHz and there's zero paperwork, making deployment quicker and cheaper.

The combination of these things drove Shenzhen Metro to connect up its signalling system at 2.4GHz, only to discover that it is now polluted with customer connections.

And Shenzhen is far from alone in its plight, as the same band is used by metro systems all over China, which will similarly fail once Mi-Fi devices become popular.

Blocking the 3G signal shuts down the devices, but it's hardly a sensible solution as it aggravates commuters. However, shifting to a licensed band will be expensive - both in terms of the equipment it will require and the frequencies in which it can operate. ®

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