In a well made geek focused viral proof of concept video, United Kingdom based PA Consulting Group, showed the world that you can build a smaller than a book GSM radio tower for less than $800 dollars using a $35 Raspberry Pi hobbyist Linux computer to do it.
PA Consulting Group PA Technology team used custom modified code to work on the Raspberry Pi. But they were able to get open source phone switch software FreeSWITCH running. Then using the OpenBTS GSM phone standards stack, some custom python scripts, and an ETTUS Research USRP B100 software programmable radio they were able to produce their own working GSM phone network. It was a network that could even assign phone numbers to hardware identifiers on phones. It could even transfer the packets of those intercepted GSM phones out to the regular cellular network.
A similar device setup done at famous Las Vegas hacker con DEFCON in 2010 showed a proof of concept of soft radio’s doing GSM phone taping, but the rig at that time cost $8000. Famously an OpenBTS and FreeSWITCH powered GSM cell network was made for the Berlin, Germany hacker camping fest called Chaos Communication Congress. At the recent 2012 DEFCON, GSM Soft radio hobbyist went so far as to make their own private cellphone network called NinjaTel. The NinjaTel GSM network had six hundred and fifty (650) customized HTC Smartphones to show off the concept of independent local GSM services.
NinjaTel Photos from Dan Goodin