There are currently (to my knowledge) two working open-source/free-software implementations of the GSM stack. OpenBTS (http://openbts.sourceforge.net/ http://o

There are currently (to my knowledge) two working open-source/free-software implementations of the GSM stack. OpenBTS (http://openbts.sourceforge.net/ http://openbts.sourceforge.net/) puts everything, beginning at the modulation/demodulation of the radio-frequency, into your PC and uses a USRP (http://www.ettus.com/products http://www.ettus.com/products) as the radio-frequency frontend (~$2000). OpenBSC (http://openbsc.gnumonks.org http://openbsc.gnumonks.org) uses a different approach and starts implementing at the Abis-interface, which is what the Base Transceiver Station (the thing with the antennae...) connects to. Here you don't need the USRP but a surplus GSM BTS (currently around or a little less than $500/€500 and the necessary E1-interface (for the Abis-link) is $200/€200, afaik). OpenBSC has been successfully tested at https://wiki.har2009.org/page/GSM https://wiki.har2009.org/page/GSM and http://events.ccc.de/congress/2009/wiki/GSM http://events.ccc.de/congress/2009/wiki/GSM using officially allocated frequencies.