Wikipedia
OpenBSC
OpenBSC is a project to develop a free software implementation of GSM protocol stack and elements. It runs on Linux and requires an E1 interface (ISDN Primary Rate Interface, via mISDN). It is written in C and licensed using the GPL (≥v2) license.
The first version implemented the GSM specification 21.12 and 08.5x, and worked for a specific Base Transceiver Station (Siemens BS11 MicroBTS). The first release of the project took place at the Chaos Communication Congress held in 2008 (25c3, Running your own GSM network).