OpenBTS replaces the conventional GSM operator core network infrastructure from layer 3 upwards. Instead of relying on external base station controllers for radio resource management, OpenBTS units perform this function internally. Instead of forwarding call traffic through to an operator's mobile switching center, OpenBTS delivers calls via SIP to a VOIP soft switch (such as FreeSWITCH or yate) or PBX (such as Asterisk). This VOIP switch or PBX software can be installed on the same computer used to run OpenBTS itself, forming a self-contained cellular network in a single computer system. Multiple OpenBTS units can also share a common VOIP switch or PBX to form larger networks
The OpenBTS Um air interface uses a software-defined radio transceiver with no specialized GSM hardware. The original implementation used a USRP from Ettus Research, but has since been expanded to support several digital radios in implementations ranging from full-scale base stations to embedded femtocells.
I need someone to make a cheap device that makes a mobile network.