Software Defined Radios(SDR) consist of function units that process the analog RF signals and of processor units. The RF-based function units can be realized by universally applicable function modules, the Universal Software Radio Peripheral (USRP).
The USRP modules are often equipped with plug-ons that open up application possibilities for different frequency bands. Since they often work with open source software, the user can further develop the software according to his needs. There is also GNU licensing for Software Defined Radios based on Linux, Windows, Mac OS X or other operating systems.