of some open source projects (e.g. Linux), it is natural to consider applying the same concept in the telecom industry. However, at the time of writing this book, most open source telecom projects focus on the core network and OAM, while open source RAN implementations appear to be in their infancy - that is, as of now there are no actual 3G, 4G, or 5G networks deployed based on open source implementations. Nevertheless, it is an interesting development that may become more important in the future. 6.3.1 Key Ideas Open source projects, e.g. Linux, have been tremendously successful in a number of domains: enterprise servers, cloud, and mobile phones to name a few. Inspired by that success, the telecom industry is interested in developing open source NG-RAN imple- mentations. Open source software is usually distributed under a number of different open source licenses. These can be categorized as "permissive" and "copyleft." While most open source licenses do not allow patent royalties, the Open Air Interface (OAI) license does. Open source RAN is made possible through the implementation of an SDR concept. In an ideal SDR implementation, everything but the RF, A/D, and D/A is implemented in software, running on general purpose (typically x86) hardware. 5 Richard Stallman (rms), is a free software movement activist. He is the original author of the GNU Software project, which produced many software packages which are in wide use to this day, e.g. the GNU Compiler Collection (gcc). 6 Linus Torvalds is the original author of the Linux kernel. 7 Linux Foundation is a non-profit organization created to support and promote Linux. It is also involved with many other open source projects. 6 Enabling Technologies A number of open source RAN projects exist, with the most advanced one being OAI, which is also the only open source project with an active 5G (i.e. NG-RAN) development. OAI is based on LTE Release-8, with select features from later releases, such as: LTE-M, NB-Internet of Things (IoT), and D2D. The code base contai