commercial offerings. As of now, they provide a good platform for research and experimentation. 6.3.5.1 srsLTE srsLTE is an open source LTE implementation, produced by a commercial entity, however released under a permissive GNU Affero General Public License. It contains the following components: srsUE - an open source UE implementation; srsENB - an open source eNB implementation; srsEPC - a lightweight open source EPC implementation with mobility management entity (MME), Home Subscriber Server (HSS), Serving Gateway (S-GW), and Packet Data Network Gateway (P-GW). The srsENB is based on Release-10 3GPP specification, implementing some basic features of an eNB, with S1 interface connectivity to EPC. 6 Enabling Technologies Since as of now it is unclear whether a 5G implementation will emerge based on this project, this is mentioned as background information about open source activities related to RAN. 6.3.5.2 OpenLTE OpenLTE is yet another open source implementation eNB, with a "built-in" simple EPC. Unfortunately, there has been little to no activity in that project since 2017 and it appears to be unlikely that new versions will be produced, let alone a 5G implementation. 6.3.5.3 OpenBTS OpenBTS is an open source 2G (GSM) base station implementation developed by Range Networks. It is distributed under the copyleft GNU Affero General Public License v3 and as such can be considered as a free software. It is perhaps the only open source cellular base station implementation that has been successfully deployed in the field. That being said, OpenBTS is limited to 2G and as of now there appears to be no inten- tion to evolve it into more advanced 3GPP releases. Therefore, it is also mentioned here for background information only. 6.3.5.4 Open Air Interface OAI is currently the most advanced open source RAN implementation and also the only one where active 5G development is taking place. It is based on LTE Release-8, with select features from later releases, such as: LTE-M, NB-IoT, and D2D. From the architecture point of