# On End-to-End Intelligent Automation of 6G Networks
## 2 6G: Preliminaries and Enabling Technologies
### Enabling Technologies
#### 2.2.3 Network Architecture Technologies  
The third component introduced by 6G networks that has a direct impact on the orchestration process is the new network architecture topologies and technologies. This because these new network architectures need to be managed by the orchestration module. As such, their characteristics need to be considered when orchestrating 6G networks. Among these new network topologies and technologies are non-terrestrial networks and special purpose networks.  
* Non-Terrestrial Networks: As the name suggests, non-terrestrial networks are defined as the set of networks that are deployed off-land. More specifically, it is the set of networks deployed under water, in the air, or even in space [51]. One example of such networks is unmanned aerial vehicle (UAV) networks [52]. In these networks, these UAVs or drones can either act as base stations, cellular users, or signal relays [52]. The main role of these NTNs is providing extended coverage to areas that are generally inaccessible or have harsh conditions. As such, by combining these NTNs with terrestrial networks, the vision of extreme coverage and extreme connectivity in 6G can be achieved.
* Special Purpose Networks: Another important type of networks that is emerging as a result of 6G is special purpose networks, commonly referred to as \"sub-networks\" [53]. These networks typically belong to one vertical industry, such as industrial automation, autonomous vehicles, and health-focused body area networks [54]. The role of such networks is to achieve the goal of converging the digital world with the physical and human worlds. As such, they are key enablers of 6G networks given that they offer added connectivity and potential intelligence to 6G networks.
* Software-Defined Networks (SDN): In addition to the aforementioned newly emerging networks, another enabling technology that already exists and will continue to play a pivotal role is Software-Defined Networks (SDNs). The basic premise of SDNs is separating the control and data planes using a centralized logical intelligence [55]. As a result, network management is facilitated, and programmability is introduced, in addition to flexibility, scalability, and robustness being offered to the network [55].  
Within the context of 6G, SDNs continue to be adopted and proposed as a key network architecture choice [56]. More specifically, the SDN controller is being proposed as the \"brain\" within the control layer of 6G networks [56]. This component is being extended to the non-terrestrial portion of 6G networks, such as space, air, and even sea-based networks.  
Hence, SDNs are still considered to be crucial to 6G networks by extending the paradigm to the new network types and components. However, a key challenge/issue to consider, particularly in 6G networks and the corresponding diverse sub-networks comprising them, is the choice of SDN deployment architecture. More specifically, choosing between a centralized controller architecture (better orchestration/management decisions at the expense of higher signaling overhead and less reliability) and a distributed controller architecture (less signaling overhead and better reliability at the expense of less optimal orchestration decisions) is a critical choice as it can have a direct impact on many of the desired performance metrics [57].