Multiple SDN Controllers Placement for Integrated Satellite/Terrestrial Network

The integration of Terrestrial Networks (TN) and Non Terrestrial Networks (NTN) has been explored within the 3GPP standardization forum, and it is now being extended toward discussions on the future 6G vision. An integrated T/NTN is highly heterogeneous and requires different communication protocols and links for each layer, resulting in increased network management and control complexity. The Software Defined Networking (SDN) paradigm can enable unified and efficient T/NTN management, allowing full resource optimization of the satellites, radio access and core network. This paper proposes an optimized SDN-based T/NTN architecture, where Low Earth Orbit (LEO) satellites are dynamically selected to act as multiple SDN controllers if the terrestrial network becomes saturated or unavailable, or to jointly operate with the terrestrial controllers under the coordination of a central terrestrial controller. From this perspective, the number of the SDN controllers and their placement are of paramount importance. A multi-controller placement strategy is evaluated for the integrated T/NTN using the Simulated Annealing (SA) plus Tabu Search methods to search for the optimal solution in terms of average latency and SDN controllers load, while accounting for the frequent topology variations inherent to LEO satellites. In addition, the design of the SDN architecture for an integrated T/NTN system, and in particular the definition of a distributed SDN control plane (CP) across both the terrestrial and satellite segments, including the specific mechanisms required to enable LEO satellites to operate as controllers, is also addressed. Extensive simulations based on realistic T/NTN topologies, specifically, the terrestrial Agis network and the Iridium NEXT satellite constellation, show that the use of multiple controllers in optimized placements both in terrestrial and satellite segments decreases the average latency and balances the load of each controller. In addition, the proposed controller switching policy adopted for the LEO segment helps to avoid frequent reassignments and improves the reliability of the overall integrated system.