# Multi-Layered Satellite Communications Systems for Ultra-High Availability and Resilience
## 4 System Model: MLSS Concept
### High-Level Architecture (HLA)  
The proposed high-level architecture of the MLSS is depicted in Figure 4. The space segment has been abstracted in this high-level architecture. The proposed HLA assumes that all constituting Access, Core, and Transport networks are virtualized (vRANs) and programmatically controlled (SDNs). The orchestration and resource management used in various networks have a hierarchical distributed architecture. Following the 5G satellite-terrestrial convergent network architecture designed for backhauls [44; 45] and extended for the new 5G NTN connectivity, the MLSS is divided into several main technology building blocks called macroblocks:  
1. User Segment (User Terminal/Equipment) and Edge;
2. MLSS Access Network, with Space Segment and Ground Segment;
3. Core and Transport Networks;
4. Service Orchestration;
5. Business Support Infrastructure (incl. Product Catalog, Order Management, Billing, etc.). The edge network is connected to the radio access network through the user terminal. The user terminal can connect to multiple types of access networks. The access networks that form the MLSS Access Network can be terrestrial networks (4G/5G) or non-terrestrial multi-orbit, multi-band, or multi-waveform satellite access networks. Each access network has its own local management functionality, but the behavior of these local managers is controlled and orchestrated by the global orchestration functionalities. Each component of RAN has its own radio resource management functionalities, but these resource managers cooperate with each other to maximize the global capacity. A centralized global resource manager module coordinates the activity of the distributed radio resource manager modules. Each macro-block can be further subdivided into component blocks or sub-systems, in line with the MLSS high-level architecture (HLA):  
**User segment and edge** consist of UT/UE modems that connect to the edge connection manager and application layer subsystem over the end user interface (EUI). The software is run on the edge HW platform. The endpoints can connect to the user segment with an ethernet or wireless connection. **MLSS access network** is responsible for the MLSS air interface. It includes the space segment, i.e., satellites, and the communications payloads in different orbits as well as terrestrial base stations. The ground segment with the RAN infrastructure provides the  
Figure 4: System architecture. Boxes with light blue color are the macro-blocks and the dark blue color boxes represent functional components or sub-systems. Circular shapes are entities outside the actual MLSS architecture. Solid lines represent connections between functional components. Dashed lines are used to highlight virtual connections and sub-blocks inside the macro blocks. control plane and data plane functionalities for the access networks. Radio resource management (RRM) is performed with global and local resource managers who manage the whole network as well as optimize local resource use. **Core and transport network** hosts the transport and NFV infrastructure, enabling SDN capabilities. It is home to the core network of the system. In addition, the SDN transport controller is in this block, and together with the local orchestrator it manages the whole SDN network and its service functions. The meet-me-point connects the MLSS network to the external networks and internet services. **Service orchestration** is the process of designing, creating, delivering, and monitoring network services in an automated way. It is performed with global and local policy framework functions that connect with the RAN control plane and resource managers. The end-to-end orchestration of network services and connections is performed by global and local orchestration blocks. The NFV controller runs the virtual machines that replace the traditional network appliance HW. **Business support infrastructure** is a critical functionality of the MLSS operator.