# A Research Study on Protocol Stack of Space-Based Optical Backbone Network
## 2 Architecture of the Protocol Stack  
The CCSDS protocol stack mentioned above includes the Advanced Orbiting Systems (AOS) space data link protocol, a Layer 2 protocol that provides a framing layer between channel coding and higher-layer link multiplexing protocols. In order to support the design of protocol stack, we expanded the protocols of the CCSDS [11]. Figure 1 shows the model of the protocol stack, where we inserted a label switching sublayer between the synchronization and channel coding sublayer and the data link protocol sublayer. Accordingly, we expanded the structure of the AOS framework. As shown in Figure 2, we inserted label, DCN (data communication network) and OAM (operation, administration and maintenance) fields in the AOS framework, the functions of which are defined as follows:  
1. Label Field: carries a label to support the AOS frame switching scheme. 2. DCN Field: delivers control and management information, such as routing protocol or network management configuration messages. 3. OAM Field: transfers information to conduct functions of network monitoring and detection. Different from the terrestrial network, the space-based optical backbone network, which is mainly composed of satellites and ISLs, is dynamic and unmaintainable because of the satellite characteristics. In addition, constrained by the quantity and volume, a satellite usually has weak performance, which could result in limited resources. That is to say, the protocol stack should support the efficient management and control of resources in order to improve resource utilization. Inspired by the architecture of the automatically switched optical network (ASON) [12], we designed the protocol stack in three function planes: management plane, control plane and transport plane. As shown in Figure 3, the functions of the three planes were designed as follows:  
1. Management Plane: manages the whole network, with functions of configuration management, performance management, and log management, which helps the network manager manage the network efficiently. 2. Control Plane: controls the network automatically, with functions of resource maintenance, routing calculation and path building, which helps the network managers improve their work efficiency. 3. Transport Plane: transports data in the network, with functions of control message transmission, data transmission, service adaptation and AOS frame switching. The management plane and control plane are highly synergistic and can both operate on the transport plane directly to implement the management and control of the network. The transport plane provides underlying support to guarantee the transmission of control information and service data. As shown in Figure 4, it can be seen that each node (satellite or ground station) consists of three parts: a network management agent, control plane modules, and transport plane modules. The internal logical relationship in each plane is designed as follows:  
Figure 1: Protocol stack model. CCSDS: Consultative Committee for Space Data Systems. Figure 2: Expanded advanced orbiting systems (AOS) frame structure. DCN: data communication network; OAM: operation, administration and maintenance. 1. Management Plane: consists of a network management center (on the ground) and network management agent (in each node). The architecture (red dotted line) is a star schema, which means that the center logically manages each agent directly. The center connects to one of the ground stations (blue lighting) and then manages the whole network. 2. Control Plane: composed of control plane modules in all nodes. The architecture (green dotted line) is distributed, which means that the modules in all nodes have the same functions and the relationships between them are logically equal in the control plane. The control plane modules in different nodes collaborate with each other to implement the functions of the control plane. 3. Transport Plane: composed of transport plane modules in all nodes and the architecture is also distributed.