introduction
As countries continue to explore outer space resources, the development and utilization of terrestrial Internet technologies continue to mature. The integration of Tiandi communication into the terrestrial Internet and the establishment of a unified data network have become a new research hotspot. This requires a set of efficient and unified space communication protocols to ensure. Under this requirement, the Space Data System Advisory Committee (CCSDS), which is composed of space system research institutions of various countries, has done a lot of work and formulated a series of agreements.
1 CCSDS protocol architecture
CCSDS series of protocols can be classified into physical layer, data link layer, network layer, transport layer and application layer, imitating the layered structure of TCP / IP. Each layer contains multiple protocols. The details are as follows:
(1) Physical layer
At the physical layer, CCSDS has developed a "Radio Frequency and Modulation System" Recommendation. The radio frequency and modulation system for near-Earth and deep-space missions with moderate communication requirements are specified.
(2) Data link layer
The data link layer includes the data link layer protocol and channel coding. The agreement mainly develops subcontracted telemetry and subcontracted remote control protocols, which are suitable for conventional spacecraft data systems and have simple operations; Advanced On-Orbit Data System Protocol (AOS), which is suitable for large spacecraft and has many types of services; short-range space links, It is suitable for space links between spacecrafts close to each other, mainly point-to-point communication. In channel coding, BCH coding, convolutional codes, RS codes, and various data structures of data links have been developed [2].
(3) Network layer
At the network layer, transport layer, and application layer, CCSDS has developed a series of recommendations parallel to the TCP / IP protocol functions, namely the space communication protocol specification SCPS. The starting point is to achieve compatibility and interoperability with Internet protocols as much as possible on the basis of adapting to space communication needs. Based on the Internet protocol, the space environment is improved. SCPS-NP is one of the network layer protocols. Compared with the IP protocol, the main improvements are reflected in the following points: different lengths of guide heads are proposed for users to choose between function and efficiency; multiple routing modes are provided, flooding addressing mode is added; provision is configured by the management mechanism End-to-end routing; in SCPS Control Information Protocol (SCMP), increased signaling for link interruption due to channel quality.
(4) Transport layer
At the transport layer, in addition to the corresponding transport layer protocol SCPS-TP, CCSDS has also developed a protocol for file transfer CFDP. CFDP provides both the transport layer function and the application layer file management function [2]. In addition, an SCPS security protocol (SCPS-SP) with end-to-end data protection capabilities has been proposed, which was developed to improve the Internet IPsec protocol.
(5) Application layer
In the application layer, CCSDS has developed document transmission protocol SCPS-FP, lossless data compression, space subcontracting protocol, etc.
2 SCPS-NP protocol modeling
In order to verify the protocol performance of SCPS-NP, OPNET software is used here for modeling and testing. OPNET adopts a three-layer modeling mechanism: the uppermost layer is the network layer, which reflects the topology characteristics of the network; the second is the node layer, which is composed of the corresponding protocol modules and reflects the characteristics of the device; the lowest layer is the process layer, which uses state machines To describe the protocol, reflecting how the specific functions of the protocol are implemented [6]. According to the three-layer modeling mechanism, the satellite-ground station communication scenario is used in the simulation. Figure 1 shows the satellite node model. The ground station node model is similar to it. Among them, the parts related to the SCPS-NP protocol are:
â‘ The scps_np_encap module, SCPS-NP protocol leader encapsulation module, encapsulates and decapsulates the data;
â‘¡scps_np module, SCPS-NP protocol implementation module, specifically implementing the functions required by SCPS-NP;
â‘¢ router_map module, routing table update module, providing static routing table or dynamically updated routing table.
Figure 1 Simulation node model.
2.1 Process design of scps_np_encap module
The main task of this module is to encapsulate or decapsulate the SCPS-NP leader, and the process design is shown in Figure 2.
Figure 2 scps_encap module process structure
First, initialize the settings in the INIT state, enter the WAIT state, and then encapsulate or decapsulate the packets according to different packet flows. SCPS-NP leader encapsulation length and format are selected by the user in the node attributes. When providing services similar to the space subcontracting protocol, the leader is 4 bytes long, and when providing services similar to the IPv4 protocol, the leader is 20 bytes long.
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