# Dynamic Routings in Satellite Networks: An Overview
## 3 System Model and Characteristics
### Key Technologies  
(1) Inter-Satellite High-Speed Transmission Technologies: Due to the high transmission rate and small terminal volume, inter-satellite laser technologies have become the leading choice for constructing inter-satellite links. Microwave signals can be modulated on an optical carrier by frequency modulation. There are two corresponding reception nodes: direct intensity detection and coherent detection. The technical scheme of intensity modulation and direct detection (IM/DD) is economical and straightforward, but it is easy to introduce noise. The modulation frequency is low, and the receiving sensitivity is relatively poor. Terahertz communication has a widely available spectrum and simple beam-tracking, making it one of the most important future technologies for high-speed transmission and networking in space.  
(2) Dynamic Routing Technologies: The movement of LEO satellites makes the topology of the whole network change constantly, and the frequent link handoffs between satellites bring new challenges to the design of spatial routing protocols [20]. A routing algorithm based on a snapshot sequence is a mature algorithm in the satellite constellation. The algorithm divides the topology of satellite networks into several individual snapshots, and the constellation topology in the snapshot is stable and predictable. Therefore, we are able to precompute the routing table in each snapshot and continue switching.  
(3) Onboard Processing Technologies: Onboard processing (OBP), onboard switching (OBS), and onboard routing (OBR) are among the technologies. In order to realize the constellation broadband system, effectively connect to the terrestrial broadband network, and meet the service level and quality of service requirements of the user, onboard processing technologies are crucial. Onboard processing technologies are used to realize the upgrading of hardware and software technologies. The hardware technologies mainly include surface acoustic wave (SAW) filtering channelization, fast switching technologies, etc. Software technologies include virtual nodes, cross-layer resource allocation, and scheduling algorithms.  
(4) Inter-Stellar Link: Inter-stellar link (ISL) is the foundation of constellation communication, and connecting satellite nodes. With the development of science and technology, laser communication technology is gradually being applied in inter-stellar link communication, resulting in a significant increase in ISL bandwidth. The laser reduces the communication delay significantly. However, the ISL of laser communication has stringent requirements for satellite attitude control. The slight instability of satellite attitudes will result in a disruption in the communication.  
(5) Dynamic Network Slicing: The motivation of network slicing in a space information networks (SIN) is to allocate as few resources as possible to satisfy the end-to-endtransmission demands. Theresource requirements vary with the types of services. For delay-sensitive services, such as video communication, high reliability and low delay paths are required to support the real-time transmission, but the delay-tolerant services, such as Earth observation, require a high data rate with a tolerant time delay. Therefore, for the different types of services, accurately acquiring the status of the available resources and the flexible scheduling of the multi-dimensional resources are the core issues of dynamic network slice design.