# Optimizing Time-Sensitive Traffic Scheduling in Low-Earth-Orbit Satellite Networks
## 2 System Model
### Network Architecture  
LEO satellite networks primarily consist of ground terminal users, LEO satellites, and ground control systems, as shown in Figure 1. To efficiently forward TSFs, this paper adopts a SDN-based architecture, decoupling the traditional network into a control plane and a data plane. The data plane handles data forwarding. Its core functions include inter-satellite data forwarding, load balancing, and data buffering, utilizing wireless and optical links to ensure efficient interconnection and low-latency communication between terminals and satellites while effectively coping with network dynamics. The control plane focuses on global network resource management, traffic scheduling optimization, and network topology adjustment.  
Figure 1: Satellite–ground network architecture model.  
The control plane consists of a globally distributed cluster of ground controllers located at gateway stations and lightweight satellite controllers (acting as local agents) deployed on each satellite. The ground controller cluster serves as the global network controller. Its primary responsibilities include managing terminal users. To achieve this, it interfaces with local network providers (LNPs) to obtain essential user context information, such as user location, access point identifiers, subscribed service level agreements (SLAs), and specific QoS requirements for TSFs. Simultaneously, the ground controller cluster leverages precise satellite orbit prediction models and periodically receives local state information (including topology segments, link load, queue status, etc.) reported by satellite controllers via STLs. It performs global optimization calculations to determine end-to-end paths and scheduling strategies. The resulting control information (e.g., routing policies and resource allocation schemes) is proactively distributed to the corresponding satellite controller via STLs during the satellite's flyover of a ground station communication window. This design avoids the additional latency, reliability risks, and bandwidth consumption associated with relaying control signaling through ISLs. The satellite controller's main tasks are to locally execute received control strategies, monitor real-time onboard status (including link state and load), manage ISLs and STLs, and perform data forwarding. Notably, when communication with the ground controller is interrupted or experiences high latency, the satellite controller can make localized decisions based on predefined policies, such as switching to backup paths or implementing local congestion control.  
The data plane comprises onboard satellite switches and ground access equipment. Its core function is to perform real-time packet forwarding, load balancing, and buffer management locally using wireless and optical links, dynamically adapting to network changes and ensuring low-latency communication. The central research focus of this paper is the queue scheduling mechanism implemented within the data plane, aiming to guarantee the efficient forwarding of TSFs. The control plane issues routing policies, resource allocation schemes, and priority parameters to data plane components via SDN protocols (primarily transmitted over STLs). This collaborative operation between the data plane and control plane ensures the effective utilization of network capacity and traffic stability, thereby efficiently supporting the forwarding of diverse traffic flows, including TSFs.