# Internal Backpressure for Onboard Crospoint-Buffered Switches with Port Multiplexing
## 1 Introduction  
The Space-Air-Ground Integrated Network, achieved through the deep integration of satellite, aerial, and terrestrial networks, possesses inherent advantages such as high throughput and robust recovery capabilities [1]. Especially in remote areas, maritime coverage, and disaster response scenarios, despite the fact that terrestrial networks possess more resources to accomplish complex tasks, their limited coverage has rendered traditional terrestrial networks inadequate to satisfy the ever-increasing demand for data transmission [2]. Consequently, the emergence of Space-Air-Ground Integrated Networks has achieved seamless global coverage, significantly enhancing the reliability and efficiency of communication systems. Within this integrated network, satellite networks serve as a pivotal component, offering a worldwide communication environment [3] and becoming the cornerstone for realizing universal connectivity. Research has indicated that in areas where terrestrial networks are inaccessible, leveraging satellites to provide internet services holds immense potential [4]. Traditional satellite communication primarily relied on circuit switching methods; however, with the exponential growth of data volume, this approach has struggled to meet the demands for efficient and flexible data transmission. Onboard switching technology [5] enables the agile processing and forwarding of data packets, thereby enhancing data transmission efficiencyand flexibility. Furthermore, it supports the concurrent handling of diverse service types, making it an indispensable element within satellite networks. The satellite network realizes the data transmission process through the onboard exchange architecture, as shown in Figure 1. In the process of data transmission, when the ground station or air platform needs to send data to other ground stations or air platforms, the data will first be sent to the satellite through the ground communication link or air communication link. After the satellite receives the data, the onboard processor will process the received data. The switching structure then selects the best transmission path based on the destination of the data and the current network state. This selection process needs to be based on complex routing algorithms and network protocols to ensure the efficiency and reliability of the data transmission. Once the transmission path is determined, the switching structure sends the data to the target satellite or ground station via interface equipment. In this process, data may be relayed and forwarded by multiple satellites, and finally, a trans-regional and trans-network transmission is realized, achieving the efficient and reliable transmission of data. In the onboard switching architecture, the performance of the switching fabric used to interconnect multiple input ports and output ports directly impacts the system's metrics such as delay and packet loss rate [6]. The switching structure is responsible for transferring packets from the ingress to the egress. The high-speed switching architecture uses fixed-length packets called cells. Incoming variable length packets are split into units at the input port and reassembled into packets at the output port [7]. Therefore, it is of great significance to study the spaceborne switching structure with high throughput [8]. The crossbar switching architecture [9], known for its expandability and non-blocking characteristics in high-speed switching environments, has become the preferred solu  
Figure 1: Transmission process of satellite network through onboard switching architecture. tion for efficient data transmission [10]. Crossbar switching structures are divided into two categories: unbuffered crossbar switching and internal buffer crossbar switching. Unbuffered crossbar switching, also known as Input Queuing (IQ) [11] switching, uses a centralized scheduler to resolve packet contention at the input and output of the switch.