## 2 Principles and Methods
### The Design of SDN-Based ISTNs
#### 2.3.2 The Working Principle of the Data Plane in the Semi-Physical Platform  
The adaptation of IP and/or non-IP data to AOS frames using a combination of TCP/IP and AOS protocol is one of the latest research topics, specifically the implementation of seamless communication and connection of ISTNs. Both terrestrial networks and on-satellite local area networks (LANs) receive and send data using the Ethernet protocol at the data link layer, which is customized to the CCSDS AOS data link layer for inter-satellite and satellite-terrestrial links, as illustrated in Figure 6.  
Figure 5: The transmission of a fixed bit length managing and controlling message in the **data communication network** field of the AOS frame. Figure 6: The protocol framework of ISTNs. In this paper, we propose the label-based AOS frame switching mechanism for the Ethernet IP data service. However, as stated in [12], four sorts of data services may be transmitted by AOS frames, i.e., IP service, bitstream service, virtual channel access service, and packet service. Figure 7 shows the functional module diagram of customizable data services over the AOS frame. The user interfaces in the ingress label edge router support various types of data access and provide data caching buffers for parallel data pre-processing, where data is classified and transmitted to the corresponding service buffers before being encapsulated as payload in the **transfer frame data field** of the AOS frame. According to the forwarding information base (FIB), the out-label is pushed to the **label** field of the AOS frame before being sent to the transmitter module, and then those parallel data are serialized to 8b/10b encoding for the transmission in the physical channel, e.g., radio and laser;  
Data are recovered to parallel encoding at the label switching router before being transmitted to the AOS switching module, where the value of the in-label of the AOS frame is swapped to the value of the out-label and the AOS frame is switched to an out-port according to the label forwarding information base. Then, the parallel-serial encoding for the AOS frame is processed;  
If the value of in-label is **0 xFFFF**, it indicates that the egress label edge router is the last node in the label switching path without further label swapping processing, and the payload in the **transfer frame data field** of the AOS frame should be de-encapsulated to the corresponding data service buffers, and then transmitted to the corresponding user interfaces, and vice versa for the backward label switching path. The FPGA board designs are as follows:  
Matching data rates. The FPGA board is designed to better match the varied data rates for services and throughput of QoS requirements:  
\[\text{Throughput}\ =\ \text{clock frequency}\ \times\ \text{width of FIFO}\ \times\ \text{depth of FIFO}. \tag{1}\]  
For example, the clock frequencies for gigabit Ethernet (GE) and 10-gigabit Ethernet (10 GE) are 125 MHz and 156.25 MHz, respectively, with parallel data widths of 8 bits and 64 bits and 1 depth of first in first out (FIFO) buffer. Furthermore, the FIFO's configurable depth is designed to fulfill line rates and the QoS requirement (i.e., throughput) for label edge routers and label switching routers along the label switching path;  
Synchronization of data. Only two oscillators with frequencies of 50 MHz and 100 MHz are used in the FPGA board for the simplicity of hardware design, i.e., the oscillator with the frequency of 100 MHz serves as the main working clock source  
Figure 7: The functional block diagram of four types of service over AOS frame. for the FPGA chip (i.e., XILINX KINTEX-7 (XC7K325)), and the oscillator with the frequency of 50 MHz serves as the input clock source for the programmable clock chip (i.e., Silicon Labs SIS5338). So, FPGA asynchronous FIFOs are used for synchronizing data across multiple clock zones.