# A Novel Decomposed Optical Architecture for Satellite Terrestrial Network Edge Computing
## 2 Literature Review and Contribution
### Motivation and Contribution  
Despite the fact that the decomposed computing architecture can provide a more flexible and efficient terrestrial network with faster application runtime for STN, the interconnection and communication among hardware cubes are still challenges, which requires low access latency and high aggregated bandwidth. Existing electrical network-based solutions is based on commercially available hardware, which needs to be compatible with the current on-board bus protocol. This inherent high latency due to the on-board peripheral I/O connection and hierarchical server-to-server interconnect network leads to degraded application performance, while repeatedly photovoltaic conversions deteriorate the energy efficiency of decomposed computing architectures. The optical switches-based network can avoid photovoltaic conversions and achieve a high aggregated bandwidth of up to hundreds of gigabits. However, the long reconfiguration time (up to milliseconds) limits the flexibility of the decomposed computing architectures. A stable and fast switching interconnect network is significant for the decomposed terrestrial computing architecture of STN. Our previous work in [25] only numerically assessed the application performance of NOS-based decomposed computing architecture, and the impact of optical network switching based on NOS as well as network performance must be experimentally investigated to validate the feasibility and scalability of the proposed decomposed architecture.  
The contributions of our work are summarized as follows:  
1. A four-node decomposed computing prototype is experimentally implemented in this work, consisting of two processor nodes, two memory nodes, and one four-port NOS. The hardware cubes are implemented utilizing FPGA chip, and two independent interconnect channels are designed between hardware cubes and NOS for sending optical payload and signal tags respectively.
2. The physical and network performance of the decomposed computing prototype are investigated in experimental assessments. In the physical assessment, the SOA-based optical gates achieve ON/OFF switch ratios larger than 60 dB and ensure low inter-channel optical signal interference. The implemented prototype provides a none-error transmission among decomposed hardware cubes with 0.5 dB power compensation at a BER of 1 \(\times\) 10\({}^{-9}\). Meanwhile, in the network performance evaluation, the prototype performs an end-to-end latency of 122.3 ns and zero packet loss after link establishing.  
3. As the NOS port count directly impacts the scalability and feasibility of decomposed architectures, we further investigate the physical performance in terms of output OSNR and required power penalty as a function of the NOS port count. The proposed decomposed architecture provides an output OSNR of up to 30.5 dB under the NOS port count of 64. Scaling the NOS port count to 64, an error-free operation with a power penalty of 1.5 dB is achieved.
4. The scalability of the NOS-based computing network with decomposed hardware is also evaluated in this work. Based on the experimentally measured parameters, the network performance of the NOS-based decomposed architecture is also numerically assessed under different network scales and link bandwidths. The results show that with a scale of 4096 hardware cubes and a memory cube access rate of 0.9, an end-to-end latency of 148.5 ns inside a rack and an end-to-end latency of 265.6 ns across racks are obtained under a link bandwidth of 40 Gb/s.