# Towards the Quantum Internet: Satellite Control Plane Architectures and Protocol Design
## 3 System Model
### Presented Architecture  
Several satellites in low orbit are considered in the presented architecture to overcome the distance problem, as shown in Figure 2[2; 22; 45]. The use of a dense constellation close to the Earth's surface allows the distance problem to be addressed, as described in [45]. In fact, as explained in [2], the probability of success in entanglement generation decreases exponentially as a function of distance, and the time required to generate the entanglement between two adjacent nodes strongly depends on this probability and the characteristics of the medium. Therefore, in our paper, we considered a single LEO constellation in which all elements can embed SDN controllers. The elements that are part of the DP--i.e., the QSRs and the GSs, as depicted in Figure 2--operate in the frequency range of FSO, described in [22]. Atmospheric factors and beam wandering limit the performance of FSO significantly [39], especially regarding Earth-to-satellite communications, but adequate performance can be achieved in fair weather conditions using the 193.415 THz frequency. Optical technology can be used not only for the quantum channel but also for the control channel. In fact, all nodes in a quantum network are assumed to have classical connectivity between each other in order to perform background protocols, such as path selection, as well as signaling protocols to set up the E2E entanglement generation [49]. Furthermore, at the considered frequencies, the space vacuum has a much higher attenuation length of 9,261,376 compared with optical fibers. For these reasons, we have considered the realization of a satellite backbone managed by an MCS on the ground with an SDN controller embedded that derives the matrices of intersatellite distances. Then, a _graph-based optimized path selection algorithm_ that calculates the best path is applied to the derived matrices [45]. The MCS calculates and manage the best path while communicating with the satellites through the southbound APIs. This architecture, shown in Figure 2, also includes the use of multiple controllers embedded in the constellation, whose proper placement helps to reduce the delays between them and the satellites acting as QRs, thus making it possible to completely avoid the terrestrial routes and, in particular, the Earth-satellite link, which is the most critical due to atmospheric phenomena. As explained in [50], the starting point from which to start generating Bell pairs [51] affects the speed of entanglement propagation over the entire path. A first attempt which considers the use of SDN technology to address this problem was made in [22], in which an architecture is described wherein the satellite _in the middle_ is detected by a single controller on the ground, which sends the necessary instructions to the selected satellites to start the propagation procedure, interfacing with them through the southbound APIs.  
However, with the proposed architecture shown in Figure 2, which uses a modular two-tier CP based on SDN, it is also always possible, thanks to the intervention of the MCS, to activate the control process on an appropriate satellite along its domain, which is composed of several satellites that compose the path. The satellites that delimit the borders between one domain and another are identified as _border QSRs_, highlighted with a red circle in Figure 2. The satellite controller manage the operations of entanglement generation and swapping, reducing both the time required for the propagation of the L2L entanglement and the propagation delays of signaling packets. The placement of the controller in the middle of the path section, as depicted in Figure 2, is fundamental to optimizing Bell pair propagation and minimizing packet delay to and from the Controller. Moreover, it is possible to avoid the satellite to ground link, which is critical. In the following section, we describe in detail the functioning of the protocol and the messages that are exchanged.