## 4 Case Study of Interworking of QKD Networks and the Role of SDN Orchestrator
### Implementation Results of SDN Controller and SDN Orchestrator for KOREN
#### 4.2.1 General Configuration and Overview of the Use Case  
The specific use case implemented in KOREN is as follows: KOREN exists solely as a separate and independent provider of QKD keys, while the actual two QKD networks generating the keys are operated by separate QKD network operators. The two operators of the QKD networks operate in disparate geographic locations. KOREN endeavors to interconnect its network infrastructures, with the aim of facilitating a more extensive dissemination of QKD keys in the wider region. In other words, in the case in which KOREN utilizes the networks of two different QKD network operators in adjacent regions where the two QKD network operators' networks are connected, it is necessary to coordinate the transfer of QKD keys. To implement this, each QKD network operator installed their own QKD SDN controller (QKDNC) responsible for managing their QKD network, and the project was carried out in the form of having a QKD SDN orchestrator (QKDNO) responsible for managing the two QKDNCs in KOREN. KOREN is a service provider that provides actual QKD keys, so this can be also viewed as a QKD provider utilizing different QKD vendors to integrate regionally separated networks from a different perspective. The network topology of the testbed implemented between 2020 and 2022 is depicted in Figure 1. The details of implementation of the testbed deployed by SK Telecom are provided in Table 1.  
Figure 1: Network topology of the testbed. The QKD network under consideration comprises multiple nodes, some of which belong to different QKD network operators. Specifically, the nodes highlighted in orange and light blue correspond to QKD nodes associated with distinct operators. Border nodes, which interconnect different QKD network operators, are also present, with nodes Pangyo and nodes Daejeon being examples of such nodes. Specifically, the use of border nodes allows for the establishment of secure connections between different QKD network operators, thereby expanding the range of available network resources and enhancing network resilience. Such interconnections can be particularly beneficial in scenarios in which a single QKD network operator may not be able to provide sufficient coverage or when interconnecting networks can lead to increased security and reliability. A QKD node configuration and the links between nodes such as the QKD link and KMS link and an encryptor link between QKD nodes are depicted Figure 2. This is a typical QKD node configuration that links nodes, such as the links between Seoul and Suwon in Figure 1. Figure 3 shows a photo of a QKD node in SK Telecom in Seoul as an example. The quantum bit error rate (QBER) and the key rate of each link which was actually measured in the testbed are shown in Figure 4.  
A use case tested for the interoperable key management configuration with a border node between two different QKD networks with each different vendor is depicted in Figure 5, which is described in clause 3. An example of this configuration corresponds to the links among Seoul-Pangyo-Icheon in Figure 1, where Pangyo is a border node. There are no QKD link or encryptor link in the border node, which contains only two KMSs which each belong to each different QKD network operator and are interconnected to deliver the keys between the two different QKD networks. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Section** & **Distance** & **Protocol** \\ \hline Seoul–Pangyo & 40 km & BB84 with decoy state \\ \hline Seoul–Suwon & 66 km & BB84 with decoy state \\ \hline Suwon–Pangyo & 39 km & BB84 with decoy state \\ \hline Suwon–Cheonan & 82 km & BB84 with decoy state \\ \hline Cheonan–Daejeon & 100 km & BB84 with decoy state \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Details of implementation of testbed implemented by SK Telecom, as described in Figure 1.  
Figure 2: QKD node configuration and QKD/KMS/encryptor links. Figure 4: _Cont_. Figure 3: QKD node in SK Telecom in Seoul. Figure 4: QBER (left side) and key rate (right side) of each link in the testbed.