# Quantum-Key-Distribution (QKD) Networks Enabled by Software-Defined Networks (SDN)
## 5 Three-Use Cases in QKD Networks
### Survivability of the Networks  
Similar to classical optical networks, the survivability of QKD networks is attracting more and more attention. The secret-key provisioning services in QKD networks can normally satisfy the security demands of users. As an inevitable problem, a single link failure will interrupt the services in failure-affected links, indirectly causing a large amount of capacity. Therefore, how to protect secret-key provisioning services in QKD networks is an important problem. This section provides two protection schemes (i.e., secret-key dedicated protection and shared protection) against quantum network failures. As shown in Figure 9, secret-key dedicated protection can allocate dedicated wavelengths in different working and protection paths for each service, and then performs QKD on two different paths simultaneously. When a link failure occurred in the working path, QKD can also be processed on the protection path to generate secret keys. Compared with the dedicated protection, secret-key shared protection can improve resource utilization by sharing the protection resources among several services. When link failures occur in the network, secret-key provisioning services can be protected by using extra pre-provisioned network resources.  
To achieve the protection, there are three sub-problems that need to be considered in the protection schemes. Specifically, the first sub-problem is that links in working path and protection path may face same type of failure risks, since the secret-key provisioning services are transmitted in a general topology. Moreover, the calculation of working path and protection with different failure risk will  
Figure 8: (**a**) The ratio of successful QKP construction, (**b**) the resource utilization.  
Figure 9: The protection in QKD networks, (**a**) a link failure occurred in the network, (**b**) the protection in the network.  
accumulate a large number of limitations about failure risks; thus, the second sub-problem is high blocking probability caused by the limitations. In addition, different secret-key rates in different links are changing over time, so how to generate secret keys in a path to meet security demands is the third sub-problem. Based on the above sub-problems, this paper proposes two protection schemes of secret-key provisioning services, and their working and protection resources are calculated by RWKA. First, a dedicated protection algorithm is proposed, which allocates working and protecting resources for the services. Second, to make full use of resources, we designed a shared protection algorithm to share protection resources among several services.  
To verify the effectiveness, we conducted a simulation of the two proposed protection schemes in terms of blocking probability, resource utilization, and secret-key consumption. As shown in Figure 10a-c, results show that the adaptive shared protection algorithm reduces the blocking probability of the dedicated protection. This is because a smaller key update cycle will increase the blocking probability, which can be reduced by increasing the maximum sharing threshold, as shown in the results. Therefore, there is a trade-off between survivability and security. In addition, these results also proved that our proposed algorithm is an effective way to provide the protection, while this process will accelerate secret-key generation.