# Survivable Deployments of Optical Sensor Networks against Multiple Failures and Disasters: A Survey
## 3 Sensing Connectivity and K-Node (Edge) Sensing Connectivity
### K-Node (Edge) Sensing Connectivity  
The definition of \(k\)-node (edge) content connectivity describes the relationship between content connectivity and the node (edge) set, which might be simultaneously affected by one disaster event. The \(k\)-node (edge) content connectivity of optical data center networks becomes an important measure of its survivability against natural disasters. [PERSON]'s theorem is the foundation of network connectivity. In contrast to \(k\)-node (edge) content connectivity, the concept of \(k\)-node (edge) sensing connectivity,  
Figure 7: Network and content connectivity: (**a**) video service provisioning through one end-to-end light path; (**b**) video service provisioning through end-to-content light paths; OXC: optical cross connect equipment.  
which ensures the connectivity between sensing data and users, is proposed for OSNs. The \(k\)-node (edge) sensing connectivity can be defined where there does not exist a set of \(k-1\) nodes (edges) whose removal disconnects the connectivity from any collector to the required sensing data, which is replicated and maintained in multiple sensors. An OSN is called \(k\)-node (edge) sensing connected when it satisfies \(k\)-node (edge) sensing connectivity requirements. We can achieve \(k\)-node (edge) sensing-connected optical sensor networks by searching independent \(k\) end-to-sensing light paths between a collector and multiple sensors where the sensing data is hosted [37]. Multiple end-to-sensing light paths are independent if they do not have any internal node in common. The \(k\)-node (edge) sensing connectivity has the advantage of sensing backup, multi-path routing, and disaster-resilience.  
**Sensing Backup**: Sensing data is replicated and maintained in multiple sensors. Multiple sensors, which are chosen as candidate sensors for sensing data deployment, can be required to be disaster-disjointed.  
**Multi-Path Routing**: There exist at least \(k\) independent end-to-sensing light paths that can be used by each collector to access the sensing data. Generally, the value of \(k\) is greater than the upper bound of network connectivity.  
**Disaster-Resilience**: The end-to-sensing light paths are independent and terminate at different sensors. The probability of sensing data inaccessibility caused by a disaster event can be significantly reduced. The end-to-sensing light paths can also be designed to be disaster-disjointed.  
The \(k\)-node (edge) sensing connectivity can be achieved by searching \(k\) independent light paths between users and multiple sensors where the sensing data is hosted. The realization of \(k\)-node (edge) sensing-connected OSNs can be decomposed into three sub-problems: flexible sensor deployment, maximum number of independent end-to-sensing light path calculations, and the routing and spectrum allocation (RSA) for \(k\) end-to-sensing light paths [135].  
**Flexible Sensor Deployment**: The sensor set is chosen where the sensing data can be obtained. There are three strategies: spectrum-aware sensor deployment, capacity-aware sensor deployment, and disaster-aware sensor deployment. Each deployment strategy can be adopted for different network scenarios and requirements.  
**Maximum Number of Independent End-to-Sensing Light Path Calculations**: The maximum number of independent end-to-sensing light paths is searched between users and multiple sensors where the sensing data are obtained. It can also be used to determine if current sensor deployment satisfies \(k\)-node (edge) sensing connectivity requirements.  
**The RSA for \(K\) End-to-Sensing Light Paths**: In elastic optical networks, we need to address the problem of RSA for each end-to-sensing light path. The RSA for each end-to-sensing light path must satisfy spectrum contiguity and spectrum continuity requirements.