# Disaster-Resilient Optical Network Survivability: A Comprehensive Survey
## 4 Disaster-Aware Network Survivability
[PERSON] et al. proved the problem of provisioning region-disjoint paths as non-deterministic polynomial time problem (i.e., NP-hard) and proposed a heuristic as a solution to this problem. Geometric probability can also be used for the geography of disasters. [PERSON] et al. (2018) proposed a geometric model based on the geometric probability for a physical network affected by a disaster. Performance metrics such as connection maintainability and network design rule are evaluated to make the network robust against disasters. [PERSON] in Reference (2018) has also analyzed the network survivability against earthquake by introducing spatial network design rules, whereas [PERSON] and [PERSON] (2018) extended the work in Reference (2018) by proposing an algorithm that determines the new additional links to enhance the network robustness against earthquakes. Disasters tend to display spatiotemporal characteristics and consequently links availability change instantly. If the spatiotemporal impact of the disaster (such as the path of a hurricane see Figure 5) can be predicted, then preventative measures can be taken to mitigate the disaster impacts on the network. [PERSON] et al. (2018) presented the spatiotemporal network resilience based on a grid-based risk profile depicted in Figure 6. Where each grid rectangle g contains an availability value representing the probability that g is free from failure for a particular time interval. The availability of a link computed as the product of the availability values of all grid rectangles it crosses. The availability of a path is the product of the availability values of its links. The most risk-averse path obtained via the shortest path algorithm, using as weights the -log of the link availabilities. Figure 5: Projected path of Hurricane Sandy 2012 (Source: National Oceanic and Atmospheric Administration). Network elements (nodes and links) contain geographical information. Spatially close links (e.g., along a bridge) have a higher chance of simultaneous failure. Previous models proposed for network resilience assumed that all links installed as straight lines, whereas links are deployed with irregular geometry because of geographical constraints, cost-effectiveness, and government rules. Figure 7 illustrates the fiber-link as a concatenation of multiple link segments of varying lengths where each link-segment is a straight line connecting two points of known geodetic locations. In Reference [94], [PERSON] et al. considered the geography of links (as indicated in Figure 7) to detect spatially close fibers using data structures like kD-tree and R-tree. The minimum separation distance between the links was constrained by a value \(\alpha\) to represent the construction-level and large-scale failures. They also proposed a polynomial algorithm based on R-tree to identify the spatially close risk groups using the minimum number of Distinct Risk Group (DRG). Extending the work of Reference [94], [PERSON] et al. in Reference [95] proposed a polynomial-time algorithm to find the spatially-disjoint pair of light paths between two network nodes using kD-tree processing and kNN (k-nearest neighbors) search algorithm. Running time and optimality of the proposed algorithm can be controlled via a heuristic \(K\) (required number of shortest paths). [PERSON] et al. proposed a novel technique in Reference [96] to boost the network robustness against natural disasters like mega quakes and tsunamis. They outlined the solutions for two imperative facets: Survivability against damages in broad-area networks and quick network recovery in confounded areas. They proposed an integrated approach of optical packet switching and circuit switching for more survivable broad-area networks, while emergency optical networks and hierarchical topologies are elaborated for quick network recovery in devastated areas. In Reference [97], [PERSON] et al. formulated an ILP model which minimizes the network resources (wavelength links) needed to handle requests for communication in data center networks.