# Disaster-Resilient Optical Network Survivability: A Comprehensive Survey
## 6 Challenges in Disaster-Resilient Optical Network Survivability  
With the high frequency of fiber cut and the tremendous traffic loss, network survivability becomes a critical concern in network design and its real-time operation. It is in ultimate need of network operators to design effective methods to recover from failures of network links and nodes. \begin{table}
\begin{tabular}{|p{42.7 pt}|p{113.8 pt} p{113.8 pt}|p{113.8 pt}|} \hline
**Ref** & **Techniques** & **Objectives** & **Limitations** \\ \hline
[120] & CASPaR Algorithm & \
\begin{tabular}{c} Pursued to enhance the PDP and reduced the latency by selecting \\ the shortest path. \\ \end{tabular} &
\begin{tabular}{c} CASPaR shows improved performance than that \\ of other protocols but in terms of packet delivery \\ only. \\ \end{tabular} \\ \hline
[122] & Centralized PCE with STAR (Self-Tuned Adaptive Routing) algorithm & \
\begin{tabular}{c} Efficient path computation \\ avoiding underutilization of \\ capacity over links and \\ congestion. \\ \end{tabular} &
\begin{tabular}{c} Introduced a novel centralized PCE with STAR \\ algorithm that discovers the paths considering \\ available link capacity and load balancing. \\ Separate path computing and path signaling \\ functions which give operators more control over \\ their network. \\ \end{tabular} \\ \hline
[123] & MILP Model to handle capacity exhaustion problem & \
\begin{tabular}{c} Investigate post-disaster static \\ traffic floods in several scenarios \\ to reduce traffic blocking. \\ \end{tabular} &
\begin{tabular}{c} The arrival of connection requests is random so \\ network traffic and lightpath establishment are \\ also dynamic in real-life networks. Should be \\ investigated with dynamic traffic floods. \\ \end{tabular} \\ \hline \end{tabular}
\end{table} 
Table 2: _Cont._Most of the research work on survivability in WDM networks focuses on the recovery from a single link or node failure. Multiple (i.e., near-simultaneous) failures are also possible in a realistic network, and appropriate recovery methods can be designed. In this paper, most of the studies proposed techniques to provision disjoint light path between two network nodes to enhance network resilience against failures and disasters. Large-scale disasters show their impact on the large geographical area. While, disjoint light path are physically disjointed but both light paths (primary as well as backup) may simultaneously fail due to the minimum spatial distance between them. We have shown, in Figure 8, how fibers can be spatially close. Algorithms for detection of spatially close fibers and grouping them using the minimum number of distinct risk groups are proposed in Reference [94]. However, light paths are established against connection requests, which are composed of fiber links in real networks. This work needs to extend from spatially close fibers to spatially close disjoint paths for risk-averse path provisioning. Similarly, Reference [101] presented the geographic proximity-aware route selection algorithm to discover backup paths with appropriate proximity factor. The study is based on a grid-based network, whereas fiber links are deployed in a zig-zag (irregular geometry) manner in real-life networks. Hence, more sophisticated techniques can be investigated. Current techniques for discovering shortest light paths and alternate light paths do not account for parameters like light path lengths, link capacities, light path capacities, connection requests, and light path establishment at the same time and accommodate one or two of them during provisioning or re-provisioning. In this way, most of the links on popular routes get congested and some remained underutilized or un-used within the network. This can be referred to as inappropriate utilization of network capacity. Un-used links and their stranded capacity of a network should also be considered for optimized utilization of network resources.