# Disaster-Resilient Optical Network Survivability: A Comprehensive Survey
## 2 Protection and Restoration
in Reference [50] proposed a combination of protection and restoration techniques for simultaneous dual-link failure to re-establish the connection with the minimum available unused capacity. [PERSON] et al. proposed a combination of protection and restoration techniques for optical networks considering two link failure [51], such that the second link failure occurs within the recovery span of the first failed link. Primary and backup paths are established using shorter and longer path lengths, respectively. Protection component of the hybrid technique reserves the backup capacity such that affected demands can restore using a pre-planned backup path, and restoration component dynamically finds another path to restore the rest of the affected demands. There have been many p-cycle based techniques proposed to protect the single link failure, however, [PERSON] et al. introduced a p-cycle based Integer Linear Programming (ILP)model in Reference [52] to mitigate the two link failure recovery problem using static load. [PERSON] et al. also proposed two other algorithms, namely Shortest Path Pair Protection (SPPP) and Short Full Path Protection (SFPP), for dynamic traffic and examined their performance for different traffic loads and capacity. Through simulation results, it is revealed that SFPP is more efficient than SPPP in terms of their capacity for incremental traffic, but has lower blocking probability at high traffic. In the restoration scheme [28; 29; 43; 53; 54; 55], backup paths are provisioned according to update link-state information of nodes and links when a failure arises. It provides higher resource utilization as compared to protection, however, recovery time is more than that of protection as this technique finds the backup path dynamically after failure occurrence. Restoration can be classified as (i) link-restoration; (ii) path-restoration; and (iii) segment-based restoration [28]. Among these restoration schemes, link restoration takes less time for fault recovery whereas path restoration takes the maximum time. Provisioning network connections with the most survivable paths (via appropriate protection and restoration schemes) increases the reliability of network connections, which is crucial to various important network services (banking, big data transfers, etc.). [PERSON] et al. [56] proposed a HCA to achieve the efficient restoration in WDM optical networks. The algorithm theme is based on the modified resource allocation to reserve a wavelength, in advance, during the establishment of a connection for a backup light path, but the model is designed to deal with single link failure only. The proactive restoration method is also proposed by [PERSON] et al. in Reference [57] that improves the network resource utilization and reduces the blocking probability. [PERSON] et al. proposed the Dynamic Load Balancing Shared Path Protection (DLBSPP) algorithm in Reference [58], which employed a traffic-aware restoration mechanism to compute link-disjoint backup paths that carry multi-link failure traffic. [PERSON] et al. [59] studied the simultaneous k link-failures on dynamic WDM optical network by proposing a routing and wavelength dimensioning approach named Fault Tolerance Method Based on Cheapest Paths (FTBCP). To avoid longer recovery times, the FTBCP computes the primary and alternate routes off-line, with maximum traffic load, and then these computed routes are assigned, on request, to reduce blocking probability with failures \(k\geq 1\).