# Disaster-Resilient Optical Network Survivability: A Comprehensive Survey
## 2 Protection and Restoration  
Several survivability mechanisms for Wavelength Division Multiplexing (WDM) based optical networks have been studied in References [27; 28; 29; 30; 31]. Several recovery techniques for mesh and ring based optical networks are summarized in Reference [27] and Reference [32]. Even a modular suite of algorithms was presented for managing the normal and protection paths in Reference [33]. Protection and restoration [34; 35; 36] are implemented either as link-based or path-based to minimize service disruptions at reasonable costs. In the protection scheme, spare capacity is reserved and kept idle until a failure occurs. The use of network capacity is thus not efficient, but recovery speed is guaranteed. However, it has been observed in Reference [37] that the aggregated impact of failures is reduced up to 40% only in backup network design, i.e., protection is not entirely effective. On the other hand, restoration refers to the arrangements where the blocked traffic is rerouted via a dynamically discovered route when some node or link fails. Resources used for recovery do not preserve, but accommodate the available unused capacity of the network. It is more capacity-efficient than a protection scheme but may require a longer recovery time and overhead. The combination of protection and restoration can also be used (known as a hybrid scheme) to take advantage of both schemes [38]. In the hybrid scheme, recovery solution is pre-designed but resources are allocated in real-time. Faults of fiber links and Optical Cross-Connects (OXCs) could cause the channel failure, thus losing a huge amount of data. [PERSON] et al. [39] studied the distributed protocols for both path restoration and link restoration to examine the protection switching and restoration times. [PERSON] et al. evaluated the adjustment between capacity utilization and vulnerability to multiple link failures. Protection schemes for different types of networks like SONET-SDH, Multi-Protocol Label Switching (MPLS), Internet Protocol (IP), etc. are proposed in Reference [40]. If a protection scheme is employed in a capacity efficient manner, it will provide much quicker recovery from failure, as proposed by [PERSON] et al. in Reference [38]. However, path protection results in lower capacity because each path is protected by a dedicated backup path [29; 41]. Dedicated protection delivers more reliability in terms of service continuity, QoS, and connection ASR, but cannot provide efficient resource utilization [42]. To mitigate this problem, shared backup path protection technique can be applied in WDM optical networks, however, dedicated protection will take less recovery time, as compared to shared protection [29; 43; 44]. It is also suggested by [PERSON] et al. [45] to protect the primary path by protecting its vulnerable segments (sub-paths) separately. Sub-path protection delivers faster recovery and higher scalability as compared to path protection at the cost of resource utilization. Link protection can be achieved using link-disjoint backup paths for each link established according to requested connections [46; 47; 48]. From the preliminary work, it is observed that most of the studies focused on a single node or single link failure at the given instant. Single-link failures are most likely the cause of dual-link failures and are more tractable than multiple failures. [PERSON] et al., in their technical report [49], studied the impact of two link failures considering of a multi-failure model for different protection schemes and proposed a restoration method for damaged and congested shared path. [PERSON] et al. also computed the capacity cost and survivability for their approach in evaluating the tradeoff. [PERSON] et al. [46] addressed the dual-link failure recovery using a failure model in which two links may fail arbitrarily. With proposed algorithms, [PERSON] et al. suggested that 100% recovery was achieved from double-link failures by increasing backup capacity unassertively. [PERSON] et al.