# Disaster-Resilient Optical Network Survivability: A Comprehensive Survey
## 3 Disaster-Based Correlated Cascading Failures  
Disaster-based failures can be correlated or cascading, and sometimes trigger the failures horizontally or vertically within the network [7]. For example, the optical layer in WDM networks provides services to the upper layers (e.g., ATM, SONET-SDH, MPLS, IP) and lacks the restoration of optical layer that may vertically disrupt the services of the upper layers. Today's smart networks are managed through software designs. The complexity level of these software designs is increasing over time. Software bugs could lead to unstable network states. For example, in 2012, a routine update of load balancing software on a Gmail server caused a partial loss of 40% of services for 18 min [60] because this update contained faulty logic. A single node was fully updated to recover this cascading error instead of partially updating all failed nodes at a time. The problem of cascading a disaster was persuasively considered after the event of the Tohoku earthquake in 2011. This problem can be viewed as a chain-sequence of interconnected failures or associated with a cause-and-effect relationship that is a vital feature of most catastrophic events. [PERSON] et al. defined and separated the cascading disasters and cascading effects by reviewing the literature in Reference [61] while considering the dynamics of disasters. [PERSON] et al. analyzed the critical elements responsible to cascade failures and concluded that cascading can be limited in the event of a disaster if vulnerability, critical infrastructure, interdependencies, amplification, and secondary disasters of the network are considered during risk reduction practices. The nature of disaster failures is much more dynamic and wide-spread than the failures discussed in Section 2. These failures may span in the form of multiple correlated cascading failures either horizontally (within a layer) or vertically (layer to layer), as depicted in Figure 3a [62]. Light paths (shown as a two-sided arrow) in the optical layer are responsible for connecting nodes and data centers of the upper layer, so disaster failures can propagate to upper layers due to functional dependencies. [PERSON] et al. [63] address some external factors and their impacts on the telecommunication networks and correlated the natural disaster with the large-scale failures. The existing capacity of the network and bandwidth demand can change pre, trans, or post-disaster events. The possible degradation in a post-disaster network offered bandwidth due to cascading failures, as is illustrated by Figure 3b. It can also be observed that requested bandwidth upsurged in a post-disaster scenario because people intend to use social media, live video streams, and TV news, etc. to get more and more information about the incident, which consequently contributes requested bandwidth in post-disaster and recovery scenarios. Optical backbone networks tend to exhibit complex behavior due to their increasing scale and interconnected components at different layers. Thereby, these networks become more susceptible to local failures, later on, turn into cascading failures. Sometimes we exaggerate the problem more instead of alleviating it in response to our actions taken to mitigate these failures and upsurge other parts of the network. [PERSON] et al. [64] proposed a model for cascading failures that includes network states as flows and the activation of link status. Under the proposed circumstances, an overload condition or disaster disruption can irreversibly inactivate a link. Coupling links can be inactive successively due to cascading failure that may or may not be adjacent. Routing policies can access the network state. An algorithm designed to assess the upper bound on the margin of resilience for directed acyclic  
Figure 3: Effects of Correlated Cascading Failures. (**a**) Horizontal and Vertical Correlated Cascading Failures; and (**b**) Pre-Disaster and Post-Disaster Typical Requested and Offered Bandwidths. graphs.