# Disaster-Resilient Optical Network Survivability: A Comprehensive Survey
## 1 Introduction
Disaster-based failures could cause multi-domain multi-layer failures that span several network domains. Disaster modeling is utilized to assess the risks involved and their corresponding physical and financial impacts. Disasters can be modeled as deterministic or probabilistic, i.e.,  
Figure 1: Types of Network Failures. whether the outcomes are precisely determined or undetermined due to the room for random variation. A large-scale disaster could cease to function multiple nodes and links which turn into multi-layer failures that can be modeled as a multilayer network. Following Reference [15; 16; 17; 18; 19] presents interesting work on disaster modeling, however, it is out of the scope of this paper. To combat disaster-based failures, intelligent network provisioning schemes are utilized to restore services with a higher priority, known as connection recovery and network recovery. Generally, approaches for connection recovery falls into proactive prevention and reactive compensation considering two factors: Routing and capacity assignment. Protection of connections can be done either by provisioning of backups proactively or by re-provisioning connections reactively after failure (path-based restoration). Protecting a connection over multiple disjoint paths has the advantage of better fault tolerance [20]. Congestion Control Service Provisioning (a basic reactive procedure) has an emphasis on capacity re-arrangement for re-provisioning of disrupted connections. As this method adapts to dynamic network events, it can handle concurrent, cascading failures. Through reactive compensation, limited network resources can re-allocate (re-provision) multiple times for the most effective usage. Multiple network components may fail simultaneously due to wide-spread geographical impacts of disasters. The focus of the research community has emerged to large-scale correlated failures with several aspects. For example, [PERSON] et al. in Reference [17] elaborated the classification of research work on disaster survivability as well as disaster impacts on optical networks using their characteristics. They also discussed several ways to combat them. Whereas References [7; 21; 22] reviewed the work on Resilient Communication services protecting end-user applications from Disaster-based failures (RECODIS). A review on network resiliency against weather-based disruptions and their impacts is provided in References [23; 24]. In References [22; 25; 26], deployment of emergency networks in the event of a large-scale disaster is addressed for post-disaster recovery. This paper is intended to present a survey of fault tolerance and management through protection and restoration, correlated cascading failures, network survivability, and congestion-aware routing in disaster scenarios. It also reviewed the recent techniques and trends adopted for network survivability. A brief review of solutions for survivability techniques regarding fault management, cascading failures, and disaster-aware survivability is presented in Sections 2-4. Section 5 elaborates on some work on congestion-aware routing and spectrum assignment. Open challenges and research directions are given in Sections 6 and 7 concludes this study. Figure 2: Overview of Disaster-based Failure Survivability Mechanisms [17].