# Disaster-Resilient Optical Network Survivability: A Comprehensive Survey
## 1 Introduction  
Optical networks provide high-capacity fiber links for the Information Communication Technology (ICT) access networks and carrying about 99% of the global internet traffic [1]. The level of bandwidth demand is rising continuously thus requires more high capacity optical links [2], which is not an economically viable option. To meet these challenges, the Dense Wavelength Division Multiplexing (DWDM) technology is gaining importance and uses multiple wavelengths in the same optical fiber link to provide multiple virtual channels. Each channel may have a capacity of 100 Gigabits per second or higher, and collectively offer a data rate of Terabits per second over these channels [3]. Network traffic moves through the optical light paths, constituted by high-capacity fiber links, which are established between two network nodes on arriving connection requests. To ensure continuous provisioning of services, survivability of network nodes and fiber links is of prime vitality [4; 5]. Network failures could be the result of misconfiguration, hardware failure, ill maintenance, power failures, and/or natural disasters, as shown in Figure 1. In a network, fiber cables are laid down in bundles swathed by a duct where each cable may carry hundreds of fiber strands. Construction or destructive natural events including earthquakes, landslides or ship anchors causes fiber cuts. All the light paths that traverse through failed fiber (fiber cut) will be disrupted and lead to massive data loss. A catastrophic event such as fires or floods can fail the central offices where OXCs are located. This is referred to as node failure which is rare but can impose severe disruptions. Similarly, channels engaged by light paths over optical links can fail due to the failure of transmitter/receiver operating on that channel and are handled either by prompt switching to another idle channel or by treating it as a link failure in absence of an idle channel [6]. Figure 1 also describes the nature of disaster-based failures which can disrupt the function of single or multiple links and/or nodes. Disasters can be categorized as natural disasters (e.g., hurricanes & tropical storms, tornadoes, earthquakes, landslides, avalanches, tsunamis, floods, wildfires, animal bites, etc.) and man-made or technological disasters (power service disruption & blackouts, human negligence or errors, anchor drag/drops, EMP attacks, nuclear explosion, sabotage, anti-corporate attacks, cyber-attacks, terrorist attacks, or vandalism). Disaster based failures can have devastating impacts on an optical network by making its services unavailable [7]. For example, the worst disaster in Japanese history was on 11 March 2011, consisting of Great East Japan earthquake of magnitude-9, along with the subsequent tsunami and nuclear accident that destroyed 1500 telecom buildings by the main shock and 700 telecom buildings experienced power outages [8; 9; 10]. The Nepal earthquake, in 2015, affected the Rural ICT infrastructure and services by collapsing of the houses, schools, ICT access centers, BTS, transmission towers, fiber backhaul, and microwave links [11; 12]. An earthquake of magnitude 7.1 struck central Mexico on 19 September 2017, caused 355 fatalities, 6100 injuries, and collapsed nearly 44,000 buildings, including telecom infrastructure [13]. Some network interruptions appear to be intentional attacks, the motivations of which are hard to observe. A later occurrence including three men associated with scuba diving down to cut off the undersea optical fiber at the bank of Egypt happened in March 2013 [14]. Many specialists feel that deliberate damage of submarine cables is unrealistic or impossible, however undersea-cable bottlenecks have the potential for serious interruptions is causing network providers to search for alternate paths to connect the continents. Figure 2 provides an overview of disaster-based failure survivability mechanisms in optical networks.