# Disaster-Resilient Optical Network Survivability: A Comprehensive Survey
## 5 Congestion-Aware Routing and Wavelength Assignment  
The last, but not least, concerning issue is the traffic congestion that may be induced due to the rerouting of disrupted or blocked traffic in the event of a disaster. The solution proposed in Reference [62], which begins with the thought that telecom networks have some spare capacity to avoid or alleviate traffic congestion. Such capacity can be utilized to give better assurance against disasters. If the network resources are consumed appropriately and the network loaded is distributed evenly over the links, the congestion will be controlled in terms of connection acceptance rate and lower blocking probability. Overloading of network resources is also known as network congestion. It means that the available capacity of the network cannot fulfill the total demand or connection requests. Congestion occurs due to several reasons like low bandwidth, multicasting, bad configuration, too many hosts in the broadcast domain, broadcast storm (can be a busy day for e-commerce or Black Friday sales) or rerouting because of disasters-based network path failures etc. Generally, congestion can be avoided by network segmentation, reconfiguring Transmission Control Protocol/Internet Protocol (TCP/IP) settings, backpressure routing, and prioritizing the network traffic. There are several studies on congestion control techniques [102; 103; 104; 105] in which network performance is evaluated under various network conditions and a range of parameters for heterogeneous networking environments. In Reference [113], [PERSON] et al. proposed a model for per link congestion control by balancing network resource allocation considering the current and future demands of light path requests. Figure 8a shows a network of six nodes and bidirectional links with lengths and channels available per link. Five light paths have established upon connection requests on the first-fit rule. Figure 8b summarizes the network traffic over each link. According to the Routing and Wavelength Assignment (RWA) problem, light paths are established considering the path lengths and the channel availability with the lowest blocking probabilities. Taking the current statistics in view, Figure 6a clearly indicates that connection requests between nodes (\(1\leftrightarrow 4\)), (\(1\leftrightarrow 3\)), and (\(3\leftrightarrow 5\)) can be established immediately because of channel availability. If a connection request arrives other than the above-mentioned requests, then it has to wait for a channel to be free which causes a delay or drop in the connection request. In the case connection request (\(4\leftrightarrow 5\)), a light path can be established immediately, however, it incorporates a delay at the cost of path length (i.e., \(4\)\(\rightarrow\)\(1\)\(\rightarrow\)\(3\)\(\rightarrow\)\(5\) as there is no channel available at \(4\)\(\rightarrow\)\(5\)), which introduces a path delay. Hence, it can be concluded that there is a tradeoff between path length and capacity while establishing a connection between two nodes of a network. Realistic networks have irregular network topologies and connection requests are random. Whenever the traffic load increases, the number of idle channels per link decreases in establishing  
Figure 8: Physical network topology with 6 nodes and 7 bidirectional links. (**a**) Network Light path Establishment and (**b**) Network Traffic Table. the connections. Connection requests start blocking when there zero channels exist over the links and can be measured as blocking probability. [PERSON] et al. [30] proposed a congestion-aware strategy dynamic in nature which seeks to minimize the blocking probability by the proportional distribution of network traffic over the links to enhance resource utilization. [PERSON] et al. in References [114; 115] developed a low complexity mathematical model to calculate and reduce the blocking probability of routes and overall network. Authors analyzed blocking probabilities against a number of links, a number of free wavelengths and length of routes.