# Disaster-Resilient Optical Network Survivability: A Comprehensive Survey
## 5 Congestion-Aware Routing and Wavelength Assignment
In Reference [116], [PERSON] et al. proposed another mathematical model for wavelength-routed WDM networks to optimize the blocking probability. Authors compared the proposed model with References [114; 115] and showed that recent model performs better than earlier models. [PERSON] et al. also address the blocking probability of ring optical networks in Reference [117] considering the total number of wavelengths in the network. They analyzed the blocking probability node by node by varying the number of wavelengths and using wavelength conversion algorithms. Results showed that blocking probability decreases with the increase of a number of wavelengths. RWA can be treated either as individual problems (i.e., routing problem or wavelength assignment problem) or as a single problem on the basis of scale, size, and other network preferences. [PERSON] et al. [118] addressed the routing problem in wavelength-routed optical networks. They studied the performance of Dijkstra and least congested path routing algorithms in terms of path lengths and blocking probability. RWA can also be utilized either in a centralized or distributed manner. For small-scale networks, there are a small number of requests; hence, the centralized RWA approach can be used to tackle the problem. However, for large-scale networks where traffic is high with a random pattern should utilize the distributed method. [PERSON] et al. [119] proposed a congestion-aware DLA routing algorithm which considered the congestion as a decision point to allocate resources. The end-to-end connectivity assurance and congestion avoidance have raised the need for robust and low latency routing protocols. [PERSON] et al. presented Congestion Avoidance Shortest Path Routing (CASPaR) algorithm in Reference [120] that pursues to enhance the Packet Delivery Probability (PDP) and reduces the latency. Continuous service availability is the major concern of disaster-resilient all-optical networks. [PERSON] et al. in Reference [121] proposed a new WRT scheme without disrupting the network services to meet the continuity constraints of wavelength and services/data loss. Re-tuning is conducted only on the backup/alternate path in case of failure of the primary path that carries the traffic under normal conditions. Authors also proposed a heuristic version of the same to intact the optimality and performance. Work on congestion-aware RWA is summarized in Table 2. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Ref** & **Techniques** & **Objectives** & **Limitations** \\ \hline \multirow{4}{*}{[30]} & Congestion control & Dynamic strategy symmetrically & Criteria for most and least used links not defined \\  & strategy in WDM & distributes the traffic on the links & before distribution of data that may be based on \\  & networks & according to the channels & a number of connection requests, connection \\ \hline \multirow{4}{*}{[117]} & Analysis of blocking & \multirow{4}{*}{\
\begin{tabular}{c} Computing and analyzing the \\ node-by-node blocking \\ performance in the ring optical \\ network \\ \end{tabular} } & \multirow{4}{*}{
\begin{tabular}{c} Computing and analyzing the \\ node-by-node blocking \\ performance in the ring optical \\ network using wavelength \\ conversion algorithms \\ \end{tabular} } & Whole work is based on the 8-node ring network. \\ \cline{1-1}  & Dijkstra Algorithm and & Studied the performance of these \\  & Least Congested Path & algorithms in terms of blocking \\  & Routing Algorithm & probability and path lengths \\ \hline \multirow{4}{*}{[119]} & \multirow{4}{*}{LC-DLA routing} & Resource allocation on the \\  & decision of network congestion in \\ \cline{1-1}  & LC-DLA routing & large-scale networks taking a \\ \cline{1-1}  & algorithm & huge number of connection \\ \cline{1-1}  & requests and heavy traffic load in \\ \cline{1-1}  & the account. & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Congestion-Aware Routing and Wavelength Assignment Techniques. All services in optical network finally converge at IP layer and move into the cloud.