# Survivable Deployments of Optical Sensor Networks against Multiple Failures and Disasters: A Survey
## 2 Existing Survivability Technologies in OSNs
### Pre-Configured Protection Schemes
#### 2.3.3 The P-Cycle Scheme  
Although 1:1, 1:N, and M:N protection schemes can improve resource efficiency, the resource efficiency of these schemes is still very high, especially when many working flows are accommodated. Figure 4: The photonic millimeter-wave bridge scheme. In 2000, [PERSON] proposed the p-cycle structure with the features of ring-like speed and mesh-like capacity [29]. They showed that p-cycle has the highest protection efficiency and the lowest resource redundancy against single failure. In other words, p-cycle is the optimal protection structure against single failure in mesh networks. As presented in Figure 5, two p-cycles were designed to protect a working flow. When a failure occurs at any link along the working flow, the interrupted working flow can be restored by the corresponding p-cycle. At present, the research on the p-cycle scheme involves cycle selection, node protection, spectrum efficiency, dual-failure protection, etc. **Cycle Selection**: For the p-cycle scheme, the first problem is how to search the required cycles. In [79], the authors proposed a heuristic approach to iteratively select and refine a set of p-cycles. The results showed that the proposed approach was within 3.5% redundancy difference of the optimal solution with very fast computation time even for large networks. In [80], the authors developed a new ILP model and a related heuristic method for failure-independent path-protecting (FIPP) p-cycle design that produced network designs with much faster runtimes. In [81], the authors proposed a new heuristic approach and a new evaluation metric to solve the p-cycle protection problem that simultaneously optimized cost, protection, length, and fairness of solutions in a mono-objective approach. In [82], the authors proposed a novel path-protecting p-cycle heuristic algorithm for survivable WDM network design. The p-cycle based path protection for span and node failure recovery was capacity efficient and robust. In [83], the authors proposed the attached node-based cycle generation algorithm for pre-computation of candidate cycles and three algorithms for light-path-protecting p-cycle selection for a protected working light-path envelope. **Node Protection**: Although the p-cycle scheme was designed for link failure, it can also be used to achieve node protection. In [84], the authors showed that the transiting path affected by the node failure is inherently restorable by ordinary p-cycle switching actions whether the respective two-hop segment was on-cycle, straddling, or partially on-cycle and partially straddling. In [85], the authors showed that high levels of node failure protection can be achieved with an ordinary set of p-cycles, which are designed in such a way that every demand that transits a node in a straddling manner is also intercepted at points upstream and downstream on its route by some other p-cycle. In [86], the authors developed capacity optimization models to support 100% restoration of transiting flows through failed nodes. Only a very small additional spare capacity was required to achieve both 100% span and intermediate node-failure restorability. **Spectrum Efficiency**: Improving the spectrum efficiency of the p-cycle scheme is an important research point. In [87], the authors studied how the efficiency of a p-cycle network increased under  
Figure 5: The p-cycle scheme: (**a**) p-cycles configuration; (**b**) a failure occurs at p-cycle A; (**c**) a failure occurs at p-cycle B.  
joint optimization of the working path routes with p-cycle placement. In [88], the authors showed how grooming decisions can be directly integrated into an overall p-cycle network design for more efficiency. In [89], the authors introduced the concept of differential capacity p-cycles as an enhancement to span protecting p-cycles. In [90], the authors provided a true comparison of span-protecting p-cycles with FIPP p-cycles, from the capacity efficiency perspective.