# Survivable Deployments of Optical Sensor Networks against Multiple Failures and Disasters: A Survey
## 2 Existing Survivability Technologies in OSNs
### Pre-Configured Protection Schemes  
#### 2.3.1 The \(1+1\) Protection Scheme and Extensions  
The \(1+1\) protection scheme is the most classic and simplest protection scheme for optical networks. It can also be applied into OSNs. As presented in Figure 3, the \(1+1\) scheme uses two shared risk link group (SRLG) disjoint light paths to transfer data simultaneously. If a failure occurs at an intermediate node or link of a light path, the source and destination nodes of this light path only need to conduct protection switching. SRLG is an important concept for survivability. It represents a group in which all links will fail simultaneously as a result of a common failure [54; 55]. For example, all links in a seismic zone can make up an SRLG because all these links will be destroyed when an earthquake occurs in this seismic zone. The \(1+1\) protection scheme has been studied by a large number of researchers involving  
Figure 2: Self-healing architecture: (**a**) FBG sensor network; (**b**) failure in the bus subnet [46] RN: remote nodes; \(S\): fiber Bragg grating, and SW: \(2\times 2\) optical switch. resource efficiency, recovery time, flexibility, etc. In [56], the authors developed a light path provisioning algorithm for wavelength-continuous optical transport networks under \(1+1/1\):1 service protection. They showed that a limited number of add/drop ports and limited transmitter tunability were sufficient to ensure that a network fully utilizes its wavelength capacity. In [57], a transmitter was modified to provide two signals without excess insertion loss. Using this transmitter, optical protection switching was demonstrated under 10 Gb/s non-return-to-zero transmission and a protection time of 5 ms was obtained. In [58], an innovative transmission technique enabling signal overlap was introduced for spectrally efficient \(1+1\) protection. The results showed that the proposed technique reduced the overall amount of occupied spectrum resources. In [59], the authors used a practical all-optical XOR network coding to leverage the \(1+1\) protection in transparent WDM optical networks. This network coding-assisted protection solution paved the way for reducing the backup capacity. The \(1+1\) scheme is also applied in optical fiber access networks. In [60], the authors compared the reactivation times of different \(1+1\) protection scenarios, and then they proposed hardware modifications to reduce protection time. Although the \(1+1\) protection scheme is the most basic and simplest protection scheme, it consumes too many spectrum resources. To improve the resource efficiency of the \(1+1\) protection scheme, the \(1\):\(1\), \(1\):N, and M:N protection schemes were developed. Compared with the \(1+1\) protection scheme, the \(1\):\(1\), \(1\):N, M:N protection schemes allow the backup resources to be shared by multiple working flows. In [61], the authors extended single failure to dual failure recovery for an elastic optical network (EON) with \(1+1\):1 network protection. In [62], the authors demonstrated an reconfigurable optical 1:N protection system using the branch state of LiNbO/sub 3/optical switches. In [63], the authors presented a 1:N protection scheme based on the cyclic property of an array waveguide grating and a specific connection pattern among the optical network units (ONUs). The proposed scheme required more fiber links among the ONUs than previous schemes, but the network resource demands were greatly reduced and the protection performance was improved. In [64], the authors presented a theoretical study on the dynamic performance requirements for the successful application of a proposed \(1\) for N wavelength protection scheme to wavelength routed trunk and branch type optical networks. In [65], the integrated MEMS 1:N protection switch, scalable to M:N protection, was proposed, which allows monitoring of the redundant network element during normal operation and of the failed network element during protection for diagnostic purposes.