## 4 The Deployment of WDM-PON Protection Schemes
### Network Availability Considerations for WDM-PON Protection Schemes  
High resilient WDM-PON networks ensuring the high availability are becoming more relevant due to an increasing number of users and demands for broadband network connections. The network availability is also an important parameter for network operators because it can ensure that the actual network configuration satisfies a requested availability level determined by the Service Level Agreement (SLA). In other way, network operators need to modernize their passive optical network for increasing a level of the network availability. Therefore, a focus on the network availability presents a practical interest especially for services with high demands [28]. A performance analysis of the WDM-PON network availability creates an important characteristic of the functionality for various protection schemes. The WDM-PON network availability represents a probability that all network connections are working properly in any time. The connection availability can be estimated on the basis of failure characteristics for all optical components involved in protection schemes. The connection fails if at least one component is mistaken [28]. For the \(n\) components connected in series, the WDM-PON network availability is the same as in the case of the TDM-PON network availability [20]. However, the WDM-PON network availability also includes components connected in parallel [8]. For this case, the total network availability \(A_{pantled}\) is presented as follows:  
\[A_{parallel}=1-\prod_{i=1}^{n}U_{comp_{i}} \tag{1}\]  
where the unavailability parameter \(U_{comp}\) represents a failure probability of each component in RBD diagrams determined mathematically using Failure In Time (FIT), MTTR and MTBF parameters defined in [25], and \(n\) represents a total number of components in the passive optical network. A deployment and operational situation of the real network infrastructure differs a lot and is depending on a specific network operator and/or area. Decreasing the time for repairing optical fibers and other network components markedly affects the network availability. Equations for the network availability calculation differ based on P2 MP and RING protection schemes due to different topologies and utilized network components. Hereby, the unavailability for each WDM-PON network can be determined. For evaluation of the total network availability for each considered WDM-PON protection scheme, RBD block diagrams, due to their simplicity and understanding given protection schemes, are used as presented in Figure 1.  
The total availability of the unprotected P2 MP WDM-PON network (Figure 1a) is calculated by using  
\[A_{P2 MP-unprotected}=1-(U_{OLT}+U_{FF}+U_{AWG}+U_{DF}+U_{ONU}) \tag{2}\]  
with unavailability parameters \(U_{\mathrm{x}}\) of the OLT module, the FF fiber, the AWG element, the DF fiber and the ONU unit for specific lengths of feeder and distribution fibers [3]. A calculation of the network availability for P2 MP protection scenarios furthermore includes a parameter of the network restoration time [20; 25]. The restoration time is a time reserved for switching to redundant components and for traffic restoration. For WDM-PON networks, values of the restoration time are considered in tens of ms. The total availability of the unprotected RING access WDM-PON network (Figure 1e) is calculated by using  
\[A_{RING~{}A-unprotected}=1-\left(U_{OLT}+\frac{FF_{A}}{2}\cdot U_{FF}+\frac{k+1} {2}\cdot U_{ONU}\right) \tag{3}\]  
where \(FF_{A}\) is the feeder fiber length in the ring, and \(k\) is a number of \(ONU\) units connected to the ring [27]. The total availability of the unprotected \(RING\) metro-access WDM-PON network (Figure 1g) is calculated by using\[A_{RING~{}M-A_{-}unproteld}=1-\left(U_{OLT}+\frac{FF_{M}}{2}\cdot U_{FF}+\frac{l+1}{2 }\cdot U_{AWG}+\frac{DF_{A}}{2}\cdot U_{DF}+U_{PSC}+U_{ONU}\right) \tag{4}\]  
where \(FF_{M}\) is the feeder fiber length in the WDM ring, and \(l\) is a number of AWG elements.