e 8.7; the wavelengths on each fiber is divided into two groups: a working group and a protec- tion group, where the working wavelengths on the outer fiber are devoted to protection in the inner fiber and vice versa. Working channels, as shown in the figure, are routed bidirectionally using both the fibers on working wavelengths. When a failure occurs, switching happens at the OCh end and the channels affected by the failures are rerouted in the opposite ring direction by using the other fiber and the set of protection wavelengths. As it is also evident from Figure 8.7, if several optical paths are routed in the ring on the same wavelengths, the corresponding protection wavelengths have to be shared among all those paths (from where comes the name shared protection). Moreover, also where a unidirectional failure occurs, that is a failure affecting only one direction, both the optical channels ends have to switch to change the ring configuration from working to protection. This means that, when a node detects a failure via a signal power off or a signal quality major alarm (e.g., an estimated BER greater than 10-10), it has to inform the other node at the end of the affected channel of the failure. Moreover also all the other nodes in the ring have to be advertised that protection switching occurred, since the protection wavelengths are no more available. This means that a control plane protocol has to be started from the CC supervisioning the node that detected the failure with the primary role to make handshaking with the note at the other end of the affected link. The messages are first sent on the other direc- tion of the path: if the failure is unidirectional, they will be detected and the other node will answer on the protection path, if the failure is bidirectional also the other node has detected the failure and after some time the handshake will start on the protection path in both directions. Before engaging the protection path for the first protection handshake, another protocol can be used to advertise