t a key point. In case of OMS, the optical transmitters remain unprotected; after the fiber cable, opti- cal transmitters are the building blocks with the lower availability, due to the presence Telecommunication Networks FIGURE 3.42 Scheme of an OChDPring: (a) path route in working mode; (b) path route after switching in protection mode due to the indicated failure (in both parts only one direction of the bidirectional path is shown, the return path run along the same fiber in the same direction). of semiconductor lasers. Thus, devising a system to locally protect them is much more important. As far as OTN rings are concerned [49,50], the situation is different from SDH/SONET since here a single wavelength represents a channel and not an MS. In Figure 3.42 an OCh dedicated protection ring (DPRing) is shown, where dedicated 1 + 1 protection at OCh level is implemented. From the figure, the different routes fol- lowed in the protection state with respect to the SDH/SONET MSDPring are clear, while in both cases a whole wavelength is switched immediately before the optical transmitter. The cause of the difference is that, while in the case of MSDPring, the SDH/SONET MS is ended at each ADMs and the ADMs adjacent to the failure react when the failure is detected, whereas in the case of OChDPring, the OCh is terminated only at the origin and terminating ADMs and thus the protection works end-to-end. In order to implement an optical multiple section protection, all the wavelengths have to be switched together by an optical switch immediately after the optical multiplexing, as shown in Figure 3.43. In this case, the OADMs adjacent to the failure react to the failure detection switching the entire OMS on the protection path. Shared protection can be implemented also in OTN rings and it is more common to see it at the OCh layer. The working of an OCh shared protection ring (SPring) is shown in Figure 3.44. Since generally a two-fiber ring is used in the OTN, the wavelength on each fiber is divided into two groups: a worki