ng group and a protection group, where the working wavelengths on the outer fiber are devoted to protection in the inner fiber and vice versa. Working chan- nels, 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 failure are rerouted in the opposite ring direction by using the other fiber and the set of protection wavelengths. Networks Fundamentals and Present Architectures OADMI FIGURE 3.43 Scheme of an OMDPring: (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). Comparing OChDPring with OChSPring several differences come out. First, in channel dedicated protection ring (ChDPring), a channel is routed unidirectionally and, consider- ing both working and protection wavelengths, occupies a single wavelength on all the ring extent and on both the fibers. Thus, in this kind of ring, whatever the traffic, the maximum number of channels that can be routed is equal to the number of available wavelengths in the WDM comb. Path 1 Path 1 Path 1 Path 1 Path 2 Path 2 Path 3 Path 2 Path 3 Path 2 FIGURE 3.44 Scheme of an OChSPring: (a) paths route in working mode; (b) paths route after switching in protection mode due to the indicated failure. Telecommunication Networks In the case of the OChSPring, the bidirectional routing algorithm allows both working and protection wavelengths to be shared among different channels. As a matter of fact, as is shown in the figure, channels whose routes do not superimpose topologically can be routed using the same couple of working wavelengths and, in case of need, can be pro- tected using the same couple of protection wavelengths. Naturally, they cannot be simul- taneously protected; otherwise, a conflict on the protection wavelength happens. This sharing mechanism i