hanged. Switchable OADMs: In this case the OADM has a certain number of add/drop ports, each of which is associated to one wavelength channel; the OADM can be configured SO to add/drop or to pass through any of these wavelength chan- nels while the channels that are not associated to one add/drop port can be only passed through. Tunable OADM: In this case the OADM has a certain number of add/drop ports and any wavelength channel can be added/dropped from any of the add/drop ports if the OADM is suitably configured. Fixed OADMs are mainly used in point-to-point systems to drop channels in an interme- diate location or to create a branching point or in access to CWDM rings. Switchable and tunable OADMs are instead used mainly in optical DWDM rings. As a matter of fact, using fixed OADMs in optical rings induces several blocking situa- tions that can be solved only via the installation of redundant client equipment and that have to be avoided in a functional network. An example is reported in Figure 7.43 where for the sake of simplicity an optical ring with only two wavelengths (the dark and the clear wavelength) is represented. The ring has four nodes where fixed OADMs are located. This fixed OADM has drop ratio equal to one; thus, they drop both the dark and the clear wavelength together. In the bottom part of the figure the traffic matrix is represented, that is the table of the connection to implement in the ring. When implementing the connection A and B on the dark wavelength two fixed OADMs are located in A and B. Thus, when implementing the connection between C and D the clear wavelength is automatically dropped in the transit nodes A and B and the connection can be realized only regenerating the optical FIGURE 7.43 Example of blocking in an OCh-DP ring due to the use of fixed OADMs. In the figure the blocking situation is removed by adding supplementary regenerators in the transit mode A. Telecommunication Networks channel in A and B and injecting again it into the ring. This is a waste of resources with res