DM realized by demultiplexing the incoming DWDM comb and managing the drop- pable channels using optical 2 X 2 switches. Since multiplexer and demultiplexer based on the AWG principle can be integrated with thermo-optic switches on a silica on silicon platform, the schemes of figures from Figures 7.42 through 7.45 are not only principle schemes, but real products are built with these architectures. Nevertheless, there are a plethora of different optical technologies competing with the AWG-based design suitable for the realization of S-OADMs. Almost all these Telecommunication Networks Passthrough channels Input DWDM comb Output DWDM comb Drop band Add band 1'2'3' 123 1'2'3' Drop-add channels FIGURE 7.46 Scheme of a tunable OADM realized by demultiplexing the incoming DWDM comb and managing the drop- pable channels using an n X n optical switch, where n < W is the number of droppable channels and W the number of DWDM channels used in the ring. FIGURE 7.47 Scheme of a tunable OADM realized by demultiplexing the incoming DWDM comb and managing all the channels using a W X W optical switch. Add channels Drop channels technologies are based on the observation that the switch fabric that is needed into an OADM is not as big as the switch fabric inserted into an OXC. Thus, both the losses are limited allowing optical transparency of the component and the cost results much more affordable. A technology that is quite diffused besides the plain use of multiplexers and demulti- plexers is the use of optical wavelength selective switches (WSS, see Chapter 5) [63,64]. The only functionality at data layer that lacks to the WSS to be a complete S-OADM is the channel Add, that in general is realized via a coupler and an external Add multiplexer to obtain the full S-OADM as shown in Figure 7.48. A brief summary of the performances of practical OADMs for different applications is reported in Table 7.2. All the considered OADMs are a set of cards to be integrated into a DWDM system; thus, all control and configuration is performed 