ere the tunable client interface is connected. By deploying a limited number of tunable client interfaces, the presence of tunable OADMs allows setup of new connections to be carried out frequently without the need of an immediate field intervention. If OADMs that are not tunable are used, but the add/drop ratio is one, a careful deploy- ment of spare tunable interfacing the network can be remotely upgraded adding several new connections without changing the equipment configuration. The different OADM types can be all realized using multiplexer, demultiplexers, and a progressively more complex switch fabric. Fixed OADMs are generally realized by demultiplexing the incoming DWDM comb and managing the channels one by one. In order not to impose a too high attenuation to pass through channels, frequently a subband demultiplexer is used for a first stage channel separation as shown in Figure 7.44. In the case of switchable OADM, a set of 2 x 2 optical switches are placed along the add/ drop lines as shown in Figure 7.45 SO that the channels of the add/drop bandwidth can be either added/dropped or passed through. In the case of tunable OADM with a reduced add/drop degree, the only variation to the architecture of a switchable device is that the set of 2x2 switches is substituted by a nonblocking 2n switch, where n is the number of accessible wavelength channels as shown in Figure 7.46. Finally, tunable OADM with unitary add/drop degree can be realized by two standard DWDM multiplexer and demultiplexer with a 2W 2W nonblocking switch fabric, where W is the number of wavelengths of the line DWDM system, as shown in Figure 7.47. Switching Systems: Architecture and Performances Passthrough channels Input DWDM comb Output DWDM comb Drop band 123 Add band 1'2'3' 1'2'3' Drop-add channels FIGURE 7.44 Scheme of a fixed OADM realized by demultiplexing the incoming DWDM comb and managing the channels one by one. Passthrough channels Input DWDM comb Output DWDM comb Drop band Add band 1'2' 3' FIGURE 7.45 Scheme of a switchable OA