channels in large groups, the channel directed toward the same output DWDM system, that are directed to the same geographi- cal location, even if on different wavelengths. On the ground of this observation is possible to introduce in the OXC switch fabric a certain degree of blocking, maintaining the property that whatever the switch state, any input channel can access every output DWDM comb, thus being able to be directed along any OXC output directions. The easiest way of doing this is represented in Figure 7.35 and it consists in separating inside the OXC channels having the same wavelength and assigning to each wavelength group a different switch fabric. In our example this corresponds to deploy 128 optical switches 4x4. These kinds of switches can be realized using MEMs 2D technology in a very compact shape also inte- grating several switches on the same chip. Also, the drivers of 2D MEMs are quite sim- pler with respect to those of 3D MEMs, SO contributing to the complexity and real estate sparing. The architecture of Figure 7.35 was first proposed for optically transparent OXC where wavelength conversion is not possible, SO that a general wavelength continuity has to be maintained across the switch [58]. However, in our case we are discussing the so-called opaque OXC, where transponders are located both at the input and at the output of the switch SO to regenerate the signal and divide from a transmission point of view input and output WDM systems. In this case, there is no need that the inner wavelength set of the OXC coincides with the wavelength set used by transmission systems. In reality, if the exact wavelength definition of each Switching Systems: Architecture and Performances 11 Switch 12 Switch 13 Switch Local Local channels channels FIGURE 7.35 Architectural scheme of an opaque OXC with a wavelength planes-based switch fabric. channel is not needed for the switches to work, there is no real need that the channels are colored inside the OXC switch fabric, since they are always spatially separated.