Chapters 4 and 7, we will not return to OADMs here. Last, but not least, the OXCs are the main switching element in the T-OTN. As a matter of fact, as we have seen at the end of Chapter 4, well connected (with a connectivity between 2 and 2.5) and very wide networks, like the LH networks of the incumbent carriers, have to be designed using mesh architecture if the potentiality of the network has to be completely exploited. The first and more intuitive transparent OXC architecture is shown in Figure 10.10 [29]. We have already commented on the fact that networks entirely realized using such OXC are divided into a number of superimposed and separated networks, one for each wavelength. The further step to avoid this separation is to install at each output port of the OXC a WC. In this way, the space division switch matrix simply determines what output direction the wavelength has to go, while it is the output WC that sets the right physical wavelength to correctly feed the output DWDM system [30]. This OXC architecture is shown in Figure 10.17. In this figure, WCs are also used on local ports, assuming to have frequency-sensitive receiver. If this is not true, as in the great part of the state-of-the-art receivers, WCs on drop ports are not needed. The scheme of Figure 10.17 has NM WCs, where N is the number of wavelengths for each output system, and M the number of output DWDM ports. In a typical OXC, it could be N = 128, M = 5, SO 640 WCs are needed. In order to reduce this number while maintaining accessibility to a WC at every incom- ing wavelength, the architecture of Figure 10.18 can be adopted [30]. In this architecture, the number of WCs is reduced by the use of the two switches that allows every incoming channel to exploit every available WC to reach every free output. A similar solution can also be adopted to incorporate electronic 3R regeneration to be exploited when needed. Telecommunication Networks FIGURE 10.17 Block scheme of a fully non-blocking Local Nonblocking Local switch fabric transparent OXC r