tails of the selection filter. In the OXC schemes that we have introduced, this effect mainly happened at the input, where a set of AWGs demultiplexes the incoming DWDM signals. The second crosstalk source is the SO called InB crosstalk and is mainly due to the non- perfect isolation of the outputs of the switch matrixes that are present in the OXC core. Let us consider the situation of Figure 10.19, where a part of the switch fabric is shown beside two paths at the same wavelength. The paths of Figure 10.19 have the same wavelength and are directed to different output ports and give rise to InB crosstalk. In this case, the interfering signal is completely transferred in the bandwidth of the use- ful signal after a simple attenuation. Its shape is thus that of a modulated signal. For this reason, it is useful to distinguish two kinds of InB crosstalk: the crosstalk from the same signal that arrives at the switch input following secondary paths besides the main one (see Figure 10.19) and the InB crosstalk coming from other channels. The first contribution can interfere with the useful signal if the phases are matched. The reciprocal phases of the two channels are to be considered random variables due to phase noise and to the different paths; thus, coherent ripples can be generated in the pulse experimenting interference (see Figure 10.20), and the distribution of the crosstalk penalty has the shape of Figure 10.21. The result is that in the presence of the same interfering channel suppression, interference can be occasionally twice stronger with respect to OutB crosstalk. In-band crosstalk at 11 due to other channels Switch In-band crosstalk at 11 FIGURE 10.19 due to the same channel Example of InB interference from the same and from other channels in the switch fabric of a transparent OXC. Telecommunication Networks Signal pulse after filtering and interference Selection filter pulse response NRZ transmitted One interfering pulse pulse mullin Normalized time (A.U.) Normalized time (A.U.) FIGURE 10.20 Time domain