 output, where N is the node degree and W the number of wave- lengths of each input DWDM system; however, only W among them can transmit a real information and the packets arriving from them all have different wavelengths. Thus, every line is simply multiplexed with the others using a passive combiner after amplification by a linear SOA or a single channel EDFA. This amplification is needed due to the fact that in a normal case in which W = 90 and N = 5, we have a 450 X 5 splitter whose loss is about 26.5 dB. The ASE introduced by the amplifier is partially compensated in the transparent path of the packets through the network by the adoption at the switch input of a regenerating WC, like a converter based on XGM in SOAs. 1 x N In NW FIGURE 10.44 Example of switch fabric based on the combination of wavelength division and space division switching. Telecommunication Networks 10.4.1.3 Digital Optical Processing Even if massive amounts of digital operations are always carried out by electronic proces- sors, the need of digital optics is unavoidable if an all-optical packet switching is considered. Even at bit rates at which electronic processing is still possible, like 40 Gbit/s, a few oper- ations on the packets payload are needed and they have to be carried out by a simple optical logic if the payload is not to be converted into the electronic form. Moreover, several prototypal implementations of all-optical packet switching has envi- sioned very high speed for the wavelength channels, at which header processing also has to be carried out by a dedicated optical logic [62-64]. If the signal speed is within 10 and perhaps also 40 Gbit/s, SOAs are very attractive to provide the nonlinear elements for all-optical logic gates. The simpler scheme adopting SOAs is shown in Figure 10.45: A single SOA is designed SO as to have very low saturation power (e.g., short and highly doped in the p and n zones surrounding the active region) can constitute a NOT gate if a probe constituted of a peri- odical sequence of pulse is inj