ch fabric is reported in Figure 10.43. The switch consists of three sections: 1. Wavelength encoder block 2. Buffer and broadcast block 3. Wavelength selector block Telecommunication Networks Wavelength encoder Buffer and broadcast Wavelength selector Out-1 Control processor Out-N FIGURE 10.43 Functional scheme of one of the switch fabrics for optical packet switching proposed in the KEOPS European project. (After Guillemot, C. et al., IEEE J. Lightwave Technol., 16(12), 2117, 1998.) The wavelength encoder block consists of WCs, one per input. Each WC encodes its pack- ets on a fixed wavelength. We have as many WCs as input wavelengths; thus, if we have four DWDM systems with 90 wavelengths at 10 Gbit/s, we have 360 WCs each of which has to accept any wavelengths in the C band and commute it on a fixed and unique wave- length at the input of the switch fabric. Even if the WCs work at a wavelength spacing of 25 GHz in C and L bands (and it is quite a difficult task), there are available no more than 180 wavelengths, and this, in practice, fixes the dimension of the KEOPS all-optical switch: no more than 180 waveleng ths input. Thus, as often happens, the all-optical design is more and more effective while the line rate of the single wavelength increases. As a matter of fact, there is no problem in accepting four fully loaded systems with 45 wavelengths at a bit rate of 40 Gbit/s or four systems each of which transport 20 wave- lengths carrying an NRZ 100 Gbit/s signal. The cell buffer block comprises K fiber delay lines followed by a space switch stage realized using SOAs as fast gates. As a matter of fact, the only role of this stage is either to block or to pass- through the respective wavelength following the commands of the central electronic processor. The last block, the wavelength selector block, consists of demultiplexers that forward the different outputs to SOA gates before the signals are recombined, thus selecting packets from the correct inputs. The broadcasting principle makes a copy of each packet a