n be adopted. ONU 1 CWDM OLT Passive node mux/demux ONU 2 ONU 3 Transmitter receiver FIGURE 11.11 Architecture of a WDM-PON using unidirectional fiber transmission. The CWDM wavelengths are assumed in the figure. Telecommunication Networks This is due to the fact that common fibers have an absorption peak around 1390nm, due to OH ions, and as shown in Figure 11.8, where the absorption profile of an SSMF fiber (ITU-T G.652) is superimposed on the CWDM frequency standard. Assuming a sensitivity of -18 dBm of the receiving PIN, a transmitted signal power of 0dBm and a loss of 5 dB from the mux/demux, an ODN attenuation smaller than 0.65 dB/km is needed in order to achieve a reach of 20 km. This requirement is not fulfilled by the CWDM channels that are around 1390 and 1410 mm, thereby reducing the number of useful channels to 16. Adoption of the CWDM standard has several advantages, from the possibility to use low cost, robust transceivers, which are produced in very high volumes mainly for datacom applications, to the availability of low cost athermal mux/demux. If a high wavelength number is to be used, DWDM is needed. In this case, it is easy to reach the number of 32 or 64 wavelengths in the third fiber transmission windows, at the expense of a higher interface cost. One of the potential problems in deploying the WDM-PON architecture depicted in Figure 11.11, is the presence of colored ONU optical interfaces. As a matter of fact, every ONU receives and transmits a different wavelength. The management of 16 different ONUs during network deployment (in the case of CWDM-PON) can increase operational costs, due both to spare parts management and in-field maintenance. Naturally, in this case also, the need of colored interfaces can be eliminated either by using tunable lasers or by injection locking in FP Lasers. 11.4 WDM-PON versus GPON and XG-PON Performance Comparison At this point, it is interesting to compare the WDM-PON and GPON on one side, where WDM-PON is based on CWDM and Gigabit Ethernet (GbE) channels on 