bands (upper and lower wavelengths) and a guard interval is maintained between the bandwidths. Let us assume that a 200 GHz spacing is used to simplify the optical system design, and that a guard interval of 400 GHz is maintained between the upstream and the downstream optical bandwidths. Thus, it results that we can put 10 channels in the upstream bandwidth and 10 channels in the downstream bandwidth, using exactly the same ODN and without in any way dis- turbing the already existing GPON. This solution is very good for the fidelity to (CCs), that means to guarantee an easy adop- tion by the carriers, and this, for sure, provides a good way of upgrading the network in pace with the penetration of broadband services. On the other hand, functionally, the capacity delivered to a single user is the same as the GPON, unless more than one ONU is allocated at the user's premises. Moreover, and definitely much more important, the overall cost of this solution heavily depends on the cost of the DWDM colored interfaces and on their market. As a matter of fact, while a traditional PON has a single OLT interface for each served ODN, this version of the NG-PON has one interface for each served wavelength. Telecommunication Networks O-band E-band S-band L-band 1260-1360 1360-1460 1460-1530 1565- 1530- Water FIGURE 11.8 ITU-T standard definition of the average attenuation of an SSMF fiber with the names of the standard band- width and the standard CWDM channels. The water absorption peak is evidenced by the dashed line. A way to alleviate the problem related to the interface cost is to adopt CWDM, whose standard grid is reported in Figure 11.8. As is evident from the figure, the water peak is unfortunately located around 1400 nm in the SSMF, almost the only fibers that are installed in the access area, which is just in the middle of the upgrade band (see Figure 11.1). This renders the CWDM almost useless for use, since the number of frequencies that can be used is really small. If the condition to interact with an already exis