 transmission can be used in WDM-PON, but comes at some cost. As a matter of fact, in order to achieve a sufficient branching ratio, DWDM is needed, for example, 32 channels with a channel spacing of 100 GHz. A pos- sible design can individuate two different bandwidths to be used upstream and downstream. They can be separated by a gap of about 800 GHz to pre- vent destructive interference from reflections. In this way, a branching ratio of 16 can be achieved. However, 100 GHz channel spacing requires cooled DFB lasers to be used both in the ONU and in the OLT. This fact, besides the greater cost of the mux/demux, clearly influences the cost of the system. In order to cope with this problem, the use of a WDM comb derived from the filtering of a single broadband noise source [20] has been proposed, but it is not clear yet if real cost advantage is achieved. Optical link budget: WDM-PON versus GPON The transmission scheme of WDM-PON is quite simple: attenuation is given by the loss of the mux/demux and by fiber propagation (taking into account con- nectors, patch panels, and other signal losing elements that can be present in the access infrastructure). Focalizing on CWDM-PON, standard CWDM optics can assure a transmitted power of 0 dBm, while the receiver sensitivity depends on the used detector. Using a PIN, the sensitivity at Gbit/s (assuming that a GbE is transmitted) can be about -18 dBm. This number increases to about -28 dBm using an APD. Assuming the use of 16 wavelengths, the worst channel experiences an attenu- ation of about 0.9 dB km. Adding the other contributions and a system margin, we can assume a loss of 1.3dB/km. Inserting these numbers in a very simple link budget evaluation, we obtain 10km using a PIN at the receiver and about 22 km with an APD. In the case of GPON, the link budgets are standardized in different GPON classes and are reported in the GPON description section. In any case, classes B The New Access Network Systems and Enabling Technologies and C power budgets are of the same order of