each wavelength, and WDM-PON and XG-PON on the other side, where WDM-PON is based on CWDM again, but each wavelength carries a 10 GbE channel, which we will call XWDM-PON. In this section, we will limit our analysis to an architectural and performance compari- son since an economic comparison is critically dependent on commercial and industrial strategies, and it is quite impossible to do if on one side there is a commercial product and on the other, a system under prototyping. However, even without an explicit calculation, the main point driving the cost structure of WDM-PON and GPON/XG-PON will be clear. In order to correctly carry out the comparison, we have to concentrate our attention on a single WDM-PON architecture; here we will consider in both cases the simple architecture of Figure 11.11, where colored ONUs and unidirectional transmission are used. Capacity: GPON versus WDM-PON The capacity per user of a WDM-PON is easily evaluated: a single wavelength is dedicated to each end user. In general, a GbE signal is transmitted on each wave- length, assigning a capacity of 1.25 Gbit/s to each end user. It is worth noting that the WDM-PON has no particular advantage if part of the signal is constituted by pure broadcast (e.g., conventional IP-TV): the broadcast signal has to be replicated by the OLT on every wavelength and independently sent to each user. The evaluation of the GPON capacity per user is not SO simple as we have seen in the dedicated section. The New Access Network Systems and Enabling Technologies Using the example we have cited there when we calculated the GPON expan- sion capacity, let us try to carry out the calculation also for the WDM-PON. Since we have to feed 400 customers through 32 ONUs placed in the basements of the buildings of the area, we use the fiber to the building (FTTB) configuration. In the case of WDM-PON, we will have 16 ONUs, thus every ONU will have twice the traffic load. Since the WDM-PON is symmetric, the evaluation is only for the downstream, which is the most critica