result from a capacity point of view. However, we must not forget that this is not for free, and whatever technology solution will be found to decrease the cost of the XWDM-PON interfaces, a certain price difference should always exist between the two categories of systems. Security and unbundling: Both cases WDM-PON assigns to each user, a dedicated wavelength. This is not exactly the same as the dedicated physical carrier that is assigned to the user in the copper network, but the situation is quite similar. Due to the presence of mux/demux in the branching point, every ONU receives only its own signal, SO that a user can- not gain access to signals directed to other users. Moreover, a wrong working of Telecommunication Networks a single ONU cannot influence the signals of other users, thus a good degree of security is guaranteed. As far as unbundling is concerned, different wavelengths could be assigned to different carriers if a suitable mechanism is implemented in the OLT. This is not a physical separation exactly, because the physical layer is common among different carriers and data security is guaranteed by wavelength separation. In the case of GPON, each user receives the signal directed to all the users SO that it is possible to gain access to the signal directed to another user by simply working on the ONU. Moreover, if the ranging protocol of a single ONU does not work, the wrongly synchronized signal interferes with the signals from other ONUs in the branching point damaging other users. As far as unbundling is concerned, it is practically impossible using GPON, unless virtual unbundling is considered. In this case, an amount of bandwidth in the shared downstream channel is assigned to a competing carrier under a defined SLA. Fiber utilization: Both cases Due to the fact that bidirectional transmission is used in the GPON case, while in our example WDM-PON unidirectional transmission is adopted, the fiber infrastructure is clearly better exploited by the GPON. As shown in Section 11.3, unidirectional