arget a downstream capacity of 40 Gbit/s GPON bandwidth allocation Isolation Wavelength Upstream Upgrade band Downstream video bands XG-PON up XG-PON down 1260-1 1575-1580 FIGURE 11.10 Wavelength plan of the XG-PON1. The New Access Network Systems and Enabling Technologies All the main elements that are present in the GPON are reproduced and adapted for the XG-PON1; thus, the adaptation for higher levels packets is the GEM, suitably adapted to work with the different TDM frames of the XG-PON1 (e.g., it contains an FEC that was not present in the GPON frame), ranging protocol managed by the OLT to prevent collision in the splitting points, DBA both in upstream and in downstream. On top of these characteristics, the XG-PON1 standard presents some aspects of nov- elty with respect to the GPON; among those a much more efficient security, with internal mechanisms for ONU and OLT registration, and a great attention to power consumption. 11.3 WDM Passive Optical Network During the first phase of the NG-PON project, FSAN has also considered the possibility of accessing the shred medium through wavelength division multiplexing. A simple architecture of a WDM-PON is shown in Figure 11.11. The OLT, placed in the local exchange, is constituted by an Ethernet switch managing a certain number of PONs and allowing grooming and QoS protocol management, and by a set of optical interfaces, one for each end customer. A different wavelength is associated to each end customer, SO that the OLT must have WDM-capable interfaces. In order to avoid problems with reflections from the fiber infrastructure, unidirec- tional transmission is generally considered. Thus, a fiber pair is needed in each branch of the PON infrastructure. A couple of athermal mux/demux is placed in the branching point, and every signal is terminated at the final user site with an ONU tuned on the user wavelength. The wavelength plan used in this architecture depends on the adopted WDM technique. If the CWDM standard is used, generally, no more than 16 wavelengths ca