 on the same ODN is also considered an acceptable solution. Loss budget: At a minimum, NG-PON is expected to operate over "Class C" ODNs (30 dB loss). Class C++ (>32 dB) operation with optical pre/post optical amplifica- tion is also considered a possible solution for long reach access. Split ratio: A minimum of 64 ways split should be supported, but for some applica- tions (e.g., office consolidation) a 256-way (or higher) split may be needed. In the last case, optical amplification is suggested to overcome power budget limitations. Reach: A 20km physical reach through a passive ODN is required and at least a 60km reach using optical amplification is recommended. If the CCs greatly simplify the carrier problem in evolving from one PON generation to the next, they pose a difficult problem to the equipment vendor. The New Access Network Systems and Enabling Technologies Power split A1, 14 Passive node mux/demux Downstream 11, 14 WDM comb A2, 15 Fiber Fiber A2, 15 Fiber 13, 16 Upstream WDM comb A3, 16 Transmitter receiver FIGURE 11.7 Architecture of the WDM overlay of a certain number of GPONs with colored interfaces. In this case, the ODN has two branching points: a WDM branching and a GPON branching. The simpler way to respect the CCs seems to be through the exploitation of WDM tech- nology starting from the ITU-T standard allocation of the fiber bandwidth in the access area summarized in Figure 11.1. The idea is to use the empty bandwidths to allocate, besides existing GPONs, other GPONs that differ from the standard today only for the value of the upstream and the downstream wavelengths. An example is the use of the 1505-1545 bandwidth (i.e., the great part of the C band) to convey a certain number of "colored" GPON on the wavelengths of the DWDM standard. The network architecture coming out from this idea is plotted in Figure 11.7. In order to try to smooth the traditional difficulties that are always present when design- ing a DWDM bidirectional system, the considered bandwidth has to be divided into two sub-