ssues here. G-PON fiber access network standards recommend that upstream traffic (from the customer to the local exchange) be in the 1260- to 1360-nm range and, as shown in Figure System Design and Implementation 6.18, it can lie close to 20 for G.652 fiber (Keiser, 2006; ITU-T, G.652). The maximum upstream datarate for which full specifications are published at the time of writing is 1.25 Gbit/s (ITU-T, G.984.1-G.984.6). The maximum recommended physical reach is 20 km, the transmitted powers are less than +7 dBm, and the downstream channels are at ~1490 and ~1550 nm on the CWDM grid. All of these values have been selected to ensure that nonlinear crosstalk is not an issue, even though the upstream wavelength can be close to 20 in some circumstances. The earlier discussion of CWDM in the section on WDM, especially with regard to Figure 6.2, raises the question of whether ~20-nm interchannel spacing is sufficient to prevent FWM when at least one of the channels is near 20. Okuno et al. (2002) show that for fiber spans of 25 km and launched powers of +4 dBm per channel at 10 Gbit/s, this is not the case, even when two of the laser wavelengths have drifted with temperature SO that the most critical channels are 17 nm apart. Therefore, CWDM can be a viable solution across the whole of the fiber's available spectrum for applications such as small-circumference metropolitan rings. The operation of longer-span DWDM systems in the vicinity of 20 is potentially more problematic, and it is an issue for operators in countries (such as Japan and Brazil) where there is a large amount of installed DSF. The usual approach is to use the L-band as much as possible. However, the fiber losses are higher beyond 1565 nm and L-band EDFAs nor- mally require longer spans of doped fiber and higher pump powers than their C-band counterparts, which can make them more expensive. For the reasons just given, it can be beneficial to place at least a few channels in the C- band. FWM-based crosstalk can be ameliorated by using unequally spaced c