DWDM systems, which would be needed for the ????»-per customer??? systems just described,
operate over a narrow band of frequencies known as the C- and L-bands (between
1530 and 1620 nm). The wavelengths must be tightly bunched together, fitting
from 32 to 128 channels in the C- and L-bands. Coarse WDM (CWDM), in contrast,
operates on a much wider range of wavelengths (1270??“1610 nm), with a maximum
of 18 channels separated by 20 nm intervals. Figure 7.6 shows a mapping of the
182 Chapter 7
ITU-T G.694.2 CWDM wavelength grid against a standard ITU-T G.652 fiber attenuation
curve. The wider spacing of the CWDM bands means that much more economical components
can be used in these systems, and currently, there is active interest in adding
CWDM to EPON systems to enhance their capacity.
Figure 7.7 presents three possible deployment scenarios for the tree-and-branch
topology WDM PON systems. Obviously, other topologies are possible, though it has
been proven already that this particular architecture minimizes both fiber deployment
as well as splitter count in the PON network [11, 14-17].
Figure 7.7a depicts a tree-and-branch topology with cascaded AWG and PSC
elements, where initial wavelength routing is performed in the AWG, multiplexing a
number of incoming downstream channels, ?»1 ??¦ ?»n, into n output ports.
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