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110 Chapter 5

cations channels. Yet the industry began to recognize the changes in
consumption patterns.The demand for multimedia communications,
video, and wide-area networks (WANs) started to erode even the
highest capacities available.
To solve this problem, researchers began to experiment with the
use of more than one light beam on the same cable. Light operates in
the frequency spectrum similar to the older cable TV systems
employing frequency-division multiplexing (FDM), as shown in Fig-
ure 5-3. By using different radio frequencies on a cable TV system,
the carriers were able to expand the number of TV channels avail-
able to them on the same coaxial systems.Why not do the same thing
with the various frequencies of light?
Wave-division multiplexing (WDM), in contrast, can carry multi-
ple bit rates, enabling multiple channels to be carried on a single
fiber. The technique quite literally uses different colors of light down
the same fiber to carry different channels of information, which are
then separated out at the distant end by a diffraction grating that
identifies each color. All optical networks employing WDM with add-
drop multiplexers and cross-connects permit this. Dense WDM
(DWDM) systems multiplex up to 8, 16, 32, or more wavelengths in
the 1,550-nanometer (nm) window, increase capacity on existing
fiber, and are data-rate-transparent.
WDM was first developed to increase the distance that signals
could be transported in long-distance networks, from 35 to 50 km to
as many as 970 km or more with optical amplifiers. Subsequently,
companies discovered that DWDM would work in metropolitan net-
works just as well.These DWDM ring systems can be connected with

Figure 5-3
Frequency-division Multiple
Channel 1
multiplexing has Channel 2 Channels
been used by cable Channel 3 FDM
Channel 4 @ different
TV operators to Channel 5
carry more Frequencies
information on
their coaxial cables.