Division Multiplexing (WDM)
In a WDM system,
each of the wavelengths is launched into the fiber, and the signals are demultiplexed
at the receiving end. Like TDM, the resulting capacity is an aggregate of the
input signals, but WDM carries each input signal independently of the others.
This means that each channel has its own dedicated bandwidth; all signals arrive
at the same time, rather than being broken up and carried in time slots. The difference
between WDM and dense wavelength division multiplexing (DWDM) is fundamentally
than does WDM, and therefore has a greater overall capacity.The limits of this
spacing are not precisely known, and have probably not been reached, though systems
are available in mid-year 2000 with a capacity of 128 lambdas on one fiber.These
include the ability to amplify all the wavelengths at once without first converting
them to electrical signals, and the ability to carry signals of different speeds
and types simultaneously and transparently over the fiber (protocol and bit rate
the carrying capacity of the physical medium (fiber) using a completely different
method from TDM. WDM assigns incoming optical signals to specific frequencies
of light (wavelengths, or lambdas) within a certain frequency band. This multiplexing
closely resembles the way radio stations broadcast on different wavelengths without
interfering with each other (see Figure 1-7). Because each channel is transmitted
at a different frequency, we can select from them using a tuner. Another way to
think about WDM is that each channel is a different color of light; several channels
thenmake up a "rainbow."
mesh networks, consisting of interconnected all-optical nodes, will require the
next generation of protection. Where previous protection schemes relied upon redundancy
at the system, card, or fiber level, redundancy will now migrate to the wavelength
level. This means, among other things, that a data channel might change wavelengths
as it makes its way through the network, due either to routing or to a switch
in wavelength because of a fault.
The situation is analogous to that of a virtual
circuit through an ATM cloud, which can experience changes in its virtual path
identifier (VPI)/virtual channel identifier (VCI) values at switching points.
In optical networks, this concept is sometimes called a light path.
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