It is the nature
of modern communications networks to be in a state of ongoing evolution. Factors
such as new applications, changing patterns of usage, and redistribution of content
make the definition of networks a work in progress. Nevertheless, we can broadly
define the larger entities that make up the global network based on variables
such as transport technology, distance, applications, and so on.
WDM increases 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.
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 then make up a "rainbow." In a WDM
system, each of the wavelengths is launched into the fiber, and the signals are
demultiplexer 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
one of only degree. DWDM spaces the wavelengths more closely 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.
has a number of other notable features. 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 independence). DWDM is the
clear winner in the backbone. It was first deployed on long-haul routes in a time
of fiber scarcity. Then the equipment savings made it the solution of choice for
new long-haul routes, even when ample fiber was available. While DWDM can relieve
fiber exhaust in the metropolitan area, its value in this market extends beyond
this single advantage. Alternatives for capacity enhancement exist, such as pulling
new cable and SONET overlays, but DWDM can do more.
What delivers additional value
in the metropolitan market is DWDM's fast and flexible provisioning of protocol-
and bit rate-transparent, data-centric, protected services, along with the ability
to offer new and higher-speed services at less cost.
Digital Audio Broadcasting
Digital Audio's Final Frontier-Class D Amplifier
Digital Light Processing
Digital Subscriber Line (DSL)
Digital Video Editing
Direct to home television (DTH)
Direct Current Machines
Distributed Integrated Circuits
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