Wavelength Division Multiplexing (WDM)
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Wavelength 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 independence).

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 (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."

DWDM 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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22-03-2012, 12:00 PM

Wavelength Division Multiplexing (WDM)

.ppt   07_WavelengthDivisionMultiplexing.ppt (Size: 2.83 MB / Downloads: 18)
Overview Sub-Lambda Multiplexing

Multiplexing a single low rate data connection onto a wavelength is very inefficient.
For example, if a FastEthernet connection is transported on a WDM wavelength, only 4% of the bandwidth is used. This assumes the wavelength is capable of transporting 2.5 Gbps.
To better utilize bandwidth many vendors support sub-lambda multiplexing.
The cards that support this are often referred to as DataMux or Muxponder cards.
Muxponder cards have multiple ports (FE, GE, etc.). These cards multiplex the data flows and transports them onto a wavelength.
Some vendor use SONET framing to multiplex the data
Others use proprietary protocols

Benefits of WDM

WDM technology allows multiple connections over one fiber thus reducing fiber plant requirement.
This is mainly beneficial for long-haul applications.
Campus applications require a cost benefit analysis.
WDM technology can also provide fiber redundancy.

Specification for point-to-point WDM System

NASA has several sites that may benefit from a small end WDM solution. As such, the following specifications were given to vendors to obtain their solution(s).
A two node system in a point-to-point configuration with no link fail-over capability.
Each node should support 4 gigabit interfaces and 8 fast ethernet interfaces.
The nodes should have redundant power supplies and processors.
The nodes should be SNMP managed.
The distance between nodes will be approximately 3 miles.

WDM provides a managed fiber service.


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