DWDM Technology In Metro Networks
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DWDM Technology In Metro Networks

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DWDM (Dense Wave Division Multiplexing) is considered as one of the best technologies to increase bandwidth over an existing fiber plant. It allows one to create multiple “virtual fibers” over one physical fiber. It does this by transmitting different wavelengths (or colors) of light down a piece of fiber. DWDM was first adopted by long distance carriers because the spending in amplification, dispersion compensation, and regeneration made up most of the network equipment cost in regional and national SONET networks. DWDM became more and more popular in metro networks when the local exchange carriers grew their networks. Other than fiber exhaustion, traffic volumme is the major economic factor for deploying DWDM technology in metro networks.

DWDM operates in the range between 1530 and 1565 nm, the so-called C-band that corresponds to the low-loss window of the optical fiber. This is also the range where the Erbium doped optical amplifier is operative.A grid of allowable wavelengths/frequencies of operation is specified by ITU-T, centered at a frequency of 193.1 THz, or a wavelength of 1553.3 nm, and all frequencies spaced at multiples of 25GHz (=0.2 nm) around that center frequency. Commercial systems can have channels at 2.5, 10, and 40Gbps (where the latter are recently becoming commercial) as well as combinations of these in the same system. The higher the bit rate, the higher needs of the power budget meaning that the lasers need to have better signal to noise figures, the amplifier spacing needs to be reduced, amplification needs to be higher for example, by employing two amplifiers in series. Typically 64 channels at 10 Gbps reach a maximum distance of around 1500 km with an amplifier spacing of around 100 km. Long distance transmission systems beyond 1500 km and up to 4500 km are also commercially available using adavanced and more expensive systems.

The DWDM layer is protocol and bit rate independent, which means that it can carry ATM, SONET, and/or IP packets at the same time. WDM technology can also be used in Passive Optical Networks (PONs) which are access networks in which the entire transport, switching and routing occurs in optical mode. With the inclusion of new 3R (reshape, retime,retransmit) devices, internal to the DWDM system, circuits utilizing only DWDM equipment are now able to be built that can span the country. New performance monitoring capabilities have been built into these devices so that maintenance and service of the links is possible. With DWDM as the transmission method, the bandwidth of the existing fiber plant is maximized.

In designing a hybrid SONET and DWDM network, the most common architecture is SONET/SDH over DWDM, where the underlying or core transport is DWDM while the SONET routing is achieved by the overlaid or integrated MSPPs (defined later). The need of SONET granularity is due to the existence of low-speed service demands such as DSI/DS3 and the lack of sub-wavelength grooming in the DWDM platforms.DWDM has a goal to carry the same traffic in as few topological structures (e.g rings) as possible. DWDM does this at the cost of allowing some traffic to traverse longer distances than it does in SONET. For a simple example, the best SONET design may generate a bunch of topolpgically different OC-48 and OC-192 rings,while the most economic DWDM transport network may consist of just a single DWDM ring. Therefore, there are two options in the topological optimization, both of which need to be decided at the SONET design stage.

Traditionally Internet traffic runs on IP which rides on ATM and SONET/SDH or IP over SDH then to an optical layer. The misconception that IP rides on ATM/SDH is that IP traffic is small and has to be combined with other services for cost-effective delivery. Contrary to this delusion the concept of IP over DWDM supports voice, video and data traffic. IP over DWDM networks work by deploying for example a STW-16 circuit directly over DWDM channel. IP over DWDM based networks will allow the carriers to use few wavelengths for carrying legacy traffic and dedicate the rest to high speed data traffic. Elimination of layer(s) (SONET/ATM) will ease the job of network management and will be cost effetive.

DWDM systems can be planned for a large number of channels, however, pay-as-you-grow strategy can be employed and channels added according to demand.The amplifier distance and overall power budget of the system needs to be calculated for the final number of channels from the beginning.

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As a global supplier of fiber optic networking solutions, FS.COM designs, manufactures, and sells a broad portfolio of optical communication products, including PON, subsystems, optical transceivers used in the enterprise, access, and metropolitan segments of the market, as well as other optical components, modules, and subsystems.

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