As discussed in the Transmitter section, lasers in CWDM networks are uncooled, so they have a much lower power consumption. In fact, DWDM transmitters consume almost 20 times more power than CWDM filters, and since heating and drifts are accepted and planned for, transmission cards can be placed closer together.

In large metropolitan areas, network service providers (NSPs) might consider going beyond the standard 80 km CWDM topology. However, when the suggested distances are exceeded, there are power/budget issues and chromatic dispersion limitations to consider. In order to extend the network successfully, a reconfigurable OADM (ROADM) must be added to the configuration. As the very essence of CWDM is to maximize capability while keeping costs low, ROADM deployments are an attractive possibility as they extend the reach of the network with only a marginal increase in the per-channel cost, therefore offering a worthwhile return on investment.

The static optical add/drop multiplexer (OADM)  as a main CWDM network component provides the capability to support a built-in add/drop facility for one or two preassigned wavelengths. It is therefore possible to add or drop any preassigned CWDM Grid wavelength at any OADM site along the way (in a linear link as well as in a ring-type topology). The OADM was designed so as to allow channel re-use in the subsequent spans of the dropped channel.

Like lasers, DWDM filters need to have high isolation, extreme central wavelength accuracy, high stability, and low insertion loss. All these factors increase the price of the filters. CWDM filters, on the other hand, are manufactured at about 40% of the price of DWDM filters.

Receivers are priced according to several factors, but the two main drivers are bandwidth and sensitivity. Receivers with narrow bandwidths and high sensitivity are most expensive. To ensure optimum quality, DWDM transmissions must be free of crosstalk, so the receivers in these networks consist of narrowband filters that, again, are centered according to the ITU-T DWDM grid. Also, since DWDM links often exceed 100 km, sensitivity is another important issue.

In terms of fiber type, metro environments contain a lot of legacy fiber; some is of unknown origin and some displays PMD levels that are too high to be used in DWDM applications. There is also G.653 dispersion-shifted fiber, which makes DWDM impossible in the C band because of four-wave mixing.

In DWDM systems, costly transmitters are typically spaced 100 GHz apart in the C band. Often, though, we hear of networks operating at 50 GHz, and 25 GHz systems are already on the way. A spacing of 25 GHz represents approximately 0.2 nm in the C band. At this level of proximity, wavelength accuracy and stability are of prime importance. The central wavelength must always be centered according to the ITU-T DWDM Grid. The full-width half-maximum (FWHM) of these wavelengths must always be maintained within strict tolerance levels and must not drift under any circumstance; otherwise, the power/signal from one wavelength will seep into adjacent channels. Ensuring that this doesn’t happen is a costly process that involves a thermo-electric cooler (TEC).

In CWDM, this expense is reduced simply by allowing a wider central-wavelength tolerance, a wider FWHM, and drift. Allowing drift implies that the TEC keeping the laser at a stable temperature (required in DWDM) is no longer necessary and can be removed.

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Coarse wavelength-division multiplexing, commonly known as CWDM, is a cost-effective solution for networks requiring fairly high bit rates, at minimal cost. This technology mainly suits short-haul metropolitan environments that have growing bandwidth requirements but do not have a substantial population to share the cost of the network.

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