PDF of this tutorialTo understand the cost issue, we can examine the typical regenerator system shown in Figure 8 of Topic 5 of this tutorial. Because the equipment at each regenerator site is essentially a duplicate of the equipment at both endpoints of the 10 Gbps system, each regenerator site adds about 100 percent to the cost of an end-to-end system without regenerators. Therefore, a 600 km end-to-end system with 2 regenerator sites would be about 3 times the cost of a system without regeneration.
As stated above, regeneration provides other benefits in addition to eliminating concerns about PMD; primary among those is the boosting of the optical signal. However, because regeneration, and therefore the cost of regeneration, is wavelength specific, the economics of regeneration get worse as more channels are added on each fiber. For this reason, end-to-end systems that do not exhibit PMD problems utilize optical amplifiers instead of regenerators to boost the power of the optical signal.
PMD compensation, like regeneration, is performed on a per-wavelength basis. This is due to significant variability in PMD over wavelength within a transmission band. However, a single PMD compensator at the receive side of a 10 Gbps signal can mitigate the PMD effects that build up over an entire transmission span. Therefore, unlike the regeneration option that may require 2 separate sites to eliminate PMD on a 600 km span, a single PMD compensator located at the destination or receive side of the optical signal can improve the end-to-end performance of the transmission path.
PMD compensation can be accomplished for a fraction of the cost of a single regenerator site's equipment costs. In addition to the equipment costs associated with regenerating optical signals, there is also the issue of the costs of the regenerator sites themselves. Since these regenerator sites must be located within certain distance ranges along the optical transmission path in order to operate most effectively, the required placement for these sites may often be in difficult geographic locations. The buildings to accommodate these sites are often more costly to construct and maintain than equivalent real-estate space in the most desirable urban locations, simply because the site is so remote.
