, space, maintenance, sparing, design, and troubleshooting standpoint, limiting their applications in the metro segment. Long-haul applications require periodic regeneration to ensure sufficient signal strength and quality at the receiving end. This results in substantial repeater bays, as shown in Figure 5-18, adding to the total transmission system cost. Although the cost of one Figure 5-17 Typical DWDM equipment rack. Control Shell Local Craft Access Panel Fiber Management Trays (5) Transport Shell Environmental Control Extension Shell (Optional) Chapter 5 OC-192 Regenerator Bay Service Figure 5-18 Terminating OC-192 Regeneration bay. Working Fibers Nonroute Network Elements Diverse Protection Fibers System Working or Protection Fibers Route Diversity Working or Protection Fibers Systems Mixed Tributaries Mixed Tributaries Up to Four Bidirectional Optical Channels Within a Single 7-Foot Bay Four-Channel Regenerator Configuration Serving Mixed Nonroute Diverse and Route Diversity Systems regenerator may appear to be small, these costs can dominate when multi- plied over the total length of a route, especially for heavy routes where there might be several systems contained within the same fiber sheath. 5.2.3 Technology Evolutions in DWDM In Figure 5-19, we see a typical evolutionary environment for DWDM sys- tems. The top portion shows a baseline transmission route that consists of four parallel SONET systems. As the need for expansion arises, WDM and TDM technology can be considered to reduce the number of fibers used in this application. WDM technology has traditionally been considered more expensive than TDM. One reason is that a WDM solution requires a sepa- rate SONET terminal for each channel in addition to the WDM terminal because of the embedded electronic regenerators, as shown at the bottom of Figure 5-19.13 One way to address this issue would be to retire these elec- tronic systems, assuming that they were fully depreciated, and replace them. However, carriers that are bottom-line positive in financia