iers. Moreover, in order to use uncooled sources, the channels are SO spaced in frequency that there is no problem in using thin film filters to select them, avoiding any distortion due to filter cascading. The challenge in designing CWDM systems is to satisfy the requirements in terms of power budget by maintaining the cost effectiveness that customers expect from this technology. There exist two types of CWDM systems on the market, that are directed to different applications: pure CWDM and mixed DWDM and CWDM. Pure CWDM systems are exclusively tailored on CWDM technology. Generally, they are simple machines whose main added value is the low cost. These systems are generally completed with regenerators and fixed ADMs based on thin film technology and are used in the metro access area and in rural areas where the local exchanges are particularly far from the hub of the regional network. Mixed CWDM and DWDM systems are instead capable to host on the same platform both CWDM and DWDM channels following the ITU-T standard channel allocation. The multiplexing stage of mixed systems is generally composed by a first CWDM mul- tiplexer, and a second DWDM multiplexer that fills with DWDM channels the CWDM passbands located in C band. This architecture is shown in Figure 6.40. Chan 17 Chan 18 Chan 19 Chan 1 Chan 20 Chan 2 Chan 21 Chan 3 Chan 22 Chan 23 Chan 24 Chan 6 Chan 7 Chan 25 Chan 8 Chan 26 Chan 27 Chan 9 Chan 28 Chan 10 Chan 29 Chan 11 Chan 30 Chan 12 Chan 31 Chan 13 Chan 32 Chan 14 Chan 15 Chan 16 FIGURE 6.40 Multistage mixed CWDM/DWDM multiplexer. Transmission Systems Architectures and Performances Regional hub LEX 2 LEX 3 LEX 1 Terminal Terminal Regeneration nodes Add/drop intermediate node 10 dB-40 km 8 dB-32 km 3 dB-12 km FIGURE 6.41 Example of CWDM system used to backhaul three small local offices to a hub. These kinds of systems have higher cost with respect to pure CWDM, but allow slower growth after the installation using CWDM channels and faster when using DWDM configuration. Even if CWDM interfaces at 10 