 numbers, the most used transmission speed in the core and metro areas is 10 Gbit/s, while 40 Gbit/s systems are starting to spread in the long-haul network, and system vendors are concentrating the research for a new generation of products on 100 Gbit/s transmission. Transmission channel Message Message Modulator Receiver source estimate Carrier Carrier source recovery FIGURE 6.1 Block scheme of a generic single-channel transmission system. Transmission Systems Architectures and Performances 6.2.1.1 Wavelength Division Multiplexing Whatever design is adopted, the available bandwidth in the third transmission window of optical fibers is SO wide that it is not possible to exploit it with a single time division multiplexing (TDM) channel. Thus, almost all metro and core transmission systems use wavelength division multi- plexing (WDM), as shown in Figure 6.2. At the link near end, several transmitters generate different optical channels using car- riers at different frequencies SO that they can be multiplexed by an optical multiplexer and sent into the fiber. At the far end, the channels are demultiplexed and processed by an array of optical receivers. When the optical channels are densely packed in the optical spectrum (e.g., 100 or 50 GHz apart from one another), the multiplexing is called dense WDM (DWDM). DWDM systems are long-reach and high-capacity systems, require high-quality compo- nents, a great design and testing effort, and, therefore, are expensive. In particular, DWDM transmitters have to be narrowband and frequency stabilized, requiring an accurate temperature control. The WDM technology can also be adopted designing wide-spaced channels, SO that thermal stabilization of transmitters is not needed. These are CWDM systems, designed mainly for application in the metro and access area. 6.2.1.2 Transmission System Performance Indicators In all transmission systems, noise and other signal distortions are introduced at various points; thus, the extraction of the transmitted symbols from the received signal