in Chapter 11. Radio bridges are not considered, even if they are still used in the network in particular situations. Radio bridge architecture and performances cannot be analyzed without a suitable review of microwave techniques; this technology is not driven by telecommunications and we feel it would be impossible to condense it in a book on telecommunication networks. The first part of the chapter is devoted to the most basic system: intensity modulation direct detection (IM-DD) systems without optical amplifiers. Even if the use of optical amplifiers is widespread, unamplified systems are still used for particular applications, especially in rural areas, when a high-capacity trunk is needed to connect local central offices that are not SO distant to justify optical amplification. The same systems are also used in the connection of mirroring data centers and in private security area networks. However, the choice to consider this system first is due mainly to their greater simplicity that allows all the elements needed to analyze the performance of an optical system to be considered with minimum added complications. In the second part, optical systems for long-haul and ultra-long-haul transmission are con- sidered. Finally, in the third part of the chapter, the hardware architecture of a transmission system is analyzed, putting into evidence those issues, like power consumption and real estate occupation, that in practice are often more important than one more channel at 10 Gbit/s. Telecommunication Networks 6.2 Intensity Modulation and Direct Detection Transmission 6.2.1 Fiber-Optic Transmission Systems The block scheme of a generic single-channel transmission system is reported in Figure 6.1 [1-3]. A transmitter receives as the input a stream of symbols containing the message to be transmitted and the so-called baseband signal. The transmitter encodes the symbol stream into the baseband signal and then uses the result to modulate the carrier; in optical systems the carrier is a continuous wave radiation emitt