impairments are somehow managed, the fiber attenuation and the receiver sensitivity set a limit to the product bit rate-distance that can be assumed as a sort of system quality factor. With an APD receiver having a sensitivity of -34 dBm at 10 Gbit/s and a transmitter that injects about 1 dBm in the fiber, the pure power budget limits the transmission to a dis- tance of 140km due to the fiber attenuation assumed to be 0.25 dB/km. Transmission Systems Architectures and Performances Using optical amplifiers to boost the signal along the line this limit is completely removed. Not only the amplifier compensates for the fiber loss, but allows transmitting less power, thus limiting nonlinear effects. Almost all the practical transmission systems that are on the market use optical ampli- fiers; thus, there is a wide variety of amplified systems depending on the application. Here we will divide the systems in three categories: 1. Long-haul and ultra-long-haul systems are characterized by an amplifier chain constituting the transmission line; they can use any type of amplifier, depending on the needs, and their reach exceeds several thousand kilometers 2. Single span systems use only booster and preamplifiers and generally they are mixed EDFA-Raman amplifiers. They are used for applications in which the dis- tances are not SO big 3. Optical ring, characterized by the ring topology, they are used mainly in the met- ropolitan network 6.3.1 Long-Haul and Ultra-Long-Haul Transmission: Performance Evaluation The principle scheme of the simpler long-haul DWDM system is shown in Figure 6.18. After the transmitter, a preamplifier brings the power to the desired level at the transmit- ter; the line is divided into spans and at each span end another amplifier is present. Since, due to the long reach of these systems, dispersion has to be compensated by some in- line device, in-line amplifiers will have the possibility to insert between the amplification stages a loss. This loss will be generally caused by a DCF, even if more comple