ed by a laser in the suitable transmission window of silica fibers. The component that performs this operation, as seen in Chapter 4, is called modulator. The modulated signal is launched in the transmission line that is constituted by a trans- mission fiber interrupted, if it is the case, by online sites where the optical signal is processed, for example, amplified, filtered, regenerated, and SO on, depending on transmission needs. At the end of the transmission line, a detector transforms the incoming optical signal into an electrical current. Starting from the electrical current, the receiver reconstructs the carrier signal using, for example, an electronic phase locked loop and uses the recon- structed carrier to extract, via a demodulator followed by a sampler and a decision thresh- old device, the original symbol stream. The method we have just described for estimating the bit stream is called hard decision [4], since it is carried out simply on the ground of the signal statistics, without any help from codes for error correction or channel estimation. When an error-correcting code is used, it is located after the decision block to correct errors in the estimated bit stream. Another procedure, could be possible where the line code is used not to correct errors after the bit stream estimation, but to help the estimation device not to make errors. This method is called soft decision and generally is few dB better than hard decision at the opti- cal error probability levels. Soft decision is not implemented generally in optical systems due to the speed required to the processing electronics by the high bit rate. This situation is changing with the big progress of complementary metal oxide transistor (CMOS) electronics, and we will see that soft decision is also entering into the optical transmission systems' designer resources. Due to the improvements in the components technology and systems design, very high- speed transmissions are possible. At present, even if 2.5 Gbit/s systems are still installed in great