5 Hardware Architecture of Optical Transmission Systems In the previous section, we have analyzed a set of possible designs of optical transmission systems for application in the long-haul and metro network. Typical quality parameters we have used to evaluate the performance have been capac- ity and BER. However, it is important to recall what is derived from Chapter 2, where we arrived to the conclusion that a main target for carriers is to reduce network OPEX. In order to reduce OPEX, it is surely important to optimize the network design using high-performing systems. As a matter of fact, decreasing the number and the complexity of central offices and points of presence is a very important task. Once the network is optimized however, often the parameters that drive the carrier in the choice of a transmission system, among all the products that have more or less similar characteristics, are directly related to the decrease of OPEX. Telecommunication Networks Things like power consumption, real estate, easy maintenance, and safety are very often the parameters making the difference in the choice of a system among the market available products. These considerations, strictly related to the engineering of the system, have driven strongly the evolution of DWDM systems especially after the telecom bubble end. In this section, we will try to put the basis for a practical analysis of the engineering of an optical transmission system. 6.5.1 Mechanical Structure of a Dense Wavelength Division Multiplexing System A typical DWDM system is mechanically hosted in one or more subracks, as shown in Figure 6.43. The same subrack is shown in Figure 6.44, without a few cards, to reveal the card positioning slides and the connectors that put the various cards in contact with the subrack backplane. A rack supports a certain number of subracks (generally from three to five, but this is only an indication and there is a wide variety of solutions) and each subrack hosts a certain number of cards. The dimension of the rack follows deta