id- ered, as can be readily demonstrated. It is interesting to notice that minimizing the noise is not the only meaningful way to optimize the span distribution in a DWDM long-haul system. Due to the importance of nonlinear effects in determining the system performances, it is also meaningful to search a condition of minimal nonlinearity for a given length of the link, span number, and transmitted power. Such a condition can be analytically expressed through the minimization of the link equivalent length that depends on the amplifier spacing. Thus, the function to minimize is now (6.73) By repeating exactly the same procedure applied in the case of the ASE noise, it is possible to demonstrate that also in this case the minimum equivalent length is achieved when the spans have all the same length. This result is quite intuitive since if a span is lon- ger the amplifier compensating that span has to launch more power in the fiber, causing more nonlinear effects and this is not compensated by the fact that the shorter span is less affected by nonlinearity due to the exponential growth of nonlinear efficiency with length. From the previous discussion, it is clear that not only the situation in which all the sys- tem spans are equally long is optimum both under a noise and from a nonlinear effect point of view, but also that the performances rapidly degrade if this condition does not hold. 6.3.2 Design of Long-Haul Transmission Systems The great part of the products offered on the market for long-haul transmission is based on DWDM and on EDFA amplifiers. In this section, we will sketch the procedure to design from an optical point of view a system of this type. A few elements on the thermal and mechanical design will be covered at the end of this chapter. Essentially the design of a long-haul DWDM system follows five steps: Transmission Systems Architectures and Performances 1. System draft design: In the first phase the main system parameters are selected like the number of channels and their bit rate, the channel sp