g fiber propagation can be described as a random rotation in the Stokes space, the property of two polarization states to be orthogonal is maintained. Telecommunication Networks Moreover, also nonlinear crosstalk caused by FWM and XPM does not take place with nearby channels since these effects both require that the beating between the interfering channels is different from zero. This technique is less sensitive to the presence of a nonlinear contribution to the evolu- tion of polarization since, as far as we are in a regime of small nonlinearity, it implies only the resurgence of a small interference percentage. Polarization diversity can be used in a different manner also. If NRZ with a small duty cycle is used, nonlinear interaction between adjacent pulses in the pulse stream at a certain frequency can manifest, with a stronger and stronger efficiency while SPM and chromatic dispersion compensation is more and more effective (see Section 9.2.4 for more details and the references). This phenomenon causes a jitter in the transmitted pulse stream that is to be controlled. One of the possibilities to control this effect is to transmit adjacent pulses on orthogo- nal polarizations. If this technique is used generally adjacent WDM channels are syn- chronized SO that pulses on adjacent channels are orthogonal in polarization in order to achieve a global interference minimization. 9.5 Design Rules for 100 Gbit/s Long Haul Transmission Systems Up to now we have analyzed "ideal" systems, where the ASE noise is the only limit to system performances. In Chapter 6, we have seen that this is not a realistic model and many other impairments have to be considered in designing a transmission system. In this section, we will face the problem of designing realistic systems in a variety of situations. In this way, we will assess the real potentialities of the different system archi- tectures when practical requirements are set. We know that a large number of long haul systems are installed all around the world, thus the requireme