ombination of a local improvement of the clock electronics and a more performing network timing distribution in case synchronous time- division multiplexing (TDM) is used. Analogously, the receiver clock jitter depends on the imperfection of the receiver clock electronics that causes the sampling instant before the decision device to fluctuate ran- domly around the average value. Also in this case, the problem can be faced with a com- bination of a local improvement of the clock electronics and a more performing network timing distribution. The propagation jitter is caused by a different phenomenon and affects systems where SPM is partially compensated by a residue chromatics dispersion [23]. This jitter Next Generation Transmission Systems contribution becomes evident for pulses with a full width at half maximum (FWHM) of the order of few ns, that could be used in return-to-zero (RZ) 100 Gbit/s systems. The propagation jitter is a combination of the jitter due to pulse interaction and to the jitter due to ASE to pulse nonlinear combination. The first effect is a reduced form of the Gordon-Haus jitter that appears in soliton propa- gation. This jitter caused by attraction between nearby pulses becomes more and more effective by increasing the Chromatic Dispersion-SPM compensation degree [24,25], and comes into play especially when strong under-compensation of chromatic dispersion is used to counterbalance SPM. The other cause of propagation jitter is the phase noise pro- duced by the nonlinear ASE to signal coupling and it is known as Gordon-Mollenauer effect [26]. 9.2.4.3 Electrical Front End Adaptation A serial 100 Gbit/s signal has a huge bandwidth and it is not easy to device receivers with a low-pass bandwidth of 100 GHz. Distortions in the photocurrent directly affect the receiver performances via ISI and loss of power in the bit interval. Great progress have been made in the last years, but this is yet an important challenge [27]. 9.3 Multilevel Optical Transmission In the previous section we have seen tha