s for the moment out of reach. This is why the use of these circuits is a consolidated practice at 10 Gbit/s, and is a chal- lenge at 40 Gbit/s, where the advantage of having a greater receiver tolerance to disper- sion has to be balanced with the greater power consumption, SO it is not feasible for serial transmission at 100 Gbit/s. To close the discussion on FFE-DFE, it is worth mentioning the proposal of nonlinear FFE-DFE based on the Volterra algorithm [77]. These nonlinear equalizers can mitigate the effects of dispersion much better than the classical FFE/DFE, but are more complicated to build. In a practical system the Volterra nonlinearity is limited to second and third order. However, the number of filter coefficients for such a nonlinear FFE of appropriate order (e.g., five taps) is still very high even if methods to reduce the complexity separating the linear from the nonlinear parts have been proposed. 5.8.3.2 Maximum Likelihood Sequence Estimation Equalizers Theoretically much more powerful with respect to FFE-DFE, the equalizers based on the MLSE algorithm, after a period of pure theoretical study, have nowadays started to become practical circuits while electronics becomes more and more fast and functionally rich [78]. An MLSE equalizer solves the problem of identifying any hidden sequence of initial states (in the transmission case the transmitted bits) starting from the observed sequence of the final states (in our case the received samples before decision). The Viterbi algorithm [79] operates on the assumption that the system under observation is a state machine, that it can be characterized by a state map composed of a finite number of states and by a set of allowed transitions that connect each state to the states that can be reached starting from it. The algorithm is based on a few key assumptions: If the system starts from one state and arrives at another state, among all the pos- sible paths connecting the two states on the state graph, one and only one is the most likely, which is called t