l cards perform all the functionalities needed to manage the incoming DWDM signal in the correct way. Essentially, we have Transponders: These are the cards where the incoming signal related to one or more channels is detected, regenerated, and retransmitted with the output power and the wavelength stability that is needed to have a correct propagation along the line. Transponders are used generally to feed the WDM system with client channels, but they can also be used (when suitable configuration is performed) to regenerate the channels in an intermediate site, when this is needed. Even if incoming and outgoing signals have essentially the same characteristics, transponders are needed in all the high-performing systems since a DWDM sys- tem has very specific requirements in terms of power and power stability, wave- length stability, and other characteristics that the client signal generally does not have. Naturally, open systems can exist, that is, systems that have to be used in con- junction of switching machines of the same vendor and that directly receive well- conditioned signals, since the DWDM interface is embedded in the client machine. In this case, the transponder is substituted by a front-end card that does not per- form optical-to electrical-to-optical conversion, but simply sends the received sig- nal online after performing measurements on the signal to make sure that it is compatible with the DWDM system. The open system solution however, up to now, does not have a great market success, especially in the incumbent carriers' networks. As a matter of fact, this technology obliges the carrier to buy a large amount of machines performing different net- work functionalities from the same vendor. Even if this brings the reduction of the amount of needed hardware, it also takes away the carrier possibility of leveraging on competition to lower the prices of different machines, since at that point the process of changing either DWDM or client vendors is very difficult and expensive. On the other hand, sma