nsidered, they enter the OXC from the add ports and are sent through the switch at the correct output port. At the output, they are multiplexed in the suitable WDM comb and sent to the transmission systems. As in the case of the DXC, the optical Channels Routing is performed by a centralized management system and the OXCs are statically configured by this central software. Since the OXC main function is located in the channel layer, the service channel convey- ing the Multiplex Layers Protocols is terminated at the OXC input and recreated at the OXC output. The availability of a suitable technology has heavily influenced the way in which OXCs are realized. After a huge amount of studies on OXCs realized with optical switching techniques, today, the commercial OXCs are almost all based on an electronic core switch. In these OXCs, after WDM demultiplexing, the optical signal is translated in the electrical domain and processed by an electronic core. At the output the signals are converted again to optical signals and WDM multiplexed. These machines are OXCs and not DXCs since they work on the signal carried on a single wavelength as a whole, without performing TDM operations. To better clarify the logical functionalities of an OXC, the detailed layer model of the OXC is shown in Figure 3.38. OCh-OH OMS-OH OMS-OH OTS-OH OTS-OH FIGURE 3.38 Layered functional scheme of an optical cross connect. The OTN layers are indicated with OTS, OMS, and OCh. Networks Fundamentals and Present Architectures Besides the OXC, also MOXC can be devised in the optical layer; they manage the OMS as a whole, without WDM Mux/Demux. A MOXC has a certain number of input line ports, where the incoming WDM signals are not demultiplexed, but for passive extraction of the information carried out by the service channel. However, the service channel is not termi- nated, but it follows the signal in the switch. As in the case of the OXC, the whole WDM combs are switched as a single entity and sent to the output ports. Clients of a MOXC are OXCs and