t to make an example, an opti- cal path could be correspondent to a single MPLS tunnel after enveloping with the ODU header to start and terminate on the ports connecting the label switching routers with the OXCs. The OCh could multiplex several MPLS tunnels with SDH MSs and ATM virtual circuits (each entity enveloped in a different ODU) and start and terminates on the end OXCs optical ports before the demultiplexing of the OCh envelop. After the OCh envelope is built, different OChs are transmitted on different wave- lengths; when all the wavelengths are multiplexed and the service channel added, we have the OMS and when the OTS header is also added in the service channel, the set of OTN envelopes is complete. A few of the key network elements of the OTN are represented in Figure 3.36, where the difference between the equipment (represented in the upper part of the figure) and the key function of this equipment that is represented in the lower part is underlined and is indicated with the equipment acronym with an F_ in front to indicate that it is a function in the layer model. In particular, in the trail represented in the figure, the following network elements are present: Channel optical cross connects (OXCs) OCh regenerators (LEM) Erbium doped optical amplifiers (OLA) Telecommunication Networks Other important network elements of the optical layer that are not present in the figure are Multiplex section optical cross connects (MOXCs) Optical add drop multiplexers (OADMs) Optical line terminals (OCLT) Raman amplifiers (ORA) The OXC [47,48] is somehow similar to the DXC, if WDM is substituted to TDM. At the OXC line input, there is a number of WDM channels; these channels are demultiplexed, channels directed to the local clients are sent to the drop ports while the others are routed through a switch toward their destination. At the switch output, all the channels hav- ing the same destination are multiplexed together and sent to the output transmission systems. As far as local clients generated channels are co