 if needed. In the architecture of Figure 3.52, since there is no switching available at the optical layer, every wavelength has to be processed by an LSR at every crossed node. This will cause a quite inefficient LSR working since several label switching operations are not really neces- sary due to the fact that the input wavelength is simply reconstructed at the output. It is possible to conceive a solution in which pass-through wavelength is not terminated on an LSR, but via a simple fiber patch panel sent to the output DWDM system. This archi- tecture is surely CAPEX effective, but causes an embedded rigidity in wavelength assign- ment that results to be unacceptable for most carriers. As a matter of fact, when a wavelength route has to be changed in such a network, manual intervention on the patch panels is needed. A most effective solution consists in enriching the optical layer of a real wavelength routing capability exploiting the full OTN definition. In order to distinguish the two solutions we will call MPLS over DWDM the solution where LSR and LER are connected directly via DWDM links and MPLS over OTN the solution in which either OXCs or OADMs (or both) realize a complete OTN layer on top of which the MPLS is built. In MPLS over OTN, wavelengths are routed into the optical layer exploiting either an optical ring or an optical mesh topology architecture. Only when the tunnels contained into a wavelength have to be resolved the wavelength is sent to the LSR for MPLS label- related operations. Networks Fundamentals and Present Architectures Fiber tunnel FIGURE 3.53 Tunnels aggregation in the signal traveling through a fiber of an MPLS over DWDM network. Intraoffice FIGURE 3.54 Representative topology of an MPLS over OTN. The model of the multiplexed traffic is shown in Figure 3.53, where the traffic traveling through an optical fiber is decomposed in wavelengths that are routed into the OTN layer. Every wavelength multiplexes several MPLS tunnels that are managed by the LSR when the wavelength is passed 