to the MPLS layer. The network architecture is schematically shown in Figure 3.54. In the figure, the OTN has a mesh architecture and OXCs are used to route whole wavelengths; however, a ring- based OTN layer can also be used where the OXCs are substituted by OADMs. The adoption of OXCs, either all optical or with an electronic core switch, is quite dif- ferent from the adoption of DXCs in the IP over SONET architecture. As a matter of fact, OXCs do not demultiplex the signal traveling on a single wavelength, but manage the wavelengths as a whole. Just to look at what happens in the aforementioned DXC example, a suitable OXC would have a switch fabric 320 x 320, able to send every input wavelength to every output port. Naturally, this implies the ability of the OXC to change the carrier wave- length during the operation, a property that is trivial for electrical OXCs, but not obvious for the all optical one as discussed in Chapter 10. 3.3.4 IP over Ethernet over OTN The alternative to MPLS to provide the network with a carrier class packet-based layer 2 is the adoption of carrier class Ethernet that brings about the substitution of LSR with carrier class Ethernet switches while at the network edges the LER is substituted by a stack of a switch and a router. Telecommunication Networks In principle, this architecture can be deployed in both the schemes shown in Figures 3.52 and 3.54, considering a possible mesh and ring architecture of the underlying opti- cal layer. In practice, even if carrier class switches could have very good performances, the appli- cation of this architecture is conditioned to the scalability limits of Ethernet, even in its carrier class version. There is a strong debate in the engineering community between Ethernet vendors and vendors of other technologies on the possible scalability of Ethernets networks and it is not the scope of this book to answer such a question, which among other things, is strongly influenced by future developments of Ethernet technology. Looking to deployed networks