ult task and essentially two solutions exists. The first relies on the adoption of the SDH/SONET frame only as an adaptation between packets and the transport layer. In this case, the GFP runs directly in the WDM equipment client interfaces just to create a TDM frame and to put MPLS packets inside. Then the frame is transmitted through the optical transmission systems up to the edge optical devices where GFP restores the packets and returns them to the MPLS layer. The second solution is of course the introduction of the OTN frame with its own framing mechanism that is generally called digital wrapper. In this case, optical network elements works with the OTN frame (see Figure 3.37). From a conceptual point of view, these are similar solutions, even if differences do exist in the efficiency of the frame and in the electronics that is needed to implement the adapta- tion functionalities at the adaptation points. The resulting network architecture is shown in Figure 3.52. From the figure, it is clear that in this kind of network architecture, all the routing of data is performed by the label switching routers (LSR) and optical equipment is simply used for transmission. In order to push this solution, a vendor has even proposed to directly integrate optical DWDM interfaces into the router equipment SO that an optical line terminal is no more needed and the DWDM system is a sort of transmission extension of the LSR. Telecommunication Networks terminal MPLS tunnel FIGURE 3.52 Representative topology and schematic layering of an MPLS over DWDM network. Since this idea seems really attractive, we will discuss it in detail while dealing with routing equipment architectures in Chapter 7, since it hides more than what seems at first glance. The architecture of Figure 3.52 is remarkably simple, but has an issue related to scaling to core networks. In a network as large as a nation or a continent, several times, a wave- length travels through a node without the need of processing but for regeneration at the transmission level