 field are some Fast Eastern countries like China and India, where the development is SO fast that the telecommunication network is also growing at a fast pace. If a new transmission system has to be added to the network to relieve a condition of traffic saturation, it is important to take into account that next generation SONET is almost always deployed using multiring topology due to resilience and high control degree of this architecture. This is due both to the fact that implementing the new functionalities in ADMs is easier than implementing it in DXCs, and to the better survivability of a multir- ing network where independent protection is present on each ring and shared protection schemes can be adopted. Moreover, integrating the next generation functionalities in such complex machines like a DXC is quite difficult and currently, the next generation DXCs are not diffused. Adding a new link in a multiring architecture is difficult and this is another issue if a SONET-based network has to be scaled [67]. Networks Fundamentals and Present Architectures As far as the Ethernet-based architecture is concerned, we have already mentioned the discussion that exists around carrier Ethernet scalability. In this case, it would be the spanning tree algorithm to limit the scalability and the presence of switches that segment the network in separated spanning tree domains provides only a partial solution to this problem. For sure, a better scalability is realized via MPLS based architectures. In MPLS over OTN architecture, if the OTN layer is realized via a multiring topology, all the dif- ficulties related to rings exist as in the SONET case. The OTN layer however is more probably realized via OXC using a mesh topology. In this case, assuming that OXCs are scalable, the overall architecture is quite scalable both at the OTN layer and at the MPLS layer. Even more scalable is the OTN over DWDM architecture, where only the transmission systems and the LER and LSR have to be scaled either when the traffic increases or when 