ical layer, they allow both protection and resto- ration (at least in the case of mesh topology) to be implemented at the physical layer. The physical layer and the layer 2 survivability mechanisms have of course to be coor- dinated in order to avoid either network instability or network freezing when a failure occurs [34], but can be done simply with hold off times that delay the intervention of layer 2 survivability mechanisms up to a moment when layer one procedures are surely ended. In this way, layer 2 survivability becomes a backup with respect to layer 1 protection and restoration. Similar mechanisms will be analyzed in Chapter 8 when dealing with multilayer convergent network. When dealing with a network survivability strategy, it is needed to consider the over- provisioning that is required to realize it. As a matter of fact, implementing multilayer survivability causes an increase of the over- provisioned bandwidth due to the fact that the protection capacity in the upper layer is routed as high-quality client data in the lower layer since it is necessary to assure its integ- rity if the layer above switches in protection. This situation is only partially mitigated by the adoption at the physical layer of shared protection rings, where the number of working channel is generally greater than the potential number of protection due to the sharing of protection capability. In Table 3.8, the capacity over-provisioning is reported for different architectures and different protection schemes. Concerning MPLS over-provisioning, we have used data reported in Refs. [34,68,69]. Obviously the numerical values of the table have to be consid- ered indicative and their value depends, among the other parameters, on the specific algo- rithms used to dimension the restoration paths both at the physical layer and at layer 2. Moreover, the cases that are compared in the table are not characterized by the same net- work availability: it is quite intuitive that a mesh network with multilayer restoration and Networks Fundamen