future period of time. Networks Fundamentals and Present Architectures Thus, the routing table used by a router is locally generated by using a local routing algo- rithm and in general the router does not know the whole trail traversed by a packet. If this trail has to be reconstructed, it is needed to start from the trail origin and to interrogate all the traversed routers, each of which knows from where the packet came in and from where it went out. At the end the packet trail can be reconstructed. In synthesis, the SDH network is based on a static and centralized traffic engineering that is realized by configuring the DXC in the network in a suitable way from the central network manager. On the contrary, the IP network performs routing via distributed algo- rithms running in the routers and the centralized control system is used for reporting and traffic engineering. The main automatic operation performed by an SDH network is protection switching. In general, traffic survivability is assured in an SDH network by two mechanisms: protection and restoration. Protection is automatic and fast, relying on control plane protocols: in particular the SDH standard prescribes a protection time smaller than 50 ms. If protection cannot recover the traffic, for example, in the case of multiple failure, restora- tion starts. It is a slow traffic rerouting carried out by the network manager that uses, if possi- ble, free network resources to find alternative routes for the traffic interrupted by the failure. Generally, restoration is implemented via DXCs only in meshed networks, because inter- connected rings networks have a good resilience also to multiple failures, with the only condition that every failure happens in a different ring. Protection mechanisms exist both at path and at multiplex section layers. When both the layers implement protection, a hierarchy is established setting a hold-off time retarding path protection. If it is possible to recover the failure at the lowest possible layer, it is done, otherwise highe