etwork paths. The main functionality of the network manager is to perform the opera- tions that characterize the life of a path: Path setup Path monitoring Path tier down Networks Fundamentals and Present Architectures During path setup, the network manager receives the request of a capacity between two endpoints and analyzes the network with an off-line tool to determine the optimum path routing taking into account the available network resources. Once the path route has been identified and the resources needed to set it up are defined, the network manager asks the element manager for a resource reservation pro- cedure. It is the element manager that matches the abstract resources considered by the network manager with the hardware present in the network and that sends a configura- tion command to the involved network elements to mark the resources as used by the considered path. When the network manager receives from the element manager the resources res- ervation acknowledgment, it sets up the path in its logical path list and starts a path verification. The path verification is first a logical operation, carried out by high-level simulation, and then it becomes a physical test of the new path carried out with test signals. Of course, in order to perform such a test, the network manager has again to interact with the element manager for the hardware operation. If all the tests are passed, the new path can be loaded with real traffic. During path working, the network manager monitors all the performances of the path and characterizes the traffic in statistical terms. Moreover, in case of fault, the network manager verifies the good working of the protection mechanism. If the traffic has to return on the working path when the failure is repaired and the protection mechanism does not implement it automatically, it is the network manager that supervises the traffic switching after a suitable test of the repaired path. At the end of a path life, the network manager verifies that no traffic is on the path and switch