each solution and how far the technology is from what would be needed for a real deploy- ment of such a network. 10.2 Optical Digital Network The idea that is at the basis of the optical digital network (ODN) is to decouple the service layer (generally layers 2 and 3) from the physical aspects of the transport layer by hiding the dense wavelength division multiplexing (DWDM) structure of optical transport with the structure that the group that first introduced the idea call bandwidth virtualization [1]. The concept of bandwidth virtualization consists essentially in the ability to offer to the client layers a wide variety of possible channel capacities to maintain always a high utili- zation of the transmission channel. The client layer requests are transformed in tributary channels by a suitable adaptation layer and are multiplexed onto the wavelengths that are used for transmission. In this way, the client layer even does not know what the wavelength capacity is, but has the possibility of occupying, each time, only the needed capacity. The channels set up as a consequence of the client requests are characterized by a quality tag, which determine among other things what kind of survivability strategy is assigned to the channel among those available at the optical layer. Moreover, since every node, even in-line sites, is able to regenerate the signal, a very good monitoring of the signal quality can be done at each transit node, and coordination with the client layers (e.g., to implement multilayer protection) is easier. Besides bandwidth virtualization, the fact that the signal is processed at the digital level in every node allows performing efficient grooming to maintain a high degree of utiliza- tion of the transmission system and to introduce flexibility whenever it is needed. As a matter of fact, tunable OADMs or wavelength converters (WCs) cannot be realized via complex analog techniques, but they can be implemented via electronic processing. From a functional point of view, the scope is the same of next 