 and to require that up to the moment in which a WC is traversed, the path have to belong all to the same wavelength plane. In practice, this is equivalent to adding to the matrix elements another index indicating the physical wavelength. This is a rigorous approach, but the problem becomes quite complicated due to the increase of the number of optimization variables (the elements of [1]R] that is multiplied by the number of wavelengths that in general is on the order of 100). A less rigorous approach, less complex under a calculation point of view, is to divide the dimensioning and wavelength assignment problem. This approach is especially useful when a few WCs can be deployed in the field. The way to proceed in this case is to divide the problem into two parts. In the first part, the dimensioning of the network will happen without taking into account the continuity of the physical wavelength. Next Generation Networking: Enabling Technologies, Architectures, and Performances In a second part, another optimization problem is set up to assign a physical wavelength to the wavelength paths. In both cases, both if the wavelength continuity is imposed directly on the complete problem, and if the problem is split in two parts, when wavelength continuity cannot be enforced, two actions can be adopted. The first is to add new cards to the present equipment, or when this is needed, a new network element. The second is to place a WC to allow the transfer of the channel on an available wavelength. Generally, WCs are placed in the OXC, but if OADMs or the line terminal that does not interface a local OXC exists, these are other possible locations for WCs. The problem of the optimum use of WCs is an important problem for transparent net- works, and a great amount of research has been devoted to pointing out the general char- acteristics of the problem and the algorithms to solve it. Also, in this case, a network optimization based on ILP can be set up by simply taking into account that the wavelength path can change physical 