pect to the transparent solution that has to be avoided. Naturally, this could be avoided if single wavelength OADMs would be available. In real rings however, there are 90 or 120 wavelengths and having such a high number of differ- ent OADMs would complicate farther the spare part management for the carriers and the product catalogue management for the vendor. Both switchable and tunable OADMs allow similar situations to be avoided. The difference between tunable and switchable OADM is essential in the process of setup of new connection. In Chapter 2 we have noted as the evolution of the service bundles proposed by carriers renders the traffic in the geographic and metro networks more and more unpredictable. Thus, when a request for setting up a new connection in the optical layer arises we can- not assume that it was foreseen on time. Depending on the traffic already routed in the ring and on the near and far ends of the new connection a suitable wavelength is first individuated to support the connection. In order to operate the setup if switchable OADMs are used two conditions are needed: the OADM covering the selected wavelength has to be present in the two end nodes and a spare client interface has to be present exactly on the port corresponding to the selected wavelength. The coincidence of both the condition is quite improbable, mainly because in the impos- sibility of following a preplanned sequence of new connections setup, it is too expensive to place spare client interfaces on all the available OADM ports in the network. Thus, if switchable OADMs are used, the setup of new connections almost surely requires a field intervention to install new cards in the present equipment. If the OADM is tunable, the second condition is removed: it is sufficient that an OADM with the selected wavelength in the add/drop bandwidth is present with at least one spare tunable client interface connected to whatever port. As a matter of fact, tunability allows the network control to send the desired wavelength to the port wh