0.16(b) [28]. From the fig- ure it results that a conversion efficiency near 0 dBm can be attained at the practical value of the frequency shift. As in the case of all WCs based on SOAs, a FWM WC also has a noise figure; however, due to the strong saturation conditions in which the amplifier generally works, it is much smaller that the noise figure of line amplifiers [28]. Next Generation Networking: Enabling Technologies, Architectures, and Performances 10.3.2 Transparent Optical Network Elements Besides the structures we have briefly reviewed, a great number of other proposals have been made to design all-optical regenerators and WCs. In the following sections, we will consider these elements as functional blocks of the network that performs a certain func- tionality while adding noise to the signal. Naturally, when considering regenerators, we will have to consider that the 2R regeneration is imperfect. From now on, we will move our attention to networking related subjects. The first point to analyze is related to the network elements of the T-OTN. The first network element is the transmission system, based on DWDM, but with an open design. This means that the input interfaces of the systems will generally receive an optical signal, and after conditioning it with an all-optical interface containing among other things a filter and perhaps a regenerator, directly launches the optical signal along the line for transmission. All the issues we raised in Chapter 7 talking about this alternative are still valid. This means that either a new type of all-optical interfaces will be standardized SO that all the DWDM systems will be produced starting from this standard, or the entire transparent part of the network will have to be produced by the same vendor. For switching in ring networks, T-OTN uses transparent OADMs. They are available today and the only transparent optical network that is today deployed in the field is com- posed by two or three transparent self-healing rings. Since we have described this solution in 