twork: Design Methods and Regenerators Placing Problem if a transparent network is very well designed, it remains that transparent transmis- sion has a limited range (even if on the order of 1000km), and if a numerous set of possible paths go beyond this "optical length," in order to set up the considered connection, regen- eration is needed. Since optical regeneration is not SO efficient to be really competitive with electronic regeneration, we will assume in this chapter to talk about electronic 3R regenerators. When 3R electronic regenerators are an important part of the transmission layer, we call the network "translucent" [49]. Three architectures have been proposed for translucent networks in the literature: 1. An architecture where electronic regenerators appear only as a support to trans- mission: This is essentially a transparent network, with a sparse number of regen- erators to aid transmission along the worst links [50]. An architecture where the nodes are translucent: This means that every node has a small electrical switch sitting on top of the optical transparent switch, as in Figure 10.31. The electrical switch can be used to locally regenerate those channels that have very far destinations, but it can also be used to perform grooming if it is based on a lower speed with respect to the wavelength channel. Thus, this version of the trans- lucent network gets nearer to a two-layer network, at least from a few points of view. Last, but not least, an architecture where transparent islands are connected via opaque gateways: This is a fully developed two-layer architecture where the fact that gateways regenerate signals is only one function of this machine, and often not the more important [51]. Among the different possible architectures of the translucent network, we consider here the nearer to the full transparent network, that is, those architectures in which regenerators Telecommunication Networks 11 Switch 12 Switch 13 Switch Local Local channels channels Electrical switch fabric FIGURE 10.31 Trans