blocking probability. another switch plane with an (S + 1) X (S + 1) switch devoted to the communication among the different switch planes. This architecture is shown in Figure 7.38. However, the situation qualitatively does not change and either dimension and cost of the optical switch core approach that of a nonblocking structure or blocking probability thwart the OXC performances for high traffic levels. 7.7.5 Optical Add-Drop Multiplexer While the OXC is designed as the transport layer switch to be used in mesh networks, the OADM is used either in point-to-point links to drop channels in an intermediate location and to create branching or in optical self-healing rings [58,61]. We will mainly refer to the second application, that is the most demanding in terms of functionalities, but we will also hold all the observation for the first application unless is explicitly said that this is not true. From a functional point of view, the OADM is mainly an OCh layer component, having network functionalities similar to those of an OXC. In particular, in an OTN network, while the optical channels pass through the OADM without electronic regeneration, the service channel is terminated SO to terminate the OM header and the OR header. The OADM also manages the OCh protection through the self-healing ring mecha- nism or through analogous mechanisms in the case of point-to-point links (see Section 3.2.7). Referring to a two fiber ring network, an OADM interfaces both the fibers and is able to add/drop optical channels from both the fibers. Thus, the principle scheme of an OADM Switching Systems: Architecture and Performances FIGURE 7.39 Position and functionality of an OADM in an OCh-DP ring. The working path of the depicted connection is called W, the protection path p. can be depicted as in Figure 7.39 where a simple OCh-P ring is shown with four OADM nodes and the path of an OCh is represented both in the working (w) and in the protection (p) fiber. The basic building block of an OADM is the so-called single-sided OADM (S