-OADM), whose functional scheme is reported in Figure 7.40. This subsystem accepts at the input a WDM comb, extracts without electronic elaboration a set of WDM channels delivering it to a local termination, and replaces them with locally generated channels. The ratio between the maximum number of add/drop channels and the number of chan- nels of the line DWDM system is called add/drop degree of the OADM. In principle, there are two possible ways of connecting two S-OADMs to make a com- plete OADM: the so-called vertical and horizontal architectures that are shown in Figures 7.41 and 7.42, respectively. In the figure TXT represents the interfaces of the local equipment OADM-single sided WDM input WDM output Channel C1 Channel C4 dropped added FIGURE 7.40 Basic building block of an S-OADM. Clockwise ring fiber West side S-OADM S-OADM East side FIGURE 7.41 Vertical architecture of an OADM Counterclockwise designed for a self-healing opti- ring fiber cal ring. Two S-OADMs are used, besides clients' transmitters (TX) and receivers (RX). Telecommunication Networks Clockwise Clockwise ring fiber ring fiber S-OADM S-OADM TXTs working working protection TXTs protection S-OADM S-OADM FIGURE 7.42 Two possible horizontal OADM Counter- Counter- architectures corresponding to clockwise clockwise different ways to connect trans- ring fiber ring fiber mitters (TX) and receivers (RX) to the two used S-OADMs. Horizontal type A Horizontal type B that, in order to manage OCh-P (either shared or dedicated), have to be divided into work- ing and protection interfaces. Moreover, in Figure 7.42 two possible ways to connect the local equipment interfaces to the OADM are considered. Even if the horizontal configuration is frequently easier to realize from a technological point of view, the vertical configuration is almost the only used in practice [57] even if it requires four S-OADMs (each block of Figure 7.41 incorporate two S-OADMs). This is due to the fact that, in case of an S-OADM failure, the vertical configuration deviates all th