e channels through the other S-OADM, automatically, due to the fact that they are switched on the other side of the ring with respect to the failure (see Chapter 3). This property allows the failed S-OADM to be replaced without disrupting the protected channel routes. This is not true for horizontal configurations, where the failure of an S-OADM com- pletely eliminates the access of the node to one of the ring fibers SO that standard OCh protection mechanisms result not to protect the S-OADM failure. It is not impossible to devise different types of routing to implement channel protection in the case where horizontal OADMs are used, but besides the fact that they should be standardized before their introduction in a product, they are all somehow underperform- ing with respect to the standard one. This can be understood even without entering into the detailed explanation of the chan- nel routing along the ring, simply looking at Figure 7.42 where the two possible ways to connect a horizontal OADM with local equipment interfaces are shown. As a matter of fact, if the solution called A in the figure is adopted, each TXT is con- nected to booth the S-OADM of the node; thus, S-OADM failure cannot be recovered by any possible routing of the channel along the ring, since both working and protection TXTs have a termination on the failed S-OADM. In the case of the configuration called B in the figure, each transponder card is connected to only one S-OADM module, but the RX and the TX paths go through different arcs of the ring, since both are routed on the same fiber. This provokes different delays in the propagation of the two directions of the same bidirectional optical channel, thing that is generally to be avoided, especially, when packet services are present in the overlay layers. Switching Systems: Architecture and Performances Depending on the flexibility in dropping and adding channels, S-OADMs can be classi- fied as follows [62,63]: Fixed OADM: In this case the set of add/dropped channels is fixed and cannot be c