nodes also send a protocol along the ring that reaches all the other nodes signaling that the protection capacity has been occupied in certain ring sections. As a matter of fact, further protection switching forced by other failures that will send different signals on the same protection fibers segment is to be avoided. If a unidirectional failure would be present, one of the nodes adjacent to the failed mul- tiplex section will receive the failure protocol and will start a handshake with the sending node using the protection capacity for the acknowledgment. After that, both the nodes Networks Fundamentals and Present Architectures perform switching, verify the new connection integrity, and then proceed as in the case of bidirectional switch. A similar procedure is performed to restore the working route when the failure is repaired. Also in this case, all the nodes have to be alerted through a specific protocol that the protection fibers are again available. 3.2.7 Optical Transport Network (OTN) With the market success of optical WDM transmission, WDM (i.e., the "optical" name for the traditional frequency division multiplexing) became the standard of transmission systems. The first step to introduce these systems in the network was to consider every wave- length channel as belonging to a separated SDH network. These brought to structure a network made of superimposed SDH rings; for example, if the transmission systems con- vey 16 wavelength channels, the network will be constituted by 16 superimposed SDH net- works and in each network node up to 16 independent SDH equipment will be deployed. This architecture, despite its intrinsic back compatibility and scalability, rapidly required a high CAPEX to multiply the number of SDH elements in every network node, SO that a different solution had to be found. As a reaction to this situation, the ITU-T standardized an independent model of the transport network based on optical WDM technology, following the same guidelines that emerged from SDH standardization, but for t