ectively. also been inserted along the protected link SO that the OMS is terminated in between, while OChs passes transparently. The consequence is that, while in the OCh protection the ADC is to be doubled, in the case of OMS protection, the protection trail ends on the ADC and only one ADC is needed. When OMS protection is used, in order to provide some resilience against transmit- ters failure, it is possible to adopt 1:N transmitter redundancy and correspondingly 1:N receiver redundancy. This kind of local protection that is shown in Figure 3.41, consists in reserving one wavelength to redundancy and in providing a set of switches that are able to direct any client signal either toward its working wavelength transmitter or toward the redundant wavelength transmitter. When a local alarm detects a failure in a transmitter, automatically the signal is locally switched on the backup wavelength after a successful handshaking between the two OCh terminals and all the other switches are blocked to avoid conflict of more than one client signal on the backup wavelength. At the receiver end, when the terminal sees no signal on the working wavelength it tries to receive the signal from the backup. If a successful handshake procedure is carried out through the backup wavelength with the sending terminal, the receiving terminal sets the receiver on the backup wavelength. Moreover, in case of bidirectional protection, that is almost the only used mechanism in OTN, the receiving terminal starts another handshak- ing on the backup wavelength of the return path and, if it is successful, the return signal is also switched on the backup wavelength. It is to underline that 1:N transmitters protection could be implemented in SDH/SONET also when the MSP is adopted. The difference is that in the case of SDH/SONET, electri- cal signal sources are unprotected in case of MSP, while optical transmitters are always protected. Since electrical signal sources have a very high reliability, devising an ad hoc protection for them is often no