ns. If one terminal detects a failure it sends a message to the other terminal, since in this case it is the source, not the destination to switch. In case of bidirectional failure, the terminals cannot communicate; thus, after a time in which each terminal does not detect a signal arriving on the protection route it identifies the bidirectional route and switches. In case of unidirectional route, the source terminal of the link interested by the failure detects a failure indication coming on the return route and switches the transmitted signal on the protection route. Also in this case, the protection can be unidirectional or bidirectional. In both cases, when the failure is repaired, the standard describes two modes of work- ing. One is the so-called nonrevertive protection, when the signal remains on the protec- tion route up to a manual switch via the management system. The other is the revertive protection. In this case, every terminal whose transmitted signal goes through the pro- tection link sends a test message periodically on the working link. When the terminal at the detection side sees the test message sends an acknowledgment on the returning working link. After a double handshake, the terminals identify the failure as repaired and automati- cally switch the signals on the working link. Besides MPS, there is also 1 + 1 dedicated path layer protection (1 + 1 PLP) and 1:1 path layer protection (1:1 PLP). The principle is the same of the analogous mechanisms in the lower layer, but the paths and not the multiplex sections are involved. In order to under- line the difference, both MSP and PLP are shown in Figure 3.30. In the figure, the MSP protection involves only one link and terminates on contiguous DXCs, due to the fact that the DXC terminates the multiplex section in each network node. On the contrary, the PSP starts and terminates on the DXCs at the path end, while the working and protection paths travel independently through the network. This is due to the fact that the path layer is accessed only 