r layer protection is activated. The simple kind of protection is 1+1 dedicated multiplex section protection (1+1 MSP), whose mechanism is shown in Figure 3.29a. An SDH multiplex section is routed along two links on different routes connecting the same endpoints. At destination, the signal on one route is detected, while the other signal is discarded. In case of failure, the receiving terminal detects a performance degradation below the threshold (or even a lack of signal) and switch to detect the signal coming from the other link. This protection mechanism is very fast, since the switch is performed by the same equipment detecting the failure. However, it is worth noting that in SDH networks Working Protection Working Protection FIGURE 3.29 Functional schemes of an SDH/SONET 1+1 dedicated linear multiplex section protection (a) and of an SDH/ SONET 1:1 dedicated linear multiplex section protection (b). Telecommunication Networks multiplex sections are always bidirectional. Thus, once a terminal switches, it sends a signaling to the other terminal on the return route to inform it of the failure. If the failure is also bidirectional, the other terminal will not receive the message, but it has already switched due to the detection of the failure. If the failure is unidirectional, the signal is received by the other terminal. In this case, the other terminal can leave the signal on the working route, implementing the so-called unidirectional protection, or it can switch too, the protection is bidirectional. Generally, bidirectional protection is preferred, due to the need of avoiding big delay differences between the two directions of the same bidi- rectional multiplex section. A better use of the protection capacity can be achieved in 1:1 MSP, whose scheme is represented in Figure 3.29b. In this case, the signal is sent only on one of the two paths connecting the same ends, called the working path. The other path is either left empty, or occupied with low-priority traffic, whose SLA accept more frequent interruptio