 logically divided into two parts: one Telecommunication Networks 4-Fiber 4-Fiber FIGURE 3.32 Scheme of an SDH/SONET path traveling through a multiple section share protection SONET/SDH ring (MSSpring) in normal conditions (a) and in protection (b). simulating a virtual working fiber and the other a virtual protection fiber, to create a logi- cal situation similar to a four-fiber ring. Due to this particular routing, while in MSDPring, a multiplex section (MS) occupies all the capacity of the fibers (on one for working on the other for protections), in the MSSPring more than one MS can be routed on the ring with the condition that they are not topo- logically superimposed. In the extreme case when only the adjacent nodes have to be con- nected, up to n MS connections can be set up, where n is the number of nodes. All these MS connections share the same protection capacity, even if when one of them is switched in protection, the other cannot be switched too, thus avoiding collisions. In this sense, the capacity is better exploited in the MSSPring with respect to the MSDPring. Let us now imagine that the outer couple of fibers are used for working traffic and the inner one for protection. In case of failure, the protection switching occurs as in Figure 3.32b. Since the protection mechanism is located in the multiplex section, the nodes adja- cent to the failure are involved in the protection procedure. In particular, each node detects the failure in the received MS and sends a multiplex section protocol to the other node to signal the failure. Since in the figure example the failure is bidirectional, no con- trol signal is received by the nodes; thus, in the absence of an acknowledgment, both the nodes assume to be in the presence of a bidirectional failure and perform an automatic protection switch. After the switch, the nodes perform a handshake on the newly setup connection to verify its integrity and, after that, they send the traffic on the new route. Once the protection switch is performed, the two switching 