at the path ends, while intermediate DXCs only pass through the paths. Passing from meshed to ring networks, the concepts of 1+1 and 1:1 protection can be applied at rings at both path and multiplex section layer, designing the so-called multi- plex section-dedicated protection ring (MSDP)ring (ULSR: Unidirectional Line Switched Ring in SONET language) and the PLPRing (UPSR: in SONET language). An example of working of ULSR is reported in Figure 3.31. In these cases, in a two-fiber ring, one fiber is devoted to working traffic and the other to protection. In the case of ring topology, also another protection mechanism can be implemented, which uses the protection capacity more efficiently. Let us imagine a four-fiber ring, as represented in Figure 3.32a. At the ring nodes there are add drop multiplexers (ADMs), essentially small DXC designed for rings. Networks Fundamentals and Present Architectures 1+1 MSP on an STM16 MS protection MS working Path working 1+1 PLP on an STM4 Path protection FIGURE 3.30 End-to-end path linear protection (PLP) versus multiple section protection (MSP) in a mesh SDH/SONET network. FIGURE 3.31 Scheme of an SDH/SONET path traveling through a 1+1 path protection SONET/SDH ring (PDPRing) in nor- mal conditions (a) and in protection (b). The first difference between the multiplex section-dedicated protection rings (MSDPring) and the multiplex section-shared protection ring (MSSPring) is in path routing. In particu- lar, comparing Figure 3.31a with Figure 3.32a it is clear that in MSDPring the two-path directions (from A to B and from B to A if A and B are the connected ADMs) are routed both on the same fiber and in the same direction (from which the name unidirectional is coined in the SONET acronyms). In MSSpring, the two-path directions are routed on dif- ferent fibers and in opposite directions (from which the name bidirectional is coined in the SONET acronyms). This routing algorithm can also be applied on a two-fiber MSSpring, exploiting the fact that the time slots of each fiber are