than the minimum (that in this case is STS3). Implements drop and continue switching, that is required to protect a set of inter- connected rings against interconnection failures. Working Working OC-48 OC-48 OC-48 Terminating Terminating OC-48 equipment equipment STS-3 switch fabric OC-48 OC-48 OC-48 Terminating Terminating OC-48 equipment equipment Protection Protection STS3 terminations STS-3 STS-1/DS3 STS-3 FIGURE 7.49 Functional scheme of an OC-48 four fiber SDH/SONET ADM. Switching Systems: Architecture and Performances Node Y Node X Drop and continue FIGURE 7.50 Double gateways protection in a double interconnected ring network using SDH/SONET. The role of drop and continue in node X is evidenced. The last functionality deserves an explanation. The drop and continue feature of an ADM is the capability to broadcast a tributary channel both on a drop port and on the output line port. These features are used in situations like that depicted in Figure 7.50, where a network is built connecting several SONET/SDH rings. If a failure occurs in a ring, normal ring protection starts SO that the whole network can cope with one failure per ring. In order to face the failure of one of the interconnection nodes or of one of the interconnection fibers, the signal exchange between each couple of rings is performed in two distinct points. To render this technique efficient it is needed that the signal that passes from one ring to another in one connection node is also sent to the other connection node to reply the same situation. In order to do that, the ADM in the first connection node has both to drop the signal (to pass it to the other ring) and to send to the line to forward it to the following connection node. Thus, drop and continue is needed. In Figure 7.50 this situation is clear imagining to set up a path from node A in ring 1 and node Z in ring 2. Working path in the two rings are represented with a full line and protec- tion paths with a dotted line (dedicated path protection is assumed and only one direction is 