e connected. Transmission Systems Architectures and Performances 6.5.1.4 Redundancies To reach the required network element availability, some cards are replicated twice in the system architecture and a 1+1 hardware protection is realized. The network element processor is one of the cards that generally are doubled. One of the two processors works as mirror of the other, being in hot standby continuously updated SO that, if the main processor fails, the second one can assure seamless network element management. Also the power supply is generally protected by a hot standby. 6.5.2 Backplane Architecture There are several possible solutions for the control architecture of a DWDM system, but the most common is that based on a bus communication structure. The block scheme of this architecture is shown in Figure 6.47. The backplane is designed to create a bus for each subrack to allow all the cards that are part of the subrack to communicate each other. This bus terminates on the subrack controller. All the subrack controllers are put in communication by the bus that connects all the subracks, that is also sustained by the backplane. In the hypothesis that more than one rack is needed, there is a bus extension that is able, from a backplane, to chain different racks. Also in the multirack case, there is a hierarchical organization among the rack control- lers SO that one of them works as network element controller. The first thing that has to be decided in designing a backplane implementing the archi- tecture represented in Figure 6.47 is how to realize its connectivity. Subrack cards Subrack controller Subrack bus Central controller FIGURE 6.47 Bus-based communication architecture among the cards of a DWDM system. Telecommunication Networks Connectivity refers to how drivers and receivers are connected to the backplane and thus to the bus that allows them to communicate. In some systems it is necessary to con- nect multiple backplanes together. This can be done from a card to a backplane or from a card in one backplan