 a switching system. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. SWITCH FABRICS SWITCH FABRICS 279 PHY/MAC TM & NPU TM & NPU PHY/MAC Potential output buffers PHY/MAC TM & NPU TM & NPU PHY/MAC PHY/MAC TM & NPU TM & NPU PHY/MAC FIGURE 14.7 The principle of passive redundancy of switch fabrics configured as N:1 redundant. If there are as many backup fabrics as active ones, it has 1:1 redundancy. aging resilience and availability becomes an application-specific issue that must be addressed in the framework of customer expectations and projected budgets. Figure 14.6 depicts a configuration of active redundancy. In this example, two parallel sets of switch fabrics carry the same traffic in duplicate. If during operation any of these two sets of fabrics runs into a problem or fault, the switching system will simply lean back on the redundant fabric and no traffic or bandwidth performance loss will occur. This approach offers 1+1 redundancy. Figure 14.8 illustrates load-sharing redundancy. This example contains multiple switch fabric cards and they all carry traffic as in active redundancy. Enough fabrics are available to handle the nor- mal traffic, but some fabrics in the combination also offer a resort in case of trouble. When a fault occurs, the problem card is isolated and traffic switches over automatically to switch fabrics where capacity is still available. Following a faulty condition, the new state of the switching system may reflect a certainly lower aggregate switching capacity, but operation continues unimpeded. This is the principle of graceful degradation, which is of paramount importance on critical links, such as those owned and managed by carriers. If the total number of available switch fabrics in the starting configuration is twice what is needed for the anticipated traffic, this is the ideal case of car- rier-grade QoS becau