buffers arbitration switch unicast VOQ's unicast arbitration buffers serdes to fabric serdes to output queues serdes output serdes input High-speed backplane FIGURE 14.9(b) A typical partitioning between the two switch fabric components. links. In a chassis-based system implementation, where typically two to four switch fabric cards are to be expected, this approach is translated into a straightforward backplane where paths do not need to cross. This type of switch fabric can be made redundant using the passive approach discussed earlier. Agere, whose switch fabrics we discuss later in this chapter, is a good example of a company using such an architecture. By distributing the switching function, we obtain a configuration where the VOQs are imple- mented on line cards and where the backplane will necessarily include more complicated crossed paths. If each output port has its own VOQ allocated at the inputs by the queue manager (to avoid the HOL problem) and on top of this each CoS is also allocated its own VOQ, then for i inputs and classes, a queue manager must be able to coordinate and manage ik queues. All of these sources will need to have access to the fabric core. With backplane, traces are physically limited to around 1,000 and the physical complexity of the backplane becomes an issue, along with the power consumption and cost that comes with the increased number of needed components to support the high-speed links. In the distributed-switching case, redundancy can be implemented as either active or passive and may or may not be based on the load-sharing principle. In fact, the in-service scalability of this approach is enabled by the load-sharing redundancy principle, which enhances the advantages of this concept beyond its impact on simple reliability and availability. The architecture can scale very well because in a single stage the scheduling problem remains manageable. Scalability also looks good different schedulers can be allowed to handle independent crossbar data paths, thereby affording a creati