CHAPTER 7
THE ABACUS SWITCH
The switches based on the knockout concept suffer from cell loss due to the lack of routing links in the switch fabric—for example, the concentrator in the knockout switch, or the multicast grouping network (MGN) in the MOBAS in Chapter 6. Although we can engineer the group expansion ratio L to achieve a satisfactory cell loss probability, say 10− 10, that is based on the assumption that the traffic from different input ports is uncorrelated and input traffic is uniformly distributed to all output ports. The latter assumption gives the worst case cell loss probability, while the former assumption may not be realistic for the applications such as Internet Web services. There may be a lot of traffic destined for the same popular site at the same time, resulting in a so-called hot-spot situation and an unacceptable cell loss probability. In order to reduce the cell loss rate, excess cells can be stored at the input buffers, which results in the switch having buffers at the input and output ports. The switch to be discussed in this chapter belongs to this category.
We describe a switch that has a similar architecture to the MOBAS but does not discard cells in the switch fabric. When the head-of-line (HOL) cells of the input ports are sent to the switch fabric, ...
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