CHAPTER 6

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KNOCKOUT-BASED SWITCHES

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As shown in Chapter 2, output buffer switches (including the shared-memory switches) provide the best delay–throughput performance. The problem of the output-buffered switches is that their capacity is limited by the memory speed. Consider the case of an ATM switch with 100 ports. We should ask ourselves what is the probability of all 100 cells arriving at the same output port in the same time slot. If the probability is very low, why do we need to have the output buffer able to receive all 100 cells at the same slot? A group of researchers in Bell Labs in the late 1980s tried to solve this problem by limiting the number of cells that can arrive at an output port in each time slot and thus the speed requirement of the memory at the output ports. Excess cells are discarded by the switch fabric. The concept is called the knockout principle. The question is how many cells should be delivered to the output port in each time slot. If it is too many, memory speed may be the bottleneck. If too few, the cell loss rate in the switch fabric may be too high to be acceptable. For a given cell loss rate, this number can be determined. The number is found to be 12 for a cell loss rate of 10−10, independent of the switch size.

This result seems very encouraging ...

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