Constructions and analysis of crosstalk-free optical queues

There are several recent advances in the constructions of conflict-free optical queues using optical switches and fiber delay lines (SDL). Many of these conflict-free optical queues are constructed by 2 times 2 switches and thus suffer from the notorious crosstalk problem. To alleviate the crosstalk problem, it is suggested in the literature that one should avoid routing two packets to the same 2 times 2 switch at the same time. A construction with such a property is called crosstalk-free. Our first contribution of this paper is to extend the two-step method by Deng and Lee to a three-step method that converts a conflict-free SDL element into a crosstalk-free SDL element. For certain optical queues, including linear compressors, linear decompressors, and non-overtaking delay lines, an additional step (the fourth step in our paper) can be taken to further reduce the construction complexity. For such optical queues, one only needs one more 2 times 2 switch than the original construction. Our second contribution of this paper is to provide worst case analysis of the signal-to-noise ratio (SNR) in both the original conflict-free SDL element and the converted crosstalk-free SDL element. Closed-form expressions for the worst case SNRs in both SDL elements are obtained. These expressions show that the gain of the worst case SNR in the converted crosstalk-free SDL element is quite substantial.