Analysis of queueing displacement using switch port speedup

Current high-speed packet switching systems, ATM in particular, have large port buffering requirements. The use of highly integrated ASIC technology for implementing high-degree and high-speed switch fabrics is facing a technology mismatch in the sense that today´s chip technology does not allow to integrate on-chip the high-speed switching fabric with the large buffering requirements. Consequently, many designs are based on the principles of queueing displacement, i.e., they attempt to move the queueing point off-chip. This is usually done by considerably speeding-up the on-chip switch output ports and placing a second external stage of buffering between the switch fabric and the outgoing link circuitry. Such designs are very popular and are used by many current ATM switch vendors. While such schemes are widely used, no rigourous analysis has so far been offered to evaluate the design trade-offs and to quantify the design points. The model we use to analyze the performance of the above system is a two-node tandem queueing system. The first node in the tandem corresponds to the internal buffer while the second node in the tandem corresponds to the external buffer. It is assumed that the internal buffer is able to transfer c₁ cells per time unit to the external buffer, while the external buffer is served at a lower rate of c₂ cells per time unit