Packet mode scheduling in buffered crossbar (CICQ) switches

Abstract

Buffered crossbars have emerged as an advantageous switch architecture mainly due to their scheduling efficiency and capacity to operate directly on variable size packets. Such operation requires crosspoint buffers at least as large as one maximum packet each. When we cannot afford that large crosspoint buffers, we are forced to segment packets. Although variable-size segments can be used to avoid padding overheads, we are still left with the cost of reassembly buffers and the associated delays. This paper applies packet mode scheduling to buffered crossbars in order to remedy these shortcomings: all segments of a variable-size packet are switched consecutively in time. We propose two scheduling schemes: probabilistic and deterministic packet mode scheduling. The probabilistic case allows cut-through forwarding and operates with independent crossbar output schedulers, but it requires reassembly buffers. Deterministic scheduling sacrifices some scheduler independence in order to eliminate reassembly buffers. Using simulation we show that it performs very close to buffered crossbars with no segmentation and large buffers at the crosspoints