Design and evaluation of a DRAM-based shared memory ATM switch

Beluga is a single-chip switch architecture specifically targeted at local area ATM networks, and it features three architectural innovations. First, an interconnection hierarchy composed of multiple switching fabrics is built into the chip to provide both low-latency cell transfer when the traffic is light and low cell drop rate under heavy load. Secondly, to improve silicon efficiency, Beluga is based on shared memory architecture, and the buffers are implemented using DRAM rather than SRAM technology. Heavy interleaving and selective invalidation are used to address long latency and periodic refreshing problems, respectively. Thirdly, Beluga supports multicast with minimal physical bit replication. It also separates support for unicast and multicast cells to optimize for the common case, where multicast cells occur infrequently. This paper describes the design details of Beluga and the results of a comprehensive simulation study to quantify the performance impact of each of its architectural features. The most important result from this research is that DRAM-based buffer implementation significantly reduces the cell-drop rate during heavy while exhibiting almost identical cell latency to SRAM-based implementation during light load. Therefore, we believe DRAM makes an attractive alternative for switch buffer implementation, especially for single-chip architecture such as Beluga.