Current issues in packet switch design

Abstract

Addressing the ever growing capacity demand for packet switches, current research focuses on scheduling algorithms or buffer bandwidth reductions. Although these topics remain relevant, our position is that the primary design focus for systems beyond 1 Tb/s must be shifted to aspects resulting from packaging disruptions. Based on trends such as increased link rates and improved CMOS technologies, we derive new design factors for such switch fabrics. For instance, we argue that the packet round-trip transmission time within the fabric has become a major design parameter. Furthermore, we observe that high-speed fabrics have become extremely dependent on serial I/O technology that is both high speed and high density. Finally, we conclude that in developing the architecture, packaging constraints must be put first and not as an afterthought, which also applies to solving the tremendous power consumption challenges. MOTIVATIONMost research on single-stage, electronic packet switches focuses primarily on high-level architectural issues such as buffering and queuing strategies and scheduling algorithms, but seldom considers all the physical issues that arise when actually building the system, a tendency that has also been noted in [1]. For instance, although many new packet-switch architectures have been proposed in the past few years, the majority of these architectural-level papers reduce memory bandwidth by eliminating the N -fold speed-up required by output queuing and instead optimizing the performance of the centralized scheduling algorithm needed in such architectures. Although reducing memory bandwidth is an important issue, it is not sufficient in itself and may render the resulting system economically infeasible. Good examples are the recent proposals of combined input-and outputqueued (CIOQ) switches with limited speed-up, which require speed-up throughout the fabric (i.e., both input and output speed-up), thus multiplying the bandwidth that must be carried across the switch core, whereas the output speed-HOTNETS '02 10/02 Princeton, NJ, USA up implemented in an output-queued switch is purely internal to the switch core.Designers of practical high-capacity packet switches face challenges on two levels: First, they must choose a design point at the architectural level, in terms of buffering and queuing strategies, scheduling algorithms, and flow-control methods. For example, for the buffering strategy, the choice to be made is between an input-, output-, or combined-queuing structure. Second, they must consider the physical level, i.e., the implementation of the architecture at the system as well as the chip level, in terms of partitioning over racks, cards, chips, and the design of the individual chips comprising the system. Switch designers have little freedom with respect to system packaging issues. On the one hand the technology imposes constraints, on the other hand customers impose their specific requirements, in addition to more general ones such as NEBS (Network Equipment Building System) compliance [2...

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