On the Stability of Isolated and Interconnected Input-Queued Switches under Multiclass Traffic

In this correspondence, we discuss the stability of scheduling algorithms for input-queueing (IQ) and combined input/output queueing (CIOQ) packet switches. First, we show that a wide class of IQ schedulers operating on multiple traffic classes can achieve 100 % throughput. Then, we address the problem of the maximum throughput achievable in a network of interconnected IQ switches and CIOQ switches loaded by multiclass traffic, and we devise some simple scheduling policies that guarantee 100 % throughput. Both the Lyapunov function methodology and the fluid modeling approach are used to obtain our results

1 Asynchronous vs Synchronous Input-Queued Switches

Abstract—Input-queued (IQ) switches are one of the reference architectures for the design of high-speed packet switches. Classical results in this field refer to the scenario in which the whole switch transfers the packets in a synchronous fashion, in phase with a sequence of fixedsize timeslots, tailored to transport a minimum-size packet. However, for switches with large number of ports and high bandwidth, maintaining an accurate global synchronization and transferring all the packets in a synchronous fashion is becoming more and more challenging. Furthermore, variable size packets (as in the traffic present in the Internet) require rather complex segmentation and reassembly processes and some switching capacity is wasted due to partial filling of timeslots. Thus, in this work we consider a switch able to natively transfer packets in an asynchronous fashion thanks to a simple and distributed packet scheduler. We investigate the performance of asynchronous IQ switches and show that, despite their simplicity, their performance is comparable or even better than those of synchronous switches. These results highlight the great potential of the asynchronous approach for the design of high-performance switches.

Strong Performance Guarantees for Asynchronous Buffered Crossbar Schedulers

Crossbar-based switches are commonly used to implement routers with throughputs up to about 1 Tb/s. The advent of crossbar scheduling algorithms that provide strong performance guarantees now makes it possible to engineer systems that perform well, even under extreme traffic conditions. Until recently, such performance guarantees have only been developed for crossbars that switch cells rather than variable length packets. Cell-based crossbars incur a worst-case bandwidth penalty of up to a factor of two, since they must fragment variable length packets into fixed length cells. In addition, schedulers for cell-based crossbars may fail to deliver the expected performance guarantees when used in routers that forward packets. We show how to obtain performance guarantees for asynchronous crossbars that are directly comparable to those previously developed for synchronous, cell-based crossbars. In particular we define derivatives of the Group by Virtual Output Queue (GVOQ) scheduler of Chuang et al. and the Least Occupied Output First Scheduler of Krishna et al. and show that both can provide strong performance guarantees in systems with speedup 2. Specifically, we show that these schedulers are work-conserving and that they can emulate an output-queued switch using any queueing discipline in the class of restricted Push-In, First-Out queueing disciplines. We also show that there are schedulers for segment-based crossbars, (introduced recently by Katevenis and Passas) that can deliver strong performance guarantees with small buffer requirements and no bandwidth fragmentation