Adapting a main-stream internet switch architecture for multihop real-time industrial networks

Abstract

As real-time industrial control systems scale up, single real-time local area network (LAN) is no longer sufficient; instead, we need real-time switches to merge many real-time LANs into real-time wide area networks (WANs). However, nowadays commercially-off-the-shelf WAN switches are designed for best-effort Internet traffic rather than real-time traffic. To address this problem, we propose a real-time crossbar switch design that minimally modifies, and even simplifies the de facto industrial standard switch design of iSLIP. Specifically, we change the iSLIP request-grant-accept negotiation to deterministic grant. The switch runs periodically with an M cell-time clock-period. Every input port runs per-flow queueing, and every output port deterministically grants input port per-flow queues according to its own M cell-time clock-period schedule. The schedules are created offline. We prove that the global scheduling can be reduced to a preemptive open shop scheduling problem; as long as every input/output needs to send/fetch no more than M cells per M cell-time clock-period, all outputs schedules do not conflict; and the scheduling algorithm takes O(N4) time (N is the number of input/output ports). Such design serves real-time periodic/aperiodic traffic in a time-division multiple-access (TDMA) fashion. This simplifies analysis, provides isolation, and results in a close-form end-to-end delay bound. We implemented the proposed real-time switch using Xilinx field programmable gate arrays (FPGAs), and built a distributed control test bed upon the switched networks. Using the test bed, we carried out experiments to compare the implemented real-time switches and iSLIP switches. The results prove the necessity of using real-time switches for real-time industrial control.