Achieving Real-Time Communication over Ethernet with Adaptive Tra c Smoothing

Ethernet continues to be one of the most popular LAN technologies. Due to the low price and robustness resulting from its wide acceptance and deployment, there has been an attempt to build Ethernet-based real-time control networks for manufacturing automation. However, it is di cult to build a realtime control network using the standard UDP or TCP/IP and Ethernet, because the Ethernet MAC protocol, the 1-persistent CSMA/CD protocol, has unpredictable delay characteristics. When both real-time (RT) and non-real-time packets are transported over an ordinary Ethernet LAN, RT packets from a node may experience a large delay due to (i) contention with non-RT packets in the originating node and (ii) collision with RT and non-RT packets from the other nodes. To resolve this problem, we designed, implemented, and evaluated adaptive tra c smoothing. Speci cally, a tra c smoother is installed between the UDP or TCP/IP layer and the Ethernet MAC layer, and works as an interface between them. The tra c smoother rst gives RT packets priority over non-RT ones in order to eliminate contention within each local node. Second, it smooths a non-RT stream so as to reduce collision with RT packets from the other nodes. This tra c smoothing can dramatically decrease the packet-collision ratio on the network. The tra c smoother, installed at each node, regulates the node's outgoing non-RT stream to maintain a certain tra c-generation rate. In order to provide a reasonable non-RT throughput, the tra c-generation rate is allowed to adapt itself to the underlying network load condition. This tra c smoother requires only a minimal change in the OS kernel without any modi cation to the current standard of Ethernet MAC protocol or the TCP or UDP/IP stack. We have implemented the tra c smoother on both the Linux and the Windows NT platforms, demonstrating signi cant reduction of the RT packet deadline-miss ratio when both RT and non-RT packets are transported over the same Ethernet. More precisely, installation of the proposed tra c smoother on every node is shown to reduce the RT message deadline-miss ratio by two orders of magnitude under a heavily-loaded condition, while lowering the non-RT throughput only by half. Index Terms | 1-persistent CSMA/CD, Ethernet, real-time communication, tra c smoothing. The work reported in this paper was supported in part by General Motors, Warren, MI, and Lawrence Livermore Laboratories under the TIMES Phase III contract.